Fiber Sensing Based on New Structures and Post-Processing Enhancement
Full text
D Fiber Sensing Based on New Structures and Post-Processing Enhancement Marta Sofia dos Anjos Ferreira Physics Physics Department 2015 Supervisor Orlando José dos Reis Frazão, Invited Assistant Professor, FCUP Co-supervisor José Luís Campos de Oliveira Santos, Full Professor, FCUP
FACULDADE DE CIÊNCIAS DA UNIVERSIDADE DO PORTO FIBER SENSING BASED ON NEW STRUCTURES AND POST-PROCESSING ENHANCEMENT Marta Sofia dos Anjos Ferreira Thesis submitted to Faculdade de Ciências da Universidade do Porto in partial fulfillment of the requirements for the degree of Ph.D. in Physics This Thesis was conducted under the supervision of Dr. Orlando José dos Reis Frazão Invited Assistant Professor of Departamento de Física e Astronomia da Faculdade de Ciências da Universidade do Porto and Prof. Dr. José Luís Campos de Oliveira Santos Full Professor of Departamento de Física e Astronomia da Faculdade de Ciências da Universidade do Porto
Bolsa de investigação da Fundação para a Ciência e a Tecnologia com a referência SFRH/BD/76965/2011, financiada pelo POPH – QREN – Tipologia 4.1 – Formação Avançada, comparticipada pelo Fundo Social Europeu e por fundos nacionais do MCTES.
Dedicated to my Mom
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement ix ACKNOWLEDGMENTS I would like to express my deepest gratitude to my supervisor, Dr. Orlando Frazão, for challenging me to pursue the dream of performing investigation in a highly technological field. His help, his knowledge and insight were decisive for the development of this work. To my co-supervisor, Professor José Luís Santos, who was a source of inspiration ever since I started working at INESC. His wise words and the valuable discussions allowed me to grow not only as a physicist, but also as a human being. Thank you, Professor. Professor António Pereira Leite thank you for all the valuable help in reviewing this Thesis. I would also like to acknowledge all my colleagues at INESC, companions on this journey, who gave me constant care and supported me right from start. Many thanks, Ricardo Silva, Paula Tafulo, Luís Coelho, Raquel Queirós, Carlos Gouveia, Paulo Caldas, João Moura, Ivo Nascimento, Rita Ribeiro, Susana Silva and Lídia Carvalho. Thank you, Paulo Roriz and Ricardo André, for those wonderful days in the lab. A special hug for you both! You rock! Thanks to all the senior researchers at INESC for the constant support and attention. To dearest Luísa, such an important figure in our inescian lives! Thank you all for the kind words, the smiles, the help and patience. Throughout this four-year journey, I was fortunate to travel and meet different research groups. I would like to acknowledge Professor Kate Sugden, from the group of Aston Institute of Photonic Technologies, in Birmingham, England. I would also like to thank Dr. Graham Lee and Dr. Neil Gordon. It was such a pleasure to work with you! To Dr. Kaiming Zhou, my deepest thanks for making the lab available for me. Finally, I would like to thank Dr. Mykhaylo Dubov, for the cup of tea and the discussion that changed the way I saw my research. To my dearest German friends at the Leibnitz Institute of Photonic Technology, in Jena, I am profoundly grateful. To Dr. Kay Schuster, who always welcomed and trusted in me. Thanks to Dr. Jörg Bierlich, for all the nice moments, the conversations and
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement xvi 4.5.1 Water Temperature and Refractive Index Relationship ............................ 71 4.5.2 Experimental Results ...................................................................................... 72 4.6 Final Remarks .......................................................................................................... 76 5 Sensing Structures Incorporated in Optical Fibers ............................................. 79 5.1 Introduction ............................................................................................................. 81 5.2 Controlling the Sensitivity of a Fabry-Perot Strain Sensor ............................... 82 5.2.1 Sensor Design and Spectral Characteristics ................................................ 83 5.2.2 Experimental Results ...................................................................................... 84 5.3 Measuring Strain at High Temperatures (Part I): Silica Tube ........................... 87 5.3.1 Sensor Design and Spectral Characteristics ................................................ 87 5.3.2 Experimental Results ...................................................................................... 89 5.4 Measuring Strain at High Temperatures (Part II): Fiber Bragg Gratings ........ 94 5.4.1 Sensor Design and Spectral Characteristics ................................................ 94 5.4.2 Experimental Results ...................................................................................... 95 5.5 Final Remarks .......................................................................................................... 99 6 Fiber Lasers for Sensing.......................................................................................... 101 6.1 Introduction ........................................................................................................... 103 6.2 Strain Sensor based on Post-Processed Fiber Bragg Grating .......................... 105 6.2.1 Theoretical Considerations .......................................................................... 105 6.2.2 Sensor Design and Spectral Characteristics .............................................. 108 6.2.3 Passive Configuration .................................................................................. 110 6.2.4 Active Configuration .................................................................................... 113 6.3 Torsion Sensor based on Figure-of-Eight Fiber Laser ...................................... 116 6.3.1 Working Principle ......................................................................................... 116 6.3.2 Sensor Design and Spectral Characteristics .............................................. 118 6.3.3 Experimental Results .................................................................................... 119 6.3.4 Final Remarks ................................................................................................ 121 7 Sensors Based on Microspheres ............................................................................ 123 7.1 Introduction ........................................................................................................... 125 7.2 Silica Microspheres Array Sensor ....................................................................... 127 7.2.1 Theoretical Considerations .......................................................................... 127 7.2.2 Sensor Design and Spectral Characteristics .............................................. 129 7.2.3 Experimental Results .................................................................................... 132
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement xvii 7.3 Fabry-Perot based on Array of Soda-Lime Glass Microspheres ..................... 133 7.3.1 Theoretical Considerations ........................................................................... 133 7.3.2 Sensor Design and Spectral Characteristics ............................................... 134 7.3.3 Experimental Results..................................................................................... 137 7.4 Final Remarks ......................................................................................................... 139 8 Final Conclusions and Future Work ..................................................................... 141 9 Appendices ................................................................................................................ 147 Appendix I – Fabrication of a Double Clad Optical Fiber ........................................ 149 Appendix II - Point-by-Point Femtosecond Laser FBG Inscription ......................... 151 Appendix III - Interferometric Excimer Laser FBG inscription ................................ 153 10 References .................................................................................................................. 155
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement xix LIST OF FIGURES Figure 2.1 – Schematic examples of fiber optic FP interferometers: a) and b) extrinsic configurations, c) and d) intrinsic configurations. ................................................................................. 12 Figure 2.2 – First page of the paper published by Fabry and Perot in 1897 [11]. ............................... 13 Figure 2.3 – Fabry-Perot interferometer experimental setup. A stands for the optical source (electric arc in the original), L, L’ and L’’ are lenses, A’ is a slit, E is the display and B corresponds to the silvered glass [11]. ............................................................................................................................ 13 Figure 2.4 – (a) Configuration proposed in 1979 by Cielo [12]. L is the light source coupled to the fiber, R corresponds to the reflectors, D is the photodetector, and S stands for the servo-control electronics. (b) Spectral response of a single cavity [12]. ....................................................................... 14 Figure 2.5 – Experimental setup for the evaluation of multiplexed FP sensors [25]. ........................ 16 Figure 2.6 - Photograph of the 112 m long in-line fiber etalon proposed by Sirkis et al. in 1993 [41]. ............................................................................................................................................................... 18 Figure 2.7 – Structure of the fiber Bragg grating FP cavity proposed by Du et al. (adapted from [57]). .............................................................................................................................................................. 19 Figure 2.8 – Structure of the FP temperature sensor proposed by Tsai et al. [63]. ............................. 20 Figure 2.9 – Configuration of an in-line hollow-core PCF etalon, proposed by Rao et al. [85]. ....... 22 Figure 2.10 – Diagram of the configuration proposed by Villatoro et al. [101]. FOC stands for fiber optic circulator, LED is the light emitting diode and OSA corresponds to the optical spectrum analyzer. ....................................................................................................................................................... 24 Figure 2.11 – Scanning electron microscope image of the FP cavity created using FIB [108] . ........ 24 Figure 2.12 – Microscope photograph of the first diaphragm-free FP cavity for gas pressure sensing [122]. ............................................................................................................................................... 25 Figure 3.1 – Numerical curves obtained for the refractive index of N2 considering (a) the dependence on wavelength, and (b) the dependence on pressure, at a temperature of 20 °C and a constant wavelength of 1550 nm. RIU stands for refractive index units. ........................................... 39 Figure 3.2 – Microscope photograph of the silica tube cross-section. ................................................. 40 Figure 3.3 – Schematic of the procedures used to fabricate the FP cavity: (a) image from the splicing machine display, evidencing the lateral offset, prior to splicing (SMF on the left and silica tube on the right), (b) image after splicing, (c) device prior to cleaving, the arrows indicate where the cleave should be done and (d) microscope image of a FP cavity produced with this method. 40 Figure 3.4 – Scheme of the experimental setup. ..................................................................................... 41 Figure 3.5 – Left: Scheme of the sensing head, highlighting the reflections occurring in the cavity. Right: cross section photograph of one sample when illuminated with a He-Ne laser. ................... 41 Figure 3.6 – Spectra of four sensing heads with different FP cavity lengths. ..................................... 42 Figure 3.7 – Spatial frequency spectra for four different cavity lengths. ............................................ 42 Figure 3.8 – Free spectral range dependence on the cavity length, considering two adjacent peaks with wavelengths close to 1550 nm. ......................................................................................................... 43 Figure 3.9 – Temperature response of the 141 m long sensing head. Inset 1 (top left): low temperatures response; inset 2 (bottom right) high temperatures response. ..................................... 44
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement xx Figure 3.10 – Sensing heads response to the applied pressure. ............................................................ 45 Figure 3.11 – Sensing heads response to the N2 refractive index change. All measurements were done at room temperature (~20 °C), and the monitored wavelength was around 1550 nm. ........... 46 Figure 3.12 - Sensing heads response to the N2 (a) pressure and (b) refractive index change. All measurements were done at room temperature (~20 °C), and the monitored wavelength was around 1550 nm. The hollow core silica tube presented an inner diameter of ~60 m. .................... 47 Figure 3.13 – Photographs of the hollow core ring fiber cross-section (a) at the scanning electron microscope (SEM) and (b) when illuminated with a He-Ne laser. ...................................................... 48 Figure 3.14 – Spectra of three sensing heads based on the HCR-PCF FP cavity. ............................... 49 Figure 3.15 – Temperature response of the 360 m long sensing head. .............................................. 50 Figure 3.16 – Sensing head response to the applied pressure, (a) atmosphere of N2, different FP cavity lengths and (b) different gas atmospheres for a cavity length of 360 m. .............................. 51 Figure 3.17 – Schematic drawing of the sensor proposed for low-pressure measurements [208]. .. 52 Figure 3.18 – Spectral response of the sensing head. Also shown the spectral shift when hydrostatic pressure is applied (step of 37.5 mmHg). ........................................................................... 53 Figure 3.19 – Sensor response to hydrostatic pressure variation. ........................................................ 54 Figure 4.1 – Schematic designs of some of the double clad optical fibers reported in the literature. ....................................................................................................................................................................... 59 Figure 4.2 – (a) Cross section of the P-doped double clad optical fiber. (b) Refractive index profile measured using a short section of the preform. ..................................................................................... 62 Figure 4.3 – Left: Microscope photos of fiber tip formation after an etching time of a) ~9 s, b) ~27 s, c) ~46 s and d) ~65 s. Right: Cavity length formation vs. time. Inset: SEM image of the etched cavity cross-section. .................................................................................................................................... 63 Figure 4.4 – Scheme of the optical fiber tip design fabrication steps. The red arrows indicate the steps to produce the diaphragm-free configuration and the blue ones are related to the configuration with diaphragm. ................................................................................................................. 64 Figure 4.5 – Scheme of the experimental setup. ..................................................................................... 64 Figure 4.6 – (a) Scheme of the FP cavity for high temperature measurement and (b) photograph of the sensing head when illuminated with a He-Ne laser. ...................................................................... 65 Figure 4.7 – Experimental (black line) and theoretical (green dashed line) spectra of the sensing head reflection response. ........................................................................................................................... 67 Figure 4.8 – Wavelength dependence on temperature. ......................................................................... 67 Figure 4.9 – FP microcavity evidencing the interface reflections. ........................................................ 68 Figure 4.10 – Scheme of the resultant wave phase variation with the amplitude of E3; E2 remains constant. ....................................................................................................................................................... 70 Figure 4.11 – Simulated spectra of the FP micro-cavity in different media. The inset shows the phase variation of the spectrum. .............................................................................................................. 71 Figure 4.12 – (a) Dependence of the refractive index of water on the operation wavelength for different temperatures and (b) refractive index of water as a function of temperature, for a wavelength of 1550 nm. ............................................................................................................................. 72
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement xxi Figure 4.13 – Spectra of the FP micro-cavity when the external medium is air (black line) and water (blue line). (a) Sensor with a thin diaphragm. (b) Sensor with a thick diaphragm. ............... 73 Figure 4.14 – Wavelength shift dependence of the sensor response with a diaphragm of 43 µm on the applied temperature, in two different media. .................................................................................. 74 Figure 4.15 – Wavelength shift dependence on temperature: (a) sensing head exposed to air (black circles) and when immersed in water (blue circles) and (b) calculated water contribution. ........... 75 Figure 4.16 – Wavelength shift variation with the water refractive index.......................................... 76 Figure 5.1 – Microscope images of (a) the HCR-PCF cross-section and (b) the 207 m long sample. ....................................................................................................................................................................... 83 Figure 5.2 – Scheme of the experimental setup. OSA stands for optical spectrum analyzer and FP cavity corresponds to the Fabry-Perot cavity. ........................................................................................ 83 Figure 5.3 – Spectra of the four samples, with different FP cavity lengths. The spectrum shift with the applied strain is also shown for each sample. .................................................................................. 84 Figure 5.4 – Identification of the lengths considered in the strain analysis. ....................................... 84 Figure 5.5 – (a) Sensors response to the applied strain. (b) Sensitivity dependence on the FP cavity length. Inset: microscope photograph of the 13 m long sensing head. ............................................. 85 Figure 5.6 - (a) Response of the 207 m long sensor cavity to strain, considering three different gauge lengths. (b) Sensitivity dependence on the gauge length (purple dots) and tendency curve (gray line). .................................................................................................................................................... 86 Figure 5.7 – Wavelength dependence on temperature for the 207 m long sensing head. .............. 86 Figure 5.8 – Cross section images of the silica tube varying the pressure during fiber drawing: (a) p = 1000 Pa, (b) p = 2300 Pa and (c) p = 3000 Pa. ...................................................................................... 87 Figure 5.9 – Photograph of one FP cavity based on the new hollow core silica tube design. .......... 88 Figure 5.10 – Spectra of the four FP cavity sensors. ............................................................................... 88 Figure 5.11 – (a) FP cavity sensors response to the applied strain. (b) Response of the 198 m long sensor to temperature. ............................................................................................................................... 89 Figure 5.12 – Response of the 70 m long FP cavity to the applied strain at different temperatures. Up and down stand for increasing and decreasing the applied strain, respectively. ....................... 90 Figure 5.13 – Wavelength shift of the 51 µm long FP cavity for an annealing temperature of 900 °C............................................................................................................................................................ 91 Figure 5.14 – Response of the 51 m long FP cavity to strain at different temperatures, after 7 hours of annealing, at 900 °C. Up and down stand for increasing and decreasing the applied strain, respectively. ..................................................................................................................................... 92 Figure 5.15 – Dependence of the strain sensitivity at different temperatures: (a) without annealing and (b) with annealing. .............................................................................................................................. 92 Figure 5.16 – Sensors response to the applied strain until rupture. The insets show the crosssection photographs of the silica tube used as sensing element in each case..................................... 93 Figure 5.17 – Microscope photograph of a fiber Bragg grating written using the point-by-point femtosecond laser technique. .................................................................................................................... 94 Figure 5.18 – Scheme of the experimental setup. OSA refers to the optical spectrum analyzer and FBG is the fiber Bragg grating (the scheme is not to scale). .................................................................. 95
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement xxii Figure 5.19 – Initial transmission spectra of the two fiber Bragg gratings (FBGs)............................. 95 Figure 5.20 – FBG1 response to temperature. ......................................................................................... 96 Figure 5.21 – FBG1 response to an annealing temperature of 900 °C. (a) Bragg wavelength shift with time and (b) Reflectivity variation with time. ................................................................................ 97 Figure 5.22 – Strain sensitivity variation with temperature (a) after annealing at 900 °C, FBG1, and (b) without annealing, FBG2. .................................................................................................................... 98 Figure 5.23 – Transmission spectra of (a) FBG1, before annealing (dotted curve), after annealing (dashed curve) and after strain at high temperatures (solid curve) and (b) FBG2, before (dotted curve) and after (solid curve) being subjected to strain at high temperatures. .................................. 98 Figure 6.1 – Scheme of a tapered FBG without strain applied (top) and under strain (bottom). Adapted from [268]. ................................................................................................................................. 105 Figure 6.2 – Numerical simulation of an etched FBG, considering three values of initial strain. (a) variation of the pitch with the length and (b) strain variation along the grating length. ............... 107 Figure 6.3 – (a) Cladding diameter variation with the chemical etching time. Also shown microscope images of the fiber tip after etching times of (b) ~30 min, (c) ~55 min and (d) ~61 min. ..................................................................................................................................................................... 109 Figure 6.4 – Reflection spectra of the FBG at different etching times. ............................................... 110 Figure 6.5 – Scheme of the experimental setup. ................................................................................... 110 Figure 6.6 – Spectra of the etched FBG tip when no strain is applied (blue line) and with 93 (pink line). .................................................................................................................................................. 111 Figure 6.7 – Response of the etched FBG tip to the applied strain by monitoring (a) the wavelength at 3 dB (solid black dots) and the peak P1 wavelength (green dots) and (b) the fullwidth at half-maximum (FWHM). ......................................................................................................... 112 Figure 6.8 – Sensor response to temperature. ....................................................................................... 113 Figure 6.9 – Scheme of the experimental setup. EDFA is the erbium doped fiber amplifier and FBG stands for fiber Bragg grating. ........................................................................................................ 113 Figure 6.10 – Laser output power as a function of the pump power. The laser stability for a constant pump power of 50 mW, over 60 min is also shown (purple line). ..................................... 114 Figure 6.11 – Response of the laser to the applied strain, regarding (a) the spectral variation and (b) the wavelength shift. The inset represents the integrated power as a function of the applied strain. .......................................................................................................................................................... 115 Figure 6.12 – Step technique to estimate the resolution of the fiber laser strain sensor. ................. 116 Figure 6.13 – Scheme of the figure-of-eight fiber laser. WDM stands for wavelength division multiplexer, PC is the polarization controller and OSA corresponds to the optical spectrum analyzer. ..................................................................................................................................................... 117 Figure 6.14 – (a) Microscopic photograph of the polarization-maintaining photonic crystal fiber used as sensing element. (b) Transmission spectrum of the sensor. ................................................. 118 Figure 6.15 – Optical power variations with the drive-in current. ................................................... 119 Figure 6.16 – (a) Variation of the laser emission with the applied torsion and (b) interferometric filter spectrum (blue line) and laser spectrum (pink line). .................................................................. 120
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement xxiii Figure 6.17 – Laser peak wavelength dependence on the applied (a) torsion angle and (b) strain. ..................................................................................................................................................................... 120 Figure 7.1 – Microspheres array sensors modeling using ZEMAX SE, considering (a) 2 microspheres, (b) 3 microspheres, (c) 4 microspheres and (d) 5 microspheres. The focal points f1, f2 and f3 for each configuration are also shown (when applicable). ...................................................... 128 Figure 7.2 – Microphotographs of the microspheres manufactured using the splicing machine. Each photo was taken after one electric arc discharge. ....................................................................... 129 Figure 7.3 – Dependence of the sphere diameter on the number of electric arc discharges. ......... 130 Figure 7.4 – Channeled spectra of light that exits the sensing heads with (a) 2 microspheres, (b) 3 microspheres, (c) 4 microspheres and (d) 5 microspheres. ................................................................. 131 Figure 7.5 – Sensors response to applied strain. ................................................................................... 132 Figure 7.6 – Response of the 3-microspheres sample to temperature variation. ............................. 133 Figure 7.7 – Scheme of the ray tracing through a ball lens. Adapted from [311]. ............................ 134 Figure 7.8 – Scheme of the experimental setup. ................................................................................... 135 Figure 7.9 – (left) Spectra of the sensing heads tested. (right) Microphotographs of the characterized sensors. .............................................................................................................................. 136 Figure 7.10 – Dependence of the spectral visibility on the number of microspheres. Inset: Microphotograph of the 4-microspheres sample when illuminated with a He-Ne laser. .............. 137 Figure 7.11 – Wavelength variation with temperature. ....................................................................... 137 Figure 9.1 – Preform fabrication using the MCVD technique, evidencing (a) the burner that moves along the tube when the layers are being deposited and (b) when the structure is being collapsed at extremely high temperatures. ............................................................................................................. 149 Figure 9.2 – Fiber drawing components: (a) the drawing furnace with the preform, (b) the UV curing lamp and (c) capstan and drum winder. ................................................................................... 150 Figure 9.3 – Experimental setup for the femtosecond laser system used to inscribe the fiber Bragg gratings. CCD stands for charged coupled device. Adapted from [317]. ......................................... 151 Figure 9.4 – Scheme of the interferometric KrF excimer laser setup. Adapted from [318]. ............ 153
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement xxv LIST OF TABLES Table 2.1 - Different fiber optic intrinsic FP sensors, with the respective characteristics, from 20092015. .............................................................................................................................................................. 28 Table 3.1 - Different fiber optic gas pressure sensors based on FP configurations, with the respective characteristics. .......................................................................................................................... 34 Table 3.2 – Sensitivity of the different sensing heads to N2 pressure and to refractive index. ........ 46 Table 5.1 – Parameters of the FBGs at room temperature. The values were obtained through the optical transmission spectra, prior to any measurements (initial values), after 7 hours thermal annealing at 900 °C and after applying strain at different temperatures (final values). The total wavelength shift is relative to the beginning and end of the measurements. .................................... 99 Table 6.1 – Properties of the interferometric filter................................................................................ 119 Table 7.1 – Strain sensitivity obtained for each sensor. ....................................................................... 132 Table 7.2 – Temperature sensitivity obtained for each sensor. corresponds to the wavelength, in nm, and T to the temperature, in °C. The correlation coefficient, r2 is also shown. ........................ 138
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 4 The main objectives relied on: the study of diaphragm-free microcavities for gas pressure sensing; characterization of sensors based on the post-processing of a purposedesigned double clad optical fiber; study of strain measurement in harsh environments, such as in high temperatures; development of fiber laser sensors and their respective characterization; manufacturing and characterization of interferometric structures based on microspheres. 1.3 Structure of the Thesis This Thesis is divided in eight Chapters, of which five are relative to experimental work developed in the PhD framework. Chapter 1 gives an overview of the Thesis structure and its framing within the fiber sensing field. It also contains the main contributions and the list of works published during the PhD. Chapter 2 provides an historical overview on Fabry-Perot based optical fiber sensors. This review results from the fact most of the sensors described in the Thesis are within this field. The emphasis is done on the cavities configurations, the measurands and the sensitivities achieved so far. Chapter 3 proposes two different Fabry-Perot cavities for the measurement of gas pressure. The devices are based on a hollow core silica tube and a hollow core ring photonic crystal fiber. The former was the first diaphragm-free Fabry-Perot sensor used in such application reported in the literature. Still in the third Chapter, a prototype for biomedical applications is addressed. In the Chapter 4, two distinct Fabry-Perot configurations based on the postprocessing of a double clad optical fiber are described. The inner cladding, doped with phosphorus, is removed through chemical etching and a tip, protected by the outer
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 5 cladding, is formed. The diaphragm free configuration is characterized in high temperature. By introducing a diaphragm in the post-processed structure, an opticalphase refractometer is obtained. The Chapter 5 is dedicated to the measurement of strain. In a first approach, the control of strain sensitivity is proposed using a hollow core ring photonic crystal fiber Fabry-Perot cavity. Smaller cavities, combined with large lengths over which the strain is applied, result in more sensitive devices. The measurement of strain at high temperatures is studied for the case of a Fabry-Perot cavity and when using a fiber Bragg grating. In both cases the influence of annealing is addressed, in order to achieve better responses to strain. On the 6th Chapter, the matter of using fiber lasers as active sensors is explored. Besides the characterization of the laser cavities, the sensing elements are subjected to variations of torsion and strain. One of the active sensors, a post-processed fiber Bragg grating, is also studied in a passive configuration, for comparison purposes. An ultrahigh sensitivity to strain is achieved with this device. The sensors described in Chapter 7 are based on microspheres. Two different configurations are explored. One consists of an array of microspheres obtained by postprocessing single mode fiber by means of fusion splicing. The second configuration is a Fabry-Perot cavity obtained by placing the soda-lime microspheres inside a hollow core silica tube, which acts as a support structure. The Chapter 8 presents some lines summarizing the main results achieved during the PhD and describes the opportunities of future work that this investigation has created. 1.4 Main Contributions From the works presented in this Thesis, it is the author opinion that three of them stand out as main contributions to the field. The first was the use of a Fabry-Perot configuration for gas pressure sensing that did not require a diaphragm. Two different hollow core fibers were successfully used in this context, a silica tube and a photonic
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 6 crystal fiber. The second contribution was the optical phase refractometer based on a post-processed Fabry-Perot cavity. With such configuration it was possible to detect spectral phase variations with the refractive index. Finally, the new sensing structures based on an array of microspheres that were tested to strain and temperature. The response to strain was dependent on the number of microspheres that constituted the array. 1.5 List of Publications From the activity developed in the framework of this PhD, a total of 10 articles were published as first author in scientific journals, one of them being an invited paper, and another was a review paper published in the Optical Fiber Technology Journal. Besides, three papers were published as co-author, as a result of collaborations outside the scope of this Thesis. A total of 8 communications in national/international conferences were published during the PhD. The list of works published as first author is presented next. 1.5.1 Scientific Journals 1. M. S. Ferreira, P. Roriz, J. Bierlich, J. Kobelke, K. Wondraczek, C. Aichele, K. Schuster, J. L. Santos, O. Frazão, Fabry-Perot cavity based on silica tube for strain sensing at high temperatures, Opt. Express, vol. 23, no. 12, 2015. 2. M. S. Ferreira, J. L. Santos, O. Frazão, Silica microspheres array sensor, Opt. Letters, vol. 39, no. 20, 2014. 3. M. S. Ferreira, J. Bierlich, S. Unger, K. Schuster, J. L. Santos, O. Frazão, Optical phase refractometer based on post-processed interferometric tip sensors, J. Light. Technol., vol. 32, no. 17, 2014. 4. M. S. Ferreira, J. Bierlich, M. Becker, K. Schuster, J. L. Santos, O. Frazão, Ultra-high sensitive strain sensor based on post-processed optical fiber Bragg grating, MDPI Fibers, vol.2, pp.142-149, 2014. Invited Paper 5. M. S. Ferreira, P. R. Oliveira, S. Oliveira Silva, J. L. Santos, O. Frazão, Next generation of Fabry-Perot sensors for high-temperature, Opt. Fiber Technol., vol.19, 2013. Review Paper
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 7 6. M. S. Ferreira, J. Bierlich, S. Unger, K. Schuster, J. L. Santos, O. Frazão, Postprocessing of Fabry-Pérot microcavity tip sensor, IEEE Photonic. Tech. L., vol.25, no.16, pp.1593-1596, 2013. 7. M. S. Ferreira, J. L. Santos, P. Mergo, O. Frazão, Torsion sensor based on a figure-ofeight cavity fibre laser, Laser Phys. Lett., vol.10, no. 4, 2013. 8. M. S. Ferreira, J. Bierlich, J. Kobelke, , J. L. Santos, O. Frazão, Fabry-Pérot cavity based on hollow core ring photonic crystal fiber for pressure sensing, IEEE Photonic. Tech. L., vol.24, no.23, pp.2122-2124, 2012. 9. M. S. Ferreira, J. Bierlich, J. Kobelke, K. Schuster, J. L. Santos, O. Frazão, Towards the control of highly sensitive Fabry-Pérot strain sensor based on hollow-core ring photonic crystal fiber, Opt. Express, vol.20, no.20, pp.21946-21952, 2012. 10. M. S. Ferreira, L. C. Coelho, K. Schuster, J. Kobelke, J. L. Santos, O. Frazão, FabryPérot cavity based on a diaphragm free hollow core silica tube, Opt. Letters, vol.36, no.20, pp.4029-4031, 2011. 1.5.2 Communications in National/International Conferences 1. M. S. Ferreira, P. Roriz, J. Bierlich, J. Kobelke, K. Wondraczek, C. Aichele, K. Schuster, J. L. Santos, O. Frazão, Measuring strain at extreme temperatures with a Fabry-Perot optical fiber sensor, OFS24, Curitiba, Brazil, 2015. 2. M. S. Ferreira, G. Lee, J. L. Santos, K. Sugden, O. Frazão, Phase-shifted fiber Bragg grating for strain measurement at extreme conditions, OSA Meeting 2014, Barcelona, Spain, 2014. 3. M. S. Ferreira, J. L. Santos, O. Frazão, New silica microspheres array sensor, OFS23, Santander, Spain, 2014. 4. M. S. Ferreira, J. Bierlich, J. Kobelke, K. Wondraczek, C. Aichele, K. Schuster, J. L. Santos, and O. Frazão, Fabry-Pérot microcavity strains sensor based on advanced silica tube, AOP2014 - II International Conference on Applications of Optics and Photonics, Aveiro, Portugal, 2014. 5. M. S. Ferreira, J. L. Santos, P. Mergo and O. Frazão, Figure-of-eight cavity fiber laser based torsion sensor, RIAO/OPTILAS 2013, Porto, Portugal.
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 8 6. M. S. Ferreira, J. Bierlich, S. Unger, K. Schuster, J. L. Santos, O. Frazão, PostProcessed Fabry-Pérot microcavity tip sensors for temperature measurement, WSOF2013 - workshop on specialty optical fibers , Sigtuna, Sweden, 2013. 7. M. S. Ferreira, J. Bierlich, K. Schuster, J. L. Santos, O. Frazão, Fabry-Pérot microcavity tip temperature sensor based on post-processing, EWOFS 2013 - 5th European Workshop on Optical Fibre Sensors, Kraków, Poland, 2013. 8. M. S. Ferreira, K. Schuster, J. Kobelke, J. L. Santos, O. Frazão, Fabry-Pérot cavity based on large hollow core photonic crystal fiber for nitrogen pressure measurements, SEON 2012 – VIII Symposium on Enabling Optical Networks and Sensors, Aveiro, Portugal, 2012.
CHAPTER TWO 2 Historical Overview of Fiber Sensors Based on Fabry-Perot Interferometry
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 11 2.1 Introduction Fiber optic sensors based on interferometry have been widely explored over the last five decades. From the most basic configurations, the Mach-Zehnder, the Michelson, the Sagnac and the low-finesse Fabry-Perot (FP) interferometers stand out [2]. Regarding the FP interferometer, it is usually composed by two parallel reflecting surfaces with a small separation between them. Light reaching the cavity with near normal incidence, will suffer several internal reflections at the interfaces, resulting in a multiple beam interferometer. This translates in a higher interaction of the measurand with the guided light, and consequently, in a higher sensitivity. Besides, this interferometer offers unique advantages, such as the compactness, reliability and the fact that it does not require the presence of an extra fiber to serve as a reference arm, since the interference occurs within a single fiber [3]. In this Chapter, the most common fiber optic configurations based on Fabry-Perot interferometry are described, followed by an historical overview of the development of these structures since the first papers were published, in the 1980s. 2.2 Fabry-Perot Based Sensors: the Basic Characteristics Since the first fiber optic FP-based sensor proof of concept, that took place in the early 1980s, there has been a great evolution in this field. In the 1980s two different categories of optical fiber sensors based on FP interferometry arose, the extrinsic and the intrinsic configurations. In the former, the cavity, which acts as the sensing element, is located outside the fiber and the two mirrors required to form the cavity can be two fiber tips placed close enough to ensure interference (Fig. 2.1 (a)) or one fiber tip and a reflective element (Fig. 2.1 (b)). In order to keep the structure stable, it can be placed inside a capillary tube, as represented in the example in Fig. 2.1 (a). When the FP cavity is contained within the optical fiber, the configuration is considered to be intrinsic. One way to produce this type of configuration can be by
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 12 splicing a hollow core fiber between two single mode fibers, as represented in Fig. 2.1 (c), or by creating two in-line partial mirrors, as in Fig. 2.1 (d). Figure 2.1 – Schematic examples of fiber optic FP interferometers: a) and b) extrinsic configurations, c) and d) intrinsic configurations. From 1990 to 2005, the focus of research was mainly on the signal acquisition systems and on the interrogation techniques. In that period, a major evolution of extrinsic FP-based sensors occurred. However, with the arising of sensors based on fiber Bragg gratings (FBGs) in 1990 [4] and the photonic crystal fibers (PCFs) in 1996 [5], the number of papers about intrinsic FP sensors slowly began to rise. From the year 2005 until 2015, the majority of works are on intrinsic FP interferometers and the focus of research turned into the cavity designs. New configurations based on the fusion splicing of special optical fibers [6], the chemical etching [7], femtosecond laser micromachining [8], excimer laser micromachining [9] and, more recently, on the focused ion beam (FIB) micromachining [10] have been investigated. These new sensor devices, besides being easy to produce and reproducible, are reliable and have low dimensions, which can be of the order of a few micrometers. In order to provide an overview of the evolution in optical fiber FP cavity sensors, a thorough description is performed next. The focus was both on the cavities design evolution that occurred throughout the years and on the sensing measurands. A Table is presented at the end of this overview, with some of the most important configurations reported from 2009 to 2015. Besides the configuration, the measurands and respective sensitivities are considered.
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 13 2.3 The First Fabry-Perot Interferometer The Fabry-Perot (FP) interferometer was firstly published in 1897 by Charles Fabry and Alfred Perot, in the Annales de Chimie et de Physique [11]. Figure 2.2 presents the first page of one of the most important papers published by the two researchers, entitled Sur les franges des lames minces argentées et leur application à la measure de petites épaisseurs d’air. Figure 2.2 – First page of the paper published by Fabry and Perot in 1897 [11]. The simple configuration, depicted in Fig. 2.3, set the basis of one of the interferometers most widely used currently in optics and photonics. Light from the electric arc optical source (A) propagates through a system of lenses (L, L’) and will suffer multiple interferences at the silvered glass (B). At the third lens (L’’), the transmitted light will be focused and the interference pattern will appear on the display E. Figure 2.3 – Fabry-Perot interferometer experimental setup. A stands for the optical source (electric arc in the original), L, L’ and L’’ are lenses, A’ is a slit, E is the display and B corresponds to the silvered glass [11].
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 20 In 2001, a new extrinsic FP configuration was proposed by Jiang et al. [62]. The end section of an optical fiber was polished at 45° and placed close to a polymer film on a steel plate. The sensor was tested to strain and temperature. In the same year, a new concept of FP interferometer-based temperature sensors was presented by Tsai et al. [63]. The structure, depicted in Fig. 2.8, was obtained by fusion splicing two fibers with different core diameters. One of the reflective mirrors was produced at the interface between the fibers, whilst the other was obtained at the fiber/air interface. Finally, Dahlem et al. proposed the use of FBGs to interrogate low-finesse extrinsic FP cavities, when subjected to displacement and temperature variations [64]. Figure 2.8 – Structure of the FP temperature sensor proposed by Tsai et al. [63]. In 2002, Chen et al. proposed the use of a white-light interferometry system based on a scanned Michelson interferometer to interrogate intrinsic FP temperature sensors [65]. The rather complex scheme yielded a resolution of 0.013 °C. In 2003, a pressure sensor based on an extrinsic FP interferometer was reported [66]. The cavity was formed between a copper diaphragm and the end face of a SMF. The sensor was embedded in epoxy and applied in the aerodynamic field. The detection of weak acoustic waves was a subject of study by Yu et al. [67]. Using an extrinsic FP configuration, these sensors became an alternative to conventional acoustic sensors for the detection of partial discharges in power transformers. In 2004, the use of extrinsic FP interferometers to measure nano-displacement, with a minimum displacement of 10 nm was reported [68]. Other parameters were also analyzed in similar configurations, such as pressure and temperature [69, 70]. In the same year, Shen et al. fabricated an intrinsic FP cavity by exposing the photosensitive fiber to UV radiation [71]. Using a metallic mask, they locally produced the Fresnel
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 21 reflectors, using the point-by-point technique. The device was subjected to temperature, strain and pressure variations. In 2005, Xiao et al. developed an extrinsic FP device for gas sensing [72]. The cavity, formed between two ferrules, was able to measure the N2 refractive index variation with a resolution greater than 10-5. A different extrinsic FP cavity was proposed by Xu et al., to measure pressure and acoustic waves [73]. On the same year, Cibula et al. developed a new type of miniature intrinsic FP sensors [7, 74]. A hollow structure with ~125 m was obtained by wet chemical etching a MMF. In a first approach, a thin polymer diaphragm was positioned inside the hollow structure [74]. Later that year, the same group developed an all-silica FP cavity [7]. Both devices were tested in pressure of liquids. The combination of different structures was also a matter of study in 2005, being further developed since then. For example, Huang et al. proposed the splicing of a MMF section between two sections of SMF [6]. The sensor was tested to temperatures up to 600 °C and to strain up to 400 . The simultaneous measurement of the refractive index of liquids and of temperature was proposed by Kim et al. [75]. In this case, a long period grating (LPG) was spliced in series to an intrinsic FP cavity whose mirrors were created by chemically etching a SMF. In 2006, Zhu et al. proposed a N2 pressure sensor for high temperature [76]. The sensor was obtained by chemically etching a MMF and using a fused silica diaphragm. Wang et al. also reported a pressure and temperature sensor based on a SMF/hollow fiber/SMF diaphragm structure [77]. In this case, pressure measurements were done with the sensor submerged in water and the device was subjected to temperatures below 600 °C. Watson et al. fabricated the FP cavities using ArF excimer laser ablation [9]. An aluminized membrane was used as diaphragm and dynamic N2 pressure measurements were performed. Dynamic strain/bend measurements were done by Cranch et al., using a multicore fiber FP with FBGs as mirrors [78]. Other strain sensors have been proposed, whose cavities were created by etching the fibers [79, 80]. In 2007, the etching of optical fibers to produce FP cavities attracted a lot of attention. Machavaram et al. reported the etching of two SMF sections that formed the intrinsic FP
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 22 interferometer when spliced together [81]. The sensors were tested in strain and a cavity length variation of 0.5 nm/ was attained. On the other hand, Cibula et al. produced a strain sensor by splicing an etched MMF section between two sections of SMF [82]. Zhang et al. proposed a microgap cavity formed by wet etching and splicing SMFs [83]. The multiplexing of two FP cavities enabled the temperature compensation of a biosensor. The micromachining of cavities in PCFs using a 157 nm excimer laser [84] or femtosecond laser [8] for strain and temperature measurement were also reported. In both cases the devices presented low sensitivity to temperature but high sensitivity to strain. In a different approach, Rao et al. presented, for the first time, a FP cavity based on a hollow-core PCF section spliced between two SMFs, whose configuration is shown in Fig. 2.9 [85]. In order to increase the reflectivity, a Ti2O3 film was coated on the leadout SMF before splicing. The cavity length was a couple millimeters long and it was tested to strain. The multiplexing of up to fifty FP cavity sensors was demonstrated by Wang et al. [86]. Two FBGs constituted the FP cavity interfaces and the devices were tested to strain and temperature. Figure 2.9 – Configuration of an in-line hollow-core PCF etalon, proposed by Rao et al. [85]. In 2008, the post-processing of optical fibers by means of laser micromachining continued to be developed [87-89]. On the one hand, the micromachining exposes the fiber core region and the FP cavities produced in this way are very sensitive to the external medium. On the other hand, the sensors are temperature independent. A different structure was proposed by Rao et al. for refractive index measurements [90]. In this case, the FP cavity was formed by fusion splicing a section of endlessly single mode PCF between two sections of SMF. The use of different fiber geometries spliced in series with SMF and/or PCF enabled high temperature [91] as well as strain [92] sensing. An humidity sensor was presented by Corres et al. [93]. The device consisted of a tip of SMF
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 23 coated with a SiO2 super hydrophilic nanofilm. An extrinsic FP temperature and strain sensors was developed by Jiang et al., by curing epoxy droplets at the end of SMF sections in order to produce micro-lenses [94]. In 2009, Liu et al. proposed a gas sensor based on a silver layer and a vapor sensitive polymer layer that were sequentially deposited at the cleaved end tip of a SMF [95]. The device was able to detect methanol vapor with a sensitivity of 3.5 pm/ppm. A pressure sensor was described by Cibula et al., whose cavity was obtained by fusion splicing a SMF to an etched Ge-doped fiber [96]. Morris et al. presented a temperature and acoustic pressure sensor, constituted by a polymer deposited at the end tip of an optical fiber [97]. Higher fringe visibility was attained through the deposition of two gold mirrors at the polymer interfaces. Two different configurations were proposed to measure refractive index variations. The first configuration was a microcavity formed near the fiber end tip by 157 nm laser micromachining [98]. Two thin films were deposited in the cavity mirrors and a microchannel was created on the fiber end, enabling the interaction between the external medium and the microcavity. A sensitivity of 1130 nm/RIU was attained with this thermal-insensitive device. The second configuration was a suspended-core fiber spliced between two sections of SMF [99], whose response was characterized in the spatial frequency domain. The same authors proposed the splice in series of a suspended core fiber (with three or four holes) and a hollow-core PCF to SMF [100]. A novel strain and temperature sensor was fabricated and characterized by Villatoro et al. [101]. An air bubble was created by fusion splicing a PCF and a SMF, as can be seen in Fig. 2.10. Strain and temperature sensitivities of 2.7 pm/ and 0.95 pm/°C were respectively achieved. In the same year, Gong et al. proposed a different FP based interferometer . The device, obtained by splicing an etched Er-doped fiber with SMF, translated in a sensor with low thermal sensitivity (~0.65 pm/°C) but with good response to strain, of 3.15 pm/. A hollow-core photonic bandgap fiber spliced between two sections of SMF was reported by Rao et al. for measurement of temperature below 600 °C [103].
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 24 Figure 2.10 – Diagram of the configuration proposed by Villatoro et al. [101]. FOC stands for fiber optic circulator, LED is the light emitting diode and OSA corresponds to the optical spectrum analyzer. In 2010, several works were related to the measurement of refractive index and temperature. The use of a dual cavity based on a small section of hollow core fiber between a MMF and a SMF [104], an etched graded index fiber (GIF) spliced to SMF [105, 106], or even a hybrid Michelson/FP structure were proposed to measure these parameters simultaneously [107]. Kou et al. reported for the first time the direct fabrication of a FP cavity in a fiber taper [10, 108]. The device, shown in Fig. 2.11, was created using a focused ion beam (FIB) and subjected to liquid refractive index [108] and high temperature [10] variations. Still in that year, two other configurations were proposed to measure refractive index. On the one hand, Deng et al. were able to measure the N2 refractive index using a SMF/hollow core fiber/PCF structure, with sensitivity of 1639 nm/RIU [109]. On the other hand, by coating a SMF tip with an epoxy-based polymer, Zhao et al. were able to measure the water refractive index variation, with a sensitivity of ~205 dB/RIU [110]. A temperature sensor based on a short section of allsilica PCF spliced to SMF was demonstrated [111]. Also to measure temperature, a dual wavelength Raman fiber laser was employed to interrogate a suspended core fiber based cavity [112]. An accelerometer based on a hollow-core PCF cantilever structure was presented by Ke et al. [113]. Finally, a pressure sensor based on an etched optical fiber with a silica diaphragm was also proposed [114]. Figure 2.11 – Scanning electron microscope image of the FP cavity created using FIB [108] .
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 25 In 2011, a temperature sensor based on a hollow core fiber spliced to a SMF with lateral offset was proposed [115]. The post-processing of optical fibers using FIB for temperature and water salinity sensing [116] or chemical etching for strain [117] and refractive index [118] measurements were developed. In the last work, the cladding was removed from a SMF and the core became in direct contact with the external medium. The cavity was formed splicing the etched fiber in between two fibers containing in-fiber mirrors. A refractometer was proposed by Zhou et al. where the FP cavities were formed by UV-written FBGs and microchannels produced by femtosecond laser and chemical etching [119]. An extrinsic FP interferometer was proposed to measure displacement over a dynamic range of 3 mm [120]. A strain sensor that could operate at high temperatures was reported by Deng et al. [121]. In this case, an air-bubble cavity was produced by fusion splicing a multimode PCF to SMF. Ferreira et al. proposed a FP cavity based on a hollow core silica tube for gas pressure measurements [122]. With this configuration, presented in Fig. 2.12, no diaphragm was used and a sensitivity of 2.61 nm/MPa was attained. Figure 2.12 – Microscope photograph of the first diaphragm-free FP cavity for gas pressure sensing [122]. In 2012, Gouveia et al. proposed the simultaneous measurement of liquid refractive index and temperature by using a FBG written in a PANDA fiber [123]. A FP cavity was formed between the FBG and the PANDA fiber cleaved end. On the other hand, Wang et al. proposed an ellipsoidal cavity formed by splicing a SMF and a PCF [124]. In this case, the external RI was measured through the fast Fourier transform (FFT) analysis, and the temperature was measured tracking the wavelength shift. A different gas refractometer was proposed by Silva et al., by splicing a fiber with an outer diameter of 50 m with a strong misalignment between two SMFs [125]. Also in that year, other configurations were proposed to measure refractive index. The FP cavities were made by femtosecond laser micromachining [126], by splicing a SMF with a large lateral offset between two
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 26 SMFs [127], by splicing an etched GIF with a SMF [128], or even using a hollow core fiber ended with a hollow core silica sphere tip [129]. In the same year, several configurations were developed to measure strain. For example, spheroidal cavities were formed by splicing a PCF and a SMF [130, 131], or by splicing one section of cleaved flat tip SMF and an arc fusion induced hemispherical tip [132]. The splice of a hollow-core ring PCF fiber between two SMFs was also proposed to measure strain and temperature [133]. Tafulo et al. proposed the use of FP cavities based on chemical etching of multimode GIFs to produce sensors for high temperature and strain [134, 135]. Zhang et al. proposed a FP cavity based on a polarization maintaining PCF to measure temperatures up to 600 °C [136]. A FP device based on an endlessly single mode PCF was also characterized in extreme temperatures, as high as 1100 °C [137]. In order to perform simultaneous measurement of gas pressure and temperature, Pevec et al. proposed two low finesse FP resonators created at the tip of an optical fiber [138]. Ferreira et al. used a short section of hollow core ring PCF spliced to a SMF to measure methane pressure variations. Two different applications were also considered in 2012. Using an extrinsic FP interferometer, Lai et al. proposed a liquid level and specific gravity sensor [139]. Wang et al. measured high intensity focused ultrasound fields by using a silica capillary tube spliced between two sections of SMF [140]. In 2013, the main focus of research was on refractive index FP sensors. For instance, by splicing a simplified hollow-core PCF between two SMFs and drilling microchannels with femtosecond laser, Wang et al. obtained a sensor with a sensitivity of ~851.3 nm/RIU that was insensitive to temperature [141]. On the other hand, Sun et al. proposed a hybrid FP/Michelson interferometer to simultaneously measure refractive index and temperature [142]. Zhang et al. fabricated a cavity by taper-drawing a microfiber at the center of a uniform FBG [143]. A FP sensor based on an ultra-thin film of gold embedded in a SMF end face was investigated for refractive index and high temperature measurements [144]. A couple of structures were also developed for temperature sensing, such as cavities based on the post-processing of a double cladding optical fiber [145] and extrinsic FP interferometers [146]. Finally, the splicing of different structures like the hollow-core microstructured fiber in between two SMFs [147], or an etched MMF
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 27 between a SMF and a silicon dioxide diaphragm were proposed for pressure sensing [148]. In 2014, the fabrication of air cavities, or micro-bubbles, inside the optical fiber was subject of extensive study [149-151]. These FP cavities were mainly applied in strain and temperature sensing, and astonishing sensitivities were achieved, as can be seen in Table 2.1. The post-processing of optical fibers, by means of chemical etching [152, 153], FIB micromachining [154], or tapering [155], also translated in new sensing FP configurations with good responses. A high speed interrogation scheme was developed for high temperature measurements [156]. A miniature configuration based on a doublecore PCF spliced to SMF was subjected to temperatures below 900 °C [157]. Liao et al. presented a sub-micron silica diaphragm based fiber tip FP interferometer that presented a response of ~1036 pm/MPa to gas pressure changes [158]. Also in this year, a novel type of sensor based on an extrinsic FP interferometer and a magnetic fluid was reported. It was observed that the refractive index of the magnetic fluid changes with the increase of the magnetic field, enabling a magnetic field sensitivity of 0.0431 nm/Gs. Until June 2015, several works were published concerning FP-based fiber sensors. Eom et al. proposed an extrinsic FP configuration constituted by a lensed fiber and a polymeric diaphragm [159]. The sensor was tested in low pressure range and was proposed for the medical field. Lee et al. reported on the measurement of the thermooptic coefficient of liquids using a structure composed by two hollow core fibers with different diameters [160]. Besides, Sun et al. designed a FP interferometer for the simultaneous measurement of pressure and temperature [161]. The sensor head was based on a polymer capped on the end face of a SMF. The simultaneous measurement of refractive index and temperature was studied by Wu et al., by using a cavity based on a multimode PCF with collapsed ends to create thin films [162]. A FP cavity based on the deposition of a magnetostrictive material in the fiber, between two FBGs, was proposed as a magnetic sensor by Li et al. [163]. The measurement of dynamic displacement was addressed by Pullteap et al. [164]. An extrinsic FP interferometer was used, where a birefringent element was introduced between the fiber and the vibration target. Also in 2015, a strain sensor based on a rectangular air bubble was proposed by Liu et al. [165].
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 28 The bubble was created by splicing two sections of SMF and tapering the splicing joint. For an air bubble of ~61 m, a sensitivity to strain of 43 pm/ was achieved. Chen et al. proposed an ultraweak intrinsic FP cavity array for distributed temperature sensing [166]. An high-resolution and fast-response temperature sensor based on a silicon FP cavity was also described [167]. A FP temperature sensor based on differential pressure resulting from thermal expansion of sealed air was developed by Liu et al. [168]. A salinity sensor was obtained by using a FP cavity with a polyimide diaphragm. With such configuration, a maximum sensitivity of 0.45 nm/(mol/L) was achieved [169]. The main characteristics of the different intrinsic FP sensors reported since 2009 until 2015 are gathered in Table 2.1. Multiple configurations have been explored in these years. The reduced dimensions of the cavities stand out, since most of them are in the order of a few hundred micrometers. Notice also the different measurands and the ranges over which the sensors were tested. The most popular ones seem to be strain and temperature, and the highest measurement range achieved was of 5000 for the former and ~950 °C for the last. Depending on the configuration, it is possible to obtain sensor with very good sensitivity to temperature (for example, -6.71 nm/°C [149]) or with extremely low sensitivity to this parameter, thus enabling the measurement of different parameters with low cross-sensitivity (as in the case of [142], for example, where the sensitivity to temperature is only 0.27 pm/°C). The wide variety of possible configurations and the high versatility of these kinds of structures are an indication that there is still room for research and development in this field, even though the first steps were taken 36 years ago. Table 2.1 - Different fiber optic intrinsic FP sensors, with the respective characteristics, from 2009-2015. Year Configuration Length (m) Measurand Range Sensitivity Ref. 2009 Air bubble between PCF and SMF 58 22 Strain Temp. 0-5000 23-500 °C 2.7 pm/ 0.95 pm/°C [101] 2009 SMF + 3 holes suspended core + hollow core PCF ~840 Strain Temp. 0-1000 23-90 °C 1.32 pm/ 7.65 pm/°C [100] SMF + 4 holes suspended core + hollow core PCF ~1000 Strain Temp. 0-1000 23-90 °C 1.16 pm/ 8.89 pm/°C 2009 SMF + etched Er-doped fiber 27 Strain Temp. 0-800 23-80 °C ~3.15 pm/ ~0.65 pm/°C [102]
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 29 2009 157 nm laser micromachining cavity + microchannel + mirrors coated with thin film 25 Liquid RI Temp. 1.33-1.405 RIU 0-70 °C ~1130 nm/RIU ~0.8 pm/°C [98] 2010 SMF + etched GIF (hybrid structure) 505 Liquid RI Temp. 1.32-1.45 RIU 15-90 °C ~160 dB/RIU ~10.4 pm/°C [106] 2010 SMF + etched GIF (hybrid structure) 25 515 Liquid RI Temp. 1.32-1.47 RIU 15-90 °C ~45.05 dB/RIU ~11.5 pm/°C [105] 2010 Fiber taper + FIB micromachining 4.4 Temp. 19-520 °C ~20 pm/°C [10] 2010 Fiber taper + FIB micromachining 3.5 Liquid RI 1.355-1.375 RIU 110 nm/RIU [108] 2010 SMF + microstructured fiber with dual core (hybrid structure) ~11500 Strain Temp. 0-1000 0-60 °C 0.89 pm/ 14.22 pm/°C [107] 2010 SMF tip coated with polymer thin film 29.9 Liquid RI 1.314-1.365 RIU ~250 dB/RIU [110] 2011 Two UV-written FBGs + microchannels inscribed by fs-laser 1000 Liquid RI 1.43-1.49 RIU 9 nm/RIU [119] 2011 Air bubble between SMF + MM PCF 44.9 Strain Temp. 0-1850 23-750 °C 2.78 pm/ pm/°C [121] 2011 SMF + hollow core silica tube 141 Gas Pressure Temp. 0-1.0 MPa 23-950 °C 2.61 nm/MPa 8.11 pm/°C [122] 2012 Spheroidal cavity between SMF and PCF 10 Strain 0-1000 10.3 pm/ [130] 2012 Microbubble between flat tip SMF + hemispherical tip SMF ~100 Strain Temp. 0-1000 100-1000 °C 4 pm/ 0.9 pm/°C [132] 2012 SMF + hollow core ring PCF 508 CH4 Pressure Temp. 0-0.8 MPa 23-500 °C 0.82 nm/MPa 3.77 pm/°C [170] 2012 SMF + PM-PCF with lateral offset ~100 Temp. 33-600 °C 13.8 pm/°C [136] 2012 SMF + large lateral offset SMF + SMF 416 Gas RI 1.0002-1.0025 RIU 1540 nm/RIU [127] 2012 Ellipsoidal cavity between SMF + PCF ~14 Liquid RI Temp. 1.332-1.45 RIU 24-95 °C ~61.74 dB/RIU 15 pm/°C [124] 2012 SMF + hollow core ring PCF + SMF ~13 Strain 0-1000 15.4 pm/ [133] 2012 SMF + 50 m diameter fiber + SMF 2000 Gas RI Temp. 1.000-1.003 RIU 0-300 °C -1375 nm/RIU 25.6 pm/°C [125] 2012 SMF + etched GIF625 105 Strain Temp. 100-1200 23-400 °C 6.99 pm/ 0.95 pm/°C [134] SMF + etched GIF50 43 Strain Temp. 100-1200 23-400 °C 4.06 pm/ -0.84 pm/°C 2013 SMF + hollow core PC+SMF + fs laser drilled channels 48 Liquid RI Temp. 1.322-1.334 RIU 100-900 °C 851.3 nm/RIU 0.27 pm/°C [141]
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 36 4, eff FP nL (3.1) where neff is the effective refractive index of light travelling within the cavity, LFP is the cavity length and corresponds to the operation wavelength. When 2m , where m is an integer, the reflection spectrum reaches a maximum, which happens for the wavelengths: 2. eff FP nL m (3.2) If there is an external perturbation to the cavity, both neff and FP L can be affected, translating into a phase change which affects the cavity channeled spectrum. When the external perturbation is caused by pressure changes (p), the sensitivity of the cavity can be determined by differentiating Eq. 3.2: . eff FP eff FP nL d dp n p L p (3.3) Since 2 2, eff FP eff FP n L n m L m (3.4) by substituting Eqs. 3.4 into Eq. 3.3, it comes that 2 2. eff eff FP FP nn LL d dp m p m p (3.5) Dividing orderly Eq. 3.5 by Eq. 3.2, the wavelength dependence on the applied pressure is given by 11 . eff FP eff FP nL d dp n p L p (3.6) Thus, the sensor interferometric spectral response to the applied pressure, which corresponds to a phase variation, is the result of two contributions: the dependence of the effective refractive index on pressure and the change of the FP cavity length with this
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 37 parameter. However, for the silica based sensors presented in this Chapter, the variation of the cavity length with pressure is extremely small and can be ignored. Thus, the dominant effect here is the change of the neff with the applied pressure. On the other hand, the refractive index of a gas is dependent on three different parameters: wavelength, temperature and pressure, as previously mentioned. There are several equations in the literature for the calculation of the gas refractive index [194-198]. However, the ranges of validity are extremely limited. Regarding the measurement of N2 with optical fiber sensors, several authors used the updated Édlen’s equation for air as a first approximation [194]. Besides the fact that this equation is only valid for wavelengths between 350 nm and 650 nm, and for low pressure (close to atmospheric pressure), air is constituted by several components: 78.09% N2, 20.95% O2, 0.93% Ar, 0.03% CO2 [194, 199]. Recently, Zhang et al. reported precision measurements for the refractive index of N2, among other gases [200]. A Mach-Zehnder interferometer setup was used, where the light source was a frequency comb. The derived equations were based on the works of Édlen [194], Birch et al. [199, 201], and Bönsch et al. [202], and were compared with the results of Peck et al. [203]. Here are summarized the main results, which set the basis for the conversion of pressure into refractive index variations for N2. According to Édlen, the refractive index of a gas can be determined by ( 1) ( 1) , Tp Tp S S D nn D (3.7) where (n-1)Tp depends on temperature, T (in °C), and pressure, p (in Pa), and (n-1)s is obtained for the refractive index under standard conditions (101325 Pa and 15 °C), and only depends on the wavelength (in m). DTp is the density factor and DS is the density factor for standard gas conditions. In order to determine DTp, it is necessary to estimate first the compressibility factor, Z. Using the equation of state as defined by Édlen [194] , , pV Z RT (3.8)
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 38 the dependence of Z on temperature at 101325 Pa (1 atm) can be obtained from the tables reported in the literature. In this Eq. Zhang and co-workers used the ones from NIST [204]. There is also an older publication [205] where these values can be found. Anyway, according to Zhang et al., 28 1 101325 0.449805 0.01177 0.00006 10 .Z T T (3.9) Édlen also stated that Z is a function of pressure and temperature, according to 1T Zp , where T is a parameter that depends linearly on temperature. Combining this relation with Eq. 3.8, Tp can be estimated and substituted in Eq. 3.10 for the density factor, which is then given by [202] 1 1 / 6 , 1 T Tp pn Dp T (3.10) where 1 1 273.15 ,K and ( 1) K /(1 )n p T is a correction factor, given by the mean values of temperature and pressure considered. K is determined through the refractive index of N2 at standard conditions. The density factor is then 28 1 0.498526 0.0119484 0.00006 10 . 1 0.0036610 Tp p T T Dp T (3.11) Thus, the relationship between the refractive index term dependent on the temperature and pressure and the refractive index term dependent on the wavelength is given by ( 1) ( 1) , 94439.27 Tp Tp S D nn (3.12) where 8 2 3073864.9 ( 1) 10 6497.378 144 1 S n (3.13) is the dispersion curve given by Peck et al. for a temperature of 15 °C [203]. The reason why this equation has been adopted is because it is valid from ~470 nm to ~2060 nm, being in better agreement with our experimental conditions than the one proposed by Zhang and co-workers. Fig. 3.1 (a) shows the numerical curve obtained for the variation
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 39 of (n-1)S with wavelength. Figure 3.1 (b) shows the value of (n-1)Tp at a constant temperature of 20 °C and a constant wavelength of 1550 nm, as a function of pressure. 0.0 0.4 0.8 1.2 1.6 2.0 2.75 2.85 2.95 3.05 3.15 3.25 (n-1)s 10-4(RIU) (m) a) 0.0 0.2 0.4 0.6 0.8 1.0 1.0000 1.0005 1.0010 1.0015 1.0020 1.0025 1.0030 b) nTp (RIU) p (MPa) Figure 3.1 – Numerical curves obtained for the refractive index of N2 considering (a) the dependence on wavelength, and (b) the dependence on pressure, at a temperature of 20 °C and a constant wavelength of 1550 nm. RIU stands for refractive index units. The refractive index dependence on the wavelength is nonlinear, decreasing as this parameter increases. However, over the operation wavelength range used in this work, the variation is very small, of the order of 10-7 RIU, and can thus be considered constant. Regarding the variation with pressure, the behavior is linear and the change is more significant, as shown in Fig. 3.1 (b). In this Section, a method to infer the variation of the refractive index of a gas with the applied pressure was described. Using the previous equations, it is possible to determine indirectly the response of a sensor to this parameter. This matter will be discussed in Section 3.3.3. 3.3 Fabry-Perot Cavity Based on a Silica Tube The Fabry-Perot (FP) cavity developed in this work was based on a hollow-core silica tube, which was produced at the Leibniz Institute of Photonic Technology (IPHT – Jena). The silica tube was made of pure silica and presented an outer diameter of (125 ± 5) µm and an inner diameter of (20 ± 5) µm. The cross-section photograph of this structure is shown in Fig. 3.2.
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 40 Figure 3.2 – Microscope photograph of the silica tube cross-section. 3.3.1 Sensor Design and Spectral Characteristics The sensing cavity was produced by fusion splicing a section of silica tube to single mode fiber (SMF). The fiber and the silica tube were placed in the splicing machine with a lateral offset, as shown in Fig. 3.3 (a) [206]. This procedure ensured that the arc discharge was mainly applied in the SMF region, preventing the collapsing of the silica tube in the splice region (see Fig. 3.3 (b)). The silica tube was then cleaved with the desired length, under a 5× magnifying lens (Fig. 3.3 (c)). Several devices were fabricated, with cavity lengths that ranged from ~140 µm up to ~1100 µm. The microscope photograph of one of the cavities produced is shown in Fig. 3.3 (d). Figure 3.3 – Schematic of the procedures used to fabricate the FP cavity: (a) image from the splicing machine display, evidencing the lateral offset, prior to splicing (SMF on the left and silica tube on the right), (b) image after splicing, (c) device prior to cleaving, the arrows indicate where the cleave should be done and (d) microscope image of a FP cavity produced with this method. The spectral response of this sensing structure was observed by connecting it to an optical circulator. A broadband optical source and an optical spectrum analyzer (OSA) were connected to the other two ports of this optical component, in a typical reflection scheme, as shown in Fig. 3.4. The optical source had a bandwidth of 100 nm, centered at 1570 nm. The readings were done with a resolution of 0.2 nm.
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 41 Optical Source Optical Circulator FP cavity Splice OSA Figure 3.4 – Scheme of the experimental setup. When the broadband optical source is used in such a reflection scheme, light travelling in the SMF will be reflected at the SMF/silica tube interface. However, a fraction of light is also transmitted onto the walls of the hollow core silica tube. When the light reaches the end face of the silica tube, Fresnel reflection takes place and a fraction of this light is recoupled once again into the SMF (see Fig. 3.5). Also shown in Fig. 3.5 is the photograph of one sample with a large length of silica tube (of the order of centimeters), when illuminated with a He-Ne laser. The reason why such a cavity length was used was to diminish the intensity of light that would travel in the hollow core region, thus enabling the clear observation of light propagating in the silica tube wall. Figure 3.5 – Left: Scheme of the sensing head, highlighting the reflections occurring in the cavity. Right: cross section photograph of one sample when illuminated with a He-Ne laser. The spectral behavior of four different samples is shown in Fig. 3.6. The spectrum of Fig. 3.6 (a), which corresponds to a cavity length of (141 ± 5) m, can be approximated to a two-wave interferometer. The increase of the cavity length translates into an excitation of more cladding modes, giving rise to one beat (Fig. 3.6 (b)) or even two different beats, as evidenced in Figs. 3.6 (c - d).
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 42 1540 1550 1560 1570 1580 1590 -28.0 -27.0 -26.0 -25.0 -24.0 d)c) b) Optical Power (dB) Wavelength (nm) LFP = 141 m a) 1540 1550 1560 1570 1580 1590 -25.0 -24.0 -23.0 -22.0 -21.0 -20.0 Optical Power (dB) Wavelength (nm) LFP = 456 m 1540 1550 1560 1570 1580 1590 -23.4 -23.2 -23.0 -22.8 -22.6 -22.4 -22.2 Optical Power (dB) Wavelength (nm) LFP = 670 m 1540 1550 1560 1570 1580 1590 -26.0 -24.0 -22.0 -20.0 -18.0 -16.0 Optical Power (dB) Wavelength (nm) LFP = 1100 m Figure 3.6 – Spectra of four sensing heads with different FP cavity lengths. This effect can also be observed in the spatial frequency spectra presented in Fig. 3.7. In the case of the shorter cavity, although several modes are excited in the silica tube, only one is recoupled in the SMF, translating into the strong mode observed in Fig. 3.7 (a). For the other samples strong cladding modes, as well as weak ones are excited and involved in the interference. The interference peak with lowest spatial frequency, present in all cases, is related to the optical source. 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0.0 0.1 0.2 0.3 0.4 0.5 d)c) b) Strong modes Strong modes Strong modes Amplitude (a.u.) Number of Fringes / nm LFP = 141 m Strong mode a) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0.0 0.1 0.2 0.3 0.4 0.5 Amplitude (a.u.) Number of Fringes / nm LFP = 456 m 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0.0 0.1 0.2 0.3 0.4 0.5 Weak modes Weak modes Amplitude (a.u.) Number of Fringes / nm LFP = 670 m 0.0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0 0.0 0.1 0.2 0.3 0.4 0.5 Amplitude (a.u.) Number of Fringes / nm LFP = 1100 m Figure 3.7 – Spatial frequency spectra for four different cavity lengths.
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 43 The spectral behavior of the cavity, and consequently, the spatial frequency spectra, are intimately related to the quality of the splices and the end face cleave. Sensors with poor quality present lower spectral visibility and the number of modes that propagate in the cavity can be reduced, as in the case of the sensor with a length of (670 ± 5) m (Figs. 3.6 (c) and 3.7 (c)). The subtraction of the wavelengths of two adjacent peaks, 21 , corresponds to the free spectral range (FSR). This parameter is related to the length of the cavity, LFP, by the equation 12 . 2eff FP nL (3.14) where it was considered that 12 , so that the effective refractive index, neff( ), was constant. Thus, from this relationship it is possible to estimate the neff within the cavity. The length of each sensing device was measured through the microscope photographs, whereas the two adjacent peak wavelengths were obtained from the sensing heads spectral response. The relationship between and LFP is shown in Fig. 3.8. From the fitting tendency curve, the neff was estimated to be (1.32 ± 0.03) RIU. The value obtained is closer to the refractive index of silica than the refractive index of air, which indicates that a significant fraction of light propagates inside the silica tube walls. 100 200 300 400 500 600 700 800 900 100011001200 0 1 2 3 4 5 6 7 Free Spectral Range (nm) Cavity Length (m) Figure 3.8 – Free spectral range dependence on the cavity length, considering two adjacent peaks with wavelengths close to 1550 nm.
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 44 3.3.2 Temperature Measurement The 141 m long sensing device was placed inside a tubular oven, in a centered position. The temperature was increased in steps of 50 °C, with an equipment resolution of 0.1 °C. The range of temperatures was between room temperature (~23 °C) and 950 °C. The wavelength shift with the applied temperature was determined by tracking the 1556.6 nm peak, and it is shown in Fig. 3.9. The experimental results were well adjusted by the second order polynomial 6 2 3 1.2 0.2 10 6.5 0.3 10 1556.3 0.1 ,TT (3.15) where λ is the wavelength, in nm, and T corresponds to the temperature, in °C. It is reasonable to divide the temperature range into two different regions: low temperatures, between room temperature and 500 °C, and high temperatures, between 550 °C and 950 °C. According to the insets in Fig. 3.9, a linear behavior is observed in both cases. The sensitivities obtained were (7.1 ± 0.2) pm/°C and (8.1 ± 0.2) pm/°C, respectively. 0100 200 300 400 500 600 700 800 900 1000 1556 1557 1558 1559 1560 1561 1562 1563 1564 Wavelength (nm) Temperature (C) 0100 200 300 400 500 600 1556 1557 1558 1559 1560 1561 Wavelength (nm) Temperature (C) 500 600 700 800 900 1000 1560 1561 1562 1563 1564 Wavelength (nm) Temperature (C) Figure 3.9 – Temperature response of the 141 m long sensing head. Inset 1 (top left): low temperatures response; inset 2 (bottom right) high temperatures response. 3.3.3 Pressure Measurements Pressure measurements were carried out on four different samples, one with (170 ± 5) m cavity length, which still presented the two wave interferometer behavior, and the remaining three were the ones presented in Fig. 3.6 (b-d). The sensors were placed inside a sealed chamber, with a gas input and a low-vacuum purge output
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 45 (p ~ 10-3 MPa). The gas under test was N2 and all experiments were done at room temperature (~20 °C). The wavelength shift dependence on pressure, for all samples, is shown in Fig. 3.10. The shift was determined by following the peak near 1550 nm. The behavior is approximately linear in all cases, and the sensitivities are reunited in Table 3.2. 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 LFP=170 m LFP=456 m LFP=670 m LFP=1100 m Wavelength Shift (nm) Pressure (MPa) Figure 3.10 – Sensing heads response to the applied pressure. Besides the relatively high sensitivity to pressure obtained with this simple diaphragm-free structure, the decrease of the sensitivity with the increase of the cavity length was somehow surprising. From the analysis of Eq. 3.6 this behavior seems explained, however, the influence of the cavity length change with pressure was considered to be very small, in agreement with [187]. Therefore, the effect of the second term in Eq. 3.6 is negligible. Thus, the sensitivity to pressure had its origin on the dependence of the gas refractive index on pressure. When pressure increases, there is a higher density of gas inside the sealed chamber, translating into a higher refractive index. Thus, there will be higher interaction between the evanescent field of light travelling in the FP cavity and the gas, which means an increase of the pressure sensitivity with the cavity length, contrary to what is observed. This points out the presence of more complex effects, a matter that will be discussed in the next Section, where a FP structure based on a photonic crystal fiber is subjected to different gases. By converting the pressure shifts to refractive index variations, according to the Eq. 3.12, the four sensors response regarding this parameter was determined. The
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 52 in the interferometric cavity, by analyzing the response of the sensor to pressure changes. The results presented here and in the previous Section indicate, for the fiber structures studied, that the sensitivity to pressure decreased with the increase of the sensing head length. At this stage it is not clear what the reasons behind such behavior are, which requires further studies. 3.4.4 Prototype for Biomedical Applications The FP cavity based on the HCR-PCF was further investigated by our group for application in low pressure measurements, creating a prototype suitable to be used in the medical or biomechanical field [208]. This Section describes briefly the sensor and the main results obtained. A sample with a cavity length of ~125 m was fabricated using the same procedures as the ones described in Section 3.3.1. At the end face of the HCR-PCF a silicone diaphragm was deposited, by repeatedly placing the tip of the sample in direct contact with a small portion of silicone. After multiple controlled steps, the silicone was cured at room temperature for 72 h. The silicone polymer used was a biocompatible commercial one, Silastic® medical adhesive silicone, type A, from Dow Corning. With the deposition method used to create the sensing device, it was not possible to control the diaphragm geometry, which can have influence on the spectral response. A scheme of the sensor design is shown in Fig. 3.17. Figure 3.17 – Schematic drawing of the sensor proposed for low-pressure measurements [208]. When illuminated with a broadband optical source, with the same reflection setup as shown in Fig. 3.4, light travelling from the SMF will be reflected at the interface between the HCR-PCF and the silicone capsule, creating a two-wave FP interferometer. In this
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 53 case, light will travel in the hollow core, and the effective refractive index is ~1.00 RIU. The spectral behavior of this interferometer is shown in Fig. 3.18. The use of a diaphragm, which will act as one of the FP cavity mirrors, translates into a higher spectrum visibility, when compared to the diaphragm free configuration (whose FP mirror is the 3.1 m thick silica ring). 1565 1570 1575 1580 1585 -18.0 -17.5 -17.0 -16.5 -16.0 -15.5 -15.0 -14.5 -14.0 Optical Power (dB) Wavelength (nm) Pressure increase Figure 3.18 – Spectral response of the sensing head. Also shown the spectral shift when hydrostatic pressure is applied (step of 37.5 mmHg). The visibility of an interferometric cavity can be determined by Eq. 3.18 [209]: max min max min , PP VPP (3.18) where Pmax and Pmin are the optical powers of two adjacent maximum and minimum of the interference signal, respectively. The diaphragm free configuration presented a visibility of ~3 %, whereas in this case its value was ~33 %. The sensor was then placed inside a hydrostatic pressure device, and pressure measurements in a range between 0 mmHg and 337.5 mmHg were carried out, with a pressure step of ~37.5 mmHg. The spectrum shifted towards shorter wavelengths (blue shift) as pressure increased (see Fig. 3.18). The sensor response was approximately linear, as shown in Fig. 3.19, and a sensitivity of (-87.0 ± 0.4) pm/mmHg was achieved, which corresponds to a sensitivity of (-652.2 ± 3.3) nm/MPa in SI pressure units. This negative response was due to the change in the cavity length, caused by the silicone diaphragm compression with the applied pressure.
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 54 050 100 150 200 250 300 350 1560 1564 1568 1572 1576 1580 1584 1588 1592 Wavelength (nm) Pressure (mmHg) Figure 3.19 – Sensor response to hydrostatic pressure variation. This sensor proved to be suitable for low pressure applications, exhibiting high sensitivity and good reproducibility. There are several physiological pressures in this range, such as blood pressure, intracranial and intra-articular pressures [208]. However, the use of the silicone capsule brings also higher thermal sensitivity, due to the high thermal expansion coefficient of this material. The typical value of this parameter is 342.0 × 10-6 /°C [208], which is more than 600 times largerr than that of silica (~0.55 × 10-6 /°C [126]). Thus, in practical applications, the sensor should be used in a controlled environment or a reference sensor should be used, to reduce the crosssensitivity effects. 3.5 Final Remarks In this Chapter, two different gas pressure sensors based on the Fabry-Perot (FP) configuration were proposed. The sensor based on a hollow core silica tube was the first reported in the literature to measure gas pressure without the use of a diaphragm. Its simplicity and the somehow surprising good response to the measurand translated into a different approach for gas sensing using micro-cavities. Different samples were analyzed, with different FP cavity lengths and also with two different hollow core diameters. In order to take the best advantage of this configuration, a compromise must be accepted: on the one hand, small cavities ensure higher sensitivity, although they
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 55 should be long enough to ensure the coupling of light in the hollow core structure; on the other hand, larger hollow core diameters are preferred. However, large enough so they remain easy to handle and mechanically robust. Regarding the sensor based on the hollow core ring photonic crystal fiber described in the second part of this Chapter, it proved to be more sensitive than the first configuration. This occurs due to the thin thickness of the silica ring, where light travels after it exits the single mode fiber. Thus, the evanescent field in this cavity is stronger, and its interaction with the external medium is larger, translating into improved sensitivity to gas measurements. The cavity was subjected not only to nitrogen pressure variations, but also to those of a mixture of nitrogen and methane, exhibiting different sensitivities. Using a model reported in the literature, the pressure measurements were converted to the gas refractive index variations, and the analysis was also done regarding this parameter. Finally, both cavities presented a non-linear behavior when subjected to temperature. The sensor based on the hollow core photonic crystal fiber was further investigated by depositing a capsule of a biocompatible silicone at the end of the sensing tip. When subjected to hydrostatic pressure, in the range of physiological activity, the diaphragm deflected, causing a phase shift in the interferometric spectrum. This new prototype can be further explored to be used in low-pressure applications, such as in biomechanics or medicine.
CHAPTER FOUR 4 Fabry-Perot Cavities Based on PostProcessed Interferometric Tips
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 59 4.1 Introduction In 1974, Kawakami et al. developed a new type of fiber, with one core and two claddings [210, 211]. This fiber was called double clad fiber (DCF) 1 and in the first work, the inner cladding presented the lowest index, followed by the index of the outer cladding. The core presented the highest refractive index. This fiber was proposed to compensate the glass dispersion, since it presented an anomalous waveguide dispersion. In this case, the parameter 22 dd , related to the signal distortion, is negative, whilst for the standard single mode fiber (SMF), this parameter is positive. Figure 4.1 (a) shows the cross-section scheme of this new type of fiber. In 1978, a new geometry was proposed for the inner cladding [212]. The fiber, birefringent and polarization maintaining, included an elliptical cladding, shown in Fig. 4.1 (c). Until the 1990s, these fibers were mainly applied for dispersion compensation. Figure 4.1 – Schematic designs of some of the double clad optical fibers reported in the literature. In 1988, the use of this kind of fibers in fiber laser configurations was proposed for the first time. For this application, the refractive index of the inner cladding needs to be higher than the one of the outer cladding, ensuring that the pump light is confined in this region. Snitzer et al. used a double clad fiber with an offset core doped with neodymium (Nd) [213]. The scheme of the fiber used is shown in Fig. 4.1 (b). Light was focused onto the inner cladding and absorbed by the Nd doped core as it proceeded 1 The acronym DCF is usually attributed, in the literature, to the dispersion compensating fiber. However, several authors also used it to designate the double clad fiber. In the works described in this thesis the dispersion compensating fiber was not used, and thus the DCF acronym is only relative to the double clad fiber.
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 60 along the fiber. By exploiting different geometries for the inner cladding, especially when the circular geometry was broken, high power lasers with outstanding efficiencies were obtained. For instance, Jeong et al. proposed the use of a D-shaped fiber, as the one presented in Fig. 4.1 (d), to obtain a 1.36 kW continuous wave output power with a launch power just over 1.6 kW [214]. In this case, a slope efficiency of 83 % and a quantum efficiency of 95 % were attained 2 . Figs. 4.1 (e-h) present several popular configurations, like the “flower”-type [216], the squared [217], rectangular [218], and hexagonal shaped [219]. The difference between the refractive indices of the layers was often attained by doping the core with lanthanide ions, thus increasing the refractive index, and using a low refractive index polymer as the outer cladding [217-219]. The employment of this kind of fibers for sensing has been recently proposed, in different configurations, and for diverse applications. For instance, in 2006, Fu et al. proposed the use of a double clad photonic crystal fiber (PCF) in a scheme to perform nonlinear optical endoscopy measurements [220]. Han et al. spliced a DCF section to SMF and wrote a long period grating (LPG) in both fibers [221]. The sensor was able to simultaneously measure refractive index and temperature. A band rejection filter was obtained by Pang et al. by splicing a DCF section between two SMFs [222]. The sensor was applied in refractive index measurements of liquids. The concatenation of two sensing structures was proposed by Liu et al. [223]. The sensor was constituted by two different DCFs. One presented an inner cladding doped with fluorine, whereas the other was doped with boron. The device was employed in temperature and refractive index measurements. Baiad et al. proposed the use of a double clad fiber coupler to capture cladding modes which were generated by a gold-coated fiber with a tilted fiber Bragg grating (FBG) [224] or by an etched FBG [225]. All these structures presented an inner cladding with a refractive index lower than the core and the outer cladding indices. It is possible to obtain an inner cladding with higher refractive index by doping it with elements such as germanium or phosphorous. In such case, the fiber needs to be 2 The slope efficiency of a fiber laser is obtained through the relationship between the absorbed pump power and the emitted laser output power, above threshold. Regarding the quantum efficiency, it is the ratio between the pump and laser photon energies [215].
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 61 post-processed, for example by means of chemical etching, in order to guide light in its core. In 2011, Pevec et al. proposed the chemical etching of a phosphorous doped DCF to obtain a FP cavity [117]. The sensor was used to measure strain and temperature. Three years later, André et al. developed a temperature and vibration sensor based on postprocessing of a P2O5 doped DCF [154]. Besides wet chemical etching, the structure was post-processed using the focused ion beam (FIB) technique. In this Chapter, a phosphorus doped DCF subjected to chemical etching is described. Two different configurations are considered, one for measurement of extreme temperatures, and the other to be used as an optical phase refractometer. Some fabrication characteristics are pointed out, as well as the development of the sensing structures. Finally, some theoretical considerations are given and the experimental results are discussed. 4.2 Design of the Double Clad Optical Fiber The phosphorus-doped double clad optical fiber (P-doped DCF) was fabricated at the Leibniz Institute of Photonic Technology (IPHT Jena), in Germany, during a short mission in the framework of COST Action TD1001. The fabrication details are provided in Appendix I. The fiber cross-section is shown in Fig. 4.2 (a). The doped region has an elliptical shape instead of being circular. This unexpected feature was a consequence of the preform fabrication (see Appendix I). The pure silica core and the outer cladding have mean diameter dimensions of (18 ± 3) m and (122 ± 3) m, respectively. In between these two regions, the layer of P-doped glass presents a thickness of (15 ± 3) m, considering the major axis of the ellipse, and (10 ± 3) m in the minor axis. This region presents a refractive index variation, n ~ 1.1 × 10-2 RIU, when compared to the undoped ones, as shown in Fig. 4.2 (b). Notice that the experimental value of the refractive index is slightly different from the expected (theoretical) one. This is mostly due to the fact that the inner cladding is not perfectly circular. Even though the inner cladding shape influences the results, as will be seen later, the fiber was successfully used in two different applications: as a sensor for extreme
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 68 4.5 Optical Phase Refractometer Using the second configuration presented in Fig. 4.4, the core of the DCF is no longer directly exposed to the external medium, which translates into a more robust cavity. The reflections that can occur in the structure are shown in the scheme of Fig. 4.9. Thus, in this situation, the interferometric behavior will be associated with the interference of three waves: one at the SMF/DCF interface (E1), due to the mismatch between the effective refractive indices on each side; the second one, caused by the same effect, will occur at the DCF/SMF interface (E2); finally, E3 corresponds to the reflection occurring at the SMF diaphragm/external medium interface. The first two waves do not change their amplitudes or their phases with the external medium index. However, E3 presents an amplitude that depends on the refractive index of the external medium. Figure 4.9 – FP microcavity evidencing the interface reflections. Considering that E1 has a phase 1 when generated at the interface, the phase difference with E2 is 2 21 4, eC nL (4.10) where ne2 is the effective refractive index inside the DCF cavity, LC corresponds to the cavity length and is the wavelength. Similarly, the phase difference between the waves E3 and E2 is 1 32 4, eD nL (4.11) where LD is the diaphragm length and ne1 is the SMF effective refractive index. This reflection at the external interface, of ~4% when the surrounding medium is air, is substantially stronger than the second one, so the interferometric behavior in this case is
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 69 essentially determined by E1 and E3. However, when the sensing head is submerged in water, due to the smaller refractive index difference between the silica and the surrounding medium, there is a considerable reduction of the amplitude of E3. Thus, both E2 and E3 contribute substantially to the interference, generating a superposition wave that depends on the relative phases of the two primary waves as well as on their relative amplitudes. This interference can be described through 2 3 02 2 03 3 023 23 sin sin sin ,E E E t E t E t (4.12) where E02 and E03 correspond to the amplitudes of the waves E2 and E3, respectively, and can be determined through the expressions: 02 0 1 2 1,E E R R (4.13) 03 0 1 2 3 1 1 .E E R R R (4.14) For the simulation purposes, the transmission losses were not taken into account. The reflection coefficients at each interface, R1, R2 and R3, are given by: 2 12 12 12 ee ee nn RR nn , 2 1 3 1 , eS eS nn Rnn (4.15) where nS is the refractive index of the surrounding medium. Using a complex notation, Eq. 4.12 comes 3 23 2( ) ( ) () 23 02 03 023 j t j t jt E E e E e E e (4.16) 3 23 2 02 03 023 . jj j E e E e E e (4.17) The amplitude E023 is obtained through 23 23 2 023 023 023 . jj E E e E e (4.18) Substituting Eq. 4.17 in the Eq. 4.18 and re-arranging, one gets: 2 3 2 3 2 2 2 023 02 03 02 03 . jj E E E E E e e (4.19) Then:
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 70 12 22 023 02 03 02 03 2 3 2 cos .E E E E E (4.20) In order to determine the phase, 23, and using Eq. 4.17, E023 can be separated in the real and imaginary parts as follows: 023 23 02 2 03 3 cos cos cos ,E E E 023 23 02 2 03 3 sin sin sin .E E E (4.21) Dividing the two equations, the following expression is achieved: 02 2 03 3 23 02 2 03 3 sin sin arctan . cos cos EE EE (4.22) A schematic diagram is shown in Fig. 4.10 that illustrates the dependence of the phase of the resultant wave on the amplitude of each wave individually. In this diagram E2 is kept constant both in phase and amplitude, whereas E3 presents the same phase but different amplitudes. It is quite clear that there is a modification on the interferometric wave, both in amplitude and phase, when the amplitude of E3 is reduced. j 2 j 3 j 23 E2 E3 E2+E3 j 2 j 3 j 23E2 E3 E2+E3 Figure 4.10 – Scheme of the resultant wave phase variation with the amplitude of E3; E2 remains constant. The simulation of the interference between the waves E2 and E3 was further investigated taking the previous equations into account. The resultant interferometric data is shown in Fig. 4.11. In the simulations, a cavity length of 90 m and a diaphragm thickness of 12 m were considered. Besides, the effective refractive indices were set ne1 ≈ 1.45, ne2 ≈ 1.41 and ns = 1.00 or ns = 1.32, for air and water as the surrounding medium. Regarding the reflection coefficients, R2 was considered to be 0.0020 and R3 was 0.0338 or
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 71 0.0023, when the surrounding medium was air and water, respectively. In addition to the clear variation in amplitude, the inset in Fig. 4.11 shows the shift that occurs when the external medium changes from air to water. 1530 1540 1550 1560 1570 1580 1590 1600 1610 -100 -80 -60 -40 -20 0 20 |E23/E0|2 (dBm) Wavelength (nm) air water 1545 1546 1547 1548 1549 1550 -150 -100 -50 0 |E23/E0|2 (dBm) Wavelength (nm) Figure 4.11 – Simulated spectra of the FP micro-cavity in different media. The inset shows the phase variation of the spectrum. If the liquid medium is subjected to temperature variations, its refractive index will also change. This variation originates a change of the silica-water reflectivity coefficient and a shift in the E23 wave phase is to be expected. The phase of the resulting interference between this wave and E1 will also be affected by the variation of the water refractive index. Therefore, with this configuration, the principle of the amplitude-phase conversion is achieved in the optical domain, i.e., the phase of the net interferometric optical signal becomes a function of the amplitude of one of the interfering waves (E3 in this case). 4.5.1 Water Temperature and Refractive Index Relationship The relationship between the refractive index of water and its temperature has been described in several works [227-230]. However, the water refractive index depends not only on the temperature, but also on other parameters, such as the density and the operation wavelength [230]. Tables were found in the literature for three different wavelengths: 430, 600 and 660 nm. The wavelength dependence can be described by the Sellmeier equation,
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 72 24 ..., BC nA (4.23) where only the first three terms were taken into account. A tendency curve was adjusted for the three wavelengths, and the parameters A, B and C were estimated. Using these values, it is possible to determine the refractive index of water at 1550 nm. This procedure was done for temperatures ranging from 10 °C to 80 °C, in steps of 10 °C, considering the values presented in [230]. This information is gathered in Fig. 4.12 (a). Notice that there is a different tendency curve for each temperature. 300 450 600 750 900 105012001350150016501800 1.314 1.318 1.322 1.326 1.330 1.334 1.338 1.342 1.346 Refractive Index (RIU) Wavelength (nm) Temperature a) 010 20 30 40 50 60 70 80 90 1.314 1.316 1.318 1.320 1.322 1.324 1.326 1.328 Refractive Index (RIU) Temperature (C) b) Figure 4.12 – (a) Dependence of the refractive index of water on the operation wavelength for different temperatures and (b) refractive index of water as a function of temperature, for a wavelength of 1550 nm. Figure 4.12 (b) shows the relationship between the water refractive index and the temperature, for an operation wavelength of 1550 nm. The refractive index diminishes as temperature increases, and the data can be well adjusted to the second order polynomial: 6 2 5 1.073 10 5.982 10 1.327.n T T T (4.24) By converting the water temperature change into its refractive index variation, it is possible to infer the sensing head response towards this parameter. This approach is only valid when the device under study exhibits a response where the different contributions to its sensitivity can be discriminated. 4.5.2 Experimental Results The experimental reflection spectra of two different cavities are shown in Fig. 4.13, for two different external media. The first cavity had a length of (80 ± 3) m and a
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 73 (13 ± 3) m thick diaphragm. Regarding the second sensor, it presented a FP cavity length of (95 ± 3) m and a diaphragm with a thickness of (43 ± 3) m. This data is in good agreement with the simulation results presented in Fig. 4.11. When the sensor is placed in water, there is an increase in the losses and the visibility diminishes. This behavior is highly dependent on the diaphragm thickness. If the thickness of the diaphragm is larger than 40 m, there is an increase of the losses, but the visibility remains nearly the same, as can be seen in Fig. 4.13 (b). 1530 1540 1550 1560 1570 1580 1590 -48 -44 -40 -36 -32 -28 -24 -20 -16 b) Air Water Optical Power (dB) Wavelength (nm) a) 1530 1540 1550 1560 1570 1580 1590 -48 -44 -40 -36 -32 -28 -24 -20 -16 Air Water Optical Power (dB) Wavelength (nm) Figure 4.13 – Spectra of the FP micro-cavity when the external medium is air (black line) and water (blue line). (a) Sensor with a thin diaphragm. (b) Sensor with a thick diaphragm. The sensing heads were subjected to temperature measurements in air and in water. In the first case, the sensor was placed inside a tubular oven and measurements were done in a range between room temperature (~23 °C) and 85 °C. Afterwards, the device was submerged in hot water (~85 °C), which was let to cool down until room temperature. In both cases, the temperature resolution was of 0.1 °C. Regarding the response of the sensor with a thick diaphragm, shown in Fig. 4.14, it is independent of the external media. The linear sensitivities obtained were of (9.6 ± 0.1) pm/°C and (9.7 ± 0.1) pm/°C when the surrounding medium was air and water, respectively. This means independence of the sensitivity from the surrounding medium. Such result, which is also visible looking at Fig. 4.13 (b) where essentially the channeled spectrum does not depend of having either air or water outside, indicates that the amplitude of E3 is residual in both situations. This points out to significant extra optical loss when the diaphragm thickness increases from ~13 m to ~43 m, probably
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 74 introduced due to a mismatch of the fabrication conditions from the established procedure. 010 20 30 40 50 60 70 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Air Water (nm) T (C) Figure 4.14 – Wavelength shift dependence of the sensor response with a diaphragm of 43 µm on the applied temperature, in two different media. The sensing device with the thinner diaphragm (of ~13 m) was subjected to the same external conditions, and the results are shown in Fig. 4.15 (a). The sensor response, in this case, is affected by the external medium, and its sensitivity is lower when submerged in water (blue hollow circles) than when in air (black solid circles). In the former situation, a sensitivity of (9.4 ± 0.1) pm/°C was obtained, whereas in the last one the sensitivity was of (13.5 ± 0.1) pm/°C. The difference between these values is an indication that the refractive index variation of water induced by the temperature has an impact on the sensor response. Thus, considering that 1 corresponds to the wavelength shift of the cavity optical spectrum when the external medium is air, we can write 11 , T kT (4.25) where 1T k stands for the temperature sensitivity. If, on the other hand, the external medium is water, the wavelength shift can be separated into two different components: one related to the diaphragm silica thermal expansion ( 1T k ), which was previously measured in air, and the other attributed to the water refractive index variation ( 2T k ).
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 75 Therefore, the wavelength shift measured in this situation is related to temperature according to the expression 2 1 2 . TT k k T (4.26) Subtracting Eq. 4.26 and Eq. 4.25, the following relationship is attained: 2 1 2 . T kT (4.27) The calculated wavelength shift due to the water contribution is shown in Fig. 4.15 (b). In this case the sensitivity is negative, with a value of (-5.8 ± 0.2) pm/°C. 010 20 30 40 50 60 70 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 2-1 (nm) T (C) b) 010 20 30 40 50 60 70 0.0 0.2 0.4 0.6 0.8 1.0 1(air) 2(water) (nm) T (C) a) Figure 4.15 – Wavelength shift dependence on temperature: (a) sensing head exposed to air (black circles) and when immersed in water (blue circles) and (b) calculated water contribution. The sensing head response to the variation of the external medium refractive index can be estimated by applying the relationship between the water refractive index and the temperature, which was described in Section 4.5.1. By diminishing temperature, the refractive index increases, and with it, there is a wavelength shift of the interferometric cavity towards red, as can be seen in Fig. 4.16. This variation was adjusted to the following second order polynomial, 2 ( ) 1758.7 560.7 4608.8 1484.1 3018.9 982.0 ,n n n (4.28) where the wavelength, , is in nm, and n corresponds to the refractive index, and comes in RIU. It is possible to extrapolate two different linear regions from Fig. 4.16, one for
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 76 lower refractive indices, between 1.319 RIU and 1.324 RIU, and another for higher refractive indices, from 1.325 RIU to 1.327 RIU. A sensitivity of (38.7 ± 2.5) nm/RIU was attained for the former, whilst a sensitivity of (56.7 ± 4.2) nm/RIU was obtained for the last region. Although the sensitivities obtained are lower than the ones reported in the literature, with this configuration there is no fiber core exposition to the external medium. It is only the interaction between the reflection of the third wave at the end of the fiber tip and the environment that ensures the spectrum variation. 1.319 1.320 1.321 1.322 1.323 1.324 1.325 1.326 1.327 1.328 -0.40 -0.35 -0.30 -0.25 -0.20 -0.15 -0.10 -0.05 0.00 (nm) Refractive Index (RIU) Temperature Figure 4.16 – Wavelength shift variation with the water refractive index. Besides the variation in the spectrum with the water temperature, a change in the visibility of the spectrum was also noticed. This is also an effect of the variation of the refractive index of water, as already mentioned. Therefore, a different kind of analysis could also have been done with this sensing head, in complement to the approach described above. 4.6 Final Remarks In this Chapter, two different sensing configurations were proposed based on postprocessing of a double clad optical fiber. The fiber did not guide light in its core, due to the higher refractive index in the P-doped inner cladding. However, after applying wet chemical etching, this layer was removed and light started to propagate in the core. In the first configuration, the micro-cavity was only constituted by a short section of such
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 77 etched fiber spliced to a SMF. The device was successfully tested to extreme temperatures and revealed to be more sensitive than more conventional fiber structures, such as the fiber Bragg grating (FBG) [231] or the hollow core silica tube [122]. In the first case, the structure was ~1.1 times more sensitive than the FBG, whereas in the last, the sensitivity was ~2 times higher. However, when the structure is placed in a liquid medium, this liquid will surround the suspended core, causing instability in the spectral response. A different configuration was then developed. In this case, a thin diaphragm was applied to the first structure by fusion splicing. This translated into a more stable device that presented different responses to temperature when the external medium was air or water. This behavior was due to the fact that the sensing head was measuring, besides the silica thermal expansion, the water refractive index variation with temperature. Thus, this sensor is an interesting design to be used in aqueous environments, namely, in biochemistry. Furthermore, if used in a multiplexed configuration, it can allow temperature compensation or even measurement of different parameters.
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 84 (35.3 ± 0.1) nm, (6.0 ± 0.1) nm and (1.3 ± 0.1) nm for the (35 ± 2) m, (207 ± 2) m and (906 ± 5) m long FP cavities, respectively. The smaller FP cavity, which had a length of (13 ± 2) m, only presents one interference peak in the spectral range considered, thus it was not possible to estimate the interferometric period from the spectral response in Fig. 5.3. 1560 1570 1580 1590 1600 1610 1620 -40 -35 -30 -25 -20 LFP=13m d) a) c) b) Strain Strain Optical Power (dB) Wavelength (nm) Strain 1540 1550 1560 1570 1580 1590 1600 1610 -24 -23 -22 -21 -20 -19 -18 LFP=35m Strain Optical Power (dB) Wavelength (nm) 1540 1544 1548 1552 1556 1560 -27 -25 -23 -21 -19 -17 LFP=207m Optical Power (dB) Wavelength (nm) 1548 1549 1550 1551 1552 -25 -23 -21 -19 -17 -15 LFP=906m Optical Power (dB) Wavelength (nm) Figure 5.3 – Spectra of the four samples, with different FP cavity lengths. The spectrum shift with the applied strain is also shown for each sample. 5.2.2 Experimental Results The devices were characterized in strain, at room temperature, being under the same test conditions. Thus, the fiber was attached to a translation stage with a resolution of 0.01 mm. The total length of fiber over which strain was applied, composed by the SMF sections and the HCR-PCF, was of (700 ± 5) mm. This length, defined as LT, is identified in Fig. 5.4, along with the other dimensions considered in this work. Fixed point FP cavitySMF SMF LSMF/2 LSMF/2LFP LT Translation stage Figure 5.4 – Identification of the lengths considered in the strain analysis.
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 85 The interferometric spectrum shifts towards longer wavelengths (red shift) as strain is applied, which means that the optical path increased with strain (see Fig. 5.3). The wavelength shift dependence on strain of each sensor is shown in Fig. 5.5 (a). There is a clear influence of the FP cavity length in the sensitivity to strain. In fact, the smaller the sensing head, the higher the sensitivity. Furthermore, the response is linear in all cases. For the 906 m long device, a sensitivity of (3.12 ± 0.01) pm/ was attained, whereas for the 207 m long sample, the sensitivity was of (3.79 ± 0.01) pm/. Decreasing the length to 35 m translated into a sensor with a doubled sensitivity, of (6.16 ± 0.01) pm/. Decreasing the length furthermore, to 13 m, resulted in a sensitivity of (15.43 ± 0.01) pm/. This means that by choosing the appropriate FP cavity length, it is possible to tailor the sensitivity most suitable for the desired application. Figure 5.5 (b) presents the estimated sensitivities as a function of the FP cavity length along with the tendency curve. The non-linear behavior shows the rapid increase of the sensitivity for smaller FP cavities, following the same tendency as observed in other configurations [237]. 0100 200 300 400 500 600 700 800 9001000 0 2 4 6 8 10 12 14 16 b) Wavelength Shift (nm) Strain () LFP= 13m LFP= 35m LFP= 207m LFP= 906m a) 0100 200 300 400 500 600 700 800 9001000 2 4 6 8 10 12 14 16 18 Sensitivity (pm/) Fabry-Perot Cavity Length (m) Figure 5.5 – (a) Sensors response to the applied strain. (b) Sensitivity dependence on the FP cavity length. Inset: microscope photograph of the 13 m long sensing head. Further investigations were carried out to understand the influence of the total length over which strain was applied (LT). The FP cavity with a length of 207 m was subjected to strain, considering three different total gauge lengths: of 706 mm, 342 mm and 170 mm. The behavior, shown in Fig. 5.6 (a), is linear in all situations and the sensitivities are quite close to each other: (3.79 ± 0.01) pm/, (3.75 ± 0.01) pm/ and (3.67 ± 0.01) pm/, for the gauge length of (706 ± 5) mm, (342 ± 5) mm and (170 ± 5) mm,
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 86 respectively. Figure 5.6 (b) shows the strain sensitivity as a function of the total gauge length, where it is explicit that as the total length increases, the sensitivity is enhanced. However, the variation is of only 0.12 pm/ for a total length increase of ~540 mm. Thus, even though there is an improvement in the sensitivity, this is not the dominant parameter for this matter. 0100 200 300 400 500 600 700 800 9001000 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 b) Wavelength Shift (nm) Strain () LT= 706 mm LT= 342 mm LT= 170 mm a) 100 200 300 400 500 600 700 800 3.64 3.66 3.68 3.70 3.72 3.74 3.76 3.78 3.80 3.82 Sensitivity (pm/) Total Length (mm) Figure 5.6 - (a) Response of the 207 m long sensor cavity to strain, considering three different gauge lengths. (b) Sensitivity dependence on the gauge length (purple dots) and tendency curve (gray line). The temperature response of the proposed sensor was analyzed by placing the same sample inside a tubular oven. The temperature was changed from room temperature to 85 °C, with a resolution of 0.1 °C. The wavelength dependence towards this parameter is shown in Fig. 5.7. 30 35 40 45 50 55 60 65 70 75 80 85 1548.40 1548.45 1548.50 1548.55 1548.60 1548.65 1548.70 1548.75 1548.80 Wavelength (nm) Temperature (C) Figure 5.7 – Wavelength dependence on temperature for the 207 m long sensing head.
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 87 The sensitivity attained was of only (0.85 ± 0.06) pm/°C. This indicates that, in most situations, the sensing device can be used to perform strain measurements without requiring temperature compensation, since the cross-sensitivity is of only 0.21 /°C [32]. 5.3 Measuring Strain at High Temperatures (Part I): Silica Tube This Section describes the characterization of a Fabry-Perot cavity based on a silica tube with a special design. Strain measurements are performed at different temperatures, which can be as high as 900 °C. The annealing effects are also addressed. 5.3.1 Sensor Design and Spectral Characteristics The silica tube used in this work was fabricated at the IPHT-Jena. All components of the silica tube were manufactured from high purity silica Heraeus Suprasil® F300. Four rods, with a diameter of 1.2 mm were placed inside the cladding tube, which had an inner diameter of 4 mm and an outer diameter of 6 mm, in exact orthogonal positions, and were sintered using the modified chemical vapor deposition (MCVD) method. The preform was drawn to the final fiber by pressurized drawing at constant temperature. The pressure inside the preform changed from 1000 Pa to 3000 Pa above atmospheric pressure. Higher pressure translated into larger silica tube hollow core area. This effect can be seen in Fig. 5.8. The final outer diameter was of 125 m and the silica tube was coated with a single layer of ultraviolet cured acrylate. Figure 5.8 – Cross section images of the silica tube varying the pressure during fiber drawing: (a) p = 1000 Pa, (b) p = 2300 Pa and (c) p = 3000 Pa. The silica tube shown in Fig. 5.8 (b) was selected to be used as a sensing element, in a FP configuration. It presented a cladding with a thickness of (14 ± 2) m, a hollow core
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 88 and the four small rods presented a diameter of (20 ± 2) m each. The presence of the four rods will have a reinforcement effect in the structure. This matter will be discussed further ahead. The FP cavity, shown in Fig. 5.9, was obtained by fusion splicing a short section of silica tube between two sections of SMF, following the same procedures as described in Section 3.3.1. The sensor was interrogated in a reflection scheme similar to the one presented in Fig. 5.2. The OSA resolution was, in this case, of 0.02 nm. Figure 5.9 – Photograph of one FP cavity based on the new hollow core silica tube design. Four different sensors were produced, with different cavity lengths. The spectrum of each sensor, presented in Fig. 5.10, is the result of a two wave interferometer. The mean effective refractive index was estimated to be ~1.00 RIU, meaning that almost all light travels inside the hollow core. 1520 1535 1550 1565 1580 1595 1610 -42 -38 -34 -30 -26 -22 d) b) c) Optical Power (dB) Wavelength (nm) L=17m a) 1540 1550 1560 1570 1580 1590 -38 -34 -30 -26 -22 Optical Power (dB) Wavelength (nm) L=51m 1535 1545 1555 1565 1575 1585 -44 -40 -36 -32 -28 -24 -20 Optical Power (dB) Wavelength (nm) L=70m 1535 1540 1545 1550 1555 1560 1565 1570 -17 -16 -15 -14 -13 -12 -11 Optical Power (dB) Wavelength (nm) L=198m Figure 5.10 – Spectra of the four FP cavity sensors.
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 89 5.3.2 Experimental Results In a first stage, the samples were attached to a translation stage with a resolution of 0.01 mm and strain measurements were carried out at room temperature. The total length over which strain was applied was kept constant throughout the experiments, with a value of (735 ± 5) mm. The FP cavities response towards the applied strain is linear, as shown in Fig. 5.11 (a). Besides, the sensitivity depends on the FP cavity length. Sensitivities of (13.9 ± 0.1) pm/, (6.0 ± 0.1) pm/, (4.6 ± 0.1) pm/ and (3.5 ± 0.1) pm/ were respectively attained for the 17 m, 51 m, 70 m and 198 m long samples, following the same trend as in the previous configuration. The cavity lengths were determined through the microscope images, with an associated uncertainty of ± 2 m. 0100 200 300 400 500 600 700 800 900 1000 0 2 4 6 8 10 12 14 L=17m L=51m L=70m L=198m Wavelength Shift (nm) Strain () a) 0100 200 300 400 500 600 700 800 900 1000 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Wavelength Shift (nm) Temperature (C) b) Figure 5.11 – (a) FP cavity sensors response to the applied strain. (b) Response of the 198 m long sensor to temperature. The 198 m long sensor was placed inside a tubular oven, with the FP cavity placed at its center. The fiber was kept straight but loose, without any tension applied. The sensor was then subjected to a temperature variation of ~900 °C. The interferometric spectrum shifted towards longer wavelengths with the increase of temperature (red shift), as can be seen in Fig. 5.11 (b). The experimental data was well adjusted to a linear fitting and a sensitivity of (0.85 ± 0.01) pm/°C was attained, which indicates that this sensor presented a cross-sensitivity of ~0.24 /°C. Considering the round-trip propagation phase shift 24 eff FP m n L , where m is the interference peak order, after some straightforward algebraic manipulation, it is possible to re-write the equation as 2/ eff FP d n dL m . Combining the two equations, one gets for the thermal sensitivity of
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 90 this sensor the value of 7 5.49 10 / FP FP L L C (at the operation wavelength of 1547 nm), which is in good agreement with the silica thermal expansion coefficient presented in the literature, of 7 5.5 10 / C [126]. Since this FP cavity presents low sensitivity to temperature, it is worthwhile to study its behavior when strain is applied under extreme temperature conditions. Thus, the 70 m long FP cavity was placed inside the tubular oven, in a centered position, and on the outside it was fixed to a translation stage. The fiber was kept straight under a slight tension. The temperature was increased from 22 °C to 750 °C in steps of 150 °C. From 750 °C to 900 °C, the steps were of 50 °C. The resolution of the oven temperature controller was of 1 °C. At each temperature step, the setup was stable for about 30 minutes. Strain measurements were then carried out, by increasing the tension in the fiber up to 1000 (up curves in Fig. 5.12) and decreasing it back to the initial state (down curves in Fig. 5.12). 0200 400 600 800 1000 1550 1552 1554 1556 1558 Wavelength (nm) Strain () Up Down a) T = 22 C 0200 400 600 800 1000 1550 1552 1554 1556 1558 b) T = 750 C Wavelength (nm) Strain () Up Down 0200 400 600 800 1000 1554 1556 1558 1560 1562 d) T = 900 C c) T = 850 C Wavelength (nm) Strain () Up Down 0200 400 600 800 1000 1554 1556 1558 1560 1562 Wavelength (nm) Strain () Up Down Figure 5.12 – Response of the 70 m long FP cavity to the applied strain at different temperatures. Up and down stand for increasing and decreasing the applied strain, respectively. Until 600 °C the behavior was nearly the same and the sensitivities obtained when increasing strain were similar as when decreasing it. However, from 750 °C on, the sensor exhibited higher sensitivity when increasing strain than when it decreased. This
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 91 fact indicates that at such high temperatures, the Young modulus of the silica tube is reduced, also associated with a certain level of induced plasticity, since the interferometric fringes did not return to the original wavelength values when strain was decreased. At 900 °C the reduction of the strain sensitivity translates into a red shift of ~1 nm. The effects of annealing were addressed by subjecting the 51 m long sensor to a temperature of 900 °C for 7 hours (see Fig. 5.13). In this case, the fiber was kept straight under a slight tension. The monitored fringe wavelength shifted 4.4 nm throughout this period of time. However, in the first 40 minutes the shift was more pronounced, with a shift rate of (0.10 ± 0.01) nm/min. After that time, the wavelength shift became slower and from 4 hours up to 7 hours, the change was of (3.0 ± 0.1) pm/min. The oven was then switched off and cooled down until it reached room temperature. 050 100 150 200 250 300 350 400 450 1534 1535 1536 1537 1538 1539 1540 Wavelength (nm) Annealing Time (min) Figure 5.13 – Wavelength shift of the 51 µm long FP cavity for an annealing temperature of 900 °C. After being subjected to the annealing at such high temperature, the sample was tested to strain at different temperatures, following the same procedure as for the 70 µm long sensor. The results, presented in Fig. 5.14, show that in this case, the difference attained in the sensitivity when increasing and decreasing strain was not as notorious as in the previous experiments. The small difference at 900 °C can be due to the fact that the annealing was not fully performed.
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 92 0200 400 600 800 1000 1550 1552 1554 1556 1558 Wavelength (nm) Strain () Up Down 0200 400 600 800 1000 1550 1552 1554 1556 1558 Wavelength (nm) Strain () Up Down 0200 400 600 800 1000 1550 1552 1554 1556 1558 Wavelength (nm) Strain () Up Down a) T = 22 Cb) T = 750 C c) T = 850 Cd) T = 900 C 0200 400 600 800 1000 1550 1552 1554 1556 1558 Wavelength (nm) Strain () Up Down Figure 5.14 – Response of the 51 m long FP cavity to strain at different temperatures, after 7 hours of annealing, at 900 °C. Up and down stand for increasing and decreasing the applied strain, respectively. The sensitivities to strain at different temperatures are gathered in Fig. 5.15. In both sensors, regardless of having been annealed, the strain sensitivity decreased as temperature increased up to 600 °C, increasing once again afterwards. This effect can be attributed to two reasons: the non-linear variation of the silica thermal expansion as temperature arises [238], as well as the dependence of the photoelastic constant on temperature. The photoelastic constant is essentially determined by the Pockels coefficient, p12, which exhibits a maximum at 600 °C [239], translating into a minimum in the strain sensitivity. Nevertheless, the difference between applying strain and reducing it is much more significant when no annealing occurred. 0200 400 600 800 1000 4.2 4.4 4.6 4.8 5.0 5.2 5.4 b) Strain Sensitivity (pm/) Temperature (C) Strain up Strain down a) 0200 400 600 800 1000 5.4 5.6 5.8 6.0 6.2 6.4 6.6 Strain Sensitivity (pm/) Temperature (C) Strain up Strain down Figure 5.15 – Dependence of the strain sensitivity at different temperatures: (a) without annealing and (b) with annealing.
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 93 The mechanical stability of this new silica tube design was compared to the one of a conventional silica tube by applying strain until rupture. Both cavities were fabricated under the same conditions. The conventional silica tube, whose cross-section is shown on the inset of Fig. 5.16, had a hollow core diameter of (57 ± 2) m, and a total crosssection area of ~2550 m2. Regarding the new design, with the four rods, its hollow cross-section area was of ~5980 m2. Two sensors were fabricated with a length of ~750 m. The sensor based on the silica tube without rods was able to measure strain up to 1500 , with a linear sensitivity of (1.95 ± 0.01) pm/(see Fig. 5.16). Regarding the sensor with the new silica tube design, it was possible to measure strain up to ~2500 , with a linear sensitivity of (3.39 ± 0.01) pm/. 0300 600 900 1200 1500 1800 2100 2400 2700 0 1 2 3 4 5 6 7 8 9 10 Wavelength Shift (nm) Strain () Figure 5.16 – Sensors response to the applied strain until rupture. The insets show the cross-section photographs of the silica tube used as sensing element in each case. The size of the hollow core area plays an important role in the sensitivity, along with the FP cavity length. Large hollow core areas and short cavity lengths translate into an enhancement of the sensitivity. This occurs due to a much reduced effective Young modulus of the hollow core structure associated with the smaller fraction of silica material in its cross-section. Therefore, most of the deformation occurs in the tube region and with a spatial rate that increases with the reduction of the tube length, therefore increasing the strain sensitivity. Besides, the presence of the four rods constitute a reinforcement to the new structure, which shows favorable sensing properties both in
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 100 sensitivities to strain. This configuration presented low sensitivity to temperature. This study was further developed by considering a different sensing element, based on a silica tube spliced between two sections of single mode fiber. Several samples were characterized in strain, and it was confirmed that the smaller the FP cavity length, the higher the sensitivity. The sensor showed low sensitivity to temperature, thus being a good candidate to perform strain measurements in high temperature environments. However, when strain was applied at temperatures higher than 750 °C, the sensitivity increased as the fiber was tensioned. When the fiber returned to its initial state, without strain applied, the sensitivity decreased to a value similar to the one found at lower temperatures. This difference was reduced through thermal annealing before subjecting the sensing head to strain at extreme conditions. Furthermore, the strain sensitivity decreased as temperature increased up to 600 °C, increasing once again for higher temperatures. This configuration was also compared to a FP cavity based on a conventional silica tube and presented not only higher strain sensitivity, but also better mechanical resilience. Fiber Bragg gratings written in standard single mode fiber were also characterized in strain at high temperatures. The fabrication process was determinant for the high thermal stability of the device, as it was produced using the point-by-point femtosecond laser technique. The Bragg wavelength shifted towards longer wavelengths and the reflectivity decreased slightly, independently of the annealing treatment. However, the sensor response became more stable after the annealing treatment, following the same trend as in the FP configuration.
CHAPTER SIX 6 Fiber Lasers for Sensing
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 103 6.1 Introduction The first works on optical fiber lasers were published in the 1960s by E. Snitzer [242]. However, it was only in 1993 that Kim et al. presented the first optical fiber laser as sensor with frequency read-out [243]. The Nd3+-doped fiber, used as the cavity active medium, was subjected to lateral stress variations that modified the fiber birefringence, thus changing the modal beat frequency. In the proposed configuration, two dichroic mirrors were employed to create the cavity. The employment of a Faraday rotating mirror was proposed by Park et al., for the measurement of magnetic-fields [244]. The use of such reflective element ensured the laser stability to strain and temperature variations. In the meantime, with the development of the fiber Bragg grating (FBG), the quintessentially wavelength filter, the external bulk mirrors fell into disuse. The FBGs were not only used as reflective elements, but also as sensing devices, thus combining the advantages of the fiber laser (such as high signal-to-noise ratio (SNR) and reduced linewidth) with the ability to perform active sensing, by exposing one mirror, or even the whole cavity, to the environmental changes to be measured. The most simple fiber laser sensor, and the one most explored by the scientific community, is the linear cavity, where the active medium (usually Er3+-doped fiber) is placed between two FBGs, constituting the distributed Bragg reflector (DBR) laser. The presence of a wavelength division multiplexer (WDM) enables the coupling of the 980 nm pump light to the cavity [245]. This kind of configuration has been used to measure dynamic strain [246], temperature [247], ultrasound [248], liquid refractive index [249], gas pressure [250] and twist [251]. Still considering the linear cavity configuration, it is possible to achieve laser action by using a single phase-shifted FBG [252]. This type of fiber laser is usually denoted as distributed feedback (DFB) laser, and it has been used for measurement of several physical parameters, such as pressure and force [252], simultaneous measurement of strain and temperature [253], or even acoustic signals [254].
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 104 A different approach to the linear cavity is the fiber ring laser. In such cavity, the Er3+-doped fiber is incorporated in a loop. The pump is coupled to the ring cavity through a WDM and the output signal is detected by using an optical coupler. In a simpler configuration, a commercial erbium-doped fiber amplifier (EDFA) is placed in the loop. The wavelength filter can be a FBG placed outside the ring, connected through an optical circulator [255], a pair of long period gratings incorporated in the ring [256, 257], a section of polarization maintaining (PM) PANDA fiber inside the ring [258] or even a phase-shifted chirped FBG [259]. The fiber ring laser has been applied in the measurement of pressure [260], bending and strain [256], vibration [255], and torsion [257]. When compared to the behavior linear cavities, the unidirectional travelling wave obtained in the ring configuration eliminates both the backscattering and the spatial-hole burning effects [261]. Thus, these cavities present good stability, flexibility, and are easy to manufacture [262]. Going further in the ring configuration, if a non-amplifying loop is added to the amplifying loop through an optical coupler, a figure-of-eight configuration is obtained. In this case, the inclusion of an optical filter in the loop can be decisive in achieving good output stability [263]. The filter can be a PM fiber [261], a twin-core photonic crystal fiber (PCF) [264], or a triangular core PCF [265]. All these works have been proposed only for laser action, not for sensing. In this Chapter, two fiber sensors are proposed, one for torsion measurements and the other for strain sensing. The first sensor is based on a post-processed FBG. The device is subjected to strain measurements, where it exhibits an ultra-high sensitivity. Two different approaches are compared, for passive and active measurements. The second configuration consists on a figure-of-eight laser, whose interferometric filter, a PM PCF, also acts as the sensing element. In this case, the torsion can be applied over a range of 180°.
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 105 6.2 Strain Sensor based on Post-Processed Fiber Bragg Grating The sensor developed in this work consisted on an etched fiber Bragg grating tip. The ultra-thin structure was characterized in strain and temperature. A theoretical model was used to better understand the device behavior. 6.2.1 Theoretical Considerations The Bragg wavelength of a fiber Bragg grating (FBG) is determined by 2, B eff n where neff and are, respectively, the effective refractive index of the propagating mode and the grating pitch [266]. When the FBG is subjected to axial strain, , its Bragg wavelength will shift according to Eq. 6.1: (1 ) , B B e p (6.1) where pe is the photoelastic coefficient for silica, considered to be ≈0.22 [267]. In the case of a tapered FBG created by means of chemical etching, the cross-section area of the fiber changes with the position z, as exemplified in Fig. 6.1. Figure 6.1 – Scheme of a tapered FBG without strain applied (top) and under strain (bottom). Adapted from [268]. Considering a linearly tapered fiber, the radius across z is given by [268] 0 0 ( ) 1 , z r z r z (6.2)
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 106 where r0 is the original radius and z0 is the point along the z axis where the radius of the fiber becomes zero. The cross-section area comes also a function of z, according to Eq. 6.3: 2 22 00 ( ) ( ) 1 .A z r z r z z (6.3) When the fiber is under a tension force N, the strain along z can be determined through 2 2 00 ( ) , 1 NN zEA z E r z z (6.4) where E is the Young modulus of silica. The strain at z = 0 is given by 2 0 (0) . N Er (6.5) Thus, substituting Eq. 6.5 in Eq. 6.4, one gets that 2 0 (0) ( ) . 1 z zz (6.6) When the fiber is under strain, the FBG pitch along the z axis becomes a function of the applied tension force, according to Eq. 6.7: 00 2 0 (0) ( ) (1 ( )) , 1/ zz zz (6.7) where 0 is the original pitch and 0 ((0) 0) is the change in the pitch at z = 0. The variation of the pitch along the FBG translates into a chirp of the spectrum. Besides, the distribution of strain is not constant in this case. The previous equations were used in a numerical simulation to better understand the consequences of having such a structure subjected to strain. Some assumptions were made, based on the etching behavior observed experimentally. The chemical etching was performed in the liquid phase; thus, there was an abrupt change of the fiber diameter at the interface between the un-etched and the etched regions. The decrease of the fiber cross-section area will induce a larger amount of strain throughout the etched region.
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 107 Considering that the whole structure is under the same amount of tension force, and using Eq. 6.5, it is possible to relate the strain applied to the un-etched structure to the one obtained at the etched region: 2 2, uu u u e e e e r AA r (6.8) where the subscripts u and e designate the un-etched and etched fiber sections, respectively. Thus, considering fiber radii of 62.5 m and 10 m (r0), for the un-etched and etched fiber regions, respectively, and that the length of un-etched fiber is much larger than the length of etched fiber, when 10 are applied to the whole structure, the amount of strain at the beginning of the tapered region is determined to be ~390 . The other parameters considered for the numerical simulations were a reduction of the fiber radius by 10-3 m by unit length, meaning that z0 = 9×103 m. It was also considered that the FBG was located ~1.5 mm after the beginning of the tapered region, and had a length of 3 mm. Besides, three different values were considered for the strain applied to the whole structure: 10, 50 and 100 . The results obtained for the variation of the FBG pitch and for the strain along the taper length are shown in Fig. 6.2. The gray rectangle indicates where the FBG was positioned along the fiber. 0 1 2 3 4 5 6 537 539 541 543 545 547 (nm) z (mm) (0)=10 (0)=50 (0)=100 a) 0123456 0 4 8 12 16 20 b) x103() z (mm) (0)=10 (0)=50 (0)=100 Figure 6.2 – Numerical simulation of an etched FBG, considering three values of initial strain. (a) variation of the pitch with the length and (b) strain variation along the grating length. Figure 6.2 (a) presents the pitch variation along the z axis, for three different values of initial strain. A maximum increase of ~4 nm was attained, when the applied strain was 100 . The dotted lines represented in the Figure are the expected variation of the FBG pitch if it was located in other regions of the taper. When the analysis is performed
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 108 considering the increase of strain as the taper becomes thinner, the values obtained are much more explanatory. In fact, according to the green curve in Fig. 6.2 (b), when the fiber is subjected to 10 , the strain in the tapered region, in z = 0 mm, is ~100 times higher. In this case, the applied strain only increases slightly with the taper length. However, when the applied strain is of 100 , the tension will be more than 5 times higher near the fiber tip than at the beginning of the taper. However, the strain experienced by the FBG increases over its length and, at its edge, the strain is twice the value determined at its beginning. Thus, although the whole structure is only strained by 100 , the tapered region is locally strained up to 20000 , an impressive amount of strain for such a thin structure. The variation of strain along the FBG length, and its induced broadening of the grating pitch translate into a chirped spectrum, which broadens as strain is applied, as a consequence of an increase in the pitch difference between the beginning and the end of the FBG. Besides, this sensor is expected to be more sensitive to the applied strain than a standard, un-etched FBG, due to the unusual distribution of strain throughout the structure. 6.2.2 Sensor Design and Spectral Characteristics The first step to produce the sensor consisted of writing the FBG in a photosensitive SMF, using a KrF excimer laser that operated at 248 nm and the phase-mask technique. The fiber, commercialized by the FBGS Company, had a core diameter of 5 m, a cladding diameter of 125 m and a numerical aperture of 0.26. The configuration for the gratings inscription, described in Appendix III, was based on a Talbot interferometer. Since the fiber was photosensitive, there was no need for prior hydrogenation. The length of the FBG was of 3 mm. The second step consisted of cleaving the end section of the sensor, guaranteeing that there were ~2 mm of SMF with no inscription between the FBG and the fiber end. Since the core of the fiber was Ge-doped, the etching rate in this region was higher than that of the cladding. Thus, it was important to ensure that the acid did not reach the core area with the FBG. The following step was to submerge the fiber tip in a 40%-hydrofluoric acid (HF) solution for around one hour, at room
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 109 temperature, thus obtaining a very thin fiber tip, with a thickness lower than 10 m. The chemical reactions involved in this step are [81] 2 2 2 6 2 2 3 6 6 2 , 6 2 . SiO HF H O H SiF GeO HF H O GeF (6.9) The chemical etching in the liquid medium favored the formation of a linearly tapered fiber tip, with a small diameter slope over the etched tip length. If the etching were performed in gas medium, a sharp conic-shaped tip would be expected [226]. Figure 6.3 (a) shows the reduction of the cladding diameter as a function of time. From the linear relationship between these two parameters, it is possible to estimate the etching rate to be ~(115 ± 1) m/h. Fig. 6.3. (b)-(d) show microscope images of the fiber at different etching times. On Fig. 6.3 (b), the fiber was submerged for ~30 min and the cladding diameter was of (65 ± 2) m. Notice the conical shape of the inner region, that corroborates the assumption that the etching will be faster in the core than in the cladding. After ~55 min, the outer diameter was of (18.2 ± 2) m (Fig. 6.3 (c)). Finally, after a time of ~61 min, the diameter was reduced to (8 ± 2) m. 010 20 30 40 50 60 70 0 20 40 60 80 100 120 140 Cladding Diameter (m) Time (min) a) Figure 6.3 – (a) Cladding diameter variation with the chemical etching time. Also shown microscope images of the fiber tip after etching times of (b) ~30 min, (c) ~55 min and (d) ~61 min. Throughout this time, there was a reduction of the length of the fiber; however, it did not reach the FBG inscription area, as evidenced by the reflection spectra, taken at different time steps (0, 30, 55 and 61 min), shown in Fig. 6.4. The Bragg wavelength shifts towards blue and, as the fiber diameter decreases below 10 m, this shift becomes more b) c) d)
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 116 deviation was of 0.01 nm. Thus, by applying Eq. 6.10, one gets a resolution of 280 n. This value was mainly limited by the signal acquisition system resolution. 0 5 10 15 20 25 30 1560.0 1560.3 1560.6 1560.9 1561.2 1561.5 Wavelength (nm) Time (arb. units) 11.61 Figure 6.12 – Step technique to estimate the resolution of the fiber laser strain sensor. 6.3 Torsion Sensor based on Figure-of-Eight Fiber Laser In this work, a figure-of-eight fiber laser is explored for torsion measurements. A section of polarization maintaining photonic crystal fiber acts both as interferometric filter and sensing element. 6.3.1 Working Principle The figure-of-eight fiber laser sensor developed in this work was obtained according to the scheme depicted in Fig. 6.13. One of the loops was constituted by a pump laser diode emitting at 980 nm, a 980/1550 nm wavelength division multiplexer (WDM), an optical isolator, a 90:10 optical coupler and an optical spectrum analyzer (OSA), with a resolution of 0.01 nm, to perform the readings. In between the WDM and the optical isolator, a section of Er3+-doped fiber (about 800 mm long) was inserted to provide the gain. The concentrations of erbium and aluminum ions in the active fiber were of 1000 ppm and 10000 ppm, respectively. The numerical aperture of this fiber was 0.27, its core had a diameter of ~5 m, and it presented a step index profile. The other fiber loop
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 117 included a polarization controller (PC) and the interferometric filter described in the following Section. Er3+-doped Fiber Optical Isolator Sensing Head (PM PCF) WDM 50:50 Coupler PC Pump Laser 90:10 Coupler OSA Figure 6.13 – Scheme of the figure-of-eight fiber laser. WDM stands for wavelength division multiplexer, PC is the polarization controller and OSA corresponds to the optical spectrum analyzer. Using the appropriate pump power, the gain medium emits in the 1525-1560 nm region, according to the erbium spontaneous emission curve [271]. Light from the amplifier section will enter the loop on the right via the 50:50 coupler. The optical isolator is placed between the Er3+-doped fiber and the optical coupler to prevent the return of light through this arm. The coupler splits the incoming light into two guided waves propagating in opposite directions: one wave will travel in the clockwise direction, with a certain velocity and polarization state; the other wave will propagate in the counter-clockwise direction, with a different velocity and polarization state. When they reach the coupler, the two beams interfere and a periodic spectrum is obtained. The sensing element, a polarization maintaining photonic crystal fiber (PM PCF), acts as an interferometric filter, selecting the wavelength at which laser emission will occur. Such selection results from the combination of two different aspects: the maximum transmission power of the filter and the region where the spontaneous emission gain is higher. Thus, the laser is expected to emit around 1530 nm. Depending on the polarization of lights travelling inside this loop, one or more lasing peaks were observed. However, all measurements were carried out with the emission of a single laser peak. The light that is recoupled at the 50:50 coupler is then redirected to the amplification loop, where a 90:10 coupler is introduced, to obtain the readings from the OSA.
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 118 6.3.2 Sensor Design and Spectral Characteristics The sensing element used to perform the torsion measurements was an interferometric filter based on a PM PCF whose cross-section is shown in Fig. 6.14 (a). The fiber used is commercially available (PM-1550-01 from Thorlabs, Inc.) and presents a pure silica core and cladding. The fiber contains two large holes, with a diameter of 4.5 m, and 82 smaller holes, with a diameter of 2.2 m. The core, placed between the two larger holes, presents dimensions of 6.6/4.3 m and the outer cladding has a diameter of 125 m [272]. The characteristics of this fiber ensure a high birefringence, that beat length is lower than 4 mm and an attenuation < 2dB/km. 1520 1530 1540 1550 1560 1570 1580 -40 -35 -30 -25 -20 -15 -10 -5 Optical Power (dB) Wavelength (nm) b) Figure 6.14 – (a) Microscopic photograph of the polarization-maintaining photonic crystal fiber used as sensing element. (b) Transmission spectrum of the sensor. The channeled spectrum shown in Fig. 6.14 (b) was obtained by removing the amplification loop (the loop on the left on Fig. 6.13) and connecting one of the 50:50 coupler arms to the broadband optical source (bandwidth of 100 nm, centered at 1570 nm) and the other arm to the OSA, in a Sagnac configuration. The properties of the interferometric filter are gathered in Table 6.1. The channel spacing was determined by calculating the distance between two maxima, in the frequency domain. Regarding the channel passband and the full-width at half-maximum (FWHM), the two parameters are relative to the same concept, i.e., the peak full width at half the maximum optical power. The former is in the frequency domain, whereas the last is in the wavelength domain. a)
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 119 Table 6.1 – Properties of the interferometric filter. Average Value FWHM (nm) 1.72 Channel Spacing (THz) 0.43 Channel Passband (THz) 0.17 6.3.3 Experimental Results The response of the laser optical power to the pump diode drive-in current is shown in Fig. 6.15. The current threshold was estimated to be (145.7 ± 5.2) mA and a maximum output power of (7.9 ± 0.1) W was achieved for a drive-in current of (229.7 ± 0.1) mA. Figure 6.15 – Optical power variations with the drive-in current. In order to perform torsion measurements, one side of the sensor was introduced in a torsion stage, which had a resolution of 0.5°, while the other side was kept fixed. It was found that both the amplitude and the central wavelength of the laser peak shifted as torsion was applied, for both negative and positive angles. This behavior, illustrated in Fig. 6.16 (a), was due to the rotation of the PM PCF cores. Starting from 0°, the peak shifted towards longer wavelengths as the torsion angles increased (red shift). Accordingly, by decreasing the angles from 0° to -90°, the laser peak shifted towards shorter wavelengths (blue shift). The laser will emit in a wavelength that corresponds to the maximum response from the combination of the erbium fiber gain curve and of the interferometric filter function, as shown in Fig. 6.16 (b). 100 120 140 160 180 200 220 240 0 1 2 3 4 5 6 7 8 9 P (W) I (mA)
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 120 1535.0 1535.7 1536.4 1537.1 1537.8 1538.5 -70 -60 -50 -40 -30 -20 b) Optical Power (dBm) Wavelength (nm) -0+ a) 1534 1535 1536 1537 1538 1539 1540 -40 -35 -30 -25 -20 -15 -10 -5 Optical Power (dB) Wavelength (nm) 0.0 0.4 0.8 1.2 1.6 2.0 2.4 Optical Power (W) Figure 6.16 – (a) Variation of the laser emission with the applied torsion and (b) interferometric filter spectrum (blue line) and laser spectrum (pink line). The response of the laser peak wavelength to the applied torsion angle is shown in Fig. 6.17 (a). The experimental data presented a linear behavior in this range, which led to a sensitivity of –(7.13 ± 0.05) pm/degree. The analysis could also have been done considering the variations of optical power, since the laser emission power varies depending on its wavelength vs. the interferometric filter response. Thus, when the wavelength corresponds to a maximum of the interferometric filter spectrum, a maximum also occurs in the laser peak emission, following the same trend as the filter itself. As the torsion is applied, besides the wavelength shift of the interferometric filter pattern, there is an increase on the filter transmission losses, which is responsible for the decrease in the laser peak power. Eventually, the losses become higher than the gain and the laser ceases to emit (black, blue and green curves in Fig. 6.16. (a)). -90 -60 -30 0 30 60 90 1536.0 1536.2 1536.4 1536.6 1536.8 1537.0 1537.2 1537.4 1537.6 Wavelength (nm) Torsion Angle () a) 0100 200 300 400 500 600 700 800 900 1000 0.00 0.05 0.10 0.15 0.20 0.25 0.30 Wavelength Shift (nm) Strain () b) Figure 6.17 – Laser peak wavelength dependence on the applied (a) torsion angle and (b) strain. Using the same active configuration, the interferometric filter was attached to a translation stage, with a resolution of 0.01 mm, and strain measurements were carried
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 121 out over a range of 1000 . The results, depicted in Fig. 6.17 (b), follow a linear trend. From the fitting, a sensitivity of only (0.30 ± 0.06) pm/ was obtained. Besides, it has been reported in the literature that this fiber has a very low sensitivity to temperature (~0.3 pm/°C) [272], meaning that with this configuration it is possible to perform strain and temperature independent torsion measurements. 6.3.4 Final Remarks In this Chapter two different fiber lasers were proposed to be applied as sensors. The first configuration was based on a post-processed fiber Bragg grating tip. The sensor was firstly characterized as a passive element, where ultra-high strain sensitivity was obtained. Three different approaches were considered for signal processing: one regarding the full-width at half maximum variation with the applied strain; the other regarding the wavelength variation of the whole spectrum, measured at 3 dB; and the third one consisted of tracking one single peak, where an outstanding sensitivity of (127.3 ± 2.4) pm/ was achieved. The sensor was also tested in temperature, where it exhibited a sensitivity similar to the one of a non-etched fiber Bragg grating. However, the signal processing of this passive sensor is more complex if a commercial interrogation system is used. When incorporated in a fiber laser, the laser peak varied not only in wavelength, but also in integrated power, following the EDFA gain curve. In fact, this was one of the major drawbacks of this configuration, as it is only suitable to be used in small strain ranges. However, the peak did not broaden with strain and the laser did not suffer from mode hopping. Nevertheless, this sensor presents a very high signalto-noise ratio, being very suitable for applications in remote sensing, and it can also be useful as vibration sensor. It is also possible to use the laser strain sensor with temperature independence, if the optical power variations are considered instead of wavelength variations. The second configuration consisted of a figure-of-eight fiber laser and had a section of polarization maintaining photonic crystal fiber which acted simultaneously as an interferometric filter and the sensing element. The active sensor was subjected to torsion measurements, and both the peak laser wavelength and the optical power changed over
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 122 a range of 180°. Since the filter presents low sensitivity to strain and temperature, in addition to its low cost, ease of fabrication and reliable results, it proves to be a good choice for performing torsion measurements.
CHAPTER SEVEN 7 Sensors Based on Microspheres
FCUP Fiber Sensing Based on New Structures and Post-Processing Enhancement 125 7.1 Introduction The incorporation of microspheres in optical fiber-based configurations has attracted great attention over the last two decades. The microspheres, due to their physical characteristics and when successfully excited, can be used in a wide variety of practical applications, from sensing to lasing or even tight focusing of light, as it shall be seen next. Two different mechanisms have been established to explain the propagation of light in microspheres. Under appropriate excitation conditions, they can provide periodical resonances in the transmission spectrum with very high Q-factors 3 and low transmission losses [273]. In this mechanism, based on the whispering gallery modes (WGMs), the light coupled to the microsphere is trapped and propagates in circular orbits close to the surface [274]. To produce the WGMs, it is necessary to couple the evanescent field adequately to and from the microsphere. When several microspheres form a linear chain, besides the tight binding of the WGMs, focusing of light is produced by each microsphere. The focused spot is designated by photonic nanojet and has an elongated shape and sub-wavelength lateral size. The nanojets present in a chain of microspheres produce periodic modes, called nanojet induced modes (NIMs) [275]. In long chains of microspheres (>10), the NIMs are the dominant effect [276]. However, the microspheres must display diameters and refractive indices that enable focusing of light at the shadow surface of the spheres, to ensure the presence of such NIMs. There are currently several techniques to produce microspheres. The first microspheres produced on a fiber tip were reported in 1973 by Kato and co-workers [277]. A hydrogen-oxygen microtorch was used and microspheres with a maximum diameter of 250 m were proposed to couple light from a laser into the fiber. A couple of years later, Paek et al. suggested the use of a CO2 laser to produce the hemispherical lens 3 The quality-factor, or Q-factor, is given by Q , where is the wavelength at which the resonance occurs and corresponds to the linewidth of the resonant wavelength. The microsphere-based resonators can present Q-factors up to 1011 [273].