Fabrication of negative curvature hollow core optical fibers capable of acoustic sensing

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IEEE ICTON 2025* The final version is available at: https://doi.org/10.1109/ICTON67126.2025.11125404 Fabrication of negative curvature hollow core optical fibers capable of acoustic sensing Ivo Barton Institute of Photonics and Electronics, Czech Academy of Sciences) Prague, Czech Republic [email protected] Ondrej Podrazky Institute of Photonics and Electronics, Czech Academy of Sciences) Prague, Czech Republic [email protected] Ali. A. Jasim Institute of Photonics and Electronics, Czech Academy of Sciences) Prague, Czech Republic [email protected] Andrei Borodkin Institute of Photonics and Electronics, Czech Academy of Sciences) Prague, Czech Republic [email protected] Martin Grabner Institute of Photonics and Electronics, Czech Academy of Sciences) Prague, Czech Republic [email protected] Yauhen Baravets Institute of Photonics and Electronics, Czech Academy of Sciences Prague, Czech Republic [email protected] Pavel Honzatko Institute of Photonics and Electronics, Czech Academy of Sciences) Prague, Czech Republic [email protected] Abstract—Negative curvature hollow-core optical fibers (NCHCFs) are one of the great inventions of modern times, as they are capable of transmitting light over a broad range of wavelengths, from UV to mid-infrared, with losses that can be lower compared to those of standard solid silica fibers. By exploiting an anti-resonance mechanism, low attenuation is achieved within the transmission windows located in the vicinity of wavelengths determined by a capillary thickness. Lately, novel applications have emerged, as these fibers show significant potential to function as distributed acoustic sensors, since their sensitivity to acoustic waves may surpass that of conventional fibers. However, the fabrication of NCHCFs often proves to be complicated due to the difficulty of controlling a fiber’s capillary diameter, fiber core diameter, and capillary wall thickness. In this contribution, we report on the fabrication process of inhouse drawn NCHCFs with different structures, the splicing process of NCHCF with conventional solid fibers, and the capability of our NCHCFs to serve as acoustic sensors. Keywords—optical fiber fabrication, hollow core fibers, fiber splicing, acoustic sensitivity I. INTRODUCTION Negative curvature hollowcore optical fibers (NCHCFs) have surpassed standard solid fibers as a guide for light in air rather than in glass, which gives them exceptional properties such as low latency, a low nonlinear coefficient, and a high damage threshold [1]. It also means that they can guide light at wavelengths where solid materials have high attenuation due to Rayleigh scattering, IR absorption, or photodarkening [2], [3]. Standard solid fibers with an optical loss of below 0.2 dB/km remain applicable in current long-range transmission systems; however, bulk material nonlinearity presents a more fundamental obstacle to capacity scaling. NCHCF exhibits low-loss guidance and a high laser damage threshold, making it suitable for delivering high-power lasers. They can handle laser powers exceeding two kilowatts without damaging the cladding microstructure [4]. HCFs can achieve exceptional polarization purity, transmitting orthogonal polarization modes with minimal cross-coupling. This property is crucial for high-performance photonics applications like interferometers and quantum information experiments. Another niche application of NCHCF is laser gas spectroscopy, which enables the control of HCF guidance performance and the air-core structure, allowing for filling any gas sample, including single gases or gas mixtures. Therefore, low-volume fiber-based gas absorption cells with optical path lengths precisely tailored to the specific application can be constructed [5]. NCHCFs can serve as effective low-volume absorption cells in laser-based gas sensors, enabling the detection of gases such as methane and carbon dioxide with high sensitivity [6]. In the majority, these fibers are drawn down from high-purity fused silica glass; however, other glasses, such as borosilicate [7] and telluride [8] have also been used. Low overlap of the guided mode with the fiber structure leads to very small Rayleigh scattering [9]. Backscattering is predominantly due to Rayleigh scattering in the air, with a backscattering coefficient of -100 dB/m. Other contributions are smaller. The backscattering coefficient from the surface of capillaries is -115 dB/m, and the backscattering coefficient from the material is -150 dB/m [10]. This low Rayleigh scattering enables low-loss propagation and makes the hollowcore fiber (HCF) less susceptible to distributed acoustic sensing, which relies on backscattered signals. This feature is particularly important in today's massively deployed passive optical networks. However, acoustic waves can still modulate the optical path in HCF. We investigated this effect for several HCFs and found that the fiber design significantly impacts the acoustic sensitivity [11]. The acoustic sensitivity of HCF is crucial for deploying precise frequency signals and controlling the vulnerability of passive optical networks to eavesdropping. NCHCF postprocessing was used to adjust the transmission bands' positions of short-length fibers [12]. This technique allows creating fiber pigtailed pass-band filters and tweaking the transmission band for a particular wavelength. In this work, we report on the fabrication procedure used to prepare NCHCFs and NANFs. We present a method for splicing both types of fibers with standard fibers that exhibit low losses. We present measurements of the normalized response of NCHCFs and NANFs to acoustic waves propagating through air, revealing the relationships between the hollow-core fibers and their acoustic sensitivity. II. FABRICATION OF NCHCFS NCHCFs are fabricated using the commonly employed stack-and-draw technique [2]. We have fabricated in-house fibers from synthetic silica glass (Heraeus F300). Following the scheme shown in Fig. 2, we depicted the fabrication process into stages A-F. We started by elongating an 18/15.2 mm silica tube into a capillary tube with dimensions 2.97/2.5 mm. In the next step, we assembled a primary preform by stacking 8 capillaries inside another tube with OD/ID=18/15.2 mm. According to the cross-sectional design shown in Fig. 3A, we used central spacers with a diameter of 9.26 mm to secure the capillaries to the wall of the outer tube and eight smaller spacers with a diameter of 2 mm to maintain equal distances between the capillaries. Fig.2. Scheme of fabrication NCHCF8 -A) Tube 18/15.2mm, B)Capillary tube 2.97/2.5 C) Primary Preform, D) Cane, E) Final preform, F) NCHCF8 fiber We elongated the assembled primary preform at 1870°C to a 4.8 mm outer diameter cane. We assembled the final preform by placing the cane inside a 10/5 mm jacketing silica tube. We used a thin silica tube with a diameter of 2/1 mm to pressurize the core of the cane and a brass T-fitting to pressurize the capillaries. We evacuated the space between the coating tube and the cane to ensure a secure connection between the cane and the jacketing tube. Finally, we have drawn the preform into a fiber with an outer diameter of 197 μm, a core diameter of 57 μm, and a capillary wall thickness of 1.3 μm at 1855°C with a drawing tension above 300 cN. The fiber core was pressurized to 1.96 kPa, and the capillaries were pressurized to 8.33 kPa. SEM photo of the drawn fiber is shown in Fig. 3B. Fig.3. A) Scheme of fabrication NCHCF8, B)SEM photo of NCFCF 8 For the second type of structure, we prepared in-house NANF with five nested rings. The fabrication process, outlined in the scheme, was divided into stages A–H. First, we elongated an F300 tube with an 18/15.2 mm diameter to a diameter of 12.5/10.6 mm at 1900 °C. Then, we took another F300 tube, 18/15.2 mm, and inserted an elongated tube inside it. In the next step, we elongated the nested tube at 1900 °C into the nested capillary, which had outer capillary dimensions of 4.8/4.4 mm and inner capillary dimensions of 3.5/3.1 mm. The primary preform (the design of the structure is depicted in Fig.5A) was then assembled by arranging five nested capillaries with two sets of five small spacers with a diameter of 2.6 mm and one central spacer with a diameter of 5.6 mm, both being 4 cm long. After assembling, the subpreform was elongated at a low temperature of 1850 °C, enabling the cane to be drawn without collapsing the capillaries inside. No pressurization was used in this step. The final preform (G) was assembled by inserting the cane into a thick 10/5 mm silica tube. Fig.4. Scheme of fabrication NANF: A) Tube 18/15.2 mm, B)Tube 12.5/10.6 mm, C) Nested tube, D) Nested capillary, E) Primary preform, F) Cane, G)Final preform, H)Final fiber Similarly, as in the case of NCHCF8, the core of the preform was pressurized through a thin 2/1 mm tube, and capillaries were pressurized through a special T-fitting that enabled the separate pressurization of both outer and inner capillaries. The space between the cane and the silica tube was evacuated to ensure the fusing of the cane and tube. The final fiber was drawn at 1850°C with a drawing tension above 300 cN. The fiber core was presurized to 0.98 kPa, and the pressure difference in the outer and inner capillaries was 4.81 kPa. A SEM photo of the NANF5 b fiber is shown in Fig. 5 B. A B Fig.5. A) Scheme of fabrication NANF5a B) Cross-section of subpreform design, C)SEM photo of NANF5a III. SPLICING NCHCFS A widespread application of HCFs requires an efficient interconnection between HCF and a standard single-mode fiber (SSMF) used in most existing optical systems. Fiber splicing is the most popular, reliable, and repeatable method for interconnecting fibers. However, there are several challenges based on the structure of hollow core fibers. HCFs typically have a large mode field diameter, which makes it necessary to use a mode field adapter (MFA) to decrease insertion loss. We employ a two-step approach to achieve simultaneous mode field and outer cladding diameter adaptation. We prepare the MFAs using standard single-mode fibers such as SMF28 and 1060XP. In the first step, we use the splicer LZM-100 LAZERMaster to expand the fiber core gradually by heating. The fiber is slowly moved in the heating region, with the heating power increasing from 20 W up to 22.5 W to achieve an adiabatic increase in the mode field diameter. The total length of the heating region is about 30 mm, and the fiber speed is 10 μm/s. We repeat this process several times to achieve a resulting mode field diameter comparable to the MFD of the HCF. In the second step, we decrease or increase the diameter of the SSMF in the thermally treated region to achieve the same cladding diameter as the HCF. It is an obligatory process in case of HCF with a diameter more than 125 μm and a thin cladding. Indeed, the only place where HCFs have enough material to splice with is cladding, so the cladding diameter of the HCF should be equal to or smaller than the cladding diameter of the fiber you splice with. Fig.6. Refractive index profile of the original SMF28 fiber and after one, two and three heating processes and three heating processes and one reverse tapering process till 160 μm. The refractive index profile was measured by the Interferometric Fiber Analyser IFA-100 in order to calculate the resulting mode field diameter (Fig.6). Moreover, transmission properties of negative curvature HCFs highly depend on the geometry of thin inner capillaries. Capillaries are quite sensitive to the splicing parameters because they can be melted and collapsed at high splicing power. To prevent such a process, we calibrate splicing power at the minimum level required to splice fibers only by cladding without any visible changes in the capillaries geometry for every HCF. We achieve the level insertion loss of 0.4 dB for the splicing of HCF with SMF28 fiber from both sides. A transmission spectrum of negative curvature HCFs contains a set of bandpass lines with positions determined by the diameter and thickness of inner capillaries. Such fibers have no transmission between these bands. In most optical applications, it is necessary to have a transmission at a specific wavelength or some spectral range, which could be out of the transmission bands of an HCF. However, we can shift these transmission bands to another position by heating or tapering a long segment of HCF in the LZM-100 splicer. In the case of heating, we achieve a regime of slightly decreasing inner capillary diameter with a corresponding increase in their thickness. As a result, the positions of the transmission lines shift to the longer wavelengths. The opposite shift can be achieved by tapering the HCF at the low temperature. The tapering proportionally decreases the full structure of the fiber, so capillaries become smaller and thinner, and transmission lines shift to shorter wavelengths. The maximum length of the shifted HCF that we achieved is 15 cm and is limited only by the movement system used. Moreover, it is possible to do a short all-fiber bandpass filter based on the HCF by shifting the transmission bands in the opposite direction on the different segments of the HCF (Fig.7). The superposition of the resulting transmission bands gives a new band with a predefined width and position. We achieve the minimum full width at half maximum of 33 nm in our laboratory [16]. Fig.7. Filtering NC-HCF’s transmission band at 1.05 μm by heating a tapering fiber segment. Black line – original transmission spectrum; red line – spectrum after heating a 2 cm segment; green line – spectrum after tapering another 2 cm segment from 125 to 112 μm. IV. MEASUREMENT OF ACOUSTIC SENSITIVITY The acoustic sensitivity of optical fibers is a key factor to consider when deploying precise frequency signals over optical fibers and planning new optical fiber installations. It is well-known that polymer fiber coatings significantly affect the acoustic sensitivity of standard optical fibers. The relationship between the thickness of fiber coatings and fiber sensitivity to acoustic waves has been exploited in constructing hydrophones [17]. We investigated how the inner structure of hollow core fibers influences their acoustic sensitivity. For this purpose, we inserted the fiber under test (FUT) into the sensing branch of a heterodyne interferometric analyzer (Fig. 1 in [11]). The FUT was placed in an acoustic chamber, well acoustically isolated from the rest of the interferometer. To preclude the influence of the supporting tray, the fiber was arranged into coils hanging inside the chamber without any contact with the chamber walls. A loudspeaker excited a sound field, and a calibrated microphone in the plane of the FUT A B measured the sound pressure level. We found a strong dependence on the thickness of the fiberglass jacket tube. The NCHCF/NANF with a thin glass jacket were observably more sensitive to acoustic waves due to their lower stiffness. Fig. 8 shows the most and least sensitive fibers' normalized acoustic sensitivity. The difference in sensitivities is 12 dB. Additionally, the response of the fiber with a thin jacket is resonantly enhanced at 5.9 kHz and is stronger by 34 dB compared to the less sensitive fiber. The normalized acoustic sensitivity of the SMF28 fiber is shown for comparison. SEM images of cross-sections of the tested fibers are also included in Fig. 8. Fig.8. (A) Normalized acoustic sensitivities of the (B) thin jacket and (C) thick jacket NANF. V. CONCLUSION In this work, we report on developing and demonstrating fabrication procedures for NCHCFs and NANFs. We demonstrated a two-step splicing technique that enables lowloss integration of hollow-core fibers with standard singlemode fibers, simultaneously adapting both mode field and outer cladding diameters. Experimental investigations into the acoustic sensitivity of NCHCFs and NANFs confirmed that acoustic waves can modulate the optical path despite the predominant air guidance in hollow-core fibers. We observed that the NCFCFs and NANFs design influences acoustic sensitivity, with a significant dependence on the thickness of the silica jacket tube. Specifically, HCFs with thinner silica jackets exhibited higher sensitivity to acoustic waves due to their reduced stiffness compared to SMF28. In the case of HCFs with thicker overcoated silica cladding, sensitivity to acoustic waves was shown to be lower than that of SMF28. The results presented highlight the critical role of fiber geometry and structural parameters in the fabrication of HCF for the acoustic response. ACKNOWLEDGEMENTS This work was supported by the Ministry of the Interior of the Czech Republic under Grant No. VK01030114 within the program OPSEC. Development of the research infrastructure was co-funded by the European Union and the state budget of the Czech Republic under the project LasApp CZ.02.01.01/00/22 008/0004573 REFERENCES [1] G. T. 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