Red Emitting DBR Laser Array with 0.1 nm Wavelength Spacing Defined by Direct Write Ebeam Lithography
Abstract
We present a distributed Bragg reflector diode laser array (DBR array) designed for digital holography on technical and biological surfaces. Compared to DBR arrays developed earlier we shift towards shorter wavelengths around 662 nm (i) for increased penetration depth into biological tissue and (ii) for better adjustment options of the holographical systems (visible light). Integration of optical gratings into edge emitting diode lasers is a prerequisite for holographic applications, because it stabilizes emission wavelengths and reduces linewidths. We apply high order surface gratings and focus on their definition with ebeam lithography, since it allows flexible control of the periods for different vertical laser structures. Controlled but small wavelengths differences are requested for digital holography to generate long synthetical wavelengths. Here we test ebeam direct write lithography to realize 0.1 nm wavelength spacing between adjacent emitters. Since current and temperature influence the emission wavelengths of DBR lasers the wavelength spacing should be obtained at similar operation points of the individual emitters.
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O. Brox, J. Fricke, R.-S. Unger, A. Maaßdorf, A. Mogilatenko, M. Beier, H. Christopher, A. Knigge, and H. Wenzel MNE2025, Photonics and Optoelectronics Paper ID54, Session 2C, 17 Sep 2025, 11:30 Red Emitting DBR Laser Array with 0.1 nm Wavelength Spacing Defined by Direct Write Ebeam Lithography
Ferdinand-Braun-Institut –Facts & Figures III-V semiconductor technologies for electronics | photonics | quantum technologies ▪Research institute, founded in 1992 ▪Staff: 390 (headcount) incl. 200 scientists & PhD candidates from 30 nationalities ▪Excellently equipped 2,000 sqm cleanrooms & labs ▪Revenue (2024): 44.5 M€ (incl. 24.9 M€ third-party funding) ▪11 Joint Labs with 5 German universities ▪Part of −FMD –Research Fab Microelectronics Germany and −APECS –European chiplet innovation, pilot line ▪ FBH gGmbH is a 100 % subsidiary of State of Berlin and a member of the Leibniz Association 2
Research Topics & Competencies 3 Optoelectronics ▪high-power diode lasers: broad area & bars ▪high-brightness & narrowband diode lasers ▪photonic components, modules & systems (cw & pulsed): from NIR to UV spectral range GaAs edge emitters (0.63 - 1.2 µm) ▪High conversion efficiency −Peak values >70% (typ. 50%) −At least a factor of two better than other lasers ▪Compactness (chip size 0.5 mm x 4 mm x 0.15 mm delivers 15 W) ▪Capability of mass production, full wafer processing (3 and 4 inch) ▪Easy excitation, reliable operation Full value chain: Design, epitaxy (MOVPE), processing, mounting, chracterisation, reliability
Applications of GaAs Edge Emitters Sensing, Spectroscopy, Instrumentation ▪custom wavelengths for material analytics ▪absorption and Raman spectroscopy Metrology ▪atomic clocks (Cs D2, K D2, Rb D2, Rb D1) ▪atom interferometry Need for single mode, small band emission 4 Optical Communication ▪pumps for space applications (satellite links) ▪free space communication Medicine (ophthalmology, dental) Measurement instrumentation ▪pulsed sources (LIDAR for driving assistance) ▪Multi wavelength coherent sources (surface inspection)
Edge Emitters – Need for Gratings 5 Periodical modulation of refractive index Integrated Bragg gratings –improved optical properties ▪Single mode operation ▪Small linewidth (kHz ... MHz) ▪Small thermal tuning (typ. 0.06 nm/K) Defined grating reflectivity DBR type: highest possible reflectivity Low losses of the gratings for high laser efficiency without with gratings DFB DBR
Gratings Types at FBH 6 Buried gratings (P. Sammeta, Poster ID 423) ▪Two-step epitaxy ▪Mainly DFB lasers ▪Typical grating order m = 2 (periods > 200nm, duty cycle 0.25) Surface gratings (this talk) ▪Single-step epitaxy ▪Mainly DBR lasers ▪Grating order m > 3 (Periods > 400 nm; duty cycle > 0.9) ▪V-shaped grating groves with depths < 2 µm
Realisation of Surface Gratings Single-step MOVPE Hardmask ▪nLOF/Ti (750/50 nm) ▪Ti for nLof patterning and conduction layer Three level dry etch-process (RIE) ▪Ti/nLOF hard mask (SF6; O) ▪semiconductor (BCl3; Ar) Design for functional surface gratings → Modelling 7 Docter et.al, Proc. Symp. IEEE/LEOS p 97, 2006 Fricke et al. Semicond. Sci. Techn. 27 (2012) 055009
Influence of Bragg Order and Duty Cycle on Reflectivity ▪Higher Bragg order gratings (larger periods) applicable for high reflectivity ▪Etched groove as small as possible for high duty cycle W/ ▪V-grooves to increase the duty cycle to > 0.9 8 0.5 0.6 0.7 0.8 0.9 435 348 261 174 87 973 974 975 976 977 978 6th order = 870 nm groove width -w (nm) duty cycle (nm) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 reflectivity 0.5 0.6 0.7 0.8 0.9 72.5 58.0 43.5 29.0 14.5 973 974 975 976 977 978 1st order = 145 nm groove width -w (nm) duty cycle (nm) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 reflectivity P. Bienstman, CAvity Modelling Framework (CAMFR), Ghent University = m L/ (2 neff) ▪grating period ▪lasing wavelength L ▪effective index neff ▪grating order m
Influence of the Etched Depth on Reflectivity Etched groove depths 1 … 2 µm ▪large aspect ratios (up to 50:1) challenging Relaxed process conditions with V-shaped grooves Maximum reflectivity > 90% 9 0.80 0.75 0.70 0.60 0.85 0.50 0.40 0.30 0.90 0.60 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 groove width / µm residual layer thickness dres / µm maximum reflectivity Fricke et al. Proc. SPIE113011H Photonics West (2020) etch depth residual layer thickness
Red DBR-Array: Experimental Results (I) L-I-curves of four emitters of a DBR array (C-mount, separated contacts) ▪Threshold currents around 60 mA and 70 mW at 150 mA ▪Single mode optical spectra with an OSMR 60 dB 16 020 40 60 80 100 120 140 0 20 40 60 80 100 optical power (mW) current (mA) LD1 LD2 LD3 LD4 0 2 4 votage (V) 658 660 662 664 666 668 -70 -60 -50 -40 -30 -20 -10 0 10 norm. intensity (dB) wavelength (nm) at 130 mA LD1 LD2 LD3 LD4
Red DBR-Array: Experimental Results (II) L-I-curves of four emitters of a DBR array (C-mount; common contact) ▪After > 1500 hours constant power 160 mW (350 mA, 25°C) ▪> 4x 50 mW at 440 mA (OSMR > 60 dB) →Peak wavelengths over pump current (spacing close to 0.1 nm) 17 0100 200 300 400 0 20 40 60 80 100 optical power (mW) current (mA) after 1500h LD1 LD2 LD3 LD4 0 2 4 voltage (V) 280 300 320 340 360 380 400 420 440 460 480 662.6 662.7 662.8 662.9 663.0 663.1 663.2 0.11 nm 0.09 nm 0.11 nm peak wavelength (nm) current (mA) after 1500h LD1 LD2 LD3 LD4
Summary Surface grating technology is a stable method to fabricate DBR lasers with small band emission Design rules for the integrations of surface gratings High-order gratings with high reflectivity possible with high duty cycles Ebeam direct write is appropriate for the definition of the surface gratings Fast, flexible mask less way to address different laser wavelengths Successful fabrication of DBR laser arrays with 0.1 nm wavelength spacing around 660 nm Promising light sources for optical surface inspection systems 18
MNE2025, Photonics and Optoelectronics Paper ID54, Session 2C, 17 Sep 2025 Thank you! [email protected] Please visit the poster ID 423, P. Sammeta et al.