GaSb based high power single spatial mode and distributed feedback lasers at 2.0 μm

GaSb based high power single spatial mode and distributed feedback lasers at 2.0 μm Clifford Frez 1, Kale J. Franz 1, Alexander Ksendzov, 1 Jianfeng Chen 2, Leon Sterengas 2, Gregory L. Belenky 2, Siamak Forouhar 1 1,, Pasadena, CA 2 Dept. of Electrical Engineering, State University of New York, Stony Brook, NY 2011. All rights reserved. 1

Seed Lasers for Atmospheric CO 2 Measurement 1. > 50 mw continuous wave optical power 2. less than 100 khz spectral linewidth 3. long term frequency stability better than 1 MHz 4. long term reliability 2011. All rights reserved. 4

Laser Structure GaSb:Be Cap Layer 1.5 µm Al 0.85 Ga 0.15 As 0.07 Sb 0.93 p cladding Be doped 0.8 µm Al 0.30 Ga 0.7 As 0.02 Sb 0.98 Barrier Layers In 0.23 Ga 0.77 As 0.10 Sb 0.90 Quantum Wells 12.5 nm / 1.45% strain 1.5 µm Al 0.85 Ga 0.15 As 0.07 Sb 0.93 n cladding Te doped GaSb:Te Substrate 2011. All rights reserved. 5

Characteristic Temperature 30 Wavelength (nm) 2070 2065 2060 2055 2050 2045 2040 1.39 nm/ºc Threshold (ma) 55 50 45 40 4 m ridge 2 mm cavity HR/AR coated cw T 1 = 215.5 K T o = 107.3 K 20 10 Peak wall-plug efficiency (%) 15 20 25 30 35 Junction Temperature (ºC) 15 20 25 30 35 40 45 50 Temperature ( o C) 0 2011. All rights reserved. 11

Laser Fabrication Ridge Waveguide Fabrication Dry etch a narrow ridge (3-4µm) Thick dielectric film to reduce optical loss Thick gold electroplating Backside contact is annealed Grating Fabrication Grating is written by electron beam lithography Grating is dry etched into the semiconductor Thick blanket film of dielectric to reduce optical loss 2011. All rights reserved. 15

Lateral Grating Laser Ridge Top View Sidewall Cross section Distributed Feedback (DFB) lasers can provide mode hop free tunability A grating etched laterally to the ridge acts as an internal optical filter that allows for single wavelength feedback. The grating linewidth is directly proportional to the wavelength of light, which for a 2µm laser is 295nm and 590nm for a 1 st order and a 2 nd order grating, respectively. The grating is defined by electron beam lithography and is etched in an ICP RIE etcher. The electronmicrographs show that there is no discontinuity in the grating, i.e. the grating completely covers the top, sidewalls, and edge of the laser ridge. 2011. All rights reserved. 16

DFB Summary 3 & 4 µm Ridge Widths Fabricated both 1 st order and 2 nd order gratings Experimented with different grating pitches DFB coupling predicted to be small based on SEM AR coated to suppress facet reflectivity Maximum output power was factor of 6 lower for 2 nd order DFB (3.2 mw vs. 20 mw) 2011. All rights reserved. 17

Linewidth Measurement Self heterodyne measurement not an option Measure frequency jitter passing through a chromatic absorber transmission voltage time wavelength wavelength 2011. All rights reserved. 22

Linewidth Measurement Self heterodyne measurement not an option Measure frequency jitter passing through a chromatic absorber CO 2 gas cell Fabry Perot etalon/interfereometer 2011. All rights reserved. 23

Data collection strategy XY Graph 5.5 Plot 0 1. Scan interferometer 2. Aim here (1/2 max) 3. Collect data for 10 ms 4. Plot histogram Signal (V) 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0-0.5 3.80562 3.9 4 4.1 4.2 4.28077 Time Interferometer driving voltage (piezo voltage) This is the measured lineshape Next page: how to interpret this? Signal (V) 2011. All rights reserved. 24

Result XY Graph 5.5 Plot 0 Detector output (V) 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 Histogram 2 4-0.5 3.80562 3.9 4 4.1 4.2 Time Piezo voltage (V) 1.5 GHz < > 13 V Full width: 1.67*10 2 V > 1.92 MHz Half width: 7.8*10 3 V > 0.8 MHz 4.28077 2011. All rights reserved. 25

Linewidth Measurement Measured 1 2 MHz linewidth (10 ms) on existing laser Linewidth limited by the temperature stability of the laser (~1.4 MHz for 10 ms data collection interval) Working on improving temperature stability and improving data collection strategy 2011. All rights reserved. 26

Summary 1. > 50 mw continuous wave optical power 2. less than 100 khz spectral linewidth 3. long term frequency stability better than 1 MHz 4. long term reliability 2011. All rights reserved. Acknowledgements The authors acknowledge Dr. Robert Menzies and Gary Spiers for their encouragement. The research described in this letter was sponsored by the Research and Technology Development Program of the,, under a contract with the National Aeronautics and Space Administration. 27