1. High Power, Uncooled InGaAs Photodiodes with High Quantum Efficiency for 1.2 to 2.2 Micron Wavelength Coherent Lidars
Shubhashish Datta and Abhay Joshi
20th June, 2018
Discovery Semiconductors, Inc.
119 Silvia Street, Ewing, NJ , USA 1

2. Outline Motivation 2 Micron InGaAs Photodiode Structure
Spectral Response
Dark Current and Diode Ideality Factor
RF Response of High Power 2 Micron Photodiode
1dB Compression
Impulse Response and Linearity
Ongoing Space Qualification of 2 Micron Photodetectors
Conclusion
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3. Motivation
1.2 to 2.2 micron wavelength covers multiple lidar designs with one photodetector
High Speed Photodiode → consolidate multiple free-space coupled low speed photodetector elements, accommodate large Doppler shifts, dual-comb spectroscopy …
High Power Photodiode → High Coherent Gain → Shot Noise Limited Detection
Multimode fiber-pigtailed module → High Optical Coupling Efficiency in free-space systems
Uncooled Photodiode Operation → Further reduction in SWaP
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4. Why In0.72Ga0.28As PDs?
Standard In0.53Ga0.47As has a bandgap of 0.74 eV corresponding to 1700 nm cut-off wavelength
Increasing Indium content to 72% extends cut-off wavelength to 2.2 mm
Lower band gap → higher absorption coefficient / quantum efficiency at 1.55 mm → Higher SNR
In0.72Ga0.28As has ~20% higher hole mobility and ~200% higher electron mobility compared to lattice-matched In0.53Ga0.47As
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5. 2 Micron InGaAs Photodiode Structure
In0.72Ga0.28As has ~1.2% lattice mismatch with InP substrate
Graded InAsP buffer layer needed to minimize dislocation defect density in absorber layer [1 - 3]
[1] A. Joshi, et al., Proc. SPIE, vol. 1715, pp. 585 – 593, 1992
[2] A. Joshi, IEEE Photon. Conf., invited paper MS1, San Francisco, CA, 2012
[3] A. Joshi, et al., Proc. SPIE, vol. 8704, paper 87042G1-8, 2013
Confidential and Proprietary © 2018 Discovery Semiconductors, Inc. All Rights Reserved.

6. Spectral Response at Room Temperature
Lattice Matched
In0.53Ga0.47As
Lattice Mismatched
In0.72Ga0.28As
In0.82Ga0.18As
[1] A. Joshi, et al., Proc. of SPIE, vol. 1715, pp. 585 – 593, 1992
Confidential and Proprietary © 2018 Discovery Semiconductors, Inc. All Rights Reserved.

7. Dark Current and Photodiode Ideality Factor
Dark current is mainly comprised of diffusion and generation-recombination current
Have achieved ideality factor close to unity (i.e. diffusion-dominated)
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8. RF Response of High Power 2 Micron PD
150 mm Diamater Terminated Photodiode
105 mm Core Diamater MMF
1m long multi-mode fiber pigtail: <0.2dB Attenuation and NA = 0.22
DC Responsivity = 1.18 A/W at 2.05 mm (73% QE)
DC Responsivity = 1.0 A/W at 1.55 mm (80% QE)
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9. 1 dB Compression of 2 Micron InGaAs PD
CW Modulation Frequency = 1 GHz, Modulation Depth ~100%
1 dB 7V bias: DC Photocurrent = 65 mA, RF output power = +16 dBm
Enables coherent sensors with local oscillator up to +18 dBm and uncooled photodiode operation
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10. Amplitude & Phase Nonlinearity in Photodiodes
RF Output Signal after Linear Photodetection t1 t2 V1 V2
Optical Input Signal P1 P2
RF Output Signal after Non-Linear Photodetection t2 t1 V1 V2
P1 > P2
Higher Optical Input Power → Lower RF Output Amplitude + Larger RF Pulse Width
Amplitude Nonlinearity leads to compression and inter-modulation distortions
Phase Nonlinearity (power to phase conversion) transfers Optical RIN to RF phase noise and causes non-linear crosstalk in time-domain multiplexed systems
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11. Impulse Response of 2 Micron InGaAs PD
PD Bias = 9 V
1550 nm wavelength Mode Locked Laser emitted 2.5 ps wide solitons with 1GHz repetition rate
Linear operation up to 3.1 Vpp RF output up to 18 pJ optical pulse energy and uncooled photodiode operation
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12. FWHM and Power-to-Phase Conversion
Pulse broadening reduced with increased photodiode bias
FWHM < 100 ps
Power-to-Phase Conversion < 10 rad/W up to 18 pJ optical pulse energy
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13. 2 Micron PIN-TIA Photoreceivers for MISSE 9
Two 10GHz Photoreceivers: 3-pin K package and Miniature Surface Mount Package
Integrated Dosimeter for in-situ TID measurements
Launched on April 2, 2018
Currently orbiting on the wake side of ISS
[4] A. Joshi and S. Datta, Proc. SPIE, vol , paper , 2018
Confidential and Proprietary © 2018 Discovery Semiconductors, Inc. All Rights Reserved.

14. Conclusion
Uncooled, fiber-pigtailed, lattice-mismatched In0.72Ga0.28As photodiode demonstrated :
Spectral Coverage: 1.2 mm to 2.2 mm
DC Responsivity: nm, and nm
-3 dB Bandwidth: 1.7 GHz
1dB Compression: up to 65 mA DC photocurrent
RF Output Power: up to +14 dBm CW
Pulsed RF Output: up to 3.1 Vpp linear
FWHM: <100 ps
Phase Nonlinearity: <10 rad/W
Enables shot noise limited coherent lidar with optical LO up to +18 dBm
105 mm core (0.22 NA) multimode fiber pigtail to maximize free-space optical coupling
Consolidate multiple channels into one high-speed photodiode
Further reduction in SWaP through uncooled photodiode operation
Confidential and Proprietary © 2018 Discovery Semiconductors, Inc. All Rights Reserved.