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 - 08628, USA www.discoverysemi.com 1

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 Confidential and Proprietary © 2018 Discovery Semiconductors, Inc. All Rights Reserved.

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 Confidential and Proprietary © 2018 Discovery Semiconductors, Inc. All Rights Reserved.

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 Confidential and Proprietary © 2018 Discovery Semiconductors, Inc. All Rights Reserved.

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.

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.

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) Confidential and Proprietary © 2018 Discovery Semiconductors, Inc. All Rights Reserved.

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) Confidential and Proprietary © 2018 Discovery Semiconductors, Inc. All Rights Reserved.

1 dB Compression of 2 Micron InGaAs PD CW Modulation Frequency = 1 GHz, Modulation Depth ~100% 1 dB Compression @ 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 Confidential and Proprietary © 2018 Discovery Semiconductors, Inc. All Rights Reserved.

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 Confidential and Proprietary © 2018 Discovery Semiconductors, Inc. All Rights Reserved.

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 Confidential and Proprietary © 2018 Discovery Semiconductors, Inc. All Rights Reserved.

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 Confidential and Proprietary © 2018 Discovery Semiconductors, Inc. All Rights Reserved.

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. 10641, paper 10641-19, 2018 Confidential and Proprietary © 2018 Discovery Semiconductors, Inc. All Rights Reserved.

Conclusion Uncooled, fiber-pigtailed, lattice-mismatched In0.72Ga0.28As photodiode demonstrated : Spectral Coverage: 1.2 mm to 2.2 mm DC Responsivity: 1.18 A/W @ 2050nm, and 1.0 A/W @ 1550 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.