High-Power Highly Linear Photodiodes for High Dynamic Range LADARs Shubhashish Datta and Abhay Joshi th June, 6 Discovery Semiconductors, Inc. 9 Silvia Street, Ewing, NJ - 868, USA www.discoverysemi.com
Outline Amplitude and Phase Nonlinearity in Photodiodes Highly Linear Photodiodes in LADARs Device Description Performance with Pulsed Stimulus Performance with Continuous Wave Stimulus Conclusion Confidential and Proprietary 6 Discovery Semiconductors, Inc. All Rights Reserved.
Amplitude & Phase Nonlinearity in Photodiodes Optical Input Signal P P RF Output Signal after Linear Photodetection V V V V t P P t τ τ RF Output Signal after Non-Linear Photodetection P > P t V V t V P τ V P τ Non-linear photodetection introduces optical power dependence on RF output signal shape 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 Confidential and Proprietary 6 Discovery Semiconductors, Inc. All Rights Reserved.
Motivation: HLPD for LADAR Signal Waveform Generator Local Oscillator Laser Modulator EDFA Circulator Telescope HLPD Signal ADC + DSP Highly Linear Photodiode (HLPD ) Reduce non-linear IMD Higher SFDR Linear behavior needed for pulsed, CW, and arbitrary waveforms High Optical Input Power High optical LO power High link gain, Low Noise Figure High RF Output Amplitude No RF postamplifier needed to maximize ENOB Sensor Type OTDR OFDR Code Correlation Waveform Type Pulse Swept Frequency (CW) PRBS Confidential and Proprietary 6 Discovery Semiconductors, Inc. All Rights Reserved.
Motivation: HLPD for LADAR Clock Waveform Generator Local Oscillator Laser Modulator EDFA Circulator Telescope HLPD Signal ADC + DSP Optical Clock HLPD Clock Optical clock distribution provides the best timing precision: optical frequency instability of ~ -8 ENOB log f S t Highly Linear Photodiode (HLPD ) Reduce AM-to-PM noise Lower Jitter Higher ENOB f S : Sampling Frequency t: Jitter Confidential and Proprietary 6 Discovery Semiconductors, Inc. All Rights Reserved. 5
Device Description: DDR Photodiode Normalized Optical Intensity Profile Anti-Reflection Coating p +.8 contact.6.9.8 p + InP i In.5 Ga.7 As Cap Layer Absorption Layer h e.. -. -..7.6.5.. i InP n + InP Drift Layer () Substrate -.6 -.8 - - -.5.5 n + contact Peak-to-Average Intensity Ratio =.5.. Transit times of electrons and holes are balanced in Dual-Depletion Region photodiodes for high-speed operation in a top-illuminated geometry Uniform radially-symmetric optical illumination profile improves device linearity by Reducing the peak space charge concentration and the resulting screening effect Distributing heat uniformly across photodiode cross-section and preventing thermal runaway Confidential and Proprietary 6 Discovery Semiconductors, Inc. All Rights Reserved. 6
Normalized RF Response (db) Device Description: TE Cooled PD Module RF Output -TE +TE +Vbd TEC Temp Sensor +Ts Out -Ts Rtn +Ts - Case GND Case GND -6-9 6 8 6 Frequency (GHz) Un-terminated PD to maximize RF power at external 5 W load Integrated Thermo-Electric Cooler to increase photodiode reliability at high power DC Responsivity =.65 A/W @ 55 nm -db Bandwidth = GHz Confidential and Proprietary 6 Discovery Semiconductors, Inc. All Rights Reserved. 7
Impulse Response Setup RF Synthesizer Trigger 55nm Mode Locked Laser EDFA Voltage Source for PD Bias GND +Vbd VOA Photodiode Module db RF Sampling Attenuator Oscilloscope +TE -TE +Ts Out -Ts Rtn +Ts Current Source for Thermo- Electric Cooler Drive Temperature Feedback Loop Control Voltmeter for Temperature Sensor +5V Voltage Source for Temperature Sensor Drive 55 nm Mode Locked Laser emits.5 ps wide optical solitons with tunable repetition rate Optical attenuator adjusted to vary optical pulse energy PD temperature maintained at 5 C RF output pulse recorded at various optical pulse energy levels, PD biases, and repetition rates Confidential and Proprietary 6 Discovery Semiconductors, Inc. All Rights Reserved. 8
Diode Output (V) Peak-to-Peak Amplitude (V) Impulse Response and Amplitude Linearity 6 PD Bias = 5V Rep Rate = GHz.8 pj mw.7 pj mw 5.5 pj mw 7. pj mw 9. pj mw pj mw pj mw 5 pj mw 6 8 Time (ps) 7 6 5 9V V 5V Compressed Regime Broadband Linear Regime 5 5 Average Optical Optical Pulse Power Energy (mw) (pj) Linearity improves with increasing photodiode bias due to reduced space-charge compensation Broadband linear operation up to Vpp at 5 V reverse bias Delivers up to 6 Vpp at 5 V reverse bias in compressed regime Confidential and Proprietary 6 Discovery Semiconductors, Inc. All Rights Reserved. 9
FWHM (ps) PPC (rad/w) FWHM and Power-to-Phase Conversion 8 7 6 5 9V V 5V 5 5 Average Optical Pulse Optical Energy Power (pj) (mw) 8 6 9V V 5V 5 5 Average Optical Optical Pulse Energy Power (mw) (pj) Pulse broadening reduced with increasing photodiode bias Power-to-Phase Conversion < rad/w at 5 V reverse bias up to 6 pj of optical pulse energy ( Vpp RF output) Broadband Linear Regime is ideal for LADAR Signal path Confidential and Proprietary 6 Discovery Semiconductors, Inc. All Rights Reserved.
Peak-to-Peak Amplitude (V) Power-to-Delay Conversion (ps) 7 6 5 Null Points in Compressed Regime 9V V 5V Compressed Regime Broadband Linear Regime 5 5 Average Optical Optical Pulse Power Energy (mw) (pj) GHz - GHz 8GHz Null Points - GHz GHz - 5 5 Average Optical Optical Pulse Energy Power (pj) (mw) Compressed regime contains null points where phase nonlinearity vanishes for certain specific sets of frequency, optical power, and photodiode bias. Compressed regime is ideal for minimizing AM-to-PM noise for LADAR Clocks Confidential and Proprietary 6 Discovery Semiconductors, Inc. All Rights Reserved.
Peak-to-Peak Amplitude (Vpp) Diode Output (V) Diode Output (V) Diode Output (V) Diode Output (V) Diode Output (V) Effect of Repetition Rate - GHz Rep Rate.5 GHz Rep Rate 5 GHz Rep Rate GHz Rep Rate - - - - 6 8 6 8 6 8 6 8 Time (ps) Time (ps) 5 Time (ps) Time (ps) Pulse shape and peak-to-peak amplitude is essentially independent of repetition rate within the flat part of the photodiode s RF response PD Bias = 9V Higher repetition rate lower average optical power & PD bias for targeted LADAR Clock power Better reliability GHz.5 GHz 5 GHz GHz 6 8 Optical Pulse Energy (pj) Confidential and Proprietary 6 Discovery Semiconductors, Inc. All Rights Reserved.
Continuous Wave Response Setup RF Synthesizer Voltage Source for PD Bias 55nm CW Laser MZM EDFA GND +Vbd VOA Photodiode Module db RF Power Attenuator Meter +TE -TE +Ts Out -Ts Rtn +Ts Current Source for Thermo- Electric Cooler Drive Temperature Feedback Loop Control Voltmeter for Temperature Sensor +5V Voltage Source for Temperature Sensor Drive CW signal with ~% modulation depth at 5 GHz modulation frequency Photodiode module biased using Bias-Tee with 7 W series resistance PD temperature maintained at 5 C RF output power recorded at different optical power levels Confidential and Proprietary 6 Discovery Semiconductors, Inc. All Rights Reserved.
RF Output Power (dbm) Continuous Wave Response Setup 6 8 - -8 - -6 - PD Bias = V Modulation Depth ~% Modulation Frequency = 5GHz 6 8 6 8 Average Optical Input Power (dbm) RF output of +6 dbm ( Vpp) achievable at V PD bias. Sufficient for maximizing ENOBs of digitizers without any post RF amplification Confidential and Proprietary 6 Discovery Semiconductors, Inc. All Rights Reserved.
Conclusion High Power, Highly Linear Photodiodes enable high coherent gain, low noise figure, and low inter-modulation distortions in LADAR signal High Power, Highly Linear Photodiodes photonically generate low phase noise (low jitter) clock signal for LADAR Have developed thermo-electrically cooled GHz photodiode module for reliable highpower operation In Broadband Linear Regime, photodiode generates up to Vpp output (+6 dbm RF power) with power-to-phase conversion factor < rad/w In Compressed Regime, null points allow generation of photonic clocks with very low phase noise Confidential and Proprietary 6 Discovery Semiconductors, Inc. All Rights Reserved. 5