Photonic Integrated Circuit FMCW Lidar On A Chip   Paul J. M. Suni, James Colosimo, Lockheed Martin Coherent Technologies John Bowers, Larry Coldren, Jonathan Klamkin, University of California Santa Barbara S.J. Ben Yoo, University of California Davis This research was developed with funding from the Defense Advanced Research Projects Agency (DARPA). The views, opinions and/or findings expressed are those of the authors and should not be interpreted as representing the official views or policies of the Department of Defense or the U.S. Government. Distribution Statement "A" (Approved for Public Release, Distribution Unlimited) © Lockheed Martin Corporation. All rights reserved.

Outline Photonic Integrated Circuits (PICs) introduction DARPA Modular Optical Aperture Building Blocks (MOABB) Program 5 year MTO program Dr. Gordon Keeler PM Lockheed Martin (LM) MOABB Team LM Coherent Technologies LM Advanced Technology Laboratory UC Santa Barbara UC Davis Program goal is developing PIC based power/aperture scalable devices and architectures for lidar, communications, illuminators, designators etc. applications Non-mechanical beam steering is critical part

Electronics vs. Photonics History Lockheed Martin Proprietary Information Lockheed Martin Proprietary Information Electronics vs. Photonics History Electronics Photonics 1 2 3 4 5 Sources: 1. https://www.quora.com/What-specific-technological-advances-have-been-made-to-make-computers-so-much-smaller 2. http://slideplayer.com/slide/7516624/ 3. https://www.nature.com/articles/s41928-017-0014-8 4. http://www.activewin.com/reviews/hardware/processors/intel/p4253ghz/cpuarch.shtml 5. https://www.tomsguide.com/us/apple-iphone-6,review-2390.html Photonics lags electronics by ~40 years

Key Enabler: Silicon Photonics Compatible With CMOS Fabrication “Strip Waveguide” Huge Δn (3.5-1.5 = 2)  Extremely tight mode confinement  High packing density w/ low cross-talk Very tight bend radii 90○ Bend With 0.5 µm Radius

Implementing Coherent Lidar in PICs? Except for high peak power source coherent systems can be built with PICs Peak power limited to ~100 mW in single Si waveguides, ~1 W in silicon nitride (SiN) Benefits of chip-scale lidar Large SWaP reductions Large cost reductions in volume Future 3D integration with signal processor/electronics

Build single point scanned system FMCW Lidar Frequency-modulated continuous wave (FMCW) uses low power Coherent detection  single photon sensitivity Range resolution = c/2Bopt 2 cm resolution  Bopt = 8 GHz Mix down Bopt to RF domain Reduces electronics bandwidth BRF Typically need BRF < 1 GHz Velocity for free from up-down chirp Target Echo Local Oscillator Transmitted Waveform 𝒇 𝒖𝒑 = 𝑩(𝝉− 𝒕 𝒐 ) 𝒕 𝒓 + 𝒇 𝑫 Time delay 𝝉 between LO and return signal produces intermediate frequency proportional to range R = c*t0/2. Velocity adds fixed frequency Doppler offset fD 𝒄𝒐𝒔 𝟐𝝅 𝑩(𝝉− 𝒕 𝒐 ) 𝒕 𝒓 + 𝒇 𝑫 𝒕 During up-chirp detector signal is proportional to where B = ramp bandwidth, tr = ramp duration During down-chirp detector signal is similar with sign switch 𝒄𝒐𝒔 𝟐𝝅 − 𝑩(𝝉− 𝒕 𝒐 ) 𝒕 𝒓 + 𝒇 𝑫 𝒕 Sum and difference of two measured frequencies extracts range and velocity MOABB goal Build single point scanned system Source: B. Krause et al., Applied Optics, 51, 8745 (2012)

Modular Optical Aperture Building Blocks (MOABB) 1 mm2 aperture, 10 m range 1 cm2 aperture, 100 m range 100 cm2 aperture

UC Santa Barbara Beam Steering Key program goal: Incorporate non-mechanical beam steering 2D steering demonstrated under DARPA SWEEPER program Tunable laser + grating steers in one dimension Transverse phase gradient steers in other dimension (optical phased array - OPA) ~24 x 12 Degree Steering Demonstration (using 16 channel test chip) Phased Array UCSB Functional Layout Source:https://en.wikipedia.org/wiki/Phased_array Source: J.C. Hulme et al., SPIE Photonics West [8989-6] (2014)

Phase 1 PIC Architecture Development Needs >50 nm tunable laser 3-8 GHz chirp over 5-20 us Detectors Efficient 1:N splitters Low power phase shifters Weak gratings with tailored emission Integration of transceiver front end to silicon on insulator (SOI) InP is flip-chip bonded to SOI Chip Assembly – Side View Emission InP Xcvr 1:N, P-shifters Grating Functional Architecture – Top View 1 mm Chirp control 1:N Splitter Detectors T/R Switch Phase shifters Grating SOA SG-DBR Laser 7 mm 1 mm 4 mm 10 mm

Component Results (UC Santa Barbara/LM) Transceiver Front End 1:480 Channel Splitter Laser Locker Interposer (dummy) w/ SOI + electronics PIC Si interposer RX Front End Weak “Fishbone” Gratings OPA PIC 25 mm

Non-Mechanical Beam Steering Demo Uses 120 channel PIC from UC Davis with thermal phase shifters External tunable laser from Freedom Photonics PIC Grating Emitter Beam Steering Demo Control Chassis Control Electronics Fiber Input

Aperture Scaling

Common transceiver front end Predicted performance Scaling Concepts Common transceiver front end 100 nm tuning range ~100 mW power 1 cm2 aperture concept ~8,000 WG / 1.3 um pitch SOAs to boost power Predicted performance 100 cm2 concept

3D Integration for Large Apertures/High Power High emission aperture fill factor requires 3D integration of photonics and electronics Low loss 3D interlayer light transport Integration of flexible 3D routing waveguides Embedding of electrical functions to interposers Low-loss pathlength-matched optical interposer with low loss coupling to tiles Uniform intensity grating emission across multi-tiles 3D “tile” structure 100 cm2 layout Input Waveguide Laser Input 1 3 2

3D Integration Elements (UC Davis) Single tile to interposer coupling with < 1.6 dB loss per interface Interlayer coupling 3D OPA with U-shaped coupler Ultrafast Laser Inscription (ULI) Arbitrary shape 3D waveguides 0.8 dB/cm loss 2mm pitch array Misalignment, imperfect etch depth ~4 dB IL ~0.5 um T=0.74, Total loss 1.3 dB Coupling to SiN

Key Challenges PICs are currently not low enough loss for complex systems – hit twice in lidar Sidelobes undesirable for lidar, secure communications, designators, and other applications No sidelobes forces trade between max steering angle and emitter pitch – left Reducing the pitch increases cross-talk and limits the length of emission gratings – right Thickness for 500 nm wide Si core Measured Phase 3 Phase 2 Phase 1 < 10% coupling in 10 mm Reduced losses and sub-λ WG pitch critical to practical lidar systems

Sub-wavelength pitch OPA (1.3 mm) Crosstalk measurements Axial steering 0.16o/nm 5 mm long coupling region Through 12 dB Drop Fabricated structures (varying pitch) Lateral steering demo Lateral steering Set 1 Set 2 Set 3 -33o +33o

Novel Component Development Uniform emission grating Measured Far-field emission β matched apodized grating 0.1o ~Diffraction limit 0.15o Uniform power splitter Star coupler with uniform power output Measured power by channel Loss <2 dB Variationall=0.46dB

Future Prospects 2-way atmospheric transmission 1.5-1.6 um Coherent lidar on a chip is feasible and highly challenging Reduce losses to make practical Reduce WG crosstalk to increase OPA steering range Need killer app to reduce cost - automotive Eliminate wavelength tuning Atmosphere not clear for large tuning ranges 2D OPAs? Current approaches require N2 controls – extremely challenging to scale… Concepts in development promise ~N controls without wavelength tuning Wavelength extensions MWIR, LWIR PIC technology far less mature Small WG pitch easier at long wavelengths Example 2D OPA (large pitch)* * Source:Jie Sun, et al., “Large-scale nanophotonic phased array”, Nature 493, pp.195 (2013)

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