19th Coherent Laser Radar Conference Non-mechanical Step and Stare Telescope for Coherent Laser Radar Applications CLRC 2018 19th Coherent Laser Radar Conference S. Serati1, C. Hoy1, D. McKnight1, L. Hosting1, J. Stockley1, K. Kluttz1 and C. Hale2 1 Boulder Nonlinear Systems 450 Courtney Way Lafayette, CO 80026 2 Beyond Photonics 6205 Lookout Rd, Ste B Boulder, CO 80301

Outline Motivation Overview of the Technology Large Aperture Development Coherent Laser Radar Demo Conclusion

Motivation for Non-mechanical Step and Stare Scanning Approach Allows multiple shots per look angle: Increases SNR Reduces pulse energy requirements on the transmitter Lower-power, smaller lasers Removes the need to lag-angle compensate due to continuously sweeping the beam Eliminates all moving parts: No need to counterbalance inertial forces Fast stepping with no slewing, overshoot, ringing, etc. Low size, weight and power (SWaP) Simplifies the beam delivery optics One telescope, one beam path

What’s Needed For Most Coherent Laser Radar Step and Stare Applications Relatively Large Aperture Higher directivity, More optical gain, Better power handling 1.5 meters or more is wanted for some applications Wide-Angle Beam Steering Hemispherical coverage is always wanted, but 45 per axis or less is generally adequate for most applications. Good Wavefront Quality < /10 rms wavefront error Minor Loss < 3 dB round trip is generally desired. Nonmechanical steering technique needs large angle-aperture product. 4

of LC Molecular Alignment Polarization Grating Overview Spatial Distribution of LC Molecular Alignment ~2 m RHCP Steering Right RHCP Geometric Phase LHCP OPD OPA Steering Left LHCP Geometric Phase OPD OPA

LCPG Steering Basics LCPG – Solid films fabricated using spin coating and UV holography LC Switches – LC halfwave or full wave retarders fabricated using standard LC gap and fill techniques Index-Matched ITO – Thin film coating with transparent conductive oxide and AR layers Glass Substrates – 0.2 mm to 2.0 mm fused silica or sheet glass Index-Matching Epoxy – Epoxy with nearly the same index of refraction as LC and glass AR-Coated End Caps – Glass substrates that mostly determine wavefront quality if index matching and LCPG holograms are perfect LCPG Steering Stage (LC Switch & LCPG) Steer Angles = 2N Where N is the number of Steering Stages Glass Substrate LCPG Grating Glass Substrate Orange Layers Index-Matching Epoxy Liquid Crystal Modulator Losses = Scatter, Fresnel, ITO Absorption, Diffraction Efficiency Loss per stage is ~2-3% for near normal incidence Yellow Layers Index-Matched ITO AR Coated End Caps 6

Example Step and Stare Telescope Configuration with LCPG Lens Monostatic Configuration Needs /4 retarder and circulator using Faraday rotator 7

Large Aperture, Wide FOR Demo - >10 cm clear aperture with 64°×64° FOR- 8

Large LCPG Diffraction Efficiency Measurement 15 cm Diameter LCPGs High diffraction efficiency (>99.5% per element)

Large LCPG and LC Switch Wavefront Characterization Wavefront errors are between /50 and /25 rms for diffractive beam from LCPGs

Measured Steering Efficiency Steering efficiency falls from 0.85 to 0.5 from center to edge Wavefront error is ~/10 rms for steered beams 11

Stack Throughput Improvement Large stack had poor throughput due to scatter from an over abundance of spacers per switch Small stack with lower scatter switches has ~91% throughput or 2.25% loss per steering stage and /4 retarder Steering efficiency was >90% across FOR 12

Coherent Lidar for testing LCPGs - Monostatic FMCW 1550 nm system with balanced mixer -

2-axis stack for lidar testing - 2 LCPGs and 3 LC switches steering in Az and El - 5 cm diameter stack 3 mm thick substrates 8 substrates + end caps Quarter wave is LC cell 0.85 dB insertion loss (measured)

Wavefront error for 2-Axis stack - Wavefront error of steered beam at different scan positions - Best Worst (+1,-1) position - /85 rms at 1550 nm (+1,+1) position - /61 rms at 1550 nm

Doppler Return through Two-axis Stack CNR Measurements - Representative Carrier-to-Noise Ratio measurements - Doppler Return through Two-axis Stack Beam Steered on Target Beam Steered off Target

Doppler lidar results - Two-stage LCPG Beam Steering Unit -

Conclusion LCPG technology enables non-mechanical, large aperture, wide angle, step and stare operation for coherent laser radar applications. Throughput is the primary problem when LCPG stacks are used for high resolution scanning. Further work is needed to increase aperture size, develop faster switches, improve steering efficiency, and reduce SWaP by using thinner LCPG and LC switch components and by replacing refractive lenses with LCPG lenses. We are grateful to AFRL and NASA for funding this work. 18

Backup Slides 19

Suppressing Ripple in LC Switches Noise suppression - LC switches produce low-frequency ripple - Suppressing Ripple in LC Switches LC Switch Return signal ripple caused by LC switch drive signal is fully suppressed when switch is driven at higher frequency No ripple artifact was observed in coherently detected return signal LCPG PBS Laser /4 Mirror Detector

Monostatic Operation - /4 waveplate is needed for return signal to retrace path -  /4 LC Switch /4 3/4

Optical quality of LCPG - LCPG wavefront and efficiency at 1550 nm - 5 cm Diameter LCPGs Single LCPG RMS wavefront error of 0.032 at 633 nm equals 0.013 (/76.5) at 1550 nm High diffraction efficiency - >99.9% Insertion Loss (one way) - 2.3% (No AR or index matching)

Coherent lidar results - Single LCPG and LC switch - Central 90% of diameter. Not including drive electronics. (3) Represents measurement uncertainty. Impact not detectable.

Grating stacks provide wide-angle beam control Beam Control with LCPGs - Co-developed with NCSU; Successful tech transfer - Grating stacks provide wide-angle beam control LCPG Beam Steering Angle deflection from each grating adds or subtracts minimizing the number of gratings needed to cover a large field of regard Thin structure minimizes beam walk-off, size, power, and weight 24