1. 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
2 Beyond Photonics
6205 Lookout Rd, Ste B
Boulder, CO

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

3. 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

4. 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

5. 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

6. 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

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

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

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

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

11. 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

12. 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

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

14. 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)

15. 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

16. 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

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

18. 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

19. Backup Slides 19

20. 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

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

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

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

24. 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