RealTime Holographic Compensation of Static and Dynamic Optical Disturbances

1
Real-Time Holographic Compensation of Static and
Dynamic Optical Disturbances

• Dr. Dennis Guthals and Dr. Rita Carbon
• Boeing Directed Energy Systems
• 8531 Fallbrook Ave., MC WB-54
• P.O. Box 7922
• West Hills, CA 91304
• Telephone [phone]
• Fax [phone]
• dennis.m.guthals_at_boeing.com

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Outline

• Introduction to Real Time Holography (Diffractive
  Wavefront Control)
• Overview of Boeing RTH Activities 1987 - Present
• Introduction to diffractive turbulence
  compensation
• Turbulence phase screens
• Atmospheric
• Aero-Optics
• Modeling diffractive adaptive optical systems
• Comparison of conventional and diffractive
  systems
• Adaptive Optics Testbed
• Develop and test concepts
• Anchor models
• Characterize and test components
• OSSim components for modeling diffractive
  compensation systems
• Self (local) referenced interferometer
• Spatial light modulators (OASLM, FPA/EASLM)
• Segmented mirror Basic MEMs model
• Diffractive compensation tests and simulations
• Demonstration of compensation for 50 waves of
  tilt
• Experimental and simulation for compensation of
  atmospheric turbulence

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What is Real-Time Holography?

• Correct an aberrated image in real-time
• -- Before aberration changes
• -- Two-step process
• - Record hologram with a point source
• - Read hologram with distorted image
• -- Subtracts phase of distortion

Distorted Image
Object
Primary Mirror (Misaligned Segments or Imperfect
Shape)
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What does Real-Time Holography add to anOptical
System?

• Beacon Leg, Interferometer, Holographic
• Recording Media
• -- Beacon samples phase distortion
• -- Interference fringes form hologram of
  aberration
• -- Hologram recorded by OASLM or FPA/EASLM
• -- Hologram transferred to LC layer as phase
  grating

Beacon or Target Return
Distorted Image
Hologram of Misfigured Primary Recorded on OASLM
by Distorted Beacon Beam
Reference
Beacon Leg
Object
OASLM (15mm)
Primary Mirror (Misaligned Segments or Imperfect
Shape)
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Where does Correction Occur?

• Beacon Leg, Interferometer, Holographic
• Recording Media
• -- Beacon samples phase distortion
• -- Interference fringes form hologram of
  aberration
• -- Hologram recorded by OASLM or FPA/EASLM
• -- Hologram transferred to LC layer as phase
  grating
• -- Distorted image diffracted by phase grating

Distorted Image
Beacon or Target Return
Hologram of Misfigured Primary Recorded on OASLM
by Distorted Beacon Beam
Reference
Beacon Leg
Hologram Read by Distorted Object Return
Object
OASLM (15mm)
Primary Mirror (Misaligned Segments or Imperfect
Shape)
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Where does Correction Occur?

• Diffraction performs phase subtraction
• -- Conjugate order contains corrected
• image

Distorted Image
Corrected Image
Beacon or Target Return
Hologram of Misfigured Primary Recorded on OASLM
by Distorted Beacon Beam
Hologram Read by Distorted Object Return Phase of
Misfigured Primary is Subtracted Corrected Image
is Obtained
Reference
Beacon Leg
Object
OASLM (15mm)
Primary Mirror (Misaligned Segments or Imperfect
Shape)
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Diffractive Wavefront Control at Boeing LEOS
SLM
Blazed Gratings
0.75 Meter Primary Mirror
Shoebox
TBL Fringes
Zernike 31
OPA Gain
TBL Phase Screens
High Optical Efficiency
Uncorrected
Uncorrected
Pump Off
Uncorrected
Fringes with Tilt
Low Signal Level
1 Mile Propagation
12l Mirror Warp
FPA Fringe Record
Fringe Transfer
Uncorrected
Corrected
Corrected
TBL Corrected
Pump On
Corrected
EASLM Fringe Transfer
High Efficiency
Dl Correction Trade
User Friendly GUI
Agile Beam Steering
Phase Preserved
SOR Field Tests
NLOS Large Optics Compensation Visible
OASLM Binary 1Khz
Aero-Optical Compensation Near IR
FPA/EASLM Binary 60Hz Blazed 10Hz RTH Comp. SRI
Local Reference Wave Optical Parametric Amplifier
Compact Adaptive Optics System Visible
FPA/EASLM Binary 30Hz
All Optical Imaging Brassboard Visible
OASLM Binary 5KHz
RTH-Based Zernike Generator on SOR 3.5m Visible
FPA/EASLM Binary 100Hz
Diffractive Beam Pointing Visible
FPA/EASLM Blazed 30Hz
1995
1987
1997
2005
1990
2003
1999
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Advanced Diffractive Turbulence Compensation

• System performance analysis requires turbulence
  phase screens of proper statistics
• Atmospheric - Exists
• Aero-Optical - Developing
• Aero-Optics compensation demands
• High spatial frequency bandwidths (gtgt50x50)
• High temporal frequency correction bandwidths
  (gt1kHz)
• Large OPD stroke (several waves)
• Ideal solution is segmented Mod 2p diffractive
  system
• Eliminates phase wrapping and branch points
• Record hologram on high-speed, high resolution
  FPA (gt1kHz, gt100x100 pixels)
• Process and transfer fringes to EASLM (gt1kHz,
  1-frame latency, use COTS PC-based interface)
• No requirement for wavefront reconstruction,
  open-loop
• Power, weight, packaging efficient
• Natural match of FPA pixel densities to LC/MEMs
  SLMs

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Turbulence Compensation Methods and Devices
Modeled with OSSim

• Two methods Conventional surface (OPD) and
  Phase correction
• Two types of phase correction Mod 2p Reflective
  and Diffractive
• Two diffractive methods Unblazed and Blazed
  phase gratings
• Turbulence screens
• Atmospheric
• Boundary layer
• Self (or local) referenced interferometer
• Three classes of devices DMs, SLMs, and MEMs
• Two types of DMs Continuous face-sheet and
  segmented
• Two types of LCSLMs OASLM and EASLM
• Two types of MEMs - Continuous face-sheet and
  segmented
• Static and dynamic optical distortions
• Optical elements and structures
• Optical path turbulence
• Results used to guide system performance modeling

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OSSIM Model Developed to Support Aero Optics
Compensation Evaluation
Kolmogorov Atmospheric Turbulence
Lorentzian Partially Developed Turbulence
Von Karman Fully-Developed Turbulence
Movie
Movie
Movie

• Anchored (wind tunnel PSDs) phase screens with
  proper statistics developed for OSSim
• Rear looking angles may require several screens
  to model correct spatial/temporal scales
• Analyzed diffractive compensation system that
  could operate at effective closed-loop bandwidths
• gt2kHz with gt100x100 equiv. actuators

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Conventional Adaptive Optical System
HEL Source Reference Source

• Separate OPD and Tilt Loops
• OPD stroke limits
• Requires phase reconstruction
• Complex, computation intensive
• Closed-loop bandwidths

Read Intensity File
Read Phase File
Flat Phase
Read Jitter File
Reference Camera
Performance (Strehl, MTF)
Far Field Propagation
Propagation Distance
Scoring Camera
Deformable Mirror
Fast Steering Mirror
Pupil Relay Optics
FSM Control
Tilt Sensor
DM Control
Wave-Front Sensor
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Diffractive Phase Correction System
HEL Source Reference Source

• Modulo-2p direct phase compensation
• No OPD stroke limits
• May eliminate tilt loop
• Requires only fringe transfer
• Very simple process
• Open-loop bandwidths

Read Intensity File
Read Phase File
Flat Phase
Read Jitter File
Reference Camera
Performance (Strehl, MTF)
Far Field Propagation
Propagation Distance
Scoring Camera
Diffractive SLM

• SLMs with over 100x100 pixels
• Frame rates gt1kHz
• Continuous trade space

Fast Steering Mirror
Pupil Relay Optics
FSM Control
Tilt Sensor
Fringe Transfer
Self-Referenced Interferometer
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Schematic Diagram of AO Testbed
Disturbance Generator
Telescope
EASLM (512x512)
Fringe Transfer (256x256)
Corrected Signal
Target
Tilt Loop FSM
SRI FPA (320x256)
Beacon Source
Shared Aperture

• OSSim Model Contains all optical elements
• 27 lenses, 2 FSMs, 2 phase wheels, FPA, SLM,
  Fringe driver, detectors

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OSSim Module for Binaryand Blazed EASLMs
Binary l/2
Blazed l 8-step
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Fringe Transfer Process
SRI FPA Intensity
Binary Fringes on EASLM
Blazed Fringes on EASLM

• FPA fringes are thresholded (30 - 50) for
  optimal contrast
• Max 40 diffraction efficiency for high-tilt
  (l/2 OPD) LC material
• Can be applied to either binary or analog LCSLMs
• Blazed fringes are processed from binarized
  fringes
• 8 phase steps per fringe gives maximum
  diffraction efficiency (gt90)
• Number of pixels per fringe sets WFE correction
  limit for both

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Signal/Reference Leg Balancing Required for High
Fringe Contrast
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Importance of SRI Sig/Ref Balancing

• Unbalanced SRI produces
• Low-contrast fringes
• Loss of grating area
• Low diffraction efficiency
• Poor wavefront correction
• Properly balanced SRI results in
• High-contrast fringes
• Maximum grating area
• Highest diffraction efficiency
• Maximum wavefront correction

IRef 10-3 ISig
IRef ISig
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OSSim Analysis of Misfigured Segmented Primary
Mirror Imaging System Basis for MEMs Modeling

• 275 element Hex Array in unobscured aperture
• Figure of merit is MTF
• Picture shows color mapped uniform random
  distribution of 0.4l P-V segment pistons
• Segment piston and tip/tilt are defined as l P-V

• Close-up view of segments shows details of
  distribution (All WFEs are OPD errors)
• WFE gt0.15l rms (1.35l P-V) Requires Correction

Uncorrected Monochromatic MTFs
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Tilt Compensation Demonstration

• Aberration - Fast Steering Mirror scanning
  through 55-waves of full-aperture tilt
• SRI Diffractive compensation system (real-time
  holography)

75-Waves
Conjugate Order Tilt Corrected
20-Waves
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Simulation Agrees with Test Results(D/r0 15)
Corrected Order
Corrected Order
Zero Order
Zero Order
Simulation
Experiment
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Diffractive Compensation of Turbulence using a
Binary l/2 EASLM - Simulation
Atmospheric Turbulence fG150Hz 3kHz frame rate 1
frame latency
Atmospheric and partially developed
Aero-optical Turbulence V0.4M
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Summary

• Introduction to Real Time Holography
• Overview of Boeing RTH activities
• Introduction to diffractive compensation of
  aero-optical turbulence
• Turbulence screens for wave-optics modeling
• Comparison of conventional and diffractive
  compensation models
• Adaptive optics testbed for testing and anchoring
• Wave-optics code components for diffractive
  compensation elements
• Demonstration of tilt and turbulence compensation
• Simulation of aero-optical compensation