TWiLiTE WB57 Aircraft Etalon Qualification

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TWiLiTE WB-57 Aircraft Etalon Qualification

• LIDAR Working Group
• Welches, OR
• June 28, 2006

Authors 1Michael Dehring, 1Scott Lindemann
2Bruce Gentry
1Michigan Aerospace Corporation 2NASA Goddard
Space Flight Center
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Discussion Outline

• Project description and overview of goals
• Etalon specifications and design strategy
• Vibration power spectrum inputs
• Test Set-Up and Description
• Vibration testing results
• Conclusions

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Project Description and Goals

• Development qualification of a tunable
  Fabry-Perot etalon capable of operation onboard a
  WB-57 aircraft for the NASA Goddard IIP TWiLiTe
  program
• TWiLiTE is an aircraft demonstration of a direct
  detection Doppler wind LIDAR
• In addition to the etalon, a digital etalon
  controller will be delivered
• Empirical vibration data provided from WB-57 test
  flights
• New development tunable FPs not typically flown
  on aircraft
• Qualification entails
• Verification of survival during takeoff and
  landing
• Verification of operation during flight profile
• Project Goal
• Etalon must maintain stability to 0.1-0.2 m/s
  for operational testing, which translates to
  plate parallelism of lt 0.1 Å .

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TWiLiTE Etalon Specifications

• Triple Aperture Stepped Air-gapped Tunable
  Etalon
• Stepped etalon plate creates three spectrally
  distinct resonant cavities

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Receiver Technology Overview
Stepped Etalon
TWiLiTE Receiver

• Two Edge Filters and Locking filter generated
  from stepped sub-apertures of etalon
• Plate parallelism is particularly important
  since filter positions must remain stable with
  respect to backscattered lineshape

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Etalon Full Field Fringe Pattern

• Steps in etalon plate were created by vapor
  deposition of fused silica.

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Etalon Design Strategy

• Take existing space qualified etalon design and
  ruggedize it for stability in a mechanically
  dynamic environment
• Accomplished through FEA analysis of etalon
• Ensured that resonant frequencies are in high
  frequency-low power region of anticipated
  spectrum
• Add vibration isolators to Doppler receiver to
  dampen the vibration imparted to the etalon
• Isolators dampen high frequencies however amplify
  low frequencies

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Input Power Spectrum for X Y-Axis Testing

• Optical axis of etalon aligned along x-axis

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Input Power Spectrum for Z-Axis Testing

• Optical axis of etalon aligned along x-axis

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Transmitted Power to Etalon

• Rope isolators shifts power from high frequencies
  to low.

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FEA of Etalon Structure

• PSD for the WB-57 indicates resonances are in
  lower power region of the transmitted spectrum.

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FEA of Etalon Structure

• PSD for the WB-57 indicates resonances are in
  lower power region of the transmitted spectrum.

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FEA of Etalon Structure
1813.4 Hz

• PSD for the WB-57 indicates resonances are in
  lower power region of the transmitted spectrum.

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Space Versus Aircraft Design

• Space qualification purely for survival
• Aircraft qualification requires optical operation
  throughout broadband vibration spectrum
• Mechanical stiffness requirements are far higher
  for an aircraft qualified etalon than a
  spacecraft version
• Increased mass over space rated version
• Reduced thermal compliance of etalon plates
  compared to space version

Space Qualified Design
Aircraft Qualified Design
Thickened Extended Cylinder Wall
Shortened Spring Arm Increases Stiffness
Lower Center of Gravity
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Vibration Profile Comparison
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Test Setup for Vibration Qualification

• CW He-Cd 355nm source used to illuminate 1 of
  etalon sub-apertures
• Si-Diodes used for detectors
• Prior to testing etalon was aligned such that the
  fringe was illuminated at the HWHH point to
  achieve max sensitivity
• Sampling rate for vibration testing was 1 KHz
• Prior to each testing sequence, pre-test data was
  recorded as well as etalon spectral response

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Vibration Test Parameters

• Input PSD profiles Derived from 3-axis
  Accelerometer Data Taken During WB-57 Flight
• Accelerometer package located in plane Bomb-bay
• Scaled to simulate worst case for survival and
  operation
• Peak amplitude plus 3-? at each frequency used
  for the various PSDs
• Integrated Instrument with Etalon Shaken in Three
  Orthogonal Axes
• Six vibration tests performed in each axis
• Sine sweep from 10-1600 Hz at 0.1 G2/Hz
• Performed before and after PSD testing in each
  axis
• Survival PSD test for 3 minutes representing
  take-off conditions
• Test performed sequentially at 0.25, 0.5 and full
  amplitude PSD
• Operational PSD test for 5 minutes

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Vibration Qualification Test Setup
Accelerometers
Signal Input Fiber
Signal Output Detector
Etalon Controller Pre-amp Board
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Vibration Qualification Test Set-Up
TWiLiTE Receiver
Fiber Input
Z-Axis
Control Accelerometer
Vibe-Table Interface Block
Wire Rope Isolators

• Testing performed at MetLabs in Baltimore, MD on
  April 28-29, 2006
• Z-Axis setup shown above

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Results of Z-Axis Testing
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Results of X-Axis Testing
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Results of Y-Axis Testing
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Results of Z-Axis Testing
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Closing Remarks

• The etalon vibration testing results for the
  TWiLiTE project are encouraging
• Represents an important risk reduction to the
  project as the Fabry-Perot is the heart of the
  receiver.
• The measured instability of the etalon was 1- 2
  m/s at 1 second sampling time for the worse
  case operational environment
• TWiLiTE will make wind measurements with 10 sec
  integration times, thus the expected noise
  injected from the etalon should drop by SQRT(10)
  or 3x.
• Data reduction is still underway to fully
  understand the implications of the results and to
  ensure analysis was performed properly.
• The wind speed errors appear to scale linearly
  with the magnitude of vibration
• Possible additional noise sources that are being
  evaluated
• Possible E-M and acoustic interference may have
  acted as a noise source in the data and skewed
  the magnitudes higher than realistic
• Final TWiLiTE etalon controller still in
  development, one used for testing not matched to
  etalon and may have contributed to slightly
  higher noise
• Rope isolators are being augmented with
  additional vibration damping to absorb low
  frequency power.
• Operation during vibration not a concern for
  space mission