TWiLiTE WB-57 Aircraft Etalon Qualification LIDAR Working Group Welches, OR June 28, Michigan Aerospace Corporation 2 NASA Goddard Space Flight Center Authors: 1 Michael Dehring, 1 Scott Lindemann & 2 Bruce Gentry

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

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 ≤ m/s for operational testing, which translates to plate parallelism of < 0.1 Å.

TWiLiTE Etalon Specifications Central Wavelength355nm Plate Spacing0.90 cm Free Spectral Range (FSR)0.555 cm -1 / GHz Reflectivity0.75 Finesse10.8 Coated Clear Aperture5.0 cm Working Aperture mm Segments PZT Dynamic Range3.1 µm Etalon Plate FlatnessLambda/150 Triple Aperture ‘Stepped’ Air-gapped Tunable Etalon –‘Stepped’ etalon plate creates three spectrally distinct resonant cavities Triple Aperture ‘Stepped’ Air-gapped Tunable Etalon –‘Stepped’ etalon plate creates three spectrally distinct resonant cavities

Receiver Technology Overview Stepped Etalon 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 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 TWiLiTE Receiver

Etalon Full Field Fringe Pattern Steps in etalon plate were created by vapor deposition of fused silica. Note Steps

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

Input Power Spectrum for X & Y-Axis Testing Optical axis of etalon aligned along x-axis x y x Z X

Input Power Spectrum for Z-Axis Testing x y x Z X Optical axis of etalon aligned along x-axis

Transmitted Power to Etalon Rope isolators shifts power from high frequencies to low.

FEA of Etalon Structure PSD for the WB-57 indicates resonances are in lower power region of the transmitted spectrum. Frequency NumberHertz

FEA of Etalon Structure PSD for the WB-57 indicates resonances are in lower power region of the transmitted spectrum. Frequency NumberHertz

FEA of Etalon Structure Frequency NumberHertz Hz PSD for the WB-57 indicates resonances are in lower power region of the transmitted spectrum.

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 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 Shortened Spring Arm Increases Stiffness Lower Center of Gravity Thickened & Extended Cylinder Wall

Vibration Profile Comparison

Test Setup for Vibration Qualification Det 2 EM Det 1 Etalon Graseby 250 Optometer Signals IN Etalon Controller Fiber input Vibration table To 16 bit AI board From 16 bit AO board He-Cd Laser FPE 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 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

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 Hz at 0.1 G^2/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

Vibration Qualification Test Setup Signal Input Fiber Signal Output Detector Etalon Controller Pre-amp Board Accelerometers

Vibration Qualification Test Set-Up Testing performed at MetLabs in Baltimore, MD on April 28-29, 2006 Z-Axis setup shown above Testing performed at MetLabs in Baltimore, MD on April 28-29, 2006 Z-Axis setup shown above Wire Rope Isolators TWiLiTE Receiver Z-Axis Vibe-Table Interface Block Control Accelerometer Fiber Input

Results of Z-Axis Testing

Results of X-Axis Testing

Results of Y-Axis Testing

Results of Z-Axis Testing

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