TWiLiTE WB-57 Aircraft Etalon Qualification

LIDAR Working GroupWelches, OR June 28, 2006

1Michigan Aerospace Corporation

2NASA Goddard Space Flight Center

Authors: 1Michael Dehring, 1Scott Lindemann & 2Bruce 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 ≤ 0.1-0.2 m/s for operational testing,

which translates to plate parallelism of < 0.1 Å .

TWiLiTE Etalon SpecificationsCentral Wavelength 355nm

Plate Spacing 0.90 cm

Free Spectral Range (FSR) 0.555 cm-1 / 16.67 GHz

Reflectivity 0.75

Finesse 10.8

Coated Clear Aperture 5.0 cm

Working Aperture 3 - 24 mm Segments

PZT Dynamic Range 3.1 µm

Etalon Plate Flatness Lambda/15024 mm working CA

Step region boundary

Reference Region

Intermediate step 20.2 nm from reference

Major step70.7 nm from reference

60 mm substrate

24 mm working CA

Step region boundary

Reference Region

Intermediate step 20.2 nm from reference

Major step70.7 nm from reference

60 mm substrate

• 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.

24 mm working CA

Step region boundary

Reference Region

Intermediate step 20.2 nm from reference

Major step70.7 nm from reference

60 mm substrate

24 mm working CA

Step region boundary

Reference Region

Intermediate step 20.2 nm from reference

Major step70.7 nm from reference

60 mm substrate

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-axisOptical 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-axisOptical axis of etalon aligned along x-axis

Transmitted Power to Etalon

Rope isolators shifts power from high frequencies to low.

profile(f)

Lens Holder Z(f): PSDRms=0.015

ETALON Side Z(f): PSDRms=0.016

ETALON Back Z(f): PSDRms=0.018

2000.003.75 10.00 100.00 1000.00

0.0398

3.98E-10

1.00E-09

1.00E-08

1.00E-07

1.00E-06

1.00E-05

0.0001

0.0010

0.0100

Frequency (Hz)

(gn)²/Hz

FEA of Etalon Structure

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

Frequency Number Hertz

1 3806

23807.

8

34231.

7

47304.

7

57569.

6

67571.

8

79821.

6

89860.

9

99865.

5

10 10362

FEA of Etalon Structure

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

Frequency Number Hertz

1 3920

23920.6

34208.2

44876.6

5 5015

65015.8

76001.4

8 6023

96032.5

106921.9

116928.3

12 7471

137496.1

147505.9

157513.5

168117.6

Three points held fixedrepresenting reference plane defined by mount

Six points held fixed in X and Yaxes while allowed to translatein Z axis, representing diaphragm tabs at top of mount.

Three points held fixedrepresenting reference plane defined by mount

Six points held fixed in X and Yaxes while allowed to translatein Z axis, representing diaphragm tabs at top of mount.

FEA of Etalon StructureFrequency Number Hertz

1 776.42

2 778.96

3 1289.8

4 1479.4

5 1661.3

6 1813.4

7 1864

8 2039.5

9 2344.1

10 2560.1

11 3108.2

12 3248.4

13 3540.4

14 3542.9

15 3932.5

16 4003.9

1813.4 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 1Etalon

Graseby 250 Optometer

Signals IN

Etalon Controller

Fiber input

Vibration table

To 16 bit AI board

Fro

m 1

6 bi

t A

O b

oard

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 10-1600 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 FiberSignal 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

0 50 100 150 200 250 300 3500

1

2

3

4

5

6Z Speed Errors (m/s): Operational, Full, Half, Quarter, Pre

File

m/s

Operational (0.682 grms)Full (2.68 grms)Half (1.34 grms)Quarter (0.67 grms)Pre (0 grms)

Results of Z-Axis Testing

0 50 100 150 200 250 3000

0.5

1

1.5

2

2.5

3X Speed Errors (m/s): Operational, Pre

File

m/s

Operational (1.09 grms)Pre (0 grms)

Results of X-Axis Testing

0 50 100 150 200 250 300 3500

0.5

1

1.5

2

2.5

3

3.5

4Y Speed Errors (m/s): Operational, Pre

File

m/s

Operational (1.09 grms)Pre (0 grms)

Results of Y-Axis Testing

0 50 100 150 200 250 300 3500

0.5

1

1.5

2

2.5

3Z Speed Errors (m/s): Operational, Pre

File

m/s

Operational (0.682 grms)Pre (0 grms)

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