# Presentation: 004623

## Active Vibration Compensated Stabilized (AVCS) Interferometry

- John Hayes, Ph.D.

## Outline

- The problem...
- Vibration is a problem when testing large mirrors
- Can we build an interferometer that cancels vibration?
- Review the concept...
- Jim Burge and his students showed it is possible
- Review how it works
- Status of the AVCS prototype project...
- Show some results
- Discuss performance & limitations

## Vibration Compensation Concept

- Example: Twyman-Green configuration
- Sense optical phase
- Feedback out-of-phase signal to phase shifter

- Jim Burge’s System
- Point sensor
- High speed temporal modulation to sense phase
- Problems
- Lower contrast signal
- “Slow”

## High Speed Phase Modulation

- Use polarization Twyman-Green configuration
- EOM changes relative phase between ‘S’ & ‘P’ components
- Fast:  200 kHz - 1 GHz response
- Conoptics model 370 with model 302 driver gives 200 kHz

## High Speed Phase Sensor

- Use polarization optics to create four phase shifted fringe patterns
- We will look at how this works shortly
- Use four single point detectors
- Align detectors to the same image point
- Compute phase in “real time” with controller
- Proportional control loop to drive the phase modulator
- 100 MHz 80486 computer running DOS (!)
- Achieves 10-12 kHz control loop speed

## Piston Correction Control Loop

## Phase Sensor Optical Layout

## AVCS Point Phase Sensor

## Optical Schematic

## Key Features of Layout

- Max tilt = 100 fringes/radius without vingetting
- Telecentric pupil imaging
- Constant pupil magnification
- < 0.10  OPD between on-axis and 100 fr/rad tilt
- 3.25” focus range
- Phase sensor in telecentric image space
- Fixed pupil magnification equal at all image planes
- Sensor can be positioned along pupil diameter
- Sensor path separate from rotating ground glass
- High speed detectors will not sense moving speckle pattern
- Integral 50 mm beam expander

## Mechanical Layout - Top View

- 10mmOutput Port

- EOM

- LASER

- R.T. Phase Sensor

- Hologram/Diverger
- Platform

- CCD Sensor + A/D

- Quad
- Cell
- Tilt
- Sensor

- Tilt Corrector

- 21.125”

- 19.750”

- Space for BS, 2nd
- zoom and camera

## AVCS Top View

- (Wires going crazy!)

## Mechanical Layout - Side View

## Source Optics, Side View

- (THIS DIDN’T WORK OUT)

## AVCS Source Optics

## Controlling Tilt

**Advantages**
- High Quality Spot on Quad Cell
- Tilt mirror near exit pupil
- No “external” components
- Mirror size not a problem
**Disadvantages**
- Works only with the Large Beam
- Small angles of tilt only (< 1 wave/radius)

## System Components and Controls

## AVCS Full System

- All the covers removed...

## AVCS with Cover

## Fringe Output

## Prototype Performance

- Dual measurement repeatability
- No Vibration (w/moderate air turbulence)
- Average of 12 single measurements
- 0.009 waves rms (surface)
- With Vibration (w/same air turbulence)
- Amplitude > 1 wave, Frequency 200-500 Hz
- Completely un-measurable without AVCS
- Average of 12 single measurements
- 0.011 waves rms (surface)
- Almost no difference in results!
- Measure to 75 fringes of tilt w/o compensation
- AVCS stable with up to 8-10 fringes of tilt

## Piston Correction Performance

- Piston correction works quite well
- Performance (rough measurement) with compensation loop running at 10-12 kHz

## Tilt Correction Performance

- Proportional control loop closed
- Works well for “macro” displacements
- Noise is about 0.3 - 0.5 Fringes
- Electronic noise probably due to cable issues
- Reference beam suppression may not be sufficient
- Polarization filter may have too much crosstalk
- May be an adjustment issue (?)
- Probably will not work well in its current state

## What Works Well...

- AVCS for piston (this works amazingly well!)
- Zoom imaging system
- Radio control system
- “Basic” alignment system
- Uncalibrated accuracy “looks about right”
- About 1/4 wave P-V

## Issues not resolved with Prototype

- Intensity is not properly matched between the two channels.
-   This affects the ability to zoom
- High frequency phase noise
- Stray light?
- Tilt QWP in both arms?
- Baffles?
- QWP quality?
- Small FOV for alignment system w/B.E.
- Need another alignment camera
- Tilt compensation not very good
- Not much tilt (or aberration) can be present at PPS
- PPS needs to be aligned
- Higher magnification in PPS imaging channel

## “Version 2” improvements

- Faster electronics and controller!
- Would like to run at 100kHz for 5-10 kHz correction
- Much smaller detectors relative to image size
- Different intensity control (at camera)
- New alignment system for B.E. (very simple)
- More room for cables (bigger box)
- Much lower noise for tilt sensor
- Better electronics configuration
- Preamps at detectors, etc.