automatic alignment using the anderson technique a. freise european gravitational observatory

19. October 2004 A. Freise

Automatic Alignment using the Anderson Technique

A. Freise

European Gravitational Observatory

Roma 21.10.2004

19. October 2004 A. Freise

Overview

Suspended bench External bench

Output Mode-Cleaner

Linear alignment

Drift control

Non-linear alignment

Simulation

Procedure/Documentation

Automation

19. October 2004 A. Freise

Linear Alignment: Status

Suspended bench External bench

Output Mode-Cleaner

Linear alignment implemented for North arm, West arm andthe recombined Michelson, using B7 and B8

Performs well for full power or reduced power (10%)

B8

B7

19. October 2004 A. Freise

Autoalignment: Why ?

Superimpose beam axesMaximize light powerStabilze optical gain

Center beam spots on mirrorsMinimize angular to longitudinal noise coupling

19. October 2004 A. Freise

Differrential wavefront sensing (analog feedback for 14 DOF in GEO)

Spot position sensing (digital feedback for 20 DOF in GEO)

Autoalignment: How ?

19. October 2004 A. Freise

The VIRGO Interferometer

N

W

EOM Injection Bench

2 Perot Fabry cavities

Recycling mirror

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‚Linear Alignment‘ for VIRGO

linear alignment : angular motion of 5 mirrors to be controlled (DC – 4 Hz)

19. October 2004 A. Freise

Modulation-Demodulation

6.26 MHz

For obtaining control signals a modulation-demodulation technique is used. Only one modulation frequency is applied to generate all signals for longitudinal and angular control of the main interferometer.

19. October 2004 A. Freise

Resonance Condition

Carrier

Upper Sideband

Lower Sideband

TEM00

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Resonance Condition

TEM01

Carrier

Upper Sideband

Lower Sideband

19. October 2004 A. Freise

Cavity Alignment

The Anderson technique uses signals in transmission of a cavity. The detectors are positioned in :

Far field

Near field

19. October 2004 A. Freise

Cavity Alignment

Sensitive to translation of the mode

The Anderson technique uses signals in transmission of a cavity. The detectors are positioned in :

Near field

Far field

19. October 2004 A. Freise

Cavity Alignment

Sensitive to tilt of the mode

The Anderson technique uses signals in transmission of a cavity. The detectors are positioned in :

Near field

Far field

19. October 2004 A. Freise

Detection

19. October 2004 A. Freise

Detection

In each of four outport ports we can set: two Gouy phases two (four) demodulation phases

to get 4x4 output signals foreach direction (horizontal/vertical)

19. October 2004 A. Freise

Detection

For tuning the telescopes one can move L2, L3, L4a and L4b. The most critical adjustment is required for L2.

19. October 2004 A. Freise

Tuning Telescopes

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Control Matrix

In total: 8 Gouy phases have to be tuned, 16 demodulation phases to be set.

This yields 32 signals to control 10 degrees of freedom (5 horizontal, 5 vertical).

Control topology (phases+control matrix) has been designed by G. Giordano.

The optical matrix has to be measured to generate two5x16 control matrices using a 2 reconstruction method.

19. October 2004 A. Freise

Example Matrix (16x5)

19. October 2004 A. Freise

Signal Amplitudes

19. October 2004 A. Freise

Alignment Control

DC: beam positions are defined by reference marks, spot position control, below 0.1 Hz

around the resonance frequencies of the suspension pendulums the beam follows the input beam from the laser bench, differential wave-front sensing, 0.1 Hz to 10 Hz

no active control at the expected signal frequencies, the two mode cleaners suppress geometry fluctuations by ~106

19. October 2004 A. Freise

The GEO 600 Detector

differential wave-front sensing

spot position control

4 degrees of freedom for MC 1

+4 for MC 2

+4 for MI common mode

+2 for MI differential mode+2 for signal recycling

16 + 32 = 48

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Signal Amplitudes in 2D

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Zero Crossings

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Angular Fluctuation

Residual fluctuations:~ 1 nrad @ 10 Hz~ <1urad RMS

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Filter design

open loop transfer function for NI/NE tx.

unity gain

3.2 Hz

19. October 2004 A. Freise

The Suspension Control

Main mirrors are suspended for seismic isolation. Active control is necessary to keep the mirrors at their operating point:

• inertial damping • local damping• local control, i.e. steering of the mirrors

Bandwidth ~5 Hz, positioning of the mirror to ~1 rad and <1 m

Good performance for operating the interferometer but more precise controls are necessary to reach the expected sensitivity of the instrument.

19. October 2004 A. Freise

Feedback

Feedback is applied to the Marionette viathe four coil-magnet actuators used alsofor the local control.

19. October 2004 A. Freise

Current Status

Suspended bench External bench

Output Mode-CleanerInterferometer currently used in recombined mode (Recycling mirror is misaligned)

North and West arm cavities are automatically aligned (to the beam) since:

North arm: December 2003

West arm: May 2004

Longest continuous lock >32h

Beam drift correction not yet implemented

19. October 2004 A. Freise

Cavity Power

AA turned ON

AA Off

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Angular Fluctuation

From Local to Global controlBandwidth ~4 Hz

AA ONAA OFF

19. October 2004 A. Freise

Angular Fluctuation

Residual fluctuations:~ 1 nrad @ 10 Hz~ <1urad RMSLimited by:

input beam jitterresonance peaks of the main suspensions (e.g. 0.6 Hz)

19. October 2004 A. Freise

Conclusion

Output Mode-Cleaner

First implementation of the Anderson technique on a large scale interferometer

Both arms of the interferometer are automatically aligned:Local controls can be switched OFF

The angular mirror motions are reduced and the power fluctuations of the arm cavities minimized

Facilitate the recombined lock acquisition

Unity gain frequency around 4Hz

32 hours continuous lock of the interferometer with automatic alignment control

Next steps Beam drifts correctionRecycling mirror automatic alignment

19. October 2004 A. Freise

End

19. October 2004 A. Freise

Global Control

Output Mode-Cleaner8 quadrant diodes yield 32 signals

Signals are linearised by the DC power on the quadrant

A static matrix is used to create 10 signals for angular control of the mirrors

Unity gain bandwidths is 3 – 5 Hz

Automatic alignment allows switch off the Local controls