automatic alignment using the anderson technique a. freise european gravitational observatory
DESCRIPTION
Automatic Alignment using the Anderson Technique A. Freise European Gravitational Observatory Roma 21.10. 2004. Overview. Output Mode-Cleaner. Linear alignment Drift control Non-linear alignment Simulation Procedure/Documentation Automation. Suspended bench. External bench. - PowerPoint PPT PresentationTRANSCRIPT
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
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Autoalignment: Why ?
Superimpose beam axesMaximize light powerStabilze optical gain
Center beam spots on mirrorsMinimize angular to longitudinal noise coupling
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Differrential wavefront sensing (analog feedback for 14 DOF in GEO)
Spot position sensing (digital feedback for 20 DOF in GEO)
Autoalignment: How ?
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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)
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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.
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Resonance Condition
Carrier
Upper Sideband
Lower Sideband
TEM00
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Resonance Condition
TEM01
Carrier
Upper Sideband
Lower Sideband
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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.
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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.
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Example Matrix (16x5)
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Signal Amplitudes
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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
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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
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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.
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Feedback
Feedback is applied to the Marionette viathe four coil-magnet actuators used alsofor the local control.
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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
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Cavity Power
AA turned ON
AA Off
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Angular Fluctuation
From Local to Global controlBandwidth ~4 Hz
AA ONAA OFF
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Angular Fluctuation
Residual fluctuations:~ 1 nrad @ 10 Hz~ <1urad RMSLimited by:
input beam jitterresonance peaks of the main suspensions (e.g. 0.6 Hz)
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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
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End
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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