The Angular Control System at LHO: Current Status
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LIGO-G030608-00-D L.Matone
The Angular Control System at LHO:Current Status
Presented by Luca Matone
LSC Conference at LHO
Nov. 13th, 2003
Paul Schwinberg, Stefan Ballmer, Matt Evans, Peter Fritschel, Nergis Mavalvala,
Virginio Sannibale, Rick Savage, Daniel Sigg
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LSC Conference – 11/13/2003 2LIGO-G030608-00-D L.Matone
Requirements
Detector’s sensitivity deteriorates with» misalignments of the Test Masses (decrease of the circulating
power, increase of contrast defect, increase of shot-noise);» beam miss-centering (angle-to-length coupling).
Need to have» Angular fluctuations of Test Masses : 10-8 rad-rms with respect to
cavity axis;» Sensing noise: 10-14 rad/rHz for f>40Hz (WFS1); » Beam centering within 1mm.
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LSC Conference – 11/13/2003 3LIGO-G030608-00-D L.Matone
How?
Cavity misalignments with respect to input beam generate couplings of higher-order modes (TEM01);
To detect and correct for the amount of TEM01, we extend the phase modulation-demodulation technique;
» Use of resonant sidebands (LSC 24MHz);» Use of non-resonant sidebands (ASC 36MHz);» Collect light beams on REFL, AS and POX.
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LSC Conference – 11/13/2003 4LIGO-G030608-00-D L.Matone
Wavefront Sensors (WFS)
Misalignment signal = product of carrier (sideband) TEM00 with sideband (carrier) TEM01;
Need to spatially resolve the amplitude modulation at the modulation frequency
» Split photodetector capable of integrating on the half-plane (WFS);
Ten (10) angular degrees of freedom to sense and control (5 Test Masses, pitch/yaw);
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LSC Conference – 11/13/2003 5LIGO-G030608-00-D L.Matone
Beam Centering (QPDs)
Beam centering on ETMs» Four (4) angular degrees of freedom
(Input beam pointing and BS, pitch/yaw);
Total of 14 angular degrees of freedom to sense and control.
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LSC Conference – 11/13/2003 6LIGO-G030608-00-D L.Matone
Row normalized; Reproducibility: ~10%
» except for WFS2 (?);
denotes expected term;
denotes feedback element;
WFS3 and 4: low sensitivity (increase of modulation depth);
Common PRM DOF: 2A->2B: wondering RF phase?
WFS Matrix - H1
dETM dITM cETM cITM RM
WFS1 1 0.5WFS2A 1 1WFS2B 1 ? ?WFS3 1 0.5
WFS4 0.3 0.3 1
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LSC Conference – 11/13/2003 7LIGO-G030608-00-D L.Matone
About calibrations…
Reproducibility: 1-2%; But systematic:
» ~20% pitch/yaw;» ~20% X/Y arm;
This generates» in the calibrated Sensing Matrix
(common/differential basis) the addition of non-existing cross-terms;
» Use of nominal values;
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LSC Conference – 11/13/2003 8LIGO-G030608-00-D L.Matone
Control Viewpoint
1/f loop (1uHz pole); Crossover with
Optical Lever servos; Unconditionally
stable.
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LSC Conference – 11/13/2003 9LIGO-G030608-00-D L.Matone
A look back at S2 – H1 only
8/10 angular degrees of freedom controlled » Problems closing WFS2B loop (dITM);
» QPDs loops not closed– input beam pointing and BS free to drift.
Low bandwidth (~100mHz)» Cross-talk between WFS prevented raising the loop gain.
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LSC Conference – 11/13/2003 10LIGO-G030608-00-D L.Matone
Observations during S2 – H1
Arm Power Dark fringe DC light level
WFS ON
WFS OFF
~9hours
The system provided» Long term arm power
stability;
» Significant reduction of first-order fluctuations.
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LSC Conference – 11/13/2003 11LIGO-G030608-00-D L.Matone
H1 WFS Commissioning after S2
10/10 angular DOF controlled (~10-100mHz)» Role of WFS3 and 4 swapped;
» WFS2 table work;
Ready for next step» Bandwidth, deal with the cross-talk;
» Include QPD;
» Noise study and loop shaping;
» BUT…
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LSC Conference – 11/13/2003 12LIGO-G030608-00-D L.Matone
Laser Power Increased - S3
To improve the modal overlap between the carrier and the sidebands;
But WFS system unstable» Tracked down to an RF phase rotation in WFS2 (?)
– Phase rotation correlated to NSPOB;– Significant decrease in WFS2B response;
Resolution: Gouy phase adjustment for WFS2» decreased the phase rotation swing (30deg);
» Recuperated WFS2B response to dITM;
WFS system for S3 (H1 and H2)» 10/10 angular DOF controlled (10-100mHz);
» Input beam pointing and BS using QPDs (<10mHz)
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LSC Conference – 11/13/2003 13LIGO-G030608-00-D L.Matone
H1: Calibrated Error Points
Dark noise limited above 80Hz» Above requirement;
» Increase of RF gain, whitening gain, NR modulation depth;
» Decrease of electronic/ADC noise;
Residual Angular fluctuations» 10-7-10-8 rad-rms
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LSC Conference – 11/13/2003 14LIGO-G030608-00-D L.Matone
Angular Control Noise Contributionto the Displacement Sensitivity (H1)
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LSC Conference – 11/13/2003 15LIGO-G030608-00-D L.Matone
H1: Beam positioning on WFS
Linear dependence between demodulated signals and DC asymmetry
» Pitch – Defined as (DC top half-DC bot half)/DC total– 4*10-8 * DC pitch asym [rad]
» Yaw– 2*10-8 * DC yaw [rad]
25% DC asymmetry corresponds to 10-8 rad offset;
Asymmetry jitter» Spectral component;» Faster than 1Hz;» Reduced as WFS bandwidth
increased?
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LSC Conference – 11/13/2003 16LIGO-G030608-00-D L.Matone
Linear dependence between demodulated signals and DC asymmetry
» Pitch/Yaw – 10-6 * DC pitch asym [rad]
10% DC asymmetry introduces 10-7 rad offset;
Feedback? Need to investigate…
H1: Beam positioning on WFS
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LSC Conference – 11/13/2003 17LIGO-G030608-00-D L.Matone
To Do…
Increase NR modulation depth; Fine tune Gouy phases (2, 3, 4)
» Eliminate cross-terms in WFS3; WFS matrix in W/urad; Increase bandwidth
» Non-diagonal terms in output matrix;» Loop shaping;
Explore beam centering on TM» Currently running off-center on ETMX;» Angle-to-length coupling;
WFS2 RF phase rotation/heating effect ? Investigate beam centering effect on WFS; Goal for S4: full bandwidth (2-3Hz) angular control.