19. October 2004 A. Freise Automatic Alignment using the Anderson Technique A. Freise European Gravitational Observatory Roma

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 and the 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 axes Maximize light power Stabilze optical gain Center beam spots on mirrors Minimize 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

19. October 2004 A. Freise ‚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 TEM 00

19. October 2004 A. Freise Resonance Condition TEM 01 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 for each 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

19. October 2004 A. Freise 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 two 5x16 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 ~10 6

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 = 48

19. October 2004 A. Freise Signal Amplitudes in 2D

19. October 2004 A. Freise Zero Crossings

19. October 2004 A. Freise Angular Fluctuation Residual fluctuations: ~ 1 10 Hz ~ <1urad RMS

19. October 2004 A. Freise 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 via the four coil-magnet actuators used also for the local control.

19. October 2004 A. Freise Current Status Suspended bench External bench Output Mode-Cleaner Interferometer 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

19. October 2004 A. Freise Angular Fluctuation From Local to Global control Bandwidth ~4 Hz AA ON AA OFF

19. October 2004 A. Freise Angular Fluctuation Residual fluctuations: ~ 1 10 Hz ~ <1urad RMS Limited by: input beam jitter resonance 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 correction Recycling mirror automatic alignment

19. October 2004 A. Freise End

19. October 2004 A. Freise Global Control Output Mode-Cleaner 8 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