Automatic Alignment using the Anderson Technique A Freise

Automatic Alignment using the Anderson Technique A. Freise European Gravitational Observatory Roma 21. 10. 2004 19. October 2004 A. Freise

Overview Output Mode-Cleaner Linear alignment Drift control Non-linear alignment Simulation Procedure/Documentation Automation Suspended bench External bench 19. October 2004 A. Freise

Linear Alignment: Status Output Mode-Cleaner B 8 Linear alignment implemented for North arm, West arm and the recombined Michelson, using B 7 and B 8 Performs well for full power or reduced power (10%) B 7 Suspended bench External bench 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

Autoalignment: How ? Differrential wavefront sensing (analog feedback for 14 DOF in GEO) Spot position sensing (digital feedback for 20 DOF in GEO) 19. October 2004 A. Freise

The VIRGO Interferometer W N 2 Perot Fabry cavities EOM Injection Bench 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 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. 6. 26 MHz 19. October 2004 A. Freise

Resonance Condition Upper Sideband TEM 00 Carrier Lower Sideband 19. October 2004 A. Freise

Resonance Condition Upper Sideband TEM 01 Carrier Lower Sideband 19. October 2004 A. Freise

Cavity Alignment 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 The Anderson technique uses signals in transmission of a cavity. The detectors are positioned in : Near field Sensitive to translation of the mode Far field 19. October 2004 A. Freise

Cavity Alignment The Anderson technique uses signals in transmission of a cavity. The detectors are positioned in : Sensitive to tilt of the mode Near field Far field 19. October 2004 A. Freise

Detection 19. October 2004 A. Freise

Detection In each of four outports we can set: § two Gouy phases § two (four) demodulation phases to get 4 x 4 output signals for each direction (horizontal/vertical) 19. October 2004 A. Freise

Detection For tuning the telescopes one can move L 2, L 3, L 4 a and L 4 b. The most critical adjustment is required for L 2. 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 5 x 16 control matrices using a 2 reconstruction method. 19. October 2004 A. Freise

Example Matrix (16 x 5) 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 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 differential wave-front sensing spot position control 19. October 2004 A. Freise

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

Zero Crossings 19. October 2004 A. Freise

Angular Fluctuation Residual fluctuations: ~ 1 nrad @ 10 Hz ~ <1 urad 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 control, i. e. steering of the mirrors ~5 Hz, of the Good performance Bandwidth for operating thepositioning interferometer mirror to ~1 necessary mrad and <1 but more precise controls are to mm reach the expected sensitivity of the instrument. 19. October 2004 A. Freise

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 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 >32 h Beam drift correction not yet implemented Suspended bench External bench 19. October 2004 A. Freise

Cavity Power AA Off AA turned ON 19. October 2004 A. Freise

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

Angular Fluctuation Residual fluctuations: ~ 1 nrad @ 10 Hz ~ <1 urad 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 4 Hz 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 8 quadrant diodes yield 32 signals Output Mode-Cleaner 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 19. October 2004 A. Freise