GEO600 – The output mode cleaner Mirko Prijatelj for the GEO600-Team Institute for Gravitational Physics Albert-Einstein-Institute Hannover / Germany

Universe Outline The Output Mode Cleaner (OMC) Control system and control logic Stability problems

Universe DC-readout LIGO-G L Controlled DFO

Universe Output mode cleaner Eliminates 99% junk light → Better sensitivity Finesse 150 Round-trip 65.8mm

Universe Output mode cleaner Junk light: Modulation sidebands & higher order modes Different resonance condition for carrier and junk light

Mode scan of GEO output GW-signal in transmitted power Need optimal transmission Control aspects: – Alignment – Length Static errors: Reduced sensitivity Dynamic errors: Noise sources Requirements derived from simulation: – Alignment: UGF << 1 Hz – Length: RMS reduction of ≈ 5 Control system requirements

Universe Control scheme Lock on OMC transmission peak Dithering – Hz – OMC 6kHz Error-signal at

New digital control system CDS = Control and Data System Similar to AdvLIGO Dithers & applies controls OMC  GEO interface via Labview Control setup CDS Control system

Universe Output mode cleaner Before OMC: After OMC:

Universe GEO stability during s6 Stable operation: GEO locks > 63h Reliable OMC lock acquisition (>> 90% success) Weeks without manual intervention

Universe Difficulty: Level 2 New seismic source Oct 17th 2010

Universe Difficulty: Level 2 Seismic increased up to a factor 10 Mostly < 10Hz Even at night!!

Universe Difficulty: Level 2

Universe Difficulty: Level 3 MSR exchange T=2% => T=10% Increased HOM

Universe OMC scan

Universe Lock acquisition Find transmission maxima in the right distances

Universe Control logic

Control setup Control infrastructure

Universe Control user interface

On switching / gain changes: Loop is nonlinear Switch ‚slowly‘ (3 steps, 3 sec apart) Hardware lowpass (2Hz) UGF ≈ 10Hz Control setup Length control

Switch ‚slowly‘ (3 steps, 3 sec apart) Control setup Length control

UGF ≈ 0.1Hz UGF below resonances in the loop => ‚Easy‘ shape Control setup Alignment control

Control setup Instability Instability after MSR change when locked to OMC power New MSR: – More HOMs leave IFO Gravel mining: – OMC alignment fluctuates more Combined effect: – More conversion from HOMs to TEM00

Universe Lets deal with it Seismic feed-forward for IFO mirrors => Reduce HOMs Faster OMC autoalignment => Less coupling of HOMs Advanced control loop design => More robust

Need to deal with pendulum resonances Rot: Pole 1.32Hz,Q=2.87 Tilt: Pole 0.78Hz,Q=2.28 Adapt servo design Control setup Faster OMC autoalignment

Compensate pendulum resonances in servo Control setup Faster OMC autoalignment

Beware actuator saturation Control setup Faster OMC autoalignment

Fluctuations are at low frequencies Use the aux diode for low frequency control Overall gain 120dB-> OMC diode gain 120dB-> No change at high frequencies Control setup Advanced control loop

Control setup Calibration fluctuations Get from [V/sqrt(Hz)] to strain [1/sqrt(Hz)] Introduce known length modulations (calibration lines) to calibrate signal DFO fluctuations => sidebands on calibration lines

Control setup Calibration fluctuations DFO spectrum 2.5,6,8Hz 2.5Hz: Pend. resonances 8Hz: Oscillating control loop

Control setup Calibration fluctuations Calibration line 305Hz DFO spectrum 2.5,6,8Hz

Universe Thank you