1. GEO – VIRGO joint noise hunting project
G. Vajente and J. Smith
ILIAS WG1 Meeting
Cascina, November 13th 2006

2. Summary Progress in Virgo Noise Hunting
Automatic longitudinal noise budget
Laser intensity noise investigations
Frequency noise investigations
Control noise investigations
Plans and ideas for noise reduction

3. Automatic longitudinal noise budget
Automatic script
Measure TFs with noise injections in the error points of the loops
Compute projections and coherences
Noise budget for WSR5
MICH = short michelson differential length
PRCL = Power Recycling Cavity length
Pr_B1_ACp = dark fringe signal

4. Laser intensity noise investigations /1
Power stabilization topology
Uses one diode after IMC
Bs_IMC_D1T_DCHF

5. Laser intensity noise investigations /2
PSTAB OFF PSTAB ON DARK NOISE
Pstab error signal reduced below dark noise (ADC)
Other diodes limited by dark (ADC) noise above ~50 Hz
Improved after diodes tuning and emphasis / de-emphasis filters implentation

6. Laser intensity noise investigations /3
NORTH ARM TRANSM.
DARK FRINGE
BS PICK-OFF
WEST ARM TRANSM.
PR REFLECTION
Coherences of dark fringe signal with ITF powers
High with DC power in the dark fringe itself

7. A possible mechanism
MICH control loop generate dark fringe power noise due to an offset
This power noise couples to ACp signal due to RMS fluctuation of dark fringe
Normal condition
MICH noise injected

8. Suspects of MICH longitudinal offset /1
DARK FRINGE
BS PICK-OFF
SIDEBANDS IN PR CAV.
ITF powers versus Michelson error signal
Powers should be maximum/minimum around the correct operating point
NORTH TRANSMISSION

9. A first experiment about MICH offset
DARK FRINGE
BS PICK-OFF
DF / (BS PICK-OFF)
B5_ACq is MICH error signal
Added an artificial offset (uncalibrated)
Dependence of ITF powers on the offset
Need for an full exploration of two-dimens. MICH-PRCL space
SIDEBANDS IN PRC
PR REFLECTION
NORTH TRANSM.

10. Frequency Noise investigations
WSR2 data
Frequency noise is limiting high frequency part of the spectrum
Coupling changes a lot in time

11. Fluctuation of freq. Noise coupling
Coherence and TF from frequency servo correction to dark fringe signal, computed while injecting noise, short periods, two different times

12. Coupling measurement with calibration line
Coupling coefficient measured with calibration lines
Lots of structures
Almost everywhere good coherence with RMS of high frequency noise

13. Origin of coupling fluctuation?
Coherences with mirrors positions read by local control systems PR BS
A good candidate is alignment… NE WE

14. Alpha technique impact
MICH correction is sent to end mirrors with opposite phases to reduce dark fringe contamination