Couplings of angular noises into dark fringe Diffused light studies

1. Couplings of angular noises into dark fringeDiffused light studies

2. Summary • Non-stationary noise couplings due to input beam jitter • Preliminary study of daily trends • Preliminary projections of angular control noise • Acoustic noise by diffused light

3. Non stationary noises

4. Temperature fluctuations in Laser Lab causing input beam jitter Powers oscillates with 20 min period Does this input beam jitter couple into high frequency noise? Search for modulated noises analyzing long locks (up to 40 h) Input beam jitter IMC transmission and PRC pick-off powers IMC end quadrants diodes

5. Temperature fluctuations in Laser Lab causing input beam jitter Input beam jitter IMC transmission and PRC pick-off powers IMC end quadrants diodes 0.7 mHz • Powers oscillates with 20 min period • Does this input beam jitter couple into high frequency noise? • Search for modulated noises analyzing long locks (up to 40 h)

6. Analyzed configurations Input laser power is stabilized using a pick-off before the IMC The “bump” was acoustic noise re-injected by diffused light at north end bench • During C6 run: • “bump” between 100 – 300 Hz (diffused light) • removed before the end of the run • During M9 minirun: • New power stabilization scheme Dark fringe spectrum with and without the “bump” Input laser power is stabilized using the power transmitted through the IMC

7. Analysis methodology • Computed DF spectrum at intervals(every 5s or 20s, 60s window length, 216 points for each FFT) • Computed band-limited RMS: [0, 2], [2, 10], [10, 30], [30, 50], [50, 200], [200, 600], [600, 1000], [1000, 10000] Hz • Time evolution and frequency analysis of these RMS

8. C6 lock (without bump) Spectrum of dark fringe band-limited RMS time evolution

9. C6 lock (without bump) Spectrum of dark fringe band-limited RMS time evolution 2 – 30 Hz Angular noise? Coherence with BS, NE, WE and ISYS angular corrections 0.6 – 1 kHz Acoustic noise from the ISYS?

10. C6 lock (with bump) Spectrum of dark fringe band-limited RMS time evolution

11. C6 lock (with bump) The “bump” is modulated at 0.38 mHz. Correlated with NE building temperature

12. New power stabilization scheme

13. New power stabilization scheme No more modulated noise! Conclusion: All these noises were acoustic/angular noises of ISYS coupled through the power fluctuations in the IMC. IMC tx IMC ty IMC trans. RFC trans.

14. Daily trends

15. Daily trends Dark fringe after OMC Dark fringe before OMC PRC pick-off power • During one weekend ITF locked continuously for more than 28 h with all 10 LA loops closed • Clear daily trend in dark fringe power • Seems to be stable between different locks

16. Effect on dark fringe Dark fringe demodulated signal Dark fringe DC signal Red: DF DC maximum Green: DF DC minimum

17. Coherences B7 DC B7 ACp B7 ACq Control noise? B8 DC B8 ACp B8 ACq Red: B1 DC maximum, Green: B1 DC minimum Diffused light B7 = NE transmitted beam B8 = WE transmitted beam

18. Projections of angular noises

19. Projections of angular noises • Strong coherence of all angular corrections with dark fringe • Can compute approximate transfer functions to DF without noise injections • All angular error signal are coherent with each other • Can “project” angular noise into DF

20. Projections of angular noises • Strong coherence of all angular corrections with dark fringe • Can compute approximate transfer functions to DF without noise injections • All angular error signal are coherent with each other • Can “project” angular noise into DF THIS MAKES SENSE ONLY FOR THE DOMINANT SIGNAL!

21. Projections of angular noises Dominant noise source is WI (under local control!) It’s possible to strongly reduce its gain (by a factor of 20)

22. Projections of angular noises These projections are automatically computed via an octave program Needed independent measurements of transfer functions!

23. Diffused light investigations

24. First step: 100 – 300 Hz bump coherent with north end signals Found several stray beams in north end bench The strongest one comes from the flip mount used to attenuate the beam in front of the quadrants Diffused light investigations Quadrants Dark fringe spectrum NE beam

25. Diffused light investigations • The beam hit the bench box and was diffused • When the box was open, the bump disappeared • Installed a cylindrical-conical aluminum beam dump • Gain in sensitivity and NS-NS horizon stability • The same beam in WE bench was dumped with black foam

26. Second step: 100 – 300 Hz coherence with WE signals Black foam was not sufficient for this beam dumping Beam hit the box not orthogonally Used black AR-coated glass h_rec [1sqrt(Hz)] h_rec [1/sqrt(Hz)] Straylight bump Diffused light investigations

27. Conclusions

28. Conclusions • Input beam jitter couples with angular/acoustic noise • disappeared using new laser power stabilization scheme • Daily trends (still input beam jitter) • will be solved with ISYS drift control • Stray light has a major effect! • Need to careful dump every stray beam… • Angular control noise • Dominant with Local Controls • AA not yet optimized. Noise re-injected up to some tens of Hz

29. Spare slides

30. VIRGO simplified optical scheme

31. Power stabilization Low pass filter at 8 Hz Unity gain at 60 kHz Reduction of power fluctuations starting 10 mHz Does not correct the slow oscillation