Couplings of angular noises into dark fringe Diffused light studies

Gabriele VajenteILIAS WG1 meeting - Perugia 20.09.05

Couplings of angular Couplings of angular noises into dark fringenoises into dark fringeDiffused light studiesDiffused light studies

ILIAS WG1 meeting 20.09.05 – Gabriele Vajente 2

SummarySummary

• 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


Non stationary noisesNon stationary noises


Input beam jitterInput beam jitter

• 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)

IMC transmission and PRC pick-off powers IMC end quadrants diodes


Input beam jitterInput beam jitter

• 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)

IMC transmission and PRC pick-off powers IMC end quadrants diodes

0.7 mHz


Analyzed configurationsAnalyzed configurations

• During C6 run:– “bump” between

100 – 300 Hz (diffused light)

– removed before the end of the run

• During M9 minirun:– New power

stabilization scheme

Input laser power is stabilized using a pick-off before the IMC

Input laser power is stabilized using the power transmitted through the IMC

Dark fringe spectrum with and without the “bump”

The “bump” was acoustic noise re-injected by diffused light at north end bench


Analysis methodologyAnalysis 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


C6 lock C6 lock (without bump)(without bump)

Spectrum of dark fringe band-limited RMS time evolution


C6 lock C6 lock (without bump)(without bump)


2 – 30 HzAngular noise? Coherence with BS, NE, WE and ISYS angular corrections

0.6 – 1 kHzAcoustic noise from the ISYS?


C6 lock C6 lock (with bump)(with bump)



C6 lock C6 lock (with bump)(with bump)

The “bump” is modulated at 0.38 mHz.

Correlated with NE building temperature


New power stabilization schemeNew power stabilization scheme


New power stabilization schemeNew 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.


Daily trendsDaily trends


Daily trendsDaily trends

• 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

Dark fringe after OMC

Dark fringe before OMC

PRC pick-off power


Effect on dark fringeEffect on dark fringe

Dark fringe demodulated signal

Dark fringe DC signal

Red: DF DC maximum Green: DF DC minimum


CoherencesCoherences

B7 DC B7 ACp B7 ACq

B8 DC B8 ACp B8 ACqControl noise?

Diffused lightRed: B1 DC maximum, Green: B1 DC minimumB7 = NE transmitted beam B8 = WE transmitted beam


Projections of angular Projections of angular noisesnoises


Projections of angular noisesProjections 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



• 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!



Dominant noise source is WI (under local control!)

It’s possible to strongly reduce its gain (by a factor of 20)



These projections are automatically computed via an octave program

Needed independent measurements of transfer functions!


Diffused light Diffused light investigationsinvestigations


Diffused light investigationsDiffused light investigations• 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 NE beam

QuadrantsDark fringe spectrum


Diffused light investigationsDiffused 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


Diffused light investigationsDiffused light investigations

• Second step: 100 – 300 Hz coherence with WE signals

h_rec [1sqrt(Hz)]h_rec [1/sqrt(Hz)]

Straylight bump

• Black foam was not sufficient for this beam dumping

• Beam hit the box not orthogonally

• Used black AR-coated glass


ConclusionsConclusions


ConclusionsConclusions

• 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


Spare slidesSpare slides


VIRGO simplified optical schemeVIRGO simplified optical scheme


Power stabilizationPower stabilization

Low pass filter at 8 HzUnity gain at 60 kHzReduction of power fluctuations starting 10 mHzDoes not correct the slow oscillation

Couplings of angular noises into dark fringe Diffused light studies

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