(Sheila, Alexa)
We made a few measurements at EX under the following configuration:
PLL Servo Board
- Input 1 pol: NEG
- Input 1 gain: 0dB
- Comm Comp: ON
- Generic Filter: ON
- Fast Option: ON
- Fast gain: -8dB
- Boost 1: OFF
- rest off
PDH Servo Board
- Input 1 pol: POS
- Input 1 gain: -1dB (this was adjusted due to the new modulation depth change...we inserted a 12dB attenuator...see alog 9466. The optical gain must be adjusted accordingly)
- Boost 1: ON
- Comm Comp: ON
- rest off
- PDH open loop transfer function (EX_PDH_OpenLoopTF_mag/phase.TXT)
- To calculate the arm cavity length we zoomed in around the FSR resonance (~37MHz) and took the open loop TF (EX_PDH_OpenLoopTF_mag/phase_zoom.TXT). We found FSR ~ 37.752 kHz, which gives a crude estimate of L~3970.6m for the arm cavity length.
- PDH error signal power spectrum (EX_PDH_Noise.TXT, EX_PDH_Noise_High.TXT, EX_PDH_Noise_Low.TXT). When plotting the power spectrum at full span we noticed a lot of variation in the measurement (we tried to collect the high and low spectrum in this variation). This variation is due to an unstable cavity lock which causes a variation in the optical gain.
- PDH shot noise (EX_PDH_ShotNoise.TXT)
(Sheila, Alexa)
With the above servo board configurations and the HEPA fans off we measured the shot and dark noise again (in a previous alog I mentioned we didnt take the correct frequency ranges for the dark noise which is why we took it again). I have attached a plot of the power spectrum of the PDH error signal, the shot noise w. the fans off and on, and the dark noise with the fan off. All these spectra's were measured out of IMON. There is a considerable difference at low frequency of the shot noise with the fan off and on. The beam size might be too large on the BBPD? Another possibilty is fringe wraping. It is also suspicious that the shot noise is not flat and seems to drift if there is movement around the table...Are we really measuring pure shot noise??
Keita, Alexa, Sheila
Some of the excess noise on the ALS end refl PD is due to fringe wrapping. We looked at spectra of REFL B from the control room while the ITM was misalinged, ( so we should have been looking at the RIN on our green beam returning from the chamber). This is motivated by seeing that the niose at Imon with the ITM mislainged is accoustic, and seems to have a fringe wrapping shelf when the HEPA filters are on. By driving the ETM longitudnaly, we see a fringe wrapping peak on the DC PD. We tried misalinging the reaction chain, and don't see any difference. At some point we need to go bow on optics on the table and try to find the scattering path.
