[Aso, Ushiba]

Abstract:
We investigated the frequency noise in ALS.
In both X and Y ALS, sharp peaks are observed at low frequency, which would disturb handover during the lock acquisition.
We haven't fully confirmed the reason yet but it could be coming from the Doppler phase noise due to the suspended optical table in PR2 and SR2 chamber.

Detail:
After fixing the glitches on GR PDH signals (klog23222), we checked the current ALS perfrmances.
Following is the brief explanation how to evaluate the ALS performance.
1. Lock IRX(Y) and GRX(Y) at the same time.
2. Measure the spectra of GRX(Y) PDH feedback signal (K1:ALS-X(Y)_PDH_SLOW_DAQ_OUT_DQ).
3 Compared the spectra with and without FNC.
Since X(Y)ARM motion can be suppressed by IR feedback loop, GR PDH feedback signals can see the differencial signals between IR and GR.

According to the VCO actuator efficiency (klog6544), calibration factors from feedback signals to GR frequency is 0.84, and 0.86 MHz/V, for X and Y, respectively.
Note that green AOMs are double path configuration, so we need to use twice of VCO efficiencies.
To calibrate from green to IR frequencies, we need to multiple 0.5 to green frequency fluctuaion.
In addition, cavity pole of PRFPMI (1.8 Hz) is necessary to be considered.

Figure 1 shows the result (top: XARM, botom: YARM).
Red and blue lines represent with and without FNC, respectively.
Below 10 Hz, a unique bump due to air disturbance can be observed, so we need to prepare windshield for ALS (POP POS, and PSL).
Also, a large peak can be observed in both X and Y ALS (0.43 Hz in XRM, 0.38 Hz in YARM).
According to Enomoto-kun's thesis, the peak is comming from Doppler phase noise but almost no coherence with TM OpLevs of the corresponding suspensions.
So, one possible source is suspended tables inside PR2 and SR2, on which GR steering mirrors are set.

Aso-san calculated the resonant frequency of suspended table in PR2 chamber and its lowest mode is about 0.4 Hz.
Also, the lowest mode cannot be damped with dammper ring effectively, so Q-factor of its mode could be high.
More inestigation is necessary.