The MICH and PRCL motions have been measured in some different configurations.
According to the measurements :
+ PRCL is always noisier than MICH.
+ MICH motion becomes noisier when the configuration is Power-Recycled Michelson (PRMI).
The next actions are :
+ check the ASPD
+ check the demodulation phases
+ try different RFPDs to lock MICH
(Motivation)
The lock of PRMI have been unstable for some reason.
One thing we wanted to check was the length fluctuations in MICH and PRCL.
(Measurement)
Four kinds of configuration were applied.
(1) Power-recycled ITMX (PR-ITMX) locked with REFL33_I, acting on PRM.
(2) Power-recycled ITMY (PR-ITMY) locked with REFL33_I, acting on PRM.
(3) Michelson locked with AS55_Q, acting on BS.
(4) Power-recycled Michelson locked with REFL33_I and AS55_Q, acting on PRM and BS.
In each configuration the spectrum of the length control signal was measured.
With the measured spectra the length motions were estimated by simply multiplying the actuator transfer function.
Therefore the resultant spectra are valid below the UGFs which were at about 200 Hz.
The BS and PRM actuator responses had been well-measured at AC (50 - 1000 Hz)
For the low frequency responses they were assumed to have the resonances at 1 Hz with Q of 5.
(Results)
The below plot shows the length noise spectra of four different configurations.
There are two things which we can easily notice from the plot.
+ PRCL (including the usual PRCL and PR-ITMs) is always noisier than MICH.
+ MICH became noisier when the power recycling was applied.
In addition to them, the MICH noise spectrum tended to have higher 3 Hz bump as the alignment gets improved.
In fact everytime when we tried to perfectly align PRMI it eventually unlocked.
I am suspecting that something funny (or stupid) is going on with the MICH control rather than the PRCL control.
(Notes)
BS actuator = 2.190150e-08 / f2
PRM actuator = 2.022459e-08 / f2 |