(PRMI locking with slightly misaligned SRM)
First I tried locking PRC and MICH with a little bit misaligned SRM. This condition allowed me to search for a good signal port for SRC.
In this locking, REFL11_I was used to lock PRC and AS55_Q was used for MICH. This is the same scheme as the current PRMI locking.
Since the alignment of SRM was close to the good alignment, I expected to see fringes from SRC in some signal ports (i.e. REFL55, POY55 and so on).
Sometimes a fringe of SRC disturbed AS55_Q and broke the MICH locking, so I had to carefully misalign SRM so that the SRC fringes are small enough to maintain the lock of MICH.
(Looking for a good signal port for SRC)
After I locked the PRMI with slightly misaligned SRM, I started looking for a good signal port for SRC.
At the beginning I tried finding a good SRC port by shaking SRM at 100 Hz and looked at the power spectra of all the available LSC signals.
I was expecting to see a 100 Hz peak in the spectra, but this technique didn't work well because SRC wasn't within the linear range and hence didn't produce linear signals.
So I didn't see any strong signals at 100 Hz and finally gave up this technique.
Then I started looking for a PDH-like signal in time series and immediately found AS55_I showed large PDH-like signals.
So I started using the AS55_I for the SRC locking and eventually succeeded.
(Two tips for the DRMI locking)
During the locking of DRMI, I found two tips that made the locking quite smooth.
- Triggered locking
Since every LSC signal ports showed large signals from PRC somehow, feeding back the signals made the suspensions crazy.
So I used triggered locking for the PRC and MICH locking to avoid unwanted kicks on BS and PRM.
If the DC of REFL goes above a certain level, the control of PRC starts. Also if the DC of AS goes below a certain level the control of MICH starts.
These triggers make the lock smoother.
- Do not use resonant gain filters
This is really a stupid tip. When I was trying to lock MICH, the lock became quite difficult for some reasons.
It looked there was an oscillation at 3 Hz every time the MICH control started. It turned out that a 3 Hz resonant gain filter had been making it difficult.
All the resonant gain filters should be off when a lock acquisition is taken place.
Eventually the DRMI was locked.
More details will be reported in the morning.