I have had a difficulty transitioning the SRCL signal from the 1f to 3f signal. Not solved yet.
After playing with ALS DIFF, I went through the initial alignment process and then moved onto the DRMI. As tested before (alog 14283), the 1f locking with REFLAIR was fine with a laser power of 10 W incident on IMC. However I kept failing in the transition from the 1f to 3f signals tonight. It was due to the SRCL loop which did not like the 3f signal tonight for unknown reason. The PRCL and MICH loops could be transitioned to RF27_I and RF135_Q respectively without a problem. But SRCL seemed to saturate the M2 stage DAC every time (??) when I tried to engaged RF135_I. I tried adjusting the demod phase, but did not seem to help. Also checked the optical gain by exciting SRCL and confirmed that the input element of -2 which was taken care by the guardian is right. Also tried it with two whitening stages on in REFL135, but this did not help either.
Note that the build up of the sideband power looked high tonight:
Also, I had the modified L2P decoupling filter engaged on the M2 stage of SRM (see alog 14304) all the time tonight.
Here is a lockloss science. Not clear what exactly was going on.
I looked at three different lock loss events from yesterday. All of them were associated with very large saturation in the M2 stage of SRM which seemed to trigger instability in SRCL and destroy all the LSC loops eventually within a 2 sec or so.
The attached plot is the one from Oct-07-2014 09:24:22 UTC. In this example, PRCL and MICH had been already controlled by the 3f signals (i.e. RF27_I and RF135_Q respectively). In the middle of the plot, the SRCL 3f signal was ramped up (though it actually ramps down because of the control sign. The 1f signal was ramping down at the same time which is not shown in the plot.) Apparently the SRM M2 stage saturated quite hard. Following the M2 stage, the SRM M3 stage hit the DAC range as well. The BS and PRM M2 stages did not saturate until the loops became completely broken. An oscillatory behavior was seen in all three length signals and it was roughly at 13 Hz. Hmmm...