today it is snowing, and we have been having ALS glitches in the X arm for the last 45 minutes or so.
We wrote a quick script to transition the green lock to the configuration where the fiber is bypassed, and switched back and forth a few times. The attached text file can just be copied and pasted into a guardian shell, to either transition to the bypassed configuration or back to the normal configuration, you will also need to change the guardain request as the text file tells you to.
In the attached screenshot you can see that when IN1EN is 1, we are locked with the laser to the PLL , and there is some fuzz on the X arm transmission which is a series of small fast glitches if the plot were more zoomed in. When IN2EN is 1 we are locked directly to the arm, and in the first couple of examples this does seem to be quieter than with the laser locked to the fiber. There was a time labeled error when we accidentally left both feedback paths on, we can ignore this.
The arm comes unlocked fairly often when we are locked with the fiber bypassed. Next time this happens and we have time to work on this we should go the end station and take some time to measure the transfer function of this lock to the arm.
We have had other difficulties locking today, with the CARM offset reduction.
Last sunday, after the computer crash we had moved the VCO compensation filter in the TR CARM loop which used to be engaged when we transitioned to TR_CARM to later in the locking sequence, and we never understood why that was necessary. Today we moved it back to earlier in the sequence and that has improved the stability of the TR CARM loop. We also have been sometimes manually deceasing the gain in this path a bit to give us more stability through the CARM offset reduction. We tried to put this into the guardian once, but that didn't work so we have removed it again.