[Sheila Craig Georgia]
This morning Sheila aligned the ALS common beat note onto the PD on ISC-T1, the beat note strength is 0 dBm, slightly lower than it used to be (+6dBm), investigation ongoing.
The PLL, which locks the ALS_COMM VCO to this beat note locked without a problem.
We had to fix a problem with the ALS_COMM guardian to finish locking ALS_COMM, where the IMC length control is transitioned from the usual IMC reflection PDH (IMC-MCL) to the common mode VCO (LSC-MCL). The guardian edges needed to be updated: in 2016 there was no PREP_FOR_HANDOFF state, and commands in this state were included in the HANDOFF_PART_1 state. The next svn commit was with Jenne's guardian revamp, somewhere between these two commits the PREP_FOR_HANDOFF state was created but the edges were not defined. We have now fixed this, and locked ALS_COMM with the X-arm. The UGF of this loop was roughly 700 Hz (note:we rerouted Craig's IMC OLTF SR785 excitation from the MC servo board to the IFO-common mode servo board (LSC-REFL_SERVO) to take this transfer function).
Meanwhile Team Hartmann are working on the ITMX SLED alignment. We've tweaked up the y-arm alignment, beamsplitter alignment (locking MICH), and SR3 alignment (locking SRY) to hopefully aid their quest.
Sheila and I went to ISCT1 to align the ALS_DIFF PD and check the ALS_COMM beatnote. The COMM beatnote pk-pk is now ~600mV (0 dBm), the DIFF beatnote is now ~360 mV (-6dBm), measured from the signals leaving the table on a scope.
We found the ALS_COMM was saturating in RF. We could produce a large beatnote by misaligning the beam til it was on the edge of the PD, and suspect that this was how it was aligned in the past. Instead we turned the optical power on the PSL SHG path down. Even though the beatnote is still much weaker than in the past it is good enough. The beam was also close to clipping in yaw on some steering mirrors and the focusing lens before the diode which we fixed.
We removed an RF amplifier from the output of the DIFF PD (measured pk-pk after RF amplifier removed) - there is extra power here now due to the improved arm cavity green finesse.