Yuta, Kiwamu, Daniel, Sheila, Dave, Evan, Stefan After we had the PRMI drive properly balanced (alog 10559), we fine-tuned the 3f-locking again: - Added a 60Hz (triggered) notch in MICH to reduce the RMS drive to BS and PRM. - AC-coupled the PR2 drive at 2Hz - it was saturating at DC. - Reset the REFLAIR_B_RF27: two stages of whitening, 27dB of whitening gain, digital gain of 1. - With that the 3f lock by itself was fairly stable - lock duration was limited only by user impatience. Next we brought up the green arm - for some reason ETMX-HEPI seems to have moved - we had to re-align both TMS and ETMX suspensions. - After manually tweaking the green arm alignment the COMM_handoff worked fine. - We parked the about 12kHz avaiy from resonance and locked PRMI. - PRMI locking im this configuration seemed more difficult. Attached is a plot of the control outputs once locked - we were again frequently saturating the BS and PRM. - Ignoring all that we switched to 3f-locking, and brought the red on resonance - it stayed locked.... .... ... but we don't understand the red build-up. It's wasn't any higher than single shot.... something aint right quite yet, but I need a beer. EDIT (after the beer): The build-up actually makes perfect sense: The reflectivity sign flip of the ITMX due to arm resonance means the Michelson is exactly anti-resonant, redirecting all the light to the dark port. effectively no light gets back to the PRM. Hence the incident power on ITMX is equal to the (PRM misaligned) single shot power. We also had rockIFO playing the PRCL signal while we put the arm on resonance, and couldn’t hear any rumbling. To do next: measure length sensing matrix in PRMI and in state 3 (which we should lock on red using the 1f-signal).
The attached plot shows time traces for the combined 3f and x-arm cavity lock. The top left pad shows the frequency dialed into the COMM VCO. It is the calibrated green laser frequency in kHz. The red beam is resonant when the green frequency was near -37 kHz and was parked 10 kHz above. This is a 5 kHz offset for the red laser frequency. We verified that this is the direction away from the higher order transverse modes.
The bottom left plot shows the green transmitted power as measured in the corner station. The x-arm was locked to the green ALS laser most of the time.
The bottom right plot shows the POP_B demodulated signal for 2f. When high, it is an indication that the 1f-sideband is resonant in the PRMI. The lock was achieved with the 1f-signals and then switched over to the 3f-signals.
The top right plot shows the red transmitted power in the x-end station. With the green locked, it will typically dither around resonance, when the COMM PLL frequency is set to the red resonance. The first region of red arm cavity locking is with the PR mirror misaligned. During the 3f-lock the arm cavity was first parked away from resonance to facilitate locking, and then brought near resonance. The PRMI stayed locked using the 3f-signals during the entire transition! The transmitted power was about equal to the power when the PR mirror was misaligned. With the PRMI locked on sidebands and only one arm locked, there is a 180º phase shift for the carrier in the Michelson. Hence, all the carrier power is guided towards the AS port.
