DriptaB and RickS

Today we investigated the 20 dB gain deficit in the Optical Follower Servo (OFS).  We found that the loop offset being set at 7.0 V may have been the source of the issue.

We followed the procedure in LIGO-T1400486 to test the performance of the OFS board and everything seemed normal.

We centered the beam on the OFS PD in the transmitter module and used the REFL output beam on the PS4 (WSH) power standard to set the loop offset to 3.94 V (V2 on the HP power supply)which gave 300 mW in the REFL beam.

We then measured the OFS OLTF (Souce at CLTF TEST IN; PD MON / ERR MON) and set the gain at 7.60 V (V1 on the HP power supply) to give a UGF of 100 kHz.  The phase margin is about 55 deg, as expected.

The loop seems to be operating normally now.   Screenshot of the OLTF measurement attached.

The system was left with the covers on the two power senors in the responsivity ratio setup, the anodized aluminum beam block at the output of the Tx module, the two external shutters closed and the Remote/Local switch in the Local position.