KAGRA Logbook

MIF (General)

tomotada.akutsu - 20:17 Wednesday 07 July 2021 (17428)

Trial of GreenX reflection from Xend Day3 PM: GrX returns

Miyo, Ushiba, T. Yamamoto, Akutsu: Cont'd from 17140, 17418, 17419

Abstract

The green beam came back from ETMX to GrRFPD on the POP table.

Details

First we misaligned ITMX so that the reflection beam from ITMX went out of the GrRFPD. Immediately after Miyo-kun started sweeping ETMX, we found a periodic and repeatable responses in the GrRFPD (see Figs 1, 2 and 3 from Miyo-kun); as reported in 17418, 17419 by Ushiba-kun and Yamamoto-kun, the signal read of the GrRFPD (K1:ALS-X_ARM_REFL_OUT16) should be improved in S/N. Then we went to the Xend and I tweaked the oplev QPD stages to zero for memorizing the "best" alignment.

Post analysis

So, it seems K1:ALS-X_ARM_REFL_OUT16 provides "0.004" when the relfection from ETMX (I believe so...) enters GrRFPD. In watt, what is this 0.004?? Unfortunately I could not see this returning beam with my eyeballs. There are several non-empty filters in the bank (see 17418) and so we need to understand what are them for proper calibration.

Memo: the conversion efficiency of the PD is ~0.3A/W (S3399 datasheet; about the modification of the RFPD from the usual IR RFPD is found in 5735)

Images attached to this report

17428_1625654974_Screenshot from 2021-07-07 14-34-11.png

17428_1625654978_Screenshot from 2021-07-07 14-51-10.png

17428_1625654982_Screenshot from 2021-07-07 15-26-38.png

Comments to this report:

takahiro.yamamoto - 20:45 Wednesday 07 July 2021 (17430)

Attachment shows the RFDC signal when the reflection light is absent and presence.
When reflection light is absent, this channel is limited by the circuit noise which is slightly larger than the ADC noise.

In the case reflection light is presence, K1:ALS-X_ARM_REFL_IN1 which is raw ADC count shows ~6.4ct = 3.9mV.
Universal whitening filter is now set as 12dB gain at DC. So the PD output should be 1mV (at differential) and 500uV (at single-end).

Considering 100Ohm as an impedance and 0.3A/W as a PD efficiency, the laser power at RFDC is ~16uW.

Images attached to this comment

17430_1625658318_Screenshot from 2021-07-07 20-25-24.png

tomotada.akutsu - 22:04 Wednesday 07 July 2021 (17432)

I might have been able to see the ETMX reflection with my eyeballs if it would be a 16 uW small spot in green... hmm.. because I can clearly see 8uW things reported in 17429. I'll try again...

kouseki.miyo - 17:28 Thursday 08 July 2021 (17440)

Yokozawa, Ushiba, Miyo
Because we got good alignment so that ETMX returns the green beam to POP table, we calibarated the TM_TILT OpLev for ETMX and ITMX.
Now, the calibrated oplev signal is available in WIT channel. This modification is committed in here.