I've roughly copied the LLO configuration for the AS power monitor (that won't saturate after lock losses) and installed an additional photodiode in the AS_AIR camera enclosure. PD output goes to H1:PEM-CS_ADC_5_19_2K_OUT_DQ for now.
GiGE used to receive ~40ppm of the power coming into HAM6. I replaced the steering mirror in front of GigE with 90:10, the camera now receives ~36ppm and the beam going to the photodiode is ~4ppm. But I installed ND1.0 in front of the PD, so the actual power on PD is ~0.4ppm.
See the attached cartoon (1st attachment) and the picture (2nd attachment).
Details:
- Replaced the HR mirror with 90:10 splitter (BS1-1064-90-1025-45P), roughly kept the beam position on the camera, and installed a Thorlabs PDA520 (Si detector, ~0.3AW) with OD1.0 absorptive ND filter in the transmission. I set the gain of PDA520 to 0dB (transimpedance=10kOhm).
- Reflection of the GigE was hitting the mirror holder of 90:10, so I inserted a beam dump there. The beam dump is not blocking the forward-going beam at all (3rd attachment).
- Reflection of the ND filter hits the black side panel of the enclosure. I thought of using a magnetic base, but the enclosure material is non-magnetic. Angling the PD to steer the reflection into the beam dump for GigE reflection is possible but takes time (the PD is in a inconvenient location to see the beam spot).
- Fil made a custom cable to route power and signal through the unused DB9 feedthrough on the enclosure. Pin 1 = Signal, Pin 6 = Signal GND, Pin 4 = +12V, Pin 5 = -12V, Pin 9 = power GND. (However, the power GND and signal GND are connected inside the PD.) All pins are isolated from the chamber/enclosure as the 1/2" metal post is isolated from the PD via a very short (~1/4"?) plastic adapter.
- Calibration of this channel (using ASC-AS_C_NSUM) seems to be about 48mW/Ct.
This is the first look of the lock loss from 60W. At least the channel didn't saturate but we might need more headroom (it should rail at 32k counts).
The peak power is in this example is something like 670W. (I cannot figure out for now which AA filter and maybe decimation filter are in place for this channel, these things might be impacting the shape of the peak.)
Operators, please check if this channel rails after locklosses. If it does I have to change the ND filter.
Also, it would be nice if the lock loss tool automatically triggers a script to integrate the lock loss peak (which is yet to be written).
Tagging Opsinfo
Also checking out the channel during the last 3 high power locklosses this morning (NLN, OMC_WHITENING, and MOVE_SPOTS). For the NLN lockloss, it peaked at ~16.3k cts 80ms after the IMC lost lock. Dropping from OMC_WHITENING only saw ~11.5k cts 100ms after ASC lost it. Dropping from MOVE_SPOTS saw a much higher reading (at the railing value?) of ~33k cts also ~100 ms after ASC and IMC lost lock.
Camilla taking us down at 10W earlier this morning did not rail the new channel, it saw about ~21k cts.
As for the filtering associated with the 2k DAQ, PEM seems to have a standard ISC AA but the most impactful filter is a 8x decimation filter (16k -> 2k). Erik told me that the same 8x filter is implemented as src/include/drv/daqLib.c: line 187 (bi-quad form) line 280 (not bi-quad) and one is mathematically transformed to the other and vice versa.
In the attached, it takes ~1.3ms for the step response of the decimation filter to reach its first unity point, which is not really great but OK for what we're observing as the lock loss peaks seem to be ~10ms FWHM. For now I say that it's not unreasonable to use this channel as is.
I added ND0.6, which will buy us about a factor of 4.
(I'd have used ND2.0 instead of ND1.0 plus ND0.6, but it turns out that Thorlabs ND2.0 is more transparent at 1um relative to 532nm than ND1.0 and ND0.6 are. Looking at their data, ND2.0 seems to transmit ~4 or 5%. ND 1.0 and ND 0.6 are closer to nominal optical density at 1um.)
New calibration for H1:PEM-CS_ASC_5_19_2K_OUT_DQ using ASC-AS_C_NSUM_OUT (after ND was increased to 1.0+0.4) is ~0.708/4.00~0.177W/counts.
