Camilla, Sheila, Georgia

Throughput measurements summary

As part of our finishing-up-in-HAM7 activities we have been using the power meter to measure the throughput for the squeezer path. We've been getting some confusing results with high variance, and it seemed like from the OPO to the periscope we were losing ~5%. This path includes SFI1, ZM1/ZM2/ZM3/FC1, then a second pass of SFI1, SFI2, ZM4, ZM5, and the beam diverter. As well as all the A and B path optics on the VOPO platform. Measurements of the ZM1/ZM2/ZM3/FC1 path showed essentially no loss. Measurements of the beam divereter showed it is also not the culprit. This afternoon Camilla and I took many measurements along the path that seem to suggest we don't have 1 particularly lossy optic - the loss is distributed along the path.

Method

For the measurements this afternoon we tweaked up the polarization into the seed fiber (this got us ~100% more power) and increased the power going into the fiber to ~60mW (measured 37mW in front of the fiber for 112 cts on the monitor PD). We halved the pump power going into the OPO to reduce green light contaminating the measurement straight out of the OPO (measured to be ~3uW). When we zeroed the power meter for the beamstraight after the OPO (between A:DC1 and A:M1) we blocked the beam on the input to the OPO, so the green power was zeroed away. We measure ~630uW of 1064 seed light out of the OPO. We turned the mega cleanroom lights off and were careful about blocking the beam upstream of the power meter when zeroing it to avoid spurrious light. Between measurements we looked at the power on the photodiode on SQZT7 and saw the power was fluctuating by +/-4 cts on an average of 45, so by +/-8%, so these measurements are probably not going to tell us the loss to the accuracy we want, but we are consistently measuring less power at the periscope than out of the OPO. We also remeasured the notorious A:M3 reflectivity to be 99.5%.

Results

We measured at a few locations: straight out of the OPO (1), at ZM1 (after A:M3) (2), the retroreflected beam on transmission of SFI1 (3), before and after SFI2 (4, 5), and at the SQZT7 periscope (6). The attached plot shows our transmission, the data is in this spreadsheet.

It looks like we pretty consistently have ~1-2% loss from the OPO to the retroreflected beam after SFI1, which maybe isn't too surprising as it double passes the SFI and A:M3. We lose another couple of % between SFI1 and SFI2 (maybe we should look more closely around this area? {ower measurements are finnicky here as it's so far in chamber and there are fibers and cables to worry about), and we lose another 1% or so between SFI2 and the periscope (not surprusing with B:BS1, a 99:1 reflector).