NAOJ GW Elog Logbook 3.2
Marc, Yuhang
As described in McKenzie PhD, a possible coupling mechanism of pump RIN coupling to squeezing measurement is the presence of a spurious IR seed.
In our case, this could come from spurious LO back-reflected from Homodyne to the OPO or from IR coming from the SHG.
We started investigation on the first case by checking if the FI after the OPO has good attenuation factor.
We removed the Homodyne BS and installed a reflective mirror before the AMC.
We injected BAB and overlapped these 2 beams.
Then, we removed BAB and check the LO power around the FI.
We checked the OPO Trans PD and found really low power. We tuned the steering mirror just before and got TEM00 peak height at about 2V.
We got about 420 uW incident, 400 uW reflected (to the top) but only 10 nW transmitted... Despite alignment tuning we could not find where the missing 20uW are..
We removed the OD filters on the LO path but we had similar power ratio.
We tried to tune the polarizer of the FI (furthest from OPO) by about 20 deg but the transmitted power did not change at all...
Marc, Matteo, Yuhang
We found out that we were only rotating the PBS circular dump and not the PBS itself..
We removed the circular dump and could see some beam. After a little realignment of the LO back-reflection to the FI, we measured :
Pinc = 3.54 mW, 12.6 uW and 8.4 uW rejected by the first PBS respectively towards top and bottom, 3.5 mW rejected by the second PBS toward the top and 0.4 uW transmitted by the FI.
This is closer to what we expect, especially because the 8.4 uW and 3.5 mW measured have some error due to space constraints to place the power-meter.
With this measurement, we can estimate the FI isolation to be about 40 dB which is in agreement with the specsheet.
We reinstalled the ND filters on the LO path, realigned the LO into the AMC, removed the steering mirror we used to increase the back-scatter light of LO (ie recovered the usual situation) and measured 3nW incident on the FI. It means that the LO seed should be about 0.3 pW.
Then, we measured the FI transmission using the BAB. We optimized the PLL frequency to be at maximum OPO transmission and got 342 uW incident on FI and 328 uW transmitted.
Finally, we were also able to remove one titled HWP on the squeezing path towards the Homodyne because we were able to use the HWP just after the FI to remove the p pol as seen in the AMC scan.