Nutsinee, Kiwamu,
WP5990
We have (re-) set up the polarization monitors on the HWS table by HAM4. We have confirmed that they are functional. For those who are interested in the polarization data, here are the channels to look at:
In theory, they should be in unit of watts as measured at the HWS table.
[Installation notes]
This time, we have newly installed a short pass optic (DMSP950L from Thorlabs) to pick off the main interferometer beam without getting too much contamination from either the SLED light (790 nm) or the ALS beam (532 nm). The short pass mirror was inserted between the bottom periscope mirror and the first iris (D1400252-v1). Looking at the green light at the table from the end stations, we learned that the beam size is already pretty small and (visually) small enough for the beams to fit into the PDA50Bs without a lens. So we decided to go without lenses as opposed to the previous setup (24046).
The short pass mirror reflects the interferometer beam toward the left on D1400252. We placed a PBS (CM1-PBS25-1064-HP) on the left side of the short pass and placed the PDA50Bs. The power reflectivity of the newly installedshort pass mirror was measured to be 5% +/-3% for 532 nm. The absolute power (assuming the Nova hand held power meter is accurate) of the reflected green light was measured to be 1 uW.
One thing we leaned today was that the green light is not so trustable to get the optimum alignment. We first aligned the optics with the green light and then noticed that the infrared beams were almost falling off of the PDA80Bs. So we then closed the shutters and aligned them with the actual infrared beam.
The manual gain settings are:
The digital gains were also changed accordingly so that the calibration of these channels should be accurate.
This is a first look at the polarization data with the new setup. Some analysis with the previous setting was reported by Aidan at 25442 back in this February with a focus on noise behaviors. This time, since we are looking for a cause of the degradation in the power recycling gain, we focused on the time series rather than the spectra.
We saw two behavior in the polarization data when PSL was ~ 40 W.
Based on the fact that the amount of S-pol decreases as a function of time (which should increase the power recycling gain at the same time, naively speaking), I am inclining to say that the variation in the polarization is not a cause for the smaller power recycling gain.
[An observation from last night, July 13th]
I have used a lock stretch from last evening starting at ~ 2016-07-13 1:00 UTC for 2-ish hours. The attached two plots show the measured polarization in time series.
At the beginning of the lock stretch, the input power was increased step by step up to 40-ish W. The power recycling gain hit 35 right after completing the power-up operation and then settled to a lower value of 29 or so. The power in P-pol was about a factor of 8 larger than that for the S-pol. Note that this is opposite to what Livingston observed (G1501374-v1) where the S-pol was bigger than the P-pol. Back-propagating the measured power to those at BS's AR surface (the ones propagating from ITMX to BS), we estimated the power ratio to be Pp/Ps ~ 2500. This separation ratio is better than what has been measured at Livingston (G1501374-v1) by a factor of roughly 13.
[Another observation from Jan 31st for comparison]
I also looked at a similar data set from Jan 31st of this year (25442) to see if the polarization in the past behaved in the same way or not. This data was with a 20 W PSL without the HPO activated. The behavior looked similar to what we have observed last night -- a slow decay in the S-pol and P-pol was larger than the S-pol by a factor of 6-ish. See the attached below.
Matt later pointed out that there is a possibility that my measurement set up could be unintentionally rotated with respect to interferometer's polarization plane. In this case, depending on the rotation angle, the S-pol can appear to decrease even though the actual S-pol in the interferometer increases. I did a back of envelop calculation and confirmed that the measurement setup needs a rotation of about 20 deg to get such confusion [ angle = atan(sqrt(1/8) )]. I don't think we have such a big rotation in our setup. So it seems that the S-pol really decreases at the beginning of the lock stretch.
Here are some photos of our set up.