40m QIL Cryo_Lab CTN SUS_Lab TCS_Lab OMC_Lab CRIME_Lab FEA ENG_Labs OptContFac Mariner WBEEShop
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Message ID: 385     Entry time: Thu Jan 26 11:49:45 2012
Author: Zach 
Type: DailyProgress 
Category: Coating Q 
Subject: Parts gathered, etc. 

[Giordon, Zach]

Yesterday, we went to the 40m and stole most of the parts we will need for the Q-measurement polarimeter. I have asked Giordon to put a list of these parts on the elog, as training for good elog routine.

We brought them to the SUS lab and started placing things on the table. The plan is to build a barebones setup, in air, to get a feel for the readout scheme. As a first pass, we will find any old piece of transparent material and drive it mechanically (read: flick it) to see if we see a low-Q ringdown. In parallel, Giordon is supposed to be working on his SNR calculations (and I am supposed to be answering some questions he has).

We set up a HeNe and some polarizing elements, but were somehow unable to linearize the polarization. This is somewhat puzzling:

  • We are using quarter- and half-wave plates that are ThorLabs-labelled (etched) for 633 nm.
  • We are using a 1-in PBS from the 40m that was unlabeled. There were three such cubes on the SP table near other visible optics, so I assumed that they could be from an old SURF HeNe setup that Rana mentioned. Using a red laser pointer, I verified that I could get reasonable visibility (say ~80-90% by eye) by rotating it with respect to the PBS. The effect was much smaller with a green pointer, so I concluded that it was a 633-nm PBS.
  • Setting the elements up in the usual way, I was only able to get contrast of ~30% or so by adjusting the waveplates. Frank looked up the HeNe head part number and found that its output was circular, but of course this is the point of the QWP.
  • We replaced the head with another one (on the off chance that the head was broken and somehow putting out a time-varying polarization), and this had no effect.
  • Frank, Vladimir and I did some independent testing of the polarization optics. We couldn't find a red laser pointer (which was the only source with which I could get the PBS working), so instead we tested them by eye with the (polarized) LCD computer monitors in the lab.
  • All the optics behaved as expected:
    • Putting the PBS between our eyes and the screen, we could achieve very high visibility by rotating it.
    • Leaving the PBS where it was, and adding the HWP between it and the screen, we could achieve very similar visibility by rotating the HWP alone.
    • Switching out the HWP for the QWP, we could set the QWP such that no orientation of the PBS blocked light from the LCD (i.e., the polarization was circularized).
    • It should be noted that we focused our attention to RED images on the screen, and that these trials did not work equally for all colors
  • So, the question is: if every component seems to be working, and switching the laser has no effect, WHAT THE HELL IS GOING ON?


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