Here are two clips recorded when rotating the polarization of the beam in front of the POP camera for the cases when

• ITMX,
• or ITMY

was aligned. Each of the clips is taken by (mostly) uniformly rotating the half-wave plate by about 180 degrees.

Spatial patterns of the S- and P-polarized beam spots are attached as well.

Hey Kiwamu,

I could not get the number of loss at ITMs. In my calculation for ITMX for instance, the loss is 1-3.82/(1455/0.9*0.1*0.5*0.1*0.5) = 5.5 %.
Do I have any stupid mistake?

Did you measure the polarization contents of the forward going beam of POP ?

Also how did you measure the polarization ?

-Hi Yoichi,

A good point. We don't have a measured number for the polarization contents of the forward propagation beam at POP.

We actually looked at the brightness of the S and P-pol. of the forward propagating beam at POP by eyes with a sensor card. The P-pol. light was so dim that we were not able to see it by eyes and therefore assumed that the incoming beam was purely S-pol.  We will try to measure it shortly.

The measurement was done by inserting a PBS plate, PBSW-1064 1 inch from Thorlabs, and measuring the amount light in the transmission and reflection sides at a time using a PDA100A. Since we didn't like to have an extra polarization sensitive optic in the measurement, such as a harmonic separator and S-pol BSs, we inserted the PBSW-1064 before the beam arrives at the periscope. The dark offset was carefully subtracted every time when we made the power measurement. In addition, we paid attention such that the incident angle of the beam on PBSW-1064 to be close to 45 deg although this is done by our eyeball. Perhaps a precision of 1 deg or smaller.

-Hi Shinji,

It is possible that the incident beam was already messed up in the polarization. You are totally right that the BS can introduce asymmetry in between the optical power in reflection and transmission if the incident beam already contained a P-pol content. As stated above, we will be assessing the polarization content of the incident beam shortly.

-Hi Masayuki,

Aha, sorry for the confusion. The trick is that we have assumed the transmissivity of PRM to be 10.35% rather than 10.00%. This 10.35% was a value that Yutaro obtained from Hirose-san through the email-exchange at one point in the past. We don't know how accurate this number is. In fact, this illuminates another issue that our estimation is quite vulnerable against uncertainties in the PRM transmissivity.

-Addendum for those who are interested in the PR2 transmissivity

According to our measurement (628 mV for the forward POP beam using a PDA100A with 20 dB setting), the optical power of the forward-propagating beam is approximately 140 uW. Here is the calculation;

P = 628 [mV] / (0.3 [A/W] * 1.5e4 [V/A]) = 139.6 uW.

Assuming that the IMC trans was 3-ish W. The design tranmissivity of 500 ppm for PR2 gives us an expected power of 150-ish uW.

Therefore, we determined that the PR2 optic was healthy in terms of the transmissivity.

Hey Kiwamu,

THanks, I got it. I was supprised that 0.35 % difference makes the change of loss by factor of 2 or more.

Another question is that is the polarization of backpropergating light linear? If so, the missed-up polarization migh be becuase of the reflection plane issue, and if not, it might be because of birefringence in some optics inside the PRC.