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AdV-SVS (Pre-commissioning)
delaurentis, sequino - 12:52 Sunday 12 September 2021 (53129) Print this report
BAB spurious p-polarization measurement on EQB1

We measured the amount of p-pol polarization that enter in HD_IR_PD_MONI photodiode:
dark 0.003 V
LO p-pol 0.016 V over 1.268 V 
BAB p-pol 0.09 V 1.254 V

the BAB has about 7% of spurious polarization reaching the fotodiode. This means that, in the calculation of the visibility, we must take into account this contribution. 

In order to investigate where this contribution comes from we also measured the p-pol component after the Faraday Isolator and after the mode matching telescope. 

Measurements performed with the power meter, placing a PBS along the beam path: 

  • after the Faraday Isolator

p-pol 0.0023mW over 0.620 mW (0.5uW dark)

  • after the MMT 

no significant p-pol power measured

Moreover we observed that any spurious polarization was introduced by the homodyne beam splitter. 

We have to investigate on the dalay-line mirrors to see if any of them is responsible for this spurious polarization. 

NB: all the measurements were done with 0.633 mW of BAB power entering the homodyne detector (0.310 mW towards PD1 and 0.319 mW towards PD2). At the 11:07 we increased the BAB power at its maximum value. 

Comments to this report:
sorrentino - 12:17 Tuesday 14 September 2021 (53141) Print this report

Yesterday I checked again the polarisation of BAB and LO beams entering the homodybne detector. I obtained different results than in entry 53129.

I placed a power meter on the beam reflected by HD_MIR, the BAB reflected from the HD BS and/or the LO transmitted through the BS. To measure the fraction of p-polarisation, I inserted a PBS in front of the power meter.

With the LO at maximum power (about 2 mW on HD, i.e. 1 mW on power meter) I measured a residual p-polarisation of about 0.05% (i.e. 500 nW over 1 mW of s-polarisation).

With the BAB at maximum power (about 2 mW on HD, i.e. 1 mW on power meter) I measured a residual p-polarisation of about 1.3% (i.e. 13 uW over 1 mW of s-polarisation).

Then I placed a waveplate at different locations along the BAB path, and I rotated the waveplate to minimise the PBS transmission. The PBS and power meter were at the same location as before, i.e. after HD_MIR.

By placing the waveplate before the delay line I could reduce the residual p-polarisation component to 0.2%.

Placing it after the first or the second mirror of the delay line I could reduce the residual p-polarisation component to 0.3%. I could reduce the residual p-polarisation component to 0.2%.

By placing the waveplate after the third mirror of the delay line I could only reduce the residual p-polarisation component to 1%.

I observed that the beam is not pefectly in plane along the delay line, i.e. the height w.r.t. the optical table changes by about 2 mm p-p along the path.

I left the waveplate between the first and second mirror of the delay line, tuned to minimize residual p-pol at HD.

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