In order to get a more precise comparison between birefringence (integrated along z) and absorption (at a given z position) measurements, we performed absorption measurements at several z positions.
Note that in that case the shinkosha 7 orientation is still the same as the measurement done by Manuel ie arrow at the top and pointing towards the imaging unit.
All results are attached to this entry where I used same colorlimit as Manuel (0 to 200 ppm/cm) and similar colormap.
Similar patterns are visibles.
However, it seems that maximum absorption is quite lower than what was measured before...
One difference with the previous measurements is that we were using 0.5s waiting time and 70mm radius..
I'm now starting new measurements with differents lockin amplifier parameters to investigate this issue.
Note that the z values indicated here correspond directly to the translation stage values (therefore different than Manuel measurements where he corrected the z value to match the real position in the mirror)
We tried to investigate possible explanations for this discrepancy.
First we performed along z scan to be sure that we are able to see the 2 surfaces of the samples.
We could find S1 at 34.8 mm and S2 at 122 mm along z.
We can see the ac/dc signal decreasing with an increase of z (same as Manuel's measurement) but the signal is roughly half of what he got.
We have the same chopper frequency, we're injecting pure s polarization, but differences are that he was injecting about 10 W vs our 8.5 W, he set the DC to about 2.5 V vs 4V now and in his computation he is using 1.16 /cm instead of the 1.04 /cm later measured and I'm not sure how the transmission was taken into account.
For reference Manuel's measurements and analysis are in the KAGRA#7 folder.
One strong possibility is that we have a too large pump beam size. Indeed Manuel found out that it could cause some factor discrepancy when he upgraded the setup.
We characterized the beam size with the razor blade as reported in figure 1. The beam waist is 48.5 um instead of the expected 35 um.
Following Jammt simulation that indicates that the beam waist of 35 um by moving the last lens by ~5mm we started to realign but without clear improvement so we'll continue on Monday.