Katsuki, Marc

Recently we found out that there are large stripes visibles in the birefringence measurements.

Furthermore, if we check either the raw or normalized s and p polarizations power, their sum is not at all constant (see figure 1).

We installed a power meter at the beginning of the imaging unit and measured the pump power without sample (fig2) with sample (fig 3) and with sample moving as a birefringence measurement (fig 4).

The power is not stable at all...

While we could see something like +/- 1 % fluctuations during the polarization calibration, we can see that the long term peak to peak fluctuations seems to be around +/- 10 % during 30min and +/- 20 % during a birefringence measurement.

This explains why we see the stripes and why the sum of s and p polarizations power is not constant during a birefringence measurement.

Note that this large power fluctuations does not affect drastically the birefringence measurements as we don't see the stripes in them.

Furthermore, it seems that these power fluctuations are far more important at low power compared to high power (few watts for absorption measurements) because we can not see the stripes in these measurements.

Another by-product of this measurement is that we could estimate the background level of each PSD to be about 1uV.