Marc, Matteo

Last Friday, we decided to slightly move the optical board hosting the birefringence readout part in order to be able to close the enclosure to reduce possible effects of scattered light.

The flipping mirror to switch to absorption measurement is not reinstalled but it should be straightforward. We checked that it is still possible to move the absorption imaging unit translation stage on all its range with this configuration. nm path but the sensor was too small.

We tried to use the photodiode for the 1310 nm path but the sensor seemed too small (1mm diameter vs 3.6 for the ones we installed). So we used a thorlabs power supply labelled 'broken' for our Thorlabs photodiode and it seemed to work fine (maybe just too noisy?)

In any case we plan to buy 2 new photodiode and 3 new powersupply.

We connected these 2 photodiodes to the lockin amplifier and could get signals.

With 2 optical density (2 and 3) there was no saturation.

We moved the HWP angle between the 2 lenses and confirmed a maximum and minimum for the photodiode in reflection of the PBS separated by 45 degrees.

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Today we started by changing the imaging unit PBS post by a more stable one and realigned the 2 photodiodes. This time, the photodiodes have a small angle to limit back-scattering.

Then, we installed walls on the birefringence board before putting the large enclosure and closing its 4 walls.

Then, we tuned the QWP angle so to be at a maximum on one of the 2 photodiode.

Then, we did a scan of the motorized HWP  with incident pump power around 30 mW) and results are reported in the attached figure.

min p pol = -0.16 mV

max p pol = 0.606 V

min s pol = 0.12 mV

max s pol = 0.588 V

where the offset without pump beam is already taken into account (p-pol = 0.6 mV, s-pol = 12 uV)

I guess we could try to add more shielding for the p pol photodiode (ie in transmission of the PBS).

Notes :

Be careful when acting on the QWP as its holder is not so stable !

Only use jog function for motorized HWP otherwise it will reset the current angle value to 0