Matteo, Eleonora, Simon

We have changed the setup of the absorption bench so to be able to measure S and P polarization of the transmitted beam.
The basic point in the new setup is that we are using two photo-sensors (PSDs, to be more specific) and a PBS together with a lens to focus and split the beam.
Since the measurements are very sensible to the setup, we needed to take several aspects into account:

• Incoming beam needs a good initial alignment in terms of polarization
  •  another PBS and a HWP were installed
• The PSDs are too sensitive for the pure transmitted beam
  • A stack of ND filters were installed between lens and PBS in the outcoming beam-path
• The setup is extremly sensitive to stray-light
  • A beam-tube is covering the PSD in the lateral beam-path from the PBS (S - polarization)
  • A beam-dump (Vantablack) is catching the reflected beams from the ND-filters
  • A razor-blade beam-dump is covering the space btw. beam-tube and ND-filters (catching scattered light)
  • A couple of obligued ND-filters is placed at the blind-side of the PBS
  • A black light-cover is placed on top of the lens to catch some ghost-beam reflections coming from the top of the sample

Especially the stray-light suppression is challenging as there are a lot of possibilities where scattered light can enter the sensors and it may be that we still find new sources.
For example, one main source of stray-light was a ghost beam created by a probe-prism which is reflected by a lens for the incoming pump-beam, and this reflection is entering the test-mass and internally reflected on the upper boundary and eventually entered the P-pol sensor from above (shielded now - see pictures)

Electronics
We are using basically the same system as already given by the absorption-bench setup.
However, we had to do some modifications:

• As we have only one channel to demodulate an AC signal on the lock-in, we have installed another lock-in to demodulate the signal from the second PSD (using the same reference signal)
• The power for the measurements is reduced to be in the mW region (2A input current)
• Of course more cables and power-sources needed for the sensors