Elli, Robert, Bram

We had another look into the RefCav. I think we found the culprit ... the free-space to fiber coupler behind the RefCav is retro reflecting back to the cavity. We use an angle-cleaved fiber cut to remidy this, but not enough. When we block the beam to the fiber coupler, intensity flickering on the CCD is gone. You can still see he peaks on the transmitted diode, but they are most likely intensity fluctuations (which is we 'immune' to).

By the lack of finding any quarter waveplates, we placed a half waveplate, a PBS and a faraday rotator after the first steering mirror behind the cavity to capture any back reflections (from the fiber coupler, photodiode and CCD).

Now we will need to relock the arm cavity to see if it made any improvements ..