[Torrey, Daniel, Sander]

While trying to lock a readout filter cavity using a setup that enabled reliable and stable locking before, we noticed mode hops (i.e. we saw different modes flashing in the cavity in random succession) during scanning and locking. There was also a much larger drift of the error signal than previously observed. I suspected this could be due to mode hops in the seed laser, caused by back-reflection into the seed laser. A seed laser output beam pick-off after the fibre-PBS is currently used as input for a Michelson setup, and this output has no Faraday isolator to prevent back reflections. Blocking this output with a beam dump resolved all problems and enabled locking of the filter cavity as before. 

Going forward we should install a Faraday isolator at the fibre output or only pick off beams after the amplifier's Faraday isolator.