I turned on both the lasers and TECs, and cleared out all extra optics/components from around the experiment.

When I re-checked the cavity alignment I saw a large beam off the side of the main beam when the main beam was not blocked, both on the third side (Mc -> Ma). I tweaked around with the alignment quite a bit and even though the beams seemed to overlap along one side (Ma -> Mb). I could not really check the third side properly.

The transmission PD shows that it is receiving power from more than one round trip as checked by blocking the outgoing light from optic Mc, but I'm still not sure if there is interference.

I also measured the beam profiles at different points on the path (will add the data later). but in summary,  the beam seems more or less collimated everywhere with a beam diameter of 2.4 mm.

[Rana, Shruti]

The fiber launch PAF2A-18C seems to have an output waist of 3.49 mm. We are ordering fiber launches with smaller waists, but in the meantime it was previously adjusted to have minimum divergence. Today Rana adjusted the three tip-tilt/ focus knobs to get the smallest waist before the entering the cavity, but it turned out result in the huge divergence within the cavity, then it was re-adjusted to a larger beam with lesser divergence, close to what it was initially.

We also experimented with a lens, but then decided against installing one after seeing that it did not help very much.

The transmission PD reading may be noisy/ somewhat incorrect because of scattered visible light.

QUESTION: Do we have filters for 1550 nm that blocks shorter wavelengths well?