Since clamping the cantilever, I began to align the triangular cavity with the cantilever.
Attachment 1 shows the setup of the cavity with the input coupler at M1, curved mirror at M2, and a cantilever with a flat optic at M3. The cantilever does not possess any knobs for fine adjustment and its initial mounting is fixed. M1 and M2 can be adjusted, but to get to TEM00 resonance, and any fine alignment, the two steering mirrors before the cavity were used.
Weird things happen while aligning:
1. While aligning, I began to see resonant peaks corresponding to the cantilever's mechanical resonance showing that the cantilever has a very high Q. I mistakenly thought the cavity was close to being aligned to resonance in this configuration while also being very stable, but later understood that the strong oscillations actually seemed more like accidental misalignments creating an optical lever. The green traces in the videos are the measured transmitted light through the cantilever. When the cavity is really aligned close to the TEM00 mode the transmission looks like the green trace here, and the reflection (from M1) is the blue trace.
2. Another time I thought it was aligned I was looking at the camera at the transmission of the cantilever [M3 in Attachment 1] and could see a single flashing spot but when I moved the camera around I actually ended up seeing two beams instead, simultaneously flashing. Minor changes in the alignment resulted in the beams individually and simultaneously turning into higher order modes. Initially I thought that the situation was as depicted in Attachment 2 and found that it was a consistent geometric solution, but later Koji pointed out that was the geometry that creates the TEM01 mode. In Attachment 2, the initial beam (red, M1->M2) overlaps not with the beam in that path from the second round trip (yellow, M1->M2) but with the one from the third round trip. Was it possible that this misalignment was too severe that instead of being TEM01 two separate, but identical, resonant cavities were created? A measurement of the FSR, which I did not do, would have proved that that was the case.
Since the transfer of fiber components into the box, I have re-aligned the cavity to the TEM00 mode. Note: replacing the fiber at the fiber launch and also changing the polarization at the fiber launch moves the beam around.
PDH locking stuff:
[more information and data coming soon]
1. The PDH signal seems too low, possibly because the LO and RF relative phase must be re-adjusted.