I’ve done some work on IOT2R in preparation for the Schnupp asymmetry measurement:

• Just so we’re clear on the optical path: periscope → 2” steering mirror → 2” lens → 1” partial reflector → PD (in transmission of the PR). The reflected beam from the PR is dumped.
• I removed the NF1811 on IOT2R. It is now in cabinet 11 in the optics lab. I also had to swap out the power supplies because the PDs require different connectors.
• I moved the partial reflector slightly closer to the lens, so that the PD can sit at the beam waist.
• I forked down the NF1611 and aligned the beam onto it. The dc voltage is −728(4) mV (the 1611 has a negative dc output, while the 1811 output is positive).
• I also measured the power with the Ophir meter. There is 0.87(1) mW total power on the PD. With the aux laser blocked, there is 0.16(1) mW. This is well below the PD’s damage threshold.
• I have not looked at the PD’s ac output. If we find that it saturates in the presence of the beat note, I suggest we turn down the aux laser power rather than stick in an ND filter.
• I also cleaned up some miscellaneous optomechanics that were lying on the table.

For the time being, I’ve turned off the PD and dumped the beam immediately after the periscope.

Next steps:

• Re-establish beat note between PSL and aux on IOT2R. Re-establish PLL to lock aux to PSL. If necessary/feasible, implement slow digital control of aux temperature.
• Install another NF1611 on ISCT1. If feasible, install it in a way that does not interfere with REFLAIR_B, so that 3f locking can proceed in parallel with this measurement. Before installing the 1611, check the beam's power level with a power meter. The manual says the damage threshold is 10 mW, and there's probably no point in having anything above 1 mW.
• Clean up the rest of the miscellaneous optics and optomechanics that are on IOT2R.