J. Kissel, S. Koehlenbeck (in person!)
D2400107 for the annotated layout of SPI Pathfinder ISIK's Transceiver Assembly.

After an easy recovery from other-the-weekend, site-wide power outages (LHO:90413), we resumed last week's investigations of what might be causing the MEAS IFO to drift (LHO:90382).

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Executive Summary: No magic bullet found. Today we ruled out the collective idea we all had that the picomotor-actuated mounts were the problem (LHO:90384) by replacing every pico-actuated mount with manually driven mounts. We also replaced the last optic we hadn't swapped in the MEAS path to the MEAS IFO -- M_B1. No impact -- we still see the MEAS IFO alone drift.
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It was only a half-day of forward investigative progress given the power outage, but having Sina on-site really helped confirm my (in)sanity and accelerate investigatory iterations.

Upon arrival in the lab Sina re-confirmed my construction and alignment of the D2400107-v5 board and the quality of things:
    - Centering on optics with IR card :: good.
    - Centering on photodiodes with IR camera :: good.
    - No stray beams (even from what would be AR coatings of beam splitters) :: good.
    - The as-we-left-off Friday assessment that with M_M2 and M_B4 down-graded to manually driven mounts, we still see drift.
    - Having one or the other REF or MEAS beam into the MEAS IFO blocked, that the power on each beam is stable without drift.
    - After an alignment drift, quickly touching the alignment of all three M_M2, M_B4, and M_M1 cannot recover the MEAS IFO efficiency.

As we continue to grasp at straws, we replaced the last optic in the MEAS path into the MEAS IFO -- M_B1. This optic splits the MEAS beam on the board feeding it to the MEAS and REF IFOs, so we were hesitant to swap it. But now that we're getting so good at re-aligning the board after changing out optics (once optic is placed, re-alignment can be done within 2-5 minutes), we figured we must. 

So, all at the same time, we
    - unlocked all manual drive set-screws on all optics (recall the REF path had been "locked down" the day I'd gotten ~95% efficiency),
    - loosened and re-tightened R_B1's 1/4-20 bolt that secured the mount to the breadboard. It wasn't "loose" but it wasn't "cranked on" either.
    - Replaced M_B1 optic 
    - Replaced the M_B1 manual-drive pitch actuator's 1/4-100 alignment screw and bushing because it was "crunchy" and "stiff" to use. This had been this way, but when I was driving solo I'd suffered thru it and locked it down the day I got 85% efficiency on the MEAS IFO and didn't look back. Now what we're touching the mount anyways, we figured we might replace it in case the issue was some sort of stick-slip thing happening.

Upon doing all these things ... things looked solid for ~10 minutes... and then the efficiency started to drift again. Especially after turning on the Clean Room fans.

This again resurrects the theory we thought debunked: "it must be the environmental conditions..."
Why did we rule this out before?
    (1) The REF IFO does not see any drift. Ever. The board is so small and so local, and both IFOs are sampling the same beams. *Very* hard to believe that the few differences in paths and path lengths would cause only ONE of the IFOs to drift. The environment should impact both IFOs equally.
    (2) We *do* see phase noise when we make environmental changes -- as expected. Path length differences caused by
         - physically manipulating the board, its mounts, or optical fibers,
         - waving hands over the board
         - talking, even at a medium volume
    (3) Any environmental change in the efficiency we can cause -- be it: 
         - clean-room fans ON/OFF, 
         - clean-room or room lights ON/OFF, 
         - people coming in-and-out of the clean room, 
         - standing in different positions around the optical table
        -- they all only some-times seem correlated, and never is it reproducible.

But we've run out of things to change, so today we're going to try to adjust the environment and/or other things that *should* be common to the IFOs:
    (a) We're gunna being in an incandescent lamp to try to create thermal gradients
    (b) We'll put on our tin-foil hats and enclose the breadboard in a temporary foil enclosure
    (c) We'll try adjusting the NPRO seed laser controller's parameters (like diode temperature)
...and keep trying all the other wild theories we can concoct.