After last week's odd behavior for the BS OpLev after a power adjustment, today I inspected the optical lever setup and took a look at the laser.

I found no issues with the setup.  The fiber was properly secured with no crimps, kinks, breaks, or crushed areas.  I briefly opened the transceiver box to look at the fiber run, the launching telescope, and the QPD and found no issues there; everything looked good with no signs of damage or any obvious component failures.  With the setup seemingly good to go, I next looked at the laser.  First thing I did was use a power meter (an Ophir Vega with a 3W-capable stick head) to measure the power out of the laser, and immediately found the problem:  The laser had failed, but was not completely dead.  It was still outputting a small amount of power, 0.34 mW, but changing the adjustment knob did not change the output power (the knob changes the amount of electrical current delivered to the laser diode); it remained at 0.34 mW regardless of knob position, until the current was too low to support lasing and the output power dropped to 0.  I don't have any guesses right now as to why the laser failed with a simple power adjustment, but the fact remains that it did.

I swapped in a spare laser to replace the old one; old laser is SN 258, new laser is SN 120-3.  I set the new laser to output ~1.5 mW, which corresponds to 0.770 V on the monitor port for the laser diode current.  I chose this because upon scanning through the full range of the laser's output power in the Optics Lab yesterday, this setting seemed to have the least amount of glitching.  With this new laser the SUM counts were reading ~47k; each QPD segment was reading ~12k counts, which while OK (it's not saturating) is much higher than we typically operate this optical lever at.  I went to the Output Configuration Switch (OCS) at the BS OpLev chassis in the biergarten and adjusted the whitening gain from 9 dB to 0 dB; the SUM counts now read ~16.5k, closer to where this OpLev has been in the past.  I've updated T1500556 to -v8 to capture this change in the BS OCS state.  The laser will take a few hours to reach thermal equilibrium in its new home, then we can start monitoring it for glitching.  We'll keep an eye on this over the next several days.

I have the old laser, SN 258, in the OSB Optics Lab.  I'll take a look at it in the coming days to see if I can find an obvious cause for the failure, but this laser will get returned to the vendor for repair.  This completes WP 11561.