J. Kissel scribing for S. Koehlenbeck, J. Freed, and R. Short
ECR E2400083
IIET 30642
WP 12453

Just writing a separate explicit aLOG for this for ease of reference in the future.

Before, during and after the SPI install we measured power along respective paths,
(1) The p-pol in-coming power into the whole ALS / SQZ / SPI pick-off path,
(2) The "ALS COMM" path: p-pol power in transmission of the ALS-PBS01 with the newly modified ALS-HWP2 position to get more power total power in the paths (3) and (4),
(3) The "SPI pick-off" path: The ~s-pol power in reflection of SPI-BS1,
(4) The "ALS/SQZ Fiber Distribution" path: ~s-pol power in transmission of SPI-BS1,
(see discussion in LHO:83978 as to why we're not so confident the reflection from ALS-PBS01 is entirely s-pol, which is why I say "~s-pol" here.)

The BEFORE vs. AFTER power in these paths is as follows:
  Path   Measured Between        Raw Power [mW]  PMC TRANS [W]     Date/Time Measured      Scaled Power [mW]  Fractional Power [%]
  (1)    ALS-L1 and ALS-HWP2         2060          103.2           2025-04-15 21:39 UTC       2095.9               ---

      % BEFORE
  (2)    ALS-M2 and ALS-L2           1970          102.8           2025-04-15 22:17 UTC       2012.2               96%
  (4)    ALS-M9 and ALS-FC2          48.7          103.0           2025-04-15 21:48 UTC         49.646              2%

      % AFTER
  (2)    ALS-M2 and ALS-L2           1790          103.3           2025-04-17 21:13 UTC       1817.7               87%
  (3)    SPI-L1 and SPI-L2            186          103.5           2025-04-16 22:43 UTC        188.7                9%
  (4)    ALS-M9 and ALS-FC2            50.5        103.5           2025-04-16 22:43 UTC         51.232              2%
where I've scaled all the raw power measurements by the rough nominal PMC TRANS power during recent observing times, 105 [W], i.e. 
(Scaled Power [mW]) = (Raw power) * (105 [W] / PMC TRANS [W])
and
(Fractional Power [%]) = 100 * {Scaled Power [mW]; paths (2)-(4)} / {Scaled Power [mW]; path 1}


As Ryan mentions in LHO:83989, for the purposes of safe long term storage, after all was said and done, we rotated SPI-HWP1 such that only 20 [mW] would go toward the SPI-FC1 fiber collimator, and it's dumped upstream just after SPI-L1. These AFTER power levels are what we anticipate running the rest of O4 in, with the SPI pick-off path safely out of commission.

2W Measurements
Head:    Ophir    10A-V2-SH SN122042 
Readout: Ophir    20C-SH    SN171175
Accuracy: +/-3%

200 mW Measurements:
Head:    Thorlabs S401C
Readout: Thorlabs PM100-D
Accuracy: +/- 7%

The attached picture summarizes all this info. The picture was take midday 2025-04-17, so you see the Ophir power meter in the middle of the ALS COMM