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

During yesterday's initial work installing the SPI pick-off path (LHO:83933), the first optic placed was SPI-BS1, the 80R/20T power beam-splitter that reflects most of the s-pol light towards the new SPI path. The pick-off is to eventually be sent into a SuK fiber collimator (60FC-SF-4-A6.2S-03), so we wanted to validate the beam profile / mode shape of this reflected beam.


The without changing any power in the ALS/SQZ/SPI pick-off path, the power now reflected from newly installed SPI-BS1 measured ~40 [mW] (see LHO:83946). This is too much for the WinCam beam profiler, so they used ALS-HWP2 to rotate the polarization going into ALS-PBS01, and thus reduced the reflected s-pol light in this ALS/SQZ/SPI pick-off path to ~10 [mW]. That necessarily means there's a little more of the ~2 [W] p-pol light transmitted and going toward the HAM1 light pipe, so they placed a temporary beam dump after ALS-M2 so as to not have to think about it.

The they set up a WinCam head on a rail and gathered the beam profile. With the WinCam analysis software on a computer stuck in the PSL, they simply gathered the profile information which I report here: 
# Distance[cm]	Radius[um]   Radius[um]
                   X             Y
    0.0           680.5         717
    17.78         465           504
    25.4          389           428.5
    30.48         346.5         368
    38.1          281.5         300.5

where "X" is parallel to the table, and "Y" is orthogonal to the table. The "0.0" position in this measurement is the "front" of the rail (the right most position as pictured in the attachment), which is Column 159 of the PSL grid. SPI-BS1 has the center of its reflective surface is set in +/- X position in Column 149 (within the existing ALS-PBS01 to ALS-M9 beam line). It's +/- Y position is set to create a reflected beam line along Row 30 of the grid, and the WinCam head and rail are centered in +/- Y on that Row to capture that beam.

Using this profile measurement, we find it to be quite different than expected from when this path was installed circa 2019 (see e.g. LHO:52381, LHO:52292, LHO:51610). Jason shared his mode matching solution from LHO:52292 with us prior to this week, and I've posted it as a comment to that aLOG, see LHO:83957.

We think we can trace the issue down to an error in the as-build drawing for the PSL:
- the whole beam path running in the +/-X direction from ALS-M1 to ALS-M2 is diagrammed to be on row 23 -- however, we find in reality, the path lies on row 25. That's 2 inches more between the (unlabeld) pick-off beam splitter just prior to ALS-M1 and ALS-M1 itself. Easily enough to distort a mode matching simulation.
- Jason confirms that he used the *drawing* to design the lens telescope for this ALS/SQZ fiber distribution pick-off path.

More on this as we work through a lens solution for the SPI path.

As of this entry, we elect to NOT create a new solution for the whole ALS/SQZ fiber distribution pick-off i.e. we *won't* adjust ALS-L1 or ALS-L5 in order to fix the true problem. But, we report what we found in the event that a case is better made to help mode matching and aligning into the ALS/SQZ fiber distribution pick-off easier -- as we have verbal confirmation that it was quite a pain.
For the record the fiber collimator used in the ALS/SQZ distribution pick-off is a Thor Labs F220 APC-1064.