jeffrey.kissel@LIGO.ORG - posted 16:03, Wednesday 16 April 2025 - last comment - 11:35, Monday 21 April 2025(83961)
SPi Pickoff Path Install Day Two
J. Kissel scribing for S. Koehlenbeck, J. Oberling, R. Short, J. Freed ECR E2400083 IIET 30642 WP 12453 Another quick summary aLOG at the end of the day, with more details to come: - With the power in the ALS/SQZ pick-off path to 10 [mW] for beam profiling, - Installed a two lens system to handle the unexpectedly different beam profile of the ALS/SQZ pick-off path - Remeasured the resulting mode after the two lens system, and we're happy enough. We're gunna call them SPI-L1 and SPI-L2. - Installed steering mirrors SPI-M1 and SPI-M2. - Rotated ALS-HWP2 to increase the s-pol light in the ALS/SQZ/SPI path to return the power transmitted through SPI-BS1 going to the ALS/SQZ fiber collimator back to 50.5 [mW]. This set the SPI path to 186 [mW] with the PMC TRANS measured at 103.5 [W]. The ALS_EXTERNAL PD in transmission of ALS-M9 measured 31 [mW] ***. - Installed SPI-HWP1 and SPI-PBS01 - Measured the power at each port of SPI-PBS01, with the intent to optimize the SPI-HWP1 position to yield maximum p-pol transmission through SPI-PBS01. *** We expect this is lower than the goal of ~45 [mW] (from LHO:83927) because we've not yet re-aligned the ALS/SQZ fiber collimator path after the install of the SPI-BS1, which translates the beam a bit due to the thickness of the beam splitter. We intend to get back to this once we're happy with the SPI path.
Small correction to above is after installing SPI-HWP1 and SPI-PBS01, we adjusted HWP1 to have 20mW in transmission of PBS1 (not maximum quite yet) to start alignment into the fiber. Using the two steering mirrors downstream of PBS1 and the collimating lens in front of the fiber, Sina maximized the transmission as measured with the output of the fiber on a spare PD. We then took power measurements of the input and output of the fiber:
This is a good start, but with a target ratio of >80%, there's still more work to be done here improving the beam into the fiber collimator. Out current mode-matching solution claims we should have 95% mode overlap into the fiber, so hopefully the issue is alignment, but it's entirely possible we'll revisit the mode-matching to see if improvements can be made there too.
The attached photo represents the optical layout as it stands as of where we stopped today, with the new SPI fiber in blue on the left (north) side of the table.