aLIGO LHO Logbook

H1 SPI (CDS, PSL)

jeffrey.kissel@LIGO.ORG - posted 12:56, Wednesday 25 March 2026 - last comment - 10:15, Monday 30 March 2026(89642)

SPI Fiber Patch Cable from PSL to SUS-R2 Installed; Verified Excellent Transmission

F. Clara, J. Kissel, S. Koehlenbeck, J. Oberling, M. Pirello
D2400110

Today we picked up where we left off with the install of SPI into H1. 
Where we last left things, we'd installed a new SPI pick-off of ALS/SQZ beam in Apr 2025 (see ECR E2400083 and results in LHO aLOGs 83989, 83996, 83978). Back then, we had ended the work with the input to the fiber collimator within the PSL dumped.

With Jason and Sina in the PSL, we confirmed that the SPI path was still blocked. 

However, we also realized/remembered/confirmed that the entire ALS/SQZ/SPI path had 25% less power -- We've been running the PSL at lower power allocation downstream of the PMC since Sep 2025 to prevent issues we'd found with the currently installed EOM after a power outage triggered a dust monitor to spew out dust into the PSL (see that saga in e.g. LHO:87109 LHO:86966). They found the power at the SPI pick-off was 140 [mW] instead of the 188 [mW] we left in Apr 2025 (see LHO:83996).

(Using labels in the half-up-to-date drawing D1300348)
Jason and Sina rotated ALS-HWP2 upstream of ALSPBS01 to restore the nominal 50 [mW] into the ALS/SQZ pick-off and ~200 [mW] (190 [mW] measured). This means there's ~50 [mW] less out to ALS / ISCT1 than before today.

Then with the SPI pickoff still dumped, we installed a 30 [m] patch cord*** from the PSL optical table, out the mouse hole between the +X wall of the PSL enclosure and HAM1, then up running along the upper racks to waterfall down at SUS-H2. The fiber sits within the typical orange tubing. Per D2400110, this is SPI_PSL_001, and it's labeled as such on both ends.

After install, I connected the SUS-R2 end to a Thorlabs S121C power meter with S120-APC2 fiber adapter. 
With this installed (making the system laser safe at SUS-R2 end), Jason/Sina unblocked the SPI pickoff input. 
With 190 [mW] in, we measure 187 [mW] out on the other end. 98% transmission, pretty excellent. Almost unbelievably excellent but we weren't rigorous with our uncertainty and systematics.

Happy with this result, we then blocked the SPI path again, and re-capped the SUS-R2 end for final dressing in the racks.
We'll unblock again when we're read to connect it to the Laser Prep Chassis.

***Patch cord details:
Manufacturer DIAMOND
DIAMOND Part Number: ENS/1094388
Customer Part Number: 9711228
Patchcord SM L=30 PM
2xFC 2mm APC (i.e. 2mm narrow key FC/APC on both ends)
tran 6,6/125/245 PAND 980nm

Comments related to this report

jeffrey.kissel@LIGO.ORG - 10:15, Monday 30 March 2026 (89693)SPI

Link

J. Oberling, S. Koehlenbeck
2026-03-27
#BelatedaLOG

During this power measurement, I made the rookie mistake of overlooking the PM100D power meter console's laser wavelength setting -- and not taking a picture of the display during the measurement.

Today, we both 
(a) retook the measurement at 1064 [nm] with 189.3 [mW] input, and found 173 [mW] output.
(b) confirmed that at 532 [nm] the output read as 188 [mW].
 
For now we set the nominal power into the laser prep chassis as 173 [mW].

After this measurement, we took this same power meter (S121C) and fiber adapter (S120-APC2) into the optics lab and instead used the fiber-coupled NPRO we'd been using to test ISIK in there. We measured the power out of the fiber
    (i) with it fiber-coupled in the same way as the SPI_PSL_001 measurement, and
    (ii) using an addition PAF2-5C collimator to project the beam into free space on to the power meter.

We set the FC-NPRO's power to 177 [mW] in the (i) configuration, but then measured 140 [mW] in the (ii) configuration.
This leads us to suspect that the S120-APC2 + S121C system -- a reflective Si diode, with a shiny metal adapter -- is errantly reporting more power than there really is. We'll repeat the measurement of SPI_PSL_001 another day with a thermal power meter to arrive at our final number.

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