G. Mansell, D. Sigg, J. Oberling, R. McCarthy, D. Brown, F. Clara

ECR E1900246

WP 8363

Today we switched the ALS fiber distribtion chassis at the ISC racks. We plugged the new PSL pick-off into the chassis and have confirmed that the end-station and squeezer PLL's can lock.

Pick-off optical layout

The new pick-off is from the ALS-bypass path, which is after the PMC inside the PSL. Previously Peter K installed a PBS and half waveplate in the ALS bypass, in preparation for the pick off. Last week Jason and I added an f=150mm mode matching lens and coupled ~82% of this light to the fiber which goes out to the ISC racks (specifically ISC R4). I have updated the PSL layout document to reflect this change. And attach a photo of the beam path. Looking at the photo made me realise that we need to update the sticker on that lens mount, it is incorrectly labelled as f=200mm.

We have 50 mW on the pick-off path, with 42 mW transmitted to the chassis. The previous path had only 10mW reaching the chassis.

Jason used the Ophir Vega 10A-V2-SH (SN 122042) to measure the power on the ALS bypass before and after the pick-off:

· Before pickoff: 1.07 W

· After pickoff: 1.01 W

Chassis modification

The old ALS path was on transmission of the ref cav, and so had a 160MHz frequency shift relative to the main PSL beam, caused by the double-passed ref cav AOM. Inside the fiber distribution chassis (D1200136) was another 160MHz AOM to undo that frequency shift before the beams were distributed to the end stations. The squeezer beam did not go through the chassis AOM so the 160 MHz shift could be used as a beat note to lock the squeezer laser. The new path in the PSL does not see the ref cav AOM and so is at the PSL carrier frequency. I modified the fiber distribution chassis so the end station beams no longer go through the AOM, while the squeezer path now goes through the AOM. An additional port was also added to delivier light to ISCT1 for Dan's phase camera. Previous and current design splitting ratios below:

I have updated the chassis test procedure (E1200140) and test results (E1201087) for the new design.

After installing the new chassis Dan and I measured the power out of each port at the chassis. with 42 mW incoming power from the PSL.

Re-locking PLLs

After the chassis switch I had to re-optimize the fiber polarization paddles to get sufficient PSL light to the end stations. I had tell the end station phase locked loops to ignore the PSL conditions (usually it checks if the ref cav is locked) and with this the X-end PLL (locking the end station ALS laser to the PSL) locked by itself. We will need to modify the PLL contions to check if the PMC is locked.

The Y-end PLL had a problem where the RF beat note power was too high (this was a sneaky problem because the error message reported that there was a noise eater error, but Sheila found the true problem). We increased the max RF power and then it locked. Previously the maximum beat note strength was 6 (... dBm?), Sheila increased it to 8, since the new value is between 6 and 7.

The squeezer TTFSS was trickier to lock. Sheila and Nutsinee had to hand-tune the laser crystal frequency to find the lock point, it was ~300MHz from the expected frequency. This could either be due to temperature excursions in the PSL enclosure, since we had a brief incursion today, or maybe I have put the AOM in the chassis in backwards. The latter problem can be fixed in software.