J. Oberling, E. Merilh, J. Bartlett
Progress report on the last 2 days of NPRO swap work.
Day 1
Day 1 was more of a prep day than anything. In the morning we began by shutting down the HPO and 35W Front End (FE) lasers. We then turned on only the NPRO and measured the current output power; measured 1.35 W. We then checked the current alignment through the 2 irises in the 35W FE, and found them both to be off; both were off to the right (if looking in the direction of beam propagation) by 1-2 beam diameters. In addition, the beam was low on iris 1 by ~1 beam diameter and high on iris 2 by ~1 beam diameter. We set 2 additional irises in the beam path to capture the current beam alignment, seeing as how the pre-installed ones were so far off. We then proceeded to check the current alignment through the irises in the HPO (each mode matching lens has an iris attached to it). The beam was slightly low and to the right on the iris on MM1, and very slightly high on the iris on MM2 (once again assuming one is looking in the direction of beam propagation). At this point we broke for lunch. The afternoon was devoted to the install of the new PSL water manifold, detailed here.
Day 2
Part of the morning was spent finishing up the manifold install. Once that was completed, we checked the NPRO beam alignment through the EOM and the Faraday isolator (FI) in the FE. We found the beam low and to the right on the EOM, almost clipping on the bottom of the entrance aperture. It was hard to judge how the beam was going into the Faraday, but on the output side it was exiting high by ~3-4 mm. I then installed another iris at the end of the mode matching rail in the HPO; not sure how useful this one will be, but it is at least capturing the current alignment. I would have liked to install a 2nd one, but there was no room in a space that would be visible. At this point we began discussing how to remove the NPRO without dismantling the FE box.
We ended up taking off the side panel by the NPRO (to access the clamp on the back leg of the NPRO) and removed 4 screws from the back panel of the FE box; we also removed the PD that monitors the NPRO power. We then carefully bent the back panel just enough to give us room to maneuver the NPRO out of the box. We removed the feet from the old NPRO and installed them onto the new one. The new NPRO was then slid into the FE box, being careful to not get caught up in the cables that route underneath it. We then broke for lunch and the 1pm safety meeting.
Once back in the PSL enclosure we hooked up the new NPRO (S/N GDP.1235944.7974), using the old power supply and cable, and set the settings on the power supply to match the data sheet for the new NPRO. For the record, these are:
The NPRO was then turned on, and there was light (yay!). After a warm up period, we measured the output power of the NPRO to be 1.864 W. To account for differences in output polarization between the 2 lasers, we then adjusted the quarter- and half-wave plates that sit directly in front of the NPRO to maximize the output power. The most we could get was 1.894 W. For alignment purposes, the operating current of the NPRO was then reduced to 0.900 A, which yielded an output power of 52 mW. The beam alignment was checked at the EOM, AOM, shutter, and FI: the beam was low and left on the EOM, low and right on the AOM, high on the shutter, and did not reach the FI (best I can guess it was bouncing off the roof of the shutter and scattering out). Not too bad for a rough "by eye" placement. The NPRO position was then carefully adjusted until the beam was horizontally centered on the EOM. Due to beam height differences between the 2 NPROs (no 2 are exactly alike), the only way to adjust the beam height on the EOM was to either move the EOM or the lens responsible for focusing the laser onto the EOM. I elected to move the lens, and centered the beam on the EOM entrance aperture. The beam was now low and centered on the AOM, and high on the shutter (but not as high as previously); a slight output through the FI can now be seen, so, progress! We called it a day at this point, alignment will continue in the morning.