R. Short, J. Oberling
Weekend work on the PSL NPRO swap, continuing on from Friday.
Saturday, 10/26
We started today by taking a beam propagation measurement with only L01 installed, to get a better idea of if it was properly positioned as our Gaussian fit for the Amp1 beam was well off what we expected from JamMT. However, the resulting Gaussian fit didn't make sense so we took another propagation measurement. This fit differed from the first, so we suspected either something was up with the profiler or something in the beam path was distorting the beam (or maybe both). To check if something was up with the profiler we switched profilers from the WinCam to the ThorLabs scanning slit; this required a different rail setup since the ThorLabs is larger than the WinCam and would be well above the beam with the rail we were using (we tried mounting it a couple different ways to the original rail we were using, to no avail). We took a profile and got a completely different result for the fit. I'm writing this from home so I don't have the numbers on me at the moment. Going back to the WinCam and looking at the beam shape on the profiler, the beam wasn't exactly good looking (see first attachment). Since we had seen the EOM change beam alignment I tried moving it around a bit while watching the WinCam and sure enough, as the EOM alignment changed the beam shape changed. At this point we broke for lunch.
After lunch we removed the EOM from the beam path and looked at the profiler again, 2nd attachment. The beam was more round and less "stretched." So we took another beam propagation measurement with the EOM removed. This one was different from the rest, and did not match what we expected based on JamMT's results with a f = +222mm lens 100.0mm from the NPRO. Now the suspicion is our initial beam propagation measurement in the lab was somehow wrong. At this point I remembered that while in the lab we needed 1.67A to get ~1.8W out of the NPRO, as measured by one of the small stick power meters; this same NPRO installed in the PSL needs 2.144A for ~1.86W. My guess is the power meter in the lab is suspect, and if so means we were under-pumping the NPRO during the initial beam propagation measurement. Under-pumping the NPRO changes the divergence of the output beam, and therefore our Gaussian fit and mode matching. To check this we set a turning mirror to direct the leakage beam from M01 into a beam profiler and took a propagation measurement. Sure enough, the results differed from what we got in the lab:
- Horizontal Waist: 153.0 µm @ -102.0 mm
- Vertical Waist: 143 µm @ -90.3 mm
Pretty different from what we had in the lab, which meant we had to redo the mode matching again. We were able to find what looked like a good solution, see final attachment. We broke for the day at this point, it had been a long one.
Sunday, 10/27
With new mode matching solution in hand we began by installing it. We checked and corrected the position of L01, then took a look at the EOM. We put a spare EOM in the beam path (with no RF) and it did not distort the beam like the in-service one did. We then put the in-service EOM back in the beam path and sure enough, it stretched the beam as we saw yesterday. Unsure if this is normal (I don't recall seeing this during installation back in 2021), but not knowing RF well enough to feel comfortable messing with it on a Sunday with almost no one onsite, we decided to remove the EOM from the beam path, continue with the mode matching install, and talk to Fil on Monday. We got the mode matching solution installed and aligned and took a beam profile measurement. It took a few iterations, but we were able to get a beam with an average waist size and position where it needs to be for the 4S-HP amplifier. Ultimately we had to move both L21 and L02 25mm away from the amplifier, increase the distance between L21 and L02 by ~3 mm (we had shorted it during install, the distance was supposed to by 86mm and we were at ~83mm), then move L01 away from the NPRO by 2.0mm. The results of our final Gaussian Fit:
- Horizontal Waist: 159.0 µm @ 1791.0 mm
- Vertical Waist: 172 µm @ 1797.0 mm
- Average: 165.6 µm @ 1794.2 mm
- Target: 165.0 µm @ 1794.0 mm
We broke here for the day. Tomorrow we will talk to Fil about the RF so we can take a look at the spare EOM and will then continue with Amp1 recovery.
