J. Oberling, E. Merilh, M. Heintze

Today we looked into the scatter issue noted yesterday.  We removed the new FE pick-off and looked at the scatter again, see attached pictures.  As can be seen, still a lot of scatter.  The going theory is that we are not optimally mode matched from the NPRO to the MOPA, and the scatter is being caused by amplified spontaneous emission (ASE) due to the unused gain from the mode mis-match.  Since the FE is running good, we have good visibility to the PMC, and the mode scan from the most recent DBB measurements indicated low higher-order mode content in the beam, our solution will be to create a spatial filter to block this scatter; this will likely be installed in front of WP02.

We then reinstalled the new pick-off and optimized the alignment, being sure to tilt it off enough to avoid saturating the PD.  It is currently not calibrated, we will have to do that at some point in the future (all PDs that measure power will be re-calibrated at the end of the install).  There was a request to take a transfer function of the FSS out to 10MHz to investigate a bump at ~5MHz noticed at LLO.  Thinking this would be an quick thing to do (famous last words...), we decided to get it in before lunch.  Unfortunately, the FSS would not lock.  After playing with several settings for tens of minutes, Matt was finally able to get the FSS locked.  We then set about tweaking the alignment, which was actually a good bit off.  At this point we broke for lunch.

After lunch, having abandoned the "quick" FSS TF to make progress on the 70W install, we began setting up for measuring the beam caustics of the FE for mode matching.  WP02 was re-installed in the system (we had to remove it to fit the Wincam for beam profile measurements during the installation of the FE pick-off, AMP_WP01 and AMP_PBS01).  We then removed all of the unnecessary optics in the FE DBB path (M09, M10, CCD01, CCD02, L05, L06, the bullseye sensor and its mirror) and set up a ruler for accurate moving of the Wincam during the caustic measurement.  Matt gave me an output coupler to use to dump the majority of the FE power, and we mounted a pump light filter (to keep any pump light from contaminating the measurement).  We also confirmed that lens L15 has a 200mm focal length, and the rough location of this focus for ease in setting up for the measurement.  This complete, we called it a day.  Tomorrow morning we will measure the beam caustics of the FE and begin modeling for a mode matching solution.