Worked on the DBB 200W beam path alignment this morning.  One side effect of the power outage that I hadn't considered was that it reset the DBB alignment PZTs to zero.  This is actually a good thing as these PZTs need to be as close to zero as possible, and were rather far off in vertical alignment due to the previous bad DBB alignment.  This reset essentially forced me to completely realign the beam to the DBB PMC, but with the added benefit of not having to tailor my alignment to also minimize the PZT voltage; it's already at zero.

For this alignment I scan the DBB PMC by ramping its PZT at 10Hz and look at the output of the DBB PMC TPD on an oscilloscope.  As a single scan of the PZT covers several FSRs of the DBB PMC the oscilloscope output shows the different higher order mode peaks that resonate in the PMC as the PZT ramps.  Alignment is then done by identifying and reducing the TEM10 and TEM01 peaks.  At first I didn't really get anywhere and the beam transmitted by the DBB PMC looked really ugly no matter what I did to the alignment, so I checked the distance between the 2 mode matching lenses.  Should be ~168mm (this is where I left it last time I worked on the DBB), was more like 187mm (not sure why this moved, didn't see the change documented in the alog).  Moved DBB_MML2 to make the distance 168mm, which then gave me a signal through the DBB PMC that I could use in alignment (had a transmitted beam that was somewhat round).  Horizontal and vertical alignment is now much better than previous, alignment peaks have almost vanished.

I then worked on the mode matching by moving one of the mode matching lenses slightly (~0.3mm, can do this with the DBB MEDM screen.  Note to self, positive movement on the MEDM screen moves the lenses AWAY from DBB_M1), realigning to the DBB PMC (as moving lenses changes alignment), then assessing whether or not this change improved the mode matching peaks in the PZT scan.  Lather, rinse, repeat.  All told I moved DBB_MML1 by +1.6mm and DBB_MML2 by +4.0mm.  This appeared to improve the mode matching peaks signficantly and the beam transmitted by the DBB PMC is much more round (although still slightly oblong in the vertical direction to my eye).  Even with these apparent improvements the DBB PMC would not lock, so there is still alignment/mode matching work to do (or something else going on that I haven't considered/worked out yet).  I will continue this work in the mornings as commissioning allows.

It should be noted that all this work was done solely on the 200W beam path.  Due to the resetting of the DBB PZTs and the subsequent realignment of the 200W path, the 35W path into the DBB will need significant realignment.  At this time the 35W path into the DBB is not usable.  I will realign/mode match the 35W beam path once the 200W path is done (as the 35W alignment depends on the 200W path being aligned/mode matched).