During yesterday's maintenance window I worked on getting the DBB up and running.  I implemented the mode matching solution shown in the first attachment and proceeded to align the DBB.  That completed I then tweaked the mode matching lens positions to optimize the mode matching to the DBB PMC.  I ended up moving the 1st mode matching lens 2.5mm towards the 70W amplifier, and the 2nd mode matching lens 11.2mm away from the 70W amp.  At this point the maintenance window was running short and further position tweaks only made the mode matching worse so I stopped here.  A single FSR scan of the DBB PMC is attached to show where I left things (see 2nd attachment).  The mode matching isn't quite as good as the last solution (implemented before the changes during the Nov vent, which rendered that solution invalid), the 20+02 peaks are higher but more equal than previously; it's possible some more lens position tweaking could improve this (will be explored during a future maintenance window).

I then tried to lock the DBB PMC, but once again it was unable to lock.  I looked into the locking threshold for the DBB PMC and then noticed something that I hadn't when I last worked on the DBB: the DBB PMC transmitted photodiode (TPD) was not registering a voltage on MEDM.  This voltage is used as the locking threshold for the DBB PMC, and it consistently read zero.  I hooked up a multimeter individually to each port of the DBB PMC TPD (0dB, 40dB, and 80dB) and all 3 were indeed outputting a voltage.  Using the DBB's Manual mode, I manually moved the DBB PMC PZT and watched the voltage from the 0dB port change accordingly: as the DBB approached resonance the TPD voltage increased, as it moved away from resonance it decreased (all expected behavior).  Plugging the DBB cables back in and repeating the same experiment showed absolutely no change on the MEDM screen.  To get a more full picture, during the next maintenance window I will repeat this for the 40dB and 80dB ports of the DBB TPD (as I only used the 0dB port due to time constraints), although this likely explains why the DBB PMC won't lock; its resonance condition can never be met if the readout is not functioning correctly.  We will start hunting this down to get it fixed.

As a last item, I did a quick 1 minute duration Mode Scan (since it doesn't involve a locked DBB PMC) just to see what the higher-order mode count of the 70W beam is (as read by the DBB PMC).  I've attached the resulting measurement (3rd attachment).  It claims the 70W beam's higher-order mode content is 41.1% +- 9.666%.  This cannot be right as at last measurement we had a PSL PMC visibility of ~89%; I also checked the PMC after seeing this result and it was transmitting ~55W and reflecting ~11W (83% power throughput, which would be impossible if the higher-order mode content of the beam was really 41%).  So something is still off here, be it alignment, mode matching, or something else.  I will try to tweak up the mode matching and alignment to see if I can get the mode matching better (at least close to the previous solution) and see how this changes the Mode Scan measurement; if this doesn't work, a new mode matching solution will be explored.  Should that fail, then we may be in the "something else is wrong here" category.  It should be noted that LLO is experiencing a similar issue with their DBB; we're currently in communication with AEI to see if we can get this figured out.