Stefan, Sheila

This morning Stefan and I returned to the PSL.  First we adjusted the AOM alingment to improve the diffraction efficency.  In the end we measured (with the filter on the power meter) 

33mW before AOM 
27.4mW after (single pass)
24.35mW after double pass

at around 74% double pass efficency, this is pretty good now.  Stefan adjusted the efficiency of the first pass by moving the AOM (mostly in pitch) and the second pass efficiency by aligning the retro reflecting mirror.  

After this the beam looked low in the apperatures of the EOM, so we moved it down, effectively undoing what Kiwamu and I did yesterday to keep the beam centered in the apperatures.  Once we had realinged the ref cav, the beam spot on the reflected camera was again in approximately the center, but now the beam spot clearly has 2 lobes when the cavity is unlocked.  Stefan removed the ND filter on the camera to make sure this wasn't an artifact, and the lobed shape remained.  

We then double checked the ref cav power budget, now making all measurements with the filter off:

23mW incident
refl (after PBS for circulator) 21.1mW (130mV on refl PD when unlocked)
locked cavity refl PD 29mV (this means 4.7mW refl unlocked)
the dark offset on this PD is 0.8mV (measured yesterday)
transmitted: 12.2mW (trans PD voltage 1.21V)

This is a visibility of 78%, but we are still lossing 25% of the power, as we were yesterday.  

We found one gost beam, which seems to be coming from the veiwport.  This light gets onto the refl PD, and causes an offset of about 1.6mV, not really significant in our power budget.

Once we were done I moved the razor blade dump that is behind the retroreflecting mirror for the AOM back about an inch so that there was room to add an iris on the leakage beam through that AOM.  
Cheryl suggested this as a way of monitoring if the beam position there drifts.  I left the iris opened so that the beam is normally dumped on the razor blade, but when the iris is closed the beam 
is right on the center. 

I also looked with the beam leveler (the aluminum one inside the PSL that Doug Cook had made, with lines scribed on it and small pin holes.)  
The path between the PMC and the ref cav is consistently silghtly below 4 inches on that leveler, but this is probably just because a different reference was used when leveling the beam.