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David Notebook: Difference between revisions
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'''6/29/15'''
I got the mode-matching working on that cavity. The cavity was slightly too long causing it to be unstable. I added a few O-rings, which pushed it into the stable region and then mode-matching was easy. Profiling the beam is a bit of a pain though--you can't just turn down the amplifier power with the beam cube because we'll get only the "bad" polarization which is pretty much in the 1,1 mode. Instead you have to send in mostly full power (which requires having it mostly aligned to the cavity) and then use a pick-off to look at a small portion of the beam. Locking worked reasonably well, but at .1 and .3 atm of pressure I saw no stokes generation--surprising, but we aren't cooling the cavity this time. In a previous paper we put out though we had plenty of stokes power at these pressures with no cooling in a similar cavity. I'm a little concerned and we wanted to look at much higher pressures to see what would happen, but we ran out deuterium and that's taken a few weeks to get in. Hopefully though we'll find the pressure threshold is just a little higher than before or maybe the gas was contaminated somehow and it just needs a fresh batch of deuterium. The gas should be in this week and then we'll know.
I've been working on more detailed calculations for the mixing efficiency of a cavity redesign--[[File:Modulation_Efficiency.zip|here's some code and generated plots]]. Current limitations of this calculation are that we assume constant intensity inside the cavity (i.e. the waist is very close to the spotsize on the mirrors--this is not a bad assumption for the geometries we're interested in since these tend to be more stable, but could be improved. It's not good for longer cavities especially. This could be corrected by doing some sort of total integrated intensity like in the previous entry). Additionally, the mode-overlap factor between the pump and the stokes and a potentially separate mixing beam is only valid at short cavity lengths (this term should depend on cavity length but we just use a constant multiplicative factor for all lengths). We also have limited knowledge of how locking performance will change with pressure and cavity length--the most we do is in one set of plots just introduce a multiplicative factor for the laser linewidth and cavity linewidth overlap.
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