I was having some issues with the beam(s) I had previously mode matched into the PMC. Apart from not having gotten great coupling to begin with, the alignment seemed to have drifted over a few days (I noticed this last week). I attributed this to 2 things: 1) the MMT I had was a pretty sensitive one, owing partly to the fact that I had to work with the beam far outside the Rayleigh zone due to the beam beat recombination being upstream, and 2) having the recombining BS in the way, I was susceptible to clipping in the output path I was using for the PMC. I don't really need the beat setup at the moment, and I can do the modulator characterization using a single laser, so I decided to rebuild the PMC test setup using a single laser.
As a first step, I simply remeasured the output beam profile of the West laser using the razor blade technique. The beam seems very circular and not astigmatic, so I only profiled in the horizontal direction. The result:
Using this, I recalculated a better MMT:
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Other solution:
mismatch: 0.00011786
w0x = 303.7849 um
w0y = 303.7849 um
lens 1: f = 103.2118 mm
lens 2: f = 206.4236 mm
Distances:
d1 = 6.161 cm
d2 = 14.3007 cm
d3 = 29.5383 cm
(Total distance = 50 cm)
I then installed this, aligned the PMC and was able to get ~96% coupling with little trouble. By locally optimizing the second lens, I pushed this to about 97.5%. While a bullseye was faintly evident on the card in the first case, it was very hard to tell what was reflected after the reoptimization.
I borrowed the RF electronics from the steel gyro PMC temporarily (splitter, mixer, bias tee and filters). For some reason, the 1-MHz dither I used with that PMC did not work with this one, but I was able to derive a nice error signal using a 300-kHz dither at 3 Vpp. I wanted to use the uPDH box I used to use before I had the digital servo for the gyro PMC, but I forgot that Eric Q had borrowed it for the 40m. Instead, I was actually able to lock robustly and stably with just an SR560 and a single pole at 10 Hz. The control signal stays within its output range over ~10 min+ time scales. (I didn't bother measuring the loop---all I needed for my phase modulator characterization is essentially a DC lock, and the bandwith was easily 10s-100s of Hz).
The transmission dither lock leaves the REFL port open so that I can measure the rejected sideband light pumped by the modulator as planned. |