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Wed Jul 23 18:42:18 2008
Alignment of AOM
[Rana, Yoichi, Jenne]
Short Version: We are selecting the wrong diffracted beam on the 2nd pass through the AOM (we use the 2nd order rather than the first). This will be fixed tomorrow.
Long Version of AOM activities:
We checked the amount of power going to the AOM, through the AOM on the first pass, and then through the AOM on the second pass, and saw that we get about 50% through on the first pass, but only about 10% on the 2nd pass. Before the AOM=60mW, after the first pass=38mW, after the 2nd pass=4mW. Clearly the alignment through the AOM is really sketchy.
We translated the AOM so the beam goes through the center of the crystal while we align things. We see that we only get the first order beam, which is good. We twiddled the 4 adjust screws on the side of the AOM to maximize the power at the curved mirror for the 1st order of the first pass, which was 49.6mW. We then looked at the DC output of the Reference Cavity's Refl. PD, and saw 150mV on the 'scope. The power measured after the polarizing beam splitter and the next wave plate was still 4mW. Adjusting the curved mirror, we got up to 246mV on the 'scope for the Refl. PD, and 5.16mW after the PBS+Waveplate. We adjusted the 4 side screws of the AOM again, and the tip/tilt of the PBS, and got up to 288mV on the 'scope.
Then we looked at the beam that we keep after the 2nd pass through the AOM, and send to the reference cavity, and we find that we are keeping the SECOND order beam after the second pass. This is bad news. Yoichi and I will fix this in the morning. We checked that we were seeing a higher order beam by modulating the Offset of the MC servo board with a triangle wave, and watching the beam move on the camera. If we were chosing the correct beam, there would be no movement because of the symmetry of 2 passes through the AOM.
I took some sweet video of the beam spot moving, which I'll upload later, if I can figure out how to get the movies off my cell phone.