Yuhang and Marc

It was verified in logbook 3081 that the main laser EOM (as shown in the attached figure) was introducing significant amount of phase noise for the squeezing measurement. However, a better way of reducing the modulation caused by EOM is to use mode cleaner to reduce it. For instance, the new EOM has modulation frequency of 88MHz, which will be filterted out with a factor of about 4000.

r1 = r2 = 0.992

fin = np.pi*np.sqrt(r1*r2)/(1-r1*r2)

l = 0.5844

FSR = c/l

spac = 88e6/FSR

1/(1+4*(fin/np.pi)**2*np.sin(np.pi*spac)**2) = 0.0002449

Thus the phase noise caused by EOM modulation should be removed to a negilible level.

On the other hand, the new wedged EOM will remove residual amplitude modulation. However, the old EOM doesn't have wedge so that the introduction of new EOM deflects the laser beam. This causes a substantial alignment change, although we expect small beam path length change. To handle the alignment change, we decide to put the wedge oriented in the horizontal direction. In this situation, the misalignment will happen without causing beam height change. In general, we would like to avoid beam height variation.

For the SHG/BAB path, we could correct alignment change by the 2-inch BS after EOM. For the LO/PLL path, we set up two one-inch mirror with small mirror mount to correct the beam tilt. Using this strategy, we started to align beam again to SHG and IRMC as the first step. In particular, we found that we resulted to have a very large beam in front of SHG by moving just the 2-inch BS to recover alignment. This is out of our expectation because we expect small beam path length change. However, now it seems to be not the situation. To solve this problem, we decided to measure beam parameter again after the 2 inch BS. This measurement will be reported in the next logbook since it's a slightly different topic.