The new OPO construction work has finished. Before replacing the old OPO, we need to characterize the old OPO. However, it was found that the p-pol and BAB matching to OPO was not great (p-pol and BAB). In the case of BAB, it matches OPO's TEM00 by about 87%. So there is a worry that the OPO BAB transmission may contain some higher-order modes, which will be different from the mode shape of squeezed vacuum. If so, the BAB homodyne alignment doesn't permit an equivalent homodyne alignment for squeezing.

According to the Airy function and the higher-order mode spacing on FSR, when a cavity is locked on TEM00, we find that the OPO cavity can suppress TEM01/10 and TEM02/20 by ~1700 in terms of power. Therefore, this 13% higher-order mode will transmit OPO as only 0.006% when OPO is locked on TEM00—in this sense, improving BAB matching to OPO maybe not be very useful.

However, a better mode matching can make more power get transmitted from a cavity. In addition, some higher-order modes may be reduced if alignment can be improved. Especially in the current OPO design, the third higher order is very close to TEM00, which can be reduced if alignment becomes better. Therefore, the improvement of BAB alignment to OPO should be beneficial.

Before this work, the BAB passed through two lenses close to their edges, as shown in the attached figure 1 and 2. This was done on purpose because a 2-inch BS was used. At the moment of exacting BAB (more than three years ago), we had no proper 1-inch BS. So we need to replace 2-inch BS with a 1-inch one. Before this replacement, I checked the power transmitance of the 2-inch BS, as shown in the attached figure 3 (incidence) and 4 (transmission). We can see that 2.2% power is transmitted. In our storage, we have BSX11, which is a 1-inch BS and can transmit 2.4% s-polarized beam. So we can use BSX11 to replace the old BS.

For the moment, a clear edge mirror mount is used for 1-inch BS. Attached figure 5 shows the moment of 1-inch BS installation on mount, which shows also that an arrow on the side of BS barrel directs to the back side of mirror mount. This arrow points to the mirror AR side as per Thorlabs. However, this mirror mount introduces scattered light since some light hits on the 'non-clear' edge of this mirror mount. As shown in the attached figure 6, 7, and 8, we have beam hitting on mirror mount in 6 and 7. Figure 8 shows the scattered light after hitting on a mirror mount. Although expensive, we can solve this problem by using BS mount from FMD.

After this replacement, the CC and BAB alignment to OPO was recovered. At the same time, BAB was made sure to go through optics center as shown in figure 9 and 10. The CC is affected because it passes through this replaced BS. The previous 2-inch BS is thicker than the one-inch BS. So CC needs to be aligned back as well.