Yuhang and Michael

We had some trouble aligning the OPO inside the holder (initially without the input coupler). A photo is shown in figure 1. The OPO assembly is contained on the white plastic mount and has a small ~ 5mm hole at both the entrance and exit. The mount past the OPO assembly is a beam splitter that goes to a camera and photodetector. The two mirrors on the bottoms left and right are the steering mirrors. However, these are used to control the horizontal and vertical alignment of the beam before placing the OPO. Our approach was to constrain the cavity axis prior to placing the OPO - we make sure the beam stays at constant height, send it to a camera and then mark the position of the beam on the TV (fig 2). The OPO cavity is placed so that the entry hole has the input beam centered and the exit hole aligns with the previous mark on the TV screen. Then, in theory, we only need one degree each of pitch and yaw (i.e. a rotation stage) to align the OPO to the cavity axis and match the incident and reflected beams. An illustration is shown in figure 3. As seen with the arrangement of the periscope, a yaw misalignment of the reflected beam when inspected at the prompt reflection translated to a pitch misalignment of the reflected beam when inspected past the periscope (fig 4).

Using the alignment setup described, we couldn't meet the alignment conditions. We tried again by moving the OPO assembly (and the OPO  curved HR surface) closer to the centre of rotation of the rotation stage. Actually, after all of this we found that the camera wasn't very stable on the mount as well, so we fixed it properly.

Initially we put the modulators on the edge of the table so that their connecting cables wouldn't get in the way during the experiment. However, in retrospect, it would have been better to put the OPO assembly on the edge of the table since the alignment of the OPO is the most delicate task.