Michael and Yuhang

We worked on replacing the phase shifter for the beam going into the IRMC, highlighted in figure 1. Afterwards we will also look at the one going into GRMC. The angle of incidence causes beam jitter noise when the phase shifter acts on the beam. We decided to replace this with a perpendicular incidence setup as per the sketch in figure 2, using a PBS and waveplates.

We have reference for the values of the IRMC (390 µm beam waist, 1.8 mW transmitted power). Using the reference waist and the distances of the holes, the beam should be collimated before the 250 mm lens (fig 3). So there is not much need for complicated rearrangement of the lens positioning. We just have to move the 250 mm forward by however much the path length is from the PBS to the phase shifter and back. Fine tuning of the lenses can be done via the mount. A rough indication of the distances is shown in fig 4.

We took the following items:

PBS: CVI PBS-1064-100

HWP: CVI QWPO-1064-08-2-R10

QWP: CVI QWPO-1064-09-4-AIR-R10

During the aligment we made sure to recheck the beam propagating along the west end of the table to the 250 mm lens, to make sure it aligned with the holes and was consistent with the more recently aligned beam height from the SHG (76 mm).

Using the power meter, the power before the PBS was 4.14 mW. After double passing through QWP and PBS, it was ~ 3.8 mW.