Rana and I were poking around on the PSL table today, getting a few more items raised to the correct height.
I checked the polarization state of the new NPRO by using a HWP to minimize the transmission through a PBS cube, and then compared the power transmitted through the cube vs. reflected. When the NPRO current was 0.772 \pm 0.001 (as read on the LCD), the transmission through the cube was 1.44mW, while the reflected was 10.53mW. The reading of the Ophir power meter with no incident light was 0.03mW. This factor of 10 means that the NPRO beam is ~10% circularly polarized and ~90% linearly polarized. In order to improve the beam, we need a Quarter Wave Plate, which it turns out we don't have. We need a QWP!
After that, using the linearly polarized part of my beam (maximizing the transmission through the PBS by rotating my HWP by 45deg), I tried to tune the angle of the polarizers that Rana pulled out of the MOPA. I think I'm confused / too tired, because I can only get the polarizer to reflect a bunch of light, and I can't get it to pass any significant amount of light through, no matter where in its actuation range I put it (It's on a rotation stage with a few degrees of range). It should just be a Brewster's Angle thing, and since I already have P-pol coming through the BS cube, this shouldn't be so hard.....
In any case, it may not be useful to do the final fine tuning of these polarizers until they are in their final places. The hacky stack of mounts that I have has some slop in the position / alignment of the base of the polarizer, so no matter what we'll have to redo the tuning after the mounts are finalized.