The 1W NPRO beam is elliptically polarized. We use a quarter-waveplate (QWP) to linearize the polarization.
7 days ago, I made some measurements of the extinction ratio of the beam for various orientations of the QWP (attenuating the beam with a half-waveplate and polarizing beamsplitter) to find the QWP orientation at which the light is linearly polarized. The minimum ratio occured when the fast axis was between 10 and 15 degrees from the vertical. I've attached a plot of the data, and I will fit it to a curve as soon as I work out which function should describe the data.