An auto-aligning photopolarimeter

A new auto-aligning photopolarimeter, that will be used to probe a liquid crystal (LC) cell, is described. As the LC cell is rotated, it steers the probing laser beam and the photopolarimeter head must be re-aligned to allow for this. To efficiently collect data this re-alignment must be done automatically. We show that using a quadrant photodetector (QD) and a two dimensional duo-lateral position sensing detector (PSD) it is possible to dynamically re-align the head. Also we introduce a new method to calibrate this auto-aligning system. We can collect data from points distributed over the Poincaire sphere and then use all of these to find the instrument matrix M-c that minimizes the error (in the least squares sense).