A Diffraction Grating Photopolarimeter

Abstract

A new division-of-amplitude photopolarimeter1 with no moving parts or modulators is described that employs the conical diffraction of light from a grating which is set for oblique incidence with the grating lines inclined with respect to the plane of incidence, Fig. 1. The grating G splits the incident light beam, whose state of polarization is to be measured, into four or more beams that are intercepted by linear photodetectors Dm (m = 0, 1, 2, …). This produces four or more signals im that represent multiple projections of the unknown Stokes vector S. S is reconstructed from its projections by matrix inversion. We have built a prototype photopolarimeter of this type using an Al-coated, 600-grooves/mm, holographic grating set at 65° angle of incidence with the grooves oriented at 45° with respect to the plane of incidence, and a 633-nm He-Ne laser as the light source. This novel photopolarimeter has been calibrated and tested, with and without the linear polarizers Pm in front of the photodetectors, using the same procedures recently established for the four-detector photopolarimeter2,3 . This polarization-analysis scheme has several advantages and degrees of flexibility. In particular, it can be adapted using detector arrays so as to perform time-resolved spectropolarimetry.