Measured performance of shadow-cast coated gratings for spectro-polarimetric applications.

We measured the efficiency of a reflective echelle grating that was specially coated in order to reduce the polarization of the diffracted light. The standard coated grating can be used between approximately 350 and 1200 nm for diffraction orders between 14 and 5. In these orders, the standard grating can be strongly polarized (up to ∼80%), which impacts its usability in spectro-polarimetric instruments. The adoption of the shadow-cast coating technique, which implies selectively depositing the reflective coating exclusively onto the blazed facets, can significantly reduce the polarization of the grating while preserving its average efficiency.