INCIDENT ANGLE DEPENDENCY OF PROPAGATING MODES IN RECTANGULAR GRATING FOR POLARIZATION-INDEPENDENT 21ST ORDER DIFFRACTION OR POLARIZATION SPLITTING

摘要

The role of the incident angle of incoming light on the excited modes in rectangular grating at the interface of air and fused silica in Littrow mounting is investigated by the modal analysis. The analysis points out that the optimum effective refractive index difference for two propagating modes varies with the angle of incidence. In polarization-independent -1st order diffraction, effective refractive index difference has the same value for the p- and s-polarizations for incident angles from 30° to 37.5°, and a ratio of one-third with the further increase of incident angle, to select the minimum aspect ratio of the grating dimensions. On the other hand, polarization splitting condition has a change of ratio of effective refractive index difference from a half to zero at an incident angle of 45.3°, where the coupling order of diffraction for p- or s-polarization is also swapped. Such ratio of effective refractive index differences are seen as the ratio of modal periods in the variation of diffraction efficiency with the grating height.