Polarization-Independent Filter Based on 2-D Crossed Grating Under Oblique Incidence

The existing polarization-insensitive filters based on two dimensional (2-D) grating are mostly under normal incidence. However, in filtering optical devices, the normal incidence regime is usually avoided because it requires the use of beam separators which yields a loss of efficiency. Herein, we present a compact method to realize polarization-independent filter under oblique incidence, which is implemented on 2-D crossed grating in the telecommunication region. Plane of incidence is set to be in xz plane and three different polarization angles (0 degrees, 45 degrees, and 90 degrees) are used to demonstrate the polarization-insensitivity. Realization of the polarization-insensitivity of the 2-D filter is based on the split feature under oblique incidence, according to the analysis of the physical mechanism. The location of the polarization-independent resonance is mainly determined by the grating period along x-axis and will right shift with increasing the period. Results show that the polarization-insensitive resonances occur at 1356.9 nm, 1372.6 nm and 1398.5 nm when the grating periods along x-axis are 830 nm, 850 nm and 870 nm, respectively. Moreover, the results and methods provided herein can be applied to search for the polarization-insensitive resonances of the 2-D grating with other planes of incidence.