Resonant gratings for narrow band pass filtering applications

A resonant grating filter can be basically described as a sub-wavelength grating inscribed on a a planar waveguide made of dielectric thin film layers. The reflectivity of such a structure presents some peaks versus the wavelength that are generated by the coupling and coupling out of different modes of the waveguide. These peaks can be tailored in order to create free-space narrow bandpass inverse (notch) filters, with a very sharp spectral response (typically a spectral bandwidth below 0.5 reflectance nm for a component working at 1550nm). We show an experimental demonstration of a high performances resonant grating filter working in reflectance under a given incidence and wavelength. This filter presents simultaneously a good angular tolerance and polarization independence with a full width at half maximum about 0.4nm. The dielectric stack reflectance configuration is chosen so that the coupling conditions for two TE reflectance guided modes are simultaneously satisfied. Moreover, the grating cell presents a complex structure of four holes with different diameters, optimized for increasing the angular tolerance of the filter while maintaining a narrow spectral width for ally polarization. Comparisons with theoretical calculations are provided.