Flat-top bandpass filters enabled by cascaded resonant gratings

Narrow bandpass filters are applied in laser systems, imaging, telecommunications, and astronomy. Traditionally implemented with thin-film stacks, there is recent interest in alternative means incorporating photonic resonance effects. Here, we demonstrate a new approach to bandpass filters that engages the guided-mode resonance effect working with a cavity-based Fabry-Perot resonance to flatten and steepen the pass band. Both of these resonance mechanisms are native to simple resonant bandpass filters placed in a cascade. Numerical examples provide quantitative spectral properties including pass-band shape and sideband levels. Thus, we compare the spectra of single-layer 1D and 2D resonant gratings with the dual-cascade design incorporating identical gratings. Two- and three-cavity designs are measured against a classic multi-cavity thin-film filter with 151 layers. Whereas these initial results show comparable and improved results achieved with sparse structures, the challenge remains of developing a suitable fabrication technology to capitalize on this promise.