Silicon optical filter with transmission peaks in wide stopband obtained by anti-symmetric photonic crystal with defect in multimode waveguides.

We experimentally demonstrate and characterize a wide optical stopband filter with transmission peaks constituted by two parallel rows of an anti-symmetric one-dimensional (1D) photonic crystal with defect in a multimode waveguide. The working principle is based on mode coupling for the wavelengths that meet the phase matching condition as the fundamental mode is coupled to a higher-order mode and then filtered out with linear tapers as they are reflected back. The defect in the photonic crystal works as a Fabry-Perot like cavity that allows for localized states in the stopband to quantify the effects of the design parameters and we show a Fabry-Perot filter that extends through the 200 nm stopband. A compact 7 μm device is demonstrated with standard fabrication techniques in Silicon-on-Insulator (SOI), and compared to Fabry-Perot filters in single mode waveguides, we obtain a larger bandwidth and lower back reflections. Applications include refractive index sensors, optical communications and on-chip spectroscopy.