Sixth-Order 2D Microring Optical Filter with Sharp Transmission Zero

Microring resonators have been shown to be extremely versatile elements for constructing high-order optical filters [1]. By coupling multiple microring resonators together, high-order optical transfer functions with flat-top passband and sharp skirt roll-offs can be realized. However, most coupled microring optical filters to date are based on the 1D serially-coupled configuration (also known as CROW filters) [1], which can only realize transfer functions with all poles and no transmission zeros. The inability of the 1D configuration to realize transmission zeros limits the rate of skirt roll-off that can be achieved in the filter spectrum. In contrast, the 2D coupling configuration provides multiple pathways of light through the lattice, which can result in complete destructive interference of the transmitted light at certain frequencies [2]. By positioning these transmission nulls near the edge of the passband, filter spectra with very sharp skirt roll-offs can be realized, which are important for applications requiring separation of two closely spaced wavelengths, such as in dense WDM communication systems, fluorescence spectroscopy and nonlinear pump-and-probe experiments.