Dual-Mode Coupled-Resonator Integrated Optical Filters

A photonic integrated architecture realizing the concept of coupled-resonator dual-mode optical filters is presented. The proposed structure consists of a chain of directly coupled microring resonators with a partial reflector embedded in the last resonator. The contradirectional coupling induced by the reflector makes the effective order of the filter double with respect to the number of coupled resonators. Since the counterpropagating modes share the same physical cavity, the resonant frequencies are intrinsically aligned, sensitivity of the filter to fabrication tolerances is reduced, and control of the structure is eased with respect to coupled resonator filters of the same order. Second- and fourth-order dual-mode filters made of microring resonators loaded with a Bragg grating are experimentally demonstrated on a silicon-on-insulator photonic platform.