Submicron-resonator-based add-drop optical filter with an ultra-large free spectral range

A low-loss add-drop microring resonator (MRR) with an ultra-large free spectral range (FSR) is demonstrated by introducing an ultra-sharp multimode waveguide bend and bent asymmetrical directional couplers (ADCs). The multimode microring waveguide is introduced to achieve a low bent loss, even with a small radius (e.g., R = 0.8 μm). The bent ADCs are used to suppress the resonance of higher-order modes. For the fabricated device, the transmission at the drop port has a narrow 3 dB-bandwidth of 0.8 nm and a low excess loss of 1.8 dB. A record large FSR of 93 nm is achieved to the best of our knowledge.