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A bandwidth-tunable flat-top optical filter based on a single reflective-type microring resonator

An optical filter with flat-top spectral response and tunable bandwidth is proposed and numerically demonstrated based on a single reflective-type microring resonator (MRR). The reflective-type MRR consists of a MRR and a reflector in the drop port of the MRR, which is equivalent to two cascaded rings with one creates a strong passband response and the other one produces a weak notch response. The combination of the two responses leads to a flat-top bandpass response. By adjusting the coupling coefficients of the two optical couplers in the MRR, the 1-dB bandwidth can be continuously tuned from 0.025 to 0.121 nm for a reflective-type MRR with a radius of 100 μm, while the insertion loss is varied within 0.63 dB.