Spectrum-Continuous Narrowband Filter with Enhanced Flat-top Passband and Sharp Roll-off Performance

We demonstrate a spectrum-continuous narrowband filter based on multi-section Mach-Zehnder interferometer (MZI) structures on a silicon nitride platform. Enhanced in-band flatness and sharp out-of-band roll-off features are achieved by cascading MZIs of specific functionalities in a series. This filter is fabricated in a single chip with a total waveguide length exceeding 237 cm. We experimentally demonstrate for the first time, to the best of our knowledge, a four-channel spectrum-continuous filter with a 3 dB bandwidth of 580 MHz, a 1 dB fluctuation bandwidth as high as 356 MHz, and an excellent 20 dB roll-off factor of 1.26 that are obtained simultaneously. Both the optical and electrical experimental results show that the proposed filter can play a key role in integrated microwave photonic front-end systems.