Programmable microwave photonic filtering with coupled ring resonators

Microwave photonic filters play a crucial role in radio frequency over fiber applications by eliminating unwanted tones following RF to optical up-conversion. This study demonstrates that coupled resonator optical waveguides (CROWs) can serve as efficient microwave photonic filters on silicon-on-insulator platforms, achieving roll-off slopes above 4 dB/GHz with less than 1 dB ripple. We demonstrate that roll-off slopes exceeding 7 dB/GHz can be achieved by serially cascading two CROW filters. This result is further validated through single-sideband signal generation using electro-optic modulators combined with these cascaded filters, achieving a measured roll-off slope of 7.5 dB/GHz. Furthermore, we show that cascading two CROW filters enables programmable filtering with two operation modes: spectral width tuning from 470 to 23 GHz by relative tuning of one filter, and broad spectral width center frequency tunability up to 1.2 THz through simultaneous tuning of both filters. We present a comprehensive experimental study of these devices, including chip-to-chip, intra-chip, and run-to-run statistical analysis. Moreover, we show that the addition of a SiN capping layer enhances fabrication tolerance and extinction ratio by 5 dB.