Demonstration of A 4th Order Pole-Zero Optical Filter Integrated in CMOS Silicon Waveguide

We demonstrate a compact, fully tunable, narrowband 4th order pole-zero optical filter that is fabricated in a silicon complementary metal oxide semiconductor foundry. The filter is implemented using silicon on oxide channel waveguides and consists of a Mach-Zehnder (MZ) interferometer with two ring resonator all- pass filters (APF) on each arm. The filter architecture is based on the sum and difference of the APFs responses. The ring resonators introduce a non-linear phase response in each arm that allows carving narrow frequency bands out of a broad spectrum. In this work, we demonstrate a 3-dB filter bandwidth of 1.0 GHz with a stop band rejection of better than 25dB. The filter free spectral range (FSR) is 16.5 GHz. Thermo-optic phase shifters are used to tune the filter. As silicon has a large thermo-optic coefficient compared to silica, the demonstrated filter requires a low tuning power of less than 300 mW. IN addition, this filter is compact with dimensions 25 times smaller than the same filter would be if it were made in standard silica with a 0.8% step index contrast