Polarization-independent Tunable Optical Filter with Variable Bandwidth Based on Silicon-on-insulator Waveguides

We propose and demonstrate an integrated polarization-independent optical filter with variable bandwidth and tunable central wavelength based on silicon-on-insulator waveguides. It consists of a polarization splitter-rotator (PSR), a polarization combiner-rotator (PCR) and two MRR filters. The PSR and PCR are based on a bi-level taper and an asymmetric directional coupler with the same structure. The MRR filter consists of two stages of second-order microring resonators with different radiuses. The two stages are serially cascaded, with each stage having a flat-top spectrum. The free spectral range (FSR) of the cascaded structure is expanded via the Vernier effect. The central wavelength of the proposed optical filter is tuned by synchronous tuning of the two stages. The 3 dB bandwidth is adjusted via intentional misalignment of the passbands of the two stages. We demonstrate a prototype of such an optical filter on the silicon-on-insulator platform. The FSR of the fabricated device is around 90 nm. We show the tuning of the central wavelength from 1460 nm to 1550 nm. We adjust the 3 dB bandwidth from 37.5 GHz to 100 GHz with a step of 12.5 GHz, with the insertion loss varying from -5.4 dB to -7.9 dB.