Polarization-independent tunable optical filter with variable bandwidth based on silicon-on-insulator waveguides

We present a polarization-independent tunable optical filter with variable bandwidth based on silicon-on-insulator (SOI) waveguides. The polarization-independent operation is achieved through the use of a polarization splitter-rotator and a polarization rotator-combiner, which are based on a bilevel adiabatic taper and an asymmetric directional coupler. Two stages of second-order microring resonators (MRRs) with different radii are used to achieve wavelength filtering with variable bandwidth and large free spectral range (FSR). Each stage of the second-order MRRs has a flat-top spectrum. The central wavelength of the 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 SOI platform. The FSR of the fabricated device is about 90 nm. We show the tuning of the central wavelength from 1460 to 1550 nm. We adjust the 3 dB bandwidth from 37.5 to 100 GHz with a step of 12.5 GHz, with the overall insertion loss varying from −5.4 to −7.9 dB.