Tunable Germanium-on-Insulator Band-Stop Optical Filter Using Thermo-Optic Effect

We demonstrate a tunable band-stop optical filter on a germanium-on-insulator photonic platform operating at 1.95 μm wavelength. The band-stop filter is implemented using parallel-coupled microring resonators with the number of microring resonators varied from one to. three. Through the use of a heating stage, the thermo-optic coefficient of germanium is measured to be +3.55 × 10
^-4
/°C. The high thermo-optic coefficient of germanium is exploited by including metal heating lines to provide localized on-chip tuning. As predicted by theoretical analysis, our experimental results show that when the number of microring resonators increases, the -3 dB bandwidth and extinction ratio of the optical filter increases. With the number of microring resonators increases from one to three, the -3 dB bandwidth of the filter increases from 0.283 to 0.661 nm and the extinction ratio from 9.56 to 22.10 dB.