C-band optical filters with micromechanical tuning
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Tunable optical filters that cover the entire range of the C-band (1530-565 nm) are designed by utilizing the Vernier effect, i.e. series coupling of microring resonators of different sizes, and the micromechanical tuning method. The micromechanical tuning method employs lateral comb-drive actuators to control evanescent coupling between the resonators and index modulators. Single crystalline silicon is used as the material for all of the main components including bus waveguide cores, resonators, index modulators, and comb-drive actuators. A finite-difference time-domain method is used for optical analysis of the filter. The simulation results show good agreement with those by analytical methods, previously reported. The width of the index modulator is found to play an important role to the filter characteristics. A wider modulator (e.g., width: 100 nm) can cover the full tuning range of 35 nm without switching among different bands owing to stronger effective index change effect, but induces significant loss to the filter, especially when it is brought close to the resonator. While a narrow modulator (e.g., width: 50 nm), on the other hand, induces moderate loss to the filter, it requires hopping among multiple bands to cover the full range since the effective index change incurred is again moderate. In order to achieve linear tuning characteristics in the cascaded-resonator filters, the shaped-finger comb-drive actuator design method is applied. The design method based on two-dimensional slice approximation is further examined by three-dimensional finite element analysis for verification. It is shown that the design method can also work for the cascaded-resonator filters, even for the one that requires band hopping. Effects of fabrication imperfections to the designed device characteristics are studied as well.