A design method of comb-drive actuators for linear tuning characteristics in mechanically tunable optical filters
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A new method is proposed in design of comb-drive actuators for specific voltage-displacement characteristics with finger gaps as the design parameters. The design method proposed by the author previously is further refined by adopting a more accurate model which considers fringe electric fields. The proposed method is applied to design comb-drive actuators with an aim to achieve linear tuning characteristics in mechanically tunable optical add-drop filters with microring resonators. To make an assessment of the accuracy of the proposed design method, three-dimensional electrostatic numerical analysis is conducted to obtain capacitances of the designed comb-drive actuators as functions of the moving finger displacement. Obtained capacitances are used to find the tuning characteristics (resonant wavelength vs. voltage) of the filter, in combination with the results from the author's other work where a relationship between the resonant wavelength and the displacement of an index modulator was studied. It is found that by employing the actuators designed by the proposed method, the maximum deviation from linearity (MDL) can be reduced by 17.2 % points (from 25.7 % of the conventional design to 8.5 % of the new design). MDL is further reduced to 4.4 % by making a few modifications in the design.