Design of Wide-Band Tunable Optical Filters with Cascaded Microring Resonators and Shaped-Finger Comb-Drive Actuators
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Utilizing the Vernier effect, series coupling of multiple microring resonators with different sizes is used to design a wide-band (free spectral range: 36 nm) tunable filter. For index modulators that shift the filter spectrum by changing effective indices through evanescent coupling, silicon waveguides are considered, which make the fabrication simpler. Effects of the index modulator width to the filter characteristics are studied. A narrower modulator (width: 50 nm) does not incur much loss to the resonator, but requires hopping among several bands since its tuning effect is moderate. On the other hand, a wider modulator (width: 100 nm) can cover the full free spectral range without band hopping, but induces severe loss when it is close to the resonator. The shaped-finger comb-drive actuator design method is applied to obtain linear drop channel control.