A highly versatile synthesis method for the design of a variety of tunable optical waveguide filters with independently variable bandwidths and tunable center frequencies and arbitrary infinite impulse response (IIR) characteristics is presented. The synthesized Mth-order tunable optical filter consists of the concatenation of M all-pole filters (APFs) with M all-zero filters (AZFs). The bandwidth and the center frequency of the designed tunable optical filter can be independently tuned by applying electric power to thin-film heaters loaded on the waveguides of both APFs and AZFs. One unique advantage of the proposed synthesis technique is that the poles and zeros of the filter can be adjusted independent of each other to enable the design of tunable optical filters with arbitrary IIR characteristics. By means of computer simulation, the effectiveness of the synthesis method is demonstrated with the design of second-order Butterworth bandpass and bandstop tunable optical filters with variable bandwidths and tunable center frequencies. To study the effects of fabrication tolerances on filter performances, the maximum allowable deviations of filter parameters from their designed values are also determined. The proposed synthesis method is general and flexible enough to enable the design of a variety of tunable optical filters with arbitrary IIR characteristics, which include Chebyshev and elliptic filter types.
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