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Butterworth, Chebyshev, and elliptic bandpass filter designs are optimal in the sense of band flatness or equiripple characteristics. A new architecture using optical all-pass filters is presented which can realize these designs exactly and efficiently using either ring resonators or reflectors such as Bragg gratings or thin-film interference filters. Design examples are given for a seventh- and eighth-order elliptic filter, and the new architecture is shown to be tolerant to loss. Previously, reflective filters could only approximate optimal responses. An order of magnitude improvement in transition width is demonstrated for an elliptic filter compared to an optimized transmission response for an individual thin-film filter.
Date of Publication: Aug. 1998