Improved channel selectivity for fiber optic add/drop wavelength division multiplexers using generalized resonant couplers

By generalizing a resonant coupler to allow for unequal delay lengths, it is possible to design an add/drop multiplexer which can select many more channels than would otherwise be possible. The new generalized resonant coupler (GRC) behaves as a finite impulse response filter whose expansion coefficients must satisfy a set of constraints. We focus attention on a family of GRCs which exhibits particularly good side lobe suppression for a given finesse. An N stage GRC of this type can have up to three times as many channels within its free spectral range as an N stage resonant coupler with comparable crosstalk