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Filter-sidelobe suppression of codirectionally coupled optical waveguides with spatially tapered coupling strength is examined. The filter responses for several taper functions with low Fourier transform sidelobes are calculated, and the resulting channel densities when used for wavelength multiplexing are compared. For a 30 dB crosstalk level, all the taper functions considered yield at least a tenfold increase in the number of carrier channels over untapered filters. The filter response for a quantized taper that results from conventional fabrication procedures is also considered.