Analysis of a rib waveguide filter response with variable separation

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Abstract
An asymmetric coupler consisting of two dissimilar waveguides can be designed to operate as a narrowband filter if the propagation constants of the individual guides coincide at one wavelength, λo. The filter response is approximately a (sinc)2 function with high sidelobes of -9 dB, undesirable for WDM applications. The sidelobes can be reduced by controlling the coupling coefficient k(z) along the coupler by varying the interguide separation t according to certain window functions. This was demonstrated theoretically by solving the coupled-wave equations1 and experimentally with one-dimensional waveguides.2

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