Narrowband InGaAsP/InP Waveguide Grating-Folded Directional Coupler Multiplexer/Demultiplexer

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Abstract
Narrowband (1,2 This contradirectional interaction provides narrowband response even for relatively short interaction lengths (< 1 mm). However, the wavelength dependent Bragg reflection within a waveguide is, by itself, not sufficient to act as a multiplexer/demultiplexer component. Spatial, not just directional, separation, of the filtered light is also required. Two techniques to efficiently provide such spatial separation in grating-based devices have been suggested. One approach, which employs a grating to achieve phase matched contradirectional coupling between mismatched waveguides has been achieved, with relatively low efficiency, in fiber couplers3 and with vertical couplers based on ARROW waveguides.4 Another possibility, well-known in the microwave domain, combines the bandpass reflectors on both output arms of a broadband 3 dB splitter.5,6 Here we report the first demonstration of this optical waveguide grating-folded coupler to achieve narrowband, spatially isolated wavelength demultiplexing. This approach has the advantage of not placing severe demands upon either the waveguides or grating.