Analysis of Finite Beam Contributions to Waveguide Wavelength Division Demultiplexer Design

Abstract : Micrograting devices have recently been implemented in chalcogenide, polymer, GaAs, and InP based materials for multichannel wavelength multiplexer/demultiplexer applications. In waveguide grating demultiplexers, the micrograting length/beam width ratio (q = d/omega o) is an important parameter in determining device performance. Finite beam Bragg diffraction effects have previously been considered in the case of thick planar gratings, but should also be accounted for in the design of waveguide demultiplexers, if beam profiles, efficiencies, and channel selectivities are to be optimized in such devices. Herein, we report on the results of a numerical study, using an impulse response technique in conjunction with coupled mode theory, that show the effects of Gaussian beam diffraction on the demultiplexer parameters of efficiency, wavelength and angular selectivities, and channel crosstalk. In the regime 1< d/omega o < 5, with d = 300 micrometers, and a center wavelength Ac = 632.8 nm, several basic results are predicted, including: (1) a departure from the plane wave behavior of (sinc)2 dependence for the diffraction efficiency, with a corresponding modification in the sidelobe features; (2) beam profile distortion for given d/wo ratios and diffraction angles alpha d; (3) a decrease in diffraction efficiency n with increasing q and diffraction angle ad; and, (4) an increase in channel crosstalk for increasing q at given detunings from the center wavelength. These results are summarized respectively, for TE - polarized light.