Analysis and design of long-period waveguide-grating couplers

By means of coupled-mode theory, an optical waveguide coupler consisting of two parallel identical waveguides, each of which contains a uniform long-period grating was analyzed. The authors show that light can be coupled effectively between the two waveguides through the gratings at specific wavelengths (the resonance wavelengths) in the absence of evanescent-field coupling. The conditions for achieving 100% coupling efficiency at the resonance wavelength under various situations are highlighted and the effects of the grating parameters on the transmission spectra of the coupler are illustrated with examples. Practical waveguide designs for the realization of the coupler are also presented. The long-period waveguide-grating (LPWG) coupler is a versatile waveguide structure and should find potential applications in optical filtering, add/drop multiplexing, and optical switching.