Long-period gratings in planar optical waveguides

We present a theoretical analysis of light propagation in a four-layer planar waveguide that consists of a long-period grating (LPG) having a period of the order of 100 mum. By means of the coupled-mode theory, we show that such a structure is capable of coupling light from the fundamental guided mode to the cladding modes at specific wavelengths (resonance wavelengths) and thus results in sharp rejection bands in the transmission spectrum of the waveguide. Our numerical results show that the resonance wavelengths as well as the transmission spectrum can be significantly changed with the waveguide and grating parameters. A waveguide-based LPG should provide a useful approach to the design of a wide range of integrated-optic devices, including wavelength-tunable filters, switches, and environmental sensors. (C) 2002 Optical Society of America.