Holographic reflection filters in photorefractive LiNbO_3 channel waveguides for applications as add/drop multiplexers

摘要

We present simulation and experimental results of an integrated optical add/drop multiplexer in a photorefractive LiNbO_3 channel waveguide device. It is experimentally confirmed that a slanted refractive index grating in a bi-modal waveguide section breaks the orthogonality of the fundamental and first excited mode. This grating couples the forward propagating fundamental mode into the backward propagating first excited mode. A mode selective directional coupler that is designed using BPM splits the reflected signal from the transmitted signal into a drop port. In a symmetrically designed device, a second coupler is used for the add port. Bragg gratings are recorded with green light in photorefractive LiNbO_3:Ti:Cu channel waveguides that are designed for infrared light around 1.55 microns. The bandwidth (FWHM) of the gratings is about 0.1 nm which is appropriate for DWDM telecommunication devices. We use the electrooptic effect to tune the Bragg wavelength by applying a bias electric field which allows fast on/off switching of the filter.