Three channel directional coupler with an optically induced grating

Abstract

Directional couplers are important elements of the integrated optics systems. The efficient power exchange between waveguides in the directional coupler occurs when the propagation constants of their guided modes are equal. The coupling between different waveguides can be obtained with the help of a grating formed by periodic changes of the refractive index along the coupler [1-5]. The grating constant K=2π/Λ (where Λ is the length of the grating period) should be equal to the difference between waveguides modes propagation constants (K=βA−βB). The gratings in the traditional grating-assisted-couplers (GAC) are usually formed during the fabrication processes and have constant, immutable parameters, what limits the operation of the coupler to one, strictly defined frequency. An optically induced grating proposed in [6] was formed by the interference of two counter-propagating external beams in a nonlinear layer separating two slab waveguides. Parameters of such grating depend on the external waves properties and can be varied during the work of the device. In particular the grating constant can be tuned to the guided modes propagation constants difference by changing the external waves frequencies or propagation directions.