Investigation of coupling between a fiber and an infinite slab

An optical waveguide system consisting of an unclad fiber core suspended at a constant distance parallel to the surface of an infinite extended dielectric slab is investigated. Fiber and slab are both embedded in a homogeneous dielectric medium. The fate of light initially launched into the fiber depends on the relationship between the refractive index values of the slab and fiber. When the refractive index of the fiber is considerably larger than that of the slab, the latter exerts little influence on the wave, so that almost no power is transferred out of the fiber. When the refractive indexes of fiber and slab are more nearly the same, a beating phenomena is observed, that is, light is exchanged periodically between the fiber and the slab. However, after some distance the beating dies out. Some of the light energy remains in the combined system of fiber and slab, which can be regarded as a ridge waveguide, while the remaining light spreads out laterally in the slab. If the refractive index of the slab is considerably larger than that of the fiber, the slab acts as a drain on the light initially launched into the fiber. The light power in the fiber decreases exponentially with the distance along the fiber axis as it is transferred to the slab where it is carried away