Light-coupling characteristics of a fiber-slab coupler using a buffer layer based on coupled-mode theory

The characteristics of light propagation in and the coupling of a fiber-slab coupler with a buffer layer were obtained through the coupled-mode theory. The results were found to highly depend not only on the refractive index of the buffer layer but also on the thickness of the buffer layer. The fiber-slab coupler was modeled as a fiber-to-slab waveguide coupler with four layers, including a fiber cladding layer, a buffer layer, a slab waveguide layer and an overlay material layer. The refractive index and the thickness of the buffer layer for effective coupling were gained by tuning. More of the light's power was transferred when the refractive index of the buffer layer was very close to that of the slab waveguide and when the thickness of the buffer layer was almost the same as that of the slab waveguide. We were able to obtain a buffer layer with favorable characteristics for achieving the desirable coupling performance.