VECTORIAL FINITE-ELEMENT ANALYSIS OF VERTICAL COUPLING BETWEEN D-FIBER AND BURIED OPTICAL CHANNEL WAVE-GUIDE

Light coupling to planar guided-wave optical circuits by means of directional coupling between D-fiber and buried square waveguide is indicated as a viable means of reducing alignment tolerances in fiber-pigtailing procedure of integrated optical devices. The geometry studied consists of a circular core fiber vertically coupled to a buried square waveguide. An accurate analysis by a vectorial formulation of the finite element method is presented. The structure is analyzed for generic cladding and core configurations. The dependence of power transfer efficiency from waveguide size and refractive index is investigated. The influence of the vertical separation between D-fiber and square waveguide on the overall coupled power is also analyzed. It is shown that because of mode mismatch at the input and output sections of the coupler between the field distributions of isolated guides and coupled guides, the power transfer is limited by radiation losses. By appropriately sizing the coupler and by choosing suitable refractive index values, the power transfer efficiency from the D-fiber to the square waveguide can approach 100%.