United model of fiber coupler based on variation technique

For the irregular shape of a fiber coupler, the coupling characteristics of it are usually studied in its different sections separately. It's necessary to build a united model to simulate the coupling characteristics of each section all together. Some appropriate geometrical configurations were employed, and modal field in a coupler was obtained by the weighted superposition of triangular distribution and Gaussian distribution, in the condition of lossless propagation. With that a united model was obtained by the variation technique, combined with the concept of local modes. It's a more refined model, which is available on condition that the core radius in the neck waist is around 1 μm and can provide theoretical guidance for the practical design and fabrication of a fused-tapered single-mode fiber coupler. In its effective area, there is a good agreement between the theoretical result and experimental result.