Abstract

The fused tapered biconical optical fiber coupler has become the key type of fiber couplers [1]. Such coupler can be designed for wavelength-multiplexing and polarization beam splitting. Fig. 1 gives a sketch of the fused section of the coupler. In the neck region of the tapered coupler the light is guided by the boundary between the (reduced) fiber cladding and the external medium which is air unless some other potting material is employed. In this paper we describe a computer model which we have established for simulating the performance of the fused couplers based on rigorous analysis of the corresponding composite waveguide mode problems. The model can treat couplers with different cross sections resulting from different fusing conditions, for example, weakly-fused coupler composed of touching circular waveguides and strongly-fused coupler with dumbbell-shaped cross section. Recently, Okamoto [2] has used the effective-index method and the step approximation to analyze the mode coupling in the fused coupler and performed a detailed theoretical investigation of light coupling phenomena in wavelength-flattened couplers. The two-dimensional coupler structure was converted into an equivalent one-dimensional slab problem in the effective- index method. In our model the two-dimensional composite waveguide problem is solved in a full-wave vectorial manner and the polarization effect is fully taken into consideration. We shall show that the full-wave simulation results are significantly different from those based on the approximate analysis.

© 1994 Optical Society of America