This paper describes a semivectorial wide-angle finite-element beam propagation method (FE-BPM) that uses the Pade approximation and that can allow efficient and accurate analyzes of the polarization dependence in arbitrary step-index optical waveguide devices. It also reports the use of this method to analyze the polarization dependence of coupling loss between a semiconductor tapered-waveguide spot-size converter and a single-mode optical fiber. It is shown that semiconductor spot-size converters having cores with a small cross-section provide low-loss and polarization-insensitive coupling to flat-end fibers. A low-loss (far less than 1 dB) and completely polarization-insensitive spot-size converter can be made using a lightly n-doped InP substrate for the tapered waveguide. These spot-size converters are consequently potentially useful for making polarization-insensitive semiconductor optical devices such as optical amplifier gate switches.
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