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.
Osamu Mitomi, Naoto Yoshimoto, Katsuaki Magari, Toshio Ito, Yosihiro Kawaguchi, Yasumasa Suzaki, Yuichi Tohmori, and Kazuo Kasaya, "Analyzing the Polarization Dependence in Optical Spot-Size Converter by Using a Semivectorial Finite-Element Beam Propagation Method," J. Lightwave Technol. 17, 1255- (1999)