1346 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 10, OCTOBER 2000 Vector Beam Propagation Analysis of Polarization Conversion in Periodically Loaded Waveguides S. S. A. Obayya, B. M. A. Rahman, and H. A. El-Mikati Abstract—In this letter, an improved design of a short and low-loss polarization converter is proposed. It consists of an asymmetrically periodic loaded rib waveguides. At a wavelength of 1.5 m, 98% polarization conversion ratio is obtained with a relatively short 0.74-mm device length and an extremely low 0.13-dB total insertion loss. The simulation results are obtained using a full vector beam propagation method based on the finite element method. Index Terms—Finite element method, optical waveguides, polar- ization conversion, vector beam propagation method. I. INTRODUCTION P OLARIZATION converters are one of the key components in optoelectronic integrated circuits for coherent optical system applications, particularly to control polarization states. Also, they are used in balanced polarization diversity hetero- dyne receiver to provide a constant 45 operation. Earlier polar- ization converters have been reported based on the electrooptic effect [1]. On the other hand, Shani et al. [2] have introduced passive converters based on the use of asymmetrically periodic loaded rib waveguides. Polarization conversion can take place at the waveguide irregularities along the axial direction and by introducing deliberately waveguide asymmetry, polarization ro- tation at each of the junction could be enhanced. Furthermore, if each segment length in a periodic structure is adjusted to match half of the beat length between the two fundamental polariza- tion modes, constructive interference between these two modes may lead to a complete polarization conversion. Although good polarization conversion ratios have been reported earlier [2], but at the expense of several mm’s device length and around 2–3 dB insertion loss. The use of guided-wave structures with tilted or angled facets [3], [4], also have been reported recently with very good polarization conversion ratios and low levels of losses. However, these structures because of tilting or slanting walls do not lend themselves to easy fabrication as a complex fabrication process required by combining both the dry and wet etching techniques. In this letter, an improved design of a polarization converter is introduced. The proposed structure consists of periodic-loaded asymmetrical semiconductor rib waveguides. The effect of rib width on the segment length, device length, polarization con- version ratio, and radiation loss is investigated thoroughly. Al- Manuscript received May 19, 2000; revised July 10, 2000. S. S. A. Obayya and B. M. A. Rahman are with the Department of Electrical, Electronics and Information Engineering, City University, London EC 1V 0HB, U.K. (e-mail: B.M.A.Rahman@city.ac.uk). H. A. El-Mikati is with the Department of Electronics and Communications, Faculty of Engineering, Mansoura University, Mansoura, Egypt. Publisher Item Identifier S 1041-1135(00)08616-X. Fig. 1. Evolution of the two polarization powers along the propagation direction in an asymmetrically loaded waveguide for different rib widths. Fig. 2. Variation of the segment length and the polarization conversion ratio in an asymmetrically loaded waveguide with the rib width. though, a semivectorial beam propagation method (BPM) can identify polarization dependence; however, only a full vectorial approach can calculate power-coupling between two polariza- tion states. Numerically simulated results reported here have been obtained by using the recently proposed [5] numerically efficient vector beam propagation method, based on the versatile finite element method (VFEBPM). The problem is formulated, in terms of only the two transverse magnetic field components, and the robust perfectly matched layer (PML) is incorporated, in order to absorb effectively the nonphysical radiation waves. Since this formulation is based on the magnetic field compo- 1041–1135/00$10.00 © 2000 IEEE