Polarization Coupling Between Strongly Guiding Waveguides Stacked Laterally

The full-vectorial beam-propagation method is applied to the assessment of the coupling characteristics between strongly guiding waveguides stacked laterally. The polarization crosstalk behavior of square waveguides stacked laterally is demonstrated by the eigenmode and beam-propagation analyses. In order to make use of the polarization crosstalk constructively, we determine the vertical spacing between the two square waveguides. Almost complete conversion can be obtained, when the two waveguides are stacked diagonally. Another coupler composed of two rectangular waveguides is studied to realize the polarization conversion without exchanging the power with the same polarization. A crossing-waveguide-type polarization converter is also developed, with a short device length of less than 80 mum at a wavelength of 1.55 mum.