Polarization‐independent guided‐wave optical hybrid coupler

Abstract

AbstractA new guided‐wave hybrid coupler is proposed and its operating analysis is described. The device structure resembles that of a conventional directional coupler. It consists of a parallel portion in which two waveguides, with identical phase constants, are placed closely in parallel and of input and output sections, in which two waveguides separated sufficiently to allow input and output from other devices, adjoin the parallel portion. Although one of the input‐output sections is symmetric with the phase constants of the two waveguides equal, another has a nonsymmetric structure with different phase constants. The optical wave coupling in this non‐symmetric part plays an important role for the device operation.In the analysis of operation, the coupling coefficient and difference of the phase constants between the waveguides are used as the waveguide parameters. The range of these parameters was obtained as necessary for stable operations in terms of difference in polarization and fabrication errors. It is shown that a rather large coupling coefficient and a large phase constant difference are needed in the case where the phase constant difference is constant. Hence, a new structure is proposed in which the phase constant difference increases gradually as the waveguide spacing is increased. This structure is analyzed. Unlike the conventional directional coupler, this structure has a large fabrication tolerance as regards the waveguide length in the parallel portion and the coupling coefficient and operation independent of the polarization can be obtained.