Broad-band optical directional couplers and polarization splitters

A method is described for designing broadband optical directional couplers. The basic idea is to determine an optimum distance between the waveguides where wavelength dependence of the coupling coefficient vanishes at a given wavelength. The method is applied to design directional coupler polarization splitters. As an example the computer design of a polarization splitter made of single-mode twin-core optical fiber is presented. The extinction ratio obtained for waves with undesired direction of polarization is better than -20 dB within a wavelength range of 80 nm. The sensitivity of the design to variations of the main parameters caused by the production process is discussed for characteristic examples