Critical dimensions of directional couplers

The critical dimensions of directional couplers include the waveguide separation at the coupling position, the coupling length, and the bending radius of the waveguides. This paper focuses on discussing the waveguide separation at the coupling position. Two directional couplers have been developed with different waveguide separation. The relationship between the waveguide separation and the performance of the couplers has been studied when the bending radius and the coupling length keep constant. It is found that the waveguide separation is strongly related to the insertion and polarization dependent loss (PDL) in the directional couplers.