The crosstalk in three-waveguide optical directional couplers

An expression for the crosstalk in three-waveguide directional couplers is derived. The crosstalk is found to be due to the uneven spacing in the distribution of the propagation constants of the three eigenmodes of the couplers. Similar to the crosstalk in the two-waveguide directional couplers, it is also proportional to the square of the field overlap of the two neighboring waveguides. This feature seems to be universal in the straight couplers.