Symmetrical directional coupler as a wavelength multiplexer-demultiplexer: theory and experiment

The simulation of a symmetrical, graded-index waveguide directional coupler as a wavelength multiplexer-demultiplexer using the effective index method is presented. The design considerations in terms of the parameters associated with the fabrication of the device are considered. The modeling is used to arrive at the fabrication conditions of a demultiplexer operating at 1.32/1.56 μm in Ag⁺-Na⁺ exchanged BK7 glass. The performance of the demultiplexer agrees very well with the modeling results. Crosstalk of -30 and -32 dB at 1.315 and 1.561 μm wavelength, respectively, and a total insertion loss of 1.25 dB for a 14.5 mm long device with propagation loss 0.15 dB/cm have been achieved. Lowest measured cross-power ratio is less than -40 dB at 1.315 μm