The optical switching behaviour of a traveling wave active directional coupler in InGaAsP/InP under optical control is investigated. The best configuration of bar and cross-state has been found by implementing two homemade BPM versions based on the method of lines and on the finite difference suitably adapted for active device analyses. The effect of the longitudinal diffusion of the charge density is accounted by the BPM, too. For a device length L=1485 /spl mu/m we have obtained a good cross-state with crosstalk CT/sub cs/-20 dB and gain G/sub cs/=17.4 dB, whereas the best bar-state, with CT/sub bs/=-9.2 dB and G/sub bs/=3.6 dB, is not completely reached.