Phase sensitivity of directional couplers

Directional couplers of various design have been treated widely, both theoretically and experimentally. However, in most cases directional couplers are designed as 1 to 2 port circuits. If both input ports are used, the phase of the guided waves has to be taken into account. The influence of the phase term on the switching behaviour of Δβ-reversal couplers is discussed as an example. It is shown that the phase difference of two waves coupled to both coupler input ports will not affect crossover and straight-through states, but all states in between. Therefore electrical controlled 2 to 2 port switches will become very sensitive, if low crosstalk is required. This effect can be used to design an all optical working switch or optical logic.