6. Theory of directional couplers

6.1 Definition

A directional coupler (figure 35 ) is an octopole which joins together two pairs of lines or guides in such a way that the lines of the same pair (1 and 3 or 2 and 4) are decoupled:

• channel 1 (or 3) is coupled to channels 2 and 4 and not to channel 3 (or 1);

• channel 2 (or 4) is coupled to channels 1 and 3 and not to channel 4 (or 2).

Let P ₁ be the power sent to channel 1 and P ₂ , P ₃ ...