Length minimization design considerations in photonic integrated circuits incorporating directional couplers

Because directional couplers involve channel waveguides which are very close to one another, transition regions to regions where channel waveguides are widely separated are utilized. The total length of a directional coupler and transition regions can be minimized for a particular degree of field confinement. Calculations presented for LiNbO3-, GaAlAs-, and SiO2/Si-based optical channel waveguides demonstrate the presence of a minimum total length corresponding to a particular degree of field confinement. The overall length at the minimum is shown to be significantly lower than for other values of field confinement allowing single-mode operation. This implies that either more devices can be integrated on a substrate or that less material is needed for an integrated optical circuit.