RADIATION LOSS OF GRATING-ASSISTED DIRECTIONAL COUPLER

We compute the coupling between the guided modes and between guided and radiation modes of two parallel slab waveguides forming a directional coupler in order to determine the radiation losses introduced by the coupling grating. Because of its complexity, we solve the problem in two stages. First, we compute the guided modes for each waveguide separately and determine the radiation modes only for the more complicated of the two waveguides, the one that is nearer to the grating. Modifications caused by the presence of the opposite waveguide are then taken into account by computing first-order correction terms. For a practical example of slab waveguides defined in GalnAsP, we find that the radiation losses per power exchange length remain below 0.02 dB for a rectangular grating of 0.01 μm depth. The corresponding power exchange length is approximately 1 cm. The loss per power exchange length increases linearly with grating depth. © 1990 IEEE