Aberration losses of the microoptic directional coupler

A theoretical analysis of the aberration losses of the microoptic directional coupler recently being used in optical communication systems has been carried out. We show that our results are consistent with those obtained experimentally and that the minimum insertion loss does not occur at the paraxial length (which has been used in the experiments), but at an optimum length, which is slightly less. However, the tolerance in the positioning of the output fiber is less for the optimum length than for the paraxial length, although the increased loss at the optimum length due to poor positional adjustments within the tolerance value is still considerably smaller than the minimum loss at the paraxial length.