TAPERED, BOTH IN DIMENSION AND IN INDEX, VELOCITY COUPLER - THEORY AND EXPERIMENT

We consider a novel, tapered, velocity coupler (TVC) that is tapered both in index and in dimension which meets the adiabatic invariance condition with sufficiently strong coupling between the fundamental modes of individual guides in the middle region of the coupler while permitting individual excitation at the input end and sorting of the modes at the output end. This approach helps reduce the device length considerably by permitting much higher taper angle. We model a TVC that consist of one tapered and another straight, graded index waveguide, by using normal modes of the entire, composite TVC structure. The analytical results are in excellent agreement with experimental results in a TVC fabricated in Ti:LiNbO3. In addition, we show that the representation of the local normal modes as the superposition of the modes of the uncoupled guides leads to erroneous results, as the avoidance of mode conversion between the local normal modes in a reasonably short length TVC invariably involves strong coupling between the guides and that the use of actual local normal modes of the TVC structure under consideration is imperative in the modeling to accurately describe the device.