Directional coupler switch with reduced voltage-length product

The phase change×interaction length product needed to switch a directional coupler is shown to be reduced by decreasing the coupling coefficient at the middle of the interaction region. To implement the structure with parallel waveguides, an additional fabrication process to produce the low index region was needed. Instead of reducing the index, enlarging the waveguide gap can be used to decrease the coupling coefficient to gain the same effect. To reduce the coupling coefficient at the middle of the interaction region, the gap between waveguides was gradually enlarged from both ends to the center. To simplify the fabrication processes, the varied waveguide gap structure was used. The single-mode waveguides were fabricated on z cut LiNbO₃ by in-diffusing 7-μm-wide ti. The design, fabrication, and measurements are discussed