Mach-Zehnder switch using an ultra-compact directional coupler in a strongly-confining rib structure

A quantum well Mach-Zehnder switch operating at 1.55 μm, fabricated in a strongly-confining rib waveguide structure, and employing a multi-mode 3-dB output coupler only 40 μm long, is described. The device yields almost 10 dB of contrast between outputs at 3 V bias, with 4.3 dB of excess coupler loss. The total cleaved device length is only 1.5 mm, and high bandwidth (15 GHz) operability is anticipated. A simple computer model of the coupler is in good agreement with the measured results, and predicts enhanced performance with a small increase in the coupler length.