Abstract:A silicon-on-insulator 2 × 2 Mach-Zehnder thermo-optical switch is developed based on strongly guided paired multimode interference couplers. The multimode-interference couplers were etched deeply for improving coupler characteristics such as self-imaging quality, uniformity and fabrication tolerance. The proposed switch achieves good performances, including a low insertion loss of −11.0dB, a fiber-waveguide coupling loss of −4.3dB and a fast response speed measured to be 3.5 and 8.8 μs for raise and fall switching time, respectively. Supported in part by the National “973” Plan (No. G2000-03-66 and No.2006CB921702), National Natural Science Foundation of China(NO. 10674185).and the Science Foundation of the Central University for Nationalities (No. CUN07A)