The cross talk and power consumption of the 2 × 2 optical switch is a key metric in the design of large-scale photonic integrated circuits (PICs). We build a theoretical model of a 2 × 2 Mach–Zehnder interferometer (MZI) optical switch, taking into account both imbalances in the arm loss and the coupler splitting ratio. The splitting ratio imbalance requirement for a given switch cross talk is summarized, which provides a guideline for the switch design. A 2 × 2 multimode interference (MMI) coupler design with low excess loss (<0.14 dB) and low imbalance (<0.13 dB) over the C-band is given. Its robustness to width variations up to ±40 nm is demonstrated. The 2 × 2 MZI switches with such an MMI are fabricated on a silicon-on-insulator (SOI) substrate using both electron beam (EB) and deep ultraviolet (DUV) lithography. The EB and DUV MMI switches exhibit cross talk of <−35 dB and <−30 dB over 40 nm, respectively. Folded configurations are used for both the silicon waveguides and the metal heaters to increase the power efficiency, resulting in a P π of ∼8 mW.