We demonstrate a type of 2 x 2 multimode interference 3 dB coupler based on
silicon-on-insulator. The fabrication tolerance was investigated by the ef
fective index method and the guide mode method. The devices with different
lengths were fabricated and near-held output images were obtained. Toleranc
es to width, length and etch depth are 2, 200 and 2 mum, respectively. The
devices show a uniform power distribution.