Mach-Zehnder waveguide interferometers have been fabricated in silicon and operation has been demonstrated at ? = 1–3/mi. The switching mechanism is based on the thermally induced variation of the refractive index of crystalline silicon. Modulation depths of 40% were obtained for switching powers of 30 mW and switching times of 50′s. © 1991, The Institution of Electrical Engineers. All rights reserved.