A single-mode optical-fibre switch which is electrically activated has been fabricated. This device is achieved by forming two couplers in series on a pair of single-mode optical fibres to provide a compact Mach-Zehnder interferometer (MZI). Power switching in the output arms is achieved by introducing a phase shift or path difference in the MZI by various means. The application of the device as a wavelength-selective switch is also considered.