Optical crossbar switch

An optical crossbar switch comprising a plurality of input devices, a plurality of output devices, an optical transpose system positioned between the input devices and the output devices, and control means for controlling the interconnections between the input devices and the output devices, wherein the optical transpose system has first, second and third stages, the first stage being such as to direct light from the input devices to the second stage, the second stage being such as to re-arrange beams input thereto from the first stage for re-direction to the third stage, and the third stage being such as to direct light input thereto to the output devices, and wherein each of the first, second, and third stages operates as a Fourier transform stage such that the optical transpose system produces at each output device a Fourier transform of a beam at the associated input device.