An optical fibre switch employing a Sagnac interferometer

Local area networks and high-bit-rate telecommunications demand fast switches for modulators, re-routing and demultiplexing. Optically-driven nonlinear optical switches based on intensity-dependent phase shifts in an optical fibre have been demonstrated at femtosecond switching speeds. An all optical switch has the benefit of compatibility with the rest of the optical network and its availability would considerably enhance the capabilities of optical transmission systems. An optical fibre switch which exploits the non-linearity in silica fibre may be polarimetric or interferometric. However, most configurations are very environmentally unstable and suffer large phase shifts due to temperature and vibration. The exception is the Sagnac interferometer, which uses the same path for both interfering beams and is therefore only sensitive to perturbations that occur in a time less than the loop transit time. We describe here the basic theory of operation of the Sagnac fibre switch, followed by an experimental demonstration of a stable switch operating on a picosecond time scale