The 100 GHz light switch

Escalating transmission rates in telecoms networks are the spur behind an international research effort in optical processing. The author reports on the work of the BT researchers who are laying the foundations for a new era of photon-based switching systems. The work is based on an interferometric switch known as the TOAD. The TOAD is a variant of a Mach-Zehnder interferometer. Light enters the interferometer, then passes through a 50:50 coupler which divides the input equally between the upper and lower arms. In general, the upper and lower light streams will experience different phase shifts, and these shifts will determine the way the original input is allocated between the two output ports. In particular, if the interferometer is suitably biased then all the light will appear at one or other of the outputs. Now, if a phase shift is introduced into one of the arms the output will switch between ports. This is the principle; its practical exploitation depends on the availability of a device that can effect a π phase shift at a rate of at least tens of GHz. In the TOAD this function is assigned to a semiconductor optical amplifier