Optical Logic Devices

Free-space digital optics relies on optical devices which may be required to play the same role as a transistor in digital electronics. In addition to being an optical port, i.e., a modulator or emitter and a detector, they may also be required to act as a thresholding device. Such devices are called optically bistable (OB). OB devices are optical elements which, over some range of light input powers, have two possible output states. The range of powers over which they are bistable corresponds to a region of hysteresis and is bounded by two discontinuities at which switching between the two states can occur. Such switching can be induced by holding close to one of these discontinuities and making an incremental change in the total light input. This power increment can derive from an independent (signal) input. It is possible to obtain a change in output larger than the signal input and hence achieve digital gain.