Optical bistable device

A bistable optical device comprising:

• light responsive means for receiving an input light signal and having an input electrode for receiving a bias signal so that an incident light input signal produces reverse current flow;
• light emitting means having an input electrode for receiving a bias signal, said light emitting means being electrically connected to said light responsive means so that said reverse current flow causes emission of light and being positioned with respect to said light responsive means so that at least a portion of the emitted light is incident upon said light responsive means to increase said reverse current flow in positive feedback so that the intensity of the output light changes from a low to a high value at a first change point value of input light and changes from a high to a low value at a second change point value of input light less than said first value;
• and
• light modulating means, disposed on a chip between said light responsive means and said light emitting means having input terminals for receiving a control signal to vary the amount of light received by said light responsive element from said light emitting element and hence continuously vary said first and second change point values.