Materials for Integrated Optics: GaAs

Abstract

Integrated optics was born out of the needs of optical communications. Communication by light waves is not a new idea. To understand its requirements, consider the early example shown in Figure 1. Simple as this system is, it has the essential elements of the most modern digital communications system. First, a source of the light wave, or carrier wave, is needed. For the smoke-signaler, this was a bonfire. In a modern system the bonfire is replaced by a laser or light-emitting diode (LED). The next step is modulation, putting the message on the light beam. Currently the smoke-signaler's blanket is likely to be replaced by an acoustoor electro­ optic modulator. Alternatively the laser may be internally modulated, as we shall discuss later. In a digital system the modulator functions much as the blanket does, stopping or passing a pulse of light (rather than a puff of smoke) . The message is sent in a prearranged code, ours being much more sophisticated than that used by the smoke-signaler, of course. The next requirement of the system is a transmission medium. The smoke­ signaler used the air, but it was, particularly over large distances, un­ reliable. Recently it is being replaced as an optical-communication medium by thin, highly transparent glass fibers. Finally, a detector is required at the receiving end, an eye in the case of the smoke-signaler, plus a decoder, the attached brain. One modern kind of detector is the photodiode, probably feeding into an electronic decoder connected ulti­ mately perhaps to a telephone receiver or TV display.