All these systems have low information capacities. A major breakthrough that led to high-capacity optic communications was the invention of the laser, in 1960. The laser provided a narrowband source of optic radiation suitable for use as a carrier of information. Lasers are comparable to the radio-frequency sources used for conventional electronics communications. Unguided optic communications systems (nonfiber) were developed shortly after the discovery of the laser. Communications over light beams traveling through the atmosphere was easily accomplished. The disadvantages of these systems include dependence on a clear atmosphere, the need for a lineof-sight path brtween transmitter and receiver, and the possibility of eye damage to persons who unknowingly look into the beam. Although somewhat limited in their use, these early applications aroused interest in to optic systems that would guide the light beam and thus overcome those disadvantages. In addition, guided beams could bend around corners and could be buried in the ground. The early work on atmospheric laser systems provided much of the fundamental theory and many of the actual components required for communications over fibers. Ironically, it is now known that laser sources are not required for all fiber systems. In many cases the broader-band light-emitting diode is suitable. (The choice of the proper light source a matter we discuss in this book. )