Optical Modulation and Coding

It can be argued that optical communications had its origins in ancient times, where modulated sunlight was often used to convey information over large distances. For example, mirrors have been used to create bright flashes of light in certain directions, producing a form of on–off modulation. Similarly, blankets used to cover a signal fire periodically produced puffs of smoke that could be seen for miles in the clear desert air, producing, in effect, a modulated signal. In modern times, navies throughout the world used bright incandescent light sources directed by a reflector and blanked by a manual shutter to send messages between ships often kilometers apart on the high seas. Before the invention of the laser, long-range optical communication was envisioned using bright flashes of light produced by intense pulses of electric current passing through an incandescent fiber placed in the focal-plane of an optical reflector. Even exploding wires that generated bursts of intense optical energy were considered for long-range applications. However, these sources were not effective in the production of intense, highly directional optical energy that could also be modulated at high enough data-rates to be seriously considered for deep-space optical communications.