The author claims that in many situations a light-emitting-diode (LED) link can provide the required optical radiation, collimated and well modulated, without the physical and electrical complexity required for laser modulation. Several breadboard models have been built, operating at ranges exceeding 300 feet and at data rates of one million bits per second. There are shown two models built at the Manned Spacecraft Center of the National Aeronautics and Space Administration, one for data transfer between a space shuttle and its booster, the other for demonstrating feasibility of replacing an umbilical cable with an optical link. Design considerations are presented for the transmitter subsystem, choice of detector, choice of optics (parabolic reflector, or this plus a lens). An approximate equation from the transmitter power required within the beam for the NASA laboratory system is given. It is pointed out that a bandpass filter matched to the LED's spectral width and center wavelength reduces optical noise.
