Trillion-unit 'hero' of fiber optics.

As light waves moving in glass fibers take over more and more of the world's communications traffic from electrical impulses in copper wires, laboratories keep striving to increase and extend fibers' performance. Those working in the field refer to these efforts as "hero" experiments, because they keep breaking records in length of distance traveled without a repeater to boost the signal or in number of bits of information transmitted per second.

Scientists usually multiply the two criteria together to get a hybrid unit, bit-kilometers per second, which they use as a figure of merit to compare different experiments. In those terms the outstanding record breaker of last week's Conference on Optical Fiber Communication '85, held in San Diego, was an experiment by AT&T Bell Laboratories, which reached 1.37 trillion bitkilometers per second, in taking a signal of 20 billion bits per second over a fiber 68.3 kilometers long. This is equivalent to carrying 300,000 simultaneous telephone conversations or 200 high-resolution television channels in the single fiber, according to Bell Labs.

More significant for future engineering is that this record was achieved by multiplexing, combining 10 different signals of 2 billion bits per second each in a single fiber. Multiplexing is a key characteristic of copper wire circuitry, and fibers must be able to match this ability to compete.

According to N. Anders Olsson of Bell Labs' Murray Hill, N.J., installation, who led the experimental group, the experimental apparatus took 10 communications channels from 10 differen lasers and combined them into a single fiber. Each of the 10 incoming channels was slightly different in wavelength from the others; they were spaced 1.35 nanometers apart over the range from 1.529 to 1.561 mulitplexer, designed by one of the group, John Hegarty of Murray Hill, consists of a linear array of 23 fibers, 22 of which can carry incoming signals. One fiber, in the center of the array, takes the outgoing signal. The incoming signals pass through a lens and hit a diffraction grating that reflects them, each different wavelength at a slightly different angle. The signals then go back through the lens. The combination of lens-grating-lens angles them in such a way that they all combine in the central fiber. This output fiber was connected to 68.3 kilometers of transmission fiber. At the other end, a similar grating served as demultiplexer. Although the apparatus can take 22 incoming channels, the experiment stopped at 10, Olsson says, because they had no more room for lasers on the table.

There were no crosstalk effects between the channels, Olsson says, and the data rate was a 10-fold improvement over previous efforts. He estimates that, working at its capacity of 300,000 simultaneous telephone conversations, such a system could ring up $8.6 million per day in revenues.