More heroes of optical communications.

If light is the wave of the future in communications, the fiber-optics technology that exploits it has to overcome a number of practical hurdles. Therefore "hero experiments" (so called because they keep breaking records) are testing not only the ultimate physical properties of the fibers and light sources but also various practical combinations of factors (SN: 2/23/85, p. 119). Several such achievements were reported at the recent meeting in San Diego of the Conference on Optical Fiber Communication '85.

Two related experiments at AT&T Bell Labs set new records for the speed at which the light signal can be modulated. Up to now the maximum rate of modulation had been 2 billion bits per sercond (2 gigabits per second), that is, 2 billion light pulses a second. Both these experiments raised to 4 gigabits per second, one using internal, the other external, means of modulation. Four gigabits per second equals 62,500 two-way voice conversations or 44 television channels.

Internal modulation means switching the light source on and off 4 billion times a second. In the past this had been difficult to do while still maintaining spectral control of the signal, that is, the precision of its wavelength. In long-distance transmission, dispersion of the signal as it passes through the fiber also hurts, causing loss of definition in the pulses. In the direct modulation experiment Alan Gnauck of AT&T Bell Labs in Holmdel, N.J., and co-workers used a specially designed distributed-feedback laser and showed that they could turn it on and off 4 billioin times a second to produce pulses with extremely stable wavelength. They trasmitted these signals over 103 kilometers of fiber without needing a repeater to boost the power and restore the signal.

In the external modulation experiment Steven K. Korotky of Holmdel and co-workers used an optical switch made of a lithium-titanium niobate waveguide element that shifted the light signal in and out of the fiber in response to an electrical signal pulsed 4 billion times a second. With the external modulation these experimenters could use a somewhat higher-powered laser, and were able to send the signal without repeaters over 117 km of fiber. People in the field often multiply the bandwidth (maximum modulation rate) times distance to get a figure by which to compare experiments. This one, Korotky says, establishes a record for a single fiber channel of 0.47 trillion bit-kilometers per second. It is also the first time, he says, that an optical switch has been used as an external modulator. The system would be economically competitive, he says, with a cost of about $300 for modulation. The other possible light sources for optical signals are light-emitting diodes (LEDs). As Paul W. Shumate Jr. of Bell Communications Research, Inc., in Murray Hill, N.J., told the meeting, using LEDs would be desirable, particularly in local loop applications (connections to subscribers' premises), because of their low cost and high reliability compared with lasers. However, potential users have feared that LEDs put too little power into the fiber.

Shumate and J.L. Gimlett, M. Stern, M.B. Romeiser and N.K. Cheung of Bellcom's Holmdel facility have shown that LEDs of both the surface-emitting and edge-emitting variety can produce enough power to send a signal of 140 million bits per second (a technologically reasonable rate) over several kilometers of fiber. This, they say, is sufficient for use in local subscriber loops.

The lasers used in most of these hero experiments are single-longtitudinal mode-emitters -- that is, they emit one very precise wavelength. Such lasers are expensive and difficult to make. More practical for long-haul transmissions would be a more-garden-variety laser that emits a short range of wavelengths. R. Goodfellow of Plessey Research (Caswell) Ltd. in Caswell, England, and co-workers used a buried heterostructure laser emitting at 1,556 micrometers to send 1,300 gigabits per second over 103 km of fiber engineered by Corning Glass Works of Corning, N.Y., to have minimum signal dispersion for the laser's wavelength. This, they say, represents the highest value yet recorded of the bit-rate-times-distance figure, 139 gigabit-kilometers per second, for a multilongitudinal mode (imprecise wavelength) laser. They see it as a practical route to long-haul, high-speed systems in the short term.