Fiber Crosses the 10-Trillion-Bit Barrier

French and Japanese engineers have squeezed more than 10 trillion bits per second through single optical fibers. This record capacity equals about 150 million simultaneous telephone conversations.

Groups from the French telecommunications giant Alcatel and the Japanese manufacturer NEC achieved the record results separately and announced them on March 22 at the Optical Fiber Communications Conference in Anaheim, CA.

Developers traditionally report record-setting “hero experiments” at the annual show. Typically, the maximum capacity of commercial systems lags behind the record-setting pace by only a few years. On March 13, the German firm Siemens A.G. announced it had transmitted 3.2 trillion bits per second through parts of WorldCom’s fiber-optic network in the Dallas area during a month-long trial that marked a record outside the laboratory.

Divide and Conquer

To set data-transmission speed records, engineers must pack signals into a fiber as efficiently as possible. To do so, they squeeze many separate transmission signals into the available optical spectrum, just as radio or television stations are packed together in the broadcast bands. The total capacity depends on both the number of separate channels and the speed at which each channel transmits.

Optical systems transmit each channel at a separate wavelength, using a technique called wavelength-division multiplexing. All channels must fit within a range limited by signal attenuation within the fiber and by the optical amplifiers needed to span long distances. Reducing space between the channels increases the number that can carry data but also increases the risk of interference between adjacent channels.

The French and Japanese groups struck different balances to crack the 10-trillion-bit barrier. Both teams used individual channels transmitting at 40 billion bits per second, the same data rate used in the WorldCom demonstration. However, Alcatel concentrated on tightly packing signals in the limited wavelength range of the erbium-doped fiber amplifiers needed for long-distance transmission. NEC instead added a new type of amplifier to extend the available wavelength range.

Alcatel Packs ‘Em In

Sebastien Bigo of Alcatel Research and Innovation in Marcoussis, France, and 14 other Alcatel engineers worked within the conventional and long-wavelength bands of standard erbium-doped fiber amplifiers. They spaced channels at alternating intervals of 50 and 75 gigahertz, corresponding to wavelength shifts of about 0.4 and 0.6 nanometers in the 1550-nanometer range of erbium amplifiers. The uneven spacing allowed them to add special optical filters, which shaved off part of each channel, so that adjacent channels did not interfere with each other. That packed 32 channels into each of two erbium fiber amplifiers operating in different wavelength bands.

The Alcatel team then doubled transmission capacity by transmitting one set of signals in one polarization and a second set in the opposite polarization, raising the total number of channels to 256 and the total speed to 10.2 trillion bits per second. They showed that the signal could travel through 100 kilometers of fiber.

NEC Plugs In an Amp

Kiyoshi Fukuchi of NEC Computer and Communication Media Research in Kawasaki, Japan, and seven NEC coworkers added a third amplifier operating at slightly shorter wavelengths, and stretched the two erbium-fiber bands. This new amplifier, based on optical fibers doped with the rare earth thulium instead of erbium, gave them a much wider range of wavelengths than the Alcatel group.

NEC researchers took advantage of this broader range to pack channels less densely. Instead of overlaying signals of different polarities at the same wavelength, they staggered the wavelengths. In this arrangement, each channel was 50 gigahertz from two channels with different polarization and 100 gigahertz from identically polarized channels, minimizing interference. Using this approach, NEC transmitted a total of 273 channels at 40 gigahertz each, setting a record of 10.9 trillion bits per second through 117 kilometers of fiber.

Both systems are well above the fastest commercial systems, which transmit up to 160 channels at 10 billion bits each. Telephone companies have yet to install transmitters in all of those slots, and most systems operate at much lower speeds.

For more than two decades, long-distance fiber-optic speed records have roughly held their own with Moore’s Law (the doubling every 18 months of the number of transistors packed on a chip). This steady growth in fiber capacity has driven down the prices of long-distance phone calls and opened the array of information pipelines that carry Internet traffic.

Still, the relentless growth of Internet traffic makes even bigger fiber-optic pipelines look very attractive.