Physicists Find Fiber's Limit



Scientists from Bell Labs have calculated the theoretical limits on the carrying capacity of glass optical fiber and concluded that there's still a long way to go before optical systems reach those limits (see Bell Labs Calculates Limits). The results were published yesterday in the journal Nature.

Partha Mitra, lead author of the paper, says his work will point the way for future research by showing which approaches are likely to come up against fundamental physical limits and which aren't. It could also aid engineers whose task it is to model the enormously complicated properties of DWDM (dense wavelength-division multiplexing) systems.

Unlike system vendors, physicists measure the information-carrying capacity of a fiber in bit/s per Hertz of spectral bandwidth (bit/s/Hz). To find the capacity in Gbit/s or Tbit/s, this number has to be multiplied by the available bandwidth of a system (that's bandwidth in the physics sense, in Hz, rather than its common telecom usage, which essentially means capacity).

Mitra and his co-author Jason Stark calculated that the theoretical limit imposed by the physical properties of optical fiber on a communication system is about 3 bit/s/Hz. This corresponds to a maximum payload of 150 Tbit/s on a single fiber, assuming that the fiber can carry signals across the wavelength range 1260 to 1620 nanometers.

Mitra points out that all existing optical systems have a lower limit of 1 bit/s/Hz. That's because they encode data using a simple on-off keying technique, which represents bits by the presence or absence of light. "We've shown that the theoretical limits are substantially greater than this," he says. "What this means is that by changing the modulation scheme, it's possible to get more data into a fiber than was thought possible."

The downside? While the work at Bell Labs suggests that fiber has plenty of room to grow, new technologies -- more complicated modulation schemes and coherent detectors, which measure both power and phase of the incoming signal -- will be needed to make the most of it.

Few would argue with Bell Labs' rather basic conclusion -- that fiber has more capacity than is currently being used. It's a no-brainer. What's new is that the researchers have been able to quantify how much surplus capacity there is, something that can't be deduced from existing communications theory.

The classical formula for calculating capacity, known as Shannon theory, predicts that capacity will increase indefinitely as the power of the optical signal goes up. That's because the signal keeps getter stronger relative to the noise, which is fixed.

In real life, however, strange "non-linear" phenomena come into play, and start creating more noise at high optical power. Mitra calls it the cocktail-party effect. "If everyone's talking at once, then you have to raise your voice in order to be heard, and if everyone raises their voice, then you can't hear anything." Much the same thing can occur among channels in the same fiber generated by DWDM systems.

The origin of non-linear effects is the fact that, rather unexpectedly, the speed of light inside a silica fiber does depend on its intensity, or instantaneous power. (Remember, the speed of light is only constant in a vacuum.) This is most likely to be observed in DWDM systems where lots channels of data are packed into the same fiber, creating very high total optical powers.

"People knew that non-linearities were doing something, but they couldn't quantify it precisely," says Mitra.

Mitra and Stark were able to include non-linearities in the calculations for the first time. Why hadn't this been done before? Simply because it required some creative mathematical thinking to reduce the equations to ones that could be solved analytically.

— Pauline Rigby, Senior Editor, Light Reading
http://www.lightreading.com
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