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Controlling Lasers with Chaos

Adding noise to telecom lasers could potentially boost bandwidth.

Researchers believe that they have found a way to speed up traffic over existing telecommunications networks. Usually, random fluctuations, or noise, in an optical or electrical system is a bad thing, and engineers strive to eliminate such unpredictable behavior from electrical current and light. But now researchers have shown that a little bit of noise can actually enhance the output of some of these systems. A collaboration between researchers from SUPELEC, in Metz, France, Free University of Brussels, Belgium, and the Public University of Navarra, Spain has resulted in controlling previously chaotic laser light, just by adding a small dose of noise to the electrical current that powers it.

Marc Sciamanna, a professor of physics at SUPELEC, says that the work is important because it could improve a popular type of laser used in telecommunications applications. These vertical-cavity surface-emitting lasers (VCSELs) are less expensive to mass-produce than other lasers, and they couple more efficiently than other lasers with the fiber-optic cables used in these networks. However, VCSELs have been a bit of a mystery to scientists and engineers because no one has been able to completely control the emitted light.

Typically, data sent using such lasers is encoded by rapidly changing the frequency or phase of the light. The polarization of the light–which describes the orientation of its magnetic field–traditionally isn’t used because this quality tends to flip unpredictably between two states. The SUPELEC team has found a way to control the polarization, which could ultimately be used to boost bandwidth in existing networks. “If one controls polarization of a laser well enough,” Sciamanna says, “then it would motivate using polarization as a degree of freedom to encode data.”

Sciamanna’s team tested a series of electrical inputs that had different noise intensities. They found that as they added more noise to the electrical current, the polarization flips became more regular; however, as the noise level increased further, the system reverted to a chaotic state.

While it may seem counterintuitive to add noise to make a system more predictable, it’s actually a concept that has been experimentally demonstrated for years. Even so, says Sciamanna, scientists aren’t sure exactly how or why it works.

One example that’s well known to researchers involves adding a noisy electrical signal to a collection of neurons, which normally fire at random, explains Claudio Mirasso, a professor of physics at the Universitat de les Illes Balears, in Palma de Mallorca, Spain. “For an intermediate noise level, the [neuron] pluses are more regular,” he says. But before Sciamanna’s work, which will be published in an upcoming issue of Physical Review Letters, no one had applied the concept to regulating the polarization of VCSELs.

The findings are especially useful for telecommunications networks, where “noise is inevitably present,” says Sciamanna, because noise is inherent in the equipment. He suspects that it wouldn’t be too difficult to take some of the existing noise in these networks and put it to good use, regulating VCSEL polarization. And since the polarization can be regulated, information can reliably be encoded onto those states, Sciamanna says.

But the research is still in the early stages, Sciamanna says. Currently, the researchers are methods to adjust the polarization fast enough to make it useful in telecommunications networks. And, as with any lab experiment, the technique still needs to prove itself in the field where variables, such as temperature changes, arise.

“While most of the time, noise is intended to be reduced to its minimum level, these studies show that some nonlinear systems can take advantage of the noise to improve their response,” says Mirasso. He adds that the team’s work “is another nice and elegant demonstration” that, when used cleverly, noise can be good for a system.

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