Using all-optical controls could speed the transmission of telecommunications data, but optical switches that can work at high bandwidths need a lot of energy to turn on and off. So the usual approach is that telecom providers deploy systems that convert light into electrical signals to process the data, then back into light for transmission.
Light work: Michelle Povinelli uses mathematical modeling to study how light moves through structures.
Now theoretical work from researchers at the University of Southern California and Stanford University suggests a way to get around this trade-off between bandwidth and power. Their simulations suggest that controlling the input beam by using new ways of shaping light pulses should allow switching at lower power. If these effects can be demonstrated in the lab, they could lead to new devices for speedier, energy-efficient telecommunications.
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Optical signal processing has high power requirements because it takes a lot of energy to get photons to interact with one another. One way to encourage these interactions is to increase the intensity of the light beams. The more intense the pulse, the more power it takes to produce, but the more likely it is to interact with another pulse.
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But researchers can now also design nano-scale structures that can confine light in a cavity for a relatively long time. They achieve this confinement by coupling optical fibers to man-made structures called photonic crystals, which are patterned with structures on the same small-size scales as the wavelength of the light with which they’re meant to interact.
The theoretical work, by Michelle Povinelli, assistant professor of electrical engineering at the University of Southern California, Shanhui Fan, an electrical engineering professor at Stanford University, and Stanford graduate student Sunil Sandhu, combines this light-trapping cavity design with a new way of controlling the incoming light pulse. They’ve calculated that shaping the pulse without increasing its intensity can encourage optical switching. The “shape” of the pulse refers to how all the different light waves within it line up with one another. Experimentalists can exercise a great degree of control over the shape of a light pulse using a method called coherent control. Povinelli and Fan suggest that it should be possible to use these shaping effects to control light interactions in optical switches. They described their work last month in the journal Applied Physics Letters.
One benefit of pulse-shaping for optical processing is power savings. Povinelli and Fan have shown that it should be possible to lower the power requirements by a factor of three without narrowing the bandwidth. Optical data is made of a series of intense pulses of light that encode 0s and 1s. The more different wavelengths, or colors, of light there are in each pulse, the more data it can carry. Without pulse shaping, the bandwidth has to be narrowed in order to operate at lower power.
The work also suggests that pulse shape can be used to very sensitively control light interactions, says Steven Johnson, associate professor of applied mathematics at MIT. “Whether this will enable new practical devices remains to be seen, and there are many obstacles to optical information processing, but every new degree of control that we have in these systems is valuable,” he says.
