Yale and Bell Labs Use Chaos Theory to Make Microlasers 1,000 Times More Powerful than Conventional Counterparts

Using chaos theory, a team of scientists from Yale University, Lucent Technologies’ Bell Labs, and the Max Planck Institute of Physics in Germany have demonstrated novel semiconductor microlasers with more than 1,000 times the power of conventional, disk-shaped microlasers. The lasers are only 0.05 millimeters in diameter, or roughly the width of a human hair.

The discovery brings scientists a step closer to developing faster computers that use light instead of electrons in some components to shuttle information. Yale applied physicist A. Douglas Stone, who first proposed the microlaser shape, said, “Just as telephone communications improved because of fiber optics, computer communications could improve dramatically with the use of some optical rather than electronic components.”

In addition to their possible use in faster computers, the new microlasers also could increase the speed of voice, video, Internet and other data transmission via existing fiber-optic networks, or could become the basis for entirely new architectures for local-area optical networks.

The microlaser design, in which light creates a bow-tie pattern as it bounces around inside an asymmetrical disk before being emitted as a laser beam, was announced by Stone and his colleagues in the June 5 cover story of the journal Science. The international team also included Yale applied physicist E. E. Narimanov; Bell Labs scientists Federico Capasso, Claire Gmachl, Deborah Sivco and Alfred Cho; Jerome Faist, formerly of Bell Labs, now with the Universite de Neuchatel in Switzerland; and Jens U. Nockel of the Max-Planck-Institut of Physics in Germany, who formerly was a graduate student with Stone.

Stone, who is chairman and professor of applied physics at Yale, said, “This was an ideal collaboration between theorists and experimentalists in which we learned from each other. This is a case study in how very basic research – in our case, in the area of chaos theory – led to a very useful invention after we forged interdisciplinary connections.”

Bow-tie pattern emerges in joint experiments

The Yale and Bell scientists at first thought the light in their microlasers operated in a “whispering-gallery” mode instead of a bow-tie pattern, Stone explained. “In a circular whispering gallery, such as the dome in St. Paul’s Cathedral in London, sound flows along the walls. A whisper can be heard by someone standing against the opposite wall, but not by someone standing in the center of the room,” he said.

Researchers had previously shown that a light ray trapped inside a perfectly round optic resonator would bounce along the perimeter in just such a fashion. Round microlasers have two drawbacks, however. First, because of excessive internal reflection, they emit only a few microwatts of optical power. Second, the direction of the emitted light is not well defined.

Several years ago, Stone, Nockel and Yale electrical engineering professor Richard K. Chang discovered that suitably deforming the resonator allows the light to bounce at slightly varying angles, in a chaotic path, until it escapes in certain controllable directions. Initial experiments and theoretical results confirming the ideas were published in 1996 and 1997 in the journals Optics Letters and Nature.

The recent experiments at Bell Labs on the semiconductor microlasers revealed that, above a critical deformation level, the light pulses will travel in a bow-tie pattern, which suffers less internal reflection and emits light in four narrow, controllable beams. Moreover, this change is accompanied by an enormous increase in power output, Stone said. Each beam has an output of 10 milliwatts of power, which increases the laser’s total output to 40 milliwatts.

The high-power microlasers offer many advantages for future applications in optical interconnects and high-density optical circuits.

Efficient light sources over a broad range

“This is a remarkable advance,” said Cherry Murray, director of the Bell Labs Physical Research Lab. “These lasers offer promise as small, efficient light sources over a broad range of the light spectrum, from the mid-infrared to the visible.”

Capasso, head of the Bell Labs Semiconductor Physics Research department, called the physics behind the new microlaser “very exciting,” adding, “It combines fundamental quantum-chaos physics with the latest advances in semiconductor laser technology.”

The new device is the latest in a long line of laser innovations from Bell Labs, where 40 years ago Arthur Schawlow and Charles Townes described the concept and design for the laser – one of the century’s greatest inventions. The Yale-Bell laser collaboration also has historical precedents as Yale physicist William R. Bennett Jr. was co-inventor of the first gas laser – the helium-neon laser – along with Bell scientists. The new laser is a type of semiconductor laser, the most widely used and versatile class of lasers.

In 1994, Capasso and Faist invented the quantum-cascade – QC –laser, a fundamentally new type of laser that operates like an electronic waterfall. The microlaser demonstrated by Yale and Bell Labs is a modified QC laser, though the technique may also be used with conventional semiconductor lasers of the type currently used in communications.

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