Rainbow Analytical chemistry can take you in directions you never expected—perhaps even across the “attosecond barrier”. t’s a mild winter afternoon on Kyushu Island (Japan), and graduate student Shuichi Kawasaki is hard at work on the near-IR laser that he is developing as a tunable source for thermal lens spectroscopy. He cranks up the laser’s output power, hoping to obtain better data. The beam is supposed to be monochromatic; but once again, he notices bright, multicolored spots. His adviser, Totaro Imasaka of Kyushu University, previously chided him that these spots were the result of carelessness. But Kawasaki is not being sloppy, and his adviser will later realize it. Imasaka will also realize that his student has stumbled on a novel technique for generating ultrashort pulses that may revolutionize data transmission. It’s 1987. Communications companies worldwide know that new methods of data transmission will soon be needed, but efforts to generate ultrashort pulses are stymied in physics labs. None of this concerns Kawasaki, because his goal is to develop a laser for spectroscopy. But he does wonder why the strange, colorful spots have reappeared. The first time he saw them, they were faint. Now they are numerous and bright. He calls to Imasaka.
Sandra Katzman with contributions from
Elizabeth Zubritsky J U LY 1 , 2 0 0 1 / A N A LY T I C A L C H E M I S T R Y
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The beautiful sight—like beads of a rainbow in long straight lines—captivates adviser and student. Unlike the continuous spectrum produced by sunlight refracting through water droplets, these are discrete spots, each a different color, arranged in rainbow order. The spots change color when the laser’s wavelength is adjusted. The yellow is too intense to stare at. All of the spots twinkle, so the researchers call them “rainbow stars”. In technical terms, they are “two-color stimulated Raman emissions” because the phenomenon requires two light beams of different colors. The researchers see no immediate application for rainbow stars, but they are curious enough to investigate the phenomenon. They learn that rainbow stars require high power: The colorful spots appear only when the laser’s wavelength is adjusted 6–7 nm away from maximum power. Thinking that rainbow stars are a nonlinear optical effect, the researchers expect the spots to become brighter when the laser is set to the maximum. Instead, the spots disappear.
Optical communications
All of the spots twinkle, so the researchers call them “rainbow stars”.
In 2001, new methods of data transmission are still needed. Despite the widespread use of optical fibers and other technological advances, “the current capability of data transmission is almost at the maximum,” says Imasaka. Data transmission rates of 10–20 GHz are possible right now. Future communications equipment will support 40- to 50-GHz transmission, but even that won’t suffice for long at the rate communications traffic is growing, so researchers continue to investigate new transmission methods. Japan wants to have a system that allows broadband data transmission as soon as possible, so the Japanese government is funding a three- to five-year initiative for developing new data transmission technology, Imasaka says. This is probably the largest national project in Japan, and it’s just as important in many other countries, he adds. But a substantial improvement in transmission capacity won’t be easy. One requirement for a high transmission capacity is achieving a high repetition rate—many pulses per second, Imasaka says. The shorter the pulse width, the more possible pulses, and the higher the data transmission capacity. In theory, researchers could keep shortening the pulses indefinitely (assuming that they could develop suitable sources). That is why researchers want to break through the attosecond barrier. If they could generate a pulse in the attosecond range (
