A laser that doesn't produce light would ordinarily be a failure. After all, the first two letters of laser stand for light amplification, not light reduction.
But a new laser created by scientists at the National Institutes of Standards and Technology and JILA, a joint institute of NIST and the University of Colorado at Boulder, shoots beams of night instead of beams of light. The "dark pulses," as the NIST scientists ominously call them, create areas absent of light.
The research could improve fiber optic communications. Pulses of light fade or degrade over long distances to cause noise and errors. Dark pulses don't have the same drawbacks, which should improve the transmission and detection of light through fiber optic cables.
"It's quite common to make a pulsed laser," that is off and occasionally turns on, said Richard Mirin of NIST, who helped lead the research and co-authored a new study in the journal Optics Express. "What is unusual is that this laser is the exact inverse of most pulsed lasers: It's on most of the time, but periodically turns itself off for a short time."
The lasers most people are familiar with are the green dots highlighting a PowerPoint presentation or the blue (actually closer to violet) light found in Blu-ray technology. Those lasers produce one wavelength of light that doesn't stop unless the power is cut.
There is another class of lasers, however, that pulse, oscillating between an "on" stage, where light is produced, and an "off" stage, when the amount of light is dramatically reduced. In these dark pulses, light is not totally eliminated, said Mirin, but it's intensity drops by around 70 percent.
These pulsed lasers are commonly used to transmit information through fiber optic cables. Most of the time the lasers are dark, producing areas of light with low intensities with the occasionally brief but brilliant flashes of high intensity light.
The new laser is the exact opposite of the traditional light pulsed laser. Instead of the occasional light pulse, this new laser has the occasional dark pulse, which lasts about one nanosecond.
The new dark pulse laser could be used to improve fiber optic communications, said Mirin.
When beams of light pass through a fiber optic cable, they tend to degrade. Pulses of light can bleed together, adding noise and degrading the signal. Dark pulses don't degrade or bleed nearly as bad as light pulses do, said Mirin. The dark pulses also transmit through certain fiber optic cables faster and with less noise that light pulses.
Don't expect telecoms to beat down the doors for dark pulse lasers, however, said Purdue University's Andrew Weiner, an expert on ultra-fast optics. Existing fiber optic technology is well developed. The new dark pulse lasers, while they have their advantages, are unlikely to be the basis for the next generation of fiber optic communications.
"What's really novel is that this laser works in the opposite mode, where most of the time the light is on, but it turns off in very brief flashes, very brief dark pulses," said Wiener. "Most people don't think of these lasers as having a dark mode; it's the first time I've heard of one."
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