Synced lasers pulse shorter

By Kimberly Patch, Technology Research News

Since Theodore Maiman built the first laser in 1960, scientists have been precisely controlling beams of synchronized lightwaves to carry out a growing number of useful tasks like etching tiny lines on silicon chips, carrying information over fiber-optic lines and very fast stop-motion photography.

During the past couple of years scientists have gained much more control over lasers, including the ability to switch the light on and off more quickly. Faster pulses mean information can be carried more quickly over fiber-optic lines, and stop-motion pictures can be taken over shorter periods of time.

A group of scientists from the University of Colorado and the National Institute of Standards and Technology (NIST) has taken a step toward generating shorter laser pulses by merging the pulses of two separate high-speed lasers of different colors into a single pulse train, or string of synchronized pulses. The method also allows pulses to span a broader range of wavelengths, or colors.

To do this, the researchers controlled the repetition rates of the lasers so that they matched each other like a pair of machine guns shooting in sync, said Jun Ye, an assistant professor of physics at the University of Colorado, and a physicist at the National Institute of Standards and Technology and at the institutions' Joint Institute for Laboratory Astrophysics (JILA).

"But this is not enough," said Ye. Light also contains an electric field, or carrier wave, that oscillates. Depending on its color, a lightwave modulates, or goes from the bottom of the wave trough to the top of its crest, at several hundred terahertz, or trillions of cycles per second. Each pulse contains many of these carrier wave oscillations, and even if two pulses have the same shape, the oscillations can occur in different places within the pulses. "We also [had to establish] a definite phase relation between the carrier waves of these two lasers," he said.

The researchers managed to lock together the repetition rates of the two pulses of light to within five femtoseconds and the phase relationship of the two lightwaves to within one femtosecond, according to Ye. A femtosecond is one millionth of one billionth of a second.

The next step in that process is locking the lasers more tightly so that the relative jitter is on the order of a few hundred attoseconds, said Ye. An attosecond is one billionth of one billionth of a second. There are one thousand attoseconds in a femtosecond.

In addition, it is theoretically possible to lock more than two laser pulse trains together this way, said Ye. The technique could also produce shorter light-pulse variations than are possible with a single laser, he said. The broader the bandwidth, or range of colors the light spans, the shorter the pulse. By using independent, synchronized lasers with different wavelengths "various beams can then be combined... to create the shortest pulse," said Ye.

The method could further expand laser applications.

For instance, the trick to capturing images of molecules in the middle of chemical reactions, and atoms in the middle of physical reactions, is creating pulses of light that occur quickly enough to show what is happening one or more times during a reaction. The trouble is, molecules react very quickly.

The blink of an eye takes about one-third of a second. A baseball passes over home plate in about one hundredth of a second. A gnat's wing-beat takes about one thousandth of a second.

Molecular changes like chemical reactions happen several orders of magnitude faster than these familiar events -- in millionths or billionths of seconds, which is within the range of existing high-speed lasers. The particles that make up atoms change even faster -- one ten millionth to as little as one hundred billion billionths of a second.

In addition to its promise for producing faster lasers, the method allows scientists to shape lightwaves by stitching together different-sized pulses in a coordinated way. This is useful because different-sized wavelengths have different effects.

Visible light wavelengths ranges from about 400 nanometers to about 700 nanometers. The two types of infrared pulses the scientists combined had wavelengths of 760 nanometers and 810 nanometers, according to Ye.

"The ultimate goal of the research is to be able to make an... optical waveforms synthesizer that can create an arbitrary optical pulse waveform on demand, much like what we are used to these days with radiofrequency pulse generators," Ye said.

Such pulses could eventually be used to control molecular motion. Shooting precisely tuned lasers at certain gases could lead to useful physical reactions like quantum entanglement among the gas atoms, according to Ye. And the method could be used to synchronize very different types of electromagnetic radiation such as laser light and x-rays, he said.

The work is an early step, but is potentially very useful, said Andrew Weiner, a professor of electrical and computer engineering at Purdue University. "This is the first time that anyone has locked together both repetition rates and phases of two [short pulse] lasers. In principle this technique can be used to generate very fast, arbitrary shaped waveforms," he said. In addition, the method has the potential to make shorter pulses with a broader bandwidth than is possible with a single laser, Weiner said.

There's a lot of work to be done before the method proves useful, he added. "At present the lasers cannot stay locked for long enough, and the total bandwidth is significantly less than can already be achieved with single, state-of-the-art short pulse lasers. It remains to be seen how far this new approach can practically be taken," he said.

The research is "attractive in principle," and may ultimately allow researchers to lock together completely different kinds of lasers in order to make very complex light waveforms, said Warren S. Warren, a professor of chemistry at Princeton University. The method also may have potential in wave shaping and in producing amplified pulses, he added.

Ye's research colleagues were Long-Sheng Ma of East China Normal University and JILA, Henry C. Kapteyn and Margaret M. Murnane of the University of Colorado and JILA, and John L. Hall of NIST, the University of Colorado and JILA. They published the research in the August 17, 2001 issue of Science. The research was funded by the National Science Foundation (NSF), the National Institute of Standards and Technology (NIST) and the Research Corporation.

Timeline:   5-10 years
Funding:   Government, Private
TRN Categories:   Optical Computing; Optoelectronics and Photonics
Story Type:   News
Related Elements:  Technical paper, "Phase-Coherent Optical Pulse Synthesis from Separate Femtosecond Lasers," Science, August 17, 2001.


October 31, 2001

Page One

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