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