pulses speed signals
Technology Research News
Optical communications, which already far
outstrip the speed of electrical signals, have been getting even faster
as researchers find ways to make laser pulses shorter and more frequent.
These pulses, and spaces between them, represent the ones and zeros of
Engineers from Purdue University and NTT Photonics Laboratories in Japan
have figured out how to make light carry information faster using a device
ordinarily used to sort lightwaves into different colors, or wavelengths.
Waveguide gratings were commercially developed to sort out the wavelengths
of slightly different colors that are used to send many sets of pulses
over the same fiber line at the same time. The different color wavelengths
are slightly different sizes and the commercial waveguide grating is used
to sort them, after their travels, into discrete sets of pulses of the
The waveguide grating consists of a fiber-optic line that shunts a light
pulse into a flat glass slab where the light fans out. Twenty curved fiber-optic
lines connect that slab to a second slab. At the other end of the second
slab, some number of fiber-optic lines carry the pulses out.
In its commercial use, the curved fiber-optic lines sort out different
wavelengths because their different sizes are tuned to the spectral, or
color ranges of the wavelengths. "A particular output will have a certain
wavelength, and a free spectral range later it will have another wavelength,"
said Daniel Leaird, a research engineer at Purdue University. The lines
out carry the signals sorted by wavelength.
Leaird and his colleagues, however, have found a way to use the device
to speed optical signals by turning one pulse into a train of 20 pulses.
"These devices have been thought of [as] wavelength filters," said Leaird.
"The inverse of.. spectral range is the time delay between guides... it's
completely valid to think of [a waveguide grating] as just an array of
The upshot is the waveguide grating can not only sort different colors
in order to increase the bandwidth, or amount of information a line can
carry at once, but also has the potential to increase the frequency of
the pulses so that a line can carry information more quickly.
In the researchers' device each output channel carries a train of 20 pulses
made from the one pulse coming in. This is possible because each line
in the device is slightly longer than the next, and the light takes slightly
longer to get through each successive line. "You can think of [the lines]
as a series of arcs, and they get longer as you go to each adjacent guide,"
The difference in length results in a rapid fire of 20 distinct pulses
as they come out the other side, to the second flat glass slab, which
passes on the 20-pulse train to all of the output guides. "If you have
10 output guides, that means you have 10 trains of 20 pulses," said Leaird.
In order to make the device work, the researchers had to start with an
input pulse with a width, or duration, that was shorter than the delay
resulting from the difference in length from one curved guide to the next.
Otherwise, the pulses would overlap, merging back into a single, wider
The researchers made two types of pulse trains at speeds of 500 gigahertz
and one terahertz. Commercial optic systems use 10-gigahertz optical signals.
Ten gigahertz is 10 billion times per second while one terahertz is one
trillion times per second.
It would take 317 years to transmit at a speed of one pulse per second
the amount of information it takes a 10 gigahertz channel to transmit
in one second, and 31,710 years to transmit at a speed of one pulse per
second the information that goes through a one terahertz channel in one
Some lasers used in research create much shorter pulses than those produced
by the Purdue-NTT device, but those lasers are too difficult and expensive
to use for the communications networks. The researchers' approach could
provide a practical method for speeding up commercial-grade communications
The researchers' work however, is just one step toward that end. In order
to transmit actual information, the researchers need to be able to mix
on and off pulses. "We'd like to go in and turn off individual [pulses]
to make not just the clock source generation but actual data transmission.
It's a pretty difficult problem but we have some ideas on ways to [use]
this kind of arrayed waveguide grating technology... with high-speed modulator
arrays," Leaird said.
Once this work is done, the device could be used to group slower electronic
signals into one very high-speed optical channel, said Leaird. Today's
fastest electrical signals, which use currents of electrons rather than
photons, are about 100 times slower than the fastest commercial optical
A super high-speed optical channel could also be used to transfer information
between different parts of a supercomputer, said Leaird.
"It's nice work," said Warren S. Warren, a professor of chemistry and
director of the Center for Ultrafast Laser Applications at Princeton University.
"These are very interesting devices [that] have a variety of potential
applications for making these very high repetition rate [pulses] from
low-frequency lasers," he said. The work could be a way to cut down on
the cost of building high repetition rate light sources, he said.
The device may also eventually prove useful in analog-to-digital conversion
in order to record the motion of very fast events, said Leaird. For example,
researchers are aiming to use ultrafast light sources to record the movement
of individual molecules. Analog signals, because they're not made up of
discrete ones and zeros like digital signals, must be broken up to be
converted. Digital signals, like frames in a movie, will always be stop-motion
samples of analog signals. The higher the sampling rate, however, the
better the digital signal represents the analog signal.
"With shorter and shorter pulses, faster and faster rates... you can stop
shorter and shorter events," said Leaird.
It will be about 10 years before waveguide gratings can be used for commercial
purposes, Leaird said.
Leaird's research colleagues were Andrew Weiner from Purdue University
and Shuai Shen, a Purdue graduate student who is now at Lucent Technologies,
and A. Sugita, S. Kamei, H. Yamada, and M. Ishii and Katsunari Okamoto
from NTT Photonics Laboratories in Japan. They presented the research
at the Conference on Lasers and Electro-Optics in Baltimore on May 5,
2001. The research was funded by the Army Research Office.
Timeline: 10 years
TRN Categories: Optical Computing, Optoelectronics and Photonics
Story Type: News
Related Elements: Technical paper, "High Repetition Rate
Flattop Pulse Trains from an arrayed Waveguide Grating," presented at
the Conference on Lasers and Electro-Optics (CLEO) in Baltimore, May 5,
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