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The Next Generation of
Optical Fibers
By Philip Ball
May 2001
New materials promise far more efficient
light pipes with a much greater capacity to
carry information. If they work, the Internet
will never be the same.
At first sight, these new
materials are simply odd:
thin as a hair, transparent
and full of holes. Like the
optical fibers that are the
mainstay of the
telecommunications
industry, they're made of
glass. But there the
similarities with
conventional materials come
screeching to a halt.
The center of each of these
novel fibers—which are
made at the University of
Bath, in England—is hollow.
In existing optical fibers,
light is transmitted through a
glass core. In the fibers made
at Bath, light travels unhindered through air.
The light beam is confined to the hollow core
by the holes in the surrounding glass material,
which looks like a honeycomb in cross
section and creates a strictly no-go region for
light. The ability to confine light in air this
way, says Philip Russell, a Bath physicist,
"could completely revolutionize
telecommunications."
The reason for the excitement is that, in
principle at least, sending light through air
rather than through glass could greatly
increase the efficiency and capacity of today's
high-speed telecom networks. These new
materials, called photonic crystal fibers,
should "leak" less light and carry more intense
light pulses without distortion, reducing the
need to constantly boost a signal—an
expensive chore in today's optical networks.
Photonic crystal fibers should be able to
convey much more information along
fiber-optic networks while lowering
installation and maintenance costs. They will
be to existing fibers as a 10-lane freeway is to
a country lane. Not only will they take more
traffic, but the journey will be smoother and
there will be less need for refueling.
It is still early in the development of this new
generation of optical fibers. Even the most
advanced of the new materials remain several
years from widespread commercial use. But
with so much at stake—optical
telecommunications is a multibillion-dollar
business—several industrial labs, including
Corning and a handful of startups, are in hot
pursuit of their own versions of photonic
fibers. While it's too soon to predict which
will prevail, rival approaches developed at the
University of Bath and at MIT are already
competing head-to-head to become the
optical fiber of tomorrow.
These efforts may bear fruit just in time for the
telecommunications industry. The huge
expansion of long-distance optical data
transmission in recent years, fed by the
growth of the Internet and its
bandwidth-hogging applications, has led
researchers to find ways to shoot more light
and more complex signals through optical
fibers (see "Wavelength Division
Multiplexing," TR March/April 1999). But
many experts believe that in the coming
decades it will become impossible to squeeze
any more performance out of the current
generation of glass fibers. Although it's
difficult to predict exactly when the roadblock
will be reached, Jim West, a scientist at
Corning's research laboratories in New York,
definitely believes "we'll run into those
limits." And that's when the next generation
of fiber optics will become crucial in feeding
the world's apparently endless appetite for
bandwidth.
Light Conversation
Although photonic fibers are
a next-generation
technology in 2001, the
history of conveying voice
data using light extends back
more than a century. After
inventing the telephone in
1876, Alexander Graham
Bell didn't rest on his laurels.
In 1880 he showed that
light, rather than electricity,
can carry a person's words
to a distant ear. Bell's
"photophone" used vibrating
mirrors to transmit sound via
sunlight. But it was an idea
long before its time. Sending
electrical signals down
copper cables proved much
more reliable, and the photophone was
largely forgotten as telephone lines enmeshed
the world.
After eight decades of the supremacy of
copper wire, the invention of the ruby laser in
1960 put light back on the communications
agenda. Here was a source bright enough to
really put light to work. Just as the transistor
ushered in the age of microelectronics, the
laser sparked the age of photonics. In 1970
Corning proudly announced that it had sent a
laser beam down a glass fiber and recovered
as much as one percent of the light at the
other end, a kilometer away (today's glass
fibers are so efficient that 80 percent of the
light will survive that distance). By the 1980s,
telephone companies began replacing copper
cables with optical fibers.
An optical fiber can carry thousands of times
more data than a copper cable: in principle, a
single fiber can transmit up to 25 trillion bits
per second. That's enough capacity to carry
all the telephone conversations taking place at
any instant in the United States—with room
to spare. Small wonder that the worldwide
web of information technology is being
woven from light-bearing glass.
In a conventional optical fiber, light is
confined in a silica inner rod by a "cladding"
of glass with a slightly different composition
than that of the core. Typically, small amounts
of germanium or phosphorus are added to the
core (a process called "doping"), giving it a
different refractive index from the cladding.
Light striking the interface between core and
cladding is reflected, so the signal bounces
back and forth and remains within the core.
Information is encoded in a series of pulses
from electronically controlled lasers and fired
down the fiber to a photodetector at the other
end, which converts the signal back into
electrical form for processing in a telephone,
computer or routing device.
Sounds great. So, where's the catch? It's a
matter of limits. As communications networks
get bigger, busier and more ambitious, the
drawbacks of conventional glass fibers are
becoming evident, and existing optical-fiber
networks will eventually be unable to cope.
One factor that limits performance is the
fading of the light signal over distance. A
certain amount of the light is
"scattered"—impurities in the silica disrupt
the transmission of some of the signal—as it
travels through the glass core; other light
simply escapes from the fiber altogether,
because the interface between glass core and
cladding is not a perfect mirror.
Unremedied, these losses would cripple
long-distance fiber-optic communications:
eighty percent transmission over a kilometer
would leave less than a ghost of a signal at
the far end of a transatlantic cable. The
answer is to amplify the light every 70
kilometers or so. But amplifiers are expensive,
and they require their own power sources
(see "5 Patents to Watch: Booster Shots").
Each amplifier typically adds a million dollars
to the price of a long-distance transmission
line. For a cable thousands of kilometers long,
that begins to add up to real money. And
when an amplifier breaks down mid-Atlantic,
there's no option but to send out a ship to
dredge up the cable. "It costs a fortune to fix
them at the bottom of the ocean," says Bath's
Russell.
This daunting economic reality is the spur for
developing the new generation of fibers.
Cambridge, MA-based OmniGuide
Communications, founded last year by several
MIT professors, claims its new fibers will be
able to squeeze losses so low there would be
no need for any amplification. What's more,
the company says, the usable bandwidth will
be substantially larger than in existing optical
fibers. The trick is to strip out the fiber's glass
core and replace it with—well, nothing at all.
Pure Air
It sounds so obvious. Light
travels through air with little
scattering. So why not just
send laser light down a
hollow glass tube? The
answer lies in physics. To
achieve the internal
reflection necessary to keep
light confined in the center
of a conventional optical
fiber, the cladding has to
have a lower refractive
index than the inner
medium. But all known
materials have a higher
refractive index than air. So
the conventional
arrangement doesn't work in making a hollow
fiber.
Which means an unconventional approach is
needed. Enter photonic crystal fibers.
Researchers worldwide are busy making
materials that act as "light insulators," which
are impassable to light just as most plastics
are impassable to electrical currents. In the
jargon of physics, these light insulators have a
"photonic band gap" corresponding to specific
wavelengths of light; those wavelengths
simply cannot enter the material. If made
correctly, these materials—unlike the
cladding in glass fibers—should permit
virtually no light to escape from an empty
core wrapped in them.
Of course, many substances will stop light
from passing through; but this is generally
because the materials simply absorb the light
rather than reflecting it. And while you might
think of metallic mirrors—silvered glass—as
good light reflectors, the truth is that they are
not nearly reflective enough to work in fiber
optics; they absorb and dissipate a small but
significant part of an incoming beam. A light
signal traveling down a silver-lined glass tube
would travel only a short distance before
dispersing entirely. Photonic-band-gap
materials, on the other hand, block all photons
of particular wavelengths; the oncoming light
is reflected almost perfectly. In other words,
they are just the thing for confining light
inside a hollow tube.
In 1998, Yoel Fink, then an MIT graduate
student, fabricated a "perfect mirror" out of a
photonic-band-gap material. Others had
previously made specialized mirrors from thin
layers of dielectric materials (materials that
contain electrically charged particles but have
insulating properties). These mirrors have
photonic band gaps, and can be extremely
efficient reflectors, but they have a major
flaw: they work only with light striking
absolutely face-on, limiting their use to
specialized applications. Fink figured out how
to make a version of a dielectric mirror that
reflects light coming at it from all angles, as
the material would have to in the core of a
fiber-optic thread.
Once you have such a mirror, seeing the
commercial potential is (for photonics
researchers, at least) obvious. Fink and a pair
of his MIT professors, physicist John
Joannopoulos and materials scientist Edwin
Thomas, along with Uri Kolodny, cofounded
OmniGuide. The company's goal is to use the
perfect mirror as cladding for an optical fiber.
Imagine taking a flat mirror and bending it
around the inside of a tube, and you have a
crude picture of an OmniGuide fiber.
So just how small are the losses of light in
such a next-generation fiber? Because the
company is still in its early stages, the
founders are keeping that information close to
their chests. "All I am free to say at this
stage," says Joannopoulos, "is that with a
hollow-tube OmniGuide [fiber] we could in
principle achieve losses less than optical
fiber." But for a telecom industry looking to
push more and more light through optical
networks—and eventually facing the limits of
current-generation fibers—even such
carefully worded pronouncements are
tantalizing.
The company is developing a series of fiber
products based on the OmniGuide concept.
These fibers are, in theory, far more efficient
in transmitting light than a standard optical
fiber. Indeed, they should be able to overcome
the current limitations of glass fibers,
achieving, among other things, less signal loss
as the light travels down the fiber. Such
heightened performance is possible, says Fink,
now an assistant professor of materials
science at MIT, "because we can achieve an
unrivaled degree of confinement."
The OmniGuide fibers should be able to
convey much more intense signals than
normal optical fibers. High-intensity light
traveling in glass fibers suffers from distortions
that can disrupt the transmission of signals at
different wavelengths, causing cross talk
between channels unless they are widely
separated in frequency. This effect limits the
number of different wavelengths you can stuff
in a conventional glass fiber, and also how
bright they can be. Because signals in air
don't suffer these effects, Fink explains, the
OmniGuide fiber can convey signals at higher
powers, with channels spaced closer together.
That is great news for telecom companies,
since stronger signals travel farther before
losses begin to compromise them, and closer
channels mean that more data can be packed
within a given wavelength range.
The MIT approach, however, is only one way
to make a photonic fiber. Other researchers
have produced photonic-band-gap materials
that, in cross section, are like a honeycomb in
which the holes form structures that refuse
entry to light of certain wavelengths. These
kinds of photonic crystals, first made in the
late 1980s, also nearly totally block out light.
The glass fibers made at Bath, for example,
are penetrated by an orderly array of holes
running parallel to the thread along its entire
length; at the center is an empty core in
which light can be nearly perfectly confined.
To give some indication of the precision
involved in making the fibers, if the long,
parallel holes were the diameter of the
Chunnel connecting England and France, the
experimental fibers made at Bath would reach
Jupiter. How does one drill such perfect
tunnels through a glass strand thinner than a
human hair?
Fortunately, the holes don't have to be drilled
at all. They are ingeniously constructed by
drawing the glass fibers from a bundle of
hollow capillary tubes. The tubes are packed
together in a hexagonal array a few
centimeters in width, and the bundle is heated
to soften the glass. As the array is pulled out
into a fine fiber, its cross section gets shrunk
by a factor of a thousand or so but remains
laced with holes.
Initially, the Bath physicists made a
light-conducting channel at the core of the
fiber by substituting a solid glass rod for the
central glass capillary. But still better than
carrying the light in a solid core would be to
send it through a hollow core—through air,
with the very low losses and absence of
distortion that entails. In collaboration with
Douglas Allan, a researcher at Corning, the
Bath team succeeded in achieving light
confinement in a hollow-core photonic crystal
fiber in 1999. Recently they have formed
optical fibers many meters long out of their
novel materials.
Photonic Finish
Taking on existing optical fibers will be a tall
order. Conventional glass fibers have been
optimized over several decades and are made
using well-entrenched technology. In
contrast, the new photonic fibers represent a
manufacturing unknown. For one thing, their
structure must be exact. "The existing
[fabrication] systems are simply not up to it,"
admits Russell.
Still, companies are lining up to meet the
commercialization challenges. Fink says
OmniGuide is working on a series of products
based on different-length fibers. Projects
include the development of active fiber-based
devices for optical switching, as well as the
development of fibers for light transmission
over 10 to 100 meters, which could be useful
for tasks such as connecting servers over
short distances. Long-haul fibers for telecom
networks will have the biggest impact, says
Fink, but these "will take a little time."
Researchers from the Bath group have
launched their own spinoff, BlazePhotonics,
and have secured funding from venture capital
firms in the United Kingdom and United
States. In Denmark a company called Crystal
Fibre, started by scientists at the Technical
University of Denmark in Lyngby who were
early collaborators with the Bath group, is
making photonic fibers with a solid glass core.
While its initial products might serve such
purposes as confining light in high-precision
lasers, no one is losing sight of the big prize.
"Telecommunications is definitely the
medium-term target," says CEO Michael
Kjaer.
Like the founders of Denmark's Crystal Fibre,
scientists at Corning have worked closely
with the Bath researchers in the past, but they
are now racing to the marketplace on their
own. Jim West reports the company can now
make photonic fibers up to a hundred meters
long. But he reserves judgment about
whether the new materials will eventually
transform the information superhighway.
Conventional optical fibers, he points out, are
a difficult act to top. "It's only when you start
working with the state-of-the-art versions that
you realize how remarkable they are."
Although sending light through air may solve
many of the limitations of today's fibers, it
poses its own problems. For one thing, the
composition of air is not uniform; as a result,
light may be transmitted differently in
different parts of the world. "Air in the U.K. is
very different from air in the Sahara," explains
West.
"It's a fascinating technology," says West of
the new generation of photonic crystal fibers,
"but there is a long way to go."
Still, if these new materials eventually fulfill
their potential of transforming long-distance
transmission in the telecommunications
industry, it will be a journey well worth
taking.
Philip Ball, a freelance science writer based in
London, has written extensively for a number
of publications, including Nature, New
Scientist, the Financial Times and the New
York Times. He is also the author of H2O: A
Biography of Water.
Related Links
OmniGuide Communications
Alexander Graham Bell's photophone, from
America's Library
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