Where Is It Headed?
The recent market downturn doesn’t mean the major fiber producers
have put research and development on hold.
At the beginning of 2001,
manufacturers of optical fiber couldn’t make the stuff fast enough. By midyear,
the economic boom’s bubble had burst, and the world’s leading suppliers
began applying the brakes to their global fiber manufacturing operations. Market
demand seemed to diminish overnight after nearly two years of unprecedented growth.
Although fiber makers may have slowed their production
temporarily, they haven’t stopped concentrating on improving their product,
according to Janice Haber, vice president for systems engineering and market development
for the OFS Div. of Furukawa in Atlanta. (Furukawa acquired Lucent Optical Fiber
Solution (OFS) last year.) “Work … in research, materials and measurements
is still going on,” she said. “We have to stay as far out in front [in
research and development] as we can. If we don’t, someone else will do the
From the deep blue
Fiber’s evolution in recent years somewhat
resembles that of the humans deploying it, albeit within a substantially compressed
time frame. Many fiber innovations make their way to terrestrial networks after
first being tested and tried in undersea networks. One example is a set of dispersion-managed
fibers for undersea applications that some are claiming can more than double the
capacity of transoceanic communications systems.
The fiber accomplishes this by combating
the effects of chromatic dispersion by using dispersion itself. Previously, undersea
systems used alternating lengths of same-slope positive- and negative-dispersion
fiber, with the cleanup of any residual chromatic dispersion being the primary responsibility
of dispersion-compensation modules in the system amplifiers. Modules are no longer
necessary because fiber manufacturers splice two fibers with inversely identical
dispersion characteristics, creating an optical fiber that flattens the dispersion
slope for hundreds of channels.
The key to making dispersion-managed
fiber is to match the negative and positive dispersion slopes “perfectly,”
Haber said. This is achieved by using proprietary “profile” controls
that are employed to make preforms from which the fiber is drawn and to draw the
fiber itself. As a result, the firm’s UltraWave dispersion-managed fiber
(Figure 1) can carry up to 64 channels at 10 Gb/s. Such capability is important
to submarine system operators because it allows adding capacity to an undersea link
without laying more fiber.
Figure 1. The inverse dispersion slope of dispersion-managed fiber cancels the detrimental effect
of dispersion slope across a wide spectrum of wavelengths, enabling a dramatic increase
in the number of DWDM channels used in ultralong-haul transmission. Courtesy of
the OFS Div. of Furukawa.
An elegant solution
Dispersion-managed fiber is, in essence, an elegant
solution to the problem of chromatic-dispersion compensation in submarine systems
because it “takes care of itself,” according to E. Alan Dowdell, new-products
manager for Corning Inc.’s Optical Fiber Div. in Corning, N.Y., which calls
its brand Vascade R1000. Another feature is inherent temperature stability; i.e.,
temperature change has no impact on dispersion because both the negative- and positive-dispersion
fibers are subject to the same temperature variations. “It’s a clamped
system,” Dowdell said, “so if one effect happens to one part of the
system, the other compensates for it automatically.”
Because a dispersion-compensation module
is not needed, system design is simplified. One benefit, said Dowdell, is a lower
noise figure for the amplifier.
Furukawa is applying some of the ideas
used to make the matched-dispersion fiber for undersea systems to its next generation
of TrueWave fiber, which was scheduled for unveiling at last month’s Optical
Fiber Conference (OFC). The goal with this fiber type is to open the S-band for
Raman amplification. “You can put your Raman pump lasers into wavelengths
that do not interfere with your transmission wavelengths,” Haber noted. “That
opens up more capacity for transmission channels and allows service providers to
transmit data 200 to 600 km farther than possible with existing fiber.”
The company is also promoting dispersion-managed
fiber for terrestrial applications. Before the market softened last year, Haber
said, there was a “strong notion” that the fiber type might be of use
in ultralong-haul terrestrial networks. One potential stumbling block, though, involves
perfect matching. Although their negative and positive dispersion slopes are matched
perfectly, the fibers’ effective areas are not. Therefore, the fibers for
submarine systems are spliced at the manufacturing facility before delivery to the
cabler. The firm has spent about a year developing field-deployable splicing capability
before introducing dispersion-managed fiber as a terrestrial solution.
The next generation of terrestrial
systems could be some form of a dispersion-managed system in long-haul networks,
Dowdell said. “I think the technology will become viable in terrestrial networks
when higher bit rates become more prevalent. People aren’t deploying 40-Gb/s
systems today, but it’s just a matter of time. As soon as that happens, you
are going to see people take a whole different look at their outside plant.”
High and dry
Another fiber type is just now coming into its
own. Lucent Technologies introduced its low-water-peak fiber under the name AllWave
in 1998. Corning and Alcatel unveiled their versions of the enhanced single-mode
fiber — SMF-28e and Enhanced SMF (E-SMF), respectively — last year.
Standard single-mode fiber absorbs
hydrogen in the 1383- to 1480-nm band. The resulting water peak degrades fiber attenuation
and blocks the use of about 30 percent of the fiber. The low-water-peak fiber is
identical to the standard type in every way, except that hydroxyl ions are removed
during the manufacturing process (Figure 2). The absence of that hydrogen opens
up 100 nm of previously unavailable spectrum to service providers.
Figure 2. Low-water-peak fiber removes attenuation contribution by
OH— and opens up the E-band for coarse WDM transmission. Courtesy of Corning Inc.
“With enhanced single-mode fiber,
you can operate across the whole region from 1260 to 1625 nm,” said Jim Ryan,
global product manager for Alcatel’s Fiber Div. in Claremont, N.C. Until recently,
however, that capability has been of little use to service providers. The reason
is simple. Because the standard fiber did not make use of the previously blocked
spectrum, equipment was not designed to do so.
Now that the situation has changed,
equipment suppliers are introducing transmission gear such as coarse wavelength
division multiplexing (WDM) equipment, which uses wider channel spacings than dense
WDM equipment and exploits the newly opened window. Furukawa and LuxN Inc. promised
to demonstrate the coarse transmission gear at OFC. LuxN is among “at least
half a dozen equipment suppliers” sending out teasers that they may roll out
such equipment this year.
In addition to the low water peak and
low attenuation (0.22 dB/km) at the 1550-nm wavelength, Alcatel’s enhanced
fiber specifies a polarization mode dispersion value of 0.08, which is less than
the 0.1 specified for the standard type. Although only 20 percent lower, this spec
extends the reach of the fiber by 50 percent, Ryan said. “That small change
will significantly impact the efficiency of 40-Gb/s transmission systems.”
Metropolitan dark-fiber provider American
Fiber Systems and other service providers are beginning to deploy the enhanced single-mode
fiber in their networks. “We use low-water-peak fiber because it is one step
better than the standard type,” said Kevin Mullaney, the company’s chief
technology officer. “If we are going to put in special fiber, we want it to
address both today’s needs and tomorrow’s.”
Although the Rochester, N.Y.-based
company has paid a premium for the low-water-peak fiber in the past — the
firm began deploying it in 2001 — the increase in suppliers means more competitive
pricing compared with standard single-mode fiber. So far, the firm has deployed
the enhanced fiber in four of the five markets in which it operates.
American Fiber does not light the fiber
it deploys. Instead, it sells dark fiber to a variety of customers with diverse
requirements. Versatility is the reason the company prefers the enhanced single-mode
fiber to the metro non-zero dispersion-shifted option, which is optimized for DWDM
in the 1550-nm band. Although Mullaney doesn’t know if any of his customers
have lit the 1400-nm window yet, “they can if they want to,” he said.
He believes that eventually most service providers will choose low-water-peak fiber
over the standard type.
On the other hand, Alcatel, which launched its
Teralight family of non-zero dispersion-shifted fiber in 1999, recommends that service
providers take into account the total length of the rings they are building before
dismissing the need for the fiber type in metro and regional fiber rings.
“In the metro environment, service
providers will be going to wavelength routing soon, so the systems will not be regenerating
signals at each node,” Ryan said. “Wavelengths will be traversing the
fiber all the way around the ring, so the total distance of the ring will become
the limiting factor.” He believes that, when total ring distance is longer
than 80 km (Figure 3), the non-zero dispersion-shifted option will offer an advantage
because of the dispersion limits of standard and enhanced single-mode fiber when
it is used with 10- and 40-Gb/s transmission systems.
Figure 3. From 80 km to the point where you reach the polarization-mode-dispersion limit
of 40-Gb/s transmission systems, non-zero dispersion-shifted fiber (NZ-DSF) effectively
lengthens the reach of such systems by almost five times that of standard single-mode
fiber (SSMF). Courtesy of Alcatel.
Corning management appears to agree.
Dowdell reported that customers have been installing two of its non-zero dispersion-shifted
fiber brands in applications where large metro and long-haul networks traverse core
metropolitan networks. The company lowered the polarization mode dispersion of its
fiber by closely controlling the circularity of the core of the fiber using a technique
called outside vapor deposition. During the process, a silica-germania compound
is deposited onto a spinning preform. “Since it’s spinning, it inherently
generates circular glass,” he said.
Besides delivering improved polarization
mode dispersion, the deposition process is very scalable. “Even lower dispersion
is better,” Dowdell said. “I don’t want to mislead you and say
we have solved the problem.” Although this parameter is controllable to a
nice level for communications fiber, the industry is focusing on lowering such dispersion
in components. Fiber in amplifiers and dispersion-compensation modules actually
contributes a significant amount of polarization mode dispersion to the network.
Overall, the big-three fiber suppliers are focusing
on the future by working to ensure that the fiber they develop is compatible with
the 40-Gb/s systems. They don’t seem to feel comfortable, however, with predicting
just when those systems will become practical reality.
“Our focus is on where transmission
systems’ capabilities are heading,” Ryan said. In the metro market,
the goal is to provide the capability to deal with wavelength routing and transparent
rings. In the medium to long haul, the emphasis is on longer distances with higher
bit rates and the ability to properly manage the chromatic and polarization mode
dispersion of all the wavelengths, especially at 40 Gb/s.
Meet the author
Annie Lindstrom is a freelance telecommunications writer based in Cape Coral, Fla.
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