Lucent, S-A Eye Optical Advance


Lucent Technologies and Scientific-Atlanta Inc. are
exploring the possibility of tapping an unused optical-wavelength "window" as a
way to lower the costs of meeting ongoing network-expansion requirements in the cable

The two companies and other firms joined in a
groundbreaking demonstration at the Optical Fiber Communications conference in San Diego
last week. They touted the use of wavelengths in the 1400-nanometer tier in conjunction
with transmissions operating at commonly used wavelengths in the 1300-nm and 1500-nm

Transmissions over all three wavelength windows were
combined for distribution over a 60-kilometer length of fiber, using Lucent's new
"AllWave" fiber technology.

"Lucent's fiber offers us an opportunity to open
a whole new territory to use as we look at ways to evolve our networks," said Don
Sieps, vice president for advanced transmission technology at S-A's network-systems

It will take several months of experimentation and further
evaluation with S-A customers before any product-development decisions are reached. But
Sieps made it clear that S-A is taking the availability of the 1400-nm window seriously as
it explores options for evolving network capabilities within budget parameters that are
acceptable to MSOs.

"You'll probably see the first prototype devices
in about a year," he said.

Despite many attractive characteristics associated with
transmitting light over single-mode fiber at the 1400-nm tier, standard manufacturing
procedures used in making fiber have created an impediment that has barred commercial use
of the wavelength.

"If it weren't for this, 1400 would probably have
been the most widely used wavelength in telecommunications," Sieps said.

One of the oddities of the physics of optical
communications is that residual oxygen/hydrogen molecules are left over from water seeping
into fiber during the manufacturing process -- the so-called water-peak phenomenon. These
have a strong absorption effect on lightwaves in the 1400-nm range, which results in much
higher signal loss than is the case at the 1300-nm or 1500-nm levels.

Signal loss in the 1400-nm window over most of the
installed fiber in use today would be in the range of 1 decibel per kilometer -- versus
0.35 dB/km for 1310 nm, the key transmission point in the 1300-nm window; and about 0.25
dB/km for 1550 nm, the 1500-nm window optimum.

Loss at 1400 nm over the newest fiber made using the
traditional process is less significant, coming in at about 0.4 dB/km, according to Curt
Weinstein, product-line manager for single-mode fiber at Corning Inc.

But Lucent, by adding a technique in the AllWave
manufacturing process, has clipped another tenth of a dB or so off the loss factor,
putting the window on a par with the other wavelength tiers for use in communications

While Corning could do the same thing, Weinstein said, it
hasn't seen a market demand for the application, although he acknowledged that one
could develop, given surging capacity needs.

But Lucent and S-A believe that the time has arrived to
begin exploiting the window. Not only does it offer a way to add to the raw carrying
capacity of fiber, but it could also lower costs for any given quantity of wavelengths
required in a given application, they contended.

In the demo at the OFC conference, the 1310-nm wavelength
was used to transmit a 1-gigabit Ethernet local-area-network signal over 60 km of fiber.
The 1400-nm window carried two wavelengths aggregating to 10 gigabits per second of data,
and the 1550-nm window carried two wavelengths of NTSC (National Television Systems
Committee) TV signals via gear supplied by S-A.

The demo offered one view of what could be done with the
AllWave capability in cable. Regional fiber linking hubs to master headends could be used
to carry multiple types of traffic without requiring that all of the wavelengths be
narrowly defined within the dimensions set for DWDM (dense-wave-division multiplexing) at
1550 nm, Sieps said.

But there are also possible applications that might be at
least equally compelling in distribution segments of the hybrid fiber-coaxial network, he

The 1400-nm option would not likely be cost-effective if it
meant replacing existing fiber with AllWave. But it could have a major impact in new fiber
installations, either as a way to cut back on how much fiber has to be installed or as a
means to assure that there will be more bandwidth available for a given fiber count, Sieps

In upstream applications, for example, operators are
searching for cost-effective ways to put several coaxial feeds coming in over the
5-megahertz to 50-MHz "subband" onto a single fiber at the node for return to
the hub and, eventually, to the headend.

DWDM is one option, but if it's done only at the
1550-nm window, it requires fairly expensive return lasers that have been optimized for
very narrowly defined wavelengths that won't interfere with each other over the
transmission path.

Adding the 1400-nm window would offer more transmission
space, which would reduce the wavelength specificity required of each transmitter, Sieps

Also, because data signals operate better with some small
amount of dispersion than with none, the slightly higher dispersion characteristics at
1400 nm versus at 1550 nm mean that lower-power lasers can be used at 1400 nm over
nondispersion shifted fiber like AllWave, said Kathy Szelag, vice president of optical
networking at Lucent.

But it's the overall cost benefits stemming from being
able to use cheap Fabry-Perot lasers at all three wavelength windows in the return path
that is most appealing to the cable industry, she added. "You don't have to
worry so much about the specific wavelength and the amount of separation between
wavelengths," she said.

Sieps said the idea was too new to have registered much
response from MSO customers.

"People are intrigued by the idea, but they need to
think about it," he added.

Vince Borelli, chairman of Synchronous Communications,
suggested that the industry's use of DWDM at 1550 nm was at a mere beginning, meaning
that the opportunities to cost-effectively make use of existing fiber to expand capacity
were much more important than applications involving the installation of new fiber.

"What we need to see is a focus on what can be done at
1550, which will lead to much more capacity at lower costs," he said.

Moreover, Borelli added, existing optical amplifiers
don't work at 1400 nm, which limits the appeal of DWDM at that window, although
Szelag said Lucent's new EDFAs (erbium-doped fiber amplifiers) will accommodate
transmissions at the high end of the 1400-nm spectrum, such as at 1480 nm.

When it comes to instances where new fiber is being
installed, however, it makes sense to at least give the 1400-nm option some thought,
Borelli said.