Surging cable-industry demand for more capacity and
efficiency in metropolitan fiber networks is pushing vendors into innovative approaches to
signal transport, including development of a powerful new optical amplifier on the part of
Scientific-Atlanta Inc. and a new approach to wavelength-based routing at C-COR.net Corp.
Chuck Crowder, vice president of operations at S-A, said
the company's new approach to optical amplification uses a specially designed type of
fiber within the "pump" segment of the amp to produce power output approaching
40 dBm (decibels per milliwatt).
That level is far greater than the output attained with the
firm's current generation of erbium-doped fiber amplifiers.
At C-COR.net, the focus is on a new type of router -- known
as the "spectrum parallel router," or "SPRouter" -- which can
distribute intelligence beyond the headend out to the fiber nodes, increasing end-to-end
capacity of the hybrid fiber-coaxial network.
"This technology gives you the option to go to a
distributed CMTS [cable-modem-termination system] architecture such as AT&T [Corp.] is
thinking about with its 'LightWire' design," C-COR.net chief technology
officer Terry Wright noted.
Crowder's and Wright's presentations at the
Society of Cable Telecommunications Engineers' 2000 Conference on Emerging
Technologies in Anaheim, Calif., last month underscored the concern that rising use of
Internet protocol-based technology to support interactive services will drive traffic
volume to levels that will require major investments in fiber and switches.
"Tomorrow, interactivity will be everywhere,"
Wright said. "Primetime entertainment could generate a lot of traffic."
By shifting to higher-power amplifiers, operators could
extend the signal reach of the multiple wavelengths that are compacted in
dense-wave-division multiplexed systems over the existing fiber infrastructure. This could
reduce the amount of signal regeneration required at primary and secondary hubs.
Extending intelligence deep into the network could reap
further capacity benefits by allowing operators to use digital-baseband techniques across
the backbone, pushing unregenerated lightwave signals even further.
S-A's new amplifier uses a specially designed piece of
fiber encased in glass cladding to open a wider pipe for the light from a pump-diode laser
to move through on its way into the chemically "doped" fiber where the light
amplification occurs.
With more light flowing through the pump fiber, there are
more photons entering the amplification segment, which generates a greater volume of
interaction with electrons in the amplifier and, therefore, a higher level of new photon
release from the energized electrons.
The trick is to design the perimeter of the pump fiber in a
way that forces as many light rays from the pump lasers into the core of the fiber as
possible, Crowder said.
S-A has had promising results with a multilobe design that
looks a little like the outline of a flower rather than a circle in the cross-section
diagram of the fiber, he noted. "If we could do everything perfectly, we could
achieve 40 dBm," Crowder said.
With so much power output, the new amplifiers will be used
with splitters so that each wavelength is sent over more than one fiber route, Crowder
noted. This avoids the problem of light scattering that comes with use of too much power.
While the noise figure (the amount of interfering light)
generated by the new amp is about 6 dB at this point, S-A is working on techniques to
bring that level down to 5 dB or less, Crowder said. He added that the new amplifier could
be on the market within six to nine months.
C-COR.net's SPRouter technology, which is patented,
was first brought to light in 1996, Wright noted. But the industry wasn't ready for
this type of device at the time, so the technology wasn't made into a product.
Now, though, MSOs are signaling that they're looking
for ways to minimize the distance signals must travel in the
quadrature-amplitude-modulation mode, which requires more costly lasers and more points of
signal regeneration along the fiber route than when signals are transported in
baseband-digital mode.
The SPRouter creates an "RF LAN" (local-area
network) within the local distribution area, where a specific wavelength carries the
information for that node, while other wavelengths are passed on to other distribution
points. The router switching can be done at layer two or three, Wright said.
"With distributed intelligence, you can also change
security at every point in the distribution hierarchy, which makes it much harder to hack
the network," he added.
