Access networks have garnered new interest because of the growing
demand for fiber-to-the-home and high-definition video.
capacity, the provider first switches
to higher bit rates, then uses
WDM to provide additional channels, and then tries to get more
bits per channel. Those choices
affect the kinds of lasers and the
type of modulation schemes used
in the network.
Nearly a decade ago, Coldren
theorizes, the advent of WDM
increased the capacity of long-haul
systems so much that the demand
for new fiber networks crashed
(the bursting of the telecom
bubble). Now, new sources of
data from high-definition video
to stock exchanges are spewing
incredible amounts of data into
networks, once again dispelling the
notion that bandwidth is free and
infinite. “We’re going to be putting
in more fiber, I think, in about five
more years, and we’re going to be
running out of fiber bandwidth
even with our advanced modulation formats,” Coldren said.
Metro networks, which run a few kilometers to a few tens
of kilometers in size, can employ either DFB lasers or VCSELs,
depending on the distance involved. Tunable VCSELs operating at 1,550 nm provide a low-cost alternative for a “local loop”
that doesn’t require high power.
Access networks have garnered new interest because of the
growing demand for fiber-to-the-home and high-definition
video. These links employ a mix of DFB and Fabry-Pérot lasers
to accommodate the blend of modern and legacy operating
wavelengths as well as the need to have downstream and upstream communications on separate frequencies. Legacy systems
usually employ time-division multiplexing (TDM); newer systems that will exceed 10 Gbps are switching to WDM or using
some sort of hybrid or other multiplexing scheme.
Optical communications in the data center
The traditional short-haul (datacom) approach has been to use
simpler lasers such as directly modulated VCSELs, Coldren
said. The small active volume of VCSELs restricts their output
to a few milliwatts, but they are inexpensive to manufacture—no small consideration when a large Internet company
needs thousands of fiber connections. Their signal rise and
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fall times, on the order of 1 ns
or less, mean that they can be
directly modulated at multi-gigabit speeds.
Instead of stuffing data
through one fiber via WDM—
the long-haul approach—
datacom systems typically
create parallel channels through
bundles of fibers. Distances
around the data center are a few
tens of meters, and the cost of
the fiber isn’t that high.
The state of the art for
transmission speeds of up to 10
Gbps is to design systems with
parallel channels. In one simple
example, a 12-fiber ribbon cable
and a 1 12 array of VCSELs,
each sending data at 10 Gbps,
provides 100-Gigabit Ethernet
(100GigE) service with a couple
of spare channels. Today, manu-
facturers are making connectors
with two 12-fiber ribbons and
24 channels, and the next leap
forward may be six such bundles to provide 72 channels. Such
examples of “space division multiplexing” are just coming onto
the market, according to Coldren.
It might be better to start looking at the long-haul approach, which takes one laser and modulates it in a more
sophisticated way, or takes an array of lasers and funnels them
via WDM into a single fiber, Coldren said.
“At datacom, parallel fibers have been a trend, but now
even the fiber cables are getting too big,” Coldren said.
“They’ve replaced electrical cables, because the fiber’s smaller,
but now as we get dozens of fibers in parallel, they’re becom-
ing as big as the electrical cables. So maybe we need to multi-
plex more optics into a single fiber, even in the data centers.”
“Channels have to be dirt-cheap in the data center,”
Coldren said. The data center doesn’t need to aggregate
different voices coming from users from all over; a company
is just processing data in one center, and when connecting
computer lines between boxes, it can’t afford to spend much
on a single interconnection.
In data centers, VCSELs power multi-mode fiber up to 10
Gbps such as in ribbon fiber cables. These cables—the optical
descendants of the multi-wire planar cables in older computers—connect the racks and switches that power the big Internet
