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Advanced Test Equipment Can Shorten TIme To Market For New Fiber Optic
Communication Gear
Raj Nair
Keithley Instruments Inc.
The recent battering of optical network and related stocks, along with those of other
technology companies, belies the underlying fundamentals of the communications
industry. In the face of financial turbulence and consolidations, the long-term outlook for
optical networking remains positive. Companies such as Marconi, Nova Crystals, Alcatel
and others continue to announce capacity expansions for opto devices. These expansions
are being coupled with vigorous cost reduction efforts to help bring down component and
system prices, which the industry recognizes as a necessity. One manufacturer predicts
that prices need to drop by as much as 40% if present growth rates are to be maintained.
Although triple digit growth rates seen by some manufacturers probably will not
be repeated, there still is a strong effort to expand fiber optic cable capacity by
increasing the use of Dense Wavelength-Division Multiplexed (DWDM)
technology. This unique technology is the most economical way to expand
bandwidth on existing networks and provide much faster transmission of data,
voice and video signals. Because of the high cost of installing new terrestrial and
undersea cable, these sectors in particular should continue to provide the
impetus for strong growth in DWDM componentry.
New Technologies in Transmitter Modules
In DWDM transmitter modules, tunable lasers are becoming a hot item. As the
number of transmitted channels increases from 16 to 400+ over the next few
years, the challenge of developing scaleable cost effective DWDM systems with
fixed wavelength lasers will increase. For the laser manufacturer, tunability
provides process and yield improvements, since a given chip can be tuned to any
one of the ITU-T channels. This eliminates the need to build an inventory of laser
diode wafers for each major wavelength. Similar benefits accrue to transmitter
module builders as well as network operators
This undoubtedly was a major factor in Nortel's 2000 acquisition of CoreTek, a
manufacturer of tunable lasers for 1550nm transmissions. CoreTek uses VCSEL
(vertical cavity surface-emitting laser) and MEMs (micro-electromechanical
systems) technology in its tunable products. This technology uses tiny movable
mirrors to change the wavelength of its semiconductor lasers. Besides its other
advantages, such tunability opens up the potential for optical networks to change
wavelengths in real time, so traffic can be monitored and re-routed to improve
performance.
Non-tunable VCSELs are coming into play for the 1300nm and above
transmission window. Nova Crystals, San Jose, CA recently announced
availability of a 1310nm VCSEL for WDM networks. Previously, VCSELs
operated reliably only at the 850nm wavelength, which limited their use to
copper-fiber interfaces for very short haul networks. Since VCSELs are much
cheaper to manufacture and test than edge emitting lasers, you can look for
manufacturers to begin offering them with higher power outputs and more
wavelengths in the 1300 - 1600nm range. These should find applications in
optical networks for LAN/WAN/MAN and access environments.
New Amplifiers and Pump Lasers for Less O-E-O
In 2001, work on the upgrading of network repeaters to higher power optical
amplifiers should intensify. This is a crucial step toward all-optical networks.
Currently, Optical-Electrical-Optical (O-E-O) regenerators in DWDM systems are
a major capital outlay. Optical amplification helps reduce equipment costs,
improve network performance, and increase system reliability.
Erbium Doped Fiber Amplifiers (EDFAs) operating at 980nm and 1480nm
wavelengths are the current workhorses of optical amplification. Although making
ever more powerful EDFAs may not be viable in the long run, there appear to be
opportunities for further improvement. Pump laser manufacturers are increasing
the drive current and power output of their products while working hard to reduce
production costs. This should extend the life of EDFA applications for a while,
particularly in the low loss 1480nm area.
However, sales of Raman amplifiers1 are expected to grow rapidly as higher
power pump lasers come on the market this year. In Raman amplification, a
continuous-wave (CW) laser signal is launched from the receiver end of the fiber
toward the source, turning the full transmission length into an amplifier. These
amplifiers, designed to work in both the 1310nm and 1550nm windows with
silica-based fiber, offer higher power and lower crosstalk potential than EDFAs.
Since Raman amplifiers are installed at the receiver end of a network, they can
actually complement EDFAs. When used together, the two types of amplifier do
not create a power boost that would cause unacceptable four-wave mixing, a
source of crosstalk. Also, Raman ampliers can selectively amplify individual
channels carried on a fiber, which is not possible with EDFAs. This allows
specific channels to be amplified by an amount that is tailored to the type of fiber
used and its channel loss profile.
Keithley's Model 2440 SourceMeter