Optical Network Components
Changing market, metro and access applications alter the measure of component value.
Since 1999, the fiber component industry has shifted its developmental resources from
volume production of leading-edge components to delivering the best component value
for the equipment manufacturer’s dollar.
This shift lies at the confluence of several other
trends. Among them is the move toward implementing equipment in the metropolitan
and access portions of the network, which will emphasize small components that are
low in cost and electrical power dissipation. Integration of optics and electronics
into higher-level modules also adds value.
Efforts to standardize components for optical networking have produced such devices as multisource
agreement transponder modules that, in their second generation, resemble a 1/2-in.-thick
credit card in dimension. Their size signifies a 60 percent reduction from first-generation modules. Also, new
devices require only 7 W of electrical power, compared with 9.5 W for their predecessors.
Courtesy of JDS Uniphase Corp.
As network demands increase value,
system requirements are calling for components that enable more flexible architectures,
longer transmission spans and greater spectral efficiency. Together, these trends
will drive development of new component technologies.
To reduce component size and cost,
manufacturers are combining multiple functions in single packages. New optical amplifier
gain blocks, for example, integrate the pump laser, erbium-doped fiber and any necessary
couplers and isolators into a module measuring just 70 x 90 x 12 mm and providing
24 dB of gain with 15 dBm of maximum output power.
Uncooled pump lasers are a recent breakthrough
in this regard. Cooling technology is expensive and bulky, and often requires more
power than the component being cooled. An uncooled pump can reduce the power consumption
in a gain block such as that described from about 4.5 W to less than 1 W. The result
is a module that is less expensive and smaller.
Semiconductor technology enables even
smaller optical amplifiers that can fit into a 14-pin butterfly package. Historically,
they have been polarization-sensitive, but innovations have addressed this drawback,
making these devices candidates for switching and receiver applications as well
The modularization of optical functions
and the development of more flexible system architectures has initiated standardization
and cooperative industry efforts. The results include multisource agreement transponder
modules, first available about a year ago. At 10 Gb/s, these transponders established
optical, electrical and mechanical standards. Since then, the second- and third-generation
models have reflected the industry trends of reduced size, cost and electrical power.
Third and future generations of these products should advance these attributes.
Modules are becoming more common in
equipment vendors’ systems because of their interchangeability and reduced
cost, but they must become even more flexible to meet the needs of metropolitan
and access applications. One challenge will be to include tunable lasers instead
of fixed-wavelength lasers without compromising the modules’ compactness and
low power requirements.
The drive to miniaturize also has led
to developments in optical switches. Typical are micro-optical mechanical switches
and microelectromechanical systems (MEMS) with dimensions approximately half of
those on the market a year ago. Vendors are addressing reliability concerns with
MEMs switch arrays, and some versions have passed Telcordia qualification.
Leading-edge developments are focused on technologies
that increase network flexibility. A variety of new products are required to enable
remote or automatic bandwidth management, or to select and route signals. Products
such as tunable lasers and filters are becoming available. More sophisticated flexible
switches, optical add/drops, variable optical attenuators, dynamic gain equalizers,
transient-suppressed amplifiers and optical network monitors are also needed for
reliable and effective operation of these networks.
Optical monitoring equipment will be
critical to the success of transparent, flexible optical networks because the status
of multiplexed signals at each element must be tracked.
The monitoring needs are trending toward
simple, lower-performance devices that not only can determine which dense wavelength
division multiplexing (DWDM) channels are present, but also can measure the optical
signal-to-noise ratio in each channel and the optical power. The need also is increasing
for optical monitors that can perform extended sets of tests on the DWDM signals,
including more accurately assessing the signal-to-noise ratio and/or bit error rate.
Raman amplification continues to power
longer transmission spans. Emerging products will combine Raman and erbium-doped
fiber amplification, both of which have the potential to select the gain profile
in each amplifier very accurately to optimize overall system performance.
Alternate modulation formats also will
increase the potential for longer transmission spans. Nonreturn-to-zero transmission
formats have been the norm for most optical systems, but return-to-zero formats
are beginning to enable ultralong-haul distances. Modulator products for either
format are increasing in efficiency.
Network architects are investigating two approaches
to higher spectral efficiency: increasing the data rate and/or increasing the channel
For higher data rates, the industry
is developing technology to enable 40-Gb/s transmission. Modulators, photodetectors,
tunable chromatic dispersion compensators, polarization mode dispersion compensators,
the electronics to drive these components and the test equipment to verify performance
are all available.
Denser WDM transmission depends on
the successful development of filters for narrower channel spacings as well as steeper
filtering to skip fewer channels in banded architectures. Filters with channel spacings
of 25 GHz are on the market, and 12.5-GHz filters are on the way. Components that
operate in the L-band have also emerged.
As we enter 2002, the fast pace of
technological change, invention and development in fiber optic components will continue.
Innovation is still the name of the game. Component manufacturers will further
refine their technologies in devices with increased value, lower cost, smaller dimensions
and more efficient power consumption.
Meet the author
Aileen Sansone is manager of technology development
for JDS Uniphase Corp. in Bloomfield, Conn.
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