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The Search for the All-Optical Switch
As the demand for bandwidth grows, the industry
races to find better and faster switching and other technologies.
It is a peculiar moment in the life of fiber optic technologies
related to communications. Potential customers face a glut
of bandwidth for the current traffic, but no one doubts that
demand will someday catch up with supply, as Internet telephony
and web use, computer-to-computer communications, streaming
digital video and audio, and other services grow in volume.
According to sources at Corning Inc., demand for bandwidth
is expected to grow at an average rate of more than 100 percent
per year for the next 10 to 15 years.
But with so much existing fiber optic cabling, new instrumentation
is required to get that cabling to yield increased bandwidth.
Most of the existing system uses optical fiber for long-distance
transmission, but the light signals must be converted to electrical
signals for switching and amplification. This process is costly,
and limits the bandwidth the systems can carry. So the industry
is searching for technologies that can do away with the electrical
repeaters and meet bandwidth demands with all-optical systems.
One approach to the problem is wavelength division multiplexing
(WDM), by which multiple optical signals are transmitted on
a single fiber strand. This process has increased the amount
of information carried on a fiber from between 1 and 2.5 gigabits
per second (Gb/s) to between 10 and 40 Gb/s, the latter capacity
just now reaching the newest installations. Researchers are
also working on 400 Gb/s carriers.
But carriers cannot realize these gains with the existing
electrical equipment that routes signals. Existing switches
cannot deal with throughput above 2.5 Gb/s. So many companies
in the industry are striving to be the first to develop an
all-optical-core switch that can route signals without the
assistance of electrical switches.
One source of funds for such development is the Ballistic
Missile Defense Office (BMDO) and its Small Business Innovation
Research department (SBIR). This agency's interest is in battlefield
communications, where data is collected, interpreted, and
communicated to various military systems, enabling complex
battle management decision-making. Synchronization between
multiple and sometimes distant ground and air subsystems is
required, and response times must be the fastest possible.
Several companies are building solid-state optical-core photonic
switches that do not rely on an electrical impulse, changing
a signal's direction based on its wavelength or polarization.
Radiant Photonics of Austin, TX, has developed a model switch
that is insensitive to polarization differences or slight
variations of wavelength in incoming signals, thereby eliminating
costly correcting equipment. It is based on polymers used
in thermo- or electro-optic prisms. These prisms can vary
the index of refraction in response to temperature or an input
voltage. The device determines where a signal hits a diffraction
grating, and thus controls which output fibers the signal
will reach. Doping the gelatin used in the prisms gives them
their refractive characteristics. Radiant's switch can operate
equally well in the C, L, and S communications bands, and
provides response speeds of just 1 nanosecond (compared with
10-15 milliseconds for competing technologies), and has an
insertion loss of less than 1 decibel.
Other companies are developing polymer switches, too, but
most rely on complicated phase delays, and are thus sensitive
to polarization differences and wavelength variations. Radiant's
switch would do without more expensive lasers and correcting
equipment. It also would have faster response times than micro-mechanical
switches, also under development. It could have as many as
50 output channels. BMDO funded the work because it needed
a polymer for high-speed fiber optic components that could
withstand the temperatures of airborne and spaceborne applications.
Meanwhile, SpectraSwitch Inc. of Santa Rosa, CA, has patent-pending
technology for using liquid crystal as the switching medium.
Its WaveWalker™ components include a low-port-count photonic
switch called the WaveWalker 1 x 2, used in single-mode transmission
in an all-optical network. The company's switch uses liquid
crystal and birefringence - the ability to refract unpolarized
light into two separate orthogonally polarized rays - with
the material's response to an electric field. A liquid crystal
cell rotates the polarization of incoming light when a voltage
is applied. SpectraSwitch's device is one of the fastest-switching
under development, reducing the current optomechanical industry
standard from 10-15 milliseconds to less than 4. Because the
material is rugged and immune to vibration degradation, the
company claims it will have a billion-cycle durability. SpectraSwitch
expects to develop a full line of WaveWalker components, including
variable optical attenuators, optical add/drop multiplexers,
polarization mode dispersion compensators, and multifunctional
modules. The technology was funded in part by BMDO.
Another company using liquid crystal technology is Chorum
Technologies of Richardson, TX. The company has developed
its PolarWave commercial line from its technology, consisting
of a fast add/drop switch and a 1 x 2 switch. The devices
use a patented fault-tolerant architecture to achieve crosstalk
and insertion loss numbers comparable to the optomechanical
specifications now in use. The switch is suited to network
protection and restoration applications that require highly
reliable switching with response times in the millisecond
domain.
Chorum's switch uses polarization manipulation to provide
the basis for switching with no moving parts. Its complimentary
polarization design allows for a high polarization extinction
ratio made possible by the use of liquid crystal. And a method
for eliminating unwanted optical energy yields a crosstalk
figure of less than -45 dB and an insertion loss of less than
1.3 dB. The PolarWave line will also include optical filters,
optical processors, and integrated optical subsystems. BMDO
awarded Chorum an SBIR Phase II contract to develop a state-of-the-art
switch for its optical signal processing applications.
OptiComp Corp. of Zephyr Cross, NV, has developed an optoelectronic
logic-array-based distributed crossbar switch that provides
terabit-level end-to-end throughput. This SmartCross™ crossbar
switch can be integrated into networks of all kinds that use
the 1.3-micron wavelength standard, reducing central switching
requirements in datacom and telecom networks. The distributed
crossbar supports high levels of fan-in and fan-out switching.
The company plans a range of components and subsystems using
the crossbar technology, targeted for integration into long-
and short-distance networks. Specifications for beta-site
testing have been solicited from industrial partners. BMDO
funds supported the production of cost-effective 1.3-micron
VCSELs for massive parallel optical interconnects.
At another California company, Templex Technologies of San
Jose, an optical encoding and decoding technology based on
fiber Bragg gratings has been developed that would increase
the optical throughput in a metro-access network and minimize
the need for costly routing equipment. The company expects
a hundredfold increase in the efficiency and capacity of today's
networks. Called SmartFBG™, the product is aimed at the market
for ultra-narrow channel-spacing dense wavelength division
multiplexing. Templex bases its expectations on a passive
all-optical routing and switching protocol using complex gratings
about the size of a microscope slide.
The gratings are formed by producing complex periodic variations
in the index of refraction of the glass lengthwise along a
fiber. The grating is designed so that the refractive index
modulation causes light of a specific wavelength. This makes
it useful for separating and switching signals, and also for
putting an optical code on every pulse of light. When a short
pulse is reflected by the grating, the light is reshaped,
delayed, and stretched into a uniquely coded pulse. The temporal
shape of this optical signal can then be used for multiplexing
and demultiplexing information, thereby eliminating electronic
conversion. Templex's optical code division multiplexing is
the only coding technology that is full compatible with dense
wavelength division multiplexing. BMDO supported Templex with
a two-year FasTrack Phase II SBIR contract to pursue novel
switching and control devices for all-optical networking.
Yet another California company, BroaData Communications of
Torrance, is marketing a series of duplex multimedia extenders
using a crossbar technology that allows simultaneous transmission
of audio, video, and data content along one fiber. The optoelectronic
crossbar combines signal fan-out and fan-in operations with
electronic modulation of laser sources. It comprises three
plane modules that can be stacked on each other to produce
a very compact device. The crossbars, with switching speeds
ranging from 0.1 to 30 microseconds, are compatible with fiber
optic communications standards. Other applications include
high-speed signal switching, reconfigurable networks, and
signal multiplexing. BMDO's SBIR program underwrote this technology
to provide fast, reconfigurable networks and to improve computer
communication systems.
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