Beaming light through the air offers the speed of optics without the
expense of fiber
Fiber Optics Without Fiber
Heinz A. Willebrand & Baksheesh S. Ghuman, LightPointe Communications
communication and most people think of fiber optics. But light travels
through air for a lot less money. So it is hardly a surprise that clever
entrepreneurs and technologists are borrowing many of the devices and
techniques developed for fiber-optic systems and applying them to what
some call fiber-free optical communication.
Known within the
industry as free-space optics (FSO), this form of delivering
communications services has compelling economic advantages. Although it
only recently, and rather suddenly, sprang into public awareness,
free-space optics is not a new idea. It has roots that go back over 30
years--to the era before fiber-optic cable became the preferred
transport medium for high-speed communication. In those days, the notion
that FSO systems could provide high-speed connectivity over short
distances seemed futuristic, to say the least. But research done at that
time has made possible today's free-space optical systems, which can
carry full-duplex (simultaneous bidirectional) data at
gigabit-per-second rates over metropolitan distances of a few city
blocks to a few kilometers.
systems require less than a fifth the capital outlay of comparable
ground-based fiber-optic technologies. Moreover, they can be up and
running much more quickly. Installing an FSO system can be done in a
matter of days--even faster if the gear can be placed in offices behind
windows instead of on rooftops. Using FSO, a service provider can be
generating revenue while a fiber-based competitor is still seeking
municipal approval to dig up a street to lay its cable.
of free-space optics are many and varied. To cite just a few:
- Metro network
extensions. Carriers can deploy FSO to extend existing
metropolitan-area fiber rings, to connect new networks, and, in
their core infrastructure, to complete Sonet rings.
access. FSO can be used in high-speed links that connect end-users
with Internet service providers or other networks. It can also be
used to bypass local-loop systems to provide businesses with
connectivity. The ease with which FSO links can be installed makes
them a natural for interconnecting local-area network segments that
are housed in buildings separated by public streets or other
- Fiber backup.
FSO may also be deployed in redundant links to back up fiber in
place of a second fiber link.
- Backhaul. FSO
can be used to carry cellular telephone traffic from antenna towers
back to facilities wired into the public switched telephone network.
acceleration. FSO can be also used to provide instant service to
fiber-optic customers while their fiber infrastructure is being
the technology was used primarily for enterprise connectivity. It shows
up mainly in local-area networks spanning multiple buildings, where
right-of-way was an obstacle to leasing copper lines or fiber-optic
Over the past year
or so, however, free-space optics has started to move into more
mainstream service. Several free-space optics companies have begun field
trials with telecommunications carriers in the United States, Europe,
Asia, South America, and the Middle East.
With capital hard
to come by and customers eager for high-speed data services, service
providers are left in an interesting position. In the United States, an
estimated 95 percent of buildings are within 1.5 km of fiber-optic
infrastructure. But at present, they are unable to access it. Connecting
them with fiber can cost US $100 000-$200 000/km in metropolitan areas,
with 85 percent of the total figure tied to trenching and installation.
and digging are not only expensive, they cause traffic jams (which
increase air pollution), displace trees, and sometimes destroy
historical areas. For such reasons, some cities, such as Washington,
D.C., are considering a moratorium on fiber trenching. Others, like San
Francisco, are hoping to limit disruptions by encouraging competing
carriers to lay fiber within the same trench at the same time.
reached this state because carriers spent billions of dollars to
increase network capacity in the core, or backbone, of their networks,
but have provided less lavishly at the network edges. Earlier this year,
chairing the NGN Ventures Conference in Burlingame, Calif., John
McQuillan, of McQuillan Ventures, Concord, Mass., a company that invests
in network infrastructure companies, summed up the explosive growth of
the Internet and network spending this way: "We keep thinking if we make
the core of the network bigger and faster, it will solve the problem.
But the real limits to growth and adoption of these new technologies
come at the network edge, where people interact with the system." It is
time to correct that imbalance, and free-space optics has a lot to
contribute to the solution.
A Single-Beam Link Head
drawing of a single-beam LightPointe transceiver shows that the
received laser beam [tan] is much wider than the transmitted
beam [red]. That's why the receiver lens is so much larger than
the transmitter lens. Both lenses, which share the same axis,
are mounted behind a glass housing with an embedded defroster
for cold environments.
A free-as-air technology
beams, which do not harm the eyes, are the means by which free-space
optics technology transmits data through the air between transceivers,
or link heads, mounted on rooftops or behind windows. It works over
distances of several hundred meters to a few kilometers, depending upon
Unlike most of the
lower-frequency portion of the electromagnetic spectrum, the part above
300 GHz (which includes infrared) is unlicensed worldwide and does not
require spectrum fees. The main limitation on its use is that the
radiated power must not exceed the limits established by the
International Electro-technical Commission (Standard IEC60825-1) or, in
the United States, the equivalent regulation promulgated by the Center
for Devices and Radiological Health (CDRH), which is part of the Food
and Drug Administration. As this is being written, by the way,
IEC60825-1 is being amended, and the CDRH is expected to adopt the
amended standard in the very near future, so there will be a single
worldwide standard for these devices.
available free-space optics equipment provides data rates much higher
than digital subscriber lines or coaxial cables can ever hope to offer.
And systems even faster than the present range of 10 Mb/s to 1.25 Gb/s
have been announced, though not yet delivered.
equipment works at one of two wavelengths: 850 nm or 1550 nm. Lasers for
850 nm are much less expensive (around $30 versus more than $1000) and
are therefore favored for applications over moderate distances.
Given that price
difference, why would a 1550-nm laser ever be chosen? The main reasons
revolve around power, distance, and eye safety. Infrared radiation at
1550 nm tends not to reach the retina of the eye, being mostly absorbed
by the cornea. Regulations accordingly allow these longer-wavelength
beams to operate at higher power than the 850-nm beams, by about two
orders of magnitude. That power increase can boost link lengths by a
factor of at least five while maintaining adequate signal strength for
proper link operation. Alternatively, it can boost data rate
considerably over the same length of link. So for high data rates, long
distances, poor propagation conditions (like fog), or combinations of
those conditions, 1550 nm can become quite attractive.
As the differences
in laser prices suggest, such systems are quite a bit more expensive
than 850-nm links. An 850-nm transceiver can cost as little as $5000
(for a 10-100-Mb/s unit spanning a few hundred meters), while a 1550-nm
unit can go for $50 000 (for gigabit-per-second setups encompassing a
kilometer or two).
its beam through an ordinary office window, this LightPointe
optical transceiver joins two offices of the law firm Cadwalader,
Wickersham & Taft located less than 100 meters apart in separate
buildings in Manhattan's financial district. The 1.25-Gb/s link
was installed early this year.
There is more than
one way of designing free-space optics equipment. Some companies include
switches and routers in their products; others offer a
physical-layer-only solution. AirFiber and Optical Access, both of San
Diego, Calif., have focused on mesh-based asynchronous transfer mode
(ATM) and Internet protocol (IP) models, respectively.
Protocol-independent physical-layer equipment that can be used in any
network topology is the focus of LightPointe, the authors' company, also
in San Diego; Optical Crossing, in Glendale, Calif.; and fSONA, in
Richmond, B.C., in Canada. Terabeam Corp., in Kirkland, Wash., is
unusual in being a provider of both equipment and communications
services. Outside North America, a handful of other free-space optics
companies in Europe and the Middle East primarily serve enterprise and
are different approaches to dealing with the various factors that can
affect link performance and reduce link availability below the five
nines (99.999 percent) figure, the Holy Grail of carrier-class
technology. By definition, carrier-class service delivers only one bad
bit out of every 10 billion it carries, and statistically is out of
service no more than 5 minutes and 15 seconds a year. For the rest of
the 12-month period (8759 hours, 54 minutes, and 45 seconds), it will be
up and running.
optics, challenges to achieving this level of performance take the shape
of environmental phenomena that vary widely from one micrometeorological
area to another. Included here are scintillation, scattering, beam
spread, and beam wander.
best defined as the temporal and spatial variations in light intensity
caused by atmospheric turbulence. Such turbulence is caused by wind and
temperature gradients that create pockets of air with rapidly varying
densities and therefore fast-changing indices of optical refraction.
These air pockets act like prisms and lenses with time-varying
properties. Their action is readily observed in the twinkling of stars
in the night sky and the shimmering of the horizon on a hot day.
systems deal with scintillation by sending the same information from
several separate laser transmitters. These are mounted in the same
housing, or link head, but separated from one another by distances of
about 200 mm. It is unlikely that in traveling to the receiver, all the
parallel beams will encounter the same pocket of turbulence since the
scintillation pockets are usually quite small. Most probably, at least
one of the beams will arrive at the target node with adequate strength
to be properly received. This approach is called spatial diversity,
because it exploits multiple regions of space. In addition, it is highly
effective in overcoming any scintillation that may occur near windows.
In conjunction with a design that uses multiple and spatially separated
large-aperture receive lenses, this multi-beam approach is even more
Dealing with fog,
more formally known as Mie scattering, is largely a matter of boosting
the transmitted power, although spatial diversity also helps to some
extent. In areas with frequent heavy fogs, it is often necessary to
choose 1550-nm lasers because of the higher power permitted at that
wavelength. Also, there seems to be some evidence that Mie scattering is
slightly lower at 1550 nm than at 850 nm. However, this assumption has
recently been challenged, with some studies implying that scattering is
independent of the wavelength under heavy fog conditions. Nevertheless,
to ensure carrier-class availability for a single FSO link in most
non-desert environments, the link length should be limited to 200-500
systems, when deployed through a network, can be engineered to provide
the high availability desired by carriers. It's done by limiting link
lengths in accordance with known local weather patterns. For example,
LightPointe, which is in lab and field trials with more than a dozen
carriers around the world, recently concluded a trial in Denver, Colo.,
in which its 1.25-Gb/s system obtained 99.997 percent availability over
a three-month period at a challenging time of the year:
disturbances, like snow and especially rain, are less of a problem for
free-space optics than fog.
One of the more
common difficulties that arises when deploying free-space optics links
on tall buildings or towers is sway due to wind or seismic activity.
Both storms and earthquakes can cause buildings to move enough to affect
beam aiming. The problem can be dealt with in two complementary ways:
through beam divergence and active tracking. The techniques are
effective, as evidenced earlier this year during an earthquake in
Seattle, where Terabeam has its free-space optical service up and
running. The company reported that only a few of its links lost
connections, and those for only a short time.
divergence, the transmitted beam is purposely allowed to diverge, or
spread, so that by the time it arrives at the receiving link head, it
forms a fairly large optical cone. Depending on product design, the
typical free-space optics light beam subtends an angle of 3-6
milliradians (10-20 minutes of arc) and will have a diameter of 3-6
meters after traveling 1 km. If the receiver is initially positioned at
the center of the beam, divergence alone can deal with many
perturbations. This inexpensive approach to maintaining system alignment
has been used quite successfully by FSO vendors like LightPointe for
several years now.
If, however, the
link heads are mounted on the tops of extremely tall buildings or
towers, an active tracking system may be called for. More sophisticated
and costly than beam divergence, active tracking is based on movable
mirrors that control the direction in which the beams are launched. A
feedback mechanism continuously adjusts the mirrors so that the beams
stay on target.
systems are also valuable for high-speed links that span long distances.
In those applications, beam divergence is not a good approach. By its
very nature, it reduces the beam power density just when receivers,
being less sensitive at high data rates, need all the power they can
Beam wander arises
when turbulent eddies bigger than the beam diameter cause slow, but
large, displacements of the transmitted beam. It occurs not so much in
cities as over deserts over long distances. When it does occur, however,
the wandering beam can completely miss its target receiver. Like
building sway, beam wander is readily handled by active tracking.
Some case studies
In one free-space
optics business case, a competitive local exchange carrier (CLEC) has an
agreement with a large property management firm to provide all-optical
100-Mb/s Internet access capability to several buildings located in an
office park. The carrier is building its network by leasing regional
dark fiber rings and long-haul capacity from a wholesale fiber provider.
It has identified a potential hub, or point-of-presence, less than a
kilometer from the office park and within sight of one of its central
offices. The CLEC currently has no fiber deployed to target customer
When fiber was
compared with free-space optics, deployment costs for service to the
three buildings worked out to $396 500 versus $59 000, respectively. The
fiber cost was calculated on a need for 1220 meters: 530 meters of trunk
fiber from the CLEC's central office to its hub in the office park plus
an average of 230 meters of feeder fiber for each of the runs from the
hub to a target building, all at $325 per meter. Free-space optics is
calculated as $18 000 for free-space optics equipment per building and
$5000 for installation. Supposing a 15 percent annual revenue increase
for future sales and customer acquisition, the internal rate of return
for fiber over five years is 22 percent versus 196 percent for
via a hub building, free-space optics can connect each of the
three buildings at the left to a competitive local exchange
carrier's central office at 100-Mb/s. This office is a node on a
metropolitan-area ring, which is connected to a regional ring by
means of conventional fiber-optics equipment.
communications networks, much money can be saved by building the network
piecemeal, adding to it as warranted by customer demand. Free-space
optical networks lend themselves to such a scalable model much better
than fiber-based networks do. With fiber, the cost of digging a trench
is so high that it makes sense to install as much fiber as possible
while the trench is open. With FSO, only the equipment absolutely needed
at any time needs to be deployed. As new customers are signed up, the
equipment needed to support them is installed. This demand-based
approach lowers the capital expenditure required to grow the customer
base and allows the service provided to immediately begin recovering
costs associated with the network equipment capital outlay. In this
scenario, not only the service provider but also the customer wins,
because he or she can be instantly online and start to benefit from the
higher bandwidth network connection.
All the same,
while free-space optics advantageously bypasses the need to dig up
streets for fiber-optic lines, its exposure to weather variations will
remain its No. 1 challenge. In an outdoor lab trial in 2000, XO
Communications, Reston, Va. (the broadband voice and data service
provider formerly known as Nextlink), used free-space optics equipment
to protect some of its fiber systems from accidentally severed cables.
In addition to providing redundancy for ground-based fiber, the FSO
equipment was used to close off a Sonet ring and to connect additional
buildings to the ring. In this scenario, free-space optics was
complementary to fiber-optic cable.
applications have been demonstrated pairing free-space optics with local
multipoint distribution systems (LMDS) radio communications networks.
According to the XO Communications trial, the diversity (in both
transport medium and traversed path) that comes from backing up fiber
with FSO may provide better protection than backing fiber up with
Clearly, then, FSO
is not the ideal choice for all communications applications. Equally
clearly, it has important roles to play both as a primary access medium
and as a backup technology. Key to its success will be a realistic
analysis of historical weather patterns in combination with customers'
needs for network availability. With proper planning, path blocks like
window washers and rooftop maintenance workers can also be dealt with,
and the technology will be able to realize its great potential.
Driven by the need
for high-speed local-loop connectivity and the costs and difficulties of
deploying fiber, the interest in free-space optics has certainly picked
up dramatically among service providers worldwide. The technology will
likely migrate deeper into the network. The authors believe that FSO
could be the ultimate solution for high-speed residential access.
Instead of hybrid fiber-coax systems, hybrid fiber-laser systems may
turn out to be the best way to deliver high data rates directly to the
home. At that point, it will certainly be true to say that technology
has caught up with the idea of providing high-capacity last-mile
connectivity via free-space optics as envisioned at the end of the
1960s. But in the meantime free-space optics continues to accelerate the
vision of all-optical networks cost effectively, reliably, and quickly
with freedom and flexibility of deployment.
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