freespace optics full report
TRANSCRIPT
INTRODUCTION
When we talk about optical communication, most people
think about optical-fiber. But optical communication is also
possible without optical-fiber. We know that light travels
through air for a lot less money. This makes possible the
optical communication without optical-fiber. Optical
communication without fiber is known as Free Space Optics.
It is used due to economic advantages. Since the introduction
of internet the backbone traffic is increasing at the rate greater
than 100%, hence the owner of the backbone infrastructure
(which is entirely based on fiber optics) are eagerly embracing
technologies that add of the capacity of the fiber optics
without adding mountains of optical cables.
FSO is not a new idea. 30-years back optical-fiber cables
are used for high-speed communication. In those days FSO
are used for high-speed connectivity over short distances.
Today’s FSO can carry full-duplex data at gigabit-per-second
rates over metropolitan distances.
What is Free Space Optics (FSO)?
Free Space Optics (FSO) is a line-of-sight technology that
uses lasers to provide optical bandwidth connections.
Currently, Free Space Optics are capable of up to 2.5 Gbps of
data, voice and video communications through the air,
allowing optical connectivity without requiring fiber-optic
cable or securing spectrum licenses. Free Space Optics require
light, which can be focused by using either light emitting
diodes (LEDs) or lasers (light amplification by stimulated
emission of radiation). The use of lasers is a simple concept
similar to optical transmissions using fiber-optic cables; the
only difference is the medium. Light travels through air faster
than it does through glass, so it is fair to classify Free Space
Optics as optical communications at the speed of light.
Free Space Optics (FSO) technology is relatively simple.
It's based on connectivity between FSO units, each consisting
of an optical transceiver with a laser transmitter and a receiver
to provide full duplex (bi-directional) capability. Each FSO
unit uses a high-power optical source (i.e. laser), plus a lens
that transmits light through the atmosphere to another lens
receiving the information. The receiving lens connects to a
high-sensitivity receiver via optical fiber. FSO technology
requires no spectrum licensing. FSO is easily upgradeable,
and its open interfaces support equipment from a variety of
vendors, which helps service providers protect their
investment in embedded telecommunications infrastructures.
HOW FREE SPACE OPTICS (FSO)
WORKS
Free Space Optics (FSO) transmits invisible, eye-safe light
beams from one "telescope" to another using low power
infrared lasers in the teraHertz spectrum. The beams of
light in Free Space Optics (FSO) systems are transmitted
by laser light focused on highly sensitive photon detector
receivers. These receivers are telescopic lenses able to
collect the photon stream and transmit digital data
containing a mix of Internet messages, video images, radio
signals or computer files. Commercially available systems
offer capacities in the range of 100 Mbps to 2.5 Gbps, and
demonstration systems report data rates as high as 160
Gbps.
Free Space Optics (FSO) systems can function over
distances of several kilometers. As long as there is a clear
line of sight between the source and the destination, and
enough transmitter power, Free Space Optics (FSO)
communication is possible
FSO: WIRELESS, AT THE SPEED OF
LIGHT
Unlike radio and microwave systems, Free Space Optics
(FSO) is an optical technology and no spectrum licensing
or frequency coordination with other users is required,
interference from or to other systems or equipment is not a
concern, and the point-to-point laser signal is extremely
difficult to intercept, and therefore secure. Data rates
comparable to optical fiber transmission can be carried by
Free Space Optics (FSO) systems with very low error
rates, while the extremely narrow laser beam widths ensure
that there is almost no practical limit to the number of
separate Free Space Optics (FSO) links that can be
installed in a given location.
Light Beam Used for FSO System
Generally equipment works at one of the two wavelengths:
850 nm or 1550 nm. Laser for 850 nm are much less
expensive (around $30 versus more than $1000) and are
favored for applications over moderate distances. One
question arises that why we use 1550 nm wavelength. The
main reason revolves 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. 1550 nm beams
operate at higher power than 850 nm, by about two orders of
magnitude. That power can boost link lengths by a factor of at
least five while maintaining adequate strength for proper link
operation. So for high data rates, long distances, poor
propagation conditions (like fog), or combinations of those
conditions, 1550 nm can become quite attractive.
Why FSO Now?
Substantial investments by carriers to augment the capacity
of their core fiber backbones have facilitated dramatic
improvements in both price and performance, and they have
also increased the capacity of these large backbone networks.
However, to generate the communications traffic and revenue
needed to fully utilize and pay for these backbone upgrades,
higher bandwidth connections must reach the end customers.
This requires substantial bandwidth upgrades at the network
edge. Essentially, to fully leverage their backbone
investments, service providers will also need to expand and
extend the reach of their metropolitan optical network to the
edge. FSO presents an opportunity that allows carriers to
achieve that goal for one-fifth the cost when compared to fiber
(if even available) and at a fraction of the time.
Increased competition: Regulation changes and
significant investments by various funds have
increased the competitive climate in these metro
networks. Each of the existing or new entrants is
racing to gain an advantage over their competition.
FSO is one of the evolutionary technologies that
allows a carrier to acquire and retain new customers
quickly and cost-effectively, thereby gaining an
entry point over competition. Metro optical
networks are expected to see $57.3 billion invested
by 2005.
International growth: Due to the growing number of
Internet-based applications, most countries are
experiencing tremendous growth in bandwidth
needs. In growing economies like Latin America
and China—where the ability to have high-
bandwidth connectivity outweighs standards for
reliability—the lack of infrastructure and rising
bandwidth demands offers a unique opportunity for
FSO.
Changing traffic patterns and protocol standards:
Multiple traffic types characterize metro networks.
Where voice was once the dominant traffic type,
data has emerged as the winner. Moreover, these
networks are also a mixture of multiple protocols
ranging from Ethernet, SONET, IP, ESCON,
FICON, etc. As a Layer One technology, FSO is
protocol agnostic.
Wireless world: With the rapid adoption and slow
deployment of wireless technologies such as LMDS
and MMDS in response to high bandwidth
communication needs in the metro area, many
service providers still find themselves short of
bandwidth to satisfy their needs. To better
understand this growing need for FSO, it is
important to understand the key drivers for FSO.
Applications of FSO
The applications of free-space-optics are many. Some of
them are as follows –
1:- 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.
2:- Last-Mile 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 business with high-speed connections.
3:- Enterprise 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 right-of-way property.
4:- Fiber Backup
FSO may also be deployed in redundant links to backup
fiber in place of a second fiber link.
5:- Backhaul
FSO can be used to carry cellular telephone traffic from
antenna towers back to facilities wired into the public
switched telephone network.
6:- Service Acceleration
FSO can be also used to provide instant service to fiber-
optic customers while their fiber infrastructure is being
laid.
FSO: Optical or Wireless?
FSO is clearly an optical technology and not a wireless
technology for two primary reasons. One, FSO enables optical
transmission at speeds of up to 2.5 Gbps and in the future 10
Gbps using WDM. This is not possible using any fixed
wireless/RF technology existing today. Two, FSO obviates
the need to buy expensive spectrum (it requires no FCC or
municipal license approvals), which distinguishes it clearly
from fixed wireless technologies. Thus, FSO should not be
classified as a wireless technology. Its similarity to
conventional optical solutions will enable a seamless
integration of access networks with optical core networks and
help to realize the vision of an all-optical network.
Free-Space Optics (FSO) Security
The common perception of wireless is that it offers less
security than wireline connections. In fact, Free Space
Optics (FSO) is far more secure than RF or other wireless-
based transmission technologies for several reasons:
Free Space Optics (FSO) laser beams cannot be
detected with spectrum analyzers or RF meters
Free Space Optics (FSO) laser transmissions are
optical and travel along a line of sight path that
cannot be intercepted easily. It requires a matching
Free Space Optics (FSO) transceiver carefully
aligned to complete the transmission. Interception is
very difficult and extremely unlikely.
The laser beams generated by Free Space Optics
(FSO) systems are narrow and invisible, making
them harder to find and even harder to intercept and
crack
Data can be transmitted over an encrypted
connection adding to the degree of security available
in Free Space Optics (FSO) network transmissions
Challenges To Free-Space Optics
Fiber-optic cable and FSO share many similarities.
However, there is a difference in how each technology
transmits information. While fiber uses a relatively
predictable medium that is subject to outside disturbances
from wayward construction backhoes, gnawing rodents and
even sharks when deployed under sea, FSO uses an open
medium (the atmosphere) that is subject to its own potential
outside disturbances. Networks with FSO must be designed to
counter the atmosphere, which can affect an FSO system's
capacity. FSO is also a line-of-sight technology and
interconnecting points must be free from physical obstruction
and able to "see" each other.
1:- Scintillation
Scintillation is 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 horizon on a hot day.
FSO communications systems deal with scintillation by
sending the same information from several separate laser
transmitters. These are mounted in the same housing, or
link head, 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.
2:- Mie-scattering
It is the scattering of beam due to fog. It is largely a matter
of boosting the transmitted power. Spatial diversity also
helps to deal with scattering. 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. But some
studies shows that scattering is independent of the
wavelength under heavy fog conditions. Other atmospheric
disturbances, like snow and especially rain, are less of a
problem for free-space optics than fog.
3:- Swaying Buildings
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 activities. Both
storms and earthquakes can cause buildings to move
enough to affect beam aiming.
The problem of swaying buildings can be dealt with in two
ways.
Beam Divergence
With beam 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 kilometer. If the receiver is initially
positioned at the center of the beam, divergence alone
can deal with many perturbations.
Active Tracking
This method is used when the link heads are mounted
on the top of extremely tall buildings or towers.
Active tracking is based on movable mirrors that
control the direction in which the beams are launched.
A feedback mechanism continuously adjust the
mirrors so that the beams stay on target. It is more
sophisticated and costly than beam divergence
method.
6:- Physical Obstructions
Flying birds can temporarily block a single beam, but this
tends to cause only short interruptions, and transmissions
are easily and automatically resumed. LightPointe uses
multi-beam systems (spatial diversity) to address this
issue, as well as other atmospheric conditions, to provide
for greater availability.
5:- Safety
To those unfamiliar with FSO, safety is often a concern
because the technology uses lasers for transmission. This
concern, however, is based on perception more than
reality. The proper use and safety of lasers have been
discussed since FSO devices first appeared in laboratories
more than two decades ago. The two major concerns
involve human exposure to laser beams (which present
much more danger to the eyes than any other part of the
human body) and high voltages within the laser systems
and their power supplies. Standards have been set for laser
safety and performance and FSO systems comply with
these standards.
Advantages Of Free-Space Optics
The FSO system requires less than one fifth of the
capital outlay of comparable ground-based fiber-optic
technologies. Optical-fibers are too costly.
Connecting the buildings with optical-fiber cost US
$100000 - $200000/km in metropolitan areas, 85
percent of the total figure tied to trenching and
installation. To install fiber you have to dig the road.
Street trenching and digging are not only expensive,
they cause traffic jams (which increase air pollution),
displace trees, and sometimes destroy historical areas.
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.
It is flexible, offers freedom, and is fast (speeds from
20 Mbps to 2.5 Gbps and beyond)
Demand for bandwidth is increasing and has been
increasing exponentially for the past few years.
Service providers have been struggling to keep up
with such demand. Service providers must extend the
reach of metro optical networks, and FSO offers
service providers the opportunity to accomplish this
objective.
The primary advantages of FSO are high throughput,
solid security, and low cost.
Conclusion
The entire face of the Free-Space Optics community is
about to change radically as driven by the need for high-speed
local loop connectivity and the costs and difficulties of
deploying fibers. FSO can be the ultimate solution for high-
speed access. Instead of hybrid fiber-coax system, hybrid
fiber-laser system may turn out to be the best way to deliver
the high capacity last-mile access. FSO provide higher
security, and throughput. FSO is capable to fulfill the
increasing demand of bandwidth.