Communication, as it has always been relied and simply depended upon speed. The faster the
means ! the more popular, the more effective the communication is Presently in the twenty-first
century wireless networking is gaining because of speed and ease of deployment and relatively
high network robustness. Modern era of optical communication originated with the invention of
LASER in 1958 and fabrication of low-loss optical fiber in 1970.
When we hear of optical communications we all think of optical fibers, what I have for u
today is AN OPTICAL COMMUNICATION SYSTEM WITHOUT FIBERS or in other words
WIRE FREE OPTICS. Free space optics or FSO –Although it only recently and rather suddenly
sprang in to public awareness, free space optics is not a new idea. It has roots that 90 back over
30 years-to the era before fiber optic cable became the preferred transport medium for high speed
communication. FSO technology has been revived to offer high band width last mile
connectivity for today‟s converged network requirements.
Free space optics or FSO, free space photonics or optical wireless, refers to the
transmission of modulated visible or infrared beams through the atmosphere to obtain optical
communication. FSO systems can function over distances of several kilometers.FSO is a line-of-
sight technology, which enables optical transmission up to 2.5 Gbps of data, voice and video
communications, allowing optical connectivity without deploying fiber optic cable or securing
spectrum licenses. Free space optics require light, which can be focused by using either light
emitting diodes (LED) 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 being the medium.
As long as there is a clear line of sight between the source and the destination and
enough transmitter power, communication is possible virtually at the speed of light. Because
light travels through air faster than it does through glass, so it is fair to classify FSO as optical
communications at the speed of light. FSO works on the same basic principle as infrared
television remote controls, wireless keyboards or wireless palm devices.
RELEVANCE OF FSO IN PRESENT DAY COMMUNICATION
Presently we are faced with a burgeoning demand for high bandwidth and differentiated data
services. Network traffic doubles every 9-12 months forcing the bandwidth or data storing capacity to
grow and keep pare with this increase. The right solution for the pressing demand is the untapped
bandwidth potential of optical communications. Optical communications are in the process of evolving
Giga bits/sec to terabits/sec and eventually to pentabits/sec. The explosion of internet and internet based
applications has fuelled the bandwidth requirements. Business applications have grown out of the
physical boundaries of the enterprise and gone wide area linking remote vendors, suppliers, and
customers in a new web of business applications. Hence companies are looking for high bandwidth last
mile options. The high initial cost and vast time required for installation in case of OFC speaks for a
wireless technology for high bandwidth last mile connectivity there FSO finds its place.
ORIGIN OF FSO
Telegraphy is a word coming from ancient greek and means in Italian “scrivere a
distanza” while in English sounds more or less like “writing to a distant place”. The human being
has from the very beginning tried to increase his capabilities to communicate with his far away
fellow men and so to transmit. Under this point of view, the mythology is full of interesting
examples with the most famous and known that is Ermes, the Gods messenger, able to move
faster than the wind and responsible to carry informations to the Gods.
First experiences in the ancient past can be found in the IVth century b.C. (before Christ),
where Diodoro Crono reports on a human chain used by the Persian king Dario I (522-486 b.C)
to transmit informations from the Capital to the Empire‟s districts. In the IVth Century b.C.,
Enea il Tattico, reports on an hydraulic telegraph probably invented by the Chartaginians. During
the Roman and Greek age, was used to place in geographical key points “fire towers” to be
switched on in case of security breaches and/or attacks on the borders. Eschilo (525-456 a.C.)
reports in the Orestea that the news about the falls of Troy arrived to Argo passing through the
Cicladi islands covering, more or less, 900 km (Eschilo, 458 b.C.). This sort of tradition
remained, for example, on the Italian territory assuming and adopting different schemes, fire or
mechanical systems, depending on the time period, the geography and the geopolicy (Pottino,
1976). In the Center-South of Italy, in particular, the use of fire based signals during night and of
smoke based signals during the daylight on the top of towers or hills, afterwards called
communications by the usage of fani, has been quite common in the XVI and XVII Centuries
a.C. (Agnello, 1963). During the day one smoke signal means the presence of one enemy vessel,
in the night was switched on a bundle of dry woods and moved up and down to inform about the
exact number of the enemy vessels. Several testimonies report on different communications links
and distances. The most interesting one has been established in 1657 between the city of Messina
and the Malta island with mid span vessels used to cover the Mediterranean sea (Castelli, 1700).
The use of mechanical systems to implement optical wireless systems is due to Claude Chappe in
1792 (Huurdeman, 2003). Chappe introduces the “optical telegraph” in France. The system was
based on a regulator, 4.5m long and 0.35m wide, to which two indicators were attached. This
systems was placed on the top of stations in LOS (Line Of Site) at 9 km each. Telescopes and
human repeaters were, of course, needed to move the regulator and the indicators via three
cranks and wire ropes. The time usage was short because the system was able to work only
during the daylight and with good weather conditions. On the other hand, it was long reach
considering an average coverage in France equal to around 4830km, with 29 cities connected
using around 540 towers. Security was to ensure by transmitting secret codes with short
preambles, this also to understand the accuracy of the transmission. Chappe introduced, infact, a
particular code in 1795, to increase the transmission speed. This system helped to reduced the
time to exchange informations from several days to minutes and has been adopted in 1794. The
subsequent studies on the electricity, the results from Volta (1745-1827) and from Ampere
(1775-1836) on the electrical pile and the introduction of the electrical telegraph in 1838
(Morse), will carry to the dismission of the Chappe system around the mid of 1800. The Chappe
system was introduced also in other European countries connecting the cities of Amsterdam,
Strasbourg, Turin, Milan and Brussels.
At the end of the 19th century, Alexander Graham Bell experienced with excellent results
the so called Photophone (Michaelis, 1965) (Bova and Rudnicki, 2001). This system worked
using the sound waves of the voice to move a mirror, responsible to send pulses of reflected
sunlight to the receiving instrument. In particular Bell modulated with his voice, by the use of an
acousto-optic transducer, a lens-collimated solar beam. Bell used to consider this invention to be
his best work, even better than “his demonstration of the telephone”. Although Bell‟s
Photophone never became a commercial reality, it demonstrated the basic
principle of optical communications. Wireless Optical Communications, becomes from this point
and year by year more important boosting the research worldwide. We can in this case divide the
wireless optical experiments in three main areas depending on the time periods: in the 60s arrives
the laser concepts and rises up the idea of wireless communications, in the 90s becomes popular
the idea of ground to satellite and satellite to ground laser communications still using red and
green sources, after 2000 the explosion of the Free Space Optical technologies (FSO) faces civil
and military applications ranging from standard telecommunications up to inter satellites & inter
planets experiments and using different wavelengths from 1 up to 10 microns.
For these reasons, essentially all of the engineering of today‟s FSO communications
systems, has been studied over the past 40 years, at the beginning for defense applications and
afterwards for civil ones. By addressing the principal engineering challenges FSO, this
aerospace/defense activity established a strong foundation upon which today‟s commercial FSO
In particular, the realization of the first LASER, based on ruby, in 1960 by Maiman
opened wider possibilities for the communications involving beams propagating over long
distances in atmosphere. Low loss optical fibers (less than 20 dB/km), infact, will arrive only in
the 70s. In 1960s NASA started to perform preliminary experiments between the Goddard Space
Center and the Gemini 7 module. In 1968 the first experiment about FSO transmission of 12
phone channels along 4km had been demonstrated in Rome (Italy) by researchers from the
Istituto P.T, CNR and Fondazione Ugo Bordoni under the management of Prof. Sette, Phisic
Insitute University La Sapienza. A red laser source (0.8 microns) was used to connect two
buildings between the Colombo and Trastevere Streets in Rome (Unknown, 1968). In the same
year Dr. E. Kube in Germany published on the viability of free space optical communications
considering both green (0.6 microns) and red (0.8 microns) laser sources (Kube, 1968). The
introduction of semiconductor light sources working at room temperature, by Alferov in 1970,
were decisive for a further development of integrated and low cost FSO systems. On the point of
view of the research, the first experiment using a quantum cascade laser (Capasso 1994) can be
considered fundamental today speaking about new transmission wavelengths for FSO (up to 10
microns). Between 1994 and 1996 years the first demonstration of a bidirectional space to
ground laser link between the ETS-VI satellite and the Communications Research Laboratory
(CRL) in Koganey (Tokio) has been accomplished. 1Mbps using 0.5 microns and 0.8 microns
emitting lasers. With the ongoing intensive and worldwide studies on FSO communications,
especially re started after the September 11 tragedy where the communications were supported
by free space optics links, the related scenarios changed extremely fast covering today different
applications and environments like the followings: atmosphere, undersea, inter satellites, deep
space. We can infact report on the SILEX experiment (Semiconductor Intersatellite Link
Experiment) in 2001 demonstrating bidirectional GEO-LEO and GEO-ground communications.
ARTEMIS satellite (GEO) using a semiconductor laser at 0.8 microns directly driven at 2 Mbps
with an average output of 10mW towards a Si-APD on SPOT-4 satellite (LEO). In the same
year, the GeoLite (Geosyncronous Lightweight Technology Experiment) experiment
successfully demonstrated a bidirectional laser communication between GEO satellites, ground
and aircraft. We cannot forget afterwards the MLCD (Mars Laser Communication
Demonstration) program started in 2003 and ended in 2005 with the aim of covering the distance
between Earth and Mars planets using an optical parametric amplifier with an average output of
5W and photon counting detectors working at 1.06 microns (Majumdar and Ricklin, 2008).
It is said that this mode of communication was first used in the 8th
century by the Greeks.
They used fire as the light source, the atmosphere as the transmission medium and human eye as
receiver.FSO or optical wireless communication by Alexander Graham Bell in the late 19th
century even before his telephone ! Bells FSO experiment converted voice sounds to telephone
signals and transmitted them between receivers through free air space along a beam of light for a
distance of some 600 feet, - this was later called PHOTOPHONE. Although Bells photo phone
never became a commercial reality, it demonstrated the basic principle of optical
communications. Essentially all of the engineering of today‟s FSO or free space optical
communication systems was done over the past 40 years or so mostly for defense applications.
WORKING OF FSO
THE TECHNOLOGY OF FSO
The concept behind FSO is simple. FSO uses a directed beam of light radiation between
two end points to transfer information (data, voice or even video). This is similar to OFC (optical
fiber cable) networks, except that light pulses are sent through free air instead of OFC cores. An
FSO unit consists 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 (laser) plus a lens that transmits light
through the atmosphere to another lens receiving information. The receiving lens connects to a
high sensitivity receiver via optical fiber. Two FSO units can take the optical connectivity to a
maximum of 4kms.
DIFFERENCE BETWEEN FIBER OPTIC AND FREE SPACE CHANNEL:
WORKING OF FSO SYSTEM
Optical systems work in the infrared or near infrared region of light and the easiest way
to visualize how the work is imagine, two points interconnected with fiber optic cable and then
remove the cable. The infrared carrier used for transmitting the signal is generated either by a
high power LED or a laser diode. Two parallel beams are used, one for transmission and one for
reception, taking a standard data, voice or video signal, converting it to a digital format and
transmitting it through free space .
Fig 2.1Mechanism of FSO device
Fig 2.2 Block Diagram of FSO operation
Today‟s modern laser system provide network connectivity at speed of 622 Mega bits/sec
and beyond with total reliability. The beams are kept very narrow to ensure that it does not
interfere with other FSO beams. The receive detectors are either PIN diodes or avalanche
photodiodes. The FSO transmits invisible eye safe light beams from transmitter to the receiver using
low power infrared lasers in the tera hertz spectrum. FSO can function over kilometers.
Characteristics of Optical Free-Space Communications
Very high data-rates (several Gbps)
Small beam divergence minimizes free-space losses
Small, light terminals (few centimetres) with low power-consumption
No regulatory issues since systems do not interfere due to the small divergence angle
Tap-proof due to minimal signal foot-print on the ground
Dependence on clear sky. Can be solved by a distributed network of optical ground
Currently available FSO hardware are of two types based on the operating wavelength –
800 nm and 1550 nm. 1550 FSO systems are selected because of more eye safety, reduced solar
background radiation and compatibility with existing technology infrastructure.
Fig 2.3: subsystems of FSO
In the transmitting section, the data is given to the modulator for modulating signal and the
driver is for activating the laser. In the receiver section the optical signal is detected and it is
converted to electrical signal, preamplifier is used to amplify the signal and then given to
demodulator for getting original signal. Tracking system which determines the path of the beam
and there is special detector (CCD, CMOS) for detecting the signal and given to pre amplifier.
The servo system is used for controlling system, the signal coming from the path to the processor
and compares with the Modulator Driver Laser Transmit optic Data in Demodulator preamplifier
detector Receive optic Data out preamplifier Special detector Tracking optic Processor Servo
systems Environmental condition.
Fig 2.4 Network of FSO
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.
Laser eye safety
A laser beam operating with an irradiance (W/cm2) above a certain level can cause
damage to the human eye. The minimum permissible exposure level (MPE) is tabulated in the
ANSI standards.16 The standards, written for direct ocular view, are given as a function of
wavelength. This is important because for wavelengths under about 1.4 _m (1400 nm), the
optical radiation that enters the eye is focused onto the retina and increased in irradiance. For
wavelengths longer than about 1.4 _m, the light is absorbed by the cornea and vitreous humor
inside the eye. Taking these considerations into account, the ANSI standards indicate that the
MPE for a 10-s exposure is about 1 mW/cm2 for 0.8-_m wavelength and about 100 mW/cm2 for
1.55-_m wavelength. The eye safety level for a LED is higher since LEDs are non coherent
sources and will not be focused to a small, diffraction-limited spot on the retina. Most FSO
systems are designed to be eye safe, or to operate in areas in which a human will not intercept the
beam. They often use beams tens of centimeters in diameter, which helps reduce beam intensity.
FREE SPACE OPTICS 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:
1.Free Space Optics (FSO) laser beams cannot be detected with spectrum analyzers or RF meters
2.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
3.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
4.Data can be transmitted over an encrypted connection adding to the degree of security
available in Free Space Optics (FSO) network transmissions
APPLICATIONS OF FSO:
Optical communication systems are becoming more and more popular as the interest and
requirement in high capacity and long distance space communications grow. FSO overcomes the
last mile access bottleneck by sending high bit rate signals through the air using laser
Applications of FSO system is many and varied but a few can be listed.
1. Metro Area Network (MAN):
FSO network can close the gap between the last mile customers, there by providing
access to new customers to high speed MAN‟s resulting to Metro Network extension.
2. Last Mile Access:
End users can be connected to high speed links using FSO. It can also be used to bypass
local loop systems to provide business with high speed connections.
3. Enterprise connectivity:
As FSO links can be installed with ease, they provide a natural method of
interconnecting LAN segments that are housed in buildings separated by public streets or other
4. Fiber backup:
FSO can also be deployed in redundant links to backup fiber in place of a second fiber
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:
Instant services to the customers before fiber being layed.
Telecommunication has seen massive expansion over the last few years. First came the
tremendous growth of the optical fiber. Long-haul Wide Area Network ( WAN ) followed by
more recent emphasis on Metropolitan Area Networks ( MAN ). Meanwhile LAN giga bit
Ethernet ports are being deployed with a comparable growth rate. Even then there is pressing
demand for speed and high bandwidth. The „connectivity bottleneck‟ which refer the imbalance
between the increasing demand for high bandwidth by end users and inability to reach them is
still an unsolved puzzle. Of the several modes employed to combat this „last mile bottleneck‟, the
huge investment is trenching, and the non- redeployability of the fiber has made it uneconomical
and non satisfying.
Other alternatives like LMDS, a RF technology has its own limitations like higher initial
investment, need for roof rights, frequencies, rainfall fading, complex set and high deployment
time. In the United States the telecommunication industries 5 percent of buildings are connected
to OFC. Yet 75 percent are with in one mile of fiber. Thus FSO offers to the service providers, a
compelling alternative for optical connectivity and a complement to fiber optics.
Free-space point-to-point optical links can be implemented using infrared laser light,
although low-data-rate communication over short distances is possible using LEDs.Infrared Data
Association (IrDA) technology is a very simple form of free-space optical communications. Free
Space Optics are additionally used for communications between spacecraft. Maximum range for
terrestrial links is of the order of 2 to 3 km (1.2 to 1.9 mi), but the stability and quality of the link
is highly dependent on atmospheric factors such as rain, fog, dust and heat. Amateur
radio operators have achieved significantly farther distances using incoherent sources of light
from high-intensity LEDs. One reported 173 miles (278 km) in 2007. However, physical
limitations of the equipment used limited bandwidths to about 4 kHz. The high sensitivities
required of the detector to cover such distances made the internal capacitance of the photodiode
used a dominant factor in the high-impedance amplifier which followed it, thus naturally forming
a low-pass filter with a cut-off frequency in the 4 kHz range.
In outer space, the communication range of free-space optical communication is currently of the
order of several thousand kilometers, but has the potential to bridge interplanetary distances of
millions of kilometers, using optical telescopes as beam expanders. In January 2013, NASA used
lasers to beam an image of the Mona Lisato the Lunar Reconnaissance Orbiter roughly 240,000
miles away. To compensate for atmospheric interference, error correction code algorithm similar
to that used in CDs was implemented. The distance records for optical communications involved
detection and emission of laser light by space probes. A two-way distance record for
communication was set by the Mercury laser altimeter instrument aboard
the MESSENGER spacecraft. This infrared diode neodymium laser, designed as a laser altimeter
for a Mercury orbit mission, was able to communicate across a distance of 15 million miles (24
million km), as the craft neared Earth on a fly-by in May, 2005. The previous record had been set
with a one-way detection of laser light from Earth, by the Galileo probe, as two ground-based
lasers were seen from 6 million km by the out-bound probe, in 1992.
Secure free-space optical communications have been proposed using a laser N-slit
interferometer where the laser signal takes the form of an interferometric pattern. Any attempt to
intercept the signal causes the collapse of the interferometric pattern. This technique has been
demonstrated to work over propagation distances of practical interest and, in principle, it could
be applied over large distances in space.
Visible light communication
Researchers used a white LED-based space lighting system for indoor local area
network (LAN) communications. These systems present advantages over traditional UHF RF-
based systems from improved isolation between systems, the size and cost of
receivers/transmitters, RF licensing laws and by combining space lighting and communication
into the same system. In 2003, a Visible Light Communication Consortium was formed
in Japan. A low-cost white LED (GaN-phosphor) which could be used for space lighting can
typically be modulated up to 20 MHz. Data rates of over 100 Mbit/s can be easily achieved using
efficient modulation schemes and Siemens claimed to have achieved over 500 Mbit/s in
2010. Research published in 2009 used a similar system for traffic control of automated vehicles
with LED traffic lights. In January 2009 a task force for visible light communication was formed
by the Institute of Electrical and Electronics Engineers working group for wireless personal area
network standards known as IEEE 802.15.7. A trial was announced in 2010 in St. Cloud,
MERITS OF FSO:
1. Free space optics offers a flexible networking solution that delivers on
the promise of broadband.
2. Straight forward deployment-as it requires no licenses.
3. Rapid time of deployment.
4. Low initial investment.
5. Ease of installation even indoors in less than 30 minutes.
6. Security and freedom from irksome regulations like roof top rights and
Unlike radio and microwave systems FSO is an optical technology and no spectrum
licensing or frequency co-ordination with other users is required. Interference from or to other
system or equipment is not a concern and the point to point laser signal is extremely difficult to
intercept and therefore secure. Data rate comparable to OFC can be obtained with very low error
rate and the extremely narrow laser beam which enables unlimited number of separate FSO links
to be installed in a given location.
Fig 3.1 FSO using High intensity LED
LIMITATIONS OF FSO:
The advantages of free space optics come without some cost. As the medium is air and the light
pass through it, some environmental challenges are inevitable.
1. FOG AND FSO
Fog substantially attenuates visible radiation, and it has a similar affect on the near-infrared
wavelengths that are employed in FSO systems. Rain and snow have little affect on FSO. Fog
being microns in diameter, it hinder the passage of light by absorption, scattering and reflection .
Dealing with fog – which is known as Mie scattering, is largely a matter of boosting the
transmitted power. In areas of heavy fogs 1550nm lasers can be of more are. Fog can be
countered by a network design with short FSO link distances. FSO installation in foggy cities
like san Francisco have successfully achieved carrier-class reliability.
2. PHYSICAL OBSTRUCTIONS
Flying birds can temporarily block a single beam, but this tends to cause only short
interruptions and transmissions are easily and automatically re-assumed. Multi-beam systems are
used for better performance.
Scintillation refers the variations in light intensity caused by atmospheric turbulence.
Such turbulence may be caused by wind and temperature gradients which results in air pockets
of varying diversity act as prisms or lenses with time varying properties. This scintillation affects
on FSO can be tackled by multi beam approach exploiting multiple regions of space- this
approach is called spatial diversity.
4. SOLAR INTERFERENCE
This can be combated in two ways.
The first is a long pass optical filter window used to block all wavelengths below 850nm
from entering the system.
The second is an optical narrow band filter proceeding the receive detector used to filter all
but the wavelength actually used for intersystem communications.
Scattering is caused when the wavelength collides with the scatterer. The physical size of the
scatterer determines the type of scattering.
When the scatterer is smaller than the wavelength-Rayleigh scattering.
When the scatterer is of comparable size to the wavelength -Mie
When the scatterer is much larger than the wavelength -Non-selective
In scattering there is no loss of energy, only a directional redistribution of energy which may
cause reduction in beam intensity for longer distance.
Absorption occurs when suspended water molecules in the terrestrial atmosphere
extinguish photons. This causes a decrease in the power density of the FSO beam and directly
affects the availability of a system. Absorption occurs more readily at some wavelengths than
However, the use of appropriate power, based on atmospheric conditions, and use of
spatial diversity helps to maintain the required level of network availability.
7. BUILDING SWAY / SEISMIC ACTIVITY
One of the most common difficulties that arises when deploying FSO 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
a. With beam divergence, the transmitted beam spread, forming optical cones which can take
b. Active tracking is based on movable mirrors that controls the direction in which beams are
Fig 3.2 New variant of FSO
System and engineering trade-offs
Besides the scientific factors to be taken into consideration in FSO design, a considerable
number of system and engineering trade-offs must be evaluated. Some of the trade-off
parameters that need to be taken into account include: ease of modulation of the laser or LED
(direct modulation through the power supply or the need for an expensive external modulator)
detector bandwidth and cooling requirements in the case of IR detectors; increased laser beam
divergence and possible need to increase laser power versus increased cost of using adaptive
optics or active alignment of a narrow laser beam; cost of laser or LED system at different
wavelengths versus advantages of availability of cheaper detector components versus penetration
of beam through fog or rain; and eye safety versus laser beam size versus divergence of beam
and beam size at detector telescope. Although a significant number of engineering trade-offs
clearly have to be made, there are usually several different ways to build a successful solution for
a specified operating condition. There is usually no single wavelength or ultimate maximized
system, but rather several that will provide the communication link required.
Fig 3.3 : Difference between Telescopic antenna and RF antenna
Free Space Optics (FSO) Features:
1. Easily upgraded
2. Roof-top or through window operation
3. No latency
4. Highly Secure (wide military applications.
5. Compatible with WDM technology
6. Low power consumption
7. Immunity from interference
8. 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) Applications:
1. LAN-to-LAN connections on campuses at Fast Ethernet or Gigabit Ethernet speeds.
2. To cross a public road or other barriers which the sender and receiver do not own.
3. Temporary network installation (for events or other purposes).
4. Reestablish high-speed connection quickly (disaster recovery).
5. For communications between spacecraft, including elements of a satellite constellation.
PERFORMANCE – TRANSMIT POWER & RECEIVER SENSITIVITY
Free Space Optics (FSO) products performance can be characterized by four main parameters
(for a given data rate):
•Total transmitted power
•Receiving optics collecting area
High transmitted power may be achieved by using erbium doped fiber amplifiers, or by noncoher
ently combining multiple lower cost semiconductor lasers. Narrow transmitting beam width
(a.k.a. high antenna gain) can be achieved on a limited basis for fixed-pointed units, with the
minimum beam width large enough to
accommodate building sway and wind loading. Much narrower beams can beachieved with an
actively pointed system, which includes an angle tracker and fast steering mirror (or gimbal).
Ideally the angle tracker operates on the communication beam, so no separate tracking beacon is
required. Larger receiving optics captures a larger fraction of the total transmitted power, up to
terminal cost, volume and weight limitations. And high receiver sensitivity can be achieved by
using small, low-capacitance photo detectors, circuitry which compensates for detector
capacitance, or using detectors with internal gain mechanisms, such as APDs. APD receivers
can provide 5-10 dB improvement over PIN detectors, albeit with increased parts co stand
a more complex high voltage bias circuit. These four parameters allow links to travel over longer
distance, penetrate lower visibility fog, or
both.In addition, Free Space Optics (FSO) receivers must be designed to betolerant to
scintillation, i.e. have rapid response to changing signal levels and high dynamic range in the
front end, so that the fluctuations can be removed in the later stage limiting amplifier or AGC.
Poorly designed Free Space Optics (FSO) receivers may have a constant background error rate
due to scintillation, rather than perfect zero error performance.
FIXED-POINTING OR ACTIVE-POINTING?
Another element of Free Space Optics (FSO) system design that must be considered by a
prudent buyer is the challenge of maintaining sufficiently accurate pointing stability. A number
of Free Space Optics (FSO) systems employ an active pointing-stabilization approach, which
represents an effective approach for addressing this challenge. However, the cost, complexity,
and reliability issues associated with active-pointing approach can be avoided in some
applications (particularly for shorter ranges and lower data rates) by utilizing the fixed-pointed
approach schematically shown in the figure. According to this approach, the transmitted beam is
broadened significantly beyond its near-perfect minimum beam divergence angle, and the
receiver field of view is broadened to a comparable extent. The broadening of the transmitted
beam and receiver field of view leads to large pointing/alignment tolerances and a very low
probability of building motion being of sufficient magnitude to take the link down. Well
engineered hardware exploits this approach of designing for loose alignment tolerances.
Therefore, it is possible to perform initial alignment of the transceivers at opposite ends of the
link during installation and then leave them unattended for many years of reliable service.
Note that this approach is facilitated for systems operating at wavelengths >1400 nm,
because the higher allowable eye-safe powers at such wavelengths allow the transmitted beam to
be significantly broadened spatially while still maintaining an adequate intensity at the receiver.
Of primary importance to prospective buyers will be selecting the right system for the situation.
We have discussed in detail how FSO technology can be rapidly deployed to provide
immediate service to the customers at a low initial investment, without any licensing hurdle
making high speed, high bandwidth communication possible.
Though not very popular in India at the moment, FSO has a tremendous scope for
deployment companies like CISCO, LIGHT POIN few other have made huge investment to
promote this technology in the market. It is only a matter of time before the customers realized,
the benefits of FSO and the technology deployed in
FSO enables optical transmission of voice video and data through air at very high rates. It has
key roles to play as primary access medium and backup technology. Driven by the need for high
speed local loop connectivity and the cost and the difficulties of deploying fiber, the interest in
FSO has certainly picked up dramatically among service providers world wide. Instead of fiber
coaxial systems, fiber laser systems may turn out to be the best way to deliver high data rates to
your home. FSO continues to accelerate the vision of all optical networks cost effectively,
reliably and quickly with freedom and flexibility of deployment.
Infrared technology is as secure or cable applications and can be more reliable than wired
technology as it obviates wear and tear on the connector hardware. In the future it is forecast that
this technology will be implemented in copiers, fax machines, overhead projectors, bank ATMs,
credit cards, game consoles and head sets. All these have local applications and it is really here
where this technology is best suited, owing to the inherent difficulties in its technological process
for interconnecting over distances.
1. Kontogeorgakis, Christos; Millimeter Through Visible Frequency Waves Through Aerosols-
Particle Modeling, Reflectivity and Attenuation
2. Analysis of Free Space Optics as a Transmission Technology, U.S. Army Information
Systems Engineering Command, page 3.
3. A 173-mile 2-way all-electronic optical contact
4. http://www.esa.int/esaTE/SEMN6HQJNVE_index_0.html 12
5. http://silicium.dk/pdf/speciale.pdf Optical Communications in Deep Space, University of
6. F. J. Duarte, Secure interferometric communications in free space, Opt. Commun. 205, 313-
7. http://www.cs.utah.edu/cmpmsi/papers09/paper1.pdf CMP-MSI: 3rd Workshop on Chip
Multiprocessor Memory Systems and Interconnects held in conjunction with the 36th
International Symposium on Computer Architecture, June 2009.