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## Finalterm paper repport on fso#w245
1. 1
A
TERM PAPER REPORT ON
FREE SPACE OPTICAL
LASER COMMUNICATION
Submitted by:
Priya Hada
B.Tech (ECE)
3rd
Semester
Under the Guidance of
Mr.Sudhir Mishra
Amity School of Engineering &
Technology
AMITY UNIVERSITY RAJASTHAN
NOV, 2012
2. 2
CERTIFICATE
This is to certify that Priya Hada, student of B.Tech. in Electronics and
Communication Engineering has carried out the work presented in the project of the
Term paper entitled “FREE SPACE OPTICAL LASER COMMUNICATION” as a
part of Second Year programme of Bachelor of Technology in of B.Tech. in Electronics
and Communication Engineering from Amity School of Engineering and Technology,
Amity University Rajasthan, under my supervision.
STUDENT GUIDE
(Priya Hada) (Sudhir Mishra)
ASET (AUR)
Date:
3. 3
ACKNOWLEDGEMENT
It has come out to be a sort of great pleasure and experience for me to work on the
project Free Space Optical Laser Communication (FSO). I wish to express my
indebtedness to those who helped us i.e. the faculty of our Institute Mr. Sudhir Mishra
during the preparation of the manual script of this text. This would not have been made
successful without his help and precious suggestions. Finally, I also warmly thanks to
all our colleagues who encouraged us to an extent, which made the project successful.
Priya Hada
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LIST OF FIGURES AND TABLE
Figure 3.1 Basic overview of FSO system
Figure 4.1 Block diagram of FSO unit
Figure 4.2 Laser Structure based on Fabry-Perot Principle
Figure 4.3 A Simplified VSCEL Laser
Figure 5.1 Block diagram of a Optical Receiver
Figure 6.1 FSO Beam through atmospheric turbulence
Table 4.1 Comparison between FB/DFB/VCSEL
Table 9.2 Losses in the FSO System
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ABSTRACT
Free Space Optics (FSO) or Optical Wireless, refers to the transmission of
modulated visible or infrared (IR) beams through the air to obtain optical
communications. Like fiber, FSO uses lasers to transmit data, but instead of enclosing
the data stream in a glass fiber, it is transmitted through the air. It is a secure, cost-
effective alternative to other wireless connectivity options. This form of delivering
communication has a lot of compelling advantages .Data rates comparable to fiber
transmission can be carried with very low error rates, while the extremely narrow laser
beam widths ensure that it is possible to co-locate multiple tranceivers without risk of
mutual interference in a given location. FSO has roles to play as primary access
medium and backup technology. It could also be the solution for high speed residential
access. Though this technology sprang into being, its applications are wide and many. It
indeed is the technology of the future...
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1. INTRODUCTION
Free Space Optics (FSO) communications, also called Free Space Photonics (FSP) refers
to the transmission of modulated visible or infrared (IR) beams through the atmosphere
to obtain optical communications. Like fiber FSO uses lasers to transmit data, but
instead of enclosing the data stream in a glass fiber, it is transmitted through the air. FSO
works on the same basic principle as Infrared television remote controls Wireless
keyboards.
It supports high bandwidth, with easy to install connections for the last-mile and
campus environments. Free space links behave similarly to fiber optic systems. Instead
of focusing the output of a semiconductor laser or Light Emitting Diode (LED) into a
strand of optical fiber, the output is broadcast in a thin beam across the sky at a 1600nm
.It is basically used to transmit data for telecommunication or computer networking. It
require no licensing and only require frequency coordination.
It also provide a line of sight link .FSO links are full duplex. Also it is unaffected by
electromagnetic interference and radio frequency interference, which increasingly plague
radio based communication systems. FSO systems are used in disaster recovery
applications and for temporary connectivity while cabled networks are being deployed.
The technology is useful where the physical connections are impractical due to high costs
or other considerations.
There has been an exponential increase in the use of FSO technology, mainly for “last
mile” applications, because FSO links provide the transmission capacity to overcome
bandwidth bottlenecks.. Fiber optics has been traditionally used for transmission of
both digital and analog signals.
FSO has now emerged as a commercially viable alternative to radio frequency and
millimeter wave wireless systems for reliable and rapid deployment of data voice
networks. The fact that FSO is transparent to traffic type and data protocol makes its
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integration into the existing access network far more rapid, but also it has atmospheric
challenges like thick fog, smoke and turbulences to attain a long range terrestrials FSO.
Unlike radio and microwave systems, 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 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 FSO links that can be
installed in a given location.
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2. HISTORY
Optical communications, in various forms have been used for thousands of years.
The Ancient Greeks polished their shield to send signals during battle. In the modern era,
wireless solar telegraphs called heliograph were developed, using coded signals to
communicate with their recipients
FSO or optical wireless communications was first demonstrated by Alexander Graham
Bell and his assistant Charles Sumner tainter in the late nineteenth century (prior to his
demonstration of the telephone!). Bell’s FSO experiment on June 3,1880 at Bell’s new
created Volta laboratory where they converted voice sounds into telephone signals and
transmitted them between receivers through free air space along a beam of light for a
distance of some 600 feet. Calling his experimental device the “photo phone,” Bell
considered this optical technology – and not the telephone – his pre eminent invention
because it did not require wires for transmission. Although Bell’s photo phone never
became a commercial reality, it demonstrated the basic principle of optical
communications.
Carl Zeiss Jena developed the direct translation: light speaking device that the German
army used in their World War II anti-aircraft defense units.
The invention of lasers in the 1960s revolutionized free space optics. Military
organizations were particularly interested and boosted their development. However the
technology lost market momentum when the installation of optical fiber networks for
civilian uses was at its peak. Many simple and inexpensive consumer remote
controls use low-speed communication using Infrared (IR) light. This is known as IR
consumer technology
The spectacular transmission of T.V signal over a 30 mile distance using GaAs LED by
researcher working in the MIT Lincolns Laboratory in 1962. The first laser link to
handle commercial traffic was built in Japan by Nippon electric company (NEC)
around 1970. The link was a full duplex He-Ne laser FSO between Yakohama and
Tamagawa, a distance of 14 km.
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FSO has also been heavily researched for deep space application by NASA and ESA
with programmes such as the then Mars Laser Communication Demonstration
Demonstration (MLCD) and the Semiconductor- laser Inter-satellite Link Experiment
(SILEX) respectively.
In the past decade, near Earth FSO were successfully demonstrated in space between
satellites at data rates of up to 10 Gbps.
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3. FSO TECHNOLOGY
FSO transmits invisible, eye-safe light beams from one "telescope" to another using
low power infrared laser in the Terahertz (1Trillion Hz) spectrum. The beams of light in
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.
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, FSO communication is possible.
(Courtesy: FSO communication Link, UCSI)
Fig 3.1 Basic overview of FSO System
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4. BASIC COMPONENT OF FSO
( Courtesy: Optical research group, NCR Lab)
Fig 4.1 Block diagram of FSO Unit
4.1 TRANSMITTER
This functional element has the primary duty of modulating the source data onto the
optical carrier which is then propagated through the atmospheric to the receiver.
The most widely used modulation type is the intensity modulation (IM) in which
the source data is modulated. This is achieved by varying the driving current of the
optical source directly in sympathy with the data to be transmitted or via an external
modulator such as electro absorption modulator The use of an modulator guarantees
a higher data rates than what is obtainable with direct modulation but an external
modulator has a non-linear response.
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Other properties of the radiated optical field such as its phase, frequency and state of
polarization can also be modulated with data/information through the use of an external
modulator.
The transmitters usually contain:
1. Optical source (laser diode)
2. Modulator (Electro Absorption)
3. Driver Circuit
4. Transmit Telescope
4.1.1 OPTICAL SOURCES (LASER)
The word laser is actually an acronym for Light Amplification by Stimulated Emission
of Radiation. A laser generates light, either visible or infrared, through a process known
as stimulated emission.
MONOLITHIC FABRY-PEROT LASERS
Monolithic semiconductor lasers with a resonance mechanism (or optical feedback)
based on the Fabry-Perot principles,growing 3-D layers of crystals with controlled
consistency and doping.It form of a straight channel (p-type AIGaAs), which is both the
active region (for stimulated emission) and the optical waveguide (to guide photons in
one direction
Fabry-Perot lasers can generate several longitudinal frequencies (modes) at once. The
semiconductor laser material, the frequency spacing, and the Fabry-Perot laser length
determine the range of frequencies. The bias current determines the threshold
frequency.
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( Courtesy: Optical Component 2nd
(Chapter 6, Light sources))
4.2 Laser structure based on the Fabry-Perot principle
.
Distributed-Feedback Laser
Distributed-feedback (DFB) lasers are monolithic devices that have an internal
structure based on InGaAsP waveguide technology and an internal grating. DFBs are an
extension of the Electro absorption-modulated lasers and take their name from their
structure. The DFB structure may be combined with multiple quantum well (MQW)
structures to improve the line width of the produced laser light (as narrow as few
hundred kilohertz). The resonant cavity may be of the Mach-Zehnder or the Fabry-
Perot type.DFB lasers are reliable sources with center frequencies in the region around
1310 nm, and also in the 1520-1565 nm range; the latter makes them compatible with
erbium-doped fiber amplifiers and excellent sources in dense wavelength division
multiplexing (DWDM) applications.
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Vertical Cavity Surface-Emitting Laser
Fabry-Perot devices, DFBs, and DBRs typically require substantial amounts of current
to operate, in the order of tens of mill amperes. Moreover, their output beam has an
elliptical cross section, typically an aspect ratio of 3:1, which does not match the
cylindrical cross section of the fiber core. Thus, a non cylindrical beam may require
additional optics. A structure that produces a cylindrical beam is known as vertical
cavity, surface-emitting.
(Courtesy: Optical Component 2nd
(Chapter 6, Light sources))
Fig 4.3 A Simplified VCSEL LASER
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TABLE 4.1 COMPARISONS BETWEEN LED/ FB/DFB/VCSEL
(Courtesy: Optical Component 2nd
(Chapter 6, Light sources))
4.1.2 ELECTRO ABSORPTION MODULATOR (EAM)
EAM is a semiconductor device which can be used for modulating the intensity of a
laser beam via an electric voltage. Its principle of operation is based on, i.e., a change in
the absorption spectrum caused by an applied electric field, which changes the band
gap energy (thus the photon energy of an absorption edge) but usually does not involve
the excitation of carriers by the electric field. For modulators in telecommunications
small size and modulation voltages are desired. The EAM is candidate for use in
external modulation links in telecommunications.
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4.1.3 DRIVER CIRCUIT
In electronics, a driver is electrical circuit an or other used to control electronic
component another circuit or other component, such as a high-power transistor. They
are usually used to regulate current flowing through a circuit or is used to control the
other factors such as other components, some devices in the circuit. The term is often
used, for example, for a specialized integrated circuits that controls high-power
switches in switched-mode power converter An Amplifier can also be considered a
driver for loudspeaker, or a constant voltage circuit that keeps an attached component
operating within a broad range of input voltages.
For example in a transistor power amplifier, typically the driver circuit requires current
gain, often the ability to discharge the following transistor bases rapidly, and low output
impedance to avoid or minimise distortion.
4.1.4 TRANSMITTER TELESCOPE
The transmitter telescope collects, collimates and direct the optical radiation toward the
receiver telescope at the other end of the channel.
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5. THE RECEIVER
The receiver helps recover the transmitted data from the incident optical field. The
receiver is composed of:
RECEIVER TELESCOPE: It collects and focuses the incoming optical radiation on to
the Photodetector. It should be noted that a large receiver telescope aperture is desirable
as it collects multiple uncorrelated radiation and focuses their average on the
Photodetector.
This is referred to as aperture averaging but a wide aperture also means more
background radiation/noise.
AN OPTICAL BAND: It contains the pass filter to reduce the amount of background
radiation.
A PHOTODETECTOR: It operates by converting light signal that hits the junction to a
voltage or current.
Photodiode- It is commonly used Photodetector. A photodiode is based on a junction of
opposite doped region (pn junction) in a sample of semiconductor. This creates a region
depleted of charge carriers that results in high impedance. The high impedance allow
the construction of detectors using silicon and germanium to operate with high
sensitivity at low impedance.
Since the light is used as an input, the diode is operated under reverse bias condition.
Photodiodes are usually made of GaAs.
PIN Photodiode: It includes an intrinsic layer in between the P and N type material. It
must be reverse bias due to high resistivity of the intrinsic layer The PIN has a layer
depletion region which allows more electron-hole pair to develop a lower capacitances.
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Avalanche Photodiode: It is operated at reverse bias close to the breakdown, which
causes photo excited change carrier to accelerate in the depletion region and produce
additional carrier by avalanching They are good for fiber optic system that require low
light levels with quantum efficiency larger than 100 percent.
POST-DETECTION PROCESSOR (decision circuit): It is the circuit where the
necessary amplification, Filtering and signal processing necessary to guarantee a high
fidelity data recovery are carried out.
(Courtesy: FSO Communication Link, UCSI)
Figure 6.1 Block Diagram of a optical receiver
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6. THE ATMOSPHERIC CHANNEL
In the optical system SNR s proportional to A (A is the receiver detector area) this
implies that for a given transmit power; a high SNR can be attained by using an large
area detector. However as A increases so does its capacitance, which has a limited
effect on the receiver bandwidth.
1. POWER LOSS
For an optical radiation traversing the atmosphere ,some of the photons are
extinguished (absorbed) by the molecular constitutes(water vapour, Carbondioxide,
ozone etc) and their energy converted into heat while other experience no loss of
energy but their initial direction of propagation changed (scattering).
a. ATMOSPHERIC CHANNEL LOSS: The atmospheric channel attenuates the field
traversing it as a result of atmosphere and scattering processes. The concentration of
matter in the atmosphere, which result in the signal attenuation vary spatially and
temporarily and will depend on the current local weather condition.
b. BEAM DIVERGENCE LAW: One of the advantage of FSO system is the ability to
transmit the a very narrow optical beam, thus , offering advanced security. But due
to diffraction, the beam spreads out. This results in a situation in which the receive
aperture is only able to collect a fraction of the beam, hence beam divergence loss.
c. OPTICAL AND WINDOW LOSS: It includes losses due to imperfect lenses and
other optical elements used in design of both transmitter and receiver. It accounts
for the reflection, absorption, scattering due to lenses in system.
d. POINTING LOSS: It occurs due to imperfect alignment of the transmitter and the
receiver.
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(Fiber Courtesy: Corning Optical,Peter Rouo)
Figure 5.1 FSO beam propagation through atmospheric turbulence
6.1 FREQUENCY MODULATION
A FSO system is based on optical FM, where the information is encoded by a time-
variable wavelength. As is well known, broadband FM systems use a transmission
bandwidth that is larger than the signal’s information bandwidth, thus enabling an
enhancement of the SNR and hence the effective information rate per unit transmitter
power. Because of the atmospheric conditions, any optical free-space communication
system, contemplated at a terrestrial level, must operate at mid-infrared wavelengths
in the range λ = 2.5-2.8 μm. Development of rapidly tunable single-frequency lasers
in this wavelength range is quite feasible, based on the current experience with
tunable telecom lasers at 1.5 μm. Nevertheless, there is no currently available optical
FM system. The main difficulty is associated not so much with the tunable optical
sources, as with the of a wavelength-discriminating receiver system that would take
advantage of the enhanced SNR. In our view, the key enabling solution is optical
super heterodyne with a local oscillator implemented as a tunable mid-infrared laser
similar to that at the source. The intermediate frequency can be tuned to lie either in a
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frequency range directly accessible to electronic limiting amplifier and frequency
discriminator.
CONCEPT
Wideband frequency-modulation (FM) systems offer a trade of the bandwidth excess
for SNR, thus relaxing the transmitter power requirement as compared to AM
transmission. Energy efficiency is essential for satellite communications, sensor
networks and mobile platforms. The FM advantage is proportional to the squared
ratio (∆F /fS)2
of the range of frequency excursion ΔF to the signal bandwidth fS ,
Thus, current direct broadcast satellite systems are made possible by using a
microwave.
To preserve the FM advantage, the signal bandwidth is limited by the inequality,
fS<<∆F<<fO
This should not be a serious limitation for optical FM in any wavelength range, since
Optical frequencies are far larger than any conceivable signal bandwidth. A more
Stringent condition limits the spectral width Δf0 of the laser emission. Line width is
not an issue in radio systems. Compared to such systems, any laser is a high-Q
resonator in the sense of ΔfO << fO However, as we shall argue below, the only
practical receiving system that can be contemplated for optical FM should be based
on optical heterodyne and since the line width is “inherited” in heterodyne detection,
one must ensure it stays well below the tuning range, viz.
Condition (2) can be viewed as an optical analog of the so-called FM threshold. This
is certainly quite feasible with single-mode semiconductor lasers.
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7. FEATURES OF FSO
1. FSO transmission links can be deployed quicker, and in some instances more
economically, than optical fiber links.
2. When compared with wireless rf links, FSO requires no licensing and provides
better link security and much higher immunity from electromagnetic interference
EMI.
3. FSO is highly invulnerable to interference from other sources of laser radiation.
4. FSO can be implemented for portable applications, e.g., movable radar dish
antennas.
5. FSO provides a viable transmission channel for transporting IS-95 CDMA signals
to base stations from macro- and microcell sites and can decrease the setup costs of
temporary microcells deployed for particular events, e.g., sporting events, by
eliminating the need for installing directional microwave or connecting cable.
6. FSO introduces a viable transmission medium for the deployment of cable
television _CATV_ links in metropolitan areas where installing new fiber
infrastructure can be relatively expensive.
7. Analog FSO can reduce the cost of transmission equipment as compared to a
digital implementation.
7.1 FSO SECURITY
Security is an important element of data transmission, irrespective of the network
topology. It is especially important for military and corporate applications security.
FSO is far more secure than RF or other wireless-based transmission technologies for
several reasons:
1. FSO laser beams cannot be detected with spectrum analyzers or RF meters.
24. 24
2. FSO laser transmissions are optical and travel along a line of sight path that cannot
be intercepted easily. It requires matching.
3. FSO transceiver carefully aligned to complete the transmission. Interception is very
difficult and extremely unlikely.
4. The laser beams generated by FSO systems are Narrow and invisible, making them
harder to find and even harder to Intercept and crack
5. Data can be transmitted over an encrypted connection adding to the Degree of
security available in FSO network Transmissions .
7.2 EYE-SAFETY
Laser beams with wavelengths in the range of 400 to 1400 nm emit light that passes
through the cornea and lens and is focused onto a tiny spot on the retina while
wavelengths above 1400 nm are absorbed by the cornea and lens, and do not focus onto
the retina, as illustrated in Figure 1. It is possible to design eye-safe laser transmitters at
both the 800 nm and 1550 nm wavelengths but the allowable safe laser power is about
fifty times higher at 1550 nm. This factor of fifty is important as it provides up to 17 dB
additional margin, allowing the system to propagate over longer distances, through
heavier attenuation and to support higher rates
7.3 COST OF DEPLOYMENT
Higher performances with little extra cost penalty, provides the best value. The key
factor that affects the cost are system design, minimization of manual labour and bulk
manufacturing. An 850 nm laser can cost up to $5000 while a 1550 nm laser can go up
to $50000.
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8. FSO-BREAKING THE BANDWIDTH BOTTLENECK
The global telecommunications network has seen massive expansion over the last few
years. First came the tremendous growth of the optical fiber long-haul, WAN followed
by a more recent emphasis on MANs. Meanwhile, LANs and gigabit Ethernet ports are
being deployed with a comparable growth rate. In order for this tremendous network
capacity to be exploited, and for the users to be able to utilize the broad array of new
services becoming available, network designers must provide a flexible and cost-
effective means for the users to access the telecommunications network. Presently,
however, most local loop network connections are limited to 1.5 Mbps (a T1 line). As a
consequence, there is a strong need for a high-bandwidth bridge (the “last mile” or
“first mile”) between the LANs and the MANs or WANs. A recent New York Times
article reported that more than 100 million miles of optical fiber was laid around the
world in the last two years, as carriers reacted to the Internet phenomenon and end
users’ insatiable demand for bandwidth. The sheer scale of connecting whole
communities, cities and regions to that fiber optic cable or “backbone” is something not
many players understood well. Despite the huge investment in trenching and optical
cable, most of the fiber remains unlit, 80 to 90 percent of office, commercial and
industrial buildings are not connected to fiber, and transport prices are dropping
dramatically.FSO systems represent one of the most promising approaches for
addressing the emerging broadband access market and its “last mile” bottleneck. FSO
systems offer many features, principal among them have being less start-up and
operational costs, rapid deployment, and high fiber-like bandwidths due to the optical
nature of the technology.
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9 .FSO ADVANTAGES AND CHALLENGES
9.1 ADVANTAGES
An FSO system offers a flexible networking solution that delivers on the promise of
broadband. Since FSO optical wireless transceivers can transmit and receive through
windows, it is possible to mount FSO systems inside buildings, reducing the need to
compete for roof space, simplifying wiring and cabling, and permitting the equipment
to operate in a very favorable environment. The only essential for FS is line of sight
between the two ends of the link.
Freedom from licensing and regulation .
Ease, high speed and low cost of deployment.
It reduces the need to compete for roof space, simplifying wiring
Only need is the line of sight between two links
Zero chances of network failure
9.2 FSO CHALLENGES
The advantages of free space optical wireless or FSO do not come without some cost.
When light is transmitted through optical fiber, transmission integrity is quite
predictable – barring unforseen events such as backhoes or animal interference.
FOG
Fog substantially attenuates visible radiation, and it has a similar affect on the near-
infrared wavelengths that are employed in FSO systems. Note that the effect of fog on
FSO optical wireless radiation is entirely analogous to the attenuation – and fades –
suffered by RF wireless systems due to rainfall. Similar to the case of rain attenuation
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with RF wireless, fog attenuation is not a “show-stopper” for FSO, because the optical
link can be engineered such that, for a large fraction of the time, an acceptable power
will be received even in the presence of heavy fog
PHYSICAL OBSTRUCTIONS
FSO products which have widely spaced redundant transmitters and large receive optics
will all but eliminate interference concerns from objects such as birds. On a typical day,
an object covering 98% of the receive aperture and all but 1 transmitter; will not cause
a FSO link to drop out. Thus birds are unlikely to have any impact on FSO transmission
POINTING STABILITY-BUILDING SWAY
Fixed pointed FSO systems are designed to be capable of handling the vast majority of
movement found in deployments on buildings. The combination of effective beam
divergence and a well matched receive Field-of-View (FOV) provide for an extremely
robust fixed FSO system suitable for most deployments. Fixed-pointed FSO systems
are generally preferred over actively-tracked FSO systems due to their lower cost.
SCINTILLATION
Scintillation is one of the effects related to turbulence. Turbulence is caused when
temperature differentials change the air particle density. Cells or hot pockets of air are
created that move randomly in space and time thus also changing the refractive index of
the air media.
Scintillation mainly causes a sudden increase in BER during very short time intervals
(typically less than a second). During hot summer days and around midday and/or in
the very early morning hours scintillation effects can be best observed.
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SOLAR INTERFERENCE
Solar interference in FSO system operating at 1550 nm can be combated in two ways.
The first is a long- pass optical filter window used to block all optical wavelengths
below 850 nm from entering the system; the second is an optical narrowband filter
proceeding the receive detector used to filter all but the wavelength actually used for
intersystem communications. To handle off-axis solar energy, two spatial filters have
been implemented in systems, allowing them to operate unaffected by solar interference
that is more than 1.5 degrees off-axis.
ATMOSPHERIC ATTENUATION
Carrier-class FSO systems must be designed to accommodate heavy atmospheric
attenuation, particularly by fog. Although longer wavelengths are favored in haze and
light fog, under conditions of very low visibility this long-wavelength advantage does
not apply. However, the fact that1550 nm-based systems are allowed to transmit up to
50 times more eye-safe power will translate into superior penetration of fog or any
other atmospheric attenuator
TABLE 9.1 Rough Estimate of Power losses in the system Infrared light (765 nm) :
Clear, still air -1 dB/km -5 dB/km
Scintillation 0 to -3 dB/km
Birds or foliage Impenetrable 0 to -20 dB/km
Window (double-glazed) -3 dB/km -1 dB /km
Light mist (visibility 400m) -25 dB/km -1 dB/km
Medium fog (visibility 100m) -120 dB/km -1 dB/km
Light rain (25mm/hour) -10 dB/km -10 dB/
29. 29
10. APPLICATIONS
METRO NETWOK EXTENSIONS – FSO is used to extend existing metropolitan area
fibers to connect new networks from outside
LAST MILE ACCESS – FSO can be used in high speed links to connect the end users
with ISPs.
ENTERPRISE CONNECTIVITY - The ease in which FSO can be installed Make them
a solution for interconnecting LAN segments, housed in building separated by public
streets.
FIBER BACKUP - FSO may be deployed in redundant links to backup fiber in place of
a second fiber link.
BACKHAUL – Used to carry cellular telephone traffic from antenna towers back to
facilities into the public switched telephone network.
FSO COMPARISONS
Free space optical communications is now established as a viable approach for
addressing the emerging broadband access market and its “last mile” bottleneck..These
robust systems, which establish communication links by transmitting laser beams
directly through the atmosphere, have matured to the point that mass- produced models
are now available. Optical wireless systems offer many features, principal among them
being slow start-up and operational costs, rapid deployment, and high fiber-like
bandwidths. These systems are compatible with a wide range of applications and
markets, and they are sufficiently flexible as to be easily implemented using a variety of
different architectures. Because of these features, market projections indicate healthy
growth for optical wireless sales. Although simple to deploy, optical wireless
transceivers are sophisticated devices.
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11. CONCLUSION
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 worldwide. 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.
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