28-10-2016, 09:55 AM
[attachment=74057]
P REFACE
Technology has rapidly grown in past two-three decades. An engineer without
practical knowledge and skills cannot survive in this technical era. Theoretical
knowledge does matter but it is the practical knowledge that is the difference
between the best and the better. Organizations also prefer experienced engineers
than fresher ones due to practical knowledge and industrial exposure of the former.
So it can be said the industrial exposure has to be very much mandatory for
engineers nowadays. The practical training is highly conductive for solid
foundation for:
Solid foundation of knowledge and personality.
Exposure to industrial environment.
Confidence building.
Enhancement of creativity
I NTRODUCTION
Free space optics (FSO) is an optical communication technology that uses light
propagating in free space to transmit data between two points.The technology is
useful where the physical connections by the means of fibre optic cable are
impractical due to high costs or other considerations.
Free Space Optics (FSO) communications or Optical Wireless, refers to the
transmission of modulated visible or infrared (IR) beams through the atmosphere
to obtain optical communications.
Free space optical communications is a line-of-sight (LOS) technology that
transmits a modulated beam of visible or infrared light through the atmosphere for
broadband communications. In a manner similar to fiber optical communications,
free space optics uses a light emitting diode (LED) or laser (light amplification by
stimulated emission of radiation) point source for data transmission. However, in
free space optics, an energy beam is collimated and transmitted through space
rather than being guided through an optical cable. These beams of light, operating
in the TeraHertz portion of the spectrum, are focused on a receiving lens connected
to a high sensitivity receiver through an optical fiber.
Unlike radio and microwave systems, free space optical communications requires
no spectrum licensing and interference to and from other systems is not a concern.
In addition, the point-to-point laser signal is extremely difficult to intercept,
making it ideal for covert communications. Free space optical communications
offer data rates comparable to fiber optical communications at a fraction of the
deployment cost while extremely narrow laser beam widths provide no limit to the
number of free space optical links that may be installed in a given location.
The fundamental limitation of free space optical communications arises from the
environment through which it propagates. Although relatively unaffected by rain
and snow, free space optical communication systems can be severely affected by
fog and atmospheric turbulence.
Free Space Optics are additionally used for communications between spacecraft.
The optical links can be implemented using infrared laser light, although low-datarate
communication over short distances is possible using LEDs. Maximum range
for terrestrial links is in the order of 2-3 km, 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 (173 miles in
at least one occasion) using incoherent sources of light from high-intensity LEDs.
However, the low-grade equipment used limited bandwidths to about 4kHz. In
outer space, the communication range of free-space optical communication is
currently in the order of several thousand kilometers, but has the potential to bridge
interplanetary distances of millions of kilometers, using optical telescopes as beam
expanders.
FREE-SPACE optical communication has attracted considerable attention recently
for a variety of applications. Because of the complexity associated with phase or
Frequency modulation, current free-space optical communication systems typically
use intensity modulation with direct detection (IM/DD). Atmospheric turbulence
can degrade the performance of free-space optical links, particularly over ranges of
the order of 1 km or longer. In homogeneities in the temperature and pressure of
the atmosphere lead to variations of the refractive index along the transmission
path. These index in homogeneities can deteriorate the quality of the received
image and can cause fluctuations in both the intensity and the phase of the received
signal. These fluctuations can lead to an increase in the link error probability,
limiting the performance of communication systems. Aerosol scattering effects
caused by rain, snow and fog can also degrade the performance of free-space
optical communication systems but are not treated in this paper.
Atmospheric turbulence has been studied extensively and various theoretical
models have been proposed to describe turbulence- induced image degradation and
intensity fluctuations (i.e., signal fading). Two useful parameters describing
turbulence- induced fading are, the correlation length of intensity fluctuations and
the correlation time of intensity fluctuations. When the receiver aperture can be
made larger than the correlation length, then turbulence-induced fading can be
reduced substantially by aperture averaging. Because it is not always possible to
satisfy the condition, in this paper, we propose alternative techniques for mitigating
fading in the regime where. At the bit rates of interest in most free-space optical
systems, the receiver observation interval during each bit interval is smaller than
the turbulence correlation time.
Throughout this paper, we will assume that and The techniques we consider are
based on the statistical properties of turbulence-induced signal intensity fading, as
functions of both temporal and spatial coordinates. Our approaches can be divided
into two categories: temporal-domain techniques and
spatial-domain techniques.
In the temporal-domain techniques, one employs a single receiver. If the receiver
has knowledge of the marginal fading distribution, but knows neither the temporal
fading correlation nor the instantaneous fading state, a maximum-likelihood (ML)
symbol-by-symbol detection technique can be used. If the receiver further knows
the joint temporal fading distribution, but not the instantaneous fading state, the
receiver can use aML sequence detection (MLSD) technique to mitigate
turbulence-induced fading.
In the spatial-domain techniques, one must employ at least two receivers to collect
the signal light at different positions or from different spatial angles. To maximize
the diversity reception gain, the multiple receivers should be placed as far apart as
possible, so that the turbulence-induced fading is uncorrelated at the various
receivers. In practice, however, it may not always be possible to place the receivers
sufficiently far apart. Hence, in this paper, making use of the spatial correlation of
turbulence-induced fading, we derive the optimal ML detection scheme for
correlated spatial diversity reception.
The major drawback, however at present time is that the glass used in fiber optics
is very lossy, amounting to a decibel per meter at the very best. In actuality, with
present glasses, the losses would amout to thousands of decibels per mile, which
makes the material clearly unsuitable for long distance communication.
The present work,then going on principally in the United States, in Britain, and in
Japan, aims at the development of very pure glass fibers. If pure enough fibers are
successfully development, dielectric waveguides might eventually be used between
switching exchanges a few miles apart within cities and towns.
By 1973, if all goes well, at least one satellite will be put aloft carrying laser
communication experiments, put up by NASA. It will have taken a full decade to
bring this about. In one way or another, many companies and have had a hand in
giving impetus to these experiments. But even yet, there has been some expression
of fear by people in the optics community that if these experiments should not
succeed, or should fail in some significant way, optical communications could be
held back for an indefinetly longer period.
H ISTORY
Optical Communication, in various forms, have been used for thousands of years.
The Ancient Greeks polished their shailds to send signals during battle. In the
modern era, semaphores and wireless solar telegraphs called Heliographs were
developed, using coded signals to communicate with their recipients.
In 1880 Alexander Graham Bell and his then-assistant Charles Sumner Tainter
created the photophone, in Washington, D.C. Bell considered it his most important
invention. The device allowed for the transmission of sound on a beam of light. On
June 3, 1880, Alexander Graham Bell conducted the world's first wireless
telephone transmission between two building rooftopsIts. First practical use came
in military communication systems many decades later.
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.
In 1966 Charles K. Kao and George Hockham proposed optical fibers at STC
Laboratories (STL), Harlow, when they showed that the losses of 1000 db/km in
existing glass (compared to 5-10 db/km in coaxial cable) was due to contaminants,
which could potentially be removed.
Optical fiber was successfully developed in 1970 by Corning Glass Works, with
attenuation low enough for communication purposes (about 20dB/km), and at the
same time GaAs semiconductor lasers were developed that were compact and
therefore suitable for transmitting light through fiber optic cables for long
distances.
After a period of research starting from 1975, the first commercial fiber-optic
communications system was developed, which operated at a wavelength around
0.8 µm and used GaAs semiconductor lasers. This first-generation system operated
at a bit rate of 45 Mbps with repeater spacing of up to 10 km. Soon on 22 April,
1977, General Telephone and Electronics sent the first live telephone traffic
through fiber optics at a 6 Mbps throughput in Long Beach, California.
The second generation of fiber-optic communication was developed for
commercial use in the early 1980s, operated at 1.3 µm, and used InGaAsP
semiconductor lasers. Although these systems were initially limited by dispersion,
in 1981 the single-mode fiber was revealed to greatly improve system
performance. By 1987, these systems were operating at bit rates of up to 1.7 Gb/s
with repeater spacing up to 50 km.
The first transatlantic telephone cable to use optical fiber was TAT-8, based on
Desurvire optimized laser amplification technology. It went into operation in 1988.
Third-generation fiber-optic systems operated at 1.55 µm and had losses of about
0.2 dB/km. They achieved this despite earlier difficulties with pulse-spreading at
that wavelength using conventional InGaAsP semiconductor lasers. Scientists
overcame this difficulty by using dispersion-shifted fibers designed to have
minimal dispersion at 1.55 µm or by limiting the laser spectrum to a single
longitudinal mode. These developments eventually allowed third-generation
systems to operate commercially at 2.5 Gbit/s with repeater spacing in excess of
100 km.
The fourth generation of fiber-optic communication systems used optical
amplification to reduce the need for repeaters and wavelength-division
multiplexing to increase data capacity. These two improvements caused a
revolution that resulted in the doubling of system capacity every 6 months starting
in 1992 until a bit rate of 10 Tb/s was reached by 2001. Recently, bit-rates of up to
14 Tbit/s have been reached over a single 160 km line using optical amplifiers.
The focus of development for the fifth generation of fiber-optic communications is
on extending the wavelength range over which a WDM system can operate. The
conventional wavelength window, known as the C band, covers the wavelength
range 1.53-1.57 µm, and the new dry fiber has a low-loss window promising an
extension of that range to 1.30-1.65 µm. Other developments include the concept
of " optical solitons, " pulses that preserve their shape by counteracting the effects
of dispersion with the nonlinear effects of the fiber by using pulses of a specific
shape.
In the late 1990s through 2000, industry promoters, and research companies such
as KMI and RHK predicted vast increases in demand for communications
bandwidth due to increased use of the Internet, and commercialization of various
bandwidth-intensive consumer services, such as video on demand. Internet
protocol data traffic was increasing exponentially, at a faster rate than integrated
circuit complexity had increased under Moore's Law.
T HEORY
Free Space Optics (FSO) systems are generally employed for 'last mile'
communications and can function over distances of several kilometers as long as
there is a clear line of sight between the source and the destination, and the optical
receiver can reliably decode the transmitted information.
There are many forms of non-technological optical communication, including body
language and sign language.Techniques such as semaphore lines, ship flags, smoke
signals, and beacon fires were the earliest form of technological optical
communication.
The heliograph uses a mirror to reflect sunlight to a distant observer. By moving
the mirror the distant observer sees flashes of light that can be used to send a
prearranged signaling code. Navy ships often use a signal lamp to signal in Morse
code in a similar way. Distress flares are used by mariners in emergencies, while
lighthouses and navigation lights are used to communicate navigation hazards.
Aircraft use the landing lights at airports to land safely, especially at night. Aircraft
landing on an aircraft carrier use a similar system to land correctly on the carrier
deck. The light systems communicate the correct position of the aircraft relative to
the best landing glideslope. Also, many control towers still have an Aldis lamp to
communicate with planes whose radio failed. Optical fiber is the most common
medium for modern digital optical communication. Free-space optical
communication is also used today in a variety of applications.
Optical fiber is the most common type of channel for optical communications,
however, other types of optical waveguides are used within computers or
communications gear, and have even formed the channel of very short distance
(e.g. chip-to-chip, intra-chip) links in laboratory trials. The transmitters in optical
fiber links are generally light-emitting diodes (LEDs) or laser diodes. Infrared
light, rather than visible light is used more commonly, because optical fibers
transmit infrared wavelengths with less attenuation and dispersion. The signal
encoding is typically simple intensity modulation, although historically optical
phase and frequency modulation have been demonstrated in the lab. The need for
periodic signal regeneration was largely superseded by the introduction of the
erbium-doped fiber amplifier, which extended link distances at significantly lower
cost.
In fiber-optic communications, information is transmitted by sending light through
optical fibers.
Fiber-optic communication is a method of transmitting information from one
place to another by sending pulses of light through an optical fiber. The light forms
an electromagnetic carrier wave that is modulated to carry information. First
developed in the 1970s, fiber-optic communication systems have revolutionized
the telecommunications industry and have played a major role in the advent of the
Information Age. Because of its advantages over electrical transmission, optical
fibers have largely replaced copper wire communications in core networks in the
developed world.
The process of communicating using fiber-optics involves the following basic
steps: Creating the optical signal involving the use of a transmitter, relaying the
signal along the fiber, ensuring that the signal does not become too distorted or
weak, receiving the optical signal, and converting it into an electrical signal.
Atmospheric Attenuation-FOG
Absorption or scattering of optical signals due to airborne particles
Primarily FOG but can be rain, snow, smoke, dust, etc.
Can result in a complete outage
FSO wavelengths and fog droplets are close to equal in size (Mie Scattering)
Typical FSO systems work 2-3X further than the human eye can see
High availability deployments require short links that can operate in the fog
Low Clouds, Rain, Snow and Dust
Low Clouds
Very similar to fog
May accompany rain and snow
Rain
Drop sizes larger than fog and wavelength of light
Extremely heavy rain (can’t see through it) can take a link down
Water sheeting on windows
Heavy Snow
May cause ice build-up on windows
Whiteout conditions
Sand Storms
Likely only in desert areas; rare in the urban core
U SAGE A ND T ECHNOLOGIES
Free Space Optics are additionally used for communications between spacecraft.
The optical links can be implemented using infrared laser light, although low-datarate
communication over short distances is possible using LEDs. Maximum range
for terrestrial links is in the order of 2-3 km, 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 (173 miles in
at least one occasion) using incoherent sources of light from high-intensity LEDs.
However, the low-grade equipment used limited bandwidths to about 4kHz. In
outer space, the communication range of free-space optical communication is
currently in the order of several thousand kilometers, but has the potential to bridge
interplanetary distances of millions of kilometers, using optical telescopes as beam
expanders. IrDA is also a very simple form of free-space optical communications.
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. Although this method has been demonstrated at laboratory distances in
principle it could be applied over large distances in space.
A PPLICATIONS
Two solar-powered satellites communicating optically in space via lasers.
Typically scenarios for use are:
• LAN-to-LAN connections on campuses at Fast Ethernet or Gigabit Ethernet
speeds.
• LAN-to-LAN connections in a city. example, Metropolitan area network.
• To cross a public road or other barriers which the sender and receiver do not
own.
• Speedy service delivery of high-bandwidth access to optical fiber networks.
• Converged Voice-Data-Connection.
• Temporary network installation (for events or other purposes).
• Reestablish high-speed connection quickly (disaster recovery).
• As an alternative or upgrade add-on to existing wireless technologies.
• As a safety add-on for important fiber connections (redundancy).
• For communications between spacecraft, including elements of a satellite
constellation.
• For inter- and intra[8]-chip communication.
The light beam can be very narrow, which makes FSO hard to intercept, improving
security. In any case, it is comparatively easy to encrypt any data traveling across
the FSO connection for additional security. FSO provides vastly improved EMI
behavior using light instead of microwaves.
A dvantages
RONJA is a free implementation of FSO utilizing high-intensity LEDs.
• Ease of deployment
• License-free operation
• High bit rates
• Low bit error rates
• Immunity to electromagnetic interference
• Full duplex operation
• Very secure due to the high directionality and narrowness of the beam(s)
• No Fresnel zone necessary
C ONCLUSION
In free-space optical communication links, atmospheric turbulence causes
fluctuations in both the intensity and the phase of the received light signal,
impairing link performance.
The potential for Free-space optical networking to solve communications
bottlenecks is making it a popular option for reliable, broadband access. A
thorough examination of the issues affecting the design of these sophisticated
systems is a useful tool when evaluating Free Space Optics (FSO) systems for
purchase. Systems that incorporate the most beneficial features, are wellengineered,
and thoroughly tested will be top performers and provide the best
value.
P REFACE
Technology has rapidly grown in past two-three decades. An engineer without
practical knowledge and skills cannot survive in this technical era. Theoretical
knowledge does matter but it is the practical knowledge that is the difference
between the best and the better. Organizations also prefer experienced engineers
than fresher ones due to practical knowledge and industrial exposure of the former.
So it can be said the industrial exposure has to be very much mandatory for
engineers nowadays. The practical training is highly conductive for solid
foundation for:
Solid foundation of knowledge and personality.
Exposure to industrial environment.
Confidence building.
Enhancement of creativity
I NTRODUCTION
Free space optics (FSO) is an optical communication technology that uses light
propagating in free space to transmit data between two points.The technology is
useful where the physical connections by the means of fibre optic cable are
impractical due to high costs or other considerations.
Free Space Optics (FSO) communications or Optical Wireless, refers to the
transmission of modulated visible or infrared (IR) beams through the atmosphere
to obtain optical communications.
Free space optical communications is a line-of-sight (LOS) technology that
transmits a modulated beam of visible or infrared light through the atmosphere for
broadband communications. In a manner similar to fiber optical communications,
free space optics uses a light emitting diode (LED) or laser (light amplification by
stimulated emission of radiation) point source for data transmission. However, in
free space optics, an energy beam is collimated and transmitted through space
rather than being guided through an optical cable. These beams of light, operating
in the TeraHertz portion of the spectrum, are focused on a receiving lens connected
to a high sensitivity receiver through an optical fiber.
Unlike radio and microwave systems, free space optical communications requires
no spectrum licensing and interference to and from other systems is not a concern.
In addition, the point-to-point laser signal is extremely difficult to intercept,
making it ideal for covert communications. Free space optical communications
offer data rates comparable to fiber optical communications at a fraction of the
deployment cost while extremely narrow laser beam widths provide no limit to the
number of free space optical links that may be installed in a given location.
The fundamental limitation of free space optical communications arises from the
environment through which it propagates. Although relatively unaffected by rain
and snow, free space optical communication systems can be severely affected by
fog and atmospheric turbulence.
Free Space Optics are additionally used for communications between spacecraft.
The optical links can be implemented using infrared laser light, although low-datarate
communication over short distances is possible using LEDs. Maximum range
for terrestrial links is in the order of 2-3 km, 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 (173 miles in
at least one occasion) using incoherent sources of light from high-intensity LEDs.
However, the low-grade equipment used limited bandwidths to about 4kHz. In
outer space, the communication range of free-space optical communication is
currently in the order of several thousand kilometers, but has the potential to bridge
interplanetary distances of millions of kilometers, using optical telescopes as beam
expanders.
FREE-SPACE optical communication has attracted considerable attention recently
for a variety of applications. Because of the complexity associated with phase or
Frequency modulation, current free-space optical communication systems typically
use intensity modulation with direct detection (IM/DD). Atmospheric turbulence
can degrade the performance of free-space optical links, particularly over ranges of
the order of 1 km or longer. In homogeneities in the temperature and pressure of
the atmosphere lead to variations of the refractive index along the transmission
path. These index in homogeneities can deteriorate the quality of the received
image and can cause fluctuations in both the intensity and the phase of the received
signal. These fluctuations can lead to an increase in the link error probability,
limiting the performance of communication systems. Aerosol scattering effects
caused by rain, snow and fog can also degrade the performance of free-space
optical communication systems but are not treated in this paper.
Atmospheric turbulence has been studied extensively and various theoretical
models have been proposed to describe turbulence- induced image degradation and
intensity fluctuations (i.e., signal fading). Two useful parameters describing
turbulence- induced fading are, the correlation length of intensity fluctuations and
the correlation time of intensity fluctuations. When the receiver aperture can be
made larger than the correlation length, then turbulence-induced fading can be
reduced substantially by aperture averaging. Because it is not always possible to
satisfy the condition, in this paper, we propose alternative techniques for mitigating
fading in the regime where. At the bit rates of interest in most free-space optical
systems, the receiver observation interval during each bit interval is smaller than
the turbulence correlation time.
Throughout this paper, we will assume that and The techniques we consider are
based on the statistical properties of turbulence-induced signal intensity fading, as
functions of both temporal and spatial coordinates. Our approaches can be divided
into two categories: temporal-domain techniques and
spatial-domain techniques.
In the temporal-domain techniques, one employs a single receiver. If the receiver
has knowledge of the marginal fading distribution, but knows neither the temporal
fading correlation nor the instantaneous fading state, a maximum-likelihood (ML)
symbol-by-symbol detection technique can be used. If the receiver further knows
the joint temporal fading distribution, but not the instantaneous fading state, the
receiver can use aML sequence detection (MLSD) technique to mitigate
turbulence-induced fading.
In the spatial-domain techniques, one must employ at least two receivers to collect
the signal light at different positions or from different spatial angles. To maximize
the diversity reception gain, the multiple receivers should be placed as far apart as
possible, so that the turbulence-induced fading is uncorrelated at the various
receivers. In practice, however, it may not always be possible to place the receivers
sufficiently far apart. Hence, in this paper, making use of the spatial correlation of
turbulence-induced fading, we derive the optimal ML detection scheme for
correlated spatial diversity reception.
The major drawback, however at present time is that the glass used in fiber optics
is very lossy, amounting to a decibel per meter at the very best. In actuality, with
present glasses, the losses would amout to thousands of decibels per mile, which
makes the material clearly unsuitable for long distance communication.
The present work,then going on principally in the United States, in Britain, and in
Japan, aims at the development of very pure glass fibers. If pure enough fibers are
successfully development, dielectric waveguides might eventually be used between
switching exchanges a few miles apart within cities and towns.
By 1973, if all goes well, at least one satellite will be put aloft carrying laser
communication experiments, put up by NASA. It will have taken a full decade to
bring this about. In one way or another, many companies and have had a hand in
giving impetus to these experiments. But even yet, there has been some expression
of fear by people in the optics community that if these experiments should not
succeed, or should fail in some significant way, optical communications could be
held back for an indefinetly longer period.
H ISTORY
Optical Communication, in various forms, have been used for thousands of years.
The Ancient Greeks polished their shailds to send signals during battle. In the
modern era, semaphores and wireless solar telegraphs called Heliographs were
developed, using coded signals to communicate with their recipients.
In 1880 Alexander Graham Bell and his then-assistant Charles Sumner Tainter
created the photophone, in Washington, D.C. Bell considered it his most important
invention. The device allowed for the transmission of sound on a beam of light. On
June 3, 1880, Alexander Graham Bell conducted the world's first wireless
telephone transmission between two building rooftopsIts. First practical use came
in military communication systems many decades later.
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.
In 1966 Charles K. Kao and George Hockham proposed optical fibers at STC
Laboratories (STL), Harlow, when they showed that the losses of 1000 db/km in
existing glass (compared to 5-10 db/km in coaxial cable) was due to contaminants,
which could potentially be removed.
Optical fiber was successfully developed in 1970 by Corning Glass Works, with
attenuation low enough for communication purposes (about 20dB/km), and at the
same time GaAs semiconductor lasers were developed that were compact and
therefore suitable for transmitting light through fiber optic cables for long
distances.
After a period of research starting from 1975, the first commercial fiber-optic
communications system was developed, which operated at a wavelength around
0.8 µm and used GaAs semiconductor lasers. This first-generation system operated
at a bit rate of 45 Mbps with repeater spacing of up to 10 km. Soon on 22 April,
1977, General Telephone and Electronics sent the first live telephone traffic
through fiber optics at a 6 Mbps throughput in Long Beach, California.
The second generation of fiber-optic communication was developed for
commercial use in the early 1980s, operated at 1.3 µm, and used InGaAsP
semiconductor lasers. Although these systems were initially limited by dispersion,
in 1981 the single-mode fiber was revealed to greatly improve system
performance. By 1987, these systems were operating at bit rates of up to 1.7 Gb/s
with repeater spacing up to 50 km.
The first transatlantic telephone cable to use optical fiber was TAT-8, based on
Desurvire optimized laser amplification technology. It went into operation in 1988.
Third-generation fiber-optic systems operated at 1.55 µm and had losses of about
0.2 dB/km. They achieved this despite earlier difficulties with pulse-spreading at
that wavelength using conventional InGaAsP semiconductor lasers. Scientists
overcame this difficulty by using dispersion-shifted fibers designed to have
minimal dispersion at 1.55 µm or by limiting the laser spectrum to a single
longitudinal mode. These developments eventually allowed third-generation
systems to operate commercially at 2.5 Gbit/s with repeater spacing in excess of
100 km.
The fourth generation of fiber-optic communication systems used optical
amplification to reduce the need for repeaters and wavelength-division
multiplexing to increase data capacity. These two improvements caused a
revolution that resulted in the doubling of system capacity every 6 months starting
in 1992 until a bit rate of 10 Tb/s was reached by 2001. Recently, bit-rates of up to
14 Tbit/s have been reached over a single 160 km line using optical amplifiers.
The focus of development for the fifth generation of fiber-optic communications is
on extending the wavelength range over which a WDM system can operate. The
conventional wavelength window, known as the C band, covers the wavelength
range 1.53-1.57 µm, and the new dry fiber has a low-loss window promising an
extension of that range to 1.30-1.65 µm. Other developments include the concept
of " optical solitons, " pulses that preserve their shape by counteracting the effects
of dispersion with the nonlinear effects of the fiber by using pulses of a specific
shape.
In the late 1990s through 2000, industry promoters, and research companies such
as KMI and RHK predicted vast increases in demand for communications
bandwidth due to increased use of the Internet, and commercialization of various
bandwidth-intensive consumer services, such as video on demand. Internet
protocol data traffic was increasing exponentially, at a faster rate than integrated
circuit complexity had increased under Moore's Law.
T HEORY
Free Space Optics (FSO) systems are generally employed for 'last mile'
communications and can function over distances of several kilometers as long as
there is a clear line of sight between the source and the destination, and the optical
receiver can reliably decode the transmitted information.
There are many forms of non-technological optical communication, including body
language and sign language.Techniques such as semaphore lines, ship flags, smoke
signals, and beacon fires were the earliest form of technological optical
communication.
The heliograph uses a mirror to reflect sunlight to a distant observer. By moving
the mirror the distant observer sees flashes of light that can be used to send a
prearranged signaling code. Navy ships often use a signal lamp to signal in Morse
code in a similar way. Distress flares are used by mariners in emergencies, while
lighthouses and navigation lights are used to communicate navigation hazards.
Aircraft use the landing lights at airports to land safely, especially at night. Aircraft
landing on an aircraft carrier use a similar system to land correctly on the carrier
deck. The light systems communicate the correct position of the aircraft relative to
the best landing glideslope. Also, many control towers still have an Aldis lamp to
communicate with planes whose radio failed. Optical fiber is the most common
medium for modern digital optical communication. Free-space optical
communication is also used today in a variety of applications.
Optical fiber is the most common type of channel for optical communications,
however, other types of optical waveguides are used within computers or
communications gear, and have even formed the channel of very short distance
(e.g. chip-to-chip, intra-chip) links in laboratory trials. The transmitters in optical
fiber links are generally light-emitting diodes (LEDs) or laser diodes. Infrared
light, rather than visible light is used more commonly, because optical fibers
transmit infrared wavelengths with less attenuation and dispersion. The signal
encoding is typically simple intensity modulation, although historically optical
phase and frequency modulation have been demonstrated in the lab. The need for
periodic signal regeneration was largely superseded by the introduction of the
erbium-doped fiber amplifier, which extended link distances at significantly lower
cost.
In fiber-optic communications, information is transmitted by sending light through
optical fibers.
Fiber-optic communication is a method of transmitting information from one
place to another by sending pulses of light through an optical fiber. The light forms
an electromagnetic carrier wave that is modulated to carry information. First
developed in the 1970s, fiber-optic communication systems have revolutionized
the telecommunications industry and have played a major role in the advent of the
Information Age. Because of its advantages over electrical transmission, optical
fibers have largely replaced copper wire communications in core networks in the
developed world.
The process of communicating using fiber-optics involves the following basic
steps: Creating the optical signal involving the use of a transmitter, relaying the
signal along the fiber, ensuring that the signal does not become too distorted or
weak, receiving the optical signal, and converting it into an electrical signal.
Atmospheric Attenuation-FOG
Absorption or scattering of optical signals due to airborne particles
Primarily FOG but can be rain, snow, smoke, dust, etc.
Can result in a complete outage
FSO wavelengths and fog droplets are close to equal in size (Mie Scattering)
Typical FSO systems work 2-3X further than the human eye can see
High availability deployments require short links that can operate in the fog
Low Clouds, Rain, Snow and Dust
Low Clouds
Very similar to fog
May accompany rain and snow
Rain
Drop sizes larger than fog and wavelength of light
Extremely heavy rain (can’t see through it) can take a link down
Water sheeting on windows
Heavy Snow
May cause ice build-up on windows
Whiteout conditions
Sand Storms
Likely only in desert areas; rare in the urban core
U SAGE A ND T ECHNOLOGIES
Free Space Optics are additionally used for communications between spacecraft.
The optical links can be implemented using infrared laser light, although low-datarate
communication over short distances is possible using LEDs. Maximum range
for terrestrial links is in the order of 2-3 km, 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 (173 miles in
at least one occasion) using incoherent sources of light from high-intensity LEDs.
However, the low-grade equipment used limited bandwidths to about 4kHz. In
outer space, the communication range of free-space optical communication is
currently in the order of several thousand kilometers, but has the potential to bridge
interplanetary distances of millions of kilometers, using optical telescopes as beam
expanders. IrDA is also a very simple form of free-space optical communications.
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. Although this method has been demonstrated at laboratory distances in
principle it could be applied over large distances in space.
A PPLICATIONS
Two solar-powered satellites communicating optically in space via lasers.
Typically scenarios for use are:
• LAN-to-LAN connections on campuses at Fast Ethernet or Gigabit Ethernet
speeds.
• LAN-to-LAN connections in a city. example, Metropolitan area network.
• To cross a public road or other barriers which the sender and receiver do not
own.
• Speedy service delivery of high-bandwidth access to optical fiber networks.
• Converged Voice-Data-Connection.
• Temporary network installation (for events or other purposes).
• Reestablish high-speed connection quickly (disaster recovery).
• As an alternative or upgrade add-on to existing wireless technologies.
• As a safety add-on for important fiber connections (redundancy).
• For communications between spacecraft, including elements of a satellite
constellation.
• For inter- and intra[8]-chip communication.
The light beam can be very narrow, which makes FSO hard to intercept, improving
security. In any case, it is comparatively easy to encrypt any data traveling across
the FSO connection for additional security. FSO provides vastly improved EMI
behavior using light instead of microwaves.
A dvantages
RONJA is a free implementation of FSO utilizing high-intensity LEDs.
• Ease of deployment
• License-free operation
• High bit rates
• Low bit error rates
• Immunity to electromagnetic interference
• Full duplex operation
• Very secure due to the high directionality and narrowness of the beam(s)
• No Fresnel zone necessary
C ONCLUSION
In free-space optical communication links, atmospheric turbulence causes
fluctuations in both the intensity and the phase of the received light signal,
impairing link performance.
The potential for Free-space optical networking to solve communications
bottlenecks is making it a popular option for reliable, broadband access. A
thorough examination of the issues affecting the design of these sophisticated
systems is a useful tool when evaluating Free Space Optics (FSO) systems for
purchase. Systems that incorporate the most beneficial features, are wellengineered,
and thoroughly tested will be top performers and provide the best
value.