CHAPTER 1 INTRODUCTION 1.1 INTRODUCTION 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.
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CHAPTER 1
INTRODUCTION
1.1 INTRODUCTION
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.
Mention optical communication and most people think of fiber optics. But light
travels through air for a lot less money. So it is hardly a surprise that clever entrepreneurs
and technologists are borrowing many of the devices and techniques developed for fiber-
optic systems and applying them to what some call fiber-free optical communication.
Although it only recently, and rather suddenly, sprang into public awareness, free-space
optics is not a new idea. It has roots that go back over 30 years--to the era before fiber-optic
cable became the preferred transport medium for high-speed communication. In those days,
the notion that FSO systems could provide high-speed connectivity over short distances
seemed futuristic, to say the least. But research done at that time has made possible today's
free-space optical systems, which can carry full-duplex (simultaneous bidirectional) data at
gigabit-per-second rates over metropolitan distances of a few city blocks to a few
kilometers.
FSO first appeared in the 60's, for military applications. At the end of 80's, it
appeared as a commercial option but technological restrictions prevented it from success.
Low reach transmission, low capacity, severe alignment problems as well as vulnerability to
weather interferences were the major drawbacks at that time. The optical communication
without wire, however, evolved! Today, FSO systems guarantee 2.5 Gb/s taxes with carrier
class availability. Metropolitan, access and LAN networks are reaping the benefits. FSO
success can be measured by its market numbers: forecasts predict it will reach a USS 2.5
billion market by 2006.
The use of free space optics is particularly interesting when we perceive that the
majority of customers does not possess access to fibers as well as fiber installation is
expensive and demands long time. Moreover, right-of-way costs, difficulties in obtaining
government licenses for new fiber installation etc. are further problems that have turned
FSO into the option of choice for short reach applications.
FSO uses lasers, or light pulses, to send packetized data in the terahertz (THz)
spectrum range. Air, ot fiber, is the transport medium. This means that urban businesses
needing fast data and Internet access have a significantly lower-cost option.
An FSO system for local loop access comprises several laser terminals, each one
residing at a network node to create a single, point-to-point link; an optical mesh
architecture; or a star topology, which is usually point-to-multipoint. These laser terminals,
or nodes, are installed on top of customers' rooftops or inside a window to complete the last-
mile connection. Signals are beamed to and from hubs or central nodes throughout a city or
urban area. Each node requires a Line-Of-Sight (LOS) view of the hub.
Free space optics (FSO) has been used for more than a decade as a short/medium
distance point-to-point (building-to-building) connectivity solution in campus enterprise
LAN markets. The license free nature of this technology combined with its high-speed
bandwidth capabilities, comparable to optical fiber, allow network administrators to
interconnect LAN segments at real networking speeds (e.g. 100 Mbps or 1000 Mbps)
without the hastle of digging to install optical fiber. Since digging to install fiber is typically
a very expensive and time-consuming process, the value proposition of using FSO can be
very appealing. Only recently has the carrier market started to look into FSO technology as
an alternative network connectivity solution. However, when considering the carrier market,
the requirements in terms of component reliability and overall weather related system
availability is much more stringent than system requirements in the enterprise market. This
paper addresses some of the issues that are most important in the design of an overall carrier
system architecture. Briefly described are the basic physics of transmission at various short
and long infrared wavelengths and their overall impact on the system design. This is
followed by an overview of basic transmitter and detector technologies. When selecting
suitable components, reliability and commercial availability of those components should
play an important factor. Eye safety is another factor that has to be taken into consideration
in a carrier class system design. Finally, the link budget will determine the overall system
availability under various weather conditions. This aspect is discussed near the close of this
document.
1.2 FSO - FREE SPACE OPTICS
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.
FSO technology is implemented using a laser device .These laser devices or
terminals can be mounted on rooftops, Corners of buildings or even inside offices behind
windows. FSO devices look like security video cameras.
Low-power infrared beams, which do not harm the eyes, are the means by which
free-space optics technology transmits data through the air between transceivers, or link
heads, mounted on rooftops or behind windows. It works over distances of several hundred
meters to a few kilometers, depending upon atmospheric conditions.
Commercially available free-space optics equipment provides data rates much
higher than digital subscriber lines or coaxial cables can ever hope to offer. And systems
even faster than the present range of 10 Mb/s to 1.25 Gb/s have been announced, though not
yet delivered.
Generally the equipment works at one of two wavelengths: 850 nm or 1550 nm.
Lasers for 850 nm are much less expensive (around $30 versus more than $1000) and are
therefore favored for applications over moderate distances. But a 1550 nm lasers are also
used. The main reasons revolve around power, distance, and eye safety. Infrared radiation
at 1550 nm tends not to reach the retina of the eye, being mostly absorbed by the cornea.
Regulations accordingly allow these longer-wavelength beams to operate at higher power
than the 850-nm beams, by about two orders of magnitude. That power increase can boost
link lengths by a factor of at least five while maintaining adequate signal strength for proper
link operation. Alternatively, it can boost data rate considerably over the same length of
link. So for high data rates, long distances, poor propagation conditions (like fog), or
combinations of those conditions, 1550 nm can become quite attractive.
As the differences in laser prices suggest, such systems are quite a bit more
expensive than 850-nm links. An 850-nm transceiver can cost as little as $5000 (for a 10-
100-Mb/s unit spanning a few hundred meters), while a 1550-nm unit can go for $50 000
(for gigabit-per-second setups encompassing a kilometer or two).
Air fiber, a major FSO vendor, says it can get a link up and running within two to
three days at one-third to one-tenth the cost of fiber (about $20,000 per building). FSO is
not only cost-effective and easy to deploy but also fast. The technology is not for everyone.
A major reason companies might not adopt FSO is its confinement to urban areas. FSO
deployments must be located relatively close to big hubs, which means only customers in
major cities will be eligible-at least initially. Businesses in more remote locations are out of
luck, unless a provider sets up hubs in their area, wh ich seems like a distant reality right
now.
When fiber was compared with free-space optics, deployment costs for service to
the three buildings worked out to $396 500 versus $59 000, respectively. The fiber cost was
calculated on a need for 1220 meters: 530 meters of trunk fiber from the CLEC's central
office to its hub in the office park plus an average of 230 meters of feeder fiber for each of
the runs from the hub to a target building, all at $325 per meter. Free-space optics is
calculated as $18 000 for free-space optics equipment per building and $5000 for
installation. Supposing a 15 percent annual revenue increase for future sales and customer
acquisition, the internal rate of return for fiber over five years is 22 percent versus 196
percent for free-space optics.
1.3 RELEVANCE OF FSO IN PRESENT DAY COMMUNICATION
Presently we are facing 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 pace 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.
1.3.1Ultra high bandwidth:
The laser systems operate in the terahertz frequency spectrum and usually
operate in the 194 THz or 375 THz range. Their performance is comparable to the best fibre
optic system available, giving speeds between 622 Mbps and 1.25 Gbps. This technology
uses devices and techniques developed for fibre optic systems.
1.4 RAPID DEPLOYMENT TIME:
Installing a FSO system can be done in a matter of days even faster if the gear
can be placed in offices behind windows instead of on rooftops. A fiber based competitor
has to seek municipal approval to dig up a street to lay its cable. Unlike most of the lower
frequency portion of the electromagnetic spectrum, the part above 300 GHz is unlicensed
worldwide. So no extra time is needed to obtain right-of-way permits or trench up the
streets or to obtain FCC frequency licenses.
1.5 ORIGIN OF FSO
It is said that this mode of communication was first used in the 8 th 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 19 th
centaury 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.
1.6 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.
CHAPTER 2
WORKING OF FSO SYSTEM
2.1 INTRODUCTION
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.
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.
2.1.1 WAVELENGTH:-
Currently available FSO hardware is 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.
2.1.2 SUBSYSTEM:-
Modulator Driver Laser Transmit
optic
Data in
De-Modulator
preamplifier detector Receive opticData out
Fig 2.1.1 Subsystem 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 environmental condition, if there is any change in
the signal then the servo system is used to correct the signal.
2.2 FSO TRANSMITTER:-
To ensure the highest performance of an FSO system, it is
important to choose a transmission wavelength within one of the two atmospheric windows
that coincide with one of the fiber optics transmission windows. Within these two
wavelength windows, namely 850 nm and 1550 nm, a suitable transmitter for a
telecommunication grade FSO system must have the following characteristics:
• Operation at higher power levels (Important for longer distance FSO systems)
• Favorable high-speed modulation characteristics (Important for high speed FSO systems)
Components small in footprint and low in power consumption (Important for the overall
system design and system maintenance)
• Capability to operate over a wide temperature range without showing major performance
decay or degradation (Important for outdoor system installation)
• Mean time between failure (MTBF) operation exceeding 10 years
preamplifier Special detector
Tracking
optic
ProcessorServo systems
For these reasons manufacturers offering carrier-class FSO equipment generally use
Vertical Cavity Surface Emitting Lasers (VCSELs) in the shorter infrared wavelength
range and Fabry Perot (FP) or Distributed Feed Back (DFB) lasers for operation in the
longer infrared wavelength range.
Fig 2.1.2 Different Types Of Fso Transmitter
Aside from general availability of high-quality components and efficient transmission
window, there are several laser types that, for a variety of reasons, are not very well
suited to FSO systems. At the current stage of development of these sources, solid-
state lasers (e.g. YdYag lasers operating at 1060 nm) or any form of gas-based lasers
fall within this category. Indeed, the majority of high power lasers operating in the
near infrared spectral range cannot fulfill the MTBF requirements of carrier-grade
systems. For example, high power GaAlAs lasers operating slightly beyond 800 nm or
slightly above 900 nm, though generally available from many vendors and at very low
cost, do not normally qualify as telecommunication grade due to insufficient MTBF
values.
2.3 FSO RECEIVER
At the receiver detectors are present. Detectors are used to convert the light
signal into electrical signal.
Detector choices are much more limited when compared to the variety of
wavelength options available. This is due to vast amounts of different semiconductor laser
compound structures. The two most common material systems used to detect light in the
near infrared spectral range are either silicon (Si) or indium gallium arsenide (InGaAs).
Detectors are based either on PIN or APD technology. A thorough discussion of these
technologies is not within the scope of this paper. More detailed information on PIN and
APD technology and their use in FSO systems can be found in the book “Free Space Optics:
Enabling Optical Connectivity in Today’s Networks” authored by H. Willebrand and B.
Ghuman and published by Sams Publishing.
Fig 2.2 Fso Receiver
Silicon is the most commonly used detector material in the visible and near infrared
wavelength range. Silicon technology is very mature and silicon receivers can detect
extremely low levels of light. Detectors based on silicon typically have a sensitivity
maximum or spectral response around 850 nm. Therefore silicon detectors are ideal
candidates for light detection in conjunction with short wavelength 850 nm VCSEL
laser. Silicon drastically loses sensitivity toward the longer infrared wavelength
spectrum; for wavelengths beyond 1 micrometer. 1100 nanometers defines the cutoff
wavelength for potential light detection and therefore silicon cannot be used as
detector material beyond this range. Silicon detectors can operate at very high
bandwidth. Recently, operation at 10 Gbps has been commercialized for use in short