Free Space Optics
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Transcript of Free Space Optics
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
wavelength 850 nm 10 GbE systems. Lower bandwidth (1 Gbps) silicon PIN and APD
detectors are widely available in a variety of mechanical packages such as TO-46
cans. Very common are also Si-PIN detectors with integrated trans-impedance
amplifiers (TIA). The sensitivity of which is a function of the signal modulation
bandwidth – decreasing as bandwidth increases. Typical sensitivity values for a Si-
PIN diode are around –34 decibel mill watts (dBm) at 155 megabits per second
(Mbps). Si-APDs are far more sensitive due to an internal amplification (avalanche)
process. Therefore Si-APD detectors are very useful for low light level detection in
free space optics systems. Sensitivity values for higher bandwidth applications can be
as low as –50dBm at 10 Mbps, -45dBm at 155 Mbps, or –38 dBm at 622 Mbps.
Silicon detectors can be quite large in size (e.g. 0.2 x 0.2 mm) and still operate at
higher bandwidths. This feature minimizes loss when light is focused on the detector
by using either a larger diameter lens or a reflective parabolic mirror. For longer
wavelength radiation, InGaAs is the most commonly used detector material. The
performance of InGaAs detectors has been constantly improved in terms of sensitivity
and bandwidth capabilities as well as the development of 1550 nm fiber optic
technology. The vast majority of longer wavelength fiber optics systems use InGaAs
as detector material. Commercially InGaAs detectors are either optimized for
operation at 1310 or 1550 nm. Due to the drastic decrease in sensitivity towards the
shorter wavelength range, InGaAs detectors are typically not used in the 850 nm
wavelength range. The main benefit of InGaAs detectors is higher bandwidth
capability. The majority of InGaAs receivers are based on PIN or PIN-TIA
technology. Typical sensitivity values for InGaAs Pin diodes are similar to those of
Si-PIN diodes (e.g. –33dBm at 155 Mbps). InGaAs diodes operating at higher speed
are typically smaller in size than Si-PIN diodes. This is because most high-speed Inga
As receivers are designed for fiber optic transmission in conjunction with 9-
micrometer core diameter, single mode (SM) fiber, and the small SM core diameter
doesn’t require a large detection surface. This makes the light coupling process a more
challenging task and overall losses that occur when the light is coupled from free-
space onto the detector surface are higher, thus impacting the link budget of the
systems. The conclusion is that both Si and InGaAs detectors are capable of fulfilling
the stringent service provider systems standard requirement since both detector
technologies are already used in carrier class fiber optic communication systems.
2.4 A SIMPLE PROPAGATION MODEL – LINK EQUATION
When taking a closer look at an FSO performance, it is important to take
several system parameters into consideration. In general, these parameters can be divided
into two different categories: internal parameters and external parameters (see Fig. 6).
Internal parameters are related to the design of a specific FSO system and can be impacted
by the system designer or engineer. Examples are: optical power, transmission bandwidth or
divergence angle on the transmitter side and receiver sensitivity, receive lens diameter or
receiver field-of-view on the receive side. Other important parameters that determine
system performance are related to external or non-system specific parameters and all of
them are related to the climate under which the system has to operate. Typical examples are
the deployment distance and visibility.
Fig2.3 Schematic explanation of internal and external FSO system design parameters.
It is important to understand that many of these parameters are linked and not
independent of each other. Two examples: 1) System availability is not only a function of
the deployment distance but also a function of the inherent atmospheric attenuation
coefficient and 2) Increasing the modulation bandwidth on the transmitter side will impact
the sensitivity figure and the BER performance of the receiver side. In general, the focus on
the improvement of one system parameter (e.g. increase of transmission power) does not
lead to an overall improved system performance. The next section demonstrates that the
ability to launch a high amount of power is certainly beneficial within the overall link
budget calculation. However, it becomes obvious that simply launching higher power levels
will not automatically result in a better performing FSO system. Many other factors have to
be considered. These factors can actually outperform the advantage of being able to launch
higher power levels. A professional FSO system designer must balance all of these
parameters. Under the assumption that the transmission source can be seen as a point
source, the simple link equation (1) below shows the impact of various system parameters
on the power received at the receiving station. The climate/weather impact on FSO system
availability is solely contained in the last part of the equation. In particular, and as can be
easily seen from this equation, the value of the atmospheric extinction coefficient’ ‘is
extremely important due to the exponential dependency on the receive power level.
2.5 ALIGNMENT IN FSO:-
In free-space optical communication using narrow laser beams, it is required to maintain the
optical alignment between the stations in spite of their relative motion. This relative motion
is caused by the mobile nature of the stations, mechanical vibration, or accidental shocks. In
order to establish and maintain a free-space optical link, a two-phase optical alignment
mechanism is required. In the first phase, a coarse alignment is achieved through the open-
loop operation of spatial acquisition. Following the coarse alignment phase, data
transmission is established and simultaneously a closed-loop fine alignment operation is
performed to precisely compensate for the persistent relative motion of the stations. A
possible scheme to achieve this fine alignment is cooperative (reciprocal) optical beam
tracking. A cooperative optical beam tracking system consists of two stations in such a
manner that each station points its optical beam toward the other one. The receiving station
continuously measures the arrival direction of its incident optical beam in order to employ it
as a guide to precisely point its own beam toward the other station. In short range
applications with negligible light propagation delay, this direction is approximately along
the line-of-sight of the stations, thus the stations transmit their optical beams along this
measured direction. In applications with a large propagation delay, the optical beams must
be transmitted within a certain angle with respect to the arrival direction in order to
compensate for the variation of the line-of-sight during the travel time of the transmitted
beams. This requires the transmitter to predict the future location of the receiver and point
its optical beam toward the predicted location. To implement the alignment scheme above,
the stations are equipped with a position-sensitive photo detector (e.g., quadrant detector)
and a focusing lens (or an arrangement of curved mirrors) to measure the azimuth and
elevation components of the beam arrival direction. In addition, each station employs an
electromechanical pointing assembly to adjust the direction of its optical devices according
to the control signals provided by a closed-loop controller. The controller incorporates the
output of the position-sensitive photo detector and generates proper azimuth and elevation
control signals. As an alternative (or complement) to adjusting the transceiver direction, the
incoming and outgoing optical fields can be directed using an arrangement of steerable flat
mirrors.
The goal of this chapter is to develop a mathematical model for a cooperative optical beam
tracking system, which includes the nonlinear effects, major disturbance sources, and light
propagation delay. For analyzing the optical alignment between two fast maneuvering
stations (e.g. aircrafts), the nonlinearity of the dynamical equations is essential; however, in
applications such as intersatellite communication in which the relative motion consists of a
predetermined large component and an unknown small component, we can linearize the
nonlinear dynamics around a nominal state trajectory.
Fig 2.4 Schematic Diagram Of A Simple Optical Receiver
In the last section, we shall describe the relative motion of the stations by means of a set of
stochastic differential equations. This stochastic model will be used for a stochastic analysis
of the system, as an alternative to the deterministic approach of System Architecture. In this
section, we first consider the structure and components of an optical transceiver and then
describe the operation of a cooperative optical beam tracking system
which employs two transceivers of this type.
2.5.1 TRANSCEIVER STRUCTURE:-
A schematic diagram of a simple transceiver used in short range free-space
optical links is illustrated in Fig. This transceiver comprises a lens, a position-sensitive
photo detector, and a narrow laser source, all installed on a rigid platform. The photo
detector surface is perpendicular to the lens axis and its center is placed at the focus of the
lens. The axes of the lens and the laser source are parallel to transceiver axis. The azimuth
and elevation of the transceiver axis can be controlled by means of an electromechanical
pointing assembly, which is mounted on the station body. The optical beam generated by
the laser source is used for two purposes: as a carrier of information and as a beacon
assisting the opposite station in its tracking and pointing operations. For the purpose of
communication, the instantaneous laser power is modulated by the information-bearing
signal, usually with a digital form of on-off-keying. The position-sensitive photo detector is
a photoelectron converter whose surface is partitioned into small regions. The output of
each region counts the number of converted electrons regardless of their location on the
region. The photoelectron conversion rate depends on the instantaneous optical power
absorbed by the region. The image of the received optical field on the surface of the photo
detector is a spot of light with a bell-shaped intensity pattern whose location depends on the
angle of arrival of the optical field with respect to the transceiver axis. Hence, using the
position-sensitive photo detector, this angle can be tracked by measuring the location of the
spot of light. Many practical optical beam tracking systems employ a quadrant detector1 as
their optical sensing device, while the low spatial resolution of a quadrant detector can be
improved using a finer partition. For instance, the authors of describe a beam tracking
system which employs a photo detector with 512 × 512 pixels.
The pointing assembly is usually a two-axes gimballed system with two independent motor
which control the azimuth and elevation of the transceiver. Gimballed pointing systems
generally suffer from low bandwidth (in order of 10 Hz) and low slew rate, while being able
to cover a large solid angle. Also, they have the disadvantage of being singular at certain
points, which limits their coverage region. To resolve this difficulty, Omni-Wrist III is an
alternative antenna pointer with double universal joints and linear actuators, which has 2π
steradian range of motion without singularity. A more sophisticated transceiver design, used
for intersatellite communication, is illustrated in Figure.
This design employs a position-sensitive photo detector, a pointing assembly,
and a laser source; however, instead of a lens, it employs a reflecting telescope. The
telescope which is shared between the receiving and the transmitting optics, consists of a
primary and a secondary curved mirror with one of the several common designs. The most
popular design, Cassegrainian telescope, employs a parabolic primary mirror and a
hyperbolic secondary mirror which share the same focus. In addition to the telescope, an
arrangement of lenses might be used for extra magnification. In design of the transceiver,
the incoming and outgoing optical fields must be isolated as much as possible, since the
backscattered photons caused by the outgoing light emerge as a source of noise for the
photo detector. This can be achieved by a combination of spectral isolation, spatial
separation, and polarization isolation.
Fig 2.5 optical transreceiver for inter satellite communication.
In the situations that these techniques
cannot provide enough isolation, two separated telescopes are required for the incoming and
the outgoing optical beams while this dual telescope approach leaves the tracking function
of the transceiver unchanged. The tracking mirror in Figure is intended to control the
direction of the incoming light toward the position-sensitive photo detector and the outgoing
light toward the target. This steerable flat mirror, which is equipped with miniature
actuators, provides a complementary (or alternative) means for the pointing assembly. The
steering machinery consists of a support plate with a single pivot and three or four
piezoelectric linear actuators (fast steering mirror). Although, the scanning region of a
steerable mirror is small (less than 5 degrees in each direction), its small mass and fast
actuators result in a high bandwidth (up to 1 kHz) and high slew rate. This provides
considerable assistance to the pointing assembly in suppressing the high bandwidth
disturbance. The point-ahead mirror is another steerable flat mirror with the purpose of
compensating for the displacement of the receiver during light propagation time. This
mirror provides an additional degree of freedom in controlling the pointing direction of the
outgoing light.
2.6 FINE ANGULAR POINTING, ACQUISITION, AND TRACKING SYSTEMS:-
The goal of the FPAT system is to complete the link, which implies that
the alignment procedure must take the received power into consideration. Also, the FSO
system, incorporated with the FPAT system, must be compact enough to be carried by the
actuator of the CPAT system. These two conditions make the spatial scan method the best
candidate, because this method (1) determines the orientation of the targets from the same
light ray that carries the information bits and (2) can be easily incorporated into a traditional
transceiver.
Pointing, acquisition, and tracking systems have been successfully
implemented in many applications ranging from short-distance cases such as human motion
track-ing to long distance applications such as missile guidance systems. Different
applications may adapt different principles of operation into the PAT system design.
Rolland et.al reviewed these techniques and further classified them into seven categories
including time of light (TOF), spatial scan, inertial sensing, mechanical linkages, phase-
difference sensing, direct-field sensing, and hybrid methods. Among these techniques, the
spatial scan method, which is based on analyzing the incoming light ray to determine the
orientation of a target, is the best match to the capabilities of an FSO system. The sensor of
the spatial scan method is usually a combination of a front-end optical system and a position
sensing diode(PSD), including coupled charge detectors (CCD), quadrant detectors (QD),
and lateral effect detectors (LEP). The CCD-based sensor can simultaneously measure the
incident angles for multiple rays, whereas the QD-based and LEP-based sensor can only
measure the angle of one ray.
2.6.1 ENHANCED FSO TRANSCEIVERS:-
An FSO transceiver consists of a transmitter and a receiver to achieve duplex
transmission. The data in the transmitter is ¯rst modulated onto an optical carrier, typically a
laser, then the laser beam is collimated through an optical system, and finally transmitted as
an optical field into the atmospheric channel. In order to comply with the PAT requirement,
beam steering capability must be incorporated into the design, which converts a simple
transmitter into a beam steerer. Gibson categorized the fine laser beam steering systems into
(1) mechanical and (2) non-mechanical. Mechanical Beam steerers have advantages in their
large steering range and inexpensive design. Non-mechanical beam steerers are useful for
eliminating potentially bulky mechanical components and can have a high pointing
accuracy. At the receiver, the arriving optical is collected through an optical front-end and
projected onto a photodiode for signal detection. For a high-speed FSO application, the
power collected from the front-end optics may not be focused onto the photodiode because
of pointing errors resulting from turbulence or misalignment. A better strategy is to utilize a
PSD to first determine the location of the focused spot and then move the photodiode to
optimize the received power using feedback control. An FSO receiver capable of estimating
azimuthal and elevation angles is defined as an angular resolver (AR).The combination of
the beam steerer and AR enhances the FSO transceiver with fundamental pointing and
tracking capability. If the beam steerer and AR are combined such that their optical axes are
identical, the resulting transceiver is denoted as mono-static as in figure otherwise it is
denoted as bi-static as in
figure. Generally, mono-static transceivers suffer from strong interference resulting from
strong energy coupled from self-reflection between the forward and backward links. Most
mono-static transceivers require additional power-isolation devices (e.g. a polarizing beam
splitter) to prevent this effect, called narcissus.
2.6.2 INTRODUCTION TO TRANSCEIVER ALIGNMENT:-
An FSO link is established if the optical axes of the local/remote beam steerer
and the remote/local AR are aligned to the vector connecting between the local/remote
beam steerer and remote/local AR, respectively. Since aligning a vector to the other vector
in general takes 2 rotations (one in azimuth and the other in elevation), it requires 4
rotations to complete a link and 8 rotations to develop a duplex channel (2 from each beam
steerer and AR). In general, the image position in the local AR is capable of providing only
the rotation angles for the local AR which optimizes the received power but not the rotation
angles which leads the local beam steerer to the remote AR. Since each link must be aligned
individually, we therefore de¯ne this alignment problem as the single alignment problem.
The details are depicted in figure If the transceivers are mono-static, and since the optical
axes for the local beam steerer and AR are identical, the alignment takes only 4 rotations.
Most importantly, the image position in the local AR
Fig 2.6 Schematic diagram of an FSO Transreceiver (a) monostatic (b) bi-static
is sufficient to determine the rotation angles for both the AR and beam steerer, which
implies that once either one of the two links is built, the other link can be automatically
aligned. Such an alignment problem is defined as a coupled alignment problem because the
two links are geometrically related. The details are shown in figure In this work, we propose
a scenario where the alignment can still be treated as a coupled alignment problem even
though the transceivers are not mono-static.
Fig 2.7 Different alignment problems for a pair of FSO transceivers: (a) Single
alignment (between two bi-static transceivers with a short link length), (b) Coupled
alignment (between two mono-static transceivers), and (c) Coupled alignment (be-
tween two bi-static transceivers with a long link length).
In this scenario, once either one of the two links is built, the other link can be
formed since the two links are related by a linear mapping, which can be calibrated in
advance. Such a scenario takes place if the following inequality is satisfied
where is the distance from the local AR to the remote beam steerer, is the
displacement between the local beam steerer and AR, is the beam divergence of the
beam steerer, and is the angle between the vector T and L. For example,
let us consider a duplex communication channel formed by a into this particular scenario.
Therefore, we can assume that most FSO transceiver alignments are
coupled alignment problems that can always be solved in three steps:
1. Apply a scanning process and point the local beam steerer to the remote AR by trial-and-
error.
2. Compute the rotation angles for the remote AR and beam steerer according to the focused
spot on the remote AR. Point the remote beam steerer back to the local AR.
3. Compute the rotation angles for the local AR and beam steerer according to the focused
spot in the local AR. Point the local beam steerer back to the remote AR.
CHAPTER 3
FSO ARCHITECTURES
3.1 POINT-TO-POINT ARCHITECTURE
Point-to-point architecture is a dedicated connection that offers higher bandwidth
but is less scalable .In a point-to-point configuration, FSO can support speeds between
155Mbits/sec and 10Gbits/sec at a distance of 2 kilometers (km) to 4km. “Access” claims it
can deliver 10Gbits/ sec. “Terabeam” can provide up to 2Gbits/sec now, while “AirFiber”
and “Lightpointe” have promised Gigabit Ethernet capabilities sometime in 2001..
Fig 3.1
point to point architecture
3.2 MESH ARCHITECTURE
Mesh architectures may offer redundancy and higher reliability with easy node
addition but restrict distances more than the other options.
Fig 3.2 Mesh Architecture
A meshed configuration can support 622Mbits/sec at a distance of 200 meters (m) to
450m. TeraBeam claims to have successfully tested 160Gbit/sec speeds in its lab, but such
speeds in the real world are surely a year or two off.
3.3 POINT-.TO-MULTIPOINT ARCHITECTURE
Point-to-multipoint architecture offers cheaper connections and facilitates node
addition but at the expense of lower bandwidth than the point-to-point option.
Fig 3.3 Point-.To-Multipoint Architecture
In a point-to-multipoint arrangement, FSO can support the same speeds as the point-
to-point arrangement -155Mbits/sec to 10Gbits/sec-at 1km to 2km.
CHAPTER 4
FREE SPACE OPTICS (FSO) SECURITY
4.1 INTRODUCTION
Security is an important element of data transmission, irrespective of the network
topology. It is especially important for military and corporate applications. Building a
network on the SONA beam platform is one of the best ways to ensure that data
transmission between any two points is completely secure. Its focused transmission beam
foils jammers and eavesdroppers and enhances security. Moreover, fSONA systems can use
any signal-scrambling technology that optical fiber can use.
The common perception of wireless is that it offers less security than wire line
connections. In fact, Free Space Optics (FSO) is far more secure than RF or other
wireless-based transmission technologies for several reasons:
4.2 INFORMATION SECURITY
Free Space Optics (FSO) laser beams cannot be detected with spectrum
analyzers or RF meters.
Free Space Optics (FSO) laser transmissions are optical and travel along a
line of sight path that cannot be intercepted easily. It requires a matching
Free Space Optics (FSO) transceiver carefully aligned to complete the
transmission. Interception is very difficult and extremely unlikely.
The laser beams generated by Free Space Optics (FSO) systems are narrow
and invisible, making them harder to find and even harder to intercept and
crack.
Data can be transmitted over an encrypted connection adding to the degree
of security available in Free Space Optics (FSO) network transmissions.
CHAPTER 5
TOPOLOGIES USED IN FSO
5.1 TOPOLOGIES:-In free space optics communication system the data transmission is
done with the help of different topologies used in computer networking. we can easily
communicate two network through free space optics .In networking the physical layer
of OSI model is used for communicating between two network. the topologies used in
free space optics are as follows :-
1. Mesh Topology
2. Ring Topology
3. Bus Topology
4. Star Topology
Fig 5.1 Mesh Topology, Star Topology, Bus Topology, Ring
Topology in FSO
CHAPTER 6
APPLICATIONS OF
FSO
6.1 INTRODUCTION
Optical communication systems are becoming more and more popular as the
interest and requirement in high capacity and long distance space communications grow.
FSO overcomes the last mile access bottleneck by sending high bit rate signals through the
air using laser transmission.
6.2 APPLICATIONS
Applications of FSO system are many and varied but a few can be listed.
1. Metro Area Network (MAN): FSO network can close the gap between the last
mile customers, there by providing access to new customers to high speed MAN’s
resulting to Metro Network extension.
2. Last Mile Access: End users can be connected to high speed links using FSO. It
can also be used to bypass local loop systems to provide business with high speed
connections.
3. Enterprise connectivity: As FSO links can be installed with ease, they provide a
natural method of interconnecting LAN segments that are housed in buildings
separated by public streets or other right-of-way property.
4. Fiber backup: FSO can also be deployed in redundant links to backup fiber in
place of a second fiber link.
5. Backhaul: FSO can be used to carry cellular telephone traffic from antenna towers
back to facilities wired into the public switched telephone network.
6. Service acceleration: Instant services to the customers before fiber being laid.
7. Satellite Laser Communication:- Fso Is Widely Used In Satellite
Communication. It provides Space-to-Ground Lasercom Link.Link distance of
communication is approx 2000 km.Its Data Transmission Rate is 1 Gbps.
Fig 6.1 satellite communication using FSO
8. Military Application of FSO :- FSO is very useful in communication between
aircraft to aircraft . Its potential for low electromagnetic emanation when
transferring sensitive data. Secure communication with submerged submarines.It
also very useful in Navigation also.
Fig 6.2 FSO used in Aircraft and Navigation
CHAPTER 7
. MARKET
Telecommunication has seen massive expansion over the last few years. First was
the tremendous growth of the optical fiber. Long-haul Wide Area Network (WAN)
followed by more recent emphasis on Metropolitan Area Networks (MAN). Meanwhile
LAN giga bit Ethernet ports are being deployed with a comparable growth rate. Even then
there is pressing demand for speed and high bandwidth.
The ‘connectivity bottleneck’ which refers the imbalance between the increasing
demand for high bandwidth by end users and inability to reach them is still an unsolved
puzzle. Of the several modes employed to combat this ‘last mile bottleneck’, the huge
investment is trenching, and the non- redeploy ability of the fiber has made it uneconomical
and non-satisfying.
Other alternatives like LMDS, a RF technology has its own limitations like higher
initial investment, need for roof rights, frequencies, rainfall fading, complex set and high
deployment time.
In the United States the telecommunication industries 5 percent of buildings are
connected to OFC. Yet 75 percent are with in one mile of fiber. Thus FSO offers to the
service providers, a compelling alternative for optical connectivity and a complement to
fiber optics.
CHAPTER 8
MERITS OF FSO
8.1 INTRODUCTION
Known within the industry as free-space optics (FSO), this form of delivering
communications services has compelling economic advantages.
Free-space systems require less than a fifth the capital outlay of comparable ground-
based fiber-optic technologies. Moreover, they can be up and running much more quickly.
Installing an 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. Using FSO, a service provider can
be generating revenue while a fiber-based competitor is still seeking municipal approval to
dig up a street to lay its cable. Street trenching and digging are not only expensive, they
cause traffic jams (which increase air pollution), displace trees, and sometimes destroy
historical areas. For such reasons, some cities, such as Washington, D.C., are considering a
moratorium on fiber trenching. Others, like San Francisco, are hoping to limit disruptions
by encouraging competing carriers to lay fiber within the same trench at the same time.
FSO works in a completely unregulated frequency spectrum (THz), unlike LMDS or
MMDS. Because there's little or no traffic currently in this range, the FCC hasn't required
licenses above 600GHz. This means FSO isn't likely to interfere with other transmissions.
Regulation could come about, however, when and if FSO carriers start to fill up the
spectrum. License free frequency band is an advantage of FSO.
Cost is one of the major advantage of this technology. Airfiber has prepared a cost
model based on deploying an FSO mesh in Boston. According to its analysis, deployment
would cost about $20,000 per building, with an average link length of 55 meters and a
maximum length of 200 meters. The mesh would also provide full redundancy. A
comparable fiber network would run between $50,000 to $200,000 per building.
With FSO, there's also no capital overhang. FSO carriers can avoid heavy build outs
by deploying laser terminals after customers have signed on. No heavy capital investments
for build out are required. Low risk investment is another advantage of FSO.
Another plus is that FSO network architecture needn't be changed when other nodes
(buildings) are added; customer capacity can be easily increased by changing the node
numbers and configurations.
High transmission capacity is an advantage of this technology.
8.2 MERITS OF FSO
1. Free space optics offers a flexible networking solution that delivers on the
promise of broadband.
2. Straight forward deployment-as it requires no licenses.
3. Rapid time of deployment.
4. Low initial investment.
5. Ease of installation even indoors in less than 30 minutes.
6. Security and freedom from irksome regulations like roof top rights and spectral
licenses.
7. Re-deploy ability.
Unlike radio and microwave systems FSO is an optical technology and no
spectrum licensing or frequency co-ordination with other users is required. Interference
from or to other system or equipment is not a concern and the point to point laser signal is
extremely difficult to intercept and therefore secure. Data rate comparable to OFC can be
obtained with very low error rate and the extremely narrow laser beam which enables
unlimited number of separate FSO links to be installed in a given location.
CHAPTER9
FSO CHALLENGES
9.1 INTRODUCTION
The advantages of free space optics come without some cost. As the medium is air
and the light pass through it, some environmental challenges are inevitable.
Despite its potential, FSO has many hurdles to overcome before it will be deployed
widely.
FSO is an LOS technology, which means nodes must have an unobstructed path to
the hub antenna. This, of course, means that interference of any kind can pose problems.
Inclement weather is the main threat. Although rain and snow can distort a signal,
fog does the most damage to transmission. Fog is composed of extremely small moisture
particles that act like prisms upon the light beam, scattering and breaking up the signal.
Most vendors know they have to prove reliability in bad weather cities in order to gain
carrier confidence, especially if those carriers want to carry voice. So these vendors try to
distinguish themselves by running trials in foggy cities. TeraBeam, for example, ran trials in
Seattle, figuring if it could make it there, it could make it anywhere.
The technology is affected badly by the environmental phenomena that vary widely
from one meteorological area to another. Some of them are scattering, scintillations, beam
spread and beam wanders.
Scintillation is best defined as the temporal and spatial variations in light intensity
caused by atmospheric turbulence. Such turbulence is caused by wind and temperature
gradients that create pockets of air with rapidly varying densities and therefore fast-
changing indices of optical refraction. These air pockets act like prisms and lenses with
time-varying properties. Their action is readily observed in the twinkling of stars in the
night sky and the shimmering of the horizon on a hot day.
FSO communications systems deal with scintillation by sending the same
information from several separate laser transmitters. These are mounted in the same
housing, or link head, but separated from one another by distances of about 200 mm. It is
unlikely that in traveling to the receiver, all the parallel beams will encounter the same
pocket of turbulence since the scintillation pockets are usually quite small. Most probably,
at least one of the beams will arrive at the target node with adequate strength to be properly
received. This approach is called spatial diversity, because it exploits multiple regions of
space.
Dealing with fog, more formally known as Mie scattering, is largely a matter of
boosting the transmitted power, although spatial diversity also helps to some extent. In areas
with frequent heavy fogs, it is often necessary to choose 1550-nm lasers because of the
higher power permitted at that wavelength. Also, there seems to be some evidence that Mie
scattering is slightly lower at 1550 nm than at 850 nm. However, this assumption has
recently been challenged, with some studies implying that scattering is independent of the
wavelength under heavy fog conditions.
One of the more common difficulties that arises when deploying free-space optics
links on tall buildings or towers is sway due to wind or seismic activity. Both storms and
earthquakes can cause buildings to move enough to affect beam aiming. The problem can
be dealt with in two complementary ways: through beam divergence and active tracking.
With beam divergence, the transmitted beam is purposely allowed to diverge, or
spread, so that by the time it arrives at the receiving link head, it forms a fairly large optical
cone. Depending on product design, the typical free-space optics light beam subtends an
angle of 3-6 mill radians (10-20 minutes of arc) and will have a diameter of 3-6 meters after
traveling 1 km. If the receiver is initially positioned at the center of the beam, divergence
alone can deal with many perturbations. This inexpensive approach to maintaining system
alignment has been used quite successfully by FSO vendors like LightPointe for several
years now.
If, however, the link heads are mounted on the tops of extremely tall buildings or
towers, an active tracking system may be called for. More sophisticated and costly than
beam divergence, active tracking is based on movable mirrors that control the direction in
which the beams are launched. A feedback mechanism continuously adjusts the mirrors so
that the beams stay on target.
Beam wander arises when turbulent eddies bigger than the beam diameter cause
slow, but large, displacements of the transmitted beam. It occurs not so much in cities as
over deserts over long distances. When it does occur, however, the wandering beam can
completely miss its target receiver. Like building sway, beam wander is readily handled by
active tracking.
9.2 CHALLENGES OF FSO
(a). FOG
Fog substantially attenuates visible radiation, and it has a similar affect on the
near-infrared wavelengths that are employed in FSO systems. Rain and snow have little
effect on FSO. Fog being microns in diameter, it hinder the passage of light by absorption,
scattering and reflection. Dealing with fog – which is known as Mie scattering, is largely a
matter of boosting the transmitted power. Fog can be countered by a network design with
short FSO link distances. FSO installation in foggy cities like San Francisco has
successfully achieved carrier-class reliability.
Fig 9.1 Interruption Of Fog In FSO
(b). PHYSICAL OBSTRUCTIONS
Flying birds can temporarily block a single beam, but this tends to cause only
short interruptions and transmissions are easily and automatically re-assumed. Multi-beam
systems are used for better performance.
(c). SCINTILLATION
Scintillation refers the variations in light intensity caused by atmospheric
turbulence. Such turbulence may be caused by wind and temperature gradients which
results in air pockets of varying diversity act as prisms or lenses with time varying
properties. This scintillation affects on FSO can be tackled by multi beam approach
exploiting multiple regions of space- this approach is called spatial diversity.
Scintillation is one of the effects related to turbulence. Scintillation cannot be characterized
using visibility. 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. Turbulence affects laser beams
propagating through the atmosphere in three different ways. First, beam wander occurs
when the refractive index changes and acts like a lens, deflecting the beam from its given
path. Second, turbulence results in a beam spread greater than diffraction theory predicts.
Third, scintillation or intensity variations (peaks and troughs across the face of the beam)
can occur that consequently change the amplitude of the beam at the receiver side.
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. Depending on the
specific system configuration, the variation in the signal strength both in time and across the
cross section of the beam can reach levels in signal variation beyond 10 dB. Scintillation
can act in both ways: Troughs can cause the signal to disappear, while peaks in amplitude
can saturate the detector. Scintillation is distance dependent andin general the system
designer has to reserve more link margin for scintillation effects over longer distances.
Research has revealed that there are several very successful geometric solutions that can
decrease the effect of scintillation significantly. One of these strategies involves the use of
multiple transmission beams that are sufficiently separated in space when they leave the
transmission aperture plane. In this way they pass through different air (refractive index)
cells, experiencing different intensity variations. The variations are averaged out when the
signals are added together at the receiving terminal where they overlap in space. By
separating multiple transmitters and by making the receiver optics sufficiently large (or
sufficiently separating smaller receiving lenses), different parts of the receiver lenses are
illuminated when the beam propagates through different air cells. As a statistical result as
this approach signal amplitude variations are averaged out at the receiver. Even though
scintillation is not physically correlated with visibility, scintillation under low visibility
conditions, usually involving wet, cooler weather, can be neglected. For high visibility
conditions that typically occur on hot and sunny days, one has to reserve the maximum loss
for scintillation in the link budget analysis.
(d). SOLAR INTERFERENCE
This can be combated in two ways:
The first is a long pass optical filter window used to block all wavelengths below
850nm from entering the system.
The second is an optical narrow band filter proceeding the receive detector used to
filter all but the wavelength actually used for intersystem communications.
(e). SCATTERING
Scattering is caused when the wavelength collides with the scatterer. The
physical size of the scatterer determines the type of scattering.
When the scatterer is smaller than the wavelength-Rayleigh scattering.
When the scatterer is of comparable size to the wavelength -Mie scattering.
When the scatterer is much larger than the wavelength-Non-selective scattering
In scattering there is no loss of energy, only a directional re-distribution of energy
which may cause reduction in beam intensity for longer distance.
(f). ABSORPTION
Absorption occurs when suspended water molecules in the terrestrial
atmosphere extinguish photons. This causes a decrease in the power density of the FSO
beam and directly affects the availability of a system. Absorption occurs more readily at
some wavelengths than others.
However, the use of appropriate power, based on atmospheric conditions, and use of spatial
diversity helps to maintain the required level of network availability.
(g). BUILDING SWAY / SEISMIC ACTIVITY
One of the most common difficulties that arises when deploying FSO links
on tall buildings or towers is sway due to wind or seismic activity Both storms and
earthquakes can cause buildings to move enough to affect beam aiming. The problem
can be dealt with in two complementary ways: through beam divergence and active
tracking
With beam divergence, the transmitted beam spread, forming optical cones
which can take many perturbations.
Active tracking is based on movable mirrors that control the direction in which
beams are launched.
CHAPTER 10
RAPIDLY ADVANCING FSO TECHNOLOGY
10.1 LIGHT POINTE:-
Light Pointe’s FSO products utilize a multi-beam sending process, which
overcomes atmospheric degradations and temporary beam obstructions by
overlapping redundant infrared beams.
Light Pointe was founded in 1998 and has become the
global market leader for high capacity wireless outdoor bridges with over 5000 systems
deployed in over 60 countries worldwide and in vertical markets such as Health Care,
Education, Military & Government networks, large and small campus enterprise
networks, Wire line and Wireless Service Provider networks. Over the last 10 years the
company has established a unique diversified product portfolio based on high capacity
Free Space Optics (FSO) and Millimeter Wave (MMW) technology. With more than 10
patents granted in the FSO, RF/MMW and in the hybrid bridging solution space Light
Pointe has established a strong IP and patent portfolio position manifesting the
company’s technology leadership position.
Light Pointe has a long list of global customers including but
not limited to Wal-Mart, DHL, Sturm Foods, Siemens, Sprint, AOL, FedEx, BMW,
Lockheed Martin, Dain Rauscher, Barclays, Nokia, Deutsche Bank, IBM, Corning,
Cisco, Hawaii just to mentioned a few.
Fig 10.1 Light Pointe In Free Space Optics
The addition of the licensed-free 60 GHz Airebeam G60 product
complements the LightPointe comprehensive product portfolio of high capacity wireless
bridges. By offering both , outdoor wireless bridges based on Free Space Optics (FSO) and
millimeter-wave technology, we can fulfill any customer's high capacity transport
requirements as far as bandwidth, distance and pricing is concerned." said Heinz
Willebrand, LightPointe CEO and President. The 60 GHz band is license-free in the USA,
Canada and soon other select countries including Europe.
About Light Pointe Communications, In Light Pointe
designs, manufactures and distributes ultra high-speed wireless point to point network
bridging solutions based on patented free-space optics (FSO) and millimeter wave (MMW)
technology. The products are used in fixed wireless last mile access for campus or
enterprise building-to-building connectivity, and in infrastructure applications such as
broadband cellular networks and wireless backhaul for WiMAX or WiFi networks. Light
Pointe installation base of high capacity wireless bridges consists of more than 5000
systems deployed in more than 60 countries. The company is recognized worldwide for the
highest standards of quality and service.
10.2. VCSEL LASERS:-
Over the last decade, VCSEL structures have gained a massive amount of popularity in the
communications industry. In addition, laser lifetime, transmission power performance and
modulation characteristics have shown dramatic improvements in the shorter 850 nm and
980 nm wavelength range. VCSELs clearly established a milestone and revolutionized the
transmission component market due to the exceptional and dramatic cost/performance
advantage over previously available technology. The success of VCSEL technology has
been so tremendous that many VCSEL laser manufactures can produce shorter wavelength
850 nm laser structures with direct modulation speeds beyond 3 Gbps at power levels in
excess of 10 mW. Direct electrical modulation of VCSEL lasers beyond 10 Gbps have been
demonstrated and commercialized for OC-48 (STM-16) and 10 gigabit Ethernet (GbE)
operations. VCSEL lasers can operate at very low threshold currents (a few mill amperes)
and the electro-optic conversion efficiency of these special semiconductor laser cavity
structures is extremely high. Power dissipation is not typically an issue and active cooling
of the VCSEL structure is not required. In addition, VCSELs emit light in the form of a
circular beam instead of an elliptical beam shape found in hetero-junction DFB lasers. The
round shape of the beam pattern perfectly matches the round core of an optical fiber strand.
Therefore, the coupling process is far easier and coupling efficiency is much higher when
compared to a standard DFB laser. Nonetheless, the most remarkable success in VCSEL
technology is certainly related to MTBF: Some tests have measured and extrapolated failure
rates below 1 FIT (1 failure in 1 billion hours) at 35 degrees Celsius junction temperature
for the first 4000 years. This corresponds to a MTTF value of more than 4*107 hours! Even
in environments that are exposed to high ambient temperature (such as outdoor FSO
equipment) where the junction temperature can reach 90°C for extended periods of time, a
MTTF value of 3.9*105 hours or 44 years was estimated. An example of short wavelength
VCSEL laser lifetime improvement since 1995 is shown in Fig. 5. Initial VCSEL laser
production showed lifetime cycles around 50,000 hours. Through constant improvements in
the fabrication process this value has been pushed beyond 5,000,000 hours for the
Honeywell VCSEL product line.
Fig 10.2 Vcsel Lasers In Fso
choosing the right transmitter is an important component of a free space optics system,
critical to satisfying telecommunications equipment requirements. Besides the transmitter,
the receiver is another important electronic component that has to be picked carefully. The
following section focuses on suitable receivers for high performance FSO systems.
10.3. TERA BEAM: -
Terabeam's FSO products have advanced beam-steering features that update beam direction
up to 300 times per second.
Terabeam is the latest in a line of high-bandwidth,
carrier-grade systems developed by Terabeam Wireless. TeraBeam is unique among our
solutions in that it operates via free-space optics and thus does not use radio frequency. The
system is a cost-effective solution for high-bandwidth connectivity at ranges less than one
kilometer, and is ideal for deployments such as mobile wireless backhaul, single customer
access, multi-tenant building access, enterprise E1/T1, Fast Ethernet extension and LAN-to-
LAN or campus connectivity.
Fig 10.3 Tera Beam In Fso
10.3.1 FEATURES OF TERA BEAM:-
Designed for outdoor installations and provides bandwidths of 125 megabits per
second (Mbps), independent of transport protocol
Ideal for dense metro deployments in the range of 20 meters to 1 kilometer
Operates at a wavelength of 850 nm and is completely eye-safe, with a Class 1
IEC/CDRH rating, which means no warning labels or access restrictions are required
License-free operation worldwide eliminates the need for spectrum licensing or
frequency planning
Provides reliable performance using high-performance lasers with a mean time
between failures (MTBF) of one million (1,000,000) hours
Lightweight, advanced industrial design includes an integrated optical scope for ease
of alignment and high/low power settings for optimized performance
Design includes advanced laser delivery technology which maximizes availability
while maintaining eye safety
Includes built-in management functionality using Simple Network Management
Protocol (SNMP) version 1
10.4. AIRFIBER: -
AirFiber's products combine FSO with 60 GHz millimeter-wave radio, makes
wireless communication possible in any weather.
The Airfiber provides low-latency, full-duplex, wireless point-to- point Gigabit
Ethernet (GbE) connectivity and combines low cost-per-bit transport and high transmission
security, all within a compact, easy to install, fully outdoor-rated unit.
The AireFiber comes equipped with a hot swappable, GbE SFP optical fiber
port as well as a GigE RJ-45 port for connecting to the network. The AireBeam G60's true
flexibility can be found in the use of these two data ports, the secondary of which can be
used as an integrated backup solution or as an add/drop port. At a total power consumption
of less than 20 W the system can be powered by either a Cat5/6 Power-over-Ethernet (PoE)
connection or by using a low voltage 48 Vdc power feed. The system supports an Ethernet
based management system for SNMP v1/2c support and comes with an integrated Web
browser agent. The system offers advanced features like a signal strength bar graph LED
and flexible mounting options to allow for easy system installation and alignment.
Fig 10.4 Air Fiber in FSO
With bandwidth and latency similar to fiber optic cable, the AirFiber G60 targets a very
rapidly increasing number of short to medium distance outdoor wireless networking
applications that require Gigabit Ethernet bandwidth. Many of these applications are in the
high capacity enterprise campus building-to-building and Metro Ethernet connectivity
market where the challenge is to interconnect buildings that have no fiber access and/or
where laying fiber simply takes too long and is cost prohibitive. The AirFiber G60 design
allows for alternative data flow when used with the LightPointe FSO, AirFiber G70 MMW
system or leased fiber for load sharing or back-up for ultimate uptime network performance.
CHAPTER 11
FSO AS A FUTURE TECHNOLOGY
Infrared technology is as secure or cable applications and can be more reliable
than wired technology as it obviates wear and tear on the connector hardware. In the future
it is forecast that this technology will be implemented in copiers, fax machines, overhead
projectors, bank ATMs, credit cards, game consoles and head sets. All these have local
applications and it is really here where this technology is best suited, owing to the inherent
difficulties in its technological process for interconnecting over distances.
Outdoors two its use is bound to grow as communications companies,
broadcasters and end users discovers how crowded the radio spectrum has become. Once
infrared’s image issue has been overcome and its profile raised, the medium will truly have
a bright, if invisible, future!
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.
BIBLIOGRAPHY
1) www.fsona.com
2) www.freespaceoptics.com
3) www.freespaceoptic.com
4) www.fsocentral.com
5) www.lightpointe.com
6) www.proxim.com
7) www.wikipedia.com
8). www.fsonews.com
9) www.cablefreesolutions.com
10) www.thefoa.org
11) www.opticsreport.com
12) www.free-space-optics.org