Running head: FSO COMMUNICATION
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CONCEPT OF FREE SPACE OPTICS COMMUNICATION
SYED MOZAFFAR CHOWDHURY
LTEC 4550
FEBRUARY 9, 2016
STEVE SMILEY
FSO COMMUNICATION
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INTRODUCTION
Free-space optical communication (FSO) is an optical communication technology that uses light
propagating in free space to transmit data for telecommunications or computer networking. Free Space
Optics (FSO) communications, also called Free Space Photonics (FSP) or Optical Wireless, refers to the
transmission of modulated visible or infrared (IR) beams through the atmosphere to obtain optical
communications. Like fiber, Free Space Optics (FSO) uses lasers to transmit data, but instead of
enclosing the data stream in a glass fiber, it is transmitted through the air. Free Space Optics (FSO) works
on the same basic principle as Infrared television remote controls, wireless keyboards or wireless devices.
HISTORY OF FSO
The engineering maturity of Free Space Optics (FSO) is often underestimated, due to a
misunderstanding of how long Free Space Optics (FSO) systems have been under development.
Historically, Free Space Optics (FSO) or optical wireless communications was first demonstrated by
Alexander Graham Bell in the late nineteenth century (prior to his demonstration of the telephone!).
Bell Free Space Optics (FSO) experiment converted voice sounds into telephone signals and transmitted
them between receivers through free air space along a beam of light for a distance of some 600 feet.
Calling his experimental device the photo phone, Bell considered this optical technology and not the
telephone his preeminent invention because it did not require wires for transmission.
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 Free Space Optics (FSO)
or free space optical communications systems was done over the past 40 years or so, mostly for defense
applications. By addressing the principal engineering challenges of Free Space Optics (FSO), this
aerospace/defense activity established a strong foundation upon which today’s commercial laser-based
Free Space Optics (FSO) systems are based.
HOW FSO WORKS
Free Space Optics (FSO) transmits invisible, eye-safe light beams from one "telescope" to
another using low power infrared laser in the terahertz spectrum. The beams of light in Free Space Optics
(FSO) systems are transmitted by laser light focused on highly sensitive photon detector receivers. These
receivers are telescopic lenses able to collect the photon stream and transmit digital data containing a mix
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of Internet messages, video images, radio signals or computer files. Commercially available systems offer
capacities in the range of 100 mega byte per second to 2.5 giga byte per second, and demonstration
systems report data rates as high as 160 giga byte per second.
Free Space Optics (FSO) systems can function over distances of several kilometers. As long as
there is a clear line of sight between the source and the destination, and enough transmitter power, Free
Space Optics (FSO) communication is possible.
FSO TECHNOLOGY
Lasers are one of the most significant inventions of the 20th century - they can be found in many
modern products, from CD players to fiber-optic networks. The word laser is actually an acronym for
Light Amplification by Stimulated Emission of Radiation. Although stimulated emission was first
predicted by Albert Einstein near the beginning of the 20th century, the first working laser was not
demonstrated until 1960 when Theodore Maiman did so using a ruby. Maiman's laser was predated by
the maser - another acronym, this time for Microwave Amplification by Stimulated Emission of
Radiation. A maser is very similar to a laser except the photons generated by a maser are of a longer
wavelength outside the visible and/or infrared spectrum.
A laser generates light, either visible or infrared, through a process known as stimulated
emission. To understand stimulated emission, understanding two basic concepts is necessary. The first is
absorption which occurs when an atom absorbs energy or photons. The second is emission which occurs
when an atom emits photons. Emission occurs when an atom is in an excited or high energy state and
returns to a stable or ground state when this occurs naturally it is called spontaneous emission because
no outside trigger is required. Stimulated emission occurs when an already excited atom is bombarded
by yet another photon causing it to release that photon along with the photon which previously excited
it. Photons are particles, or more properly quanta, of light and a light beam is made up of what can be
thought of as a stream of photons.
A basic laser uses a mirrored chamber or cavity to reflect light waves so they reinforce each
other. An excitable substance gas, liquid, or solid like the original ruby laser is contained within the
cavity and determines the wavelength of the resulting laser beam. Through a process called pumping,
energy is introduced to the cavity exciting the atoms within and causing a population inversion. A
population inversion is when there are more excited atoms than grounded atoms which then lead to
stimulated emission. The released photons oscillate back and forth between the mirrors of the cavity,
building energy and causing other atoms to release more photons. One of the mirrors allows some of
the released photons to escape the cavity resulting in a laser beam emitting from one end of the cavity.
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FSO COMMUNICATION CHALLENGES
FOG
Fog substantially attenuates visible radiation, and it has a similar affect on the near-infrared
wavelengths that are employed in laser communications. Similar to the case of rain attenuation with RF
wireless, fog attenuation is not a show-stopper for optical wireless, because the optical link can be
engineered such that, for a large fraction of the time, an acceptable power will be received even in the
presence of heavy fog.
PHYSICAL OBSTRUCTIONS
Laser communications systems that employ multiple, spatially diverse transmitters and large receive
optics will eliminate interference concerns from objects such as birds.
POINTING STABILITY
Pointing stability in commercial laser communications systems is achieved by one of two methods. The
simpler, less costly method is to widen the beam divergence so that if either ends of the link moves the
receiver will still be within the beam. The second method is to employ a beam tracking system. While
more costly, such systems allow for a tighter beam to be transmitted allowing for higher security and
longer distance transmissions.
SCINTILLATION
Performance of many laser communications systems is adversely affected by scintillation on bright
sunny days. Through a large aperture receiver, widely spaced transmitters, finely tuned receive filtering,
and automatic gain control, downtime due to scintillation can be avoided.
FSO ADVANTAGE
The FSO system offers a flexible networking solution that delivers on the promise of broadband.
Only free space optics or Free Space Optics (FSO) provides the essential combination of qualities
required to bring the traffic to the optical fiber backbone virtually unlimited bandwidth, low cost, ease
and speed of deployment. Freedom from licensing and regulation translates into ease, speed and low
cost of deployment. Since Free Space Optics (FSO) optical wireless transceivers can transmit and receive
through windows, it is possible to mount Free Space Optics (FSO) systems inside buildings, reducing the
need to compete for roof space, simplifying wiring and cabling, and permitting the equipment to
operate in a very favorable environment. The only essential for Free Space Optics (FSO) is line of sight
between the two ends of the link. Freedom from licensing and regulation leads to ease, speed and low
cost of deployment. Since FSO units can receive and transmit through windows it reduces the need to
compete for roof space, simplifying wiring and cabling.
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FSO SECURITY
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 Sunbeam
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. 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:
1. Free Space Optics (FSO) laser beams cannot be detected with spectrum analyzers or RF
meters.
2. Free Space Optics (FSO) laser transmissions are optical and travel along a line of sight path
that cannot be intercepted easily.
3. Free Space Optics (FSO) requires a matching transceiver carefully aligned to complete the
transmission. Interception is very difficult and extremely unlikely.
4. 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.
5. Data can be transmitted over an encrypted connection adding to the degree of security
available in Free Space Optics (FSO) network transmissions
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. Unlike radio and microwave systems, FSO is an optical technology and no
spectrum licensing or frequency coordination with other users is required, interference from or to other
systems or equipment is not a concern, and the point-to-point laser signal is extremely difficult to
intercept, and therefore secure. Data rates comparable to optical fiber transmission can be carried by
FSO systems with very low error rates, while the extremely narrow laser beam widths ensure that there
is almost no practical limit to the number of separate FSO links that can be installed in a given location.
FSO COMMUNICATION
REFERENCES
1. WIKIPEDIA URL: HTTP://EN.WIKIPEDIA.ORG/WIKI/FREE-SPACE_OPTICAL_COMMUNICATION
2. IEEE COMMUNICATIONS MAGAZINE. AUGUST 2000, FREE SPACE
LASER COMMUNICATIONS :LASER CROSS-LINK SYSTEMS AND TECHNOLOGY
BY: DAVID L. BEGLEY, BALL AEROSPACE & TECHNOLOGIES CORPORATION
3.
CHAOTIC FREE-SPACE LASER COMMUNICATION OVER A TURBULENT
CHANNEL BY: N. F. RULKOV,1 M. A. VORONTSOV, AND L. ILLING
INSTITUTE FOR NONLINEAR SCIENCE, UNIVERSITY OF CALIFORNIA,
SAN DIEGO, LA JOLLA, CALIFORNIA 92093
ARMY RESEARCH LABORATORY, ADELPHI, MARYLAND 20783
4. FREE SPACE OPTICS OR LASER COMMUNICATION THROUGH THE AIR
BY: DENNIS KILLINGER OPTICS & PHOTONICS NEWS, OCTOBER
2002
5.
HIGH DATA-RATE LASER TRANSMITTERS FOR FREE-SPACE LASER
COMMUNICATIONS. BY:A. BISWAS, H. HEMMATI AND J. R. LESH
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