20-09-2014, 10:02 AM
FREE SPACE OPTICS
FREE SPACE OPTICS.pdf (Size: 358.99 KB / Downloads: 194)
INTRODUCTION
When we talk about optical communication, most people think about optical-fiber. But
optical communication is also possible without optical-fiber. We know that light travels through
air for a lot less money. This makes possible the optical communication without optical-fiber.
Optical communication without fiber is known as Free Space Optics. It is used due to economic
advantages. Since the introduction of internet the backbone traffic is increasing at a greater rate,
hence the owner of the backbone infrastructure (which is entirely based on fiber optics) are
eagerly embracing technologies that add of the capacity of the fiber optics without adding
mountains of optical cables.
FSO is not a new idea. 30-years back optical-fiber cables are used for high-speed
communication. In those days FSO were used for high-speed connectivity over short distances.
Today’s FSO can carry full-duplex data at gigabit-per-second rates over metropolitan
distances
MEANING OF FREE SPACE OPTICS
Free Space Optics (FSO) is a line-of-sight technology that uses lasers to provide optical
bandwidth connections. Currently, Free Space Optics are capable of transporting upto 2.5 Gbps
of data, voice and video communications through the air, allowing optical connectivity without
requiring fiber-optic cable or securing spectrum licenses. FSO requires light, which can be
focused by using either light emitting diodes (LEDs) or lasers (light amplification by stimulated
emission of radiation)[1]. The use of lasers is a simple concept similar to optical transmissions
using fiber-optic cables; the only difference is the medium. Light travels through air faster than it
does through glass, so it is fair to classify Free Space Optics as optical communications at the
speed of light.
USAGE AND TECHNOLOGIES
Free Space Optics is used for communications between spacecraft. The optical links can
be implemented using infrared laser light, although low-data-rate communication over short
distances is possible using LEDs. Maximum range for terrestrial links is in the order of 2-3
km[2], but the stability and quality of the link is highly dependent on atmospheric factors such as rain, fog, dust and heat. Amateur radio operators have achieved significantly farther distances
(173 miles in at least one occasion) using incoherent sources of light from high-intensity LEDs.
[3]However, the low-grade equipment used limited bandwidths to about 4 kHz. In outer space,
the communication range of free-space optical communication is currently in the order of several
thousand kilometers[4] , but has the potential to bridge interplanetary distances of millions of
kilometers, using optical telescopes as beam expanders[5]. IrDA is also a very simple form of
free-space optical communications
FSO: WIRELESS AT THE SPEED OF LIGHT
Unlike radio and microwave systems, Free Space Optics (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[4]. Data
rates comparable to optical fiber transmission can be carried by Free Space Optics (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 Free Space Optics (FSO)
links that can be installed in a given location
LIGHT BEAM USED FOR FSO SYSTEM
Generally equipment works at one of the two wavelengths: 850 nm or 1550 nm. Laser for 850
nm are much less expensive (around $30 versus more than $1000) and are favored for
applications over moderate distances. One question arises that why we use 1550 nm wavelength.
The main reason revolves 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. 1550 nm beams
operate at higher power than 850 nm, by about two orders of magnitude[6]. That power can
boost link lengths by a factor of at least five while maintaining adequate strength for proper link
operation. So for high data rates, long distances, poor propagation conditions (like fog), or
combinations of those conditions, 1550 nm can become quite attractive.
CHALLENGES OF FREE SPACE OPTICS
Fiber-optic cable and FSO share many similarities. However, there is a difference in how
each technology transmits information. While fiber uses a relatively predictable medium that is
subject to outside disturbances from wayward construction backhoes, gnawing rodents and even
sharks when deployed under sea, FSO uses an open medium (the atmosphere) that is subject to
its own potential outside disturbances[5]. Networks with FSO must be designed to counter the
atmosphere, which can affect an FSO system's capacity. FSO is also a line-of-sight technology
and interconnecting points must be free from physical obstruction and able to "see" each other.
MIE-SCATTERING
It is the scattering of beam due to fog. It is largely a matter of boosting the transmitted power.
Spatial diversity also helps to deal with scattering. 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[4]. But some studies shows that scattering is independent of the wavelength under heavy
fog conditions. Other atmospheric disturbances, like snow and especially rain, are less of a
problem for free-space optics than fog.
CONCLUSION
The entire face of the Free-Space Optics community is about to change radically as driven by the
need for high-speed local loop connectivity and the costs and difficulties of deploying fibers.
FSO can be the ultimate solution for high-speed access. Instead of hybrid fiber-coax system,
hybrid fiber-laser system may turn out to be the best way to deliver the high capacity last-mile
access. FSO provide higher security, and throughput. FSO is capable to fulfill the increasing
demand of bandwidth.