Free Space Optics (FSO) communications
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20-09-2009, 03:55 PM

Free Space Optics (FSO) communications, also called Optical Wireless (OW)
Bo Rong, Yi Qian, Member, IEEE, and Kejie Lu, Member, IEEE

Imagine a technology that offers unsurpassed reliability and

high-speed connectivity. A technology that can be installed globally is

easy to deploy, license-free and offers a fast, high ROI. That

technology is free-space optics (FSO).

Free Space Optics (FSO) communications, also called Optical

Wireless (OW) or Infrared Laser, 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 IRDA ports on laptops or

cellular phones
Speed of fiber flexibility of wireless

Optical wireless, based on FSO-technology, is an optical

technology that provides the speed of fiber, with the flexibility of

wireless. It enables optical transmission at speeds of up to 2.5 Gbps

and, in the future, 10 Gbps using WDM. This is not possible with any

fixed wireless/RF technology today. Optical wireless also obviates the

need to buy expensive spectrum, which further distinguishes it from

fixed wireless technologies. Moreover, its narrow beam transmission

is typically two meters versus 20 meters and more for traditional,

even newer radio-based technologies such as millimeter-wave radio.

Optical wireless products similarities with conventional optical

solutions enable the seamless integration of access networks with

optical core networks and help to realize the vision of an all-optical

FSO: Wireless Links 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-topoint

laser signal is extremely difficult to intercept, and therefore

secure. 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.

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!).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.

Although photophone never became a commercial reality, it

demonstrated the basic principle of optical communications.

Essentially all of the engineering of today, Free Space Optics (FSO)

or free space optical communications systems was done over the

past 40 years or so, mostly for defense application

Originally developed by the military and NASA, FSO has been

used for more than 30 years to provide fast communication links in

remote locations. Cable-Free has extensive experience in this area:

its experts were in the labs developing prototype FSO systems in

Europe mid â„¢90s, before others had even started to think about the


While fiber-optic communications has gained acceptance in the

telecommunications industry, FSO communications is still relatively

new. FSO technology enables bandwidth transmission capabilities

that are similar to fiber optics, using similar optical transmitters and

receivers and even enabling WDM-like technologies to operate

through free space.
The technology behind Free Space Optics (FSO) technology

FSO technology requires light, which can be focused by using

either light emitting diodes (LEDs) 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 is the medium. Light travels through air faster than it

does through glass, so it is fair to classify FSO technology as optical

communications at the speed of light.

FSO technology is surprisingly simple. Itâ„¢s based on connectivity

between FSO-based optical wireless units, each consisting of an

optical transceiver with a laser transmitter and a receiver to provide

full-duplex (bi-directional) capability. Each optical wireless unit uses a

high-power optical source (i.e. laser), plus a lens or telescope that

transmits light through the atmosphere to another lens receiving the

information. At this point, the receiving lens or telescope connects to

a high-sensitivity receiver via optical fiber

FSO transmits invisible, eye-safe light beams from one

telescope to other using low power infrared lasers 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 of

Internet messages, video images, radio signals or computer files.

Commercially available systems offer capacities in the range of 100

Mbps to 2.5 Gbps, and demonstration systems report data rates as

high as 160 Gbps.

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.
The FSO Laser technology

Laser (Light Amplification by Stimulated Emission of Radiation)

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 leads 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.
Laser Safety:

Laser communications systems can be designed to be eye-safe,

which means that they pose no danger to people who might happen

to encounter the communications beam. Laser eye safety is classified

by the International Electro-technical Commission (IEC), which is the

international standards body for all fields of electro-technology.

However, that the eye safe limits vary with wavelength Terrestrial

Laser Communications
Applications of Free Space Optics (FSO) technology

A number of advantages of using Free-Space-Optics (FSO) has

been purposed, such as:

1. Enterprise Network

2. Service Providers
3. Broadcast & Cctv
Enterprise Network

A. Campus connectivity solution

B. Disaster recovery and emergency services:
A. Cable-free solves Campus Connectivity Problems:

A number of separate buildings, separated by roads or other

obstacles, between which communications links are frequently in

demand. Traditional connectivity solutions for links between buildings

include leased-line, fiber-optic, copper cable or microwave links but

all of these have associated penalties.

Cable-free systems use optical technology to offer a number of


_ No site or frequency licenses unlike radio or microwave


_ No disruption from trench digging - roads, rivers or railways are

no obstacle

_ No risk of interference, signal reflections or airport radar


_ No leased-line rental or connection charges

_ No limitation on bandwidth up to 1500Mbps with current

technology Quick to install

_ On-site upgrades ensure future-proof network provision

_ Permanent or temporary use, medium-term leasing options also


_ Cost-effective solution
B. Cable-free connections for Disaster Recovery and Emergency

Natural disasters, terrorist attacks and emergency situations are

by their nature unpredictable and require flexible and innovative


Mobile control centres are often brought in, with vehicles that

need to be connected to a number of other centres, observation

points or services, but local infrastructure may be damaged,

inadequate or unreliable.

In urban environments, Microwave or radio solutions require

licensing and often suffer from interference, multipath and signal

reflections and public health fears in urban areas are voiced with

increasing frequency.

Computer and telecommunications networks may need to be set

up in a short space of time in Disaster Recovery and Emergency

situations, and operators cannot depend on local telecommunications

providers to help especially with response teams that may be sent a

considerable distance to operate.

Cable-free offers state-of-the-art communications systems ideally

suited for rapid provision of network connections, with industrystandard

data interfaces from 2Mbps to 622Mbps. Installations are

quick and costs competitive with limited-bandwidth radio and

microwave solutions. With no need for frequency allocation or

licensing, Cable-free can be deployed to expand networks rapidly

anywhere in the world.

Cable-free offers an innovative solution offering full-bandwidth

network connectivity:

_ Data rates up to 622Mbps

_ Line-of-sight connections in excess of 2,000m

_ No site or frequency licenses required

_ No leased-line rental or connection charges

_ Trench digging and long runs of fiber-optic or copper cable are

not needed

_ No risk of interference, signal reflections or airport radar


_ No ignition hazard in flammable environments

_ Suited for permanent or temporary use

_ Plug-in modules allow on-site upgradeability of data bandwidth

or format

_ Cost-effective, safe and reliable solution
Service Providers

A. Cable-free solves network problem

B. virtual point to multi point
A. Cable-free solves network problems:

Telecommunication operators have made huge investment in

fixed infrastructure, providing coverage of urban and industrial areas

with voice, data and video services connected usually from a

backbone SDH fiber network using copper, fiber or wireless


Cable digging, increasingly unpopular in cities, is regulated by

the local authority that may restrict re-digging frequency of roads -

and the cost may be prohibitive in any case, especially if a river or

railway is in the way. Using copper telephone lines only provides

2Mbps access per pair - too slow for power users requesting Ethernet

at 10/100Mbps or ATM services at 155 or 622Mbps. Cable Modems

and xDSL are often not reliable on all installations, depending on the

age and condition of the copper cables.

Point-to-point Microwave or radio solutions for Ëœwireless local

loopâ„¢ may require licensing and often suffer from interference,

multipath and signal reflections - and public health fears in urban

areas are voiced with increasing frequency.

Cable-free offers an innovative solution offering full-bandwidth


_ Data rates up to 622Mbps

_ Line-of-sight connections in excess of 1,000m

_ No site or frequency licenses required

_ No leased-line rental or connection charges

_ Trench digging and long runs of fiber-optic or copper cable are

not needed

_ No risk of interference, signal reflections or airport radar


_ No ignition hazard in flammable environments

_ Suited for permanent or temporary use

_ Plug-in modules allow on-site upgradeability of data bandwidth

or format

_ Cost-effective, safe and reliable solution
B. Cable-free Virtual-Point-to-Multipoint:

Cable-free V-PMP systems enable implementation of Licensefree

Local Multipoint Distribution Service (LMDS) networks using

Optical Wireless technology. Services using LMDS technology

include high-speed Internet access, real-time multimedia file transfer,

corporate local area network extensions, interactive video, video-on-

demand, video conferencing, and telephony amongst many other

potential applications.

Cable-free Solutions has applied its proven expertise in Optical

Wireless to create License-free Next-Generation LMDS solutions

using Virtual-Point-to-Multipoint technology. Key benefits of this

technology are:

_ License-free operation

_ Non-shared bandwidth for each subscriber

_ High symmetric bandwidths up to 1.5Gbps per subscriber

_ Rapid network deployment and fast new-user connection

_ Low cost of start-up

_ No frequency planning

_ Data Security against interference and interception

_ Redundant 1+1 connection options including equipment and

path diversity

Cable-free V-PMP combines Point-to-Point Optical Wireless

technology and Intelligent Routers to create high-performance cellular

networks offering high symmetrical bandwidths up to 1.5Gbps to endusers

within the coverage area.
Broadcast & Cctv

A. Outside broadcast application

B. satellite uplink connections
A. Outside Broadcast Applications:
Outside Broadcast and Television News reporting demand

innovative solutions to communications problems and with todays alldigital

environments, high integrity feeds and very high data rates

such as 270Mbps SDI are often required.

Connecting remote cameras to OB vans, mixing/edit facilities or

hopping feeds between difficult-to-access areas and tall buildings

may not be suited to traditional solutions such as leased-line, fiberoptic,

copper cable or microwave links all of which have associated


With Cable-free many of these problems are solved:

_ Microwave links are limited to 34Mbps, Cable-free easily

achieves 270Mbps at 2,000m

_ Copper 270Mbps SDI connections limited to 300m, Cable-free

offers 2,000m

_ High quality analogue Composite Video option offers 9MHz with

67dB SNR

_ No site or frequency licenses required

_ No leased-line rental or connection charges

_ Long runs of fragile fiber-optic and expensive Tri-axial cable are

not needed

_ Deployment time is reduced compared to running long fiber or

copper cables

_ No risk of interference, signal reflections or airport radar


_ No ignition hazard in flammable environments

_ Mains or +12V battery power options

_ Low manpower costs and rapid response capability

_ Suited for permanent or temporary use, leasing options


_ Cost-effective solution
B. Cable-free connects to Satellite Uplinks:

Satellite uplinks and downlinks are frequently used for

telecommunications and broadcast television applications where

temporary connection has to be made such as television news

reporting and coverage of live sporting event.

In such applications the uplink may be a suitably equipped vehicle

with line-of-sight coverage of the satellite orbit, or a permanently

installed antenna which cannot be moved. The terrestrial problem is

then connecting the source to the uplink terminal, and the downlink

terminal to the recipient this is traditionally performed with leased-line,

fiber-optic, copper cable or microwave links all of which have

associated penalties.

Cable-free offers a new, high-performance alternative for the

terrestrial links:

_ Cable-free offers 622Mbps, compared with Microwave links

typically 34Mbps

_ Copper 270Mbps SDI connections limited to 300m, Cablefree

offers 2,000m

_ No site or frequency licenses required

_ No leased-line rental or connection charges

_ Long runs of fragile fiber-optic and expensive Tri-axial video

cable are not needed

_ Deployment time is reduced compared to running long fiber or

copper cables

_ No risk of interference, signal reflections or airport radar


_ No ignition hazard in flammable environments

_ Mains or +12V battery power options

_ Low manpower costs and rapid response capability

_ Suited for permanent or temporary use, leasing options


_ Cost-effective solution
Cable-Free links to Satellite Television feeds:

High-quality television signals need to be relayed from a camera to

the satellite up-link, often when high-bandwidth network connections

are unavailable locally or will take too long to organize.

Cable-free offers an ideal solution which carries high-quality

analogue or uncompressed digital pictures with a system that can be

rapidly deployed on tripods and powered from 12-volt batteries. With

no need for frequency allocation or licensing, Cable-free can be

deployed anywhere in the world and even used in war zones with no

risk of interference with military communication systems.

FSO is free from licensing and regulation which translates into

ease, speed and low cost of deployment. Since Free Space Optics

(FSO) 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 Free Space Optics (FSO) equipment to

operate in a very favorable environment. The only essential

requirement for Free Space Optics (FSO) or optical wireless

transmission is line of sight between the two ends of the link.
Why FSO?

The global telecommunications network has seen massive

expansion over the last few years. First came the tremendous growth

of the optical fiber long-haul, wide-area network (WAN), followed by a

more recent emphasis on metropolitan area networks (MANs). In

order for this tremendous network capacity to be exploited, and for

the users to be able to utilize the broad array of new services

becoming available, network designers must provide a flexible and

cost-effective means for the users to access the telecommunications

network. Presently, however, most local loop network connections

are limited to 1.5 Mbps. As a consequence, there is a strong need for

a high-bandwidth bridge between the LANs and the MANs or WANs.

FSO systems represent one of the most promising approaches

for addressing the emerging broadband access market. Free Space

Optics (FSO) systems offer many features, principal among them

being low start-up and operational costs, rapid deployment, and high

fiber-like bandwidths due to the optical nature of the technology.

Free Space Optics (FSO) products performance can be

characterized by four main parameters (for a given data rate):

1. Total transmitted power:

High transmitted power may be achieved by using erbium doped

fiber amplifiers, or by non-coherently combining multiple lower cost

semiconductor lasers.

2. Transmitting beamwidth:

Narrow transmitting beamwidth can be achieved on a limited

basis for fixed-pointed units, with the minimum beamwidth large

enough to accommodate building sway and wind loading.

3. Receiving optics collecting area:

Larger receiving optics captures a larger fraction of the total

transmitted power, up to terminal cost, volume and weight limitations.

This allow links to travel over longer distance, penetrate lower

visibility fog, or both.

4. Receiver sensitivity:

Free Space Optics (FSO) receivers must be designed to be

tolerant to scintillation, i.e. have rapid response to changing signal

levels and high dynamic range in the front end, so that the

fluctuations can be removed in the later stage limiting amplifier or


Every customer wants to know the expected failure rate of the

equipment they are investing in, for outdoor or industrial applications

the ruggedness of a system becomes even more important. A system

can be engineered and designed for exceptional reliability.

Engineering a product for long-life includes selecting top-quality,

long-life components from reliable vendors. Telecom grade

components are preferred, as are low-stress electronics. The system

must also be designed to maintain an optimum operating

environment for the selected components and sub-systems. A

rugged, environmentally-sealed housing is the first defence of a

system against the elements. Appropriate heating and cooling

mechanisms should be also in place in order to maintain optimum

temperature and humidity within the device. In addition, a system

design that incorporates a mechanism for reducing laser power

during clear weather will extend the life of the laser drivers and the

product itself. Active cooling of each laser will further enhance the

lifespan of these relatively expensive sub-systems. If these

considerations are taken into account, the system should have an

impressive MTBF (mean time before failure).
Figure Of Merit (FOM):

A figure of merit (FOM) can be used to compare competing

systems, based on

the basic physics of this equation:

Figure of Merit = (Power*Diameter2)/(Divergence2*Sensitivity); where

Power = Laser power in milliwatts

Diameter = effective diameter in cm (excluding any obscuration


Divergence = beam divergence in millirad

Sensitivity = receiver sensitivity in nanowatts

While cost is always a consideration when procuring telecom

products, many buyers are interested in obtaining the best value

proposition in the medium to low cost range. For example, higher

performance, with little extra cost penalty, often provides the best

value. The key factors that affect cost are system design (i.e. choice

of components and their configuration), minimization of manual labor

(especially for optical alignment), and volume manufacturing to

reduce procurement costs and amortize non-recurring costs

Currently available Free Space Optics (FSO) hardware can be

classified into two categories depending on the operating wavelength:

systems that operate near 800 nm and those that operate near 1550

nm. Each vendor manages to make huge claims that their own

chosen wavelength is best. But as we point out below, itâ„¢s actually

real world link margin, not market-eering that matters.

Contrary to claims, there are only a few compelling reasons for

selecting 1550 nm Free Space Optics (FSO) systems, and many

against. One argument in favour is about laser eye safety, but ignores

the effect of increased transmit aperture used by a competing 980nm

solution from a vendor such as CableFree. There is reduced solar

background radiation at 1550nm, but the receiver devices are much

less sensitive than the enhanced silicon at 800-980nm, completely

negating any advantage. The true argument for 1550nm is about the

existence of EDFAs, optical amplifiers which can boost transmit

signals to whole watts of power, and the existence of DWDM

components which enable multi-channel multi-gigabit systems for the

future. But the cost penalty associated with 1550nm makes it


The common perception of wireless is that it offers less security

than wireline connections. In fact, Free Space Optics (FSO) is far

more secure than RF or other wireless-based transmission

technologies for several reasons:

1. FSO laser beams cannot be detected with spectrum analyzers or

RF meters

2. 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

3. Data can be transmitted over an encrypted connection adding to

the degree of security available in Free Space Optics (FSO) network


The advantages of free space optical wireless or Free Space

Optics (FSO) do not come without some cost. When light is

transmitted through optical fiber, transmission integrity is quite

predictable; barring unforseen events such as backhoes or animal

interference. When light is transmitted through the air, as with Free

Space Optics (FSO) optical wireless systems, it must contend with a

a complex and not always quantifiable subject - the atmosphere.

Attenuation, Fog:

Fog substantially attenuates visible radiation, and it has a similar

affect on the near-infrared wavelengths that are employed in Free

Space Optics (FSO) systems. Similar to the case of rain attenuation

with RF wireless, fog attenuation is not a show-stopper for Free

Space Optics (FSO) 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:

Free Space Optics (FSO) products which have widely spaced

redundant transmitters and large receive optics will all but eliminate

interference concerns from objects such as birds. On a typical day,

an object covering 98% of the receive aperture and all but 1

transmitter; will not cause an Free Space Optics (FSO) link to drop

out. Thus birds are unlikely to have any impact on Free Space Optics

(FSO) transmission.

Pointing Stability, Building Sway, Tower Movement

Only wide-beamwidth fixed pointed Free Space Optics (FSO)

systems are capable of handling the vast majority of movement found

in deployments on buildings. Narrow beam systems are unreliable,

requiring manual re-alignment on a regular basis, due to building

movement. ËœWide beamâ„¢ means more than 5milliradians. Narrow

systems (1-2mRad) are not reliable without a tracking system

The combination of effective beam divergence and a well

matched receive Field-of-View (FOV) provide for an extremely robust

fixed pointed Free Space Optics (FSO) system suitable for most

deployments. Fixed-pointed Free Space Optics (FSO) systems are

generally preferred over actively-tracked Free Space Optics (FSO)

systems due to their lower cost.


Performance of many Free Space Optics (FSO) optical wireless

systems is adversely affected by scintillation on bright sunny days.

Some optical wireless products have a unique combination of large

aperture receiver, widely spaced transmitters, finely tuned receive

filtering, and automatic gain control characteristics. In addition,

certain optical wireless systems also apply a clock recovery phaselock-

loop time constant that all but eliminate the affects of

atmospheric scintillation and jitter transference.

Solar Interference:

Solar interference in Free Space Optics (FSO) free space optical

systems can be combated in two ways. Optical narrowband filter

proceeding the receive detector used to filter all but the wavelength

actually used for intersystem communications. To handle off-axis

solar energy, sophisticated spatial filters have been implemented in

CableFree systems, allowing them to operate unaffected by solar

interference that is more than 1 degree off-axis.
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