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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 Palm® devices.
History of Free Space Optics (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’s
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 “photophone,” Bell considered this optical technology –
and not the telephone – his preeminent invention because it did not
require wires for transmission.
Although Bell’s photophone 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
Free Space Optics (FSO) Works
Free Space Optics (FSO) transmits invisible, eye-safe light beams from one
"telescope" to another 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.
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: 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. 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.
How Free Space Optics (FSO) can help you
FSO’s freedom from licensing and regulation 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.
For Metro Area Network (MAN) providers the last mile or even feet can be the
most daunting. Free Space Optics (FSO) networks can close this gap and allow
new customers access to high-speed MAN’s. Providers also can take advantage
of the reduced risk of installing an Free Space Optics (FSO) network which
can later be redeployed.
The Market. Why FSO? Breaking the Bandwidth Bottleneck
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). Meanwhile, local area networks (LANs) and
gigabit ethernet ports are being deployed with a comparable growth rate. 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 (a T1 line). As a consequence,
there is a strong need for a high-bandwidth bridge (the “last mile”
or “first mile”) between the LANs and the MANs or WANs.
A recent New York Times article reported that more than 100 million miles
of optical fiber was laid around the world in the last two years, as carriers
reacted to the Internet phenomenon and end users’ insatiable demand
for bandwidth. The sheer scale of connecting whole communities, cities and
regions to that fiber optic cable or “backbone” is something not
many players understood well. Despite the huge investment in trenching and
optical cable, most of the fiber remains unlit, 80 to 90% of office, commercial
and industrial buildings are not connected to fiber, and transport prices
are dropping dramatically.
Free Space Optics (FSO) systems represent one of the most promising approaches
for addressing the emerging broadband access market and its “last mile”
bottleneck. 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.
Broadband Bandwidth Alternatives
Access technologies in general use today include telco-provisioned copper
wire, wireless Internet access, broadband RF/microwave, coaxial cable and
direct optical fiber connections (fiber to the building; fiber to the home).
Telco/PTT telephone networks are still trapped in the old Time Division Multiplex
(TDM) based network infrastructure that rations bandwidth to the customer
in increments of 1.5 Mbps (T-1) or 2.024 Mbps (E-1). DSL penetration rates
have been throttled by slow deployment and the pricing strategies of the PTTs.
Cable modem access has had more success in residential markets, but suffers
from security and capacity problems, and is generally conditional on the user
subscribing to a package of cable TV channels. Wireless Internet access is
still slow, and the tiny screen renders it of little appeal for web browsing.
Broadband RF/microwave systems have severe limitations and are losing favor.
The radio spectrum is a scarce and expensive licensed commodity, sold or leased
to the highest bidder, or on a first-come first-served basis, and all too
often, simply unavailable due to congestion. As building owners have realized
the value of their roof space, the price of roof rights has risen sharply.
Furthermore, radio equipment is not inexpensive, the maximum data rates achievable
with RF systems are low compared to optical fiber, and communications channels
are insecure and subject to interference from and to other systems (a major
constraint on the use of radio systems).
Free Space Optics (FSO) Advantages
Free
space optical (FSO) systems 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.
Free Space Optics (FSO) Security
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:
- 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.
Free Space Optics (FSO) Challenges
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.
Fog and Free Space Optics (FSO)
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. Note that the effect of fog on Free Space Optics (FSO) optical wireless
radiation is entirely analogous to the attenuation – and fades –
suffered by RF wireless systems due to rainfall. 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. Free Space Optics (FSO)
optical wireless-based communication systems can be enhanced to yield even
greater availabilities.
Physical Obstructions and Free Space Optics (FSO)
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.
Free Space Optics (FSO) Pointing Stability –
Building Sway, Tower Movement
Fixed pointed Free Space Optics (FSO) systems are designed to be capable of
handling the vast majority of movement found in deployments on buildings.
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.
Scintillation and Free Space Optics (FSO)
Performance of many Free Space Optics (FSO) optical wireless systems is adversely
affected by scintillation on bright sunny days; the effects of which are typically
reflected in BER statistics. 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 phase-lock-loop
time constant that all but eliminate the affects of atmospheric scintillation
and jitter transference.
Solar Interference and Free Space Optics (FSO)
Solar interference in Free Space Optics (FSO) free space optical systems operating
at 1550 nm can be combatted in two ways. The first is a long-pass optical
filter window used to block all optical wavelengths below 850 nm from entering
the system; the second is an 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, two spatial filters have
been implemented in SONAbeam systems, allowing them to operate unaffected
by solar interference that is more than 1.5 degrees off-axis.
Free Space Optics (FSO) Reliability
Employing an adaptive laser power (Adaptive Power Control or APC) scheme to
dynamically adjust the laser power in response to weather conditions will
improve the reliability of Free Space Optics (FSO) optical wireless systems.
In clear weather the transmit power is greatly reduced, enhancing the laser
lifetime by operating the laser at very low-stress conditions. In severe weather,
the laser power is increased as needed to maintain the optical link - then
decreased again as the weather clears. A TEC controller that maintains the
temperature of the laser transmitter diodes in the optimum region will maximize
reliability and lifetime, consistent with power output allowing the FSO optical
wireless system to operate more efficiently and reliably at higher power levels.
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