LASER TRANSMISSION

LASER TRANSMIISSIION
Arelatively new category of wireless communication uses
laser, sometimes called “free-space optics,” operating in the
near-infrared region of the light spectrum. Utilizing coherent
laser light, these wireless line-of-sight links are used in campus
environments and urban areas where the installation of
cable is impractical and the performance of leased lines is too
slow. Laser links between sites can be operated at the full
local area network (LAN) channel speed. And unlike
microwave transmission, laser transmission does not require
a Federal Communications Commission (FCC) license, and
data traveling by laser beam cannot be intercepted.
Performance Impairments
The lasers at each location are aligned with a simple bar
graph and tone lock procedure. Fiberoptic repeaters are used
to connect the LANs to the laser units. Alternatively, a bridge
equipped with a fiberoptic to attachment unit interface (AUI)
transceiver may be used. Connections to and from the laser
are made using standard fiberoptic cable, protecting data
from radio frequency interference (RFI) and electromagnetic
interference (EMI). Monitors can be attached to the laser
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units to provide operational status, such as signal strength,
and to implement local and remote loop-back diagnostics.
The reason that laser products are not used very often for
business applications is that transmission is affected by
atmospheric conditions that produce such effects as absorption,
scattering, and shimmer. All three can reduce the
amount of light energy that is picked up by the receiver and
corrupt the data being sent.
Absorption refers to the ability of various frequencies to
pass through the air. Absorption is determined largely by the
water vapor and carbon dioxide content of the air along the
transmission path, which, in turn, depends on humidity and
altitude. The gases that form in the atmosphere have many
resonance bands that allow specific frequencies of light to
pass. These transmission windows occur at various wavelengths,
such as the visible light range. Another window
occurs at the near-infrared wavelength of approximately 820
nanometers (nm). Laser products tuned to this window are
not greatly affected by absorption.
Scattering has a much greater effect on laser transmission
than absorption. The atmospheric scattering of light is
a function of its wavelength and the number and size of scattering
particles in the air. The optical visibility along the
transmission path is directly related to the number and size
of these particles. Fog and smog are the main conditions that
tend to limit visibility for optical-infrared transmission, followed
by snow and rain.
Shimmer is caused by localized differences in the air’s
index of refraction. This is caused by a combination of factors,
including time of day (daytime heat), terrain, cloud
cover, wind, and the height of the optical path above the
source of shimmer. These conditions cause fluctuations in
the received signal level by directing some of the light out of
its intended path. Beam fluctuations may degrade system
performance by producing short-term signal amplitudes
that approach threshold values. Signal fades below these
threshold values result in error bursts.
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LASER TRANSMISSION 197
Vendors have taken steps to mitigate the effects of absorption,
scatter, and shimmer. For example, such techniques as
frequency modulation (FM) in the transmitter and an automatic
gain control (AGC) in the receiver can minimize the
effects of shimmer. Also, selecting an optical path several
meters above heat sources can greatly reduce the effects of
shimmer. However, all of these distorting conditions can vary
greatly within a short time span or persist for long periods,
requiring onsite expertise to constantly fine-tune the system.
Many businesses simply cannot risk frequent or extended
periods of downtime while the necessary compensating
adjustments are being made. As if all this were not enough,
there are other potential problems to contend with, such as
thermal window coatings and the laser beam’s angle of incidence,
both of which can disrupt transmission. These problems
are being overcome with newer lasers that operate in
the 1550-nanometer (nm) wavelength. A1550-nanometer
delivery system is powerful enough to go through windows,
can deliver signals under the fog blanket, and is safe enough
that it does not blind the casual viewer who happens to look
into the beam. Up to 1 Gbps of bandwidth is available with
these systems—the equivalent bandwidth capacity of 660 T1
lines (Figure L-1).
There is also a distance limitation associated with laser.
The link generally cannot exceed 1.5 kilometers (km), and 1
kilometer is preferred. With 1550-nanometer systems, the
practical distance of the link is only 500 meters.
Summary
Despite its limitations, laser, or free-space optics, can provide
a valuable last link between the fiber network and the end
user—including as a backup to more conventional methods,
such as fiber. Free-space optics, unlike other transmission
technologies, are not tied to standards or standards development.
Vendors simply attach their equipment into existing
fiber-based networks and then use any laser transmission
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methods they like. This encourages innovation, differentiation,
and speed of deployment.