Thanks Frank, here's some additional info to supplement the links you provided:
{as well as some other links:
airlinx.com
airlinx.com
}
snippets from Telecommunications, Feb 1996 v30 n2 p37(3)
Author Jutila, John M.
TRANSMISSION RANGE
Commercial Class IIIB lasers have diode power output less than 500 milliwatts and do not require
special technician licensing for safe operation and handling. These systems can support data rates
up to 155 Mbps at distances of up to 4000 feet (three-quarters of a mile), although there is a trade-off
between data rate and distance. Higher power diodes provide greater signal to noise performance for
longer range transmission, but do require special precautions to ensure eye safety. For example, military
laser systems using 10-watt diodes have a communication range of 10 miles. Beyond this range, terrestrial
line-of-sight transmission is prohibited by the curvature of the earth.
Class IIIB lasers with a laser diode output power of 20 milliwatts require eye safety precautions only within
about 50 to 75 feet of the transmitter. New designs with wide-diameter lenses spread the laser power
density over a larger surface area so that eye safety is greatly enhanced. Transmission up to 4000 feet can
be provided with a 10-1 bit error rate (BER) at data rates of up to 34 Mbps. Therefore, Class IIIB lasers are
suitable for T1, E1, Ethernet, Token Ring, and E3 networking. At higher speeds, effective transmission
distances must be reduced to maintain the same [10.sup.9] BER. For example, at 155 Mbps the effective
transmission range is 900 feet (see Figure 1). Longer ranges are possible at any data rate, but only at the
expense of BER or network availability.
FACTORS AFFECTING LINK PERFORMANCE
Commercial laser transmission is line-of-sight using infrared light with a typical installation as shown in
Figure 2. A clear line of sight is essential for a reliable link. Unlike microwave, which requires
added clearance from the direct bore sight between transmitter and receiver to prevent multipath
signal distortions, laser transmission requires only a clear line of sight along the narrow beam path from
the transmitter to receiver. Many campus or across-the-street network applications offer line-of-sight from
one rooftop or window to another. Network planners should also consider environmental factors that may
impact line-of-sight such as power lines that may sag with temperature changes or treetop foliage that
denser in summer months.
A laser system is similar to a radio transmission system in that a specific signal power is emitted and then
received some distance away. The power density is reduced as the laser beam spreads during transmission.
It is also reduced by attenuation (signal loss) due to the atmosphere and other distortions along the path.
Laser transmission systems are designed with enough fade margin to overcome such attenuation and
ensure reliable communication under most conditions. The
Fade margin is measured in decibels (dB) of power loss and represents the maximum amount of light
power that can be lost between the transmitter and receiver over a specified distance while still
maintaining target performance specifications, such as a [10.sup.9] BER at 10 Mbps. A 10-dB loss in signal
strength means that only 10 percent of the transmitted power is received. A 20,dB attenuation results in
only 1 percent of the signal getting through (see Table 1). Fade margin can be measured using neutral
density filters (which block transmitted light in standardized dB increments) by increasing the attenuation
at the receiver on a clear day until reliable transmission is no longer possible. Some attenuation occurs
naturally due to absorption of light by gases and water vapor in the atmosphere. Specific wavelengths are
chosen (e.g., 820 nanometers) to minimize such absorption.
TABLE 1
FADE MARGIN
Percent of light received Power loss (dB)
10.00% 10 dB
1.000% 20 dB
0.100% 30 dB
0.010% 40 dB
0.001% 50 dB
EXAMPLE: A LASER LINK WITH A 20-dB FADE MARGIN
CAN WITHSTAND A 99 PERCENT LOSS OF SIGNAL
POWER BEFORE TRANSMISSION PERFORMANCE IS
AFFECTED. A FADE MARGIN OF 15 dB AT 1-KILOMETER
RANGE IS A STANDARD MINIMUM FADE MARGIN
FOR LASER TRANSMISSION AND ENABLES UP TO
95 PERCENT SIGNAL LOSS ACROSS THE LINK.
Precipitation also attenuates laser transmission by scattering light rays, but in most applications network
availability will exceed 99 percent as long as the fade margin is 15 dB or better. Heavy rain, fog, or snow
has the potential to interrupt laser transmission, though typically only for short sporadic time periods. A
fade margin of 15 dB or more is sufficient for reliable transmission at one-half-mile range in heavy rainfall
up to 3 inches per hour, in wet snowfall up to 2 inches per hour, and in dry snowfall up to 1 inch per hour.
It is also sufficient to penetrate fog at a distance of approximately 110 percent of the actual visibility.
Scintillation due to heat can also impact light transmission. Rising hot air creates vortexes that diffract
light rays. This phenomenon is often observed as rippling waves rising above hot asphalt roads or
parking lots. Scintillation typically occurs only within a few feet of dark horizontal surfaces such as
roads or rooftops. Scintillation can also form on the sides of tall buddings in which the entire
surface creates a hot airflow draft upwards to the roof. Laser transmitters and receivers must be located to
minimize scintillation effects by ensuring that the laser beam path is more than 10 feet above potential
problem surfaces. Laser units may also be mounted on overhangs or rooftop comers to minimize the
impact of updraft scintillation on tall buildings.
Sunlight, which emits infrared light waves, may also impact laser performance by shining directly into the
receiver field of view. Sunlight shining directly into the transmitter may also interfere with automatic gain
control (AGC) power feedback circuits, causing a reduction in diode power output. This may affect
transmission performance. Some products use filters and optical channels to minimize such sunlight
interference. In worst cases, laser links may have to be relocated along a different path or in front of
obstacles such as rooftop utility sheds that shield the opposite unit from sunlight.
Other potential sources of interference include birds, dust storms, or anything else that may block light
along the path. In most applications, short intervals of downtime can be accommodated.
INSTALLATION AND MAINTENANCE
A narrow laser beam transmitting over a long distance is sensitive to movement or vibration. A 1 milliradian
angular motion will cause a laser beam to move sideways by 1 meter at the end of a 1-kilometer
transmission. Some manufacturers have attempted to use auto-tracking serve mechanisms or parabolic
reflectors to maintain alignment, though at great expense and complexity. A more practical and economical
approach is to use a wider beam. For example, a 2-meter diameter beam at the receiver will enable motion
up or down or to each side of up to 1 milliradian at the source transmitter without losing alignment along a
1-kilometer path. This approach only requires that laser units be mounted on reasonably stable surfaces
such as concrete or steel structures. Rooftop comers offer the best stability.
Laser transmission systems can be installed quickly, which is one reason that they are the subject of
increased interest. Laser systems can be used for emergency restoration, although most commercial
systems are used as permanent installations for day-to-day network applications. Installation and
alignment can be performed very quickly. Running power and communication cables to the laser usually
require the most time. Indoor installation from window to window is also possible, although attenuation
caused by the glass must be considered during network planning. Expect a signal loss of about 4 percent
through each clear glass window surface.
Installation requires rough alignment using a tilt and swivel mount, and then fine tune alignment using
vertical and horizontal adjustment screws. Infrared viewers may be used for alignment, although some
products have built-in signal strength meters and other alignment aids. Sighting telescopes or "rifle scopes"
are still sometimes used but require special precautions with higher power lasers to ensure eye safety. Once
aligned, neutral density filters are used to measure the link fade margin to ensure proper performance.
Laser communication systems are solid state devices, and, therefore, offer long-term reliability. Regular
maintenance includes occasional realignment and lens cleaning, and periodic replacement of the laser
diodes. Commercial laser diodes rated to 50 milliwatts have a typical life of 2.5 years or more. Higher
power diodes have shorter lifetimes.
APPLICATIONS
Laser transmission is suitable for nearly all short-hand communication applications (see Figure 3). Several
thousand commercial systems have been installed worldwide, and that number is expected to grow
dramatically as awareness increases. Laser transmission is often considered as an alternative in
applications where rights of way cannot be obtained or natural obstacles prevent cable runs.
Laser transmission is often preferred over wireless microwave because the laser provides higher data rates
and can be installed without obtaining a radio frequency license. Laser transmission is also
becoming increasingly favored over spread spectrum radio systems because of much higher data
throughput. Light emitting diode (LED) technology, a close cousin of laser systems, is also used for
point-to-point optical transmission in applications requiring very short range networking where the higher
fade margin possible with collimated laser light is not required. Laser transmission systems have been
installed primarily in campus environments, including government or corporate office clusters, universities,
schools, military bases, industrial sites, hospitals, utilities, stadiums, museums, and shipyards. Systems are
also used to provide bypass access to voice and data networks, to connect wireless cell sites to mobile
telecommunications switching offices (MTSOs), or just to connect offices across the street from each other.
Laser network equipment can offer a quick financial payback when used as an alternative to leased
communication circuits. In fact, customers are now combining laser transmission with digital multiplexers
to enable many applications to share bandwidth across the link, such as combined LAN, voice trunk, and
analog video traffic. Laser systems are available with a wide range of network interfaces to interconnect with
private automatic branch exchanges (PABXS), routers, bridges, asynchronous transfer mode (ATM) switches,
closed channel television (CCTV) video transmitters, and many other devices (see Figure 4).
|
