Laser Transmission

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.

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 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.

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.

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.

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 methods they like. This encourages innovation, differentiation, and speed of deployment.