Free-space optics leaps last mile

While the fiber-optic backbone is already widely installed, only 5 percent of commercial buildings in the United States have fiber to their door, even though most are within a mile of a fiber-optic line, and many are within a few hundred feet. The "last mile" is proving to be the major bottleneck to expanding broadband services to many potential customers, and it offers an obvious opportunity for free-space optics (FSO) systems.

FSO systems are fast. Data rates of 1.25 Gbits/second are commonly supported and vendors have recently been promoting 10-Gbit/s systems. Systems up to 160 Gbits/s are reportedly in development. FSO operates in an unregulated section of the spectrum, so no Federal Communications Commission permits are required. In addition, since trenching is eliminated, so are all of the permits associated with such activities as digging up the streets and disrupting traffic, which greatly speeds up installation.

Security is generally not a concern. Not only would the optical signal be very difficult to intercept (requiring a receiver to somehow be placed up in the air directly in the narrow, invisible light path), as soon as it was, the connection would drop out. The optical signal is also relatively immune to jamming, in contrast to radio frequency systems. As a testament to the level of security offered, the U.S. Ballistic Missile Defense Organization is investigating FSO communications for "undisclosed" applications.

The primary limitation to FSO system performance is interference due to weather, especially fog. Fog interference in optical transmission is similar to rain fading in radio transmission, a well-known phenomenon. The fading is the result of scattering of the beam as it encounters particles in the atmosphere, and is a combination of two primary processes, Rayleigh and Mie scattering.

Rayleigh scattering results from particles less than about one-tenth of a wavelength and mainly consists of scattering off the surfaces of particles in the air. Rayleigh scattering is highly wavelength-dependent. It is the phenomenon that causes the sky to appear blue because blue light is scattered around four times as much as red light, and the refracted blue light makes the sky look blue to an observer. Rayleigh scattering due to rain is commonly the dominant process responsible for signal fading at radio wavelengths, but has very little effect at FSO wavelengths. Raindrops are typically about 0.5 to 3 mm in diameter, and radio signals, especially high-frequency GHz signals, with wavelengths in the millimeter to centimeter range, are susceptible to rain fading by Rayleigh scattering. The wavelengths of FSO signals, on the order of 0.001 mm, are too short to be much affected by Rayleigh scattering.

Fog interference

Mie scattering is the predominant process responsible for fog interference at FSO wavelengths. Mie scattering is caused by particles of a similar size or larger than a given wavelength, and is less wavelength-dependent than Rayleigh scattering. Mie scattering is the reason clouds and fog appear white, because all wavelengths of visible light are equally scattered by the moisture particles of the cloud. FSO signals, which are relatively close to the wavelength of visible light, are scattered by fog in exactly the same manner. FSO wavelengths are much smaller than raindrops and pass through relatively unaffected. FSO signals are unfortunately close enough to the size of fog particles, which range from 0.01 to 0.05 mm, to be significantly scattered by Mie scattering. Heavy fog can reduce the signal by up to about 300 dB/km.

Dealing with fog, however, is a relatively straightforward matter. Transmitted power is maximized, up to the limits of eye safety, and link lengths are designed for a specified level of availability, considering the local fog statistics. High-performance micro-optics at the diode can be used to correct for aberrations and astigmatism, optimizing the output beam quality for greater fog-penetrating power.

Beam alignment problems are dealt with by the divergence of the optical beam, which results in beam diameters on the order of tens of centimeters over common link lengths. With the receiver centered in the beam, most movement is passively accommodated. In applications where movement exceeds what beam divergence can accommodate, an active alignment system can maintain connectivity. Objects such as birds passing through the optical path usually only block the beam for a few milliseconds. This can have the effect of momentarily slowing down the data rate, but it will not normally cause the connection to drop out.

See related chart

New laser transmitters are now addressing fog interference, the most significant drawback that has hindered the widespread acceptance of FSO systems. Laser diodes with correcting micro-optics can circularize and remove all aberrations from the laser beam, resulting in an optimized beam with significantly improved fog-penetrating ability.

Many prospective users meet the requirements for an FSO installation. Free-space optics systems can be installed along any line of sight (up to a kilometer or two), providing an appealing option to laying fiber. While last-mile access probably represents the majority of the market for FSO, there are several other opportunities. Cellular networks can utilize FSO to make connections. Mobile or temporary network connections for sporting events can be rapidly deployed. FSO can be instantly deployed for emergency response or disaster recovery situations, in remote areas or where fiber lines have been disrupted.