New applications put an old idea back in the air

Atmospheric optical communication is undergoing a revival as a new generation of developers tries to fill difficult niches in metropolitan telecommunications. The demand for bandwidth is growing faster than network infrastructure, creating bottlenecks between high-speed networks inside of businesses and high-speed backbone systems. Optical fibers can provide the needed capacity, but only at the cost of time and considerable money to dig up the streets and lay new cables.

Sending laser beams through the air rather than through optical fibers can be an attractive alternative. Free-space laser transmission systems were first developed decades ago, but the technology languished with few applications. Now engineers have developed a new generation of atmospheric laser transmission systems. Ranging from point-to-point links to complex networks, they offer a new way to deliver high-speed communications in environments such as congested urban areas where lines of sight are much easier to find than cable routes.

Recycling an old idea

Sending optical signals through the air is not a new idea. Alexander Graham Bell demonstrated it using reflected sunlight with his Photophone in 1880. Nearly a century later, Bell Labs hoisted an early laser to the top of a microwave tower at its Murray Hill, NJ, headquarters to send pulses 40 km to its Holmdel site. However, experimenters quickly learned that air is nowhere near as clear as it seems. Air turbulence can bend beams far from distant receivers. Fog, snow, rain, haze and even passing birds can interrupt the beam, reducing the reliability of a long point-to-point link below the standards for backbone telecommunications. By 1964, a British military engineer bluntly concluded, "The atmosphere is completely inimical to laser transmission systems."¹

In time, optical communication became almost synonymous with fiberoptics. Yet a scattering of short atmospheric links earned their place in specialized applications, typically connecting pairs of nearby buildings. Sending a laser beam through the air could avoid the high cost of laying a dedicated cable. Atmospheric laser links could be put into place quickly, and unlike point-to-point microwave systems, they did not require licenses from the Federal Communications Commission. However, reliability problems limited their applications.

The new generation of atmospheric systems has made great strides in improving reliability. Developers have demonstrated "four nines" or 99.99% reliability, meaning no more than one hour a year of outages. That falls short of the "five nines" or 99.999% target for telecommunications links, but it's good enough for many applications, particularly when atmospheric links are part of networks with other backup services. (It's also better than either my local telephone company or electric company managed this year.)

Atmospheric laser technology

Conceptually, an atmospheric optical link is simple, with a transmitter directing a laser beam at a receiver. The narrow beam concentrates power and makes eavesdropping very difficult, but it must be directed precisely to the receiver. Systems may include active alignment features, in which feedback from the receiver is used to help direct the transmitter beam to the optimum point. That both aids in installation and helps stabilize transmission between the tops of tall buildings, which can sway in the wind or in earthquakes. Typical beam divergence in a free-space laser link is 3 to 6 mrad, so the beam expands to only a few meters in diameter after traveling a kilometer. The collecting optics must strongly exclude ambient light lest sunlight or other illumination overwhelm the laser beam. In practice, the receiving optics have a narrow field of view and include narrow-pass filters that select the laser wavelength while blocking other wavelengths (see Fig. 1).

Transmission requires a clear line of sight between terminals. Usually this means the transmitter and receiver must be mounted on the roofs or sides of multistory buildings, or placed inside windows well above the ground. The beam should not pass close to trees or other obstructions that can move in the wind; it also should pass well above any passing vehicles and out of the paths of people. All outdoor equipment must be environmentally sealed to withstand extremes of weather, and external optics may need a defroster to remove frost or dew. Outdoor equipment also must be designed to discourage birds from perching on it, and to avoid collecting dirt or debris that could attenuate the optical signal.

The transmitters and receivers can use components adapted from their fiberoptic counterparts, with different beam-focusing and collecting optics. As in fiberoptic systems, two-way transmission requires one transmitter and one receiver at each end; they may be separate or built into the same package.

Current systems operate at a variety of wavelengths; the usual choices are 780, 850 and 1550 nm. Wavelength-division multiplexing is possible, but rarely used. The main concerns in the choice of wavelength are eye safety and laser cost. The eye does not transmit the 1550-nm wavelength of long-haul fiberoptic systems to the retina, so lasers emitting up to 10 mW at that wavelength are considered as eye-safe Class 1 products. On the other hand, 1550-nm lasers are far more expensive than lasers emitting at 780 or 850 nm, in which the maximum power for Class 1 operation is a fraction of a milliwatt. Somewhat higher powers can be used as long as the beams are kept out of human reach, but such transmitters require automatic power-reduction systems, which monitor received power and quickly reduce output to Class 1 level if anything interrupts the beam. The main concern is maintenance workers on rooftops rather than passing birds.

Point-to-point access systems

The main selling points of atmospheric laser links are their lower cost and much faster installation time than dedicated fiberoptic links with the same bandwidth. Current atmospheric links transmit signals from about 100 m to a few kilometers at standard transmission rates such as 155 Mbit/s and 622 Mbit/s; systems for Gigabit Ethernet and higher speeds are in development. Atmospheric links have standard fiberoptic interfaces at both ends, so the rest of the system doesn't "know" if the signals go through the air or through a fiber.

Considerable engineering goes into increasing reliability. Small-scale atmospheric turbulence causes laser beams to fluctuate in both intensity and direction, an effect we see as laser speckle. To prevent this effect from introducing noise into the signal, multiple lasers can be mounted in a single laser head, spaced about 20 cm apart. All lasers are modulated with the same signal, but the beams experience different scintillation because they travel different paths, so at least one of them should reach the receiver with adequate power.

Performance also improves with adaptive power control, which adjusts output power to match local conditions. When the air is clear, the transmitters send a low-level signal with a safety margin to ensure it will be received reliably. If the weather starts to attenuate the beam, the receiver detects reduced power and signals the transmitter to increase its output to compensate for the higher attenuation, ensuring that enough power reaches the receiver.

Applications of such point-to-point atmospheric links can go well beyond the stereotypical example of linking two tall office buildings on opposite sides of a busy highway. Several links can radiate from one building with dedicated fiberoptic line to buildings that don't have their own fiber connections. Atmospheric links also can serve as protection circuits for fiberoptic systems; failure of the fiber circuit switches the signal to the atmospheric link. The free-space backup could ease the anxieties of network administrators who tend to worry every time they see a backhoe.