The telecommunications business has always involved some risks. But now two of telecom’s largest companies are investing in thin air.

Lucent Technologies is spending $450 million on a joint venture with Seattle-based TeraBeam Networks to build communications systems that will transmit light directly between buildings, skipping optical fibers altogether. Not to be outdone, rival Nortel Networks is developing a line of similar equipment with San Diego-based AirFiber. The goal of both ventures: shoot laser beams between medium and large businesses in downtown areas or office parks, providing vastly more voice and data capacity than ordinary phone lines without the expense and delay of laying fiber-optic cable.

The explosion in the Internet means that businesses have an ever-growing appetite for bandwidth. Fiber optics, which can carry data at gigabit speeds, can readily provide that capacity, but less than 5 percent of downtown office buildings are currently “wired” with fiber. New installations take time, and construction costs can be staggering. Ever try digging up the sidewalk in mid-town Manhattan?

Don’t expect the new technology to replace fiber-optic networks. But for businesses needing from 10 to several hundred megabits per second of bandwidth at one-tenth the cost of installing fibers, it could be a boon. “We don’t consider it revolutionary. But it’s a useful concept,” says Jeff Montgomery, chairman of ElectroniCast, a telecom consulting firm in San Mateo, Calif.

Shooting laser beams through the air between buildings is not a new idea. In fact, laser communication through open air was demonstrated in the early 1960s. But the technique lost out to fiber optics; in earlier systems anything from bad weather to passing birds could interrupt the pencil-thin beams and destroy the line of communication.

The new systems are designed to be more reliable-pigeon-proof. Both TeraBeam and AirFiber use redundant beams, each spread over a larger area, so interruption would require almost complete blockages of multiple large beams. AirFiber arranges an interconnected mesh of rooftop transmitters and receivers, spaced 200 to 500 meters apart, depending on clarity of the local atmosphere; at least one node in the mesh connects to a fiber-optic backbone. Each transmitter aims beams of light at three or four receivers, building up a redundant mesh with multiple interconnections. TeraBeam puts a base transmitter in a strategic window in a building served by a fiber-optic network.

The companies have demonstration systems up and running-TeraBeam in Seattle, and AirFiber in Madrid, Tokyo and Dallas.