Optical laser-communications systems carve niche in metro markets

As more bandwidth capacity becomes necessary in crowded metropolitan business centers and for use in premise and local area networks (LANs), more network operators are considering free-space optical communications as a less-expensive alternative to deploying fiber. Coupled with improvements in speed and distance capabilities, these systems offer a newly useful communications alternative.

The degree to which free-space systems have progressed became apparent with the recent debut of dense wavelength-division multiplexing (DWDM) technology for this market with Lucent Technologies' recent announcement of its WaveStar OpticAir system.

According to Frank Galuppo, director of Lucent's OpticAir product line, Lucent made the decision to develop this technology last March and the first products are scheduled to hit the market by next March, just a 12-month development interval. The first product will support a single wavelength at speeds of up to 2.5 Gbits/sec. A four-wavelength system with a maximum capacity of 10 Gbits/sec for distances up to 5 km is planned for commercial availability in the summer of 2000. Global Crossing Ltd. will conduct field trials before December.

"We will be trialing a number of systems in the December 1999 time frame with customers in addition to Global Crossing," adds Galuppo. "The interest has been extremely high. In fact, we're probably going to have problems limiting the number of trials we can do because of the interest."

In August 1998, Lucent teamed with AstroTerra Corp., a manufacturer of wireless optical communications systems headquartered in San Diego, CA. The two companies successfully tested a prototype wireless optical data transceiver, running an error-free data link at 2.5 Gbits/sec over a distance of 2.5 km, a new record for the highest data rate attained in wireless optical communications. Last February, Lucent announced it had successfully built a laser-communications system capable of transmitting 10 Gbits/sec using AstroTerra's telescope transceivers.

AstroTerra, founded in 1992, has been developing its own suite of products known as TerraLink. The company markets TerraLink as an "alternative form of optical data transfer when fiber is too costly or logistically impossible to install." It is also developing products to reach distances of 5 km to run at higher speeds for use in enterprise markets and last-mile telecommunications.

"Companies with multiple buildings using high-speed computer networks have a few options available to them right now," says Eric Korevaar, president of AstroTerra. "They could install their own fiber-optic cable. They could try to lease bandwidth from an independent provider. They could have a microwave link or a laser link. But when you get up into higher speeds, the leasing of bandwidth becomes problematic and the microwave links become more difficult. So when you get down to the choice of a fiber-optic link or a laser link, the cost becomes the factor, and a free-space laser link can be much more cost-effective."

Transmitting information through the air using optical laser light beams is hardly a new concept. Alexander Graham Bell experimented with the technology more than 100 years ago. Although there are a large number of companies with their own unique free-space optical communications products on the market, they all use similar technology.

Each point-to-point system comprises two main communications components: the sender and the receiver, otherwise referred to as the link heads. One link head sends a transmission beam, or laser, through the air to a receiver link head. Information is transmitted across the laser beam. The only real requirement is having a direct line of sight between the transmitter and receiver. The main limitations are distance, bit rates, and weather conditions.

The problem commonly cited with laser links is how well--or how poorly--they overcome these limitations. Korevaar points out that there is a major difference between reliability and availability when talking about optical laser communications.

"In the laser-communications realm, we refer to reliability as whether the actual equipment is prone to break down," says Korevaar. "Customers should understand that there is nothing inherently wrong with the equipment in these systems that makes them any less reliable than other equipment. Reliability is just as high. We use the word 'availability' to indicate the percentage of time laser equipment will operate due to weather conditions. At long distances, you may have 98% availability. As you get to shorter distances, the availability can be much higher."

In an enterprise network, the customer may be comfortable with an availability of 98%. In fact, if there is a critical need to have a permanent connection, some may opt to back up the laser link with a leased 1.5-Mbit/sec T1 line from the telephone company. That way, they have a high-speed link most of the time and, in the event of weather-related downtime, they'll still have network connectivity...just temporarily not as fast.

The other major difficulty faced by laser-communications vendors is achieving longer distances. Ron Jenkins, marketing-communications manager at SilCom Manufacturing Technology Inc. (Toronto, ON, Canada), points out that a 5-km link is fairly long for a laser because of the dispersion of the beam over distance. If a receiver of sufficient size is needed to pick up the beam and focus it back to a receive diode, you can't have a high amount of dispersion, probably less than 1/10th of one degree.

"Over 5 km, you're talking about something that's opened out very wide, and you'd need to have a very sensitive receive diode or some other way to maintain the alignment," says Jenkins. "Although it can be done, it's very expensive. When you have such a narrow angle involved, things like expansion and contraction of a building during the course of a day can put the link out of alignment. Longer distances, such as a kilometer or more, need some sort of active ongoing alignment process, and that complicates the mechanics of things quite a bit and would also raise the price of such a system significantly."

Putting cost aside, products have been on the market for about two years that can track the movements involved between two locations and compensate for the alignment, according to AstroTerra's Korevaar. His company has measured building sways with its equipment through internal video cameras that can look at the received signal from the other end of a link.

"We can watch the signal move around on the camera, so we can know what kind of angle movements we have on particular buildings," says Korevaar. "Short distances have beamwidth that's large enough to compensate for those movements. To get to longer distances, such as 5 km, you have to make the beamwidth narrower to get enough power from the transmitter to the receiver. In this case, we put an automatic tracking system on the equipment and use this internal camera to get directional feedback. We use that feedback to control some electronic actuators that maintain the alignment."

Canon USA is another manufacturer that is introducing new systems with the auto-tracking feature that provides higher availability by maintaining alignment despite the sway of buildings, vibrations from ventilation systems, and wind factors encountered on building rooftops.

"Auto-tracking, by focusing rather than spreading the beam, makes the system more resilient to environmental or weather issues, since the beam maintains more power," says Ken Ito, product manager for the broadcast equipment division at Canon. "Today, the need for fast backbones is much more common and availability of these links is most critical. In order to maintain this highly critical link, auto-tracking capability is a real necessity. Customers with experience in optical laser transmission without auto-tracking are becoming our best advertisement as they realize the advantages in terms of distance and availability."

With technology overcoming more and more hurdles for laser communications, the one remaining hurdle may be the sheer newness of the systems themselves. Vendors say customers simply aren't aware of this technology.

Most of the advantages of using a point-to-point laser system can be boiled down to cost. It's cheaper than deploying fiber across town. It's easier than getting rights-of-way and digging trenches. There are normally no licensing requirements. There are no lease or rental costs involved or a need for additional equipment.

Lastly, there is the advantage of security. According to SilCom's Jenkins, the systems have many military applications because a laser link is highly secure when compared to a radio.

"You can intercept a radio signal pretty much anywhere you want, even if it's a directional antenna," says Jenkins. "A laser link cannot be intercepted without breaking the beam. That affords a rather high level of security."

The equipment can be used to temporarily provide a communications link for a network where a fiber cable has been displaced due to a flood or other event, preventing revenue loss for a provider. Communications between islands can be achieved without the deployment of expensive underwater fiber cable. An entire system can be up and operational in less than a day, typically in about three hours. It can be installed indoors for large manufacturing plants. In fact, there are probably as many uses for laser-communications products as there are manufacturers.

With the data surge increasing capacity demands in the metropolitan, enterprise, premises, and LAN arenas, vendors of laser-communications products are racing to bring newer and better products to market. The good news is that these systems are being built to work hand-in-hand with existing fiber-optic networks. It would seem that our fiber-optic telecommunications industry might begin making preparations to welcome a "new kid on the block."