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Free-space data links: time for a reality check

30 Jul 2002

With no sign of an end to the telecoms downturn, the low installation costs of free-space optical links are looking increasingly attractive. Michael Hatcher finds the industry playing down long-range links and focusing on reliability.

From Opto & Laser Europe June 2002

Nobody doubts the huge potential of free-space optical communications. Ease of installation and low costs give the technology an advantage over fixed-line fibre-optic links, while its speed, the secure transmission it offers and the absence of spectrum licence issues gives it the edge on wireless microwave transmission.

However, free-space optics (FSO) equipment manufacturers have come to realize that for all the benefits of the technology, it is link reliability that customers value most.

In the past, FSO companies have been guilty of a certain amount of posturing, with the various players vying to offer the fastest or longest link. Some firms, including PAV Data Systems in the UK and Laserbit Communications in Hungary, claimed to offer links with a 5 or 6 km range.

Realistic targetsThat kind of stance appears to be changing now. The general consensus is that, at least in the cloudier skies of Europe, 2 km is a more realistic target and 4 km the upper limit. This is one facet of what PAV's marketing director Richard Redgrave sees as a maturation in the industry. "We have withdrawn the 6 km link from [PAV's] product brochures," he told OLE.

The "magic" figure is 99.999% availability - equivalent to a few minutes of downtime a year. But FSO links are susceptible to water droplets in the atmosphere, which both absorb and scatter data-carrying photons. Fog is the biggest problem, but despite this sensitivity, thousands of FSO links have been deployed worldwide.

Hermann Lentke is general manager of Communication by Light (CBL), an FSO manufacturer based in Germany. Lentke is even more sceptical about the distances that a link can cover. He says that in central Europe, a conservative upper limit is 800 m, and told OLE: "It makes no sense to expand the link distance to more than 2000 m. Physics is working against FSO. We made detailed calculations and concluded that FSO should be limited to 2 km. Some vendors try to sell links for longer distances - but how satisfied are their customers?"

Stephen Patrick is chief technology officer of Cablefree, an FSO manufacturer in the UK. He points to a customer in central London as evidence of a working link operating over almost 4 km. "The Victoria & Albert Museum had a radio link between its headquarters in Kensington and a storage site in Olympia, 3.7 km away. When the link failed, we replaced it with a 155 Mbit/s optical link, which has had no outages yet."

It seems that the squeeze in telecoms spending, which is forcing carriers to look at more cost-effective solutions, has benefited Cablefree in the past year. "Three years ago, FSO was merely a viable solution, but now it's a very exciting market," said Patrick.

Current estimates point to a global market worth around $200 m in 2001. Caught up in the telecoms hyperbole of 2000, Nortel Networks of Canada estimated that today's global FSO market would be worth at least five times that figure. Despite this apparent under-performance, however, both Cablefree and PAV are making profits on their modest revenues.

PAV has recently undergone big changes. Company founders Mike Turner and Nick Birchall sold their stakes in the company and businessman Neal Kelly took over in May 2001. Headquarters have since moved from the tranquil lakeside environment of Windermere to the more urban surroundings of Warrington, near Manchester.

Redgrave is the sole surviving member of the original management team, and says that there has been a sea-change in the company's style since the takeover. Like Cablefree, all systems manufacturing is now outsourced, and only the R&D operation and its distribution centre remain in Windermere. "The original management took the company from a two-man operation to a 50-strong company, and it was time for a change," Redgrave told OLE. 'We have a new business model, a record number of orders in the pipeline and a positive cashflow. Up until September 11, everything was looking fantastic.'

Since then, a combination of the management change and the turbulent global economy has seen PAV's business stall at a yearly turnover of $10 m. According to Redgrave, business in Europe and the US has slowed dramatically, with PAV having to rely predominantly on sales in China.

By contrast, Cablefree says its upward business trend continues, with revenues continuing to double each year and a ten-fold increase in its workforce since 1997.

Although they are concentrating on reliability, improving the speed and range of FSO remains a priority for both Cablefree and PAV. While recent moves to gigabit speeds have seen a shift to the use of avalanche photodiodes in place of pin diodes for detection, the key to long-term improved performance is moving to longer-wavelength laser sources than the current standard components, which operate at around 800 nm.

"The next generation of FSO links will be at 1550 nm," said Patrick. The indium phosphide-based lasers offer greater modulation speeds, which will enable the step up to 10 Gb/s, while the physics of scattering means that 1550 nm sources should theoretically improve transmission losses.

However, CBL has carried out some studies suggesting that in terms of light attenuation, 1550 nm may not offer any great improvement on 800 nm links. Its investigation suggests that in clear weather, reduced scattering at 1550 nm does offer a marginally better connection, but in heavy fog, rain, or snow, attenuation is so bad that both wavelengths are seriously affected.

This wavelength change will be some time coming, however. PAV's Redgrave says that 1550 nm optical components currently cost 40 times more than their 800 nm counterparts, which must change before any widespread deployment is seen. That said, Redgrave is convinced that 1550 nm links will become the industry standard: 'The longer wavelength is essential to improve bandwidth in the future.'

Safety standard

The longer wavelength is also safer than sources operating nearer to the visible spectrum, as it is not transmitted onto the retina. With the increase in FSO demand, developments on the safety aspects of FSO are also under way. There is no current safety standard dedicated to FSO, but the International Engineering Consortium (IEC) is tailoring new standard IEC 60825-12 specifically for the industry.

Safety expert Brian Tozer of Lasermet, UK, told OLE: "The new guidelines will provide guidance on how to implement an open-beam system over both short and long ranges. High powers will be allowed, but only in places where it is impossible for people to be affected by the beam. This will require automatic power reduction (APR) systems." The new standard is currently a work in progress, and is expected to come into force at the end of next year.

The sole company to offer 1550 nm links at present is Canada-based Fsona, which was set up by former BT executive Theresa Carbonneau. The long-term strategy employed by Fsona and many of its fellow Canadian and US FSO companies is in stark contrast with European firms, which are concentrating on current market demand.

Although most FSO companies have showcased their top-of-the-range 2.5 Gb/s products within the last year, Redgrave is sceptical about deployment of this link speed. "As an industry, we still have to prove acceptable availability. I think we will have to wait until 2004 before we see any volume sales of 2.5 Gb/s equipment."

PAV has limited itself to a 1 Gb/s system for the moment, and is keeping R&D focused on more near-term solutions such as extending the range of its 622 Mb/s product from 1 to 2 km. Networking heavyweights Nortel, Lucent and Cisco have all made significant moves into FSO. Nortel-backed AirFiber is concentrating on network systems, rather than point-to-point links, and is reputedly yet to sell any.

However, the Cisco-financed Lightpointe, which was founded by German FSO technology guru Heinz Willebrand and has a manufacturing base in Dresden, Germany, has made inroads into Europe. It has distribution agreements with partners in Greece, Italy and most eastern European countries for its 850 nm links. Along with Cablefree, Lightpointe claims to have an installed base of more than 1000 FSO systems.

PAV's response has been to take the fight back over the Atlantic. It is said to be poised to sign a deal with a major US distributor.

Adaptive optics

Long-term, both PAV and Cablefree are looking to adaptive optics to extend the reach of their systems. Cablefree is working with US company Aoptix, which is developing an adaptive optics system using wavefront sensors and miniature deformable mirrors to correct beamshapes distorted by atmospheric conditions and windows.

The technique means that a highly collimated beam is needed for adaptive correction, whereas in current FSO transmission beams are allowed to reach metre-scale diameters at the receiver to ease alignment. Some clever beam-tracking and alignment technology will be needed if the adaptive-FSO technology is to work.

Recent developments in quantum cascade lasers may turn out to be significant for FSO. Matthias Beck and colleagues at the University of Neuchatel, Switzerland, have developed a room-temperature-operating 9.1 µm device that the US-based Maxima Corporation has licensed for communication applications. This much longer wavelength could greatly enhance transmission through both wet and dry atmospheres, owing to reduced scattering effects at longer wavelengths, and the fact that water does not absorb in this mid-infrared region.

For more information
The FSO Report, Engalco www.engalco.com

ABTechSPECTROGON ABPhoton Lines LtdLASEROPTIK GmbHCeNing Optics Co LtdLaCroix Precision OpticsECOPTIK
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