British technology group acquires EDFA patents

British technology group acquires EDFA patents

DAVE WILSON

BTG plc, London, an international technology transfer organization, has acquired worldwide patents of erbium-doped fiber amplifiers (edfas) invented at the University of Southampton, UK. Patent rights have been acquired in the United States, United Kingdom and Australia, and BTG is pursuing similar rights in other nations. License agreements are also pending for the technology.

British Telecommunications, General Electric Co. plc, Northern Telecom, the University of Southampton and BTG will share the revenues earned from licensing the edfa terrestrial and submarine technology--a market estimated at $250 million today and expected to grow to $1 billion by the year 2000.

The device was developed in the course of a U.K. government-sponsored program, which was jointly supported and funded by British Telecommunications, General Electric plc, Plessey-UK Ltd. and Standard Telephone and Cables plc, now owned by Northern Telecom in Canada.

According to Jim Strutt, a BTG licensing executive in London, the concept of edfa patents has been "a little cloudy." Many companies currently manufacture edfas but may not be fully aware of patent implications. He hopes that the recent transfer of ownership will clarify the associated royalty issues. This will help to financially benefit the University of Southampton, where the edfa was created in 1985.

At that time, Professor David Payne and his team at the University of Southampton demonstrated the first practical optical amplifiers for communications applications. Strutt claims the invention of the edfa laid the foundation for all-optical communications systems and subsequently led to increased use of fiber optics worldwide. The amplifiers have also been the key to the recently commissioned TAT-12 and TAT-13 transatlantic cable systems.

Awards to inventors

In 1990, Professor Emmanuel Desurvire of Holmdel, NJ-based Bell Laboratories--an authority on edfas--acknowledged the importance of the work of the Southampton University inventors, which preceded the work of his group at AT&T Bell Labs in the United States. The Southampton contribution was further recognized by the prestigious Japanese Computers and Communications Award, which was presented to Professor Payne in 1993.

The edfa patent in the United States, awarded in 1990, deals with the work performed at the Optoelectronics Research Center of the University of Southampton on edfas and lasers based on neodymium and other rare-earth materials. The patent cites erbium as the most commonly used dopant in the design of rare-earth doped optical amplifiers and lasers.

Edfas make possible the transmission of optical signals over long distances without the need for regenerative repeaters, allow simpler and more-reliable amplification than electronic systems, and permit more of the bandwidth of the optical fiber to be used. When used with wavelength-division multiplexing (WDM) technologies, these amplifiers can increase the information- carrying capacity of long-distance cables, which can be thousands of miles long.

The amplifier consists of optical fiber containing a small amount of a rare-earth element, typically erbium, which is spliced into the main signal-carrying fiber. Light of a different wavelength from a laser-diode pump source is then introduced into the doped fiber to excite the erbium ions. The signal passing down the main fiber stimulates the erbium ions to emit light. Amplification is achieved as energy from the pump source is transferred to the input signal.

Several years ago, Payne noted that although the progress in edfa technology has been rapid, its arrival has been too late to exploit the bandwidth of 40 million km of installed fiber already in the ground in technologically advanced nations. He noted that the challenge today is to apply the new fiber-amplifier technology to update existing transmission capacity.

There was a worldwide search for such an amplifier that used a special fluoride glass composition. However, most engineers regarded the reported pump efficiency of 4% as too low, and interest turned instead to upgrading the installed fiber base by operating it at a wavelength of 1.5 microns, where the edfa can be used.

According to Professor John Marsh in the Department of Electronics and Electrical Engineering at the University of Glasgow in Scotland, the praseodymium dopant used in 1.3-micron amplifiers is not efficient. More importantly, the amplifiers require a special fluoride glass as the host for the praseodymium, which is not easy to splice into the silica fiber.

Payne said that the strategy of operating at a wavelength of 1.5 microns in existing standard high-dispersion fiber, rather than the dispersion minimized fiber usually used at this wavelength, results in large pulse dispersion and therefore, a bandwidth penalty. Upgrading in this way limits the transmission span to 60 km at 10 Gbits/sec. At that time, he said that to increase the capacity of the installed fiber network involves the choice between developing a viable 1.3-micron amplifier or employing the edfa and developing compensation.

As it turned out, this was not a simple decision. Other factors such as distance and cost-effectiveness are important. As Marsh explains, "Much of the British Telecom trunk work in the United Kingdom was installed with 1.3-micron fiber, but has now moved to 1.5 microns because of lower losses and faster data rates; yet it uses few edfas because of high costs."

Fortunately, edfas are not necessary in the United Kingdom because major population centers are close together. But in Australia, for example, population centers can be thousands of miles apart, so there are different system considerations.

In addition to transmission systems, another important application of edfas is in broadcast distribution, such as in cable TV, where signal strength needs a boost as the light is divided down the distribution network to home and office subscribers.

"Edfas are going to become more important in WDM applications, regardless of dispersion-compensation effects," says Marsh. "If broadcasting signals are made at all wavelengths to all ends of a fiber system, then amplifiers are needed to compensate for the loss that occurs every time the signal is split."

To perform this amplification, the signal must be detected, processed digitally and then retransmitted. "If WDM is used at every repeater, then every wavelength is separated individually and retransmitted using a range of lasers for each wavelength." he adds. But by using an edfa--provided all the wavelengths lie between the gain spectrum of the amplifier--all the repeaters can be replaced by the edfa, and it will amplify all the wavelengths together.

However, there are still difficulties. For example, the gain of the amplifier must be flattened so that all the wavelengths get amplified equally. "There are a lot of technological tricks to get it to work well, but it looks promising," says Marsh. q

Dave Wilson writes from London.

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