EMMANUEL DESURVIRE, RANDY GILES AND DAVID PAYNE

The optical telecommunications sector took a giant leap forward between the mid-1980s and 1990s. During this period, devices called erbium-doped fibre amplifiers (EDFAs) were born, revolutionizing the world of long-distance communications and benefiting people worldwide.

The innovators behind the EDFA are Professor Emmanuel Desurvire, Dr. Randy Giles and Professor David Payne. Professor Payne at the University of Southampton and Professor Desurvire and Dr. Giles, both of whom were then working at Bell Labs, reported the first results of optical signal amplification in EDFAs, developed new computer models in order to optimize EDFA performance, and addressed a myriad of engineering challenges to make EDFAs practical. These outstanding technologists are to be thanked for the broadband Internet and voice connections that enable communications among people worldwide.

Emmanuel Desurvire has returned to France and is working as the head of Thales’ physics research unit in Palaiseau near Paris. "Research people are very hard to manage in a conventional sense, but I think I’m quite good at it, because I was once one of them. It’s very exciting to be in a position where I can inspire and motivate new generations of researchers."

Randy Giles continues to work at Bell Labs, now part of Alcatel-Lucent, and has achieved many advances in laser technology - including the first practical optical switch. One of his current projects is an ultra-small projector based on three small laser diodes.

David Payne is currently a director of the Optoelectronics Research Centre (ORC) at the University of Southampton in the UK. One of the world’s leading institutes for photonics research, ORC will be moving into a brand new building in the near future to replace an earlier lab that was destroyed by fire some years ago.

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Optical amplifier in wikipedia

Innovation

The principle behind the optical fibre amplifier is quite simple: take some optical fibre that incorporates optical material with special properties and, using a laser, target light on it. Optical amplifiers can also be considered to be lasers without the feedback. While a laser’s purpose is to generate coherent light; the optical amplifier boosts the actual quantity of light transmitted.

Physically, optical amplifiers are just a laser source (i.e. a laser diode or an array of laser diodes) and can be regarded as specially-doped optical fibre, reeled into a coil with the optical isolators and filters required to shepherd the light. For scientists the challenge was a threefold one: finding the right doping material, incorporating it into the fibre and constructing a suitable pump laser.

Amplification

The basic principle behind optical amplification has been known since the era of Albert Einstein. When certain dopant ions in a material are targeted using an intense laser source, their energy state jumps from lower (ground) to higher (excited). The ions spontaneously drop back to ground state by emitting the extra energy as a photon (or light quanta) that corresponds to the energy difference in levels.

When the correct energy levels are used, photons emitted by the dopant ions have the same wavelength as the signal light that needs to be amplified. If this signal is input to the medium then the excited ions are forced to release their emery by stimulated emission. The resulting output signal is therefore more powerful than the input signal. Amplification thus results from the stimulated emission of photons from dopant ions in the doped fibre. The dopant ions are maintained in the excited state by a pump laser, which creates an energy reservoir for the amplification process.

Erbium

Erbium (Er) is a chemical element with an atomic number of 68, making it one of the heaviest elements in the periodic table before the line of radioactive metals. Discovered by Carl Gustaf Mosander in 1843 in Ytterby, Sweden, the salts of this rare earth metal are rose-colored (which is used in artistic glassware). Apart from optical telecommunications, the 1.55 microns laser wavelength of erbium ions is has the “eye-safe” property, which allows many non-hazardous applications such as telemetry, laser imaging, and surgery for skin, eye and ear.

When the core of a silica fibre is doped with trivalent Erbium ions (Er+3) and is efficiently pumped with a laser at either 980 nm or 1480 nm, it exhibits gain in the 1550 nm region. Erbium was perfect for silica-based optical fibre communications, because standard single-mode optical fibres have minimal loss at wavelengths of 1525–1565 nm. Erbium also works very well at 1570-1610 nm, another widely-used transmission window.

The most recent version of the optical amplifier is the Raman amplifier, in which the coil of erbium-doped fibre can be much shorter than in a traditional Erbium amplifier. At 500 mW or more than 1 W of optical power, the pumping power required for Raman amplification is higher than that required by the traditional Erbium amplifiers. The principal advantage of Raman amplification is its ability to provide distributed amplification within the transmission fibre, thereby increasing the spans between amplifier and regeneration sites.

Applications

Underwater and terrestrial cables

Erbium-doped fibre amplifiers, which amplify signals in optical fibres, made it possible to dramatically expand the bandwidth of the fibre-optic networks carrying the vast majority of today's voice and data communication signals. The worldwide demand for voice and high-speed Internet connectivity and greater communications capacity is growing at an unrelenting pace made possible through the ubiquitous deployment of erbium-doped fibre amplifier technology.

Nowadays, optical amplifiers are widely used in all kinds of optical networks – both terrestrial and underwater. Their importance is increasing as optical cables gradually replace the older copper cables and domestic use expands. The smallest amplifiers are no bigger than a matchbox, while the underwater repeaters are several meters long.

“If you take a map of the world today and look at the oceanic cables, it resembles a spider’s web criss-crossing the world,” says David Payne. And erbium-doped fibre amplifiers are everywhere. “EDFAs are fairly ubiquitous today,” says Randy Giles. “Show me a fibre-optic cable above or below ground – it won’t take me long to find an erbium-doped fibre amplifier.”

Other applications

Optical amplifiers are used also in industries where high-power lasers are used for cutting, marking and machining, and amplifiers are widely used in surgical lasers.

Industrial applications are also increasing steadily as the power of the laser beams available increases. While lasers are of course used in heavy industry, they also enable many smaller businesses with to offer highly personalized services and allow small production series or special orders to be produced in an economic way.

New kinds of optical amplifiers will be needed when quantum computers and their associated optical systems arrive.

While optical amplifiers are now produced all over the world, only a few companies manufacture erbium-doped optical fibre. SPI Lasers, co-founded by Professor Payne, is one of these companies and also a leader in the manufacture of industrial and surgical lasers.