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19 July 2011
Researchers at Chalmers University of Technology in Sweden have demonstrated an optical amplifier with record low noise, which could extend the reach that optical signals can travel before needing amplification by a factor of four.  The phase-sensitive amplifier (PSA) has a noise figure of 1.1 dB; an improvement of 3dB compared to most erbium-doped fibre amplifiers (EDFAs).
“This is the ultimate optical amplifier,” claimed Professor Peter Andrekson, who led the research team working on device. “It enables connecting cities, countries, and continents more efficiently by placing the amplification hubs at much greater intervals.”  Bold claims, it must be said.
The key component of the PSA is a highly nonlinear singlemode fibre.  Amplification occurs by power transfer from a high-power pump laser (operating at a wavelength near the zero dispersion point of the fibre) to the signal wave by the process of four-wave mixing inside the nonlinear fibre. To make the device phase sensitive, an idler wave is injected, which has a constant phase with respect to the other two waves.
PSAs have been studied extensively before. In principle, noise-free amplification can be achieved in a device that amplifies the different phases separately, but it has taken what Andrekson describes as “a lot of engineering” to make an amplifier that approaches this ideal. 
The PSA developed by the team at Chalmers also uses a new, simplified configuration that doesn’t require an idler wave at each amplification stage.  The device can be extended to work with multiple wavelength channels with “modest system complexity”, according to the researchers.
Chalmers is looking at various specific commercial opportunities in the relative short term, both in telecom and other fields where very low light needs to be amplified.  The cost of such a device is likely to be higher than EDFAs, but should be offset by the savings from increased network performance.
The work was reported in the 21 June edition of Nature Photonics.
By Pauline Rigby