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A group of researchers led by a Monash Univeristy PhD student has demonstrated an all-optical technique for dealing with nonlinearity – something that considerably boosts the throughput of an optical system.

The demonstration is important for two reasons. One is that fibre optic cables used to transport signals over long distances need occasional boosters. As you'll read below, this demonstration renders those unnecessary, a boon for network operators. The second is speed. At 1.8 Tbps on a single fibre path, this technology has the potential to upgrade even modest extant fibre systems (that typically have many such paths) to hundreds of terabits per second.

The technique that makes this all possible is mid-span spectral inversion (MSSI), and has been known since the 1990s. By inverting the spectrum of the optical signal – that is, changing the long wavelengths to short and vice-versa – MSSI adds a “pre-emphasis”, shaping the signal in such a way that non-linearities in the second half of the fibre “reverse” the effect of non-linearities in the first half.

In other words, by reversing the time order of the wave-front, the fibre's nonlinearity – which distorts what started out as a nice optical pulse – helps reconstruct the original pulse.

The downside is that this usually requires active electronics to be inserted into the path, which may not always be convenient.

In his paper, Monash PhD student Monir Morshed, working with collaborators from CUDOS in Sydney and Melbourne, describes an all-optical MSSI implementation, eliminating the active electronics. Instead, an all-photonic optical phase conjugation unit (the bit that performs the inversion) was inserted at the midpoint of the total 800 km link.

By reversing the wavefront, the optical nonlinearities

that distort the signal help restore it

The paper, which was deemed important enough to be accepted as a post-deadline presentation at the recent Kyoto OptoElectronics and Communications Conference (OECC), states: “Using 1.21 Tbit/s through 10×80-km fibers with EDFA amplification we show that MSSI improves the nonlinear threshold by 2.8 dB”.

For an 800 km link on a single fibre, Monash University claims the technique offers more than ten times the capacity of current systems.

Because it's an all-optical approach, the key component – the optical phase conjugator – could be retrofitted to any suitable fibre. ®

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