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Off-the-shelf optics kit tweaked for bonkers performance

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A couple of Australian optics labs have joined up with vendor Finisar to demonstrate an energy-efficient optical system transmitting 10 Tbps over 850 km.

On its own, 10 Tbps isn't anything to crow about: terabit systems are, after all, routine in the long-haul market. However, doing it on one fibre and cutting down the energy – and achieving the result with off-the-shelf kit – is another matter.

By way of comparison, back in 2011 ZTE indulged in a certain amount of chest-beating that it had sent 10 Tbps down 640 Km of single fibre – and to do that, it needed 112 power-hungry lasers and patented carrier generation technology. The CUDOS / Monash University demonstration achieved its result using just one laser; even though it's pulsed at higher power, it yields an overall power saving to get the same performance over a greater distance.

Conventional WDM systems need a guard band between wavelengths, and since today's photonic systems occupy the 1530–1565 nanometre band, gaps between bands represent wasted capacity, Monash University's professor Arthur Lowery told The Register, the need for that band gap is eliminated.

Optical OFDM is a popular research angle at the moment, Professor Lowery noted: both Melbourne University and Monash submitted papers on the topic to last week's Optical Fiber Communications conference in California (the CUDOS / Monash work was accepted as a post-deadline paper).

Getting performance at this level, said Jochen Schroeder of Sydney University's CUDOS facility, was a three step process: broaden the pulsed laser spectrally (to increase its bandwidth); split the resulting pulse into multiple pulses; and then modulate each of the resulting pulses.

At the heart of the experiment was the use of standard equipment – a Finisair Wavelength Selective Switch (WSS) – doing something that might have been unexpected for its manufacturer.

“We use the WSS to perform a special filter function, equivalent to a discrete fourier transform. It's a switching device used for WDM to switch different WDM channels to different fibre outputs – we've adjusted this to work for OFDM,” explained Schroeder.

He likened the use of the WSS to how FPGAs are used in the world of electronics: it provides a reprogrammable platform. “We can create optical reconfigurable optical circuits, creating an optical circuit that emulates the functionality of a discrete fourier transform,” he said.

There's one more energy saving worth mentioning, Schroeder added: the OFDM is carried out entirely in the optical domain. In most current optical OFDM work, the modulation is carried out in electrical circuits and the result encoded onto the optical carrier, but in the CUDOS / Monash experiment, “the fourier transform, the parallel-serial conversion, cyclic prefix insertion – we do all this in optics”. ®

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