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What's the ultimate limit of speed with optical fibre?

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May 22, 2017

Here's a good question that some people may be wondering: what's the ultimate limit of speed today when optical fibre is concerned?

As with many other topics in the IT industry, the answer depends on who you ask the question. Fiber optics is a technology that was developed in the early 1990s, and looks like it will soon be improving submarine fibre speeds around the globe.

This comes from Dr Laurent Schmalen, department head at Nokia/Bell Labs's fibre division. Schmalen spoke about “constellation shaping” and the future of fibre research in the rapidly-changing communications industry.

He asserted that two questions are on Bell Labs' mind at the moment-- 1) what's the ultimate limit of fibre, and 2) what are the near-term speeds can we achieve over the long run?

In the short term however, Dr Schmalen asserted that the 'constellation shaping' technology it recently tested in conjunction with various social sites looks somewhat promising.

Dr Schmalen said that the testing was a culmination of technology we've developed over two-to-three years that's quite mature now.

To be sure, and by definition, constellation shaping seems counter-intuitive, simply because it delivers better performance from a specific channel by ignoring some of its available frequencies, some might comment.

Dr Schmalen then explained that the conventional use of constellations, in the QAM (quadrature amplitude modulation) 16 or QAM 64 that helped the 1990s-era modems to work, each so-called slot is used equally.

Some stars are more equal than other, he said. Constellation-shaping simply picks out the best one to utilize in the process.

In constellation shaping, we use the points that are closer to the middle of the constellation more frequently than those at the bottom, because bad points need much higher transmission power, he asserted.

Using the best points lets designers stretch the constellation at the same transmit energy. That he added, “makes the constellation more resilient against ambient noise”.

He then theorized that this comes back to Shannon-era information basics. Claude Shannon is noted for having founded information theory with a landmark paper: 'A mathematical theory of communication' that he published in 1948. He was also well known for founding digital circuit design back in 1937, when as a 21-year-old master's degree student at the Massachusetts Institute of Technology, he wrote his thesis demonstrating that electrical applications of Boolean algebra could construct any logical, numerical relationship.

Shannon also contributed to the field of cryptanalysis for national defense during World War II, including his fundamental work on codebreaking and secure telecommunications. Shannon had a reputation of being a visionary.

The downside of using the bad channels is that their need for high transmit power increases the overall noise floor of the system, including the noise observed in the good channels.

Dr Schmalen added: 'We can stretch the constellation at the same transmit energy, and make the constellation more resilient against noise. A small gain of just 1 dB, which improves the performance of existing systems could be interpreted as an improvment'.

The other reason so much research is going into constellation shaping today is that it only needs to be deployed at endpoints-– a very important consideration when you're looking at squeezing more out of existing submarine fibreoptic systems.

Dr Schmalen asserted: “We can simply upgrade existing systems because everything is done at the endpoints. The repeaters don't need to be changed”.

He also discussed a topic that's arisen from time to time-- IT technology researchers are now borrowing multiple-in, multiple-out (MIMO) techniques that have their roots in wireless technology, one of the most promising ways to improve the capacity of optical cable, he highlighted.

To be sure, this is a long-term goal because it does need fibre cables to be replaced over time. Today's cables only have a single physical path, and spatial multiplexing needs multiple paths to be truly efficient at higher speeds.

But it's also necessary, because Dense Wavelength Division Multiplexing (DWDM) is running close to capacity, mostly because we're very close to the practical speed limits of optical amplifiers.

Source: Nokia Bell Labs.

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