Fiber optic cables have long been the arteries of global communication - but still have weaknesses. (Image: Thinkstock)
Reading aloud They connect continents over thousands of kilometers, but are only a few centimeters thick. Fiber optic cables have long been the backbone of our global information society. 15 million simultaneous phone calls or the content of 250 DVDs in one second are no problem for the fast-moving ladder. On very long distances, however, there are still strong losses in the transmission over optical fibers. Because physical effects ensure that the light signal attenuates quickly and is superimposed by interference. A new method to reduce distortion and signal loss over long distances has now been tested by Bell Labs researchers in New Jersey. They were able to transmit data at 400 megabits per second over 12, 800 kilometers - ten times further than previously possible. Fiber optic cables have long been the method of choice when it comes to sending large amounts of digital data. "They've made the exponential growth in communications capacity possible, " said Xiang Liu of Bell Labs and his colleagues. With transmission rates of hundreds of megabits to even terabits per second, optical fibers are already superior to satellites in terms of data capacity and transmission speed.

The principle of optical transmission is relatively simple: The actual conductor usually consists of a quartz glass fiber, which is surrounded by a jacket of less refractive material. At the beginning of the optical fiber link, a light emitting diode or laser diode converts the incoming electrical signals into light waves. The light enters the core of the fiber and expands. It is reflected by the walls of the core and thereby kept on track. At the other end, the receiving photodiode converts it back into electrical signals. The big advantage is that unlike electrons in the copper cable, the light moves almost without resistance. It therefore hardly loses momentum in thousands of kilometers of glass fiber cables and is still traveling at almost the speed of light there. In addition, the different wavelengths of light can be provided with different information, which then go on the journey at the same time.

Moved copy as equalization helper

Unfortunately, the whole thing has a catch: Due to various physical effects, the light signal in the fiber is weakened and distorted relatively quickly. Depending on the wavelength and data rate, the range of such a transmission is therefore very limited. To bridge longer distances, the signal must be modified at regular intervals along the line by amplifiers and signal corrections. Liu and his colleagues have now tested a method to reduce or effectively filter out distortions. display

Their trick: they sent their signal not in a ray of light on the journey, but in two copies. These were sent but slightly offset, so that the waves do not swing in common mode, but exactly against each other: Whenever one reached a high, the other had a low. The disturbing effects acting at a certain time trigger slightly different distortions for the two waves. "This allows us to filter out the interference by re-aligning and superimposing the waves at the end of the signal path, " the researchers explain. Disorders then appear as rashes that do not match in both. If you remove these, the original form of the signal can be reconstructed. This method reduces nonlinear distortion by more than 8.5 decibels, the researchers said.

What that means in practice was demonstrated by Liu and his colleagues in a long-distance trial. They sent eight such shifted pairs of signals with a data rate of 400 megabits per second in succession through a loop out of an 80-kilometer-long fiber-optic cable. As it turned out, they were able to extract the signal from the noise even after traveling a total of 12, 800 kilometers in this test track.

Xiang Liu (Bell Labs, Holmdel) et al., Nature Photonics, doi: 10.1038 / nphoton.2013.109 © - === Nadja Podbregar


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