A new method of processing signals via fiber optic cables could vastly increase the distance at which error-free data is transmitted via submarine cables without additional signal amplification. As the technique is capable of correcting corrupted or distorted data being transmitted, it may also assist in increasing the capacity of all optical fiber communications.

With demand for internet connectivity running at an all time high – and only increasing – the fiber optic cables over which much of the data flows draw ever-closer to reaching capacity. Short of laying more cables, growing demand is being increasingly met by boosting the number of available frequency channels on which the data, in the form of encoded light signals, is transmitted. This is often achieved using a variety of compression and error-correction techniques, as well as employing methods designed to overcome nonlinearity in long lengths of optical fiber.

Unfortunately, given that many of these techniques are reaching the limit of their capaabilities to transmit and receive light without adjacent light signal overlap and subsequent interference, data is often received with distortion errors.

To address this problem, researchers from University College London (UCL) have created a way of avoiding such interference by using a set of frequencies that are coded using amplitude, phase, and frequency to create an exceptionally large-capacity, high-quality optical signal.

In detail, the research team used a laser to generate the optical carrier and passed this through a comb generator to form seven equidistantly-spaced, frequency locked signals in the form of Quadrature Amplitude Modulation (QAM) – specifically a 16QAM super-channel. This super-channel signal was then introduced into one end of a fiber optic cable and captured at the other end with a high-speed super-receiver. Employing a range of new signal processing techniques specifically developed by the researchers, reception of all the channels was received intact and without error.

"By eliminating the interactions between the optical channels, we are able to double the distance signals can be transmitted error-free, from 3,190 km (1,982 mi) to 5,890 km (3,660 mi), which is the largest increase ever reported for this system architecture," said Dr Robert Maher of UCL's Electronic & Electrical Engineering department. "The challenge is to devise a technique to simultaneously capture a group of optical channels, known as a super-channel, with a single receiver. This allows us to undo the distortion by sending the data channels back on a virtual digital journey at the same time."

Using the full 70 GHz bandwidth available to the optical super-channel, the maximum reach the system was able to achieve was an experimentally-confirmed 5,890 km transmission distance equivalent. This is the largest distance gain ever reported that incorporates digital back-propagation.

Following on from the successful testing of the new method, the researchers now intend to conduct research on incorporating these techniques in denser super-channels commonly used in digital cable TV, cable modems, and Ethernet connections.

“We’re excited to report such an important finding that will improve fiber optic communications," said Professor Polina Bayvel, also of UCL Electronic & Electrical Engineering. "Our method greatly improves the efficiency of transmission of data – almost doubling the transmission distances that can be achieved, with the potential to make significant savings over current state-of-the art commercial systems. One of the biggest global challenges we face is how to maintain communications with demand for the internet booming – overcoming the capacity limits of optical fiber cables is a large part of solving that problem.”

The results of the research have been published in the journal Scientific Reports.

Source: UCL

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