FEATURE DWDM AND ROADM NETWORKS


Learn from experience


As data demand ramps ever higher, researchers are looking to innovative amplifier designs to help transport a broader light spectrum through optical fibres, finds Andy Extance


U


nbeknown to most people, the erbium-doped fibre amplifier (EDFA) is a superhero that keeps the world talking and communicating. But


almost three decades aſter its invention, it is struggling to do the job alone. ‘Te internet wouldn’t exist without EDFAs,’


stressed Nick Doran, from Aston University, UK. Te world was ‘lucky that there were readily available amplifiers’ to boost optical communication signals in the right wavelength range, he says. In that range, we encode signals onto light beams whose colours are least susceptible to losses as the glass fibres they travel through absorb and scatter light. Yet still, the signals eventually grow faint, and so EDFAs boost them when necessary. ‘Without EDFAs we would be lost,’ Doran says. ‘But now, the next generation expects even more bandwidth, so we have to go beyond EDFAs.’ Tere are many strategies for keeping data


flowing, and avoiding a ‘capacity crunch’, Doran underlined. Different fibre types and encoding technologies, such as spatial multiplexing, boost spectral efficiency – the speed with which data can be transmitted over a given bandwidth. Tey do this by improving the optical signal-to-noise ratio (OSNR) of transmission, but a logarithmic physical relationship means that large OSNR improvements give only small spectral efficiency gains. By contrast, ‘if we double the bandwidth of our amplifiers we double the bandwidth of all systems,’ Doran stressed. Tat is a ‘factor of two or three’ more effective in increasing data flow, he adds. Terefore,


14 FIBRE SYSTEMS Issue 20 • Summer 2018


In March these systems, which include a hybrid EDFA/Distributed Raman Amplifier, were used in collaboration between Xtera and UCL that achieved 120Tb/s capacity on a single fibre


researchers are seeking a new generation of amplifiers to help – or even replace – EDFAs.


A hybrid solution But they won’t be retiring just yet. EDFAs have excellent amplification properties in the C-band from 1530nm-1565nm, observed Wayne Pelouch, vice president of photonics at Xtera in Allen, Texas. ‘Supplementing the EDFA with other amplification technologies can provide a better hybrid solution for many applications,’ he says. To provide a solution that can be industrialised, Xtera is therefore combining EDFAs with another existing technology, distributed Raman amplification, working together with University College London (UCL). In March 2018, the collaboration announced


a hybrid EDFA/Distributed Raman Amplifier achieving record-breaking 120Tb/s capacity on a single fibre. Te demonstration covered nine 70km spans, transmitting wavelengths from


1525nm-1615nm, covering the C-band and L-band. ‘Raman amplification provides both extended wavelength range and improved noise figure (NF), with relatively few additional optical components,’ Pelouch observed. ‘It is optimal for standard effective area (Aeff) fibre, which is less expensive. Very long fibre spans, for example 160km, can be supported within the C-band over 3,000km, or wide spectral bandwidth of 90nm can be supported at shorter 80km spans with high spectral efficiency over transatlantic distances.’ Raman amplification exploits how light very


occasionally excites molecules in a fibre to a higher energy state. Te molecules relax to their original state by releasing both vibrational energy and light. Because some energy is released as vibration, emitted light has less energy than the previously absorbed light, and is therefore a different colour. In silica fibre, the light wavelength emitted is around 100nm longer, and consequently a 1450nm pump laser


@fibresystemsmag | www.fibre-systems.com


Xtera


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