Single-mode fibres with low loss and a large transmission bandwidth are a key enabler for long-haul high-speed optical communication and form the backbone of our information-driven society. However, we are on the verge of reaching the fundamental limit of single-mode fibre transmission capacity. Therefore, a new means to increase the transmission capacity of optical fibre is essential to avoid a capacity crunch. Here, by employing few-mode multicore fibre, compact three-dimensional waveguide multiplexers and energy-efficient frequency-domain multiple-input multiple-output equalization, we demonstrate the viability of spatial multiplexing to reach a data rate of 5.1 Tbit s −1 carrier −1 (net 4 Tbit s −1 carrier −1 ) on a single wavelength over a single fibre. Furthermore, by combining this approach with wavelength division multiplexing with 50 wavelength carriers on a dense 50 GHz grid, a gross transmission throughput of 255 Tbit s −1 (net 200 Tbit s −1 ) over a 1 km fibre link is achieved. W ith the persistent exponential growth in Internet-driven traffic, the backbone of our information-driven society, based on single-mode fibre (SMF) transmission, is rapidly approaching its fundamental capacity limits 1 . In the past, capacity increases in SMF transmission systems have been achieved by exploiting various dimensions, including polarization and wavelength division multiplexing, in tandem with advanced modulation formats and coherent transmission techniques 2 . However, the impending capacity crunch implies that carriers are lighting up dark fibres at an exponentially increasing rate to support societal capacity demands 3 . To alleviate the corresponding costs and increased energy requirements associated with the linear capacity scaling from using additional SMFs, spatial division multiplexing (SDM) within a single fibre can provide a solution 4,5 . By introducing an additional orthogonal multiplexing dimension, the capacity, spectral and energy efficiency, and therefore the cost per bit, can be decreased, which is vital for sustaining the business model of major network stakeholders. To fulfil the SDM promise, a new paradigm is envisaged that allows up to two orders of magnitude capacity increase with respect to SMFs 6 . SDM is achieved through multiple-input multiple-output (MIMO) transmission, employing the spatial modes of a multimode fibre (MMF) 7 , or multiple single-mode cores, as channels 8-13 . Recently, a distinct type of MMF, the few-mode fibre (FMF), has been developed to co-propagate three or six linear polarized (LP) modes 14-17 . Driven by rapid enhancements in high-speed electronics, digital signal processing (DSP) MIMO techniques can faithfully recover mixed transmission channels 18 , allowing spectral efficiency increases as spatial channels occupy the same wavelength. State-of-the-art single-carrier FMF transmission experiments have demonstrated capacity increases in a single fibre by exploiting six spatial modes, achieving 32 bit s −1 Hz −1 spectral efficiency 17 . By using multicore transmissi...
Abstract:Space-division multiplexing (SDM), whereby multiple spatial channels in multimode 1 and multicore 2 optical fibres are used to increase the total transmission capacity per fibre, is being investigated to avert a data capacity crunch 3,4 and reduce the cost per transmitted bit. With the number of channels employed in SDM transmission experiments continuing to rise, there is a requirement for integrated SDM components that are scalable. Here, we demonstrate a cladding-pumped SDM erbium-doped fibre amplifier (EDFA) that consists of six uncoupled multimode erbium-doped cores. Each core supports three spatial modes, which enables the EDFA to amplify a total of 18 spatial channels (six cores × three modes) simultaneously with a single pump diode and a complexity similar to a singlemode EDFA. The amplifier delivers >20 dBm total output power per core and <7 dB noise figure over the C-band. This cladding-pumped EDFA enables combined space-division and wavelength-division multiplexed transmission over multiple multimode fibre spans.The bandwidth demands for optical networks are growing exponentially and will soon exceed the maximum achievable capacity of a single-mode fibre (SMF) due to fibre nonlinearities 4 , resulting in a 'capacity crunch' 3 in the near future. The simplest way to avert the 'capacity crunch' is to use multiple SMF transmission systems in parallel. However, this most basic form of SDM requires duplication of all the optical amplifiers, reconfigurable optical add-drop multiplexers and transponders for each additional fibre, and is subject to linear scaling of cost and complexity. Alternatively, SDM that uses the spatial
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