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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. A few-mode, multicore fibre allows ultra-high-speed data transmission on a single wavelength of light.

A photonic integrated mode coupler based on silicon-on-insulator is employed for mode division multiplexing (MDM) over a 193 m 19-cell hollow-core photonic bandgap fibre (HC-PBGF) with a −3 dB bandwidth >120 nm. Robust MDM transmissions using LP01 and LP11 modes, and two degenerate LP11 modes (LP11a and LP11b) are both experimentally verified.

A 40GHz clock with low jitter is recovered from a 640Gbit/s optical time-division multiplexed (OTDM), single-polarised, return-to-zero PRBS data signal. The clock recovery circuit is an injection-locked loop which contains a 40GHz Gunn oscillator, a 40GHz pulse source, and a semiconductor optical amplifier (SOA) assisted by a filtered chirp as optical phase comparator.

An all-optical flip-flop memory with separate set and reset inputs is presented. The flip-flop is realised from two coupled polarisation switches. The concept is explained and experimental results are presented that demonstrate that a contrast ratio of 20 dB and a switching power of < -3 dBm can be obtained. The all-optical flip-flop can be utilised in all-optical packet switches.

Error-free demultiplexing of 40 Gbit/s channels out of a 160 Gbit/s optical time-division signal is demonstrated using four-wave mixing in a semiconductor optical amplifier.