The performance of direct-detection transceivers employing electronic dispersion compensation combined with DSP-based receiver linearization techniques is assessed through experiments on a 4 × 112 Gb/s wavelength-division multiplexing direct-detection single-sideband 16 quadratic-amplitude modulation Nyquist-subcarrier-modulation system operating at a net optical information spectral density of 2.8 b/s/Hz in transmission over standard single mode fiber links of up to 240 km. The experimental results indicate that systems with receiver-based dispersion compensation can achieve similar performance to those utilizing transmitter-based dispersion compensation, provided it is implemented together with an effective digital receiver linearization technique. The use of receiver-based compensation would simplify the operation of a fiber link since knowledge of the link dispersion is not required at the transmitter. The recently proposed Kramers-Kronig receiver scheme was found to be the best performing among the receiver linearization techniques assessed. To the best of our knowledge, this is the first experimental demonstration of the Kramers-Kronig scheme.

Abstract-A new decision-directed (DD) synchronization scheme is proposed for joint estimation of carrier frequency offset (CFO) and sampling clock frequency offset (SFO) in orthogonal frequency-division multiplexing (OFDM) systems. By exploiting the hard decisions, we report accurate estimators of residual CFO and small SFO. The performance analysis and simulation results indicate that the proposed novel DD scheme achieves much better performance than the conventional pilot-based schemes in both additive white Gaussian noise and frequency-selective channels.Index Terms-Carrier frequency offset (CFO), fast Fourier transform (FFT), orthogonal frequency-division multiplexing (OFDM), sampling clock frequency offset (SFO), synchronization.

The achievable transmission capacity of conventional optical fibre communication systems is limited by nonlinear distortions due to the Kerr effect and the difficulty in modulating the optical field to effectively use the available fibre bandwidth. In order to achieve a high information spectral density (ISD), while simultaneously maintaining transmission reach, multi-channel fibre nonlinearity compensation and spectrally efficient data encoding must be utilised. In this work, we use a single coherent super-receiver to simultaneously receive a DP-16QAM super-channel, consisting of seven spectrally shaped 10GBd sub-carriers spaced at the Nyquist frequency. Effective nonlinearity mitigation is achieved using multi-channel digital back-propagation (MC-DBP) and this technique is combined with an optimised forward error correction implementation to demonstrate a record gain in transmission reach of 85%; increasing the maximum transmission distance from 3190 km to 5890 km, with an ISD of 6.60 b/s/Hz. In addition, this report outlines for the first time, the sensitivity of MC-DBP gain to linear transmission line impairments and defines a trade-off between performance and complexity.

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We investigate the effect of varying the DSP resampling rate and the carrier-to-signal power ratio on the performance of direct-detection Kramers-Kronig receivers, through experiments on 4×112 Gb/s SSB Nyquist-SCM transmission over 240 km.

The rapid growth of data transferred within data centres, combined with the slowdown in Moore's Law, creates challenges for the future scalability of electronically-switched data-centre networks. Optical switches could offer a future-proof alternative and photonic integration platforms have been recently demonstrated with nanosecond-scale optical switching times. End-to-end switching time, however, is currently limited by the clock and data recovery time, which typically takes microseconds, removing the benefits of nanosecond optical switching. Here we show a clock phase caching technique that can provide clock and data recovery times of under 625 ps (16 symbols at 25.6 Gb/s). Our approach uses the measurement and storage of clock phase values in a synchronised network to simplify clock and data recovery versus conventional asynchronous approaches. We demonstrate our technique using a real-time prototype with commercial transceivers and validate its resilience against temperature variation and clock jitter, based on measurements from a production cloud data centre. Main T he rate of data transmitted between servers within data centres has rapidly increased over the last few years [1], driven by cloud adoption and data-intensive cloud workloads such as data analytics and machine learning. Cloud providers have been able to accommodate this fast growth by relying on Moore's Law for networking: every two years the electronic switch integrated circuits (ICs) double their bandwidth at same cost and power. The long-term sustainability of this trend, however, is being questioned by two upcoming challenges: Firstly, similar to processor ICs, scaling transistor density on electronic switch ICs is fundamentally limited by power dissipation as few-nm transistor sizes are approached [2]. Secondly, electronic high-speed serial transceiver data rates are predicted to be hard to scale beyond 112 Gb/s due to the steep increase in dielectric loss when operating at high frequencies [3, 4]. Consequently, increasing the aggregate switch capacity will require a proportional increase in the number of serial transceivers surrounding the chip, resulting in greater power density and packaging complexity. Although continued bandwidth scaling in the near future could be supported by architectural optimisations such as co-packaged optics [5], preserving cost neutrality in the medium-to-long term appears very challenging. This uncertainty has motivated research in optical switches as a viable alternative to electronic switches [6]. Optical switches simply redirect the incoming signals onto output ports without any optical/electronic conversion or digital processing and, hence, they do not suffer from the limitations of transistor or transceiver technology. They could, therefore, provide a future-proof solution for bandwidth scaling within the data centre [7].

A major cause of faults in optical communication links is related to unintentional third party intrusions (normally related to civil/agricultural works) causing fiber breaks or cable damage. These intrusions could be anticipated and avoided by monitoring the dynamic strain recorded along the cable. In this work, a novel technique is proposed to implement real-time distributed strain sensing in parallel with an operating optical communication channel. The technique relies on monitoring the Rayleigh backscattered light from optical communication data transmitted using standard modulation formats. The system is treated as a phase-sensitive OTDR (ΦOTDR) using random and non-periodical non-return-to-zero (NRZ) phase-shift keying (PSK) pulse coding. An I/Q detection unit allows for a full (amplitude, phase and polarization) characterization of the backscattered optical signal, thus achieving a fully linear system in terms of ΦOTDR trace coding/decoding. The technique can be used with different modulation formats, and operation using 4 Gbaud single-polarization dual PSK and 4 Gbaud dual-polarization quadrature PSK is demonstrated. As a proof of concept, distributed sensing of dynamic strain with a sampling of 125 kHz and a spatial resolution of 2.5 cm (set by the bit size) over 500 m is demonstrated for applied sinusoidal strain signals of 500 Hz. The limitations and possibilities for improvement of the technique are also discussed.