Lightwave communications is a necessity for the information age. Optical links provide enormous bandwidth, and the optical fiber is the only medium that can meet the modern society's needs for transporting massive amounts of data over long distances. Applications range from global highcapacity networks, which constitute the backbone of the internet, to the massively parallel interconnects that provide data connectivity inside datacenters and supercomputers. Optical communications is a diverse and rapidly changing field, where experts in photonics, communications, electronics, and signal processing work side by side to meet the ever-increasing demands for higher capacity, lower cost, and lower energy consumption, while adapting the system design to novel services and technologies. Due to the interdisciplinary nature of this rich research field, Journal of Optics has invited 16 researchers, each a world-leading expert in their respective subfields, to contribute a section to this invited review article, summarizing their views on state-of-the-art and future developments in optical communications.

The use of multiple laser transmitters combined with multiple photodetectors (PDs) is studied for terrestrial , line-of-sight optical communication. The resulting multiple -input/multiple-output channel has the potential for combatting fading effects on turbulent optical channels. In this paper, the modulation format is repetition-ary PPM across lasers, with intensity modulation. Ideal PDs are assumed, with and without background radiation. Both Rayleigh and log-normal fading models are treated. The focus is upon both symbol-/bit-error probability for uncoded transmission, and on constrained channel capacity. Index Terms-multiple-input/multiple-output (MIMO) processing , optical communication, optical modulation/demodulation.

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Abstract-We study the use of multiple laser transmitters combined with multiple photodetectors for atmospheric, line-of-sight optical communication, and focus upon the use of multiple-pulseposition-modulation as a power-efficient transmission format, with signal repetition across the laser array. Ideal (photon counting) photodetectors are assumed, with and without background radiation. The resulting multiple-input/multiple-output channel has the potential for combating fading effects on turbulent optical channels, for which both log-normal and Rayleigh-fading models are treated. Our focus is upon symbol error probability for uncoded transmission, and on capacity for coded transmission. Full spatial diversity is obtained naturally in this application.Index Terms-Multiple-input/multiple-output (MIMO) systems, optical communications, pulse-position-modulation (PPM).

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In this paper, we study the joint resource allocation in flexible-grid networks based on a nonlinear physical layer impairment model. An optimization problem is formulated to assign resources and guarantee the signal quality for every channel. Compared with the resource allocation in fixed-grid wavelength-division multiplexing scenario, our method achieves significant bandwidth reduction and transmission distance extension in flexible-grid networks. The maximum spectrum usage is shown to be insensitive to the ordering of channels. We also analyze the relation between modulation formats and transmission distance based on the results of the proposed method. Finally, we demonstrate the performance and scalability of the proposed algorithm in ring networks.

A multistage multiuser detector (MMD) is presented for frequency hopping/code division multiple access (FH/CDMA.) The MMD reduces the bit error rate over the conventional detector by exploiting prior knowledge of the addresses and energies of the users. This detector is a conservative multiuser detector which is robust to unknown users and is only linear in complexity in the number of users. The performance analysis of the synchronous MMD (SMMD) includes both theoretical and simulation bit error rates for the noiseless case and simulation results in the presence of noise. The MMD is then extended to the asynchronous case. This asynchronous multistage multiuser detector (AMMD) is compared via simulation to the conventional detector.