This Review summarizes the simultaneous transmission of several independent spatial channels of light along optical fibres to expand the data-carrying capacity of optical communications. Recent results achieved in both multicore and multimode optical fibres are documented.

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Space-division multiplexing in optical fibres

Orthogonal frequency division multiplexing (OFDM) is a modulation technique which is now used in most new and emerging broadband wired and wireless communication systems because it is an effective solution to intersymbol interference caused by a dispersive channel. Very recently a number of researchers have shown that OFDM is also a promising technology for optical communications. This paper gives a tutorial overview of OFDM highlighting the aspects that are likely to be important in optical applications. To achieve good performance in optical systems OFDM must be adapted in various ways. The constraints imposed by single mode optical fiber, multimode optical fiber and optical wireless are discussed and the new forms of optical OFDM which have been developed are outlined. The main drawbacks of OFDM are its high peak to average power ratio and its sensitivity to phase noise and frequency offset. The impairments that these cause are described and their implications for optical systems discussed.

OFDM for Optical Communications

Optical wireless communication (OWC) refers to transmission in unguided propagation media through the use of optical carriers, i.e., visible, infrared (IR), and ultraviolet (UV) bands. In this survey, we focus on outdoor terrestrial OWC links which operate in near IR band. These are widely referred to as free space optical (FSO) communication in the literature. FSO systems are used for high rate communication between two fixed points over distances up to several kilometers. In comparison to radio-frequency (RF) counterparts, FSO links have a very high optical bandwidth available, allowing much higher data rates. They are appealing for a wide range of applications such as metropolitan area network (MAN) extension, local area network (LAN)-to-LAN connectivity, fiber back-up, backhaul for wireless cellular networks, disaster recovery, high definition TV and medical image/video transmission, wireless video surveillance/monitoring, and quantum key distribution among others. Despite the major advantages of FSO technology and variety of its application areas, its widespread use has been hampered by its rather disappointing link reliability particularly in long ranges due to atmospheric turbulence-induced fading and sensitivity to weather conditions. In the last five years or so, there has been a surge of interest in FSO research to address these major technical challenges. Several innovative physical layer concepts, originally introduced in the context of RF systems, such as multiple-input multiple-output communication, cooperative diversity, and adaptive transmission have been recently explored for the design of next generation FSO systems. In this paper, we present an up-to-date survey on FSO communication systems. The first part describes FSO channel models and transmitter/receiver structures. In the second part, we provide details on information theoretical limits of FSO channels and algorithmic-level system design research activities to approach these limits. Specific topics include advances in modulation, channel coding, spatial/cooperative diversity techniques, adaptive transmission, and hybrid RF/FSO systems.

/pdf/survey-on-free-space-optical-communication-a-communication-239v68hfyw.pdf

Survey on Free Space Optical Communication: A Communication Theory Perspective

Digital coherent receivers have caused a revolution in the design of optical transmission systems, due to the subsystems and algorithms embedded within such a receiver. After giving a high-level overview of the subsystems, the optical front end, the analog-to-digital converter (ADC) and the digital signal processing (DSP) algorithms, which relax the tolerances on these subsystems are discussed. Attention is then turned to the compensation of transmission impairments, both static and dynamic. The discussion of dynamic-channel equalization, which forms a significant part of the paper, includes a theoretical analysis of the dual-polarization constant modulus algorithm, where the control surfaces several different equalizer algorithms are derived, including the constant modulus, decision-directed, trained, and the radially directed equalizer for both polarization division multiplexed quadriphase shift keyed (PDM-QPSK) and 16 level quadrature amplitude modulation (PDM-16-QAM). Synchronization algorithms employed to recover the timing and carrier phase information are then examined, after which the data may be recovered. The paper concludes with a discussion of the challenges for future coherent optical transmission systems.

Digital Coherent Optical Receivers: Algorithms and Subsystems

Orthogonal Frequency Division Multiplexing (OFDM) has recently been proposed as a modulation technique for optical networks, because of its good spectral efficiency, flexibility, and tolerance to impairments. We consider the planning problem of an OFDM optical network, where we are given a traffic matrix that includes the requested transmission rates of the connections to be served. Connections are provisioned for their requested rate by elastically allocating spectrum using a variable number of OFDM subcarriers and choosing an appropriate modulation level, taking into account the transmission distance. We introduce the Routing, Modulation Level and Spectrum Allocation (RMLSA) problem, as opposed to the typical Routing and Wavelength Assignment (RWA) problem of traditional WDM networks, prove that is also NP-complete and present various algorithms to solve it. We start by presenting an optimal ILP RMLSA algorithm that minimizes the spectrum used to serve the traffic matrix, and also present a decomposition method that breaks RMLSA into its two substituent subproblems, namely 1) routing and modulation level and 2) spectrum allocation (RML+SA), and solves them sequentially. We also propose a heuristic algorithm that serves connections one-by-one and use it to solve the planning problem by sequentially serving all the connections in the traffic matrix. In the sequential algorithm, we investigate two policies for defining the order in which connections are considered. We also use a simulated annealing meta-heuristic to obtain even better orderings. We examine the performance of the proposed algorithms through simulation experiments and evaluate the spectrum utilization benefits that can be obtained by utilizing OFDM elastic bandwidth allocation, when compared to a traditional WDM network.

https://www.ceid.upatras.gr/webpages/faculty/manos/kchristodou/journals/ofdm_jlt_revision_2.pdf

Elastic Bandwidth Allocation in Flexible OFDM-Based Optical Networks

The presence of mode-dependent loss (MDL) in mode-division multiplexed systems detrimentally impacts the system capacity. We review different techniques for increasing the MDL tolerance.

Mode-Dependent-Loss Mitigation for Mode-Division Multiplexed Systems

We experimentally verified the effectiveness of a novel digital pre-emphasis technique to compensate limited DAC bandwidth for next generation flexible systems. A gain larger than 1dB/0.1nm OSNR at the 7%-FEC limit has been experimentally verified for 8, 16 and 32QAM.

Low-complexity digital pre-emphasis technique for next generation optical transceiver

Employing three subcarriers per terabit of 37GBd serial line-rate PM-64QAM modulated signals, we report 32Tb/s C-band transmission over ∼1000km. Large Area Pure Silica Core Fiber (LA-PSCF) and hybrid EDFA-Raman amplification were used to achieve a SE of 6.67bit/s/Hz.

32 × 1Tb/s C-band transmission over 1000km employing three subcarriers PM-64QAM for regional applications

We review the applications of fiber nonlinearity compensation (NLC) in homogeneous and heterogeneous networks, (non)dispersion-managed links, and terabit transmission systems. Practically viable intra-channel NLC is shown to allow ∼3.7dB gains, well beyond conventionally acknowledged bounds.

Fiber nonlinearity compensation: Practical use cases and complexity analysis

Disclosed herein is a modulator ( 50 ) for polarization-division multiplexing (PDM) transmission. The modulator ( 50 ) comprises first and second DP-MZMs ( 12, 28 ) associated with first and second polarizations, each DP-MZM ( 12, 28 ) having an input for an in-phase and a quadrature driving signal for modulating the in-phase and quadrature components of an optical signal according to respective transfer functions, and a detector ( 58 ) suitable for detecting light comprising at least a portion of the light outputted by the first DP-MZM ( 12 ) and a portion of the light outputted by the second DP-MZM ( 28 ). The modulator ( 50 ) is adapted to superimpose a first pilot signal on one of the in-phase and quadrature driving signals of the first DP-MZM ( 12 ) and on one of the in-phase and quadrature driving signals of the second DP-MZM ( 28 ), and a second pilot signal on the respective other of the in-phase and quadrature driving signals of the first and second DP-MZMs ( 12, 28 ). Further, the first and second pilot signals are chosen such that the signal detected by said detector ( 58 ) is indicative as to whether the slopes of the transfer functions are different for the in-phase and quadrature components of one of the first and second DP-MZMs ( 12, 28 ) and identical for the other of the first and second DP-MZMs ( 12, 28 ).

Bernhard Spinnler

Papers

Mode-Dependent-Loss Mitigation for Mode-Division Multiplexed Systems

Low-complexity digital pre-emphasis technique for next generation optical transceiver

32 × 1Tb/s C-band transmission over 1000km employing three subcarriers PM-64QAM for regional applications

Fiber nonlinearity compensation: Practical use cases and complexity analysis

Spectral inversion detection for polarization-division multiplexed optical transmission