Randomly-coupled multi-core fiber (MCF) technology has come to attract lots of attention because of its strong applicability to long-haul transmission systems. Compared with weakly-coupled MCFs with independent cores, it can simultaneously realize higher spatial channel density and ultralow transmission loss using existing ultralow-loss single-mode fiber (SMF) core designs. The strong mode coupling characteristics of randomly-coupled MCFs can provide favorable optical properties, such as suppressed accumulation of modal dispersion (MD), mode-dependent loss (MDL), and nonlinear impairments. This article gives an overview of randomly-coupled MCF technology advancements. First, we describe the classification and design of randomly-coupled MCFs and explain what the randomly-coupled MCFs are and how they are designed. State-of-the-art randomly-coupled MCFs can accommodate four, seven, or 12 cores in a standard 125-$\mu \text{m}$cladding while achieving ultralow transmission loss and/or small MD, which are very promising for long-haul transmission media. Next, we present the methods to characterize the optical properties of randomly-coupled MCFs and the difference compared to conventional SMF measurements. We also show the low-loss low-MDL connectivity of this type of MCF and the cabling that can suppress MD. A field-deployed randomly-coupled MCF cable testbed is also presented, which confirmed the favorable optical properties of randomly-coupled MCFs after deployment. Then, multi-core amplifier technologies are briefly summarized, and finally, we discuss the performance improvements in the transmissions over randomly-coupled MCFs and suitable application areas.

It is well established that physical layer impairments significantly affect the performance of optical networks. The management of these impairments is critical for successful transmission, and may significantly affect network layer routing decisions. Hence, the traditional divide-and-conquer layered approach is sub-optimal, which has led to work on cross-layer techniques for routing in optical networks. Apart from fiber loss, one critical physical layer impairment that limits the capacity of optical networks is fiber nonlinearity. Handling nonlinearity introduces significant complexity to the traditional cross-layer approaches. We formulate and solve a joint routing and power control problem to optimize the system throughput that takes into consideration both fiber loss and nonlinearity. The joint power control and routing problem considered is a nonlinear integer programming problem. By characterizing the feasible solution space of the power control problem, we find a set of universal power settings that transform the complex power control and routing problem into a constrained path routing problem. We then propose an efficient fully polynomial time approximation scheme to solve the constrained path routing problem. Simulation results show that our proposed algorithm significantly improves network throughput and greatly outperforms greedy heuristics by providing a guaranteed performance bound.

It is well established that physical layer impairments significantly affect the performance of optical networks. The management of these impairments is critical for successful transmission, and may significantly affect network layer routing decisions. Hence the traditional divide-and-conquer layered approach is sub-optimal, which has led to work on cross-layer techniques for routing in optical networks. Apart from fiber loss, one critical physical layer impairment that limits the capacity of optical networks is fiber nonlinearity. Handling nonlinearity introduces significant complexity to the traditional cross-layer approaches. We formulate and solve a joint routing and power control problem to optimize the system throughput that takes into consideration both fiber loss and nonlinearity. The joint power control and routing problem considered is a nonlinear integer programming problem. By characterizing the feasible solution space of the power control problem we find a set of universal power settings that transforms the complex power control and routing problem into a constrained path routing problem. We then propose an efficient Fully Polynomial Time Approximation Scheme (FPTAS) to solve the constrained path routing problem. Simulation results show that our proposed algorithm significantly improves network throughput and greatly outperforms greedy heuristics by providing a guaranteed performance bound.

Since the first deployments of fiber-optic communication systems three decades ago, the capacity carried by a single-mode optical fiber has increased by a staggering 10 000 times. Most of the growth occurred in the first two decades with growth slowing to ten times in the last decade. Over the same three decades, network traffic has increased by a much smaller factor of 100, but with most of the growth occurring in the last few years, when data started dominating network traffic. At the current growth rate, the next factor of 100 in network traffic growth will occur within a decade. The large difference in growth rates between the delivered fiber capacity and the traffic demand is expected to create a capacity shortage within a decade. The first part of the paper recounts the history of traffic and capacity growth and extrapolations for the future. The second part looks into the technological challenges of growing the capacity of single-mode fibers by presenting a capacity limit estimate of standard and advanced single-mode optical fibers. The third part presents elementary capacity considerations for transmission over multiple transmission modes and how it compares to a single-mode transmission. Finally, the last part of the paper discusses fibers supporting multiple spatial modes, including multimode and multicore fibers, and the role of digital processing techniques. Spatial multiplexing in fibers is expected to enable system capacity growth to match traffic growth in the next decades.

The mutual information between a complex-valued channel input and its complex-valued output is decomposed into four parts based on polar coordinates: an amplitude term, a phase term, and two mixed terms. Numerical results for the additive white Gaussian noise (AWGN) channel with various inputs show that, at high signal-to-noise ratio (SNR), the amplitude and phase terms dominate the mixed terms. For the AWGN channel with a Gaussian input, analytical expressions are derived for high SNR. The decomposition method is applied to partially coherent channels and a property of such channels called “spectral loss” is developed. Spectral loss occurs in nonlinear fiber-optic channels and it may be one effect that needs to be taken into account to explain the behavior of the capacity of nonlinear fiber-optic channels.

Fiber-optic communication systems form the high-capacity transport infrastructure that enables global broadband data services and advanced Internet applications. The desire for higher per-fiber transport capacities and, at the same time, the drive for lower costs per end-to-end transmitted information bit has led to optically routed networks with high spectral efficiencies. Among other enabling technologies, advanced optical modulation formats have become key to the design of modern wavelength division multiplexed (WDM) fiber systems. In this paper, we review optical modulation formats in the broader context of optically routed WDM networks. We discuss the generation and detection of multigigabit/s intensity- and phase-modulated formats, and highlight their resilience to key impairments found in optical networking, such as optical amplifier noise, multipath interference, chromatic dispersion, polarization-mode dispersion, WDM crosstalk, concatenated optical filtering, and fiber nonlinearity