Mode coupling is a key to overcoming challenges in mode-division-multiplexed transmission systems in multimode fiber. This chapter provides an in-depth description of mode coupling, including its physical origins, its effect on modal dispersion (MD) and mode-dependent loss or gain (MDL), and the resulting impact on system performance and implementation complexity. Strong mode coupling reduces the group delay spread from MD, minimizing the complexity of digital signal processing used for compensating MD and separating multiplexed signals. Likewise, strong mode coupling reduces the variations of MDL arising from transmission fibers and inline optical amplifiers, maximizing average channel capacity. When combined with MD, strong mode coupling creates frequency diversity, which reduces the probability of outage caused by MDL and enables the outage capacity to approach the average capacity. The statistics of strongly coupled MD and MDL depend only on the number of modes and the variances of MD or MDL, and can be derived from the eigenvalue distributions of certain random matrices.

Wavelength-division-multiplexed passive optical network (WDM PON) has long been considered as an ultimate solution for the future optical access network capable of providing practically unlimited bandwidth to each subscriber. On the other hand, it is still considered to be too expensive for the massive deployment. To solve this problem and meet the ever-increasing demand for bandwidth, there have been numerous efforts to improve the competitiveness of WDM PON. This chapter reviews the current status and future direction of these WDM PON technologies. We first review various colorless light sources, which are critical for the cost-effective implementation of the optical network units (ONUs), and several representative network architectures proposed for WDM PONs. We then review the recent research activities for the realization of high-speed (>10Gb/s) and long-reach WDM PONs. Various fault-monitoring and protection techniques are also reviewed as they can be increasingly important in the future high-capacity WDM PONs.

The transmission of multi-band radio signals through optical fibers has drawn great attention recently for its potential in cellular backhaul networks, mobile cloud computing, and wireless local-area networks. As wireless services and technologies evolve to multi-gigabit radio access networks, their speed is increased but the wireless coverage of a single access point is inevitably reduced dramatically. As a result, the importance of >10GHz radio-over-fiber techniques has been emphasized for the capability of expanding wireless coverage feasibility, and in the meantime reducing system complexity and operation expenditure, especially in the high-speed millimeter-wave regime. In this chapter, we introduce the radio-over-fiber technique and its challenge of handling optical millimeter-wave generation, transmission, and converged multi-band system. By exploring real-world, system implementation and characterization, the unique features and versatile applications of radio-over-fiber technologies are investigated and reviewed to reach next-generation converged optical and wireless access networks.

This chapter provides a global network service provider’s view on the technology development and product commercialization of 100-Gb/s for optical transport networks. Optical channel capacity has been growing over the past four decades to address traffic demand growth and will continue this trend for the foreseeable future to meet the ever-increasing bandwidth requirements. In this chapter, optical channels are re-classified into three basic design types. Commercial 100-Gb/s channel development experienced all three types of channel designs before eventually settling on the single carrier PM-QPSK format with coherent detection, which appears to be the optimal design in the industry. A series of 100-Gb/s channel related field trials were performed in service providers’ networks to validate the technical merits as well as business advantages of this new capacity standard before its deployment. Introduction of the 100-Gb/s channel brings new opportunities to boost fiber capacity, accommodate increases in client interface speed rates, lower transmission latency, simplify network management, and speed up the realization of next generation optical add/drop functions.

The increasingly important role of Internet-based, “cloud” service delivery is motivating the evolution of the Internet to a flatter hierarchy of more densely interconnecting networks that shall cost-effectively scale to Zettabytes of bandwidth with improved operational efficiency, under increased traffic variability, and forecast unpredictability. This chapter reviews the implications of this evolution in its underlying metro regional and core transport network architectures, and evaluates the most important innovations in photonics, optical transport, routing, and traffic engineering technologies enabling it. Most notably, 1) a new generation of coherent DWDM systems with more than 2 b/s/Hz spectral efficiency is scaling the existing fiber infrastructure, albeit at a significantly higher proportion, typically more than 50%, of the total transport network cost, while 2) the convergence of IP/MPLS with flexible DWDM promises the most cost-efficient transport evolution, in open architectures that combine advancements in photonics, routing, multi-layer control-plane and management coordination, with interoperability, to improve operation, automate provisioning and restoration, and may optimize network utilization.

At the beginning of an exciting new era in optical communications, we review fundamentals as well as practical experimental aspects of MIMO-SDM: we discuss the importance of selectively addressing all modes of a coupled-mode SDM channel at transmitter and receiver in order to achieve reliable capacity gains and show that reasonable levels of mode-dependent loss (MDL) are acceptable without much loss of channel capacity. We then introduce MIMO-DSP techniques as an extension of familiar algorithms used in polarization-division multiplexed (PDM) digital coherent receivers and discuss their functionality and scalability. Finally, we review the design of mode multiplexers (MMUXs) that allow for the mapping of the individual transmission signals onto an orthogonal basis of waveguide mode, and discuss their performance in experimental demonstrations.

This chapter represents an overview of advanced coding techniques for optical communication. Topics include: codes on graphs, coded modulation, rate-adaptive coded modulation, and turbo equalization. The main objectives of this chapter are: (i) to describe different classes of codes on graphs of interest for optical communications, (ii) to describe how to combine multilevel modulation and channel coding, (iii) to describe how to perform equalization and soft-decoding jointly, and (iv) to demonstrate efficiency of joint de-modulation, decoding, and equalization in dealing with various channel impairments simultaneously. Codes on graphs of interest for next-generation FEC for high-speed optical transport include turbo codes, turbo-product codes and low-density parity-check (LDPC) codes. We describe both binary and nonbinary LDPC codes, their design and decoding algorithms. We also discuss an FPGA implementation of decoders for binary LDPC codes. We demonstrate that an LDPC-coded turbo equalizer is an excellent candidate to simultaneously mitigate uncompensated chromatic dispersion, PMD, fiber nonlinearities and I/Q-imbalance. For completeness of presentation, we also provide the information capacity study of optical channels with memory.

Radio-over-fiber (RoF) link technology has been developed to support multiple wireless signals over optical fiber applications such as mobile backhaul networks and WLANs. Given that conventional wireless networks use packet-switch links between the central office and remote base stations, the cost and complexity of the high-speed wireless networks for data and video transmission increase exponentially while the area of effective coverage decreases. These wireless systems become more inefficient as the data rates and the carrier frequencies required for delivering multi-gigabit wireless services climb higher. However, radio-over-fiber link systems utilizing lightwave to carry multiple analog RF signals through optical fibers can greatly extend the cellular sizes while transparent to the bit rates, modulation formats, and protocols. As a result, the complexity of wireless system to deliver multi-band, multi-gigabit wireless services can be simplified by taking advantages of microwave photonics in optical wireless network design and system integration. The end-to-end RoF systems, from the generation schemes of 1–100 GHz optical wireless signals in the central office to the design of transceivers for base stations and radio access units, are reviewed and investigated in this chapter. Various radio-over-fiber link technologies and the optical wireless interface specifications required to build a converged multi-service, gigabit wireless access network are introduced. The system impairment and its mitigation of radio-over-fiber link transmission will be investigated and analyzed.

Rapidly increasing network demand based on unpredictable services has driven research into methods to provide intelligent provisioning, efficient restoration and recovery from failures, and effective management schemes that reduce the amount of “hands-on” activity to plan and run the network. Integrating the service-oriented IP layer together with the efficient transport capabilities of the optical layer is a cornerstone of this research. Converged IP-optical networks are being demonstrated in large multi-carrier and multi-vendor venues. Research is continuing on making this convergence more efficient, flexible, and scalable. In this chapter, we review the current key technologies that contribute to the convergence of IP and optical networks, describing control and management plane technologies, techniques and standards in some detail. We also illustrate current research challenges, and discuss future directions for research.

This chapter starts by providing some statistics on traffic demand in optical networks and the capacity scaling over time of commercial optical communication systems. Next there is a brief review of the basic results of information theory. We then describe the stochastic nonlinear Schrödinger equation (SNSE), the equation that governs nonlinear propagation in SMFs. This is followed by calculations of nonlinear capacity limit estimates for the SSMF, and advanced fibers with improved transmission characteristics are then presented along with an analytical formula of nonlinear capacity. We then introduce a set of coupled partial differential equations (PDEs) describing nonlinear propagation of polarization-division multiplexed (PDM) signals in SMFs along with nonlinear capacity estimates for these systems. This followed by a focus on multimode fibers (MMFs) and multicore fibers (MCFs). The rest of the chapter then focuses on nonlinear effects in MMFs and MCFs, with an emphasis on MMFs and FMFs. The chapter concludes by reporting experimental observations of two important effects involving nonlinear effects between spatial modes: inter-modal cross-phase modulation (IM-XPM) and inter-modal four-wave mixing (IM-FWM).