Quantum cryptography could well be the first application of quantum mechanics at the individual quanta level. The very fast progress in both theory and experiments over the recent years are reviewed, with emphasis on open questions and technological issues.

Quantum Cryptography

We describe a method to estimate the capacity limit of fiber-optic communication systems (or ?fiber channels?) based on information theory. This paper is divided into two parts. Part 1 reviews fundamental concepts of digital communications and information theory. We treat digitization and modulation followed by information theory for channels both without and with memory. We provide explicit relationships between the commonly used signal-to-noise ratio and the optical signal-to-noise ratio. We further evaluate the performance of modulation constellations such as quadrature-amplitude modulation, combinations of amplitude-shift keying and phase-shift keying, exotic constellations, and concentric rings for an additive white Gaussian noise channel using coherent detection. Part 2 is devoted specifically to the "fiber channel.'' We review the physical phenomena present in transmission over optical fiber networks, including sources of noise, the need for optical filtering in optically-routed networks, and, most critically, the presence of fiber Kerr nonlinearity. We describe various transmission scenarios and impairment mitigation techniques, and define a fiber channel deemed to be the most relevant for communication over optically-routed networks. We proceed to evaluate a capacity limit estimate for this fiber channel using ring constellations. Several scenarios are considered, including uniform and optimized ring constellations, different fiber dispersion maps, and varying transmission distances. We further present evidences that point to the physical origin of the fiber capacity limitations and provide a comparison of recent record experiments with our capacity limit estimation.

Capacity Limits of Optical Fiber Networks

Silicon has long been the optimal material for electronics, but it is only relatively recently that it has been considered as a material option for photonics1. One of the key limitations for using silicon as a photonic material has been the relatively low speed of silicon optical modulators compared to those fabricated from III–V semiconductor compounds2,3,4,5,6 and/or electro-optic materials such as lithium niobate7,8,9. To date, the fastest silicon-waveguide-based optical modulator that has been demonstrated experimentally has a modulation frequency of only ∼20 MHz (refs 10, 11), although it has been predicted theoretically that a ∼1-GHz modulation frequency might be achievable in some device structures12,13. Here we describe an approach based on a metal–oxide–semiconductor (MOS) capacitor structure embedded in a silicon waveguide that can produce high-speed optical phase modulation: we demonstrate an all-silicon optical modulator with a modulation bandwidth exceeding 1 GHz. As this technology is compatible with conventional complementary MOS (CMOS) processing, monolithic integration of the silicon modulator with advanced electronics on a single silicon substrate becomes possible.

A high-speed silicon optical modulator based on a metal–oxide–semiconductor capacitor

Conjugated polymers and indium arsenide–based nanocrystals were used to create near-infrared plastic light-emitting diodes. Emission was tunable from 1 to 1.3 micrometers—a range that effectively covers the short-wavelength telecommunications band—by means of the quantum confinement effects in the nanocrystals. The external efficiency value (photons out divided by electrons in) is ∼0.5% (that is, >1% internal) and is mainly limited by device architecture. The near-infrared emission did not overlap the charge-induced absorption bands of the polymer.

http://science.sciencemag.org/content/sci/295/5559/1506.full.pdf

Efficient near-infrared polymer nanocrystal light-emitting diodes.

Erbium-doped fiber amplifiers are modeled using the propagation and rate equations of a homogeneous two-level laser medium. Numerical methods are used to analyze the effects of optical modes and erbium confinement on amplifier performance, and to calculate both the gain and amplified spontaneous emission (ASE) spectra. Fibers with confined erbium doping are completely characterized from easily measured parameters: the ratio of the linear ion density to fluorescence lifetime, and the absorption of gain spectra. Analytical techniques then allow accurate evaluation of gain, saturation, and noise in low-gain amplifiers (G >

Modeling erbium-doped fiber amplifiers

It is now widely recognized that erbium-doped fiber amplifiers have revolutionized optical fiber communications. EDFAs not only made single-channel, multigigabit-rate, long-distance optical communications possible, but they also opened up a wide variety of additional possibilities Such as soliton generation and transmission and multichannel wavelength-division multiplexing communications. While at AT&T Bell Labs (he is now at Alcatel-Alsthom Recherche in a suburb of Paris, France) Emmanuel Desurvire became heavily involved in and contributed enormously to the theoretical and experimental investigation of EDFA characteristics and system applications. His pioneering work has been internationally recognized. In my view, Desurvire is one of those best qualified to cover the subject of EDFAs, in Erbium-Doped Fiber Amplifiers: Principles and Applications, he has accepted the challenge. According to the author, the purpose of the book is "to provide the basic materials of a comprehensive introduction to the principles and applications of EDFAs." The book is divided into three major parts, which to some extent can be considered independently. Nonetheless, it keeps its cohesion throughout. It provides a thorough understanding of the fundamentals in optical amplification while considering the practical issues related to the device and system performance of EDFAs. The first part of the book explores all the fundamental issues related to EDFAs. It introduces the main concepts necessary for the modelling of the erbium atomic transition. The analysis is detailed and covers such parameters as field distributions and overlap integrals under different operating conditions. This section and the numerous relevant appendices contain a number of useful generalizations of existing models that are published for the first time. The author also considers the fundamental quantum properties of noise generation and accumulation in single- and multiple-stage amplification of classical light. The analysis discusses in great depth the nature, origin and inevitability of noise associated with optical amplification - it also provides useful engineering formulas for the measurement of the noise introduced by amplification. I found the treatment of noise and photon statistics particularly detailed and original. Researchers working on this subject can benefit enormously from the analysis. The second part is primarily experimental and focuses on EDFA device characteristics. However, when specific characteristics of the erbium transition are discussed, the necessary theoretical modifications and additions, supplementary to the general formulations given in the first part, are provided. I found that on the important issue of pulse amplification requirements, the book considers briefly only the special (but very exciting) case of solitons and misses the many problems associated with general pulse amplification. The third and final section of the book, on applications, is primarily concerned with some of the up-to-date linear and nonlinear communication systems and local area networks and the enormous impact that EDFAs have had on their successful implementation. The significance of the EDFAs in optically preamplified receivers is stressed. The most significant digital (linear) and soliton (nonlinear) system experiments performed to date are also reviewed. Initations imposed on linear systems by fiber nonlinearities and dispersion are briefly mentioned. The ground that Desurvire sought to cover in the third part is quite diverse and could well have been the subject of several separate volumes. Therefore, its inclusion in this book is inevitably of a review type. However, the book clearly points out how and to what degree these applications are benefited or enhanced by EDFAs. Overall the book gives one of the most comprehensive and detailed accounts of the physics and fundamental principles of erbium-doped fiber amplifiers published so far. I have not the slightest doubt that the book will be of great help to all scientists and engineers working in the field who are struggling to understand EDFAs. The unified and in-depth presentation of the subject will benefit in particular researchers and graduate students who are dealing with problems involving optical amplification. The book imparts the fundamental concepts quite skilfully and can be used as collateral reading The sections dealing with modelling and the entire second part could well be used in undergraduate courses. I do not hesitate to recommend the book enthusiastically to anybody having an interest in EDFAs and their applications.

Erbium‐Doped Fiber Amplifiers: Principles and Applications

Erbium-doped fiber amplifiers

Traveling-wave amplification of a lambda = 1.53 microm signal with +22-dB gain is achieved at 295 K in an Er(3+)-doped single-mode fiber using a lambda = 514.5 nm pump source. The optimum fiber length for maximum gain is determined experimentally. A limit in signal-to-noise ratio that is due to concurrent amplification of spontaneous emission is observed. By cooling the fiber to 77 K, the amplifier gain is increased to +29 dB as a result of depopulation of the lower laser level.

High-gain erbium-doped traveling-wave fiber amplifier

Amplification of spontaneous emission (ASE) in erbium-doped single-mode fiber amplifiers operating at lambda =1.53 mu m is studied theoretically and experimentally. The ASE noise spectra obtained from the theory are found to be in excellent quantitative agreement with the experimental data. The observed changes in ASE spectral shapes under different population inversion conditions are also explained. The model may be used to evaluate the performance of erbium-doped fiber lasers as well as to assess the noise characteristics of erbium-doped fiber amplifiers as applied to wavelength-division multiplexing optical communications. >

Emmanuel Desurvire

Papers

Modeling erbium-doped fiber amplifiers

Erbium‐Doped Fiber Amplifiers: Principles and Applications

Erbium-doped fiber amplifiers

High-gain erbium-doped traveling-wave fiber amplifier

Amplification of spontaneous emission in erbium-doped single-mode fibers