Introduction to the First Edition
Optoelectronics is a remarkably broad scientific and technological field that supports a multibillion US dollar per annum global industry, employing tens of thousands of scientists and engineers. The optoelectronics industry is one of the great global businesses of our time.
In this Handbook, we have aimed to produce a book that is not just a text containing theoretically sound physics and electronics coverage, nor just a practical engineering handbook, but a text designed to be strong in both these areas. We believe that, with the combined assistance of many world experts, we have succeeded in achieving this very difficult aim. The structure and contents of this Handbook have proved fascinating to assemble, using this input from so many leading practitioners of the science, technology, and art of optoelectronics.
Today’s optical telecommunications, display, and illumination technologies rely heavily on optoelectronic components: laser diodes, LEDs, liquid crystal, and plasma screen displays, etc. In today’s world, it is virtually impossible to find a piece of electrical equipment that does not employ optoelectronic devices as a basic necessity—from CD and DVD players to televisions, from automobiles and aircraft to medical diagnostic facilities in hospitals and telephones, from satellites and space-borne missions to underwater exploration systems—the list is almost endless. Optoelectronics is in virtually every home and business office in the developed modern world, in telephones, fax machines, photocopiers, computers, and lighting.
“Optoelectronics” is not precisely defined in the literature. In this Handbook , we have covered not only optoelectronics as a subject concerning devices and systems that are essentially electronic in nature, yet involve light (such as the laser diode),
but we have also covered closely related areas of electro-optics, involving devices that are essentially optical in nature but involve electronics (such as crystal light-modulators).
To provide firm foundations, this Handbook opens with a section covering “Basic Concepts.” The “Introduction” is followed immediately by a chapter concerning “Materials,” for it is through the development and application of new materials and their special properties that the whole business of optoelectronic science and technology now advances. Many optoelectronic systems still rely on conventional light sources rather than semiconductor sources, so we cover these in the third chapter, leaving semiconductor matters to a later section. The detection of light is fundamental to many optoelectronic systems, as are optical waveguides, amplifiers, and lasers, so we cover these in the remaining chapters of the Basic Concepts section.
The “Advanced Concepts” section focuses on three areas that will be useful to some of our intended audience, both now, in advanced optics and photometry, and now and increasingly in the future concerning nonlinear and short-pulse effects.
“Optoelectronics Devices and Techniques” is a core foundation section for this Handbook, as today’s optoelectronics business relies heavily on such knowledge. We have attempted to cover all the main areas of semiconductor optoelectronics devices and materials in the eleven chapters in this section, from LEDs and lasers of great variety to fibers, modulators, and amplifiers. Ultrafast and integrated devices are increasingly important, as are organic electroluminescent devices and photonic bandgap and crystal fibers. Artificially engineered materials provide a rich source of possibility for next-generation optoelectronic devices.
At this point, the Handbook “changes gear”— and we move from the wealth of devices now available to us—to how they are used in some of the most important optoelectronic systems available today. We start with a section covering “Communication,” for this is how the developed world talks and communicates by Internet and email today—we are all now heavily dependent on optoelectronics. Central to such optoelectronic systems are transmission, network architecture, switching, and multiplex architectures—the focus of our chapters here. In communication, we already have a multi-tens-of-billions-of-dollarsper-annum industry today.
“Imaging and displays” is the other industry measured in the tens of billions of dollars per annum range at the present time. We deal here with most if not all of the range of optoelectronic techniques used today from cameras, vacuum and plasma displays to liquid crystal displays and light modulators, from electroluminescent displays and exciting new three-dimensional display technologies just entering the market place in mobile telephone and laptop computer displays to the very different application areas of scanning and printing.
“Sensing and Data Processing” is a growing area of optoelectronics that is becoming increasingly important—from non-invasive patient measurements in hospitals to remote sensing in nuclear power stations and aircraft. At the heart of many of today’s sensing capabilities is the business of optical fiber sensing, so we begin this section of the Handbook there, before delving into remote optical sensing and military systems (at an unclassified level—for here-in lies a problem for this Handbook—that much of the current development and capability in military optoelectronics is classified and unpublishable because of its strategic and operational importance). Optical information storage and recovery is already a huge global industry supporting the computer and media industries in particular; optical information processing shows promise but has yet to break into major global utilization. We cover all of these aspects in our chapters here.
“Industrial Medical and Commercial Applications” of optoelectronics abound, and we cannot possibly do justice to all the myriad
inventive schemes and capabilities that have been developed to date. However, we have tried hard to give a broad overview within major classification areas, to give you a flavor of the sheer potential of optoelectronics for application to almost everything that can be measured. We start with the foundation areas of spectroscopy—and increasingly important surveillance, safety, and security possibilities. Actuation and control—the link from optoelectronics to mechanical systems is now pervading nearly all modern machines: cars, aircraft, ships, industrial production, etc.—a very long list is possible here. Solar power is and will continue to be of increasing importance—with potential for urgently needed breakthroughs in photon to electron conversion efficiency and cost of panels. Medical applications of optoelectronics are increasing all the time, with new learned journals and magazines regularly being started in this field.
Finally, we come to the art of practical optoelectronic systems—how do you put optoelectronic devices together into reliable and useful systems, and what are the “black art” experiences learned through painful experience and failure? This is what other optoelectronic books never tell you, and we are fortunate to have a chapter that addresses many of the questions we should be thinking about as we design and build systems—but often forget or neglect at our peril.
In years to come, optoelectronics will develop in many new directions. Some of the more likely directions to emerge by 2010 will include optical packet switching, quantum cryptographic communications, three-dimensional and large-area thin-film displays, high-efficiency solar-power generation, widespread biomedical and biophotonic disease analyses and treatments, and optoelectronic purification processes. Many new devices will be based on quantum dots, photonic crystals, and nano-optoelectronic components. A future edition of this Handbook is likely to report on these rapidly changing fields currently pursued in basic research laboratories.
We are confident you will enjoy using this Handbook of Optoelectronics, derive fascination and pleasure in this richly rewarding scientific and technological field, and apply your knowledge in either your research or your business.
Editors
John P. Dakin, PhD, is professor (Emeritus) at the Optoelectronics Research Centre, University of Southampton, UK. He earned a BSc and a PhD at the University of Southampton and remained there as a Research Fellow until 1973, where he supervised research and development of optical fiber sensors and other optical measurement instruments. He then spent 2 years in Germany at AEG Telefunken; 12 years at Plessey, research in Havant and then Romsey, UK; and 2 years with York Limited/York Biodynamics in Chandler’s Ford, UK before returning to the University of Southampton.
He has authored more than 150 technical and scientific papers, and more than 120 patent applications. He was previously a visiting professor at the University of Strathclyde, Glasgow.
Dr. Dakin has won a number of awards, including “Inventor of the Year” for Plessey Electronic Systems Limited and the Electronics Divisional Board Premium of the Institute of Electrical and Electronics Engineers, UK. Earlier, he won open scholarships to both Southampton and Manchester Universities.
He has also been responsible for a number of key electro-optic developments. These include the sphere lens optical fiber connector, the first wavelength division multiplexing optical shaft encoder, the Raman optical fiber distributed temperature sensor, the first realization of a fiber optic passive hydrophone array sensor, and the Sagnac location method described here, plus a number of novel optical gas sensing methods. More recently, he was responsible for developing a new distributed acoustic and seismic optical fiber sensing system, which is finding major applications in oil and gas exploration, transport and security systems.
Robert G. W. Brown, PhD, is at the Beckman Laser Institute and Medical Clinic at the University of California, Irvine. He earned a PhD in engineering at the University of Surrey, Surrey, and a BS in physics at Royal Holloway College at the University of London, London. He was previously an applied physicist at Rockwell Collins, Cedar Rapids, IA, where he carried out research in photonic ultrafast computing, optical detectors, and optical materials. Previously, he was an advisor to the UK government, and international and editorial director of the Institute of Physics. He is an elected member of the European Academy of the Sciences and Arts (Academia Europaea) and special professor at the University of Nottingham, Nottingham. He also retains a position as adjunct full professor at the University of California, Irvine, in the Beckman Laser Institute and Medical Clinic, Irvine, California, and as visiting professor in the department of computer science. He has authored more than 120 articles in peer-reviewed journals and holds 34 patents, several of which have been successfully commercialized.
Dr. Brown has been recognized for his entrepreneurship with the UK Ministry of Defence Prize for Outstanding Technology Transfer, a prize from Sharp Corporation (Japan) for his novel laserdiode invention, and, together with his team at the UK Institute of Physics, a Queen’s Award for Enterprise, the highest honor bestowed on a UK company. He has guest edited several special issues of Applied Physics and was consultant to many companies and government research centers in the United States and the United Kingdom. He is a series editor of the CRC Press “Series in Optics and Optoelectronics.”
Contributors
takao a ndo
Research Institute of Electronics
Shizuoka University
Hamamatsu, Japan
Dominique Chiaroni
NOKIA Bell Labs
Paris-Saclay, France
John P. Dakin
Optoelectronics Research Centre
University of Southampton
Southampton, United Kingdom
Fernando a rau jo de Castro
Materials Division
National Physical Laboratory
Middlesex, United Kingdom
Michel Digonnet
Stanford Photonics Research Center
Stanford University
Stanford, California
Uzi Efron
Holon Institute of Technology
Holon, Israel
Günter Gauglitz
Department of Analytical Chemistry University of Tübingen Tübingen, Germany
ron Gibbs
Gibbs Associates
Dunstable, United Kingdom
Nicholas Holliman
University of Durham
Durham, United Kingdom
Kazuo Hotate
University of Tokyo
Tokyo, Japan
Michel Joindot
Laboratoire FOTON, UMR
CNRS
Lannion, France
J. Cliff Jones
School of Physics and Astronomy
University of Leeds
Leeds, United Kingdom
George K. Knopf
Department of Mechanical and Materials
Engineering
University of Western Ontario Ontario, Canada
ton Koonen
Department of Electrical Engineering
Technische Universiteit Eindhoven
Eindhoven, the Netherlands
John N. Lee
Naval Research Laboratory
Washington, District of Columbia
robert a . Lieberman Lumoptix Inc
Redondo Beach, CA
Makoto Maeda
Home Network Company
SONY
Kanagawa, Japan
Michael a . Ma rcus
Lumetrics Inc.
Rochester, New York
tom Markvart
University of Southampton
Southampton, United Kingdom
Susanna Orlic Department of Optics Technische Universität Berlin Berlin, Germany
tsutae Shinoda Home Network Company
SONY Kanagawa, Japan
a nt hony E. Smart Scattering Solutions, Inc. Costa Mesa, California
Euan Smith Light Blue Optics Cambridge, United Kingdom
Masayuki Sugawara
Kochi University of Technology Tokyo, Japan
Kenkichi tanioka
Kochi University of Technology Tokyo, Japan
Heiju Uchiike Home Network Company
SONY Kanagawa, Japan
J. Michael Vaughan Worcestershire, United Kingdom
Enabling technologies for communications
Optical transmission
MICHEL JOINDOT
1.3 General structure of optical transmission systems 7
1.3.1 Modulation and detection: RZ and NRZ codes 7
1.3.2 Basic architecture of amplified WDM communication links 8
1.3.3 Basic architectures of repeaterless systems 9
1.3.4 Optical reach and amplification span 9
1.4 Limitations of optical transmission systems 10
1.4.1 Noise sources and bit error rate 10 1.4.1.1 Amplifier noise 10 1.4.1.2 Photoreceiver thermal noise 11
1.4.1.3 Relationship between bit error rate and noise 11
1.4.1.4 Accumulation of noise 12
1.4.2 Signal distortions induced by propagation 13
1.4.2.1 Chromatic dispersion 13
1.4.2.2 Nonlinear effects 15
1.4.2.3 Self-phase modulation 15
1.4.2.4 Cross-phase modulation 16
1.4.2.5 Four-wave mixing 17
1.4.2.8 Polarization mode dispersion 20
1.5 De sign of an optical WDM system 20
1.5.1 Global performance of a system: BER and OSNR 20
1.5.2 Critical parameters and tradeoffs for terrestrial, undersea, and repeaterless systems 21
1.6 St ate of the art and future of the WDM technology 22
1.6.1 State-of-the-art WDM system capacity and distance 22
1.6.2 Forward error-correcting codes 23 1.6.3 Ultralong- haul technology: New problems arising 23
1.6.4 Raman amplification 24
1.6.5 Diversification of fibers and international telecommunications union fiber standards 25
1.6.6 Toward the future: WDM 40G systems and beyond 26
1.6.6.1 Increasing the number of channels by increasing the amplification bandwidth 26
1.6.6.2 Increasing the number of channels with a closer channel spacing 27
PROLOGUE
Optics has become the unique transmission technology in backbone networks, providing capacities absolutely unknown before, a very high transmission quality and a reduction of operational costs per transmitted bit. This is due to the development of wavelength division multiplexing (WDM) introduced in 1995 and allowing to transmit around 800 Gbit/s (80 and 10 Gbit/s channels) over one single mode fiber in 2005. This chapter describes this history, puts in evidence the basics of optical transmission and the development of WDM technology, and shows how the capacity of WDM systems can be increased by extending the used bandwidth, increasing the channel number or the bit rate per channel.
In 2005, coherent reception which had been explored by a many academic and industrial laboratories in the 1980s came back in foreground, but receivers were implemented quite differently from what had been proposed 30 years before. In fact, this very important technological step is closely related to the progress of electronics, allowing the implementation of complex and powerful digital signal processing (DSP) algorithms. The receiver consists now in a very “simple” optical front end (local oscillator and photodiodes), analogue to digital converters and a DSP unit compensating for all the transmission impairments due to the propagation over the fiber. Due to the fact that the optical demodulator just translates the channel transfer function into baseband, the baseband transfer function to be compensated for by the DSP unit is just the transfer function of the optical channel, which is not true with a quadratic detection. Coherent reception opens the way to complex and more than binary modulation schemes, which results into an increase of the transmitted bit rate within a given bandwidth.
Chromatic dispersion is no more in line compensated, which eliminates the dispersioncompensation fiber in each amplification site, polarization mode dispersion (PMD), which was a very serious problem can be compensated for very efficiently. Moreover, both polarizations can be used, which doubles the potential capacity of each channel by using polarization division
1.6.6.3 Modulation schemes 28
1.6.6.4 Selected recent results 28
References 30
multiplexing in conjunction with WDM. And DSP can cancel the interference between polarizations and then separate them without any problem.
Commercial coherent systems transmitting 80 100 Gbit/s channels in C band (8 Tbit/s over one single mode fiber) have been available since 2011 and are installed in backbone networks to face the always increasing traffic demand, while 200 and beyond 400 Gbit/s per channel are actively investigated in industrial laboratories.
1.1 INTRODUCTION
The enormous potential of optical waves for highrate transmission of information was recognized as early as the 1960s. Because of their very high frequency, it was predicted that light waves could be ultimately modulated at extremely large bit rates, well in excess of 100 Gbit·s−1 and orders of magnitudes faster than possible with standard microwave-based communication systems. The promise of optical waves for high-speed communication became a reality starting in the late 1980s and culminated with the telecommunication boom of the late 1990s, during which time a worldwide communication network involving many tens of millions of miles of fiber was deployed in many countries and across many oceans. In fact, much of the material covered in this handbook was generated to a large extent as a result of the extensive optoelectronics research that was carried out in support of this burgeoning industry. The purpose of this chapter is to provide a brief overview of the basic architectures and properties of the most widely used type of optical transmission line, which exploit the enormous bandwidth of optical fiber by a general technique called WDM. After a brief history of optical network development, this chapter examines the various physical mechanisms that limit the performance of WDM systems, in particular, their output power [which affects the output signal-to-noise ratio (SNR)], capacity (bit rate times number of channels), optical reach (maximum distance between electronic regeneration), and cost. The emphasis is placed on the main performance-limiting effects, namely fiber optical nonlinearities, fiber chromatic and
group velocity dispersions (GVDs), optical amplifier noise and noise accumulation, and receiver noise. Means of reducing these effects, including fiber design, dispersion management, modulation schemes, and error-correcting codes, are also reviewed briefly. The text is abundantly illustrated with examples of both laboratory and commercial optical communication systems to give the reader a flavor of the kinds of system performance that are available. This chapter is not meant to be exhaustive, but to serve as a broad introduction and to supply background material for the following two chapters (optical network architecture and optical switching and multiplexed architectures), which dwell more deeply into details of system architectures. We also refer the reader to the abundant literature for a more in-depth description of these and many other aspects of optical communication systems (see, for example, [1,17,32,34]).
1.2 HISTORY OF THE INTRODUCTION OF OPTICS IN BACKBONE NETWORKS
Enabling the implementation of the optical communication concept required the development of a large number of key technologies. From the 1960s through the 1980s, many academic and industrial laboratories around the world carried out extensive research towards this goal. The three most difficult R&D tasks were the development of reliable laser sources and photodetectors to generate and detect the optical signals, of suitable optical fibers to carry the signals, and of the components needed to perform such basic functions as splitting, filtering, combining, polarizing, and amplifying light signals along the fiber network. Early silica-based fibers had a large core and consequently carried a large number of transverse modes, all of which travel at a different velocity, leading to unavoidable spreading of the short optical bits that carry the information and thus to unacceptably low bit rates over long distances. Perhaps, the most crucial technological breakthrough was the development of single-mode fibers, which first appeared in the mid-1970s and completely eliminated this problem. Over the following decade, progress in both material quality and manufacturing processes led to a dramatic reduction in the propagation loss of these fibers, from tens of decibels per kilometer in early
prototypes to the amazingly low typical current loss of 0.18 dB·km 1 around 1.5 μm used in submarine systems (or an attenuation of only 50% through a slab of glass about 17 km thick!). The typical attenuation of fibers used in long-distance terrestrial networks today is around 0.22 dB·km 1 at 1550 nm.
Fiber components were developed in the 1980s, including such fundamental devices as fiber couplers, fiber polarizers and polarization controllers, fiber wavelength division multiplexers [5,48], and rare-earth-doped fiber sources and amplifiers [17,18,41]. The descendants of these and several other components now form the building blocks of modern optical networks. Interestingly, the original basic research on almost all of these components was actually done not with communication systems in mind, but for fiber sensor applications, often under military sponsorship, in particular for development of the fiber optic gyroscope [5]. Parallel work on optoelectronic devices produced other cornerstone active devices, including high quantum efficiency, low-noise photodetectors, efficient and low-noise semiconductor laser diodes in the near infrared, in particular distributed-feedback (DFB) lasers, as well as semiconductor amplifiers, although these were eclipsed in the late 1980s by rare-earth-doped fiber amplifiers. The development of high-power laser diodes, began in the 1980s to pump high-power solidstate lasers, in particular for military applications, sped up substantially in the late 1980s in response for the growing demand for compact pump sources around 980 and 1480 nm for then-emerging erbiumdoped fiber amplifiers. Another key element in the development of optical communication networks was the advent of a new information management concept called Synchronous Optical NETworks [31], especially matched to optical signals but also usable for other transmission technologies.
Up until the mid-1980s, long-distance communication network systems were based mostly on coaxial cable and radio frequency technologies. Although the maximum capacity of a single coaxial cable could be as high as 560 Mbit·s 1, most installed systems operated at a bit rate of 140 Mbit·s 1, while radio links could support typically eight 140 Mbit·s 1 radio channels. Intercontinental traffic was shared between satellite links and analogue coaxial undersea systems; digital undersea coaxial systems never existed. The switch to optical networks was motivated in part by the need for a much greater capacity, in part by the need for improved security
and reliability of radio-based and cable-based systems. These systems were commonly affected by two different types of failures, namely signal fading and cable breaks due to civil engineering work, respectively. The first optical transmission systems were introduced in communication networks in the mid-1980s. Early prototypes were classical digital systems with a capacity that started at 34 Mbit·s−1 and rapidly grew to 140 Mbit·s−1, i.e., comparable to established technologies. Optical communication immediately outperformed the coaxial technology in terms of regeneration span, which was tens of kilometers compared to less than 2 km for high-capacity coaxial-cable systems. However, there was no significant advantage compared with radio links, in terms of either capacity or regeneration span length, the latter being typically around 50 km. One could thus envision future long-distance networks based on a combination of secure radio links and optical fibers. Soon after optical devices became reliable enough for operation in a submerged environment, optical fiber links rapidly replaced coaxial-cable systems. The very first optical systems used multimode fibers and operated around 800 nm. This spectral window was changed to 1300 nm for the second generation of systems, when lasers around this wavelength first became available. In Japan, where optical communication links were installed early on, prior to the development of the 1550-nm systems, many systems operate in this window. Most of the current systems for backbone networks, especially in Europe and the United States, operate in the spectral region known as the C-band (1530–1565 nm). This has become the preferred window of operation because the attenuation of silica-based single-mode fiber is minimum around 1550 nm. The first transatlantic optical cable, TAT-8, was deployed in 1988. Containing two fiber pairs and a large number of repeaters, it spans a distance of about 6600 km under the Atlantic Ocean between Europe and the United States and carries 280 Mbits of information per second. In 1993, optical transmission systems carrying 2.5 Gbit·s−1 (16 × 155 Mbit·s−1) over a single fiber with a typical regeneration span of 100 km began to be added to the growing worldwide optic–optic network. In terms of both capacity and transmission quality, radio-based systems could no longer compete, and optics became the unique and dominating technology in backbone networks.
The single most important component that made high-speed communication possible over
great distances (≫100 km) without electronic regeneration is the optical amplifier. Although the loss of a communication optic around 1.5 μm is extremely small, after a few tens of kilometers, typically 50–100 km, the signal power has been so strongly attenuated that further propagation would cause the SNR of the signal at the receiver to degrade significantly, and thus the transmission quality, represented by the bit error rate (BER), to be seriously compromised. The SNR can be improved by increasing the input signal power, but the latter can only be increased so much before the onset of devastating nonlinear effects in the optic, in particular stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), and four-wave mixing (FWM). Moreover, the gain in distance would be limited: a transmission over 200 km i nstead of 100 km of current optic would require the input power to be increased by roughly 20 d B!
This distance limitation was initially solved by placing optoelectronics repeaters along the optical line. Each repeater detects the incoming data stream, amplifies it electronically, and modulates the current of a new laser diode with the detected modulation. The modulated diode’s output signal is then launched into the next segment in the optic link. This approach works well, but its cost is high and its bit rate is limited, on both counts by the repeaters’ high-speed electronics. A much cheaper alternative, which requires high-speed electronics only at the two ends of the transmission line, is optical amplification. Each electronic repeater is now replaced by an in-line optical amplifier, which amplifies the low-power signals that have traveled through a long optic span before their SNR gets too low and then reinjects them into the next segment in the optic link. The advantage of this alloptical solution is clearly that the optical signal is never detected and turned into an electronic signal, until it reaches the end of the long-haul optical line, which can be thousands of kilometers long. Because the noise figure (NF) of optical amplifiers is low, typically 3–5 dB, the SNR can still be quite good even after the signals have traveled through dozens of amplifiers.
Starting as early as the 1960s, much research was devoted to several types of in-line optical amplifiers, first with semiconductor waveguide amplifiers [51], then with rare-earth-doped optic amplifiers [18], and more recently Raman optic amplifiers [28].
Semiconductor amplifiers turned out to have the highest wall-plug efficiency. However, at bit rates under about 1 Gbit·s−1, in WDM systems they induce cross-talk between signal channels. Although solutions have been recently proposed, semiconductor amplifiers have not yet entered the market in any significant way, partly because of the resounding success of the erbium-doped optic amplifier (EDFA). First reported in 1987 [42], this device provides a high small-signal gain around 1.5 μm (up to ∼50 dB) with a high saturation power and with an extremely high efficiency—the record is 11 dB of small-signal single-pass gain per mi lliwatt of pump power [52]. EDFAs used in telecommunication systems operate in saturation and have a lower gain, but it is still typically as high as 20–30 dB. The EDFAs can be pumped with a laser diode, at either 980 or 1480 nm, and they are thus very compact. Another key property is their wide gain spectrum, which stretches from ~1475 to ~1610 nm, or a total bandwidth of 135 nm (~16.4 THz!). For technical reasons, a single EDFA does not generally supply gain over this entire range, but rather over one of three smaller bands, called the S-band (for “short,”
~1480–1520 nm), the C-band (for “conventional,”
~1530–1565 nm), and the L-band (for “long,”
~1565–1610 nm). Amplification in the S-band can also be accomplished with a thulium-doped fiber amplifier (TDFA) [49]. Gain has been obtained over the S- and C-band by combining an EDFA and a TDFA [50].
Perhaps more importantly, EDFAs induce negligible channel cross-talk at modulation frequencies above about 1 MHz. These unique features make it nearly ideally suited for optical communication systems around 1.5 μm. Since the mid-1990s, it has been the amplifier of choice in the overwhelming majority of deployed systems, thus eliminating the electrical regeneration bottleneck.
The very large gain bandwidth of EDFAs and other optical amplifiers also provided the opportunity of amplifying a large number of modulated optical carriers at different wavelengths distributed over the amplifier bandwidth. This concept of WDM had of course already been applied in radio links. One significant advantage of WDM optical systems is that the same amplifier amplifies many optical channels, in contrast with classical regenerated systems, which require one repeater per channel. Optical amplifiers thus reduce the installation
cost of networks in two major ways. First, the WDM technique results in an increase in capacity without laying new fibers, which reduces optic cost. Second, the cost of amplification is shared by a large number of channels, and because the use of a single optical amplifier is cheaper than implementing one regenerator per channel, the transmission cost is reduced proportionally. This critical economic advantage provided the final impetus needed to displace regenerated systems and launch the deployment of the worldwide optical WDM backbone networks that took place at the end of the 1990s.
1.3 GENERAL STRUCTURE OF OPTICAL TRANSMISSION SYSTEMS
1.3.1 Modulation and detection: RZ and NRZ codes
While radio systems use a wide variety of modulation formats in order to improve spectrum utilization, in optical systems data have been so far transmitted using binary intensity modulation. A logic 1 (resp. 0) is associated to the presence (resp. absence) of an optical pulse. Two types of line codes are mainly encountered: nonreturn to zero (NRZ), where the impulse duration is equal to the symbol duration (defined as the inverse of the data rate) and return to zero (RZ), where the impulse duration is significantly smaller than the symbol duration. This property explains why the name “return to zero” is used; if the impulse duration equals roughly one half of the symbol time, the modulation format is designed as RZ 50%. So at a bit rate of 10 Gbit·s−1, NRZ uses impulses with a width of approximately 100 ps, while RZ 50% or RZ 25% will use 50 and 25 ps wide pulses, respectively. RZ has, for a given mean signal power, a higher signal peak power. This property can be used to exploit nonlinear propagation effects, which under certain conditions can improve system performance. Details will be provided further on.
Research is actively being conducted to investigate new modulation schemes for future high-bitrate systems. For instance, duobinary encoding, a well-known modulation scheme in radio systems, has been proposed because of its higher resistance to chromatic dispersion. Carrier-suppressed
RZ, an RZ modulation format with an additional binary phase modulation, is also extensively studied, as well as recently differential phase shift keying (DPSK); both provide a higher resistance to nonlinear effects. However, only NRZ and RZ are used in installed systems today.
Detection of the optical signals at the end of a transmission line is performed with a photodetector, which is typically a PIN or an avalanche diode. Photons are converted in the semiconductor in electron–holes pairs and collected in an electrical circuit. The generated current is then amplified and sent to a decision circuit, where the data stream is detected by comparing the signal to a decision threshold, as in any digital system. Detection errors can occur in particular because of the presence of noise on the signal and in the detector. The error probability is a measurement of the transmission quality. In practice, the error probability is estimated by the BER, defined as the ratio of error bits over the total number of transmitted bits.
Several sources of noise are typically present in the detection process of an optical wave. Shot noise, the most fundamental one, arises from the discontinuous nature of light. Thermal noise is generated in the electrical amplifiers that follow the photodetector. In PIN receivers, thermal noise is typically 15–20 dB larger than the quantum limit, and if the optical signal is low, thermal noise dominates shot noise. In the case of amplified systems under normal operating conditions, the amplified spontaneous emission (ASE) noise of the in-line amplifiers
is largely dominant compared to the receiver noise, which can thus be neglected.
1.3.2 Basic architecture of amplified WDM communication links
A typical amplified WDM optical link is illustrated in Figure 1.1. The emitter consists of N lasers of different wavelengths, each one representing a communication channel. The lasers are typically DFB semiconductor lasers with a frequency stabilized by a number of means, including temperature control and often Bragg gratings. Each laser is amplitude modulated by the data to be transmitted. This modulation is performed with an external modulator, such as an amplitude modulator based on lithium niobate waveguide technology. Direct modulation of the laser current would be simpler and less costly, but it introduces chirping of the laser frequency, which is unacceptable at high modulation frequencies over long distances [1,29]. The fiber-pigtailed laser outputs are combined onto the optical fiber bus using a wavelength division multiplexer, then generally amplified by a booster fiber amplifier.
The multiplexer can be based on concatenated WDM couplers (for low number of channels) or arrayed waveguide grating multiplexers. This is the technology of choice for high channel counts, in particular in the so-called dense wavelength division multiplexed systems: although this term has no precise definition, it applies usually to systems with a channel spacing less than 200 GHz. In-line
Figure 1.1 Structure of an amplified WDM optical system.
optical amplifiers are distributed along the fiber bus to periodically amplify the power in the signals, depleted by lossy propagation along the fiber. Ideally, each amplifier provides just enough gain in each channel to compensate for the loss in that channel, i.e., such that each channel experiences a net gain of unity. Because the gain of an optical amplifier and to a smaller extent the loss along the fiber are wavelength dependent, the net gain is different for different channels. If the difference in net gain between extreme channels is too large, after a few amplifier/bus spans the power in the strongest channel will grow excessively, thus robbing the gain for other channels and making their SNR at the receiver input unacceptably low. This major problem is typically resolved by flattening or otherwise shaping the amplifier spectral gain profile, or equivalently equalizing the power in the channels, using one of several possible techniques, either passive (for example, long-period fiber gratings) [58] or dynamic (e.g., with variable optical attenuators). While the first WDM systems that appeared in the mid-1980s used basic amplifiers without control system, the new very long reach systems include complex gain flattening devices that compensate for the accumulation of gain tilt. The gain of an in-line amplifier ranges approximately from 15 to 30 dB per channel. The distance between amplifiers is typically 30–100 km, depending on fiber loss, number of channels, and other system parameters. In deployed undersea systems, the amplifiers are equally spaced, whereas in terrestrial networks the amplifier location depends on geographical constraints, for instance, building location and amplifiers tend to be unevenly spaced. At the output of the transmission line, a wavelength division demultiplexer separates the N optical channels, which are then sent individually to a receiver, then electronically processed.
1.3.3 Basic architectures of repeaterless systems
A repeaterless communication system aims to accomplish a very long optical reach without inline amplifiers. A common application is connecting two terrestrial points on each side of a straight or narrow arm of sea, in which case it is generally not worth incurring the cost of undersea amplifiers. Some deployed repeaterless systems are extremely long, as much as hundreds of kilometers,
and consequently they exhibit a high span attenuation, up to 50 or even 60 dB. The problem is then to ensure that the output power at the receiver is high enough, in spite of the high span loss, to achieve the required SNR at the end of the line.
This goal has been achieved with a number of architectures. A common solution involves using a preamplifier, i.e., an amplifier placed before the receiver to increase the detected power and reduce the receiver NF, as is also often done in classical WDM systems. Another one is to use a highpower amplifier, i.e., an amplifier placed between the emitter and the transmission fiber to boost the signal power launched into the fiber. Although such a booster amplifier is also present in some of the amplified WDM systems described before, the typical feature in repeaterless systems is the high power level, which can reach up to 30 dBm. In both cases, the amplifier can be either an EDFA or a Raman amplifier, or a combination of both. This solution, as we will see further on, is limited by nonlinear effects in the fiber, although they can be somewhat mitigated with proper dispersion management.
A third solution specific to repeaterless systems is to place an amplifier fiber in the transmission fiber itself and to pump it remotely with pump power launched into the transmission fiber from either end. The amplifier fiber can then be a length of Er-doped fiber; the entire transmission fiber can be lightly doped with erbium (the so-called distributed fiber amplifier); or the transmission fiber can be used as a Raman amplifier. The drawback of this general approach is that it requires a substantially higher pump power than a traditional EDFA, and it is therefore more costly. The reason is that the pump must propagate through a long length of transmission fiber before reaching the amplifier fiber, and because the transmission fiber is much more lossy at typical pump wavelengths than in the signal band, some of the pump power is lost. A fourth general solution, which is not specific to repeaterless systems, is to use powerful errorcor recting schemes [12].
1.3.4 Optical reach and amplification span
Two important features of a WDM communication system are its total capacity, usually expressed as N × D, where N is the number of optical channels
and D the bit rate per channel, and the optical reach, which is the maximum distance over which the signal can be transmitted without regeneration. Even in amplified systems with a nominally unity net gain transmission, due to the accumulation of noise from the optical amplifiers and signal distortions, after a long enough transmission distance the bit error becomes unacceptably high, and the optical signals need to be regenerated. In practical deployed WDM systems in 2001, this limitation typically occurs after about seven amplifier spans with a loss of roughly 25 dB per span.
Another important parameter is the amplification span, i.e., the distance between adjacent amplifiers. The performance of an optical WDM system cannot be expressed only in terms of optical reach; the number of spans must also be introduced. As an example, the optical reach of commercially available terrestrial systems in 2002 was around 800 km, compared to 6500 km i n transatlantic systems. A key difference between them is the amplification span, as will be explained in the next section. In the following, WDM systems with a bit rate per channel of 2.5, 10, and 40 Gbit·s−1 will be designated as WDM 2.5G, WDM 10G, and WDM 40G, respectively.
1.4 LIMITATIONS OF OPTICAL TR ANSMISSION SYSTEMS
1.4.1
Noise sources and bit error rate
1.4.1.1 AMPLIFIER NOISE
Amplification cannot be performed without adding noise to the amplified signals. In optical amplifiers, this noise originates from ASE [17], which is made of spontaneous emission photons emitted by the active ions (Er3 + in the case of EDFAs) via radiative relaxation subsequently amplified as they travel through the gain medium. The spectral power density of the ASE signal per polarization mode is given by
γ= nhvG –1 ASEsp (1.1)
where G is the amplifier gain, h Planck’s constant, and v the signal optical frequency. n sp is a dimensionless parameter larger or equal to unity called the spontaneous noise factor. It depends on the amplifier’s degree of inversion, and it approaches unity (lowest possible noise) for full inversion of
the active ion population. The ASE is a broadband noise generated at all frequencies where the amplifier supplies gain, and its bandwidth is nominally the same as that of the amplifier gain. The ASE power coming out of the amplifier, concomitantly with the amplified signals, is obtained by the integration of γASE over the frequency bandwidth of the gain. As an example, in a particular C-band EDFA amplifying ten signals equally spaced between 1531 and 1558 nm and with a power of 1 μW each, and with a peak gain of 33 dB at 1531 nm, the total power in the amplified signals is 5.5 mW, whereas the total ASE output power is 0.75 mW, i.e., more than 10% of the signals’ power.
The photodetectors used in receivers are the socalled quadratic detectors, i.e., they respond to the square of the optical field. Detection of an optical signal S corrupted by additive noise N (ASE noise in the case of amplified systems) in a photodetector thus gives a signal proportional to |S + N |2 . Expansion of this signal gives rise to the signal |S|2 (the useful signal) plus two noise terms. The first term (2SN ) is the beat noise between the signal and the ASE frequency component at the signal frequency; it is called the signal–ASE beat noise. The second term (N2) is the beat noise between each frequency component of the ASE with itself (the ASE–ASE beat noise). The signal–ASE beat noise varies from channel to channel, but the ASE–ASE beat noise is the same for all channels. A third and fourth noise terms are of course the shot noise of the amplified signal and the shot noise of the ASE, and to this must be added a fifth term, namely the receiver noise discussed earlier. In high-gain amplifiers with low input signals, which are applicable to most in-line amplifiers in communication links, the dominant amplifier noise term is the signal–ASE beat noise. In the amplifier example given at the end of the previous paragraph, the SNR degradation (also known as the NF) is ~3.4 d B for all 10 signals, and it is due almost entirely to signal–ASE beat noise. This noise term is typically large compared to the receiver noise, which can usually be neglected. Note that the NF is defined as the SNR degradation of a shot-noise-limited input signal. The SNR degradation at the output of an amplifier is therefore equal to the NF only when the input signal is shot-noise limited. In a chain of amplifier, this is true for the first amplifier that the signal traverses. However, after traveling through several amplifiers, the signal is no longer
shot-noise limited but dominated by signal–ASE beat noise, and the SNR degradation is smaller than the NF. Refer to “Accumulation of Noise” section for further detail on noise accumulation in amplifier chains.
1.4.1.2 PHOTORECEIVER THERMAL NOISE
As mentioned earlier, the photoreceiver thermal noise is generally fairly large compared to shot noise. However, it can become negligible when the signals are amplified with a preamplifier placed before the detector. To justify this statement, consider a receiver consisting of an optical preamplifier of gain G followed by a photodetector. The optical signal and ASE noise powers at the receiver input are proportional to G and G 1, respectively (in practice, G is very large and G 1 ≈ G). Because the thermal noise does not depend on G, and because it is typically 15–20 dB worse than the quantum limit, it is clear that if the preamplifier gain is large enough, say 20 dB, the thermal noise is negligible compared to the signal–ASE beat noise. This is exactly the same phenomenon as in electronics, where the high-gain first stage of a receiver masks the noise of the following stages. This property illustrates another advantage brought by optical amplifiers: optical preamplifiers allow to get away from the relatively poor NF of electronic circuits and thus to achieve much better performance.
1.4.1.3
RELATIONSHIP BETWEEN BIT ERROR R ATE AND NOISE
How does the error rate at the receiver depend on the noise level, or more exactly on the optical signal-to-noise ratio (OSNR), of the detected signal? To answer this question, we must make some assumptions regarding both the signal and the noise. First, because the signal–ASE beat noise depends on the signal power, it also depends on the state of the signal, i.e., on the transmitted data. If we assume an ideal on–off keying (OOK) modulation, signal–ASE beat noise is present only when the signal is on, whereas the ASE–ASE beat noise is present even in the absence of signal. Because the data can be assumed to be equally often on and off, the mean signal power is equal to half the peak power.
Second, to obtain an analytical expression for the bit error probability requires another assumption, common in communication theory, which is that the noise has a Gaussian statistics. This is true
for signal–ASE beat noise, as a result of the linear processing of Gaussian processes, but it is not true for ASE–ASE beat noise. However, under normal operating conditions of amplified systems (i.e., with a sufficiently high OSNR), the influence of ASE–ASE beat noise remains relatively small. After a large number of optical amplifiers, however, the ASE–ASE beat noise component, which depends on the total ASE noise, can become significant. An effective way to reduce this noise component is then to place before the receiver an optical filter that cuts down the ASE power between the optical signals. This can be accomplished with a comb filter or with the demultiplexer that separates the channels. Such a filter reduces the ASE–ASE beat noise, but of course it does not attenuate either the signals or the ASE at the signals’ frequencies, so it does not affect the signal–ASE beat noise. In the following, we assume that such a filter, with a rectangular transmission spectrum of optical bandwidth B a , is placed before the receiver.
Third, because the noise variance is not the same conditionally to the transmitted data, the best decision threshold is not just at equal distance between the two signal levels associated with the two possible data values at the sampling time, but rather some other optimum threshold value that depends on signal power. Assuming that this optimum threshold value is used, the bit error probability (or BER) can be expressed as [30]:
where erfc is the complementary error function and the SNR, R is the ratio of the mean signal power to the ASE power within the electrical bandwidth B, i.e., γASE B. The electrical bandwidth B is the bandwidth of the electronic post-detection circuits. The parameter m is the normalized optical filter bandwidth, m = B a /B. Equation 1.2 can be easily derived by computing the variances of the signal–ASE beat noise and ASE–ASE beat noise contributions. For the computation of the first term, the average power of the signal is used. An ideal rectangular optical filter is assumed, as well as a rectangular electrical filter
The SNR is usually measured not within the signal bandwidth, but over a much larger bandwidth B 0, corresponding generally to 0.1 nm in wavelength (or 12.5 GHz near 1550 nm). Calling this
parameter the optical SNR R0, the bit error probability can be rewritten as
where Q is the quality factor:
β is the ratio B/B 0 of the electrical to measurement bandwidths, R0 = βR a nd μ = mβ2 . An error probability of 10 −9 (resp. 10−15) requires, for example, Q = 6 (resp. 8). Neglecting the ASE–ASE beat noise contribution (μ → 0), the BER can be simply expressed as
of successive amplifiers that can be used, and thus the optical reach. Assuming a link with N equally spaced amplifiers of mean output power per channel P0, an inversion parameter nsp, and a gain G compensating exactly for the attenuation of the fiber span between them, the total noise power Pn (including both polarization modes) within a bandwidth B0 at the last amplifier output is given by [29]:
From Equation 1.4, the value of R0 needed to achieve a given quality factor Q 0 is given by
where L is the total length of the link, Z a the length of the amplification span, and α the attenuation factor of the fiber. The SNR R is
or in dB:
As an example, consider a 10 Gbit·s−1 system with an optical bandwidth B a of 50 GHz, an electrical bandwidth B of ~7 GHz (as a rule of thumb, the electrical bandwidth is taken to be 70% of the bit rate), and a filter bandwidth B 0 of ~12.4 GHz (0.1 nm). Then β = 0.56 and m = 7, a nd a BER of 10 −15 (Q 0 = 8) requires an OSNR R0 of ~17 d B.
Based on the various degradation mechanisms that induce power penalty along the transmission system, equipment vendors specify a minimum SNR required by a given system. Typically, for WDM 10G systems, the minimum OSNR is in the range of 21–22 dB. It must be noted that the required OSNR increases with increasing electrical bandwidth, which is proportional to the bit rate. For instance, by going from 2.5 to 10 Gbit·s−1 the OSNR needs to be increased by about 6 dB (see Equation 1.6). This is a very important constraint, for system designers as well as operators.
1.4.1.4
ACCUMULATION OF NOISE
The accumulation of noise generated by successive amplifiers along an optical line degrades the OSNR. This accumulated noise limits the number
These relationships show that for a given launched power and a given amplification span, the maximum transmission distance, represented by the maximum number of spans, is limited by the minimum required OSNR at the receiver input. For a given distance L , the OSNR increases when G decreases, i.e., when the amplification span becomes shorter. As an example, Equation 1.7 shows that for a link with a fixed length L and a fiber attenuation of 0.2 dB·km−1, an OSNR improvement of 7 dB is achieved when the span length is reduced from Z a = 100 km (a span loss of 20 dB) to Z a = 50 k m (a span loss of 10 dB). The link with the 50-km span length does require twice as many amplifiers, but the gain they each need to supply is reduced by 10 dB. So is their output noise power, and as a result the OSNR is improved. This illustrates how important a parameter the fiber attenuation is. For a given amplification span and a given number of spans, any reduction in this attenuation will result in a better OSNR simply because the amplifier gain G will be smaller. Equivalently, a lower fiber loss allows increasing the optical reach. For example, a fiber loss reduction as small as 0.02 dB·km−1 from 0.23 to 0.21 dB·km−1, in 100-km spans, will allow to reduce the gain by 2 dB and thus to improve the
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industrial school, I suppose."
"That is what the policeman said; but, grandpa, that boy, with his sweet face and gentle ways, to mix with boys of the worst sort—"
"Probably he is one of that class himself, Edith. Depend upon it, the young rogue knows the value of those gentle ways. Now what do you want me to do—not adopt the lad, surely?"
"No, grandpa, not that," said Edith, smiling; "but I was thinking, if we could send him to Mr. Mouncey, he would know what to do with him."
"Mouncey! What made you think of him? Upon my word, that's not a bad suggestion. Mouncey's fond of ragamuffins. He'd make something of him, if any one could. But why should we trouble about it? Better leave him to the magistrate."
"Grandpa, that little boy was in my class one Sunday afternoon; I felt strangely drawn to him; I do still. I am sure he is a good boy."
Colonel Carruthers was silent for a few moments. Across his mind had come again the vision of that other boy, with his broad, fair brow, his open, innocent gaze. His face contracted as if with pain. A heavy sigh escaped him ere he said—
"Do not trust too much to appearances, Edie. These sweet, innocent-looking children are very disappointing. This boy will most likely turn out a scoundrel. But I'll think about it; I will see what I can do."
The colonel did think about it, with the result that he decided to send Gus to Rayleigh, a village in Kent, where the colonel had a small estate, and consign him to the care of the Rev. Sebastian Mouncey, a clergyman who took a special interest in orphan lads, and had instituted a cottage home for them in his parish. But the police desired that Gus should be detained at hand for a day or two, that he might be ready to identify Lucas and his son if they were arrested. But though the police made every effort to track the burglars, and expressed confidence in their ultimate success, the two men appeared to have got well out of their reach.
Gus did not feel particularly glad when Edith told him of the plan that had been made for him. He would rather have gone back to his old life at Sally Dent's. It was a poor home he had with her, but he could hardly remember the days when he had known a better one. Sally had been kind to him in her way, and now he knew he was not to return to them, he was conscious of a strange drawing of the heart towards Sally and her children, and all the people with whom he had lived at Lavender Terrace. He became aware that he had an affection even for the baby—the heavy, fretful, exacting baby, who had so often made his arms to ache, and tried his patience to the utmost.
And Lucy—Gus thought of her with an aching heart. He longed to know what had become of her, and wondered sadly if he should ever see her again. Gus begged that he might be allowed to go and say good-bye to Sally and her children ere he went away into the country.
Edith thought the wish quite to his credit, and persuaded her grandfather to grant it. But he was still suspicious of the lad, and thinking it might be a ruse to
effect an escape, he resolved to accompany Gus to Glensford.
Though Sally had declared that Gus should never darken her doors again, she received him kindly, being impressed by his appearance in the neat new suit of clothes in which Edith had seen him arrayed, and also by the stateliness of the gentleman who accompanied him.
The colonel did not enter the house, but stood on the doorstep, in a position to see that Gus escaped neither by window nor door from Sally's room, into which she had drawn him.
Sally questioned him eagerly as to all that had happened since he left her house; she admired his clothes, she pressed him to drink from her black bottle, whilst the children gathered round him, pleased to see their old friend again.
"Mrs. Dent," said Gus, when at last Sally paused, "I wish you would give me that Bible of father's. I should like to take it with me where I am going; I have nothing that belonged to him. You have it still, have you not?"
Mrs. Dent hesitated. She glanced at the high shelf on which the Bible lay, and then through the half-open door at the colonel's stately form. She longed to refuse the request. She never read the Bible, but none the less she wished to keep that handsome copy. But she knew she had no right to keep it from the boy, and she believed that if she refused, Gus would call the colonel to insist upon her giving him his own. So reluctantly she climbed on a chair, and lifted down the book. She undid the paper in which it was wrapped, and looked regretfully at the embossed covers.
"One don't often see a Bible like this," she remarked, and with that she opened it.
Strange to say, it opened at the very page on which she had last closed it, and again the warning words met her glance, "The wages of sin is death."
She started, closed the book, and thrust it from her.
"Take it!" she cried. "Take it, and welcome. I don't want it. It's not the book for me. I can't bear to be reminded of death and the grave, and all dismal things."
And of sin—she would fain forget that there was such a thing as sin, and that she was a sinner.
"What have you in that parcel?" asked the colonel, when the boy presently came out of Sally's room.
"My father's Bible, sir."
The colonel's person stiffened visibly. He elevated his chin, drew his military cloak about him with an air of annoyance, and with a commanding gesture signed to Gus to precede him as they passed out of Lavender Terrace.
"That's cant," he said to himself; "wants to do the religious, does he? His father's Bible, indeed! As if a boy whose father believed in the Bible would ever have fallen so miserably low!"
A profound distrust of those whom he was wont to describe as the "lower orders" was engrained in the colonel's character, and especially was he doubtful of any such if they professed to be religious.
CHAPTER XV. RAYLEIGH.
WITH that visit to Lavender Terrace, Gus' old life came to an end. The next morning Edith and her grandfather saw him into the train for Rayleigh.
Mr. Mouncey, the energetic young vicar of Rayleigh, would meet him at the other end of his journey. Edith had no fear for him, since he was going to Mr. Mouncey; yet the boy had already won for himself such a place in her heart that she felt parting with him.
"We shall come to Rayleigh in the spring, I hope," she said, "so I shall see you then, Gus."
The boy smiled his sweet, winsome smile, but tears rose suddenly in his eyes. He was leaving this kind friend, he was leaving behind every one he had ever known, and going to a place of which he knew nothing, and his heart sank within him at the thought.
"Look here, young sir," said the colonel sharply, "mind you do your duty where you are going. You attend to what Mr. Mouncey says, and he will make a man of you."
Make a man of him! What sort of a man? Gus wondered, as Miss Edith's form receded from his view, and he knew that the train was bearing him out of London. A
gentleman—such a gentleman as his father had wished him to be? That was what Gus desired to become.
Rayleigh was a long, straggling village, with few houses that were not the dwellings of working people. A deep, still river ran through the place, and supplied the need of its chief industry, the large paper mill that stood upon its banks.
Most of those who dwelt in the small brick cottages were engaged in the mill. The long lines of these cottages were not picturesque, but the village could boast the beauty of a fine old church, with an ivy-mantled tower. Close to the church was the vicarage, a large square house, once the home of a numerous family; but the former vicar had removed to another living, and his successor, being young and unmarried, seemed out of place in the big house. But he was a warm-hearted, genial man, and soon gathered plenty of life about himself.
Within a stone's throw of the vicarage was his cottage home for orphan lads, a veritable home, where, under the care of a good-natured, motherly widow, the boys lived a free and happy life. The vicar showed them a kindness which fell little short of that of a father. The boys had wellnigh the run of the vicarage. They dug and tended the garden, which was one of the best in the neighbourhood, and were rewarded with the finest of the fruit and vegetables.
Mr. Mouncey supplemented the instruction they received at the village school with informal classes held of an evening in his large old dining-room. He had had no intention of founding an orphanage. A desire to befriend a poor woman who had lost her husband, and two young lads who had been deprived of both father and elder brother by
an accident at the mill, had led to the opening of the cottage. Other orphan lads in the neighbourhood, who must otherwise have been sent to the nearest workhouse, were, as time went on, placed in it. Sometimes friends at a distance asked the vicar to take charge of a forlorn lad; but, as a rule, the inmates of the cottage were boys whose antecedents were well known to Sebastian Mouncey. Gus was the first who came there with a stigma upon him—a boy who had associated with thieves.
But Mr. Mouncey had a welcome for him, not alone because of the generous way in which the colonel, whilst prophesying to Mr. Mouncey the disappointment of his hopes, had been ever ready to open his purse to increase the funds by which the cottage home was maintained; but also because the utter friendlessness of the boy was a sure passport to Sebastian's heart.
He kept to himself all that the colonel had told him of Gus' history. The boy should not begin his new life at Rayleigh with an ill name. And whatever fears the vicar might have concerning his new charge, he showed no suspicion of him. Nothing could be kinder than the way in which he welcomed Gus.
Gus, as he looked into his face, and met the glance of Mr. Mouncey's kind, earnest eyes, said to himself, "A gentleman—one of the right sort." And this conviction deepened in the boy's mind as he came to know Sebastian Mouncey better. His was indeed the gentle heart from which springs the gentle life.
As for Mr. Mouncey, he was delightfully surprised at the appearance of the boy committed to his care. Simplehearted as a child himself, and frank almost to eccentricity, he was quick to feel the charm of the boy's artless grace.
Gus was not at all the kind of boy he expected to see. He was not a boy of a low type. There was no sign of deceit, meanness, or rascality on his small, well-formed features. The clergyman looked at him, and marvelled.
It was curious how quickly the two came to understand each other. A bond was forged between them from the hour of their meeting.
Gus got on well with the boys in the cottage home. He was friendly with them all, but he made no special friend of any one. It was the vicar who was his friend.
Mr. Mouncey soon discovered that the boy had unusual abilities. He took trouble and sacrificed time in order to give Gus further instruction than he obtained at the village school. He began to teach him Latin, and was astonished at the rapidity with which he mastered the rudiments of that language. Once when the vicar was teaching him, Gus let fall a word which showed that his father had understood Latin.
"Your father?" said Mr. Mouncey in surprise. "Then he was an educated man?"
"My father was a gentleman once," said Gus, with unconscious dignity.
"Once!" repeated the vicar. "What do you mean?"
"He was a gentleman once," said Gus again; "but then —" The boy paused, and a deep flush of shame dyed his face.
"An educated man who lost his character, and sank to be the companion of thieves and vagabonds," thought the
vicar, and forbore to question the boy further. All he said was, "You must be a true gentleman, Gus."
The boy looked at him with quick, questioning glance.
"Howe'er it be, it seems to me It's only noble to be good—"
Quoted Mr. Mouncey.
Gus' eyes flashed a quick, comprehensive response, but he made no other reply.
The first winter which Gus spent in the country was a happy one. He now made acquaintance with the real country, which is very different from the suburban country on which London so greedily encroaches.
Gus thoroughly enjoyed the long rambles Mr. Mouncey would sometimes take with the boys on a clear, frosty day. Gus loved to see the grass and hedges all glittering with hoar-frost, or to hear the crisp silvery leaves crack beneath his tread as they walked through the woods. He thought the trees looked beautiful, with their great branches, bare save for ivy or lichen, outlined against the blue sky. He loved to watch the birds and squirrels, and even hares made tame almost by severe cold, and to learn all Mr. Mouncey could tell him of their habits. How Gus enjoyed, too, the novel delight of learning to skate on the frozen river, and the effects of the first heavy snowfall, the work of clearing the church and vicarage paths, and the snowballing, which the boys were not likely to omit.
And no less, though in a different way, he enjoyed the bright Sunday services in the beautiful old church, and the class in the vicar's dining-room on Sunday afternoons, when he talked to the boys of the one perfect life of the God-man, through the knowledge of whom we may become "partakers of the Divine Nature."
In the spring, Colonel Carruthers and his granddaughter, accompanied by Miss Durrant, came to Rayleigh.
The colonel's house was at some distance from the vicarage, at the opposite end of the village. It was an oldfashioned thatched house, with a flower garden in front of it. A fir plantation stretched to the right, and behind, just beyond the well-stocked kitchen garden, rose a bit of breezy, furzy common, the top of which commanded a view all over the village.
Looking down, one saw the river stealing along the meadows with a gentle curve, now to this side and now to that, till it reached the large stone buildings, pierced by numerous small windows, where so many hands were employed in paper-making. Near the mill stood a large house, also of stone, and an imposing structure in its way, with bay windows and turrets. This was the residence which the owner of the mill had built for himself. Observing the house and the well-laid-out gardens which surrounded it, one could hardly doubt that the business was a prosperous concern.
Yet, if rumour could be trusted, the mill had not paid well of late, and the proprietor was beset by difficulties. It was feared that a day might come when the working of the mill would be suspended; an event which threatened ruin to the hands employed. But months had passed since these
rumours began to circulate, and the mill had gone on working all the same. And now that the winter was over, and signals of summer's approach were beginning to appear everywhere, anxiety ceased to burden the minds of the working folk.
On the April evening following that of their arrival at Rayleigh, Edith and her grandfather were walking across the common, enjoying the beauty of the sunset, when Mr. Mouncey climbed over the low stone wall, accompanied by two or three of his boys, Gus being one of the party. It was the colonel's first meeting with the vicar, and he greeted him heartily.
"This is never Gus," said Edith, turning to look at the boys, when she had shaken hands with the clergyman.
The boy had indeed greatly changed since she saw him. He had grown taller, and though slender, looked strong and well-formed. His hair, though closely cut, still showed a tendency to curl; his cheeks had now the bright hue of health; his eyes were blue as ever, and the smile which lit up his face as Edith spoke to him had all the old sweetness. But whilst it was the same sweet, boyish face, the seriousness of its expression had deepened. Gus had learned much and thought much since he left London, and his countenance revealed the quickened mental and spiritual life.
The colonel had nodded kindly to Gus. His eyes now took quick, observant survey of the boy as he stood talking to Edith. His glance seemed to sadden as it rested on the child.
"He has improved," he remarked in an undertone. "What do you think of him, Mouncey? How will he do?"
"Well," was the prompt reply; "I have not a doubt of it. I never had such a boy as Gus in my home before."
"Indeed! How does he differ from the others?"
"In every way. They are good lads, most of them, but their minds are dull and slow, their manners rough and boorish. There is a peculiar gentleness about Gus, a goodness of heart, an unselfishness, an innate charm, I hardly know how to describe. Then as to his mind—he can learn anything; he grasps my ideas in a moment. Oh, I have the highest hopes of him."
"I would not have, if I were you," said the colonel drily. "You are too sanguine, as I often tell you. Depend upon it, you will be disappointed."
"I am not afraid," said Mr. Mouncey, with a smile. "I can tell you, Gus is a little gentleman."
"Gus—Gus what?" asked the colonel abruptly. "I suppose he has another name?"
"Gus Rew," said Mr. Mouncey.
"Rew! You can't make much of that," said Colonel Carruthers. "It is not an aristocratic patronymic. But every one is a gentleman nowadays. The grand old name is indeed—
"Defamed by every charlatan, And soiled with all ignoble use.
"Still, to apply it to the son of a burglar does seem to me going too far."
"I do not think that Gus' father was a burglar," replied Mr. Mouncey.
The colonel shrugged his shoulders impatiently.
Mr. Mouncey thought it advisable to introduce another subject.
"Have you heard of the change we are to have here?" he asked, pointing towards the mill.
"I have heard nothing; I only arrived last night."
"Mr. Gibson has sold the mill. You know, perhaps, that for some time past he has been carrying it on at a loss. Now he has made over the whole concern to some one else."
"And who is the purchaser?"
"Some one from London. Philip Darnell is his name."
As it passed his lips, Mr. Mouncey saw that the name was not unknown to the colonel. The old soldier gave a slight start, his colour changed, his brow contracted, as if with pain.
"You know this gentleman, perhaps?" Mr. Mouncey ventured to suggest.
"By hearsay only," replied Colonel Carruthers stiffly. "He is of your complexion politically, Mouncey. He came forward as a candidate for one of the suburban boroughs some time ago, but was not elected. You will perhaps find him a sympathetic companion."
Mr. Mouncey shook his head, and smiled goodtemperedly. "He will hardly sympathise with my politics, I fear," he said. "He must be a rich man, and few rich men can tolerate such an out-and-out Radical as I am."
Another one present had caught the name of Philip Darnell, and was no less startled by it than the colonel. Gus stood near enough to hear it, and the sound sent a thrill through his boyish frame. Philip Darnell! His father's enemy! The man he had seen on that summer morning nearly a year ago, when he had made that long tramp with his father in search of employment, and his father had refused the work when found because it was to be done for this man.
In the night that followed his father had died, and every incident of that last day together was vividly imprinted on Gus' memory. This was the man who had wrought his father's ruin, the man on whom he had promised to be revenged, if ever it was in his power. Was the chance coming to him now?
Gus remembered every word which his father had uttered concerning Philip Darnell; but other words which his father had said to him had almost faded from his mind. He had hardly given a thought since his father's death to the fact that Rew was not his real name. He had forgotten the slip of paper inserted within the lining of the old Bible; but now it suddenly flashed on his mind, and he wondered what was the long name his father had written that day.
At that moment Mr. Mouncey, was saying, "Goodevening, Colonel Carruthers."
Gus had heard the colonel's name often before; but a novel thought came to him as he heard it now. Had not the name his father had declared to be his sounded something
like that—something like Carruthers? Could it be? But, no, it must be his fancy; it seemed impossible that a poor boy, such as he was, should have the same name as a gentleman like the colonel. And what did it matter what his right name was? Gus Rew was a nice, easy little name, which did very well for him.
