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The use of stimulated Raman scattering (SRS) as a means of amplifying signals in telecommunication systems has been demonstrated since 1976 [1]. Yet despite its advantages over erbium-doped fiber, Raman amplification was not used in the first generation of deployed optically amplified systems. One of the principal reasons for this was the lack of reliable high-power pump sources needed for Raman amplification. It was in this environment that the cascaded Raman fiber laser (RFL) was invented. [ABSTRACT FROM AUTHOR]

After discovery of a new physical effect, the speed to widespread commercialization is dictated by complementary technologies and market need. In the case of the Raman effect, the spectral shift of scattered light from "molecules in dust-free liquids or gases" observed in 1928 [1, 2] has long been used for the spectroscopic characterization of materials. Yet application of it to amplification in optical fiber has had a long wait: both for the development of the key enabling technologies, namely, high-power pump lasers and low-loss optical fiber, and for the commercial need for significantly reduced cascaded optical noise buildup in fiber transmission systems. [ABSTRACT FROM AUTHOR]

In this chapter we focus on the use of discrete or lumped Raman amplifiers in the short-wavelength S-band. Recent advances in data communications have led to requirements for higher throughput of wavelength-division-multiplexed (WDM) transmission systems. Two approaches have been pursued to address the increasing throughput requirements, namely, increasing the spectral efficiency of WDM systems within existing transmission bands, and increasing optical bandwidth to utilize much more of the low-loss window in silica fiber than presently supported on WDM systems. The first approach has been pursued through coding and modulation techniques. The second approach to increasing throughput involves making available new transmission bands beyond those supported by conventional erbium-doped fiber amplifiers (EDFAs), which presently define the optical bandwidth of most WDM systems. [ABSTRACT FROM AUTHOR]

High-capacity terrestrial optical transmission systems are now being deployed, offering aggregate capacities of 1 Tb/s or higher over distances of more than 1000 km. These systems use dense wavelength-division-multiplexing (DWDM) operating with more than 100 channels at a 10 Gb/s line rate and channel spacing of 50 GHz in both the C- and L-bands. In order to offer scalable solutions for future traffic growth in the backbone network, the 40 Gb/s line rate appears to be the natural successor to 10 Gb/s. However, with the downturn in the telecommunications industry in 2001 and 2002, natural questions to ask are: why are 40 Gb/s line rates needed, and why are researchers pursuing 40 Gb/s technologies and transmission? The answers are twofold: 40 Gb/s line rates will help to meet increased bandwidth demands and 40 Gb/s line rates reduce cost. [ABSTRACT FROM AUTHOR]

In most chapters of this book, stimulated Raman scattering (SRS) is invoked intentionally. Pump radiation is coupled into the fiber carrying the signal radiation to generate Raman gain. The Raman gain can be used very advantageously, for example, to improve the optical signal-to-noise ratio (OSNR) budget by distributed amplification in the transmission fiber. However, SRS also occurs unintentionally in WDM transmission systems. Due to the large number of channels inside the Raman gain bandwidth, total power can add up to levels where considerable amounts of SRS are generated, with the signal channels acting as pumps. In contrast to the beneficial effects of intentional Raman pumping, the unintended generation of SRS usually degrades system performance. [ABSTRACT FROM AUTHOR]

In the late 1990s, the telecommunications industry began to see dense WDM as the way to provide for the seemingly explosive growth in demand for transmission capacity. The usual industry practice of using electronic regeneration at every node point (typically, once every 400 to 600 km; see Fig. 18.1), however, promised to use far too much capital equipment and office space, especially if the net transmission rates were to be at terabit levels. For example, a system carrying one terabit/s in each direction, at the practical and increasingly popular per-channel rate of 10 Gbit/s, would require no less than 200 (expensive and bulky) regenerators, or OT units per node (one for each direction and channel). In the meantime, it was already well known, principally from undersea practice, that such dense WDM could be successfully carried out, without regeneration, over transoceanic distances, at least under the special conditions of the undersea environment. Thus the idea of developing an all-optical terrestrial system (which had in fact been advanced many years ago [1-3]) began to take root and to undergo engineering development by many firms. [ABSTRACT FROM AUTHOR]

Optical communication is the science of transmitting information over a distance using light. The engineering difficulties vary greatly because the distances to be traversed differ significantly. For some, the task may be the need to interconnect electronic integrated circuits within a computer. For others, the distance to be covered may be between computers in a building or between the buildings in a city. The differences between these cases are the distances over which information must be carried. These differences dictate the nature of the technologies required to implement the connections. [ABSTRACT FROM AUTHOR]

This chapter describes the technologies needed for cascading an erbium-doped fiber amplifier (EDFA) and a fiber Raman amplifier (FRA or RA) to create a hybrid amplifier (HA), the EDFA/Raman HA. Two kinds of HA are defined in this chapter: the narrowband HA (NB-HA) and the seamless and wideband HA (SWB-HA). The NB-HA employs distributed Raman amplification in the transmission fiber together with an EDFA and provides low noise transmission in the C- or L-band. The noise figure of the transmission line is lower than it would be if only an EDFA were used. The SWB-HA, on the other hand, employs distributed or discrete Raman amplification together with an EDFA, and provides a low-noise and wideband transmission line or a low-noise and wideband discrete amplifier for the C- and L-bands. The typical gain bandwidth (Δλ) of the NB-HA is ~30 to 40 nm, whereas that of the SWB-HA is ~70 to 80 nm. [ABSTRACT FROM AUTHOR]

Lightwave communication systems carry information that is encoded onto the intensity, phase, or polarization of light from one point to another along an optical path. When designing such systems, many mechanisms that degrade the transfer of information must be taken into account. Until the late 1990s, the main causes of signal degradation in transmission were fiber nonlinearity and amplified spontaneous emission (ASE) from optical amplifiers. More recently, however, a third type of system degradation, involving the unwanted beating of the signal with a number of weak interferers, has become increasingly important. With reference to Fig. 15.1(a), such interferers can result from imperfect extinction of the drop-signal in optical cross-connects and add-drop multiplexers, which are both key elements for flexible and transparent optical network architectures [1,2]. Also, single-Rayleigh backscattering in bidirectional transmission systems [3, 4] can lead to unwanted interferers at the receiver. Although these two examples involve interferers that are independent of the main signal, the important class of multiple-path interference (MPI) involves interferers that are delayed replicas of the main signal. In the case of MPI, additional (unwanted) optical paths, with losses orders of magnitude greater than the main path, lead to interfering signals at the receiver, and can have a significant impact on system performance. With reference to Figs. 15.1 (b) and 15.1(c), MPI is encountered for discrete reflections within or surrounding optical amplifiers [5],double-Rayleigh scattering in optical amplifiers [6, 7],double-Rayleigh scattering in the transmission span [8], orunwanted transverse mode mixing in higher-order mode dispersion compensators[9]. [ABSTRACT FROM AUTHOR]

Distributed Raman amplification can be achieved by optical pumping at either end of the fiber. In the copumped Raman configuration, the pump is launched at the front end and copropagates with the optical signal along the transmission span. In the counter-pumped architecture that is widely deployed, the optical pump and signal launch at the opposite ends. Finally, Raman pumping at both ends of the transmission span characterizes the bidirectional scheme. The latter term is also used for optical links that support two-way signal traffic and often leads to confusion. To avoid this ambiguity, we designate optical transmission as unidirectional or bidirectional, independently of any amplification considerations. In unidirectional transmission lines, all signals travel in the same direction. Bidirectional network functionality is supported by a separate fiber that carries signal traffic propagating in the opposite direction. In contrast, bidirectional transmission can be used to realize two-way traffic within a single fiber line: counterpropagating signal traffic is launched and received at the opposite ends of the optical link. A bidirectionally pumped fiber span can support both uni- and bidirectional signal transmission. A unidirectionally pumped span, however, almost exclusively supports unidirectional signal traffic. [ABSTRACT FROM AUTHOR]

This chapter describes designs and experiments that apply the Raman effect to wideband amplifiers (WBAs). In the context of this chapter, wideband corresponds to a bandwidth of approximately 50 to 100 nm or more. We start by explaining the need for WBAs, and briefly review some of the key enabling technologies for wideband systems. Section 14.2 describes several approaches for WBA, including the erbium-doped fiber amplifier (EDFA) and Raman amplifier combinations as well as all-Raman amplifiers. Section 14.3 summarizes the advantages and challenges of the all-Raman approach, the focus of this chapter. Section 14.4 identifies the key physical principles that need to be considered in the design of all-Raman WBAs. Then, perhaps the most important section of this chapter, Section 14.5 describes engineering design rules for construction of all-Raman WBAs that satisfy gain and noise figure performance requirements of typical long-haul and ultra-long-haul fiber-optic transmission systems. Several WBA experiments that use either EDFA/Raman amplifier combinations or all-Raman amplifiers are illustrated in Section 14.6, and exemplary wideband system experiments are described in Section 14.7. Finally, we summarize and conclude the chapter in Section 14.8. [ABSTRACT FROM AUTHOR]

The time response of the Raman effect is associated with the vibrations of the molecules in the gain medium and is on the order of several hundred femoseconds [1]. Compared to current data rates, this energy transfer is practically instantaneous resulting in very fast-gain dynamics. Fast-gain dynamics in semiconductor amplifiers results in a large amount of crosstalk between signal channels, even at high frequencies, and poor system performance. However, the Raman effect in optical fibers is quite weak such that the gain cavity needs to be several kilometers long. [ABSTRACT FROM AUTHOR]

This chapter discusses issues surrounding the pump laser diodes for broadband Raman amplifiers, which range from fundamentals to industry practices of Raman pump sources based on the so-called 14XX nm pump laser diodes. It also refers to design issues of wavelength-division-multiplexed (WDM) pumping for realizing a broad and flat Raman gain spectrum over the signal band. Section 5.1 introduces fundamentals of pump laser diodes. Section 5.2 refers to the principle and design issues of WDM pumping technique. Section 5.3 discusses details of pump laser diodes and their efficiently combining and depolarizing technologies. Section 5.4 describes practical Raman pump units. And Section 5.5 briefly touches upon ongoing issues on copumped Raman amplifiers and their pumping sources. [ABSTRACT FROM AUTHOR]

The Raman scattering of light is one of the oldest and most well-studied optical phenomena. It was discovered more than 70 years ago and was named after one of the authors of this discovery. In 1928 C. V. Raman and K. S. Krishnan published the paper in which they described light scattering in liquids, the frequency of the scattering light being less than the frequency of the initial light [1]. The same year Russian physicists G. S. Landsberg and L. I. Mandelstam observed independently a similar light scattering in quartz [2]. [ABSTRACT FROM AUTHOR]

Dispersion-compensating fibers (DCF) are the most widely used technology for dispersion compensation. The idea to additionally use the DCF as a Raman gain medium was originally proposed by Hansen et al. in 1998 [1]. This was quickly followed by Emori et al., [2], who demonstrated a broadband lossless DCF using multiple-wavelength Raman pumping. DCF is a good Raman gain medium, due to a relatively high germanium doping level and a small effective area. Normally, a discrete Raman amplifier will contain several kilometers of fiber, adding extra dispersion to the system that must be handled in the overall dispersion management. Dispersion-compensating Raman amplifiers integrate two key functions, dispersion compensation and discrete Raman amplification, into a single component. [ABSTRACT FROM AUTHOR]

Spontaneous emission is the inevitable consequence of gain in an optical amplifier. In this chapter, the definition of noise figure is shown to be useful only in characterizing shot noise and signal-spontaneous beat noise. The noise characteristics of both discrete and distributed Raman amplifiers are then presented. The choice of discrete amplifiers alone, or together with distributed optical amplifiers results as a trade-off between maximizing optical signal-to-noise ratio at the expense of increases in nonlinear distortion of the signal due to high signal intensities. Hansen et al. [1] showed that distributed amplification could be used to obtain a significant improvement in system margin that could be used to upgrade the transmission capacity, either in terms of more channels, or a faster line rate. [ABSTRACT FROM AUTHOR]

Raman scattering was discovered independently and almost simultaneously in 1928 by groups in India and Russia [1, 2]. If C.V. Raman had not published first we might know Raman scattering as the Landsberg-Mandelstam Effect. Raman was awarded the 1930 Nobel Prize for the discovery, which was not shared with the Russians. Neither group was actually looking for what we now know as the Raman effect [3]. Landsberg and Mandelstahm were looking for a small wavelength shift due to scattering from thermal fluctuations, now called "Brillouin scattering." Raman was seeking an optical analogue of the Compton effect. It was quickly understood that Raman scattering is a shift in the frequency of scattered light due to interaction of the incident light with high-frequency vibrational modes of a transparent material. It was later pointed out that the correct interpretation had been predicted by A. Smekal in an obscure 1923 theoretical paper [4]. [ABSTRACT FROM AUTHOR]

The potential of stimulated Raman scattering to turn transmission fiber spans into their own, distributed, low-noise amplifiers has been recognized and generally understood for several decades [1,2]. Indeed, before the erbium-doped fiber amplifier became available in the late 1980s, the Raman effect provided just about the only practical form of optical gain for fiber transmission, and was used in the first successful long-haul demonstrations [3]. Nevertheless, for early all-optical transmission systems, which tended to involve just one, or at most a handful, of WDM channels, the lumped erbium-doped fiber amplifier became the technology of choice, largely on the issue of the required pump powers. That is, when the total signal power was at most a few mW, there was little incentive to incur the trouble and expense of the hundreds of mW of pump power required for Raman amplification, when a very simple erbium fiber amplifier, pumped with just a few tens of mW, could provide the necessary narrowband gain. Thus the issue of pump power tended to completely overshadow the nearly contemporaneous understanding that distributed Raman amplification could provide for greatly reduced growth of ASE noise, especially in the context of the long (80 to 100 km) amplifier spans typical for terrestrial systems [4]. With the more recent advent of dense WDM and its demands for flat gain over wide bandwidths and greatly increased net signal power, however, the erbium fiber amplifiers rapidly became very complex, and began to require pump powers little different from those required for Raman gain! The consequent commercial development of high-powered, 1480 nm diode lasers (easily generalized to 14XX nm) tended to remove the principal obstacle to the use of Raman gain. Thus Raman amplification is now being rediscovered, not only for its superior noise performance, but equally for the ease with which it enables the establishment and maintenance of flat gain over the wide bands of dense WDM. [ABSTRACT FROM AUTHOR]

In the early 1970s, Stolen and Ippen [1] demonstrated Raman amplification in optical fibers. However, throughout the 1970s and the first half of the 1980s, Raman amplifiers remained primarily laboratory curiosities. In the mid-1980s, many research papers elucidated the promise of Raman amplifiers, but much of that work was overtaken by erbium-doped fiber amplifiers (EDFAs) by the late 1980s. However, in the mid to late 1990s, there was a resurgence of interest in Raman amplification. By the early part of the 2000s, almost every long-haul (typically between 300 and 800 km) or ultra-long-haul (typically longer than 800 km) fiber-optic transmission system uses Raman amplification. There are some fundamental and technological reasons for the interest in Raman amplifiers that this book explores. [ABSTRACT FROM AUTHOR]