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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]

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]