Abstract—Raman amplifiers are being deployed in almost
every new long-haul and ultralong-haul fiber-optic transmission
systems, making them one of the first widely commercialized
nonlinear optical devices in telecommunications. This paper
reviews some of the technical reasons behind the wide-spread
acceptance of Raman technology. Distributed Raman amplifiers
improve the noise figure and reduce the nonlinear penalty of fiber
systems, allowing for longer amplifier spans, higher bit rates,
closer channel spacing, and operation near the zero-dispersion
wavelength. Lumped or discrete Raman amplifiers are primarily
used to increase the capacity of fiber-optic networks, opening up
new wavelength windows for wavelength-division multiplexing
such as the 1300 nm, 1400 nm, or short-wavelength -band. As
an example, using a cascade of -band lumped amplifiers, a
20-channel, OC-192 system is shown that propagates over 867 km
of standard, single-mode fiber. Raman amplifiers provide a simple
single platform for long-haul and ultralong-haul amplifier needs
and, therefore, should see a wide range of deployment in the next
few years.
Index Terms—Optical amplifiers, optical fiber amplifiers, optical
fiber communications, Raman lasers, nonlinear optics.
I. INTRODUCTION
I
N 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 ampli-
fiers, but much of that work was overtaken by erbium-doped
fiber amplifiers (EDFAs) by the late 1980s [2]. However, in the
mid- to late 1990s, there was a resurged interest in Raman am-
plification. By the early part of 2000s, almost every long-haul
(typically defined 300 to 800 km) or ultralong-haul (typ-
ically defined above 800 km) fiber-optic transmission system
uses Raman amplification. There are some fundamental and
technological reasons for the interest in Raman amplifiers that
this paper will explore.
The first section of this paper reviews the physics of Raman
amplification in optical fibers. The second section of the paper
focuses on a distributed Raman amplifiers (DRAs). We show
that a DRA can improve the signal to noise ratio and nonlinear
penalty of amplifiers. Then, the third section discusses discrete
or lumped Raman amplifiers. We show that discrete Raman
amplifiers are often used to open up new wavelength bands in
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