05-09-2012, 04:31 PM
Raman Amplifiers for Telecommunications
[attachment=33462]
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.
INTRODUCTION
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 [2]. However, in the
mid- to late 1990s, there was a resurged interest in Raman amplification.
By the early part of 2000s, almost every long-haul
(typically defined 300 to 800 km) or ultralong-haul (typically
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.
RAMAN AMPLIFICATION IN OPTICAL FIBERS
Raman gain arises from the transfer of power from one optical
beam to another that is downshifted in frequency by the energy
of an optical phonon . The Raman gain spectrum in fused
silica fibers is illustrated in Fig. 1(a) [1]. The gain bandwidth is
over 40 THz wide, with the dominant peak near 13.2 THz. The
gain band shifts with the pump spectrum, and the peak value of
the gain coefficient is inversely proportional to the pump wavelength.
In the telecommunications bands are around 1500 nm
13.2 THz corresponds to approximately 100 nm.
Fig. 1(b) illustrates the polarization dependence of Raman
gain [18]. The copolarized gain is almost an order of magnitude
larger than the orthogonal polarization gain near the peak
of the Raman curve. Nonetheless, a polarization-independent
Raman amplifier can be made by using polarization diversity
pumping to avoid polarization dependent loss. Furthermore, the
mixture of modes in a nonpolarization-maintaining fiber helps
to scramble the polarization dependence.
Sources of Noise in Raman Amplifiers
There are four primary sources of noise in Raman amplifiers.
The first is double Rayleigh scattering (DRS), which corresponds
to two scattering events (one backward and the other
forward) due to the microscopic glass composition nonuniformity.
Amplified spontaneous emission (ASE) traveling in the
backward direction will be reflected in the forward direction by
DRS and experience gain due to stimulated Raman scattering.
This in addition to ASE experiencing multiple reflections, will
degrade the signal-to-noise ratio (SNR). Furthermore, multipath
interference of the signal from DRS will also lower the SNR.
DRS is proportional to the length of the fiber and the gain in the
fiber, so it is particularly important in Raman amplifiers due to
the long length of fiber, where lengths of several kilometers are
typically required. From a practical viewpoint, DRS limits the
gain per stage to approximately 10 to 15 dB. Higher gain amplifiers
can be obtained through the use of isolators between the
multiple stages of amplification. For example, a 30-dB discrete
Raman amplifier has been demonstrated with two stages of amplification
and a noise figure less than 5.5 dB [4].
DISCRETE RAMAN AMPLIFIERS
Discrete Raman amplifiers refer to a lumped element that
is inserted into the transmission line to provide gain. Unlike a
DRA, all of the pump power is confined to the lumped element.
For example, Fig. 7 shows the typical setup for a lumped
Raman amplifier. In this particular case, counterpropagating
pump power is confined within the unit by the use of isolators
surrounding the amplifier. Compared with Fig. 4, no pump
power enters the transmission line.
The primary use for discrete Raman amplifiers is to open new
wavelength bands in fused silica fibers. For example, different
wavelength bands are illustrated in Fig. 8. EDFAs operate in
the -band, which stretches from 1530 to 1565 nm, and the
-band, which stretches from about 1565 to 1625 nm. There
is also the -band, which stretches from roughly 1480 to 1530
nm, which has at least as low loss as the EDFA bands. In addition,
the band extends from approximately 1430 to 1480
nm. Earlier transmission systems were deployed in the 1310-nm
band, which can stretch between from 1280 and 1340 nm. There
is also a 1400-nm band, which is only useful in new fibers that
use special drying techniques to reduce the water peak absorption
around 1390 nm. Thus, Raman amplifiers can be used to
open up wavelengths between about 1280 and 1530 nm, a wavelength
range that is inaccessible by EDFAs.
SUMMARY
There has been a revived interest in Raman amplification due
to the availability of high pump powers and improvements in
small core size fibers. Two general categories of Raman amplifiers
are DRAs and lumped or discrete Raman amplifiers. DRAs
improve the noise figure and reduce the nonlinear penalty of the
amplifier, allowing for longer amplifiers spans, higher bit rates,
closer channel spacings, and operation near the zero-dispersion
wavelength. DRAs are already becoming commonplace in most
long-haul networks.
[attachment=33462]
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.
INTRODUCTION
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 [2]. However, in the
mid- to late 1990s, there was a resurged interest in Raman amplification.
By the early part of 2000s, almost every long-haul
(typically defined 300 to 800 km) or ultralong-haul (typically
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.
RAMAN AMPLIFICATION IN OPTICAL FIBERS
Raman gain arises from the transfer of power from one optical
beam to another that is downshifted in frequency by the energy
of an optical phonon . The Raman gain spectrum in fused
silica fibers is illustrated in Fig. 1(a) [1]. The gain bandwidth is
over 40 THz wide, with the dominant peak near 13.2 THz. The
gain band shifts with the pump spectrum, and the peak value of
the gain coefficient is inversely proportional to the pump wavelength.
In the telecommunications bands are around 1500 nm
13.2 THz corresponds to approximately 100 nm.
Fig. 1(b) illustrates the polarization dependence of Raman
gain [18]. The copolarized gain is almost an order of magnitude
larger than the orthogonal polarization gain near the peak
of the Raman curve. Nonetheless, a polarization-independent
Raman amplifier can be made by using polarization diversity
pumping to avoid polarization dependent loss. Furthermore, the
mixture of modes in a nonpolarization-maintaining fiber helps
to scramble the polarization dependence.
Sources of Noise in Raman Amplifiers
There are four primary sources of noise in Raman amplifiers.
The first is double Rayleigh scattering (DRS), which corresponds
to two scattering events (one backward and the other
forward) due to the microscopic glass composition nonuniformity.
Amplified spontaneous emission (ASE) traveling in the
backward direction will be reflected in the forward direction by
DRS and experience gain due to stimulated Raman scattering.
This in addition to ASE experiencing multiple reflections, will
degrade the signal-to-noise ratio (SNR). Furthermore, multipath
interference of the signal from DRS will also lower the SNR.
DRS is proportional to the length of the fiber and the gain in the
fiber, so it is particularly important in Raman amplifiers due to
the long length of fiber, where lengths of several kilometers are
typically required. From a practical viewpoint, DRS limits the
gain per stage to approximately 10 to 15 dB. Higher gain amplifiers
can be obtained through the use of isolators between the
multiple stages of amplification. For example, a 30-dB discrete
Raman amplifier has been demonstrated with two stages of amplification
and a noise figure less than 5.5 dB [4].
DISCRETE RAMAN AMPLIFIERS
Discrete Raman amplifiers refer to a lumped element that
is inserted into the transmission line to provide gain. Unlike a
DRA, all of the pump power is confined to the lumped element.
For example, Fig. 7 shows the typical setup for a lumped
Raman amplifier. In this particular case, counterpropagating
pump power is confined within the unit by the use of isolators
surrounding the amplifier. Compared with Fig. 4, no pump
power enters the transmission line.
The primary use for discrete Raman amplifiers is to open new
wavelength bands in fused silica fibers. For example, different
wavelength bands are illustrated in Fig. 8. EDFAs operate in
the -band, which stretches from 1530 to 1565 nm, and the
-band, which stretches from about 1565 to 1625 nm. There
is also the -band, which stretches from roughly 1480 to 1530
nm, which has at least as low loss as the EDFA bands. In addition,
the band extends from approximately 1430 to 1480
nm. Earlier transmission systems were deployed in the 1310-nm
band, which can stretch between from 1280 and 1340 nm. There
is also a 1400-nm band, which is only useful in new fibers that
use special drying techniques to reduce the water peak absorption
around 1390 nm. Thus, Raman amplifiers can be used to
open up wavelengths between about 1280 and 1530 nm, a wavelength
range that is inaccessible by EDFAs.
SUMMARY
There has been a revived interest in Raman amplification due
to the availability of high pump powers and improvements in
small core size fibers. Two general categories of Raman amplifiers
are DRAs and lumped or discrete Raman amplifiers. DRAs
improve the noise figure and reduce the nonlinear penalty of the
amplifier, allowing for longer amplifiers spans, higher bit rates,
closer channel spacings, and operation near the zero-dispersion
wavelength. DRAs are already becoming commonplace in most
long-haul networks.