Signal Amplifiers: All within the Fiber

Nobody really had an idea then that the phenomenon of scattering of light
called Raman Effect, the Nobel prize winning discovery of Indian scientist CV
Raman in 1928, will become so important and critical to the design of
high-capacity photonic networks. What a way of amplification of optical signals
when the optical fiber itself becomes the amplifying medium! A process called
Raman Amplification is now emerging as a very useful technology in order to
extend the transmission span distances considerably in building large-capacity
optical networks more cost-effectively.

Optical signals throughout the length of a transmission optical fiber cable
in a photonic network are enhanced due to the phenomenon of scattering of
pump-laser wavelength from atoms in the optical fiber that changes
pump-wavelength to that of the optical signal, thereby amplifying the signal.
This process of amplification of optical signals by transforming the fiber
itself into an amplifier is called Raman Amplification, named after CV Raman.
This kind of scattering is called Stimulated Raman Scattering (SRS) or simply
Raman scattering. Raman amplification is usually accomplished as a ‘distributed
process’ since it happens throughout the length of the actual transmission
fiber rather than in one place and hence is called more appropriately as
Distributed Raman Amplification (DRA).

The
basic premise of Raman scattering is based on injection of a lower wavelength
(lower than the signal to be amplified) high-power pump laser light into the
fiber that traverses along the length of the transmission fiber. It scatters off
the atoms in the fiber, losses some energy to the atoms, and then continues its
journey with the same wavelength as the data signal to be amplified. Therefore,
the data signal has additional photons of light representing it and is thus
amplified.

There are various ways of pumping the light into the fiber to accomplish
Raman amplification. If the pump-laser is inserted at the beginning of the
fiber, it is known as co-pumping or forward pumping. If the pump-laser is
inserted at the far-end of the fiber, it is known as counter-pumping or backward
pumping. Another way could be co-counter, the combination of these two. Most
commonly, backward pumping is used since the power amplitude fluctuations due to
the pump noise are averaged out unlike in forward pumping.

Application Areas

There are new challenges that need to be overcome in the traditional
configuration of DWDM systems. Dramatically burgeoning service demand generated
by the stupendous growth of the Internet world has become the major cause of
such challenges today. In order to achieve these demanding requirements, future
systems need to conform to the enhanced performance criteria such as:

• Transmission of higher total payload through increased bit rate per
  channel and number of channels multiplexed together

• Cost-reduction by minimizing the number of amplifiers
  required permitting longer spans, and

• Reduction of signal distortion to allow long-distance
  transmission links.

Raman amplification in an optical network would help meet
these challenges efficiently. It increases the span distance between the optical
amplifiers that are used to boost the optical signals and permits higher data
capacity by spacing wavelength channels closer together. This results in
remarkable reduction of the capital expenditure in building an optical network.
According to an estimate a four-fold increase in network capacity employing this
system would result in cutting down service provider cost by the order of 30
percent. In a conventional optical network system, signals are amplified by
Erbium Doped Fiber Amplifiers (EDFAs) deployed at every 80 km, typically. Using
a 40 gigabit technology in a conventional manner, without Raman Amplification,
will require four times the launched power creating distorted signals and
cross-talk between channels.

Raman amplification enjoys several distinguished advantages
over EDFA-based technology, including the following.

• Raman amplifiers are topologically simpler to design.
  They do not require a special medium, and direct signal amplification can be
  achieved in the existing optical fiber

• Low noise

• Raman amplification can potentially be achieved in every
  region of transmission window of the optical fiber. Since Raman gain is
  dependent only on pump wavelength, signal frequencies can be assigned
  flexibly

• Broad gain bandwidth is achievable by multi-wavelength
  pumping that is combining the amplification effects of several pump waves
  that are carefully selected in the wavelength spectrum

Although, Raman amplification also has certain limitations,
yet it is proving superior to the EDFA-based technology, topologically, for
building optical networks resulting in substantial reduction in expenditure. The
main drawback of Raman amplification is the need of high pump power to provide
reasonable high gain. The selection of pump power and wavelengths as well as the
number of wavelengths and separation of pump wavelengths should be done
carefully as it determines the wavelength behavior of Raman gain and noise.
Raman gain depends strongly on two components–the pump power, and the
wavelength-offset between the pump and the signal. Raman gain response is
asymmetrical. It increases almost linearly with the wavelength-offset between
pump and signal at about 100 nm and then suddenly drops with increased offset
after the gain peak. Also, the flat gain response characteristics of Raman
amplifier are achieved by carefully selecting the pump wavelengths.

While designing complex WDM photinic network systems with
Raman amplification that require a throughput of high capacity, it is mandatory
to take into account many aspects. These aspects include the non-linear
propagation effects namely amplifier noise, cross phase modulation, and
four-wave mixing when deciding on optimum signal and pump power. There can be
some degradation effects due to cross talk since, in addition to pump wave some
WDM channel may amplify the other WDM channel. Apart from these negative
effects, effects of spontaneous Raman scattering and Rayleigh scattering must be
considered for accurate analysis of advanced WDM systems.

Attributed to its superiority, Raman amplification can be
deployed in a host of network applications as listed below.

• As a means to partially compensate fiber attenuation
  using the Raman Effect and thus to increase EDFA spacing

• Raman pump wave can be conveniently placed at EDFA
  locations. This results in less number of EDFAs and less number of sites
  that are required to be maintained, and thus decreases the cost.

• EDFA—Raman Hybrid amplification, characterized by a
  flat gain over a large bandwidth, can be employed to build special repeaters
  that compensate non-flatness of the EDFA gain. EDFA structures cover a
  defined region of wavelength for amplification while Raman amplification is
  used for signal amplification in transmission windows that cannot be covered
  by these EDFAs. This topology is especially beneficial in existing network
  systems where it provides for upgradation by merely opening a new window for
  Raman amplification.

Ramdev Sharma

head (product marketing), Huawei Technologies India