Raman Amplifiersfor Long HaulTransmissionPresented By:George HarrisMike McDonough

IntroductionRaman amplifiers are being used in almost every new long-haul and ultra-long-haul fiber-optic transmission system, making them one of the first widely commercialized nonlinear optical devices in telecommunications.

Raman ScatteringRaman amplifiers take advantage of stimulated Raman scattering (SRS), a nonlinear process that occurs in all optical fibers.SRS can be used to amplify an optical signal at a certain wavelength by the use of a strong radiation at a lower wavelength, called the pump.

Raman Scattering (Contd)Raman scattering is the interaction of a photon with an optical phonon.The phonon releases energy that creates one or multiple phonons with the difference energy and momentum.

Raman AmplifiersRaman amplifiers provide a simple, single platform for long-haul and ultra-long-haul amplifier needs.Raman amplifiers are broad-band and wavelength independent.Raman amplifiers can serve as low-noise preamplifiers, or they can meet the full amplifier needs in all-Raman systems.Raman amplifiers can be distributed, lumped or discrete, or hybrid.

Advantages of Raman AmplifiersFirst, Raman gain exists in every fiber, which provides a cost-effective means of upgrading from the terminal ends.Second, the gain is nonresonant, which means that gain is available over the entire transparency region of the fiber ranging from approximately 0.3 to 2 m.

Advantages of Raman AmplifiersA third advantage of Raman amplifiers is that the gain spectrum can be tailored by adjusting the pump wavelengths. For instance, multiple pump lines can be used to increase the optical bandwidth, and the pump distribution determines the gain flatness.

Advantages of Raman AmplifiersAnother advantage of Raman amplification is that it is a relatively broad-band amplifier with a bandwidth > 5 THz, and the gain is reasonably flat over a wide wavelength range.Raman amplifiers have a distributed nature rather than being lumped such as EDFAs, and can be used to keep the signal power in a fiber approximately constant.

Challenges for Raman AmplifiersCompared to the erbium-doped fiber amplifiers (EDFAs), Raman amplifiers have relatively poor pumping efficiency at lower signal powers.Although a disadvantage, this lack of pump efficiency also makes gain clamping easier in Raman amplifiers.

Challenges for Raman AmplifiersRaman amplifiers require a longer gain fiber.This disadvantage can be mitigated by combining gain and the dispersion compensation in a single fiber.

Hybrid Amplifier SystemIt is also possible to combine EDFAs and Raman amplifiers into a hybrid amplifier system, where the Raman amplification in the transmission fiber serves as a low-noise pre-amplifier and an EDFA as a booster.

Distributed Raman AmplificationIn todays long-haul and ultra-long-haul networks, distributed Raman amplification (DRA) has demonstrated its ability to enable greater span distances without electrical regeneration.In DRA, the transmission-line fiber serves as the gain medium.

Distributed Raman AmplificationBy reverse pumping at the tail-end, the fiber induces a Raman gain that achieves an equivalent SNR over a longer transmission distance.Using distributed Raman amplifiers, which are powered by 500- to 600-mW pumps at each tail-end site, the head-end can transmit a lower-power signal, improving the SNR by approximately 5-6 dB in each span.

Distributed Raman pump at the tail-end

Discrete Raman AmplifiersDiscrete 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.The primary use for discrete Raman amplifiers is to open new wavelength bands in fused silica fibers.

S-band, C-band and L-band

Opening Up the S-BandThe S-band has comparable or better attenuation characteristics in standard single-mode fibers (SMFs) than the L-band.Also, the S-band has far less sensitivity to attenuation caused by bending during cabling and installation than the L-band.In addition, the S-band has better dispersion characteristics in standard SMFs than C- and L-band.

Opening Up the S-BandAdding S-band channels can significantly reduce the overall cost of expanding a network versus lighting new fiber.The capacity of most fibers can almost be doubled by opening up the S-band, which is inaccessible by EDFAs.Raman amplifiers are the only fused-silica-based amplifier solution for the S-band.

SummaryRaman amplifiers provide a simple single platform for long-haul and ultra-long-haul amplifier needs.The noise performance of a typical Raman amplifier is excellent, translating into larger achievable bit rates and error-free transmission distances.Raman amplifiers are an important enabling technology for the future and should see a wide range of deployment in the next few years.

Communications industry watchers agree that with network traffic doubling every few months, carriers need new strategies to economically expand the capacity of existing optical fiber. Fiber bandwidth can be extended [through DWDM (Dense Wave Division Multiplexing)] by increasing channel bit rates or by tightening channel spacing.In crystal and glasses such as silica, one vibrational mode of the lattice can be categorized by the different atoms vibrating with a phase shift of Pi, that is, the different atoms move against each other. This vibrational mode is known as an optical phonon. In other words, a phonon is a piece of vibrational energy. In materials with ionic bonds such as silica, the optical phonon is associated with an electrical field whose wavelength is often in the range of light, hence the name.

In the optical spectrum, the scattered photons give rise to a new peak at a higher wavelength called the Stokes line. Raman scattering is a very fast process that takes on the order of femtoseconds and according to the uncertainty principle, the uncertainity in the photon energy is very large, corresponding to a large range in possible wavelenght shifts of the incident photon. As a result, Raman scattering is a very unpredictable process at low pump powers. At higher pump powers, stimulated Raman scattering (SRS) sets in. In SRS, the wavelength of the scattered photon is preferably increased to a value at which the optical spectral density is already large, making the scattering process more deterministic. In Raman amplification, this second peak is the signal.As currently constituted, the transport wavelengths within the C-band (conventional band) will not suffice to meet demand for expansion. The L-band (long-wavelength band) provides obvious expansion opportunities, because EDFAs (erbium-doped fiber amplifiers) can be employed for both C-band and L-band. Less obvious but more promising, the S-band (short-wavelength band) has now shown to provide sound opportunities for expansion. Though EDFAs cannot function in the S-band (Erbium doped amplifiers do not amplify below approximately 1525 nm), advances in Raman amplification now make expansion into the S-band attractive and practical.