Raman Amplifiers

Amplifiers based on Raman gain, which results from the effect of stimulated Raman scattering.

The Raman-active medium is usually an optical fiber, although it can also be a bulk crystal, or a cell with a gas or liquid medium. An input signal can be amplified while co- or counterpropagating with a pump beam, the wavelength of which is typically a few tens of nanometers shorter. For silica fibers, maximum gain is obtained for a frequency offset of 10–15 THz between pump and signal, depending on the core composition.

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Raman amplifier

A type of amplifier used in long haul, single-mode (SMF) fiber optic transmission systems (FOTS). Raman amplification usually occurs throughout the length of the transmission fiber itself in a process known as distributed amplification, rather than in a discrete amplification, or lumped amplification configuration such as that employed by an erbium-doped fiber amplifier (EDFA). Raman amplification occurs as a high-energy pump wavelength is sent in the reverse direction from the output end of the fiber span, where the incoming signal is weakest. The pump wavelength, which generally is in the 1450 nm range (E-Band), interacts with and excites atoms in the crystalline lattice of the fiber core.The atoms absorb the photons, and quickly release photons with energy equal to the original photon, plus or minus atomic vibration. In other words, a frequency/wavelength shift occurs as the pump wavelength propagates along the fiber in the reverse direction.The energy lost in the pump wavelength shifts to longer-wavelength (within about 100 nm) signals, generally in the 1550 nm window (C-Band), in the forward direction, thereby serving to amplify them. Raman amplifiers offer the advantage of amplifying signals in the broad range extending from 1300 nm to 1700 nm. Further, they perform better than EDFAs in terms of signalto-noise ratio (SNR). Raman amplifiers often are used as preamplifiers to enhance the performance of EDFAs in dense wavelength division multiplexing (DWDM) systems.

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Raman Amplifier

A Raman optical amplifier is not a "black box" like an EDFA. It consists of little more than a high-power pump laser, usually called a Raman laser, and a WDM or directional coupler. The optical amplification occurs in the transmission fiber itself. The optical amplification is distributed along the transmission line. Optical signals are amplified up to 10 dB in the network optical-fiber. The Raman optical amplifiers have wide gain bandwidth (up to 100 nm). They can use any installed transmission optical-fiber (single-mode optical fiber, TrueWave, etc.). In effect, they reduce the effective span loss and improve noise performance of the transmission line by boosting the optical signal in transit. They can be combined with erbium-doped fiber amplifiers (EDFA) to achieve very wide optical gain flattened bandwidth. Figure 3 shows the topology of a typical Raman optical amplifier. The two key elements of the Raman optical amplifier are the pump laser and the directional coupler. The pump laser, in this case, has a wavelength of 1535 nm. The circulator provides a convenient means of injecting light backwards in to the transmission path with minimal optical loss.
shows the optical spectrum of a forward-pumped Raman optical amplifier. In this case, the pump laser is injected at the transmit end rather than the receive end as shown in Figure 3. The pump laser has a wavelength of 1535 nm. As is usually the case, the amplitude of the pump laser is much greater than the data signals. Figure 5 shows the received signal after the same length of fiber used in the SRS example above. While the amplitude of the pump laser is significantly decreased, the amplitude of the six data signals is now much stronger and they all have roughly equal amplitudes. In this case, a great deal of energy was robbed from the 1535 nm pump laser signal and redistributed to the six data signals.

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