One of the most recent and interesting developments includes the constructive usage of the so-called Raman effect in optical fibers. A Raman amplifier uses intrinsic properties of silica fibers to obtain signal amplification. This means that transmission fibers can be used as a medium for amplification, and hence that the intrinsic attenuation of data signals transmitted over the fiber can be combated within the fiber. An amplifier working on the basis of this principle is commonly known as a distributed Raman amplifier (DRA).

The physical property behind DRAs is called SRS. This occurs when a sufficiently large pump wave is co-launched at a lower wavelength than the signal to be amplified. The Raman gain depends strongly on the pump power and the frequency offset between pump and signal. Amplification occurs when the pump photon gives up its energy to create a new photon at the signal wavelength, plus some residual energy, which is absorbed as phonons (vibrational energy) as shown in Figure 2.

Figure 2. Energy States during SRS

As there is a wide range of vibrational states above the ground state, a broad range of possible transitions are providing gain. This is shown in Figure 2 by means of the shaded region. Generally, Raman gain increases almost linearly with wavelength offset between signal and pump peaking at about 100 nm and then dropping rapidly with increased offset. Figure 3 shows a typically measured Raman gain curve. The usable gain bandwidth is about 48 nm.

Figure 3. Typical Raman Gain Curve versus Wavelength Offset

The position of the gain bandwidth within the wavelength domain can be adjusted simply by tuning the pump wavelength. Thus, Raman amplification potentially can be achieved in every region of the transmission window of the optical transmission fiber. It only depends on the availability of powerful pump sources at the required wavelengths. The disadvantage of Raman amplification is the need for high pump powers to provide a reasonable gain.

This opens a new range of possible applications. It is possible, for instance, to partially compensate fiber attenuation using the Raman effect and, thus, to increase the EDFA spacing. The Raman pump wave can be conveniently placed at the EDFA locations. This saves costs as less EDFAs are needed on the link, and the number of sites to be maintained is reduced.

Another application of the Raman effect is given with hybrid EDFA/Raman amplifiers characterized by a flat gain over especially large bandwidths. Repeaters can be built that compensate the nonflatness of the EDFA gain with a more flexible Raman gain. Multiwavelength pumping could be used to shape the Raman gain such that it equalizes for the EDFA gain shaping.

Also, the Raman effect on its own might be used for signal amplification in transmission windows that cannot be covered properly by EDFAs. Some frequency regions of a wideband WDM signal could be amplified by common EDFA structures, while others are amplified using the Raman effect and proper pumping. The upgrade of already existing systems by opening another transmission window where Raman amplification is applied could be an attractive application.