Raman Amplifiers

A Raman amplifier is an optical amplifier based on Raman gain, which results from the effect of stimulated Raman scattering in some Raman gain medium. That medium is often an optical fiber (possibly a highly nonlinear fiber), although it can also be a bulk crystal, a waveguide in a photonic integrated circuit, 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 composition of the fiber core.

For applications in telecom systems, fiber Raman amplifiers compete with erbium-doped fiber amplifiers. Compared with those, their typical features are:

• Raman amplifiers can be operated in very different wavelength regions, provided that a suitable pump source is available.
• The gain spectrum can be tailored by using different pump wavelengths simultaneously. For example, very broadband amplification — a gain bandwidth e.g. well beyond that of an EDFA — would be feasible with a proper combination of pump sources.
• A Raman amplifier requires a high pump power (order of 1 W, possibly raising laser safety issues) and high pump brightness; it can also provide high signal output powers. Pump sources may be multiple laser diodes (at different wavelengths) or fiber lasers.
• A much greater length of fiber is required — normally several kilometers. However, instead of making a lumped Raman amplifier, the transmission fiber in a telecom system may be used, so that no additional fiber is required.
• Raman fiber amplifiers can have a lower noise figure. On the other hand, they more directly couple pump noise to the signal than laser amplifiers do.
• They also have a fast reaction to changes in the pump power, particularly for co-propagating pump, and very different saturation characteristics.
• If the pump wavelength is polarized, the Raman gain is polarization-dependent. This effect is often unwanted, but can be suppressed e.g. by using two polarization-coupled pump diodes or a pump depolarizer.

A telecom Raman amplifier is pumped with continuous-wave light from a diode laser. Efficient amplification of ultrashort pulses is also possible using copropagating pump pulses. However, the phenomenon of group velocity mismatch then severely limits the useful interaction length, particularly for pulse durations below 1 ps.

Fibers used for Raman amplifiers are not doped with rare earth ions. In principle, any ordinary single-mode fiber could be used, and in practice the transmission fibers themselves are often suitable (→ distributed amplifiers). However, there are special fibers with increased Raman gain, resulting from certain dopants (e.g. germania) for enhanced Raman cross-sections, or simply from a small effective mode area. Such highly nonlinear fibers are used for lumped Raman amplifiers, where a shorter piece of fiber is dedicated to amplification only. Also, there are phosphorous-doped fibers, for example, offering a much increased Raman shift (in terms of optical frequency), or alternatively a gain peak with very low Raman shift.

While distributed Raman amplifiers offer excellent noise performance, their achievable gain is practically limited by double Rayleigh backscattering (DRB). A small fraction of the signal light is backscattered (Rayleigh scattering) in the (typically long) fiber and then amplified; if this light is backscattered a second time, it travels in the same direction as the signal, acting as a noise source (multipath interference). This effect becomes significant at high gain levels (e.g., above 15–20 dB), creating a noise floor that degrades signal quality despite high optical power. Consequently, Raman amplifiers are usually operated at moderate gain levels or combined with lumped amplifiers (like EDFAs) in hybrid configurations. Note that lumped amplifiers (including some Raman amplifiers) are much less subject to this problem since they utilize much shorter fibers.

Raman amplifiers, as used particularly in optical fiber communications, may be implemented using forward pumping, backward pumping or a combination of both (bidirectional pumping), referring to the direction of pump light propagation relative to the signal. The choice of pumping configuration has a significant impact on noise performance, gain distribution, nonlinear effects and system design.