Raman amplifier

Raman scattering in silica shifts light down in frequency by the energy of molecular vibrations, and when a strong pump and a weaker signal share the fiber, the scattering becomes stimulated: the pump's photons are converted into photons at the signal wavelength, and the signal grows. The Raman gain spectrum of silica is broad, several terahertz wide, with its peak about 13.2 THz below the pump frequency. For a signal at 1550 nm that places the pump near 1451 nm, which is why 1450 nm pump lasers are the standard part.

Because the gain comes from the glass rather than from a dopant, a Raman amplifier has no fixed gain band: moving the pump moves the gain, and several pumps at different wavelengths, with their powers adjusted, sum to a flat gain spectrum wider than any single pump produces. That property lets Raman amplification cover the S, C and L bands and fill regions where erbium-doped amplifiers do not reach. The price is efficiency. Raman gain is weak per unit length, scaling as the gain coefficient divided by the fiber's effective area, so useful gain needs hundreds of milliwatts of pump and kilometers of fiber.

That is why the dominant form is distributed: the pump is launched into the transmission span itself, usually counter-propagating from the receiving end, and the span becomes the amplifier. The signal is amplified before it has decayed to its lowest level, so the noise it picks up is smaller than if the same gain were added at the end of the span. Expressed as the noise figure of a discrete amplifier that would give the same OSNR, a distributed Raman amplifier can have an effective noise figure that is low or even negative, and hybrid Raman plus EDFA spans are standard in long-haul terrestrial links. Lumped, or discrete, Raman amplifiers use a coil of small-core fiber and are chosen for bands without a doped-fiber alternative.

Raman gain also brings its own impairments. The gain responds in femtoseconds, so pump intensity noise transfers directly to the signal; co-propagating pumps are therefore used only with low-noise pumps, and counter-propagation averages the noise over the transit time. Double Rayleigh backscatter within the amplified span creates a delayed copy of the signal that acts as crosstalk, and at high gain it limits how much of the span loss can be recovered. The comparison of SOA, EDFA and Raman amplification for a given link is worked in SOA vs EDFA vs Raman.

References: J. Bromage, J. Lightwave Technol. 22, 79 (2004); C. Headley and G. P. Agrawal, eds., Raman Amplification in Fiber Optical Communication Systems (Elsevier, 2005).