  In its usual implementation, the Raman amplifier features only one pump
carrier frequency. However, pulses with well-separated frequencies can also be
Raman amplified while compressed in time. Amplification with
frequency-separated pumps is shown to hold even in the highly nonlinear,
pump-depletion regime, as derived through a fluid model, and demonstrated via
particle-in-cell (PIC) simulations. The resulting efficiency is similar to
single-frequency amplifiers, but, due to the beat-wave waveform of both the
pump lasers and the amplified seed pulses, these amplifiers feature higher seed
intensities with a shorter spike duration. Advantageously, these amplifiers
also suffer less noise backscattering, because the total fluence is split
between the different spectral components.
