Competition between spontaneous scattering and stimulated scattering in an injection-seeded Raman amplifier

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Abstract
The amplification of a visible laser-diode beam in a gain-guided, multipass Raman amplifier is studied as a function of laser-diode and pump-laser powers. At a relatively high laser-diode power (178 nW) the amplified output grows as a simple exponential from very low gains to amplifier saturation. At a relatively low laser-diode power (2.3 nW) the behavior is more complicated, since the seeded growth and the growth from spontaneous scattering are comparable in power. The experiments, which also include an experiment with no injected laser-diode light, are modeled by a nonorthogonal-mode theory that accounts for the three-dimensional nature of the amplification process. The theory and data are found to be in close agreement in all experimental regimes studied, except at low gains with low input seed power. © 1996 The American Physical Society.