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

It has been shown that only a few photons of input signal into a H2 Raman amplifier are enough to ensure that the amplified signal is larger than the amplified spontaneous scattering. This low noise property of Raman amplifiers makes it feasible to perform injection seeding experiments using a low power continuous wave laser diode. In this paper we study, experimentally and theoretically, the amplification of a laser diode beam in an optically pumped Raman amplifier. It is found that for larger laser diode powers the signal grows nearly exponentially in the pump laser power. At low laser diode powers (<EQ 5 nW), on the other hand, the behavior is more complicated because the spontaneously scattered light which is then amplified is comparable in power to the amplified signal. We also find that due to an increased number of spatial spontaneous scattering modes, the ratio of amplified spontaneous scattering to amplified signal is larger at low pump powers than high pump laser powers. The experiments, including experiments with no laser diode input, are described by a 3D theory of a gain-guided amplifier. This theory expresses the amplified electric field as a linear combination of nonorthogonal modes and, in slightly different forms, has been used to describe a wide range of amplifiers from Raman amplifiers to x-ray lasers.