**1. Introduction**

Spontaneous Raman is an inelastic light scattering by which a fraction of the light incident upon a transparent material is shifted in frequency. Linear Raman scattering is a weak process (approximately 1 in 107 photons) involving the collective behavior of many atoms, behaving independently (see **Figure 1(a)**) and leading to nearly isotropic emission. Raman spectra, given by the superposition of stochastic totally independent vibrations from individual molecules, are unique to the material [1].

Stimulated Raman scattering (SRS) phenomenon occurs when there is a transfer of energy from a high power pump beam to a probe beam (copropagating or counterpropagating) through SRS. In particular, this energy exchange occurs when the frequency difference between the pump and the Stokes laser beams matches a given molecular vibrational frequency of the sample under test; the SRS effect occurs in the form of a gain of the Stokes beam power (stimulated Raman gain, SRG) and a loss of the pump beam power (stimulated Raman loss, SRL. See **Figure 1(b)**) [2]. Because of its coherent nature, the molecular bonds oscillate in phase and interfere constructively inside the focus area of the laser beam. As a consequence a SRS signal, which is orders of magnitude bigger than spontaneous Raman scattering, is generated (about 20–30% of the incident laser radiation can efficiently be converted into SRS) (see **Figure 1(c)**).
