In Eq. (4) there are three terms with a different
value of frequency of scattering radiation. For correct
understanding of Raman-DTS behavior are important the
second and the third terms of equation. These terms
correspond to Stokes and anti-Stokes part of Raman
scattering. Stokes part is characterized by lower
frequency than frequency of excitation radiation, anti-
Stokes part is characterized by higher frequency (Fig. 1).