The principle of DTS systems is based on the principle
of optical time domain reflectometry (in Fig. 1). It
means that very short optical impulse of width about 10 ns
in time is launched into the fiber and the system measures
the amount of power returned back depending up to scattering
and reflections. The wavelength of the pulse
launched into the fiber is 975 nm, 1064 nm or 1550 nm
depending on the DTS system construction. Some part of
the optical power returns back with the same wavelength
(the elastic (Rayleigh) scattering and reflections) and some
power returns with different wavelength (the nonelastic
scattering). The nonelastic effects causing the change of
wavelength are named Raman and Brillouin stimulated
scattering [2] [4].