The distributed fiber optic temperature sensor system (DTS) is currently basically based on the optical time-domain reflectometry (OTDR) principle and the Raman scattering effect of optical fiber. The fundamental principle of fiber optic temperature measurement is the backward Raman scattering effect: the laser pulse interacts with the fiber molecules, producing various kinds of scattering such as Rayleigh scattering, Brillouin scattering and Raman scattering. Theoretically, all three kinds of scattering could serve as reference signals for temperature measurement. However, long-term research on distributed linear temperature-sensing fiber has found that the intensity of Rayleigh scattering is extremely unstable with respect to fiber loss. If Rayleigh scattered light were used as the reference signal for a long time, the loss characteristics of the fiber would change unpredictably, system stability would decline, and there might eventually be a risk that the system could not operate. Therefore, when selecting the reference signal, the scattering most sensitive to ambient temperature changes is chosen — that is, Raman scattering.

Raman scattering is caused by the thermal vibration of fiber molecules. It generates a Stokes light with a longer wavelength than the light source and an anti-Stokes light with a shorter wavelength. The modulation of the fiber by the external ambient temperature changes the anti-Stokes light intensity in the fiber. The ratio of anti-Stokes to Stokes gives an absolute indication of temperature. Using this principle, distributed measurement of the temperature field along the fiber can be achieved. Combined with a high-quality pulse light source and high-speed signal acquisition and processing technology, accurate temperature values at all points along the fiber can be obtained.