According to the backscattering principle, distributed fiber optic temperature measurement systems can be divided into three types: Rayleigh scattering, Raman scattering and Brillouin scattering. At present, the most mature one with products applied in engineering is the distributed fiber optic temperature measurement system based on Raman scattering. Its sensing principle is mainly based on the optical time-domain reflectometry (OTDR) principle of optical fiber and the backward Raman scattering temperature effect of optical fiber.

I. Basic principle of distributed fiber optic temperature measurement. According to the backscattering principle, distributed fiber optic temperature measurement systems can be divided into Rayleigh scattering, Raman scattering and Brillouin scattering. The currently most developed is the Raman-scattering-based distributed fiber optic temperature measurement system with products used in engineering. Its sensing principle is mainly based on the OTDR principle and the backward Raman scattering temperature effect.

(1) OTDR principle: when a laser pulse is transmitted in the fiber, scattering occurs due to the macroscopic inhomogeneity of the refractive index in the fiber. In the time domain, the time required for the incident light to return to the fiber output end after backward scattering is t, and the path traveled by the laser pulse in the fiber is 2L, where v is the propagation speed of light in the fiber, C is the speed of light in vacuum, and n is the refractive index of the fiber. When the time t is measured, the distance L from the light source can be obtained.

(2) Backward Raman scattering temperature effect of optical fiber: when a laser pulse enters the fiber from one end, this light pulse propagates forward along the fiber. Because the light pulse undergoes momentum-conserving inelastic collisions with the molecules inside the fiber, reflection occurs at every point during propagation, and a small part of the reflected light travels in exactly the opposite direction to the incident light (also called backward). The intensity of this backward reflected light has a certain correlation with the temperature of the reflection point: the higher the temperature of the reflection point (the ambient temperature where the fiber is located), the stronger the reflected light intensity. Using this phenomenon, if the intensity of the backward reflected light can be measured, the temperature of the reflection point can be estimated — this is the basic principle of temperature measurement with optical fiber.