Distributed fiber optic temperature measurement refers to a temperature monitoring system that uses the Raman scattering effect of optical fiber and optical time-domain reflectometry (OTDR) to obtain spatial temperature information. Some physical parameters in life, such as temperature, pressure and tension, can affect the glass fiber and partially change the light transmission characteristics of the fiber. Because scattering can cause light loss in quartz fiber, the location of external physical effects can be determined, allowing the fiber to act as a linear sensor. The thermal effect causes lattice oscillation in the quartz solid of the fiber. When light falls on these thermally excited molecular oscillations, light scattering occurs from the interaction between photons and the electrons of the crystal molecules — also known as Raman scattering. Unlike the incident light, the spectral shift of this scattered light is equivalent to the natural frequency of the lattice oscillation.

The light scattered back from the fiber includes three different spectra: Rayleigh scattered light, Stokes light and anti-Stokes light. Anti-Stokes light has a strong temperature dependence, while Stokes light is almost independent of temperature. When the fiber is affected by external temperature, the intensity of the anti-Stokes light in the fiber changes. The ratio of anti-Stokes intensity to Stokes intensity can be used to calibrate the temperature. Using this principle, distributed measurement of the temperature field at each point along the fiber can be achieved.

OTDR (optical time-domain reflectometry) works as follows: the distributed fiber monitoring technology sends a light pulse into the measured fiber, producing a Raman scattering effect, and the generated backward scattered light propagates back to the injection end of the fiber. Because each backward scattered signal corresponds to a scattering point on the fiber, the location of the scattering point can be determined according to its transmission time.