This type of sensor consists of a multi-mode optical fiber and a temperature-sensitive material. These features of optical fibers make them a useful tool for various sensing applications including in medicine, automotives, biotechnology, food quality control, aerospace, physical and chemical monitoring. Among all the reported applications, optical waveguides have been widely exploited to. High-definition temperature sensing based on the natural Rayleigh backscatter in optical fiber delivers a virtually continuous line of temperature measurements with sub-millimeter spatial resolution. They can be based on different operation principles as explained in the following.