1. Principle of optical fiber in temperature measurement and communication: In the low-temperature region (below 400℃), the radiation signal is weak, and the system turns on the light-emitting diode (LED) to make the fluorescent temperature measurement system work. The LED sends the excitation light through the condenser lens to the branch end of the Y-type fiber, then through the Y-type fiber and fiber coupler to the fiber optic temperature sensor head. The excitation light stimulates the first end of the sensor to emit fluorescence. The fluorescent signal is guided out by the fiber, emitted from the other end of the Y-type fiber through the fiber coupler, and received by the photodetector. The optical signal output by the photodetector is amplified and processed by the fluorescent signal processing system, which calculates the fluorescence lifetime and thereby obtains the measured temperature value. In the high-temperature region (above 400℃), the radiation signal becomes strong, the radiation temperature measurement system works and the LED is turned off. The radiation signal passes through the sapphire fiber and is output through the Y-type fiber, converted into an electrical signal by the detector, and the system calculates the measured temperature by detecting the radiation signal.

2. Functions of optical fiber in temperature measurement and communication: (1) When electromagnetic waves pass through the fiber material, part of the light is converted into heat, causing power loss. (2) The inherent absorption of high-temperature measuring optical cables (such as SiO2) is not caused by impurities or defects; therefore, intrinsic absorption basically defines the minimum absorption loss of a material. (3) Additional absorption loss caused by impurities in the fiber material (dust, metal ions, etc.). (4) Due to defects or inhomogeneity in the material, shape and refractive index distribution of the cable temperature measurement device, light transmitted in the fiber is scattered, resulting in loss. (5) When light passes through an inhomogeneous substance in density or refractive index, light can be seen in directions other than the propagation direction — this is called light scattering. (6) Additional fiber loss and scattering caused by fiber bundling into optical cables, cable laying under various conditions, fiber splicing, and coupling and connection as part of the system.