>Introduction to the principle of distributed optical fiber temperature measurement
The distributed optical fiber temperature sensing system DTS is based on the fiber Raman scattering phenomenon. The light pulses emitted by the laser light source interact with the fiber molecules and scatter. There are many types of scattered light, such as: Rayleigh scattering, Brillouin scattering, and Raman scattering. Among them, Raman scattering is related to the thermal vibration of the fiber molecules, so it is sensitive to temperature and can be used for temperature measurement. In the optical fiber, the scattered signal is continuous. By using a high-speed signal acquisition technology to measure the time interval between the incident light and the Raman scattered light, the position where the Raman scattered light occurs can be obtained. The corresponding temperature distribution can be measured along the fiber.
E1 and E2 in Figure 2 represent the ground state and excited state of molecular vibration, respectively. Assume that the frequency of the laser injected into the fiber is υ0, and the energy of the photon is hυ0; when the molecule is excited by the incident light from the vibration ground state E1 (or the vibration excited state E2) to the imaginary state of the energy level E1 + hυ0 (or E2 + hυ0), it returns to vibration The ground state E1 (or vibrationally excited state E2) scatters photons with frequency υ0. This process is called Rayleigh scattering. υ)的拉曼散射, 散射光子的频率为υ0υ,这种散射称为斯托克斯(Stokes)散射;另一种是处于振动激发态的分子被入射光激发到虚态, 然后回到振动基态E1, 产生能量为h(υ0+υ)的拉曼散射, 散射光子的频率为υ0+υ, 这种散射称为反斯托斯(Anti2Stokes)散射。 When the molecules in the ground state of the vibration are excited to the virtual state by the incident light, and then return to the vibration excited state E2, Raman scattering with energy h (υ0- υ) is generated, and the frequency of the scattered photons is υ0υ. This scattering is called Stoke Stokes scattering; the other is that the molecules in the excited vibrational state are excited to the imaginary state by the incident light, and then return to the ground state of vibration E1, which generates Raman scattering with energy h (υ0 + υ), and the frequency of the scattered photons is υ0 + υ, this scattering is called Anti-Stokes scattering. The distribution of Stokes and Anti2Stokes scattered light on the spectrogram is roughly symmetrical. Anti2Stokes scattered light is sensitive to temperature and its intensity is modulated by temperature. Stokes scattered light is basically independent of temperature. Temperature dependent. Therefore, Anti2Stokes Raman scattering is usually used as a signal channel as the main basis for calculating temperature. And Stokes Raman scattering is usually used as a reference channel to eliminate the influence of other factors such as noise. By detecting the ratio of the two light intensities, the temperature information of the scattering region can be demodulated, and the instability of the light source and the coupling loss, fiber connector loss, fiber bending loss, and fiber transmission loss in the fiber transmission process can be effectively eliminated. influences.
FIG. 3 is a structural diagram of a distributed optical fiber temperature measurement system. Under the trigger of a synchronous control module, the light emitted by the laser is formed by a light pulse modulator to form pulse light with a specific repetition frequency and width. The pulsed light is connected to the thermostatic bath and the sensing fiber through an optical coupler. During the transmission of pulsed light, the scattered light signals at different test points will be partially returned to the optical coupler along the transmission optical path. Optocouplers couple approximately 50% of the Raman scattered light to the light processing subsystem. Raman scattered light contains two frequencies of light: Stokes light and Anti2Stokes light. Their frequencies are distributed on both sides of the frequency of the incident light. Through the optical splitter, two different frequencies of light are separated and entered into different optical paths for processing. Because the scattered light is also mixed with other scattered light and interference light, it is necessary to perform a certain bandpass filtering process on the two lights to obtain Anti2Stokes Raman scattered light with temperature information and backscattered reference signal. The backscattered light of these two channels is photoelectrically converted and amplified by the respective APD (avalanche photodiode), and the data processing and display software processes the collected data to obtain the spatial distribution of temperature and display it in the form of graphics or tables come out. Due to the influence of the loss and noise of the entire system, multiple measurements need to be performed, and the data is accumulated and averaged to obtain a true temperature curve that better reflects the measured temperature field, and then transferred to a computer for storage in a database.