Study On Cr³⁺: MgAl₂O₄ Optical Fiber Fluorescence Lifetime Temperature Sensor

Optical fiber sensing is a technology that measures the temperature, displacement, velocity, acceleration, speed, pressure, liquid level, current, humidity, and other physical quantities by the use of the wavelength, intensity, polarization, phase and other characteristics of light. Compared to the conventional senor, the fiber senor has the advantages of anti-electromagnetic interference, electrical insulation, distributed sensing and good chemical stability. The fiber sensor, which has very broad application prospects, is especially suitable for the physical quantities measurement of the occasions with poor environment or strong electromagnetic interference.The paper is focused on the research of optical fiber temperature sensor, more specifically, a kind of optic fiber fluorescence lifetime temperature sensor using Cr3+ ions doped spinel as sensor material. So the main object is to study the fluorescence characteristics, including that temperature dependence of fluorescence intensity and lifetime.This paper elaborates the theoretical analysis of the fluorescence temperature dependence of fluorescence lifetime of Cr3+doped different crystals. Combining with spinel crystal structure, spinel crystal field strength, energy levels splitting of Cr3+in the spinel and the Tanabe-Sugan diagram, etc., this paper theoretically analyzes the MgAl2O4:Cr3+ fluorescence with temperature.The Cr3+ ions doped spinel crystals have been grown by using laser heated pedestal growth technique (LPGH). Specimens were prepared from stoichiometric composition (4N)Al2O3, (4N)MgO and (4N)Cr2O3 powders. Cr3+concentrations included 0.5 at.%,1 at.%,2 at.%,3 at.%,4 at.%and 5 at.%are carried out in the experiments.In this paper, it is reported that the spectra of MgAl2O4:Cr3+ is different from that of Al2O3:Cr3+ due to the difference of their crystal structures. With the Cr3+ concentration varying from 0.5 at.%to 5 at.%, the spectra has slight change, while there is no difference between fluorescence pumped by 405nm LED and 520nm LED. Furthermore, after testing the fluorescence spectra of different sensor materials, we know that fluorescence intensity decreases as the temperature rises, and the temperature dependence of fluorescence intensity has been fitted by least square method.The temperature dependence of fluorescence lifetime, which the optical fiber fluorescence lifetime based on, are studied carefully. Compared with optical fiber fluorescence intensity-based temperature sensor, the fluorescence lifetime based temperature senor is independent of the pump light intensity and optical transmission loss. It is suitable for the low temperature measurement. We have tested the lifetime of sensors with different Cr3+ doping concentrations at the temperature range from 20℃to 400℃,and sample a experiment data for each 10℃. Combining the experiment data and theoretical analysis of fluorescence lifetime relation with the temperature, we fit the temperature dependence of fluorescence lifetime by least square method. The fluorescence lifetime of Cr3+ doped spinel probe varies from 13.8ms to 7.2ms with the Cr3+ concentrations from 0.5 at.% to 5 at.%, so the 0.5 at.% is best. With the temperature from room temperature to 440℃, the fluorescence lifetime of probe with the Cr3+ doping concentration 0.5 at.% decreases from 13.8ms to 0.6ms and the temperature coefficient varies from-0.043ms/℃to-0.0049ms/℃. Lifetime of the spinel crystal is 3 times lager than that of ruby crystal(τ=4ms),so the Cr3+ doped spinel crystal is an obvious candidate for the thermometer applications using fluorescence decay. Analysis shows that the difference of lifetime between the MgAl2O4:Cr3+ and ruby results from the symmetry of their crystal structures.In this article, due to the disadvantage of experimental conditions and experimental design, temperature accuracy and temperature range of the fiber fluorescence lifetime temperature senor is not perfect. This affects the accuracy of experimental results, So we still need to do something deeply. In addition, at present there are a few studies on other materials with spinel structure, so we can try more.