Optical Temperature Sensor Based On Fluorescence Intensity Ratio Of Spectral Thermal Coupling Level

Temperature is an important parameter to measure the basic state of the object,so it is particularly important to accurately measure the temperature.The traditional contact temperature measurement technology measures the temperature by directly contacting the sensor with the object,which has many drawbacks.For example,the instrument is susceptible to signal interference,resulting in a large temperature deviation of the measurement.In addition,there are problems such as poor timeliness,insufficient dynamics,and large limitations.With the progress of science and technology and the continuous development of emerging industries,people have put forward higher requirements for temperature detection devices.For example,higher sensitivity,smaller volume,faster detection speed,etc.In recent years,optical temperature measurement technology has flourished.As a non-contact measurement method,it has the advantages of strong real-time performance,non-invasive detection and high sensitivity.Fluorescence temperature measurement technology is a kind of optical temperature measurement technology.Because the luminescent properties of phosphors are affected by temperature,the change of temperature can be monitored by measuring the parameters of the luminescent center under external stimulation.The temperature distribution can be monitored by analyzing the temperature-related luminescence properties of luminescent materials,such as luminescence intensity,bandwidth,luminescence decay time,peak position,etc.This technology has the characteristics of short acquisition time(>1%K^-1)and high spatial resolution（<10μm）,and can be operated remotely.It can accurately measure fast moving objects and objects under strong magnetic field conditions.Among them,the fluorescence intensity ratio temperature measurement technology is a self-reference technology,which has the advantages of being not affected by non-temperature factors in experimental conditions,non-invasive,and high sensitivity.Therefore,it has attracted many researchers to further explore ways to improve the sensitivity of temperature measurement.This paper comprehensively expounds the principle and formula derivation of luminescence intensity temperature measurement,fluorescence intensity ratio temperature measurement and fluorescence decay life temperature measurement.For the fluorescence intensity ratio temperature measurement technology,the thermal coupling energy level of fluorescence intensity ratio optical temperature measurement is described in detail,and some examples are compared and analyzed.For the quantitative comparison of the performance of the thermometer,the specific criteria such as sensitivity,temperature resolution,reproducibility and spatial and temporal resolution are described.Two common temperature measuring devices and instruments,optical fiber fluorescence thermometer and non-contact fluorescence thermometer,are described.Er³⁺/Yb³⁺rare earth ion pairs are widely considered to be the most effective upconversion luminescence system to date.Er³⁺/Yb³⁺co-doped molybdates exhibit bright green upconversion luminescence,which is different from other oxides.Therefore,we have deeply explored the luminescence mechanism.In this paper,Er³⁺/Yb³⁺co-doped Gd₂Mo₄O₁₅ up-conversion phosphors were prepared and characterized by X-ray diffractometer,Raman spectrometer,spectrometer and scanning electron microscope.Under 980 nm laser excitation,Gd₂Mo₄O₁₅:Er³⁺/Yb³⁺upconversion fluorescence spectrum shows two strong green emissions and a weak red emission,which is due to the ²H_11/2→⁴I_15/2,⁴S_3/2→⁴I_15/2,⁴F_9/2→⁴I_15/2 transitions of Er³⁺ions at 520 nm,555 nm and 672 nm,respectively.Studies have shown that the weak red upconversion emission is due to the high cutoff phonon energy of Gd₂Mo₄O₁₅ at960 cm^-1,and its emission efficiency is low.In addition,the time evolution of Er³⁺and Yb³⁺luminescence with Er³⁺and Yb³⁺concentration after 980 nm pulse excitation of Gd₂Mo₄O₁₅:Er³⁺/Yb³⁺was also studied.The ²H_11/2 and ⁴S_3/2 energy levels of Er³⁺ions are thermally coupled energy levels,and the population ratio of electrons at these two energy levels satisfies the Boltzmann distribution.These two energy levels are also commonly used in the field of luminescence temperature measurement based on the principle of fluorescence intensity ratio.The combination of fluorescence intensity ratio thermometry and upconversion thermometry can exert the advantages of both,especially in biomedicine and environments with strong autofluorescence.Er³⁺/Yb³⁺co-doped molybdates usually exhibit bright green upconversion luminescence,which is helpful for the application of²H_11/2 and ⁴S_3/2 level fluorescence thermometry of Er³⁺ions.The green emission mechanism of Gd₂Mo₄O₁₅:Er³⁺/Yb³⁺sample was analyzed,which provides a theoretical basis for the strong green upconversion luminescence of Er³⁺ions ²H_11/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2 transitions for temperature detection based on the principle of fluorescence intensity ratio.The optical temperature sensing properties of Gd₂Mo₄O₁₅:Er³⁺/Yb³⁺samples excited by 980 nm laser were studied by fluorescence intensity ratio technique.By studying the optical temperature measurement behavior of different materials,the absolute sensitivity of different materials is compared.In this process,we estimate the sensitivity by comparing the diffuse reflectance spectra and simplify the analysis process.