What is optical temperature sensing?

Definition and basic concepts of optical temperature sensing

Optical temperature sensing is a technology that utilizes optical properties to measure temperature. It is based on the principle that the optical properties of certain materials (such as fluorescence intensity, luminous wavelength, light absorption or scattering, etc.) will change accordingly when the temperature changes, and by detecting the changes in these optical properties, it can realize the measurement and monitoring of temperature.

1、the background and development of optical temperature sensing With the increase in demand for high-precision

non-contact temperature measurement in science and technology, traditional temperature sensors (such as thermocouples, thermistors and other sensors based on the principle of electricity) in a number of special scenarios (eg, a strong electromagnetic interference environment, miniaturization and high-precision requirements of biomedical fields, etc.) there are limitations, the optical temperature sensing technology came into being. The optical principles involved provide a novel and effective way to measure temperature. Optical temperature sensing technology is a product of multidisciplinary crossover, integrating the knowledge of optics, materials science, physics and other related disciplines, which has prompted many researchers to continuously explore and develop optical temperature sensing materials and devices with better performance. For example, from the simple use of the thermoluminescent properties of optical substances, the development of today's use of a variety of complex optical effects and advanced optical materials for high-precision temperature detection. At present, optical temperature sensing technology has been widely used in many fields, such as scientific research, industry, biomedicine, environmental monitoring, etc., and has become an important branch of modern temperature measurement technology.

2、optical temperature sensing in the relevant optical elements

Luminescent materials and energy level jump

In optical temperature sensing, many times it involves special luminescent materials, such as rare earth ion-doped compounds. Pr³⁺ ion-doped SrZn₀. ₃₃Nb₀. ₆₇O₃ material as an example.22 The Pr³⁺ ions have different energy levels, and under the excitation of UV light, different energy level jumps are generated. Such leaps correspond to emission peaks at specific wavelengths, such as the 491 nm emission peak for the 3P₀→3H₄ leap, the 619 nm emission peak for the 1D₂→3H₄ leap, and the 651 nm emission peak for the 3P₀→3F₂ leap. Optical properties such as the intensity of these emission peaks change with temperature, which forms the basis of optical temperature sensing.

The occurrence or not of energy level jumps and the change in intensity are closely related to temperature. At the microscopic level, temperature affects the thermal state of motion of particles (ions, etc.), which in turn affects their distribution among different energy levels, ultimately leading to changes in optical properties such as luminous intensity and wavelength.

Intensity, wavelength and temperature of light

The fluorescence intensity ratio is a commonly used parameter in optical temperature sensing. For example, in some materials, the fluorescence intensity ratio of two specific emission peaks shows a significant positive correlation with temperature. For example, the fluorescence intensity ratios of the emission peaks at 619 nm to those at 491 nm and 651 nm, respectively, in the Pr³⁺ ion-related materials mentioned earlier show such a relationship with the temperature, which can be used for temperature monitoring. When the temperature is increased, factors such as the corresponding energy level transition efficiency or the absence of radiative transitions change, leading to changes in the fluorescence intensity at different wavelengths, thus affecting the intensity ratio.

In addition to the intensity ratio, the wavelength of light itself may also drift with temperature. For example, in the fiber Bragg grating (FBG) temperature sensor, when the temperature changes, due to the thermal expansion effect of the optical fiber and thermo-optic effect, will lead to changes in the wavelength of light reflected at the grating, according to the amount of this wavelength change can be deduced inversely to get the value of the temperature change. The period and effective refractive index of a Bragg grating change when the temperature changes, according to the Bragg condition λ = 2nₑ₍ₑ₎₄Λ (where Λ is the grating period, λ is the vacuum wavelength, n ₑ₍ₑ₎₄ is the effective refractive index of light in the fiber), the effect of temperature on the grating period and refractive index will be reflected in the change of the reflected wavelength.

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