The basic principle of fluorescence fiber optic temperature measurement

When a substance is excited, such as by short wavelength visible or ultraviolet light, electric field excitation, or chemical reactions, it will absorb and store energy, and transition to an excited state with a certain vibrational energy level through the ground state. But the excited state is not stable and may return to equilibrium, and when the external excitation source stops working, the luminescence phenomenon will continue for a period of time, which is called afterglow. In addition, during the molecular deactivation process of fluorescent substances, unstable excited molecules will pass through energy levels

Transition, from high energy level to low energy level, and excess energy will emit light in the form of electromagnetic radiation. Therefore, according to the different excitation methods, it can be divided into photoluminescence, electroluminescence, chemiluminescence, and bioluminescence. According to Planck's law, when the energy of incident light is received by the luminescent material, it will excite electrons in the material to produce electron level transitions, and this process will produce a wavelength of λ  The emitted light. The formula for the energy difference between high and low energy levels is:

E2-E1=k λ V=kf

In the formula: E1- the energy of the electron at a lower energy level;

E2- Energy of electrons at higher energy levels;

K - Planck constant;

V - the propagation speed of light in vacuum;

F - frequency of light;

λ——— The wavelength of the emitted light.

Due to E1 and E2 being in different energy bands, they represent light in a certain band, while the energy in the molecule includes rotational energy generated by electrons and vibrational energy caused by axial elastic vibrations between nuclei. Therefore, when molecules absorb light radiation, after quantum transitions, the electron energy will rise from the ground state to a higher energy level, and the rotational energy and vibrational energy will change simultaneously, causing the three energies to interact with each other. After the incident light disappears, the luminescent material will continue to emit light for a period of time, and the emitted light is fluorescence; High frequency and short wave light energy will excite long wave and low frequency light, and follow Stokes' law.

The basic theoretical basis of fluorescence measurement method is that when the intensity of the fluorescence linear spectrum shows monotonicity with temperature, and the temperature of the fluorescent substance determines the time of the light quenching process, fluorescence temperature measurement can be carried out. Therefore, it is generally divided into fluorescence intensity temperature measurement method, fluorescence lifetime temperature measurement method, and laser induced fluorescence method. Among them, the fluorescence lifetime temperature measurement method is not easily affected by the intensity of the excitation light source, coupling degree, and fiber transmission efficiency in the temperature measurement process, and has more obvious advantages in use. The temperature measurement principle is:

After removing the excitation light source, the duration of fluorescence emission by the fluorescent substance is called fluorescence lifetime, which depends on the lifetime of the excited state. The fluorescence lifetime of fluorescent substances is related to the corresponding temperature within a certain temperature range. Fluorescence lifetime refers to the time it takes for the fluorescence intensity to decay to 1/e of its original intensity when the excitation light source is cut off, and its relationship with temperature can be expressed as:

τ (T) =1+e - Δ E/(KT) Rs+RTe - Δ E/(KT)

In the formula: Rs, RT, K Δ E - constant;

T - thermodynamic temperature.

From this, it can be seen that the time constant of fluorescence afterglow decay is a single valued function of temperature and is only related to temperature.

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