Quasi-distributed optical fiber temperature sensor based on fluorescence quenching

Abstract

The invention belongs to the technical field of optical fiber sensing, and particularly relates to a structure and system design of a quasi-distributed optical fiber temperature sensor based on fluorescence quenching, which can modify quantum dots with different particle sizes on the surfaces of optical fiber sensing units at different sites to form sensing areas. The characteristic that quantum dots are sensitive to temperature is utilized, and temperature sensing is achieved by detecting fluorescence signal changes of evanescent waves transmitted back to a fiber core through a signal acquisition module optical fiber spectrometer. Due to the fact that the wavelengths of fluorescence emitted by the quantum dots of the same material but different particle sizes are different, quasi-distributed sensing of temperature can be achieved by monitoring fluorescence intensity changes of different emission peaks, short-distance multi-site distributed sensing is achieved, the sensor is low in signal processing difficulty, the demodulation complexity degree is greatly reduced, and the sensing accuracy is improved. The temperature sensor has an anti-electromagnetic interference capability, can adapt to most temperature monitoring occasions, and can carry out distributed temperature sensing in special occasions with electromagnetic interference.

Description

Technical Field

[0001] The present invention belongs to the technical field of fiber optic sensing, and more specifically relates to a quasi-distributed fiber optic temperature sensor based on fluorescence quenching. Quantum dots with different particle sizes can be modified on the surface of the fiber optic sensing area at different sites to form a fiber optic sensing unit. By utilizing the temperature-sensitive characteristics of quantum dots, temperature sensing is achieved by detecting the change in the fluorescence signal transmitted to the fiber optic spectrometer through the evanescent wave. Quasi-distributed sensing of temperature is realized by monitoring the change in the fluorescence intensity of different emission peaks, and short-distance multi-site distributed sensing can be performed. The sensing system can adapt to most temperature monitoring scenarios and can perform distributed temperature sensing in special scenarios with electromagnetic interference. Background Art

[0002] Fiber optic temperature monitoring technology is one of the key technologies developed in the field of physical quantity detection today. Nowadays, the temperature detection technology has been very mature. As one of the most common and influential physical quantities in human life, temperature needs to be controlled during the industrial manufacturing process to ensure the quality of the final product; during agricultural production, temperature needs to be monitored to ensure that crops can adapt to the environment; in daily life, the temperature of items or the environment needs to be detected to infer whether it is suitable for human survival: all these aspects reflect the importance and necessity of temperature detection. Against this background, the ability to detect whether the temperature of the environment and equipment is within the required range at multiple points quickly, efficiently, and accurately has attracted wide attention.

[0003] As a common temperature detection method, fiber optic sensing has the characteristics of fast response speed and anti-electromagnetic interference ability, and can sense the temperature of the environment or objects in a complex environment. The current fiber optic temperature sensing technology is relatively mature. In order to meet the current demand for simultaneous multi-point temperature measurement, it is gradually developing towards the direction of fiber optic distributed temperature sensing. The current main fiber optic distributed sensing technologies mainly include: fiber optic Bragg grating (FBG)-based sensing technology, fiber optic interference-based sensing technology, and fiber optic scattering-based sensing technology. Among them, the fiber optic Bragg grating-based sensing technology mainly uses the reflection wavelength of the Bragg grating for temperature monitoring, with good sensitivity. However, when the number of distributed monitoring sites increases, multiple Bragg gratings are required for monitoring, which greatly increases the complexity of signal demodulation. Moreover, the FBG itself is easily affected by factors such as stress and generates cross-sensitivity problems. The fiber optic interference-based sensing technology mainly uses the wavelength of the interference peak for temperature sensing, and is mostly used for temperature monitoring of a single point. It has high sensitivity. Distributed temperature can be monitored by connecting multiple interference-structured optical fibers at the same time, but it also increases the complexity of demodulation. Fiber optic scattering distributed temperature sensing is mainly based on the principle of backscattering such as Raman scattering, Brillouin scattering, and Rayleigh scattering to perform distributed temperature sensing, and can effectively judge the temperature change site and the amount of temperature change. It is the most common fiber optic distributed temperature sensing technology at present and has good performance in long-distance temperature sensing. However, its accuracy decreases in short-distance distributed temperature sensing, and there are also certain errors in the perception of temperature change sites. Therefore, it is used for long-distance distributed temperature monitoring. The geological temperature monitoring distance in many large industrial equipment or production activities is relatively short. Only by solving the problems of insufficient positioning accuracy of temperature change points and complex demodulation, the fiber optic distributed temperature sensing technology is expected to achieve more extensive applications in the future. Summary of the Invention

[0004] Aiming at the problems that the existing fiber optic sensing system is difficult to achieve high-precision monitoring of short-distance distributed temperature and the technical problem of complex signal demodulation, the present invention proposes a quasi-distributed fiber optic temperature sensor based on fluorescence quenching.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] A quasi-distributed optical fiber temperature sensor based on fluorescence quenching, which includes a laser light source module, a transmission optical fiber, an optical fiber sensing unit, a signal acquisition module and a computer. The output wavelength band of the laser light source in the laser light source module is within the ultraviolet absorption spectrum range of the fluorescent material quantum dots, as close as possible to the optimal fluorescence absorption wavelength of the quantum dots, so as to maximize the generated fluorescence signal. The transmission optical fiber is used to transmit the excitation light to each optical fiber sensing unit, and then collect the fluorescence signal generated by the optical fiber sensing unit to the fiber core in the form of evanescent wave and finally transmit it to the optical fiber spectrometer of the signal acquisition unit. The optical fiber sensing unit is encapsulated with a stainless steel capillary tube, and the optical fiber sensing optical fiber fixed with quantum dots is encapsulated inside the capillary tube. The excitation light signal excites the quantum dots on the surface to generate fluorescence signals in the form of evanescent wave in the optical fiber sensing unit. During the sensing process, the external heat (temperature) is transferred to the quantum dots on the surface of the sensing area of the optical fiber sensing unit through heat transfer. The change in temperature will cause the change of its fluorescence signal. Since different-sized quantum dots can generate different-wavelength fluorescence emission peaks under the same excitation wavelength condition and their fluorescence sensitivity to temperature remains unchanged, different-sized quantum dots can be modified at the optical fiber sensing units distributed at different sites to achieve distributed temperature sensing. In the signal acquisition module, an optical fiber spectrometer is mainly used to collect the fluorescence signals in the optical fiber. Finally, the computer visualizes the fluorescence signal spectrum and calculates the temperature of each site using the calibrated relationship between fluorescence intensity and temperature to achieve short-distance distributed temperature sensing without the need for an additional demodulation process.

[0007] For further optimization of this technical solution, the optical fiber sensing unit can be a tapered optical fiber, a D-shaped optical fiber or an optical fiber with a bare fiber core after corrosion cladding as a substrate to enhance sensitivity. The methods of modifying quantum dots on the optical fiber surface can be physical deposition, chemical modification, sputtering thin films, etc. According to the application scenarios and the requirements for sensor performance, the modification range on the optical fiber surface and the number of quantum dots can be changed to improve the corresponding sensitivity.

[0008] For further optimization of this technical solution, specific substances can be additionally modified on the surface of the quantum dots to increase or decrease their sensitivity to temperature, so as to increase their sensing range and the resolution of temperature sensing, and the temperature can be applied to occasions with special requirements.

[0009] For further optimization of this technical solution, the fluorescent material can be a fluorescent substance with an approximate absorption band, different emission bands and fluorescence temperature sensitivity characteristics, which is suitable for low-temperature and high-temperature sensing to improve the sensitivity of the sensing system.

[0010] For further optimization of this technical solution, the encapsulating material can be replaced with materials having special properties such as light shielding (light transmittance lower than 5%), high temperature resistance (able to withstand high temperatures up to 500 °C or more without deformation), corrosion resistance (no chemical reaction in strong acid and strong base environments), good heat transfer performance (transferring the ambient temperature to the inside of the structure within 10 s), and good biocompatibility (no reaction or very low reaction rate with biological secretions in a biological environment), etc., to improve the applicable range of the sensing system and the temperature measurement range, and meet the distributed temperature monitoring in more scenarios and even the temperature monitoring of different parts of a living body.

[0011] For further optimization of this technical solution, a liquid with good heat transfer effect (transferring the temperature of the capillary encapsulation structure to the sensing area of the fiber optic sensing unit within 10 s) is used to fill the encapsulation structure. The type of the liquid is adjusted according to the type of the fluorescent material to adapt to the fluorescent material and prevent the fluorescent material from falling off the sensing unit, increasing the storage time of the fluorescent material to extend the service life of the sensor. In addition, the solvent effect can be used to improve the fluorescent properties (including fluorescence intensity, fluorescence quantum yield, and fluorescence lifetime, etc.) of the sensing unit to achieve special sensing effects.

[0012] For further optimization of this technical solution, in long-distance distributed sensing, some specially doped optical fibers or optical fiber devices with signal amplification gain can be added to the transmission optical fiber to enhance the effects of exciting optical signals and amplifying fluorescent signals, and reduce the influence of signal attenuation caused by long-distance transmission.

[0013] Different from the prior art, the above technical solution has the following advantages:

[0014] 1. The present invention uses fluorescence detection technology to achieve distributed temperature sensing. By utilizing the characteristics that quantum dots of the same type but different sizes have the same absorption spectrum and different emission spectra, modifying quantum dots of different sizes at different sites of the sensing optical fiber can achieve distributed sensing, and the generated fluorescence emission will not provide an excitation signal for the subsequent sensing unit, avoiding the crosstalk problem. Therefore, the influence of the absorption spectrum of quantum dots on temperature changes under non-ultra-high conditions is very small, and stress and pressure also have very little influence on the fluorescent signal of the sensing unit, maximizing the avoidance of the cross-sensitivity problem of fluorescent signals caused by other physical quantities.

[0015] 2. The present invention uses a capillary liquid core to encapsulate the fiber optic sensing unit. The material and size of the capillary can be flexibly adjusted according to the sensing application scenario, the internal encapsulated liquid, and the size of the fiber optic sensing unit, having good adaptability, and can protect the sensing unit from other factors except temperature in the outside world, endowing the sensing system with stronger anti-interference ability; the liquid core encapsulation can also be replaced according to needs, which can not only achieve rapid heat transfer but also play a role in protecting the sensing unit, and can also adjust the sensing performance of the sensing unit by using the solvent effect, enabling the sensing system to adapt to more occasions and meet more performance requirements.

[0016] 3. In the sensing system of the present invention, multiple fiber optic sensing units are cascaded on the same fiber to prepare a sensor. The signal acquisition module uses a fiber optic spectrometer to demodulate and receive signals, without an additional signal demodulation process. All fluorescence signals can be presented in the same spectrum, simplifying the signal processing process. Since the increase in temperature will cause fluorescence quenching of quantum dots, a calibration relationship can be formed between the temperature and the reduction amplitude of different fluorescence emission peaks. According to the change of the fluorescence signal, the temperature can be accurately detected, and according to the fluorescence emission peak wavelength, the area where the sensing unit with temperature change is located can be accurately positioned, thereby realizing distributed temperature sensing and optimizing the signal demodulation and processing process. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic diagram of a fiber optic fluorescence distributed temperature sensing system;

[0018] Figure 2 It is a schematic diagram of fluorescence temperature sensing principle;

[0019] Figure 3 It is a structural diagram of a fiber optic sensing unit;

[0020] Description of the reference numerals:

[0021] 1: Laser light source module 2: Computer 3: Signal acquisition module 4: Transmission fiber 5: Fiber optic sensing unit

[0022] 6: Object or environment to be measured 7: Laser light source 8: Laser excitation 9: Quantum dots with different particle sizes

[0023] 10: Stimulated emission transition 11: Core 12: Curing glue 13: Capillary encapsulation structure

[0024] 14: Heat transfer liquid 15: Sensing area of the fiber optic sensing unit 16: Quantum dots 17: External temperature DETAILED DESCRIPTION OF THE EMBODIMENTS

[0025] To describe in detail the technical content, structural features, achieved objectives, and effects of the technical solution, the following is a detailed description in conjunction with specific embodiments and with reference to the accompanying drawings.

[0026] Refer toFigure 1 As shown in the figure, it is a schematic diagram of an optical fiber fluorescence distributed temperature sensing system. The quasi-distributed optical fiber temperature sensor based on fluorescence quenching includes a laser light source module 1, a transmission optical fiber 4, an optical fiber sensing unit 5, a signal acquisition module 3, and a computer 2. The laser light source module 1 inputs a laser with a central wavelength of 350 - 400 nm into the entire sensing system as a signal to excite fluorescence of the fluorescent substance. The bandwidth range of the laser overlaps with the optimal absorption wavelength range (350 - 400 nm) of the fluorescent substance (quantum dots), which can maximize the intensity of the excited fluorescence signal for subsequent acquisition and processing. The transmission optical fiber 4 serves as a transmission medium for the laser signal and the fluorescence signal, transmitting the laser from the laser light source module 1 through the transmission optical fiber 4 to the optical fiber sensing unit 5, generating a fluorescence signal in the form of an evanescent wave, and then collecting the fluorescence signal in the form of an evanescent wave and transmitting it to the signal acquisition module 3 for acquisition and processing. The optical fiber sensing unit 5 uses the fluorescent quantum dots as temperature-sensitive materials and modifies and fixes them on the surface of the optical fiber sensing area through physical modification, chemical deposition, etc. Utilizing the temperature-sensitive characteristics and good repeatability of the fluorescence signal of the quantum dots for temperature sensing, the entire optical fiber sensing unit is encapsulated in a capillary tube matching the size of the optical fiber in the form of a capillary liquid core. The capillary encapsulation of the optical fiber can not only achieve a sealing effect but also play a role in protecting the sensing unit and isolating external environmental interference. The material of the capillary tube can be adjusted accordingly according to different detection environments; the liquid core encapsulation method can quickly conduct heat to the quantum dots on the surface of the optical fiber sensing area through liquid heat transfer, affecting the fluorescence signal. The physical and chemical properties of the encapsulation liquid itself can also protect the fluorescent substance from reacting with substances in the external environment, thereby extending the service life of the optical fiber sensing unit and adjusting the sensing performance of the sensor. The optical fiber sensing unit 5 is placed in the object or environment 6 to be measured for detection. The environment or object conducts heat to the quantum dots on the sensing unit through heat transfer with the capillary tube and the internal liquid. According to the temperature change, the fluorescence signal changes accordingly. The greater the temperature change, the more obvious the influence on the fluorescence signal. According to the change degree of the fluorescence signal, the temperature of the corresponding object or environment to be measured can be measured.

[0027] The optical fiber spectrometer in the signal acquisition module 3 can collect the fluorescence signal transmitted by the transmission optical fiber and output it to the computer 2 for analysis and calculation, and finally visualize it as a spectrum. The fluorescence intensity of the fluorescence emission peak is corresponded with the temperature to calculate the calibration curve. By measuring the fluorescence intensity of the fluorescence emission peaks at different positions, the temperature corresponding to this position can be calculated, realizing quasi-distributed temperature sensing with precise positioning. All the optical fiber sensing units 5 are cascaded on the same transmission optical fiber. Therefore, only the same fluorescence signal needs to be demodulated and signal processed, simplifying the complex distributed optical fiber sensing demodulation and the cumbersome signal processing problems, and being able to realize the identification of the precise positioning of the temperature change site, meeting the requirements of short-distance multi-site temperature sensing in harsh environments.

[0028] See Figure 2 As shown, it is the schematic diagram of fluorescence temperature sensing. Fluorescent materials such as quantum dots have good fluorescence quantum yields and temperature-sensitive characteristics. They can be excited by the laser source 7 to generate excitation light corresponding to their optimal absorption wavelength, resulting in laser excitation 8, and then fluorescence emission is generated. Quantum dots 9 with different particle sizes have the same absorption spectrum and optimal absorption wavelength due to the same constituent substances but can produce different fluorescence emission wavelengths. When excited by the excitation light, stimulated emission transitions 9 will occur to produce fluorescence. In the case of the same material, the wavelength of the fluorescence emission peak is related to the particle size of the quantum dot. The larger the particle size, the longer the fluorescence emission wavelength. After quantum dots with different particle sizes are modified on the sensing regions of different fiber optic sensing units and are excited by excitation light with the same excitation wavelength, different fluorescence will still be generated. Moreover, the wavelength of the fluorescence emission peak of the quantum dot itself has a large gap from the absorption spectrum and does not overlap. Therefore, the fluorescence signals generated by other fiber optic sensing units will not be used as possible subsequent excitation light signals, thus avoiding the crosstalk problem. Different degrees of temperature changes will cause different degrees of fluorescence quenching of the quantum dot fluorescence in the corresponding sensing unit, which is not related to the fluorescence intensity of the quantum dots in other cascaded fiber optic sensing units. Therefore, on the final emission spectrum of the optical path after cascading multiple fiber optic sensing units, multiple significantly different fluorescence emission peaks will appear, and the intensity of each fluorescence emission peak can change independently according to the change of the ambient temperature of the corresponding fiber optic sensing unit. Utilizing this point, distributed fiber optic fluorescence temperature sensing can be realized. By detecting the change of the fluorescence intensity of different fluorescence emission peaks, temperature sensing can be achieved, and the corresponding temperature change sites can be identified by the wavelength of the fluorescence emission peak.

[0029] See Figure 3As shown, it is a structural diagram of the optical fiber sensing unit. The entire structure is composed of the optical fiber sensing unit sensing area 15, the heat transfer liquid 14, and the capillary packaging structure 13 from the inside to the outside. The optical fiber sensing unit sensing area 15 is located inside the capillary packaging structure 13 and is fixedly connected to the inlet and outlet of the optical fibers at both ends of the curing structure by the curing glue 12. The heat transfer liquid 14 is filled in the gap between the capillary packaging structure 13 and the optical fiber sensing unit sensing area 15 to achieve the effect of heat transfer. The optical fiber sensing unit sensing area 15 is prepared by corroding the optical fiber cladding. Quantum dots of different particle sizes are fixed in the corroded area by physical deposition, chemical modification, ion sputtering or filling. The uncorroded area is the transmission optical fiber 4 not used for sensing. Its main function is to transmit the excitation light signal to the sensing area of ​​the optical fiber sensing unit as a medium and transmit the fluorescence signal generated by it to the signal acquisition module. A capillary packaging structure 13 matching the diameter of the transmission optical fiber 4 is inserted into the transmission optical fiber, and the sensing area 15 of the optical fiber sensing unit is moved to the center of the packaging structure. The capillary effect of the capillary is used to absorb the heat transfer liquid 14 to form a liquid core structure. Finally, the two sides of the capillary are sealed with a curing glue 12. To ensure the sealing, ultraviolet curing glue with good waterproof properties is often used for rapid sealing. After sealing, the packaging of the optical fiber sensing unit is completed.

[0030] After the package is completed, the optical fiber sensing unit 5 is placed in the environment to be tested or close to the object to be tested. The external temperature 17 is transferred from the capillary to the heat transfer liquid and then to the quantum dots 16 modified on the surface of the sensing area 15 of the optical fiber sensing unit by heat transfer. When the excitation light passes through the fiber core 11, it transmits energy to the quantum dots in the form of evanescent waves to generate fluorescence signals. The generated fluorescence signals are then returned to the fiber core 11 in the form of evanescent waves and transmitted to the optical fiber spectrometer for signal acquisition and demodulation. The quantum dots generate changes in fluorescence signals due to changes in the ambient temperature, so temperature sensing can be achieved based on the changes in fluorescence signals. Since the fluorescence wavelengths generated by quantum dots of different particle sizes are not within the absorption band of quantum dots, cascading multiple optical fiber sensing units on the same optical fiber will not affect the absorption of the excitation light by each optical fiber sensing unit, and thus will not affect the intensity of its fluorescence. Therefore, the influence of signal crosstalk can be effectively alleviated. Finally, by detecting the temperature change of the peak intensity of different fluorescence, the position of the temperature change is determined according to the wavelength of the fluorescence emission peak, and distributed temperature sensing is achieved.

[0031] It should be noted that in this text, relative terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or terminal device comprising a series of elements not only includes those elements, but also includes other elements not expressly listed, or further includes elements inherent to such process, method, article or terminal device. Without further limitation, elements defined by the statement "comprising..." or "including..." do not exclude the presence of additional elements in the process, method, article or terminal device comprising the said elements. In addition, in this text, "greater than", "less than", "more than" etc. are understood not to include the base number; "above", "below", "within" etc. are understood to include the base number.

[0032] Although the above-described embodiments have been described, those skilled in the art can make additional changes and modifications once they learn the basic creative concept. Therefore, the above are only embodiments of the present invention and do not limit the patent protection scope of the present invention. Any equivalent structure or equivalent process transformation made by using the content of the specification and drawings of the present invention, or directly or indirectly applied in other related technical fields, shall be included in the patent protection scope of the present invention by the same token.

Claims

1. A quasi-distributed optical fiber temperature sensor based on fluorescence quenching, characterized in that: It includes a laser light source module, a transmission optical fiber, an optical fiber sensing unit, a signal acquisition module and a computer. The laser light source module provides an excitation light signal to excite fluorescent substances to generate a fluorescence signal; the transmission optical fiber transmits the excitation signal to the optical fiber sensing unit and collects the evanescent wave fluorescence signal generated on the surface of the optical fiber sensing unit; the optical fiber sensing unit uses an optical fiber sensing unit modified with quantum dots of different particle sizes to perform multi-point optical fiber fluorescence distributed temperature sensing, detects the temperature change through the change of the fluorescence signal, and identifies the specific location of the temperature change through the wavelength of the fluorescence emission peak; the signal acquisition module is used to collect the fluorescence signal and process it and transmit it to the computer; the computer presents the spectrum and calculates the calibration relationship between the intensity change of each fluorescence emission peak and the temperature, and then calculates the temperature change according to the relationship to realize distributed temperature sensing.

2. The quasi-distributed optical fiber temperature sensor based on fluorescence quenching according to claim 1, characterized in that: The optical fiber sensing unit uses a tapered optical fiber with enhanced sensitivity, a D-type optical fiber, or an optical fiber with a bare core after cladding corrosion as a substrate, and the method of modifying quantum dots on the optical fiber surface includes physical deposition, chemical modification, and sputtering film.

3. The quasi-distributed optical fiber temperature sensor based on fluorescence quenching according to claim 1, characterized in that: The laser wavelength range is 350-400nm, which is the optimal absorption wavelength range of quantum dots. The excitation light wavelength can be changed according to different fluorescent materials.

4. The quasi-distributed optical fiber temperature sensor based on fluorescence quenching according to claim 1, characterized in that: The optical path connected by the transmission optical fiber can be doped with gain materials in the transmission optical fiber or an optical fiber device can be added in the optical path to assist in signal amplification, enhancement and filtering functions.

5. The quasi-distributed optical fiber temperature sensor based on fluorescence quenching according to claim 1, characterized in that: The optical fiber sensing unit is packaged by a capillary tube corresponding to the size of the optical fiber, and the interior of the optical fiber sensing unit is filled with a liquid core.

6. The quasi-distributed optical fiber temperature sensor based on fluorescence quenching according to claim 1, characterized in that: The fluorescent substance in the optical fiber sensing unit is modified with a temperature-sensitive fluorescent substance using fluorescent properties.

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