Furfural detection system and method based on moire enhanced differential photothermal lens spectrum

Abstract

The application discloses a system and method for detecting furfural based on a Moire-enhanced differential photothermal lens spectrum, wherein a quantum cascade laser is used as a pumping light source, a helium-neon laser is used as a detection light source, and a dichroic mirror is used to realize coaxial incidence of the pumping light and the detection light; the pumping light is chopped and modulated and focused into a fixed optical path liquid pool to excite a thermal lens effect; the detection light is diffracted by a sample to generate a diffraction circle; the diffraction circle is subjected to spatial beat frequency amplification by a Moire circle grating; then, the diffraction circle enters a silicon bias photodetector through a variable diaphragm; a detection signal is differentially demodulated by a lock-in amplifier and processed by an upper computer. By using the spatial beat frequency amplification effect of the Moire circle grating, optical amplification is realized, and the sensitivity of the detection system is greatly improved; in combination with a differential detection structure, common-mode noise caused by light source power fluctuation, environmental vibration and temperature drift is effectively suppressed, and the signal-to-noise ratio and long-term stability of the detection system are greatly improved.

Description

Technical Field

[0001] This invention relates to the field of transformer oil testing technology, specifically to a furfural detection system and method based on Moiré enhanced differential photothermal lens spectroscopy. Background Technology

[0002] Large power transformers are core equipment in power grids, and the aging state of their internal insulation paper directly affects the safe and stable operation of the power system. During long-term operation, the insulation paper undergoes aging and degradation under the influence of heat, electricity, and mechanical stress, generating characteristic compounds such as furfural. Furfural has good thermal stability, is readily soluble in transformer oil, and its concentration shows a strong negative correlation with the degree of polymerization of the insulation paper. Therefore, detecting the furfural content in transformer oil can effectively assess the aging state of the transformer's solid insulation. Currently, the main methods for detecting furfural in transformer oil include high-performance liquid chromatography (HPLC) and spectrophotometry. While these methods offer high precision, they generally suffer from drawbacks such as complex sample pretreatment, long analysis cycles, and difficulty in achieving in-situ monitoring.

[0003] Photothermal lensing spectroscopy, a highly sensitive absorption spectroscopy technique, can measure weak light absorption. Currently, there are schemes using photothermal lensing spectroscopy to detect furfural in oil. The basic principle is to reflect the concentration of the analyte by measuring the change in light intensity in the central region of the probe light. To improve detection sensitivity, existing schemes typically employ strategies such as increasing the pump light power or extending the optical path of the liquid cell. However, transformer oil has a low thermal conductivity (approximately 0.11 W / (m·K)). Under the aforementioned enhancement conditions, the thermal lensing effect is significantly amplified, causing the light intensity at the center of the probe light to approach zero, accompanied by the formation of diffraction rings. At this point, existing photothermal lensing spectroscopy methods become ineffective, making it difficult to accurately extract furfural concentration information. This physical bottleneck severely restricts the practical application of existing photothermal lensing spectroscopy methods in the detection of trace amounts of furfural in transformer oil. Summary of the Invention

[0004] The purpose of this invention is to address the problems existing in the prior art by providing a furfural detection system and method based on Moiré enhanced differential photothermal lens spectroscopy. This system aims to effectively avoid detection failure caused by light intensity collapse in low thermal conductivity media, thereby achieving ultra-high sensitivity detection of trace furfural.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: In a first aspect, a furfural detection system based on Moiré-enhanced differential photothermal lens spectroscopy is provided, comprising: A quantum cascade laser, wherein the quantum cascade laser is used to emit pump light, which is reflected and transmitted by an infrared beam splitter to form a first pump beam and a second pump beam; A helium-neon laser, wherein the helium-neon laser is used to emit probe light, which is reflected and transmitted by a visible light beam splitter to form a first probe beam and a second probe beam; The first visible-infrared dichroic mirror, the first pump beam and the first detector beam are combined by the first visible-infrared dichroic mirror, and then pass sequentially through the first fixed optical path liquid cell, the first moiré ring grating, the first variable aperture and the first photodetector, which are arranged in the output direction of the first visible-infrared dichroic mirror. The second visible-infrared dichroic mirror, the second pump beam and the second detector beam are combined by the second visible-infrared dichroic mirror, and then pass sequentially through the second fixed optical path liquid cell, the second moiré ring grating, the second variable aperture and the second photodetector, which are set in the output direction of the second visible-infrared dichroic mirror. The first fixed optical path liquid cell is used to hold pure oil without furfural, and the second fixed optical path liquid cell is used to hold the oil sample to be tested. The difference in the thermal lensing effect excited by the two will be caused only by furfural. The first photodetector and the second photodetector are respectively connected to a lock-in amplifier via cables, and the lock-in amplifier is connected to a host computer.

[0006] Furthermore, the wavelength of the pump light output by the quantum cascade laser matches the characteristic absorption peak of furfural, which is located in the mid-infrared band.

[0007] Furthermore, a half-wave plate and a chopper are sequentially provided between the quantum cascade laser and the infrared beam splitter. The chopper is connected to a chopper controller, and the chopper controller is connected to the lock-in amplifier. After passing through the half-wave plate, the pump light is modulated by the chopper. The modulated linearly polarized pump beam is then split into a first pump beam and a second pump beam with equal power after passing through the infrared beam splitter.

[0008] Furthermore, by rotating the half-wave plate to change the polarization direction of the pump light emitted by the quantum cascade laser, the beam splitting ratio of the infrared beam splitter is adjusted to 50:50.

[0009] Furthermore, a plurality of first visible light reflectors are provided between the helium-neon laser and the visible light beam splitter, and a visible light plano-convex lens is also provided between the first visible light reflectors and the visible light beam splitter.

[0010] Furthermore, a second visible light reflector is provided between the visible light beam splitter and the first visible light infrared dichroic mirror or the second visible light infrared dichroic mirror.

[0011] Furthermore, a first infrared plano-convex lens is provided between the infrared beam splitter and the first visible infrared dichroic mirror, and an infrared reflector and a second infrared plano-convex lens are provided sequentially between the infrared beam splitter and the second visible infrared dichroic mirror.

[0012] Furthermore, the optical path lengths of both the first fixed optical path liquid cell and the second fixed optical path liquid cell are greater than 0.1 mm. The first fixed optical path liquid cell and the second fixed optical path liquid cell are respectively mounted on a slide to adjust the sensitivity. The power of the pump light is greater than 100 mW, and the power of the probe light is 0.5 to 5 mW.

[0013] Secondly, a detection method for a furfural detection system based on Moiré-enhanced differential photothermal lens spectroscopy is provided, the detection method comprising the following steps: Turn on the quantum cascade laser and preheat it to a stable operating temperature; Pure transformer oil samples without furfural were respectively poured into the first fixed optical path liquid pool and the second fixed optical path liquid pool, and the output of the lock-in amplifier was recorded to determine the zero point and noise level. A furfural reference sample of known concentration was added to the second fixed optical path liquid cell, and the measurement response was recorded and a calibration curve of furfural concentration was established. The oil sample to be tested is placed in the second fixed optical path liquid cell and the first fixed optical path liquid cell is used as a control. Moiré enhancement and differential detection are performed. The host computer collects the output of the lock-in amplifier after differential detection and calculates and outputs the furfural concentration in the oil sample to be tested according to the calibration curve.

[0014] Further, the steps for performing moiré enhancement and differential detection are as follows: the first probe beam and the second probe beam pass through the corresponding first moiré ring grating and the second moiré ring grating respectively to achieve spatial beat frequency amplification, converting the diffraction ring difference caused by the thermal lensing effect into moiré fringe changes, and then sending the two probe signals to the lock-in amplifier for differential demodulation after being collected by the first variable aperture, the first photodetector and the second variable aperture and the second photodetector.

[0015] Compared with the prior art, the beneficial effects of this invention are: 1. This transformer oil furfural detection system, by setting a moiré ring grating and introducing a moiré fringe enhancement mechanism, transforms the minute displacement of the diffraction ring of the probe light caused by the photothermal lensing effect into significant moiré fringe changes, achieving optical amplification and thus greatly improving the sensitivity of the detection system, overcoming the technical defects of existing technologies that are difficult to effectively detect under strong thermal lensing conditions; 2. This transformer oil furfural detection system also incorporates a differential detection structure, effectively suppressing common-mode noise introduced by light source power fluctuations, environmental vibrations, and temperature drift, greatly improving the signal-to-noise ratio and long-term stability of the detection system, providing a reliable guarantee for continuous online monitoring of furfural in transformer oil; 3. The The first fixed optical path liquid cell and the second fixed optical path liquid cell are respectively irradiated by the first pump beam and the second pump beam to excite the thermal lensing effect; at the same time, the first probe beam and the second probe beam also synchronously pass through their respective fixed optical path liquid cells, forming two diffraction rings respectively; using two sets of moiré ring gratings, the diffraction rings caused by thermal lensing are magnified by producing moiré fringes through the gratings; 4. Differential demodulation is used to suppress common-mode noise, including light source power fluctuations, environmental vibrations and temperature drift, to improve the signal-to-noise ratio and long-term stability; the host computer is used to record and process the differential signal output by the lock-in amplifier, and to calibrate based on the response difference between the control oil sample and the test oil sample to obtain the furfural concentration. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the furfural detection system based on Moiré-enhanced differential photothermal lens spectroscopy of the present invention. Figure 2 This invention provides the light spot before the detection light passes through the moiré ring grating. Figure 3 This is a schematic diagram of the moiré enhancement principle of the present invention.

[0017] In the diagram: 1. Quantum cascade laser; 2. Half-wave plate; 3. Chopper; 4. Chopper controller; 5. Infrared beam splitter; 6. Pump beam; 7. First pump beam; 8. Second pump beam; 9. Infrared reflector; 10. First infrared plano-convex lens; 11. Second infrared plano-convex lens; 12. First visible-infrared dichroic mirror; 13. Second visible-infrared dichroic mirror; 14. First fixed-path liquid cell; 15. Second fixed-path liquid cell; 16. Helium-neon laser Optical device; 17. Probe beam; 18-19. First visible light reflector; 20. Visible light plano-convex lens; 21. Visible light beam splitter; 22. Second probe beam; 23. First probe beam; 24. Second visible light reflector; 25. Second moiré ring grating; 26. First moiré ring grating; 27. Second variable aperture; 28. First variable aperture; 29. ​​Second photodetector; 30. First photodetector; 31. Cable; 32. Lock-in amplifier; 33. Host computer. Detailed Implementation

[0018] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are merely some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] In the description of this invention, it should be noted that the terms "middle," "upper," "lower," "left," "right," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention. Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0020] Example 1: A furfural detection system based on Moiré-enhanced differential photothermal lens spectroscopy is provided, such as... Figure 1 As shown, it includes: A quantum cascade laser 1 is used to emit pump light 6, which is reflected and transmitted by an infrared beam splitter 5 to form a first pump beam 7 and a second pump beam 8. A helium-neon laser 16 is used to emit a probe light 17, which is reflected and transmitted by a visible light beam splitter 21 to form a first probe beam 23 and a second probe beam 22. The first visible-infrared dichroic mirror 12, the first pump beam 7 and the first detector beam 23 are coaxially combined through the first visible-infrared dichroic mirror 12, and then sequentially pass through the first fixed optical path liquid pool 14, the first moiré ring grating 26, the first variable aperture 28 and the first photodetector 30, which are set in the output direction of the first visible-infrared dichroic mirror 12. The second visible-infrared dichroic mirror 13, the second pump beam 8 and the second detector beam 22 are coaxially combined by the second visible-infrared dichroic mirror 13, and then sequentially pass through the second fixed optical path liquid pool 15, the second moiré ring grating 25, the second variable aperture 27 and the second photodetector 29, which are set in the output direction of the second visible-infrared dichroic mirror 13. The first fixed optical path liquid cell 14 is used to hold pure oil without furfural, and the second fixed optical path liquid cell 15 is used to hold the oil sample to be tested. The difference in the thermal lensing effect excited by the two will be caused only by furfural. The first photodetector 30 and the second photodetector 29 are respectively connected to the lock-in amplifier 32 via cable 31, and the lock-in amplifier 32 is connected to the host computer 33.

[0021] This transformer oil furfural detection system introduces a moiré fringe enhancement mechanism by setting up a moiré ring grating. This transforms the minute displacement of the diffraction ring of the probe light caused by the photothermal lensing effect into significant changes in the moiré fringes, achieving optical magnification and thus greatly improving the sensitivity of the detection system. This overcomes the technical deficiency of existing technologies that are difficult to effectively detect under strong thermal lensing conditions.

[0022] This furfural detection system in transformer oil also incorporates a differential detection structure, which effectively suppresses common-mode noise introduced by light source power fluctuations, environmental vibrations, and temperature drift. This greatly improves the signal-to-noise ratio and long-term stability of the detection system, providing a reliable guarantee for continuous online monitoring of furfural in transformer oil.

[0023] The first fixed optical path liquid cell 14 and the second fixed optical path liquid cell 15 are respectively irradiated by the first pump beam 7 and the second pump beam 8 to excite the thermal lensing effect. Due to the setting of the first and second visible and infrared dichroic mirrors, the first detection beam 23 and the first pump beam 7 can be combined and propagated coaxially, and the second detection beam 22 and the second pump beam 8 can be combined and propagated coaxially. In this way, when the first pump beam 7 and the second pump beam 8 excite the thermal lensing effect through their respective fixed optical path liquid cells, the first detection beam 23 and the second detection beam 22 also pass through their respective fixed optical path liquid cells simultaneously, forming two diffraction rings respectively.

[0024] The first and second sets of moiré ring gratings are respectively positioned on the two probe light propagation paths after the two fixed optical path liquid pools. They are used to amplify the moiré fringe effect caused by the diffraction rings induced by the thermal lens through the gratings. The first and second variable apertures are positioned between their respective moiré ring gratings and photodetectors to filter stray light and adjust the light spot. The first photodetector 30 and the second photodetector 29 are two silicon-biased photodetectors, each used to receive the probe light passing through its respective sensor. The differential input terminals of the lock-in amplifier 32 are connected to the output terminals of the two silicon-biased photodetectors via BNC cables, enabling the demodulated differential signal to be sent to the host computer 33 via a USB interface.

[0025] The system converts the minute differences in the diffraction rings caused by the thermal lensing effect into changes in moiré fringes through spatial beat frequency amplification of the first and second moiré ring gratings, and suppresses common-mode noise through differential detection to achieve the detection of furfural.

[0026] Furthermore, the quantum cascade laser 1 is a continuous-wave quantum cascade laser used as a pump source, and the wavelength of its output pump light matches the characteristic absorption peak of furfural, which is located in the mid-infrared band.

[0027] Furthermore, a half-wave plate 2 and a chopper 3 are sequentially provided between the quantum cascade laser 1 and the infrared beam splitter 5. The chopper 3 is connected to a chopper controller 4, and the chopper controller 4 is connected to the lock-in amplifier 32. The pump light 6 is modulated by the chopper 3 after passing through the half-wave plate 2. The modulated linearly polarized pump beam is split into a first pump beam 7 and a second pump beam 8 with equal power after passing through the infrared beam splitter 5.

[0028] The half-wave plate 2 is used to adjust the polarization direction of the pump light, and the chopper 3 and its controller are used to modulate the pump light; the infrared beam splitter 5 is used to split the modulated pump light into two equal-power first pump beam and second pump beam. The modulation frequency of the chopper is 20 Hz to 200 Hz.

[0029] Furthermore, by rotating the half-wave plate 2 to change the polarization direction of the pump light emitted by the quantum cascade laser, the beam splitting ratio of the infrared beam splitter 5 is adjusted to 50:50.

[0030] Furthermore, a plurality of first visible light reflectors are provided between the helium-neon laser 16 and the visible light beam splitter 21, and a visible light plano-convex lens 20 is also provided between the first visible light reflectors and the visible light beam splitter 21.

[0031] In this embodiment, two first visible light reflectors, 18 and 19, are provided to adjust the direction of probe light propagation so that the probe light is reflected and propagated to the visible light plano-convex lens. The visible light plano-convex lens 20 is used to focus the probe beam.

[0032] Furthermore, a second visible light reflector 24 is provided between the visible light beam splitter 21 and the first visible light infrared dichroic mirror 12, which is also used to adjust the direction of probe light propagation.

[0033] Furthermore, a first infrared plano-convex lens 10 is provided between the infrared beam splitter 5 and the first visible infrared dichroic mirror 12; an infrared reflector 9 and a second infrared plano-convex lens 11 are sequentially provided between the infrared beam splitter 5 and the second visible infrared dichroic mirror 13; the first and second infrared plano-convex lenses are used to focus the pump beam, and the infrared reflector is used to reflect the pump light to adjust the propagation direction of the pump light.

[0034] Furthermore, the optical path lengths of both the first fixed optical path liquid cell 14 and the second fixed optical path liquid cell 15 are greater than 0.1 mm, with 0.1 to 1 mm recommended; the power of the pump light is greater than 100 mW to excite the diffraction ring for detection, and higher power can be used with thermal management if necessary; the power of the probe light is 0.5 to 5 mW.

[0035] In some embodiments, the first fixed optical path liquid cell 14 and the second fixed optical path liquid cell 15 are respectively mounted on a slide block for adjusting the sensitivity within a small range. Furthermore, the ring spacing and shape of the first moiré ring grating 26 and the ring spacing and shape of the second moiré ring grating 25 are selected according to the size of the diffraction rings, and the propagation distance between the moiré ring grating and the silicon bias photodetector can be adjusted to optimize the spot size and contrast.

[0036] Furthermore, the host computer 33 is used to record and process the differential signal output by the lock-in amplifier 32, and to calibrate based on the response difference between the control oil sample and the test oil sample to obtain the furfural concentration.

[0037] Furthermore, the differential demodulation is used to suppress common-mode noise, including light source power fluctuations, environmental vibrations, and temperature drift, in order to improve the signal-to-noise ratio and long-term stability.

[0038] Example 2: A detection method for a furfural detection system based on Moiré enhanced differential photothermal lens spectroscopy is provided.

[0039] The detection method includes the following steps: (1) Preheating: Turn on the quantum cascade laser 1 and preheat to a stable operating temperature; (2) Blank measurement: Pure transformer oil samples without furfural were respectively placed into the first fixed optical path liquid pool 14 and the second fixed optical path liquid pool 15, and the output of the lock-in amplifier 32 was recorded to determine the zero point and noise level; (3) Calibration measurement: Add a furfural reference sample of known concentration to the second fixed optical path liquid cell 15, record and establish a calibration curve of measurement response versus furfural concentration; (4) Measurement to be measured: The oil sample to be measured is loaded into the second fixed optical path liquid cell 15 and the first fixed optical path liquid cell 14 is used as a control. The modulated first pump beam 7 and the second pump beam 8 are used to excite the thermal lens effect in each fixed optical path liquid cell, so that each probe beam forms a diffraction ring behind each cell. (5) Moiré enhancement and differential detection: The first detection beam 23 and the second detection beam 22 are respectively passed through the corresponding first moiré ring grating 26 and the second moiré ring grating 25 to achieve spatial beat frequency amplification, converting the diffraction ring difference caused by the thermal lens effect into moiré fringe changes, and the two detection signals are collected by the first variable aperture 28, the first photodetector 30 and the second variable aperture 27, the second photodetector 29 and then sent to the lock-in amplifier 32 for differential demodulation; (6) Data processing: The host computer collects the output of the lock-in amplifier after differential detection and calculates and outputs the furfural concentration in the oil sample to be tested according to the calibration curve in step (3).

[0040] This detection method, through the above steps, introduces a moiré fringe enhancement mechanism, which optically amplifies the minute changes in the diffraction ring of the probe light caused by the photothermal lensing effect through a moiré ring grating. This effectively avoids the detection failure problem caused by light intensity collapse in low thermal conductivity media. Combined with differential processing technology, it achieves ultra-high sensitivity detection of trace furfural.

[0041] Example 3: This example further provides a furfural detection system in transformer oil based on photothermal lens spectroscopy, illustrating its principle and method.

[0042] Combination Figure 1As shown, the pump source of the detection system uses a continuous-wave quantum cascade laser, whose output wavelength precisely matches the characteristic absorption peak of furfural in the mid-infrared band. To ensure a significant thermal lensing effect under a fixed optical path, the output power generally needs to be greater than 100mW. The pump optical path includes a half-wave plate 2 and an optical chopper 3. The chopper is driven by a dedicated chopper controller 4, with a typical modulation frequency in the range of 20 to 200Hz, used to intensity modulate the continuous laser for subsequent phase-locked detection. The infrared beam splitter 5 splits the modulated pump light 6 into a first pump beam 7 and a second pump beam 8 with equal power. The splitting ratio can be precisely adjusted by rotating the half-wave plate 2 to achieve a 50:50 equal distribution. The second pump beam 8 is reflected by the infrared reflector 9. Then, the two pump beams are focused by the first infrared plano-convex lens 10 and the second infrared plano-convex lens 11, respectively, and reflected by the first visible-infrared dichroic mirror 12 and the second visible-infrared dichroic mirror 13 into the first fixed optical path liquid cell 14 and the second fixed optical path liquid cell 15. The optical path of the two liquid cells is usually greater than 0.1 mm. The first fixed optical path liquid cell is filled with a control oil without furfural, and the second fixed optical path liquid cell is filled with the oil sample to be tested. The cells are mounted on a slide and their positions can be finely adjusted to optimize the detection sensitivity.

[0043] The detection source is a helium-neon laser 16 with a wavelength of 632.8 nm. The visible light output (detector beam 17) is guided by the first visible light reflectors 18 and 19, then focused by the visible light plano-convex lens 20, and then split into a first detection beam 23 and a second detection beam 22 by the visible light beam splitter 21. The first detection beam 23 is reflected by the second visible light reflector 24. The two detection beams are then combined with the pump beams of the corresponding paths by the first visible-infrared dichroic mirror 12 and the second visible-infrared dichroic mirror 13, respectively, to achieve coaxial propagation. A moiré ring grating (second moiré ring grating 25 and first moiré ring grating 26) is arranged in the propagation direction of each detection beam. When the pump beam excites the thermal lensing effect in the sample cell, the diffraction ring formed after the detection beam passes through the thermal lensing region is amplified by the moiré ring grating, resulting in a spatial beat frequency amplification effect and causing a significant change in the moiré fringes. The probe beam propagates a distance after the moiré ring grating to adjust the spot size, and then passes through the first variable aperture 28 and the second variable aperture 27 respectively before being incident on the silicon bias photodetector.

[0044] The output signals of the two silicon bias photodetectors are connected to the differential input of the lock-in amplifier 32 via a BNC cable. The lock-in amplifier differentially demodulates the two signals to effectively suppress common-mode noise and extract the thermal lens signal synchronized with the modulation frequency. The output of the lock-in amplifier 32 communicates with the host computer 33 via a USB interface. The host computer is responsible for data recording, processing phase and amplitude information, and calibration based on the response differences between the control oil and the test oil sample, thereby achieving high-sensitivity detection of furfural content in the oil.

[0045] The pump light generates a photothermal effect in the sample, forming a localized thermal lens. This thermal lens causes central intensity collapse on the probe light, producing diffraction rings. The two control oil samples and the test oil sample exhibit slightly different diffraction ring morphologies due to differences in furfural content. Figure 2 As shown. After the diffraction rings pass through the moiré ring grating, due to the spatial beat frequency amplification effect, the minute ring displacement is converted into significant moiré fringe changes, thus producing measurable intensity changes, such as... Figure 3 As shown.

[0046] Differential detection: After the two probe beams pass through two identical optical paths and gratings, the silicon detector output is demodulated by a lock-in amplifier and differentially processed, which effectively suppresses common-mode noise such as light source power fluctuations, environmental vibrations and temperature drift, and improves the signal-to-noise ratio and long-term stability.

[0047] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A furfural detection system based on Moiré-enhanced differential photothermal lens spectroscopy, characterized in that, include: A quantum cascade laser, wherein the quantum cascade laser is used to emit pump light, which is reflected and transmitted by an infrared beam splitter to form a first pump beam and a second pump beam; A helium-neon laser, wherein the helium-neon laser is used to emit probe light, which is reflected and transmitted by a visible light beam splitter to form a first probe beam and a second probe beam; The first visible-infrared dichroic mirror, the first pump beam and the first detector beam are combined by the first visible-infrared dichroic mirror, and then pass sequentially through the first fixed optical path liquid cell, the first moiré ring grating, the first variable aperture and the first photodetector, which are arranged in the output direction of the first visible-infrared dichroic mirror. The second visible-infrared dichroic mirror, the second pump beam and the second detector beam are combined by the second visible-infrared dichroic mirror, and then pass sequentially through the second fixed optical path liquid cell, the second moiré ring grating, the second variable aperture and the second photodetector, which are set in the output direction of the second visible-infrared dichroic mirror. The first fixed optical path liquid cell is used to hold pure oil without furfural, and the second fixed optical path liquid cell is used to hold the oil sample to be tested. The difference in the thermal lensing effect excited by the two will be caused only by furfural. The first photodetector and the second photodetector are respectively connected to a lock-in amplifier via cables, and the lock-in amplifier is connected to a host computer. The first detection beam and the second detection beam are respectively passed through the corresponding first and second moiré ring gratings to achieve spatial beat frequency amplification, converting the diffraction ring differences caused by the thermal lensing effect into moiré fringe changes. The two detection signals are then collected by the first variable aperture, the first photodetector, the second variable aperture, and the second photodetector and sent to the lock-in amplifier for differential demodulation.

2. The furfural detection system based on Moiré-enhanced differential photothermal lens spectroscopy according to claim 1, characterized in that, The pump light output by the quantum cascade laser has a wavelength that matches the characteristic absorption peak of furfural, and is located in the mid-infrared band.

3. The furfural detection system based on Moiré-enhanced differential photothermal lens spectroscopy according to claim 1, characterized in that, A half-wave plate and a chopper are sequentially arranged between the quantum cascade laser and the infrared beam splitter. The chopper is connected to a chopper controller, and the chopper controller is connected to the lock-in amplifier. The pump light is modulated by the chopper after passing through the half-wave plate. The modulated linearly polarized pump beam is split into a first pump beam and a second pump beam with equal power after passing through the infrared beam splitter.

4. The furfural detection system based on Moiré-enhanced differential photothermal lens spectroscopy according to claim 3, characterized in that, By rotating the half-wave plate to change the polarization direction of the pump light emitted by the quantum cascade laser, the beam splitting ratio of the infrared beam splitter is adjusted to 50:

50.

5. The furfural detection system based on Moiré-enhanced differential photothermal lens spectroscopy according to claim 1, characterized in that, A plurality of first visible light reflectors are provided between the helium-neon laser and the visible light beam splitter, and a visible light plano-convex lens is also provided between the first visible light reflectors and the visible light beam splitter.

6. The furfural detection system based on Moiré-enhanced differential photothermal lens spectroscopy according to claim 1, characterized in that, A second visible light reflector is provided between the visible light beam splitter and the first or second visible light infrared dichroic mirror.

7. The furfural detection system based on Moiré-enhanced differential photothermal lens spectroscopy according to claim 1, characterized in that, A first infrared plano-convex lens is provided between the infrared beam splitter and the first visible infrared dichroic mirror, and an infrared reflector and a second infrared plano-convex lens are provided sequentially between the infrared beam splitter and the second visible infrared dichroic mirror.

8. The furfural detection system based on Moiré-enhanced differential photothermal lens spectroscopy according to claim 1, characterized in that, The optical path lengths of both the first fixed optical path liquid cell and the second fixed optical path liquid cell are greater than 0.1 mm. The first fixed optical path liquid cell and the second fixed optical path liquid cell are respectively mounted on a slide to adjust the sensitivity. The power of the pump light is greater than 100 mW, and the power of the probe light is 0.5 to 5 mW.

9. The detection method of the furfural detection system based on Moiré enhanced differential photothermal lens spectroscopy according to any one of claims 1 to 8, characterized in that, The detection method includes the following steps: Turn on the quantum cascade laser and preheat it to a stable operating temperature; Pure transformer oil samples without furfural were respectively poured into the first fixed optical path liquid pool and the second fixed optical path liquid pool, and the output of the lock-in amplifier was recorded to determine the zero point and noise level. A furfural reference sample of known concentration was added to the second fixed optical path liquid cell, and the measurement response was recorded and a calibration curve of furfural concentration was established. The oil sample to be tested is placed in the second fixed optical path liquid cell and the first fixed optical path liquid cell is used as a control. Moiré enhancement and differential detection are performed. The host computer collects the output of the lock-in amplifier after differential detection and calculates and outputs the furfural concentration in the oil sample to be tested according to the calibration curve.

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