Photoacoustic spectroscopy detection system based on direct acoustic wave coupling-multi-path photoacoustic cell

Abstract

The present invention claims protection for a photoacoustic spectrum detection system based on a direct coupling of acoustic waves and a multi-path photoacoustic cell. The system includes a modulator, a DFB laser light source, an isolator, a direct coupling of acoustic waves and a multi-path photoacoustic cell, a circulator, a low-coherence superluminescent diode light source, a charge-coupled device (CCD), and a computer. The excitation light is reflected multiple times on the inner wall of the direct coupling of acoustic waves and a circular multi-path photoacoustic cell. The detection light is loaded with acoustic wave information in the FPE structure in the photoacoustic cell, and the two beams of light reflected in the FPE structure interfere with each other. The interference signal is demodulated to obtain an acoustic wave signal, and finally the concentration information of the trace gas is obtained by the standard gas calibration method. In the present invention, the photoacoustic spectrum is detected by directly coupling the acoustic wave with the rigid FPE structure inside the photoacoustic cell and by multiple reflections of the excitation light, which greatly improves the sensitivity and dynamic range of low-frequency detection of the photoacoustic signal and improves the vibration resistance.

Description

Technical Field

[0001] The invention belongs to the technical field of optical fiber sensing and relates to a photoacoustic spectrum detection system based on a novel sound wave direct coupling-multi-optical path photoacoustic cell. Background Art

[0002] With the rapid development of the economy, the further advancement of urbanization and industrialization, environmental pollution problems have become increasingly prominent, seriously affecting people's health and quality of life. In recent years, environmental protection issues have received more and more attention from the country and the whole society. More sensitive, accurate, real-time and effective monitoring of gases that pollute the environment has become one of the urgent problems to be solved. At the same time, the demand for trace gas detection technology in the fields of coal mine safety production monitoring, gas leak detection, electrical equipment failure gas analysis and medical breath diagnosis is also increasing. High-sensitivity detection of the gas to be tested helps accurate analysis and early warning.

[0003] There are many means of detecting trace gases. Traditional detection methods include mass spectrometry, semiconductor sensing, and electrochemical sensing. They generally have problems such as low sensitivity, cross-interference, short life, and frequent calibration, which make it difficult to meet the needs of accurate monitoring of trace gases. Since the near- and mid-infrared spectral regions are the characteristic absorption bands of most small and medium-sized gas molecules, these gases can be measured using a light source of appropriate wavelength and a spectral detection method. Compared with non-optical detection methods, spectral detection methods have the advantages of high sensitivity, good selectivity, fast response, and no need for carrier gas, and are particularly suitable for accurate monitoring of trace gases. Photoacoustic spectroscopy is a typical method for gas detection using this principle. Photoacoustic spectroscopy is an indirect absorption spectroscopy technique that inverts the concentration of the target gas by measuring the intensity of the sound wave generated by the optical absorption of the gas. Photoacoustic spectroscopy does not directly measure the changes in the transmission spectrum, but measures the sound waves caused by the light absorption of the gas. Since the scattering, reflection and other losses in the optical path will not generate sound wave signals, the photoacoustic detection method can accurately measure the gas absorption. The concentration information of the gas to be measured can be obtained by collecting and processing the generated sound wave signals.

[0004] In the photoacoustic spectroscopy system, the lowest cost and most effective way to improve the excitation efficiency is to optimize the design of the photoacoustic cell to improve the excitation efficiency, such as optimizing the inner diameter of the photoacoustic cell, using multiple reflections to increase the effective absorption range, etc. In addition, amplifying the sound wave using the principle of acoustic wave resonance can also improve the detection sensitivity, but this type of photoacoustic cell is often large in size, requires a large amount of detection gas, and can also cause the resonant frequency to drift due to temperature changes, thereby affecting stability.

[0005] In the research of acoustic wave detection technology, quartz tuning forks, optical cantilevers and fiber microphones that can achieve high-sensitivity acoustic wave detection are the current research hotspots. In 2004, FK Tittel et al. from Rice University in the United States first proposed a technical solution based on quartz tuning fork enhanced photoacoustic spectroscopy (QEPAS), and many scholars at home and abroad have conducted in-depth research on it. In 2022, Yang Zhifei et al. from Jinan University used high-power erbium-doped fiber amplifiers in the QEPAS system and achieved a high-sensitivity detection of 7ppb for C2H2 using a 1536nm near-infrared laser. The QEPAS system uses a high-Q quartz tuning fork to improve the detection sensitivity of the system and reduce the volume of the gas detection unit. However, due to the small spacing between the tuning forks and the high operating frequency (generally around 32kHz), the QEPAS system has certain requirements for the laser beam quality and the gas transition relaxation rate. Compared with the quartz tuning fork, the optical cantilever has the advantages of good low-frequency response, high sensitivity and large linear range. In 2004, J. Kauppinen and others from the University of Turku in Finland designed an optical cantilever acoustic wave sensor based on Michelson interferometer and proposed the cantilever enhanced photoacoustic spectroscopy (CEPAS) scheme. In 2018, T. Tomberg and others adopted the CEPAS technical solution and used an optical parametric oscillator as a light source to achieve sub-ppt detection sensitivity for HF gas. The application of optical cantilever greatly improves the detection sensitivity of the system, but the structure of Michelson interferometer makes this type of acoustic wave detector have the disadvantages of poor vibration resistance, large size, complex structure and difficulty in matching with small-volume photoacoustic pool. The transmission mechanism of traditional optical acoustic wave sensors such as optical cantilever beams and fiber microphones is an indirect coupling acoustic wave sensor that converts the mechanical vibration of the cantilever beam or film caused by the sound wave into an optical signal. This type of acoustic wave sensor is limited by factors such as materials, size, thickness and process, and has the disadvantages of small frequency response flatness range and sound pressure linear range, and difficulty in achieving large bandwidth and dynamic range; moreover, the photoacoustic spectrum detection system of optical acoustic wave sensors based on cantilever beams and film structures is easily affected by external vibrations and produces measurement errors. In addition, due to the disadvantages of the resonant photoacoustic cell, such as the large amount of gas to be measured and the existence of resonance drift, the non-resonant photoacoustic cell with small size and strong robustness has become the best choice for the photoacoustic spectroscopy system in practical applications. However, the non-resonant photoacoustic cell usually works in a low-frequency range close to infrasound, and the traditional microphone is affected by the physical properties of the diaphragm and has a relatively weak response in the low-frequency band, which affects the sensitivity of photoacoustic signal detection. Through the overview of photoacoustic detection technology and the summary of current problems, it is urgently needed to have a new photoacoustic detection technology with high low-frequency sensitivity, large sound pressure linearity and dynamic range, not affected by temperature changes, and with certain vibration resistance.

[0006] After searching, application number 201810337667.9, a detection instrument and method for a miniaturized multiple-reflection photoacoustic cell enhanced laser photoacoustic spectroscopy, belongs to the field of trace gas detection technology. The instrument includes a laser light source, a light source driver, an optical collimator, a miniaturized multiple-reflection photoacoustic cell, an air inlet valve, an air outlet valve, a microphone, a signal acquisition and processing circuit, and a touch screen. The wavelength of the laser light source is modulated, and the laser is reflected multiple times in the radial direction on the surface of the pool wall of the miniaturized multiple-reflection photoacoustic cell. The use of second harmonic detection technology can eliminate the interference of the fundamental frequency photoacoustic signal generated by the solid photoacoustic effect on the surface of the pool wall, thereby realizing background-free and highly sensitive detection of trace gases. The miniaturized multiple-reflection photoacoustic cell in the present invention can greatly increase the effective power of the photoacoustic excitation light by multiple reflections of the laser on the inner wall while reducing the volume of the pool, thereby reducing the requirements for the amount of gas sample and improving the detection sensitivity and stability of the system. The defect of this patent is that it uses a traditional microphone as a sound signal detection device, then processes the detected sound signal, and finally obtains the concentration information of trace gas through the standard gas calibration method. Non-resonant photoacoustic cells usually work in a low-frequency range close to infrasound. Traditional microphones are affected by the physical properties of the diaphragm and have relatively weak response in the low-frequency band and are easily affected by external vibrations, which affects the sensitivity, dynamic range and stability of photoacoustic signal detection.

[0007] Patent of this invention: A detection method based on direct acoustic wave coupling-multi-optical path photoacoustic cell is proposed. The bottom surface of the photoacoustic cell body and the output end surface of the detection light collimator are innovatively formed into a rigid FPE structure to replace the traditional microphone as a sound wave detection device. The sound wave signal is detected through the direct acoustic wave coupling technology (the vibration of the acoustic wave signal changes the refractive index of the gas in the photoacoustic cell, and the change of the refractive index is loaded on the detection light propagating in the FPE structure to change the optical path of the detection light. The detection light reflected by the bottom surface of the photoacoustic cell body and the light reflected by the output end surface of the detection light collimator return to the optical fiber to interfere. The change in refractive index will eventually cause the interference light intensity to change proportionally), thereby improving the sensitivity and dynamic range of low-frequency detection of photoacoustic signals and improving vibration resistance. Summary of the invention

[0008] The present invention aims to solve the above problems of the prior art. A photoacoustic spectroscopy detection system based on direct acoustic wave coupling-multi-path photoacoustic cell is proposed. The technical solution of the present invention is as follows:

[0009] A photoacoustic spectrum detection system based on a novel acoustic wave direct coupling-multi-optical path photoacoustic cell comprises a modulator, a DFB laser light source, an isolator, an excitation light collimator, an acoustic wave direct coupling-multi-optical path photoacoustic cell, a detection light collimator, a low-coherence superluminescent diode (SLD) light source, a circulator, a charge coupled device (CCD), and a computer. The DFB laser light source is used as an excitation light source for trace gases. Driven by the modulator, the excitation laser is emitted, passes through the isolator, enters the excitation light collimator, and finally is emitted in parallel to the acoustic wave direct coupling-multi-path photoacoustic cell. The acoustic wave direct coupling-multi-path photoacoustic cell is reflected multiple times to fully react with the measured gas to generate an acoustic wave signal. The acoustic wave signal reacts to the trace gas to be measured in the photoacoustic cell to change the refractive index of the gas. The low-coherence superluminescent diode (SLD) light source emits a detection light, passes through the circulator, and enters the detection light collimator (the transmission end face of the detection light collimator is coated with a reflection-enhancing film, which can achieve partial reflection of the transmitted detection light) and is emitted in parallel to the interior of the acoustic wave direct coupling-multi-path photoacoustic cell. The acoustic wave signal reacts to the trace gas to be measured in the photoacoustic cell to change the refractive index of the gas. The invention is used to obtain the information of the change of gas refractive index caused by the trace gas to be measured. The optical path of the detection light in the photoacoustic cell is changed and loaded onto the detection light. The detection light is reflected on the inner surface of the acoustic wave direct coupling-multi-optical path photoacoustic cell. The reflected light at the bottom of the photoacoustic cell and the reflected light at the transmission end face of the detection light collimator are re-coupled into the transmission optical fiber at the tail of the detection light collimator through the detection light collimator, and then irradiated onto the charge coupled device CCD through the circulator to cause interference. Finally, the electrical signal converted by the charge coupled device CCD is transmitted to a computer for processing to obtain the trace gas refractive index change information carried by the reflected light, and the acoustic wave information of the trace gas refractive index change is further demodulated. Finally, the concentration information of the trace gas is obtained by inversion, thereby realizing the detection of the trace gas.

[0010] Furthermore, the acoustic wave direct coupling-multi-optical path photoacoustic cell is a circular structure, and the bottom and surrounding areas of the cell are coated with a high-reflection film. The left side is coupled with the excitation light collimator to introduce an excitation laser, and the excitation laser can be reflected multiple times in the photoacoustic cell to fully interact with the trace gas to be measured. The upper part is an air inlet and an air outlet to introduce the gas to be measured, and the lower part is coupled with the detection light collimator to introduce the detection laser. The transmission end face of the detection light collimator and the upper surface of the photoacoustic cell form a rigid Fabry-Perot etalon (FPE) structure, and the detection laser is reflected on the upper surface of the photoacoustic cell and returns to the detection light collimator.

[0011] The advantages and beneficial effects of the present invention are as follows:

[0012] The most important innovation of the direct coupling of sound waves - multi-path photoacoustic cell is to use the bottom surface of the photoacoustic cell body and the emission end surface of the detection light collimator to form a rigid FPE structure to replace the traditional microphone as a sound wave detection device. The sound wave is directly coupled into the detection light by directly loading the change of the gas refractive index caused by the sound wave vibration into the detection light, thereby realizing the measurement of the sound wave signal. Compared with the traditional photoacoustic spectrum detection system that uses a microphone to detect sound waves, the present invention can greatly improve the sensitivity and dynamic range of low-frequency detection of photoacoustic signals and improve vibration resistance. Secondly, the photoacoustic cell is designed as a non-resonant annular structure, and its inner wall surface is coated with a gold film, which can realize multiple reflections of the laser on the inner surface of the photoacoustic cell, increase the effective optical path of the excitation light, and compared with the traditional resonant photoacoustic cell, the present invention can greatly reduce the volume of the photoacoustic cell and improve the detection sensitivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is a structural block diagram of a photoacoustic spectroscopy detection system based on a novel acoustic wave direct coupling-multi-optical path photoacoustic cell according to a preferred embodiment of the present invention.

[0014] Attached Figure 2 The annular photoacoustic cell simulates the optical fiber path of the embodiment of the present invention.

[0015] Attached Figure 3 This is the structure diagram of the optical fiber FPE type acoustic wave direct coupling-multi-path photoacoustic cell of the embodiment of the present invention. The symbols and corresponding component names in the attached figure are:

[0016] 1- modulator, 2- DFB laser light source, 3- isolator, 4- direct acoustic wave coupling - multi-path photoacoustic cell, 5- circulator, 6- low coherence superluminescent diode (SLD) light source, 7- charge coupled device CCD, 8- computer, 9- excitation laser, 10- photoacoustic cell body, 11- excitation light collimator, 12- air inlet, 13- rigid FPE structure, 14- air outlet, 15- gold film, 16- detection light collimator DETAILED DESCRIPTION

[0017] The following will describe the technical solutions in the embodiments of the present invention in detail in conjunction with the accompanying drawings in the embodiments of the present invention. The described embodiments are only part of the embodiments of the present invention.

[0018] The technical solution of the present invention to solve the above technical problems is:

[0019] Furthermore, the acoustic wave direct coupling-multi-optical path photoacoustic cell is a circular structure, and the bottom and surrounding areas of the cell are coated with a high-reflection film. The left side is coupled with the excitation light collimator to introduce an excitation laser, and the excitation laser can be reflected multiple times in the photoacoustic cell to fully interact with the trace gas to be measured. The upper part is an air inlet and an air outlet to introduce the gas to be measured, and the lower part is coupled with the detection light collimator to introduce the detection laser. The transmission end face of the detection light collimator and the upper surface of the photoacoustic cell form a rigid Fabry-Perot etalon (FPE) structure, and the detection laser is reflected on the upper surface of the photoacoustic cell and returns to the detection light collimator.

[0020] As attached Figure 1 As shown, the photoacoustic spectroscopy detection system based on the novel acoustic wave direct coupling-annular multi-path photoacoustic cell of this embodiment includes a modulator 1, a DFB laser light source 2, an isolator 3, an acoustic wave direct coupling-annular multi-path photoacoustic cell 4, a circulator 5, a low-coherence superluminescent diode (SLD) light source 6, a charge-coupled device CCD 7, and a computer 8.

[0021] The DFB laser light source 2 emits excitation light under the action of the modulator 1, passes through the isolator 3 and finally enters the acoustic wave direct coupling-annular multi-path photoacoustic cell 4. Figure 2 The excitation laser 9 is reflected multiple times on the inner wall 10 of the annular multi-path photoacoustic cell, and fully reacts with the trace gas in the acoustic wave direct coupling-annular multi-path photoacoustic cell 4 to generate a sound signal, and the sound signal reacts to the trace gas to change the refractive index of the trace gas. The low-coherence superluminescent diode (SLD) light source 6 emits a detection laser through the circulator 5 and the detection light collimator 16 to enter the acoustic wave direct coupling-annular multi-path photoacoustic cell 4, as shown in the attached Figure 3 The detection laser shown is collimated by the detection light collimator 16 and then incident on the acoustic wave direct coupling-multi-path photoacoustic cell 4. The inner wall of the multi-path photoacoustic cell body 10 is plated with a gold film 15, so the reflectivity is very high. The transmission end face of the detection light collimator 16 is plated with a reflective film to achieve partial reflection and partial transmission. Therefore, the transmission end face of the detection light collimator 16 and the inner wall of the multi-path photoacoustic cell form a rigid FPE structure 13. After the detection laser is loaded with the refractive index change information of the trace gas in the photoacoustic cell, it is reflected on the inner wall of the photoacoustic cell. The reflected light on the two surfaces of the rigid FPE structure 13 is re-coupled into the optical fiber through the detection light collimator 16 and then passes through the circulator 5 to interfere at the charge coupled device CCD 7. The interference signal is detected by the charge coupled device CCD 7 is converted into an electrical signal and transmitted to the computer 8 for processing to obtain the trace gas refractive index change information carried by the reflected light, and the acoustic wave information carried by the trace gas refractive index change is further demodulated. Finally, the concentration information of the trace gas is obtained through the standard gas calibration method, thereby realizing the detection of the trace gas.

[0022] Attached Figure 2 The multi-path photoacoustic cell of this embodiment simulates the optical fiber path. The inner surface of the multi-path photoacoustic cell is gold-plated to have high reflectivity. The excitation laser 9 is incident on the inner wall 10 of the multi-path photoacoustic cell and is emitted multiple times to increase the optical path contacting the trace gas in the photoacoustic cell.

[0023] Attached Figure 3 It is a fiber FPE type acoustic wave direct coupling-multi-path photoacoustic pool structure of the embodiment. The inner surface of the multi-path photoacoustic is coated with a gold film, which can reflect the excitation light injected by the excitation light collimator 11 and the detection light injected by the detection light collimator 16. The air inlet 12 and the air outlet 14 can meet the input of the gas to be measured. The inner surface of the photoacoustic pool and the end face of the detection light collimator constitute a rigid FPE structure 13. The detection light collimator 16 is fixed on the photoacoustic pool. The collimator adopts a special structural design (a partial emission film is plated on the exit end face of the detection light collimator, so that it can realize partial reflection of the detection light on the exit end face). The end face has no anti-reflection film and has a certain reflectivity, and the end face reflected light and the reflected light in the pool can be converged into the transmission optical fiber.

[0024] It should also be noted that the terms "include", "comprises" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, commodity or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, commodity or device. In the absence of more restrictions, the elements defined by the sentence "comprises a ..." do not exclude the existence of other identical elements in the process, method, commodity or device including the elements.

[0025] The above embodiments should be understood to be only used to illustrate the present invention and not to limit the protection scope of the present invention. After reading the contents of the present invention, technicians can make various changes or modifications to the present invention, and these equivalent changes and modifications also fall within the scope defined by the claims of the present invention.

Claims

1. A photoacoustic spectroscopy detection system based on direct acoustic wave coupling-multi-path photoacoustic cell, characterized in that: The invention comprises a modulator (1), a DFB laser light source (2), an isolator (3), a direct acoustic coupling-multi-optical path photoacoustic cell (4), a circulator (5), a low-coherence superluminescent diode light source (6), a charge coupled device (CCD) (7), and a computer (8). The modulator (1), the DFB laser light source (2), the isolator (3), the direct acoustic coupling-multi-optical path photoacoustic cell (4), the circulator (5), and the low-coherence superluminescent diode light source (6) are connected in sequence. The circulator (5) is also connected to the charge coupled device (CCD) (7), and the charge coupled device (CCD) (7) is connected to the computer (8). The modulator (1) is used to modulate the DFB laser light source. The DFB laser light source (2) is used to emit laser light as the excitation light of the detection system. The isolator (3) is used to ensure the unidirectional transmission of the excitation light and prevent the excitation light from being reflected and transmitted to the DFB laser. The excitation light is used to excite the gas to be tested to generate an acoustic wave signal and to load the acoustic wave onto the detection light by direct coupling of the acoustic wave. The circulator (5) is used to determine the transmission direction of the laser. The low-coherence superluminescent diode light source (6) is used to emit laser as the detection light of the detection system. The charge-coupled device CCD (7) is used to detect and convert the received interference light signal into an electrical signal. The computer (8) is used to process the electrical signal detected by the charge-coupled device CCD (7). The acoustic wave direct coupling-multi-optical path photoacoustic cell (4) comprises a photoacoustic cell body (10), an excitation light collimator (11), an air inlet (12), a rigid FPE structure (13), an air outlet (14), a gold film (15) and a detection light collimator (16), wherein the excitation light collimator (11) is arranged at the top of the photoacoustic cell body (10), the air inlet (12) and the air outlet (14) are arranged at the upper and lower positions of one side of the cell body, respectively, the detection light collimator (16) is arranged at the middle position of the cell body, the inner surface of the photoacoustic cell and the end face of the detection light collimator (16) constitute the rigid FPE structure (13), the four walls of the photoacoustic cell body (10) are plated with a gold film (15), wherein the excitation light collimator (11) is used to collimate the excitation light so that it is incident in parallel into the acoustic wave direct coupling-multi-optical path photoacoustic cell, the air inlet (12) The method is used to pass the gas to be measured into the acoustic wave direct coupling-multi-path photoacoustic cell. The rigid FPE structure (13) is used to make the detection light reflect in the rigid FPE structure. The detection light is loaded with the refractive index change information of the gas to be measured, and then returns to the optical fiber together with the light reflected from the end face of the detection light collimator to interfere. The gas outlet (14) is used to discharge the gas to be measured from the acoustic wave direct coupling-multi-path photoacoustic cell. The gold film (15) is plated on the inner surface of the acoustic wave direct coupling-multi-path photoacoustic cell to realize the reflection of the excitation light and the detection light. The detection light collimator (16) is used to collimate the detection light so that it is incident in parallel into the acoustic wave direct coupling-annular multi-path photoacoustic cell, and at the same time, part of the detection light is reflected back at the end face.

2. The photoacoustic spectroscopy detection system based on direct acoustic wave coupling-multi-path photoacoustic cell according to claim 1, characterized in that: The invention also comprises an excitation laser (9), wherein the excitation laser (9) is reflected multiple times on the inner wall of the photoacoustic cell body (10), and is fully directly coupled with the sound wave to react with the trace gas in the multi-path photoacoustic cell (4) to generate a sound signal, and the sound signal reacts with the trace gas to change the refractive index of the trace gas.

3. The photoacoustic spectroscopy detection system based on direct acoustic wave coupling-multi-path photoacoustic cell according to claim 1, characterized in that: The acoustic wave direct coupling-multi-optical path photoacoustic cell is a ring structure.

4. The photoacoustic spectroscopy detection system based on direct acoustic wave coupling-multi-path photoacoustic cell according to claim 1, characterized in that: The reflected light from the two surfaces of the rigid FPE structure is re-coupled into the optical fiber through the detection light collimator (16) and then passes through the circulator (5) to interfere at the charge coupled device CCD (7). The interference signal is converted into an electrical signal by the charge coupled device CCD (7) and transmitted to the computer (8) for processing to obtain the trace gas refractive index change information carried by the reflected light, and further demodulates the acoustic wave information carried by the trace gas refractive index change. Finally, the concentration information of the trace gas is obtained by the standard gas calibration method.

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