Gas detection method and system based on off-axis integral output cavity enhanced photoacoustic spectroscopy

Abstract

The invention discloses a gas detection method and system based on off-axis integral output cavity enhanced photoacoustic spectrometry. According to the method, an off-axis integral output cavity is combined with a photoacoustic cell, the photoacoustic cell is arranged in the off-axis integral output cavity structure, laser forms optical resonance in the off-axis integral output cavity and interacts with gas in the photoacoustic cell at the same time, and a photoacoustic signal is generated; therefore, the off-axis integral output cavity improves an effective absorption path, the photoacoustic cell realizes photoacoustic signal detection, the influence of optical noise is reduced, and the signal-to-noise ratio of the system is improved. The system comprises a laser controller, a laser, a collimator, an H-type photoacoustic cell, an off-axis integral output cavity, a microphone module, a lock-in amplifier, an acquisition card and an upper computer. The system has the advantages of insensitivity to optical noise, low detection limit, high sensitivity and the like.

Description

Technical Field

[0001] The present invention belongs to the field of laser technology, and relates to a gas detection method and system based on off-axis integrated output cavity enhanced photoacoustic spectroscopy. Background Art

[0002] Photoacoustic spectroscopy has the advantages of being structurally compact, having a fast response speed, low consumption of sample gas, high sensitivity, and being able to continuously, real-time and in-situ monitor, etc., so it has been widely studied.

[0003] Off-axis integrated output cavity spectroscopy is a laser spectroscopic gas detection technology that realizes enhanced long optical path laser absorption by off-axis laser coupling into coaxial arranged mirrors. Its core advantages include: (1) High-sensitivity detection: By multiple reflections (such as dozens to hundreds of times), the optical path is significantly extended (the equivalent optical path can reach the kilometer level), enhancing the absorption of the target gas to the laser and improving the detection sensitivity (ppb level). (2) Structural compactness and flexibility: The off-axis optical path design avoids the strict alignment requirements of traditional coaxial cavity enhanced gas detection technology, reduces the assembly difficulty, and at the same time supports a compact cavity. (3) Anti-environmental interference ability: The off-axis optical path reduces the interference noise of the direct reflected light of the mirror surface, reduces the sensitivity to vibration and temperature fluctuations, and is more suitable for on-site applications. Although off-axis integrated output cavity spectroscopy has significant advantages, its technical bottlenecks still focus on optical field noise and mode noise. The optical intensity noise comes from the laser power fluctuation, the interference fringes (etalon effect) caused by multiple reflections, and the scattering caused by the surface defects of the cavity mirror; the mode noise comes from the resonance mode drift caused by the change of cavity parameters (such as temperature, pressure), which destroys the optical field integration stability. These noises will mask the weak absorption signal, limit the signal-to-noise ratio (SNR) of the system, and cause baseline drift in long-term measurements, requiring frequent calibration. Summary of the Invention

[0004] In order to overcome the deficiencies of the prior art, the purpose of the present invention is to provide a gas detection method and system based on off-axis integrated output cavity enhanced photoacoustic spectroscopy. The present invention first proposes to integrate photoacoustic spectroscopy technology into an off-axis integrated output cavity spectroscopy system. Photoacoustic spectroscopy detects by converting light energy into acoustic signals, which can effectively avoid the optical noise problem of the off-axis integrated output cavity spectroscopy system. In the photoacoustic effect, the target gas absorbs the modulated light energy and generates a pressure wave (acoustic signal), which is detected by a microphone or a piezoelectric sensor, rather than directly measuring the transmitted light intensity. While obtaining the increased effective absorption optical path of the off-axis integrated output cavity, the system can be immune to optical intensity noise and improve the signal-to-noise ratio of the system. The high sensitivity of the off-axis integrated output cavity and the anti-noise characteristics of photoacoustic spectroscopy are complementary. By combining the two spectroscopic detection technologies, the present invention synchronously solves the contradiction between the long optical path requirement and noise suppression, and can break through the performance bottleneck of traditional gas detection.

[0005] To achieve the above object, the technical solution adopted by the present invention is: A gas detection method based on off-axis integrated output cavity enhanced photoacoustic spectroscopy, which uses an off-axis integrated output cavity combined with a photoacoustic cell. The photoacoustic cell is placed inside the off-axis integrated output cavity structure. The laser forms an optical resonance in the off-axis integrated output cavity and at the same time interacts with the gas in the photoacoustic cell to generate a photoacoustic signal. Thus, the off-axis integrated output cavity increases the effective absorption path, and the photoacoustic cell realizes the detection of the photoacoustic signal, reducing the influence of optical noise and thereby improving the system signal-to-noise ratio.

[0006] The photoacoustic cell includes a resonant cavity, a buffer cavity, and an optical window. The geometric shape of the resonant cavity is configured to amplify the photoacoustic signal of the target gas through acoustic resonance. The buffer cavity suppresses the laser noise at the optical window, and the optical window allows the laser to enter the photoacoustic cell.

[0007] The off-axis integrated output cavity is composed of two highly reflective concave mirrors with a reflectivity of >99.99% that are placed coaxially and at a certain distance from each other. The laser is coupled into the cavity in an off-axis manner to form an optical resonance in the off-axis integrated output cavity, increasing the effective absorption optical path of the system.

[0008] A gas detection system based on off-axis integrated output cavity enhanced photoacoustic spectroscopy, including an off-axis integrated output cavity and an H-type photoacoustic cell. The off-axis integrated output cavity increases the effective absorption path, and the H-type photoacoustic cell realizes the detection of the photoacoustic signal, reducing the influence of optical noise in the system and improving the system signal-to-noise ratio.

[0009] The gas detection system includes a laser controller, a laser, a collimator, an H-type photoacoustic cell, an off-axis integrated output cavity, a microphone module, a lock-in amplifier, a data acquisition card, and a host computer. The laser controller controls the laser, and the laser emits laser light that enters the off-axis integrated output cavity through the collimator. The laser forms an optical resonance in the off-axis integrated output cavity. The H-type photoacoustic cell is placed inside the off-axis integrated output cavity so that the laser that forms an optical resonance by reflecting back and forth in the off-axis integrated cavity also passes through the H-type photoacoustic cell. The microphone module is placed in the center of the H-type photoacoustic cell to collect the photoacoustic signal, and the measured signal is transmitted to the lock-in amplifier. The lock-in amplifier transmits the demodulated result to the data acquisition card, and then the data acquisition card sends it to the host computer. The data acquisition card is respectively connected to the laser controller and the lock-in amplifier and is responsible for the output of the modulation signal and the reference signal. The gas detection system is used to realize laser control, excitation and resonance enhancement of the photoacoustic signal, improvement of the effective absorption path, detection of the photoacoustic signal by the microphone module, lock-in amplification detection, and display of the gas concentration inversion result based on the LabVIEW program.

[0010] The described H-type photoacoustic cell includes a resonant cavity, two buffer cavities, and two optical windows. The buffer cavities are on both sides of the resonant cavity, and the optical windows are arranged on both sides of the buffer cavities. The length of the resonant cavity, the radius of the resonant cavity, the length of the buffer cavity, and the radius of the buffer cavity are 80 mm, 5 mm, 40 mm, and 35 mm respectively. The off-axis integrated output cavity consists of two identical high-reflectivity concave mirrors with a focal length of 2000 mm, a diameter of 25.4 mm, a thickness of 6.35 mm, a reflectivity greater than 99.99%, and a polished back surface. The two concave mirrors are coaxially positioned and are 160 mm apart.

[0011] Advantages of the present invention: A gas detection method based on off-axis integrated output cavity enhanced photoacoustic spectroscopy of the present invention: 1) The photoacoustic cell realizes the resonance enhancement of photoacoustic signals; 2) The off-axis integrated output cavity realizes the enhancement of the effective absorption path; 3) The introduction of the off-axis integrated output cavity system in photoacoustic spectroscopy reduces the influence of optical intensity noise in the off-axis integrated output cavity system, and realizes the detection of gases with high sensitivity and extremely low minimum detection limits.

[0012] The gas detection system based on off-axis integrated output cavity enhanced photoacoustic spectroscopy built by the present invention realizes the detection of ammonia gas with high sensitivity and extremely low minimum detection limits. Description of the drawings

[0013] Figure 1 It is a schematic structural diagram of a gas detection system based on off-axis integrated output cavity enhanced photoacoustic spectroscopy.

[0014] In the figure, there are a laser controller 1, a laser 2, a collimator 3, an H-type photoacoustic cell 4, an off-axis integrated output cavity 5, a microphone module 6, a lock-in amplifier 7, a data acquisition card 8, and a host computer 9. Detailed implementation manners

[0015] In order to make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the coverage scope of the present invention.

[0016] Embodiment 1: Building a gas detection system based on off-axis integrated output cavity enhanced photoacoustic spectroscopy To achieve the detection of ammonia gas with a low minimum detection limit, a gas detection system based on off-axis integrated output cavity enhanced photoacoustic spectroscopy is built. Under the same cavity length, compared with the traditional off-axis integrated output cavity system, the noise of the off-axis integrated output cavity enhanced photoacoustic spectroscopy system of the present invention is reduced by 80%. This system has excellent linearity (R 2 = 0.9999) for ammonia gas in the range of 0 - 5 ppm, and realizes the minimum detection sensitivity of ppb level for ammonia gas.

[0017] Step 1. Through COMSOL multi-physics simulation, under the conditions of the same cavity length and resonator tube radius, the sound pressure distribution uniformity of the H-type photoacoustic cell (standard deviation σ = 0.8 Pa) is better than that of the cylindrical (σ = 2.1 Pa) and T-type (σ = 1.5 Pa), and the quality factor of the H-type photoacoustic cell is the best, that is, its acoustic resonance enhancement ability is the strongest. Therefore, the H-type photoacoustic cell is selected for this system. Further optimize the size of the H-type photoacoustic cell, which has a resonant cavity, two buffer cavities and two optical windows. The buffer cavities are on both sides of the resonant cavity, and their length is 1 / 4 of the resonant sound wavelength, with an acoustic filtering function, which can be used to suppress the laser window noise. The optical windows are set on both sides of the buffer cavities for coupling the laser. Optimize the shape of the H-type photoacoustic cell through COMSOL multi-physics simulation, including the resonator tube length, buffer cavity length, resonator tube radius and buffer cavity radius, to enhance the resonance enhancement effect of the photoacoustic signal and the "acoustic-optical coupling", and make it compatible with the off-axis integral output cavity. During optimization, the finite method is used to solve the FLNS equation to simulate the absolute pressure distribution inside the H-type photoacoustic cell. Finally, the optimized resonator cavity length, resonator cavity radius, buffer cavity length and buffer cavity radius of the H-type photoacoustic cell are 80 mm, 5 mm, 40 mm and 35 mm respectively.

[0018] Step 2. Design an off-axis integral output cavity, which consists of two identical high-reflectivity concave mirrors with a focal length of 2000 mm, a diameter of 25.4 mm, a reflectivity greater than 99.99%, and a polished back surface. The two concave mirrors are coaxially positioned and 160 mm apart. The laser is incident at a small angle to form an optical resonance in the off-axis integral output cavity. According to the fact that the effective absorption optical path of the off-axis integral output cavity system is equal to the cavity length divided by the transmittance of the mirror, that is L eff = L / (1 - R ), the effective absorption optical path of the system is increased by 1000 times.

[0019] Step 3. Build a gas detection system based on the off-axis integral output cavity enhanced photoacoustic spectroscopy, and its structure is as Figure 1As shown in the figure. The gas detection system includes a laser controller 1, a laser 2, a collimator 3, an H-type photoacoustic cell 4, an off-axis integrated output cavity 5, a microphone module 6, a lock-in amplifier 7, a data acquisition card 8, and a host computer 9. The laser controller 1 controls the laser 2. The laser 1 emits laser light that enters the off-axis integrated output cavity 5 through the collimator 3, and optical resonance is formed inside the off-axis integrated output cavity 5. The H-type photoacoustic cell 4 is placed inside the off-axis integrated output cavity 5, so that the laser light that forms optical resonance by reflecting back and forth inside the off-axis integrated cavity 5 also passes through the H-type photoacoustic cell 4 at the same time. The microphone module 6 is placed in the center of the H-type photoacoustic cell 4 to collect photoacoustic signals. The measured signals are sent to the lock-in amplifier 7. The lock-in amplifier 7 transmits the demodulation result to the data acquisition card 8, and then the data acquisition card 8 sends it to the host computer 9 (computer). The data acquisition card 8 is respectively connected to the laser controller 1 and the lock-in amplifier 7, and is responsible for the output of modulation signals and reference signals.

[0020] The temperature of the laser is stabilized at 20 °C through a laser temperature controller. The bias current of the detection laser is made 140 mA through a laser current driver, so that its central wavelength is located at the absorption peak of ammonia gas. The data acquisition card provides modulation and scanning signals. A modulation signal composed of a 0.1 Hz sawtooth wave and a high-frequency sine wave is generated and sent to the laser controller to drive the laser. The temperature controller and the current driver are used to stabilize the laser temperature and convert the modulation signal into a laser drive current.

[0021] Step 4: After being collimated by an optical fiber collimator, the laser beam is coupled into the off-axis integrated output cavity. Two high-reflectivity concave mirrors are coaxially positioned and are 160 mm apart. Through a high-precision XYR translation stage, the laser beam enters the off-axis integrated output cavity at a small angle, realizing enhanced optical resonance, and the effective absorption optical path of the system is increased by 1000 times.

[0022] Step 4: After ammonia gas is introduced into the photoacoustic cell, it absorbs the laser light and generates photoacoustic signals, which are collected by the microphone module.

[0023] Step 5: The signals collected by the microphone module are input into the lock-in amplifier. The lock-in amplifier demodulates the secondary harmonic signals according to the reference signals provided by the data acquisition card and collects them to the host computer by the data acquisition card. The LabVIEW program of the host computer realizes: (1) Concentration inversion: The amplitude of the secondary harmonic signals output by the lock-in amplifier is brought into the calibration curve to obtain the gas concentration; (2) Phase synchronization: By locking the phase of the reference signal and the modulation signal (the lock-in loop bandwidth is set to 10 Hz), the phase noise introduced by laser frequency drift is eliminated; (3) Noise suppression: The original signals are applied with a moving average filter (window width 50 ms) and a band-pass filter (cut-off frequency 20 Hz) to reduce the influence of environmental vibration and electromagnetic interference.

[0024] The technical features of the above-described embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the various technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered to be within the scope described in this specification.

[0025] The above-described embodiments merely represent one implementation manner of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention patent shall be subject to the appended claims.

Claims

1. A gas detection method based on off-axis integrated output cavity enhanced photoacoustic spectroscopy, characterized in that: An off-axis integrating output cavity is combined with a photoacoustic cell, and the photoacoustic cell is placed inside the off-axis integrating output cavity structure. Laser forms optical resonance in the off-axis integrating output cavity and interacts with the gas in the photoacoustic cell to generate photoacoustic signals. Thus, the off-axis integrating output cavity improves the effective absorption path, and the photoacoustic cell realizes photoacoustic signal detection, reduces the influence of optical noise, and thus improves the signal-to-noise ratio of the system.

2. The method according to claim 1, characterized in that The photoacoustic cell comprises a resonant cavity, a buffer cavity and an optical window. The geometry of the resonant cavity is configured to amplify the photoacoustic signal of the target gas through acoustic resonance. The buffer cavity is used to suppress laser noise at the optical window. The optical window allows laser light to enter the photoacoustic cell.

3. The method according to claim 1, characterized in that The off-axis integral output cavity is composed of two concave mirrors with a high reflectivity of >99.99% that are placed relatively coaxially and at a certain distance. The laser is coupled into the cavity in an off-axis manner, forming optical resonance in the off-axis integral output cavity, thereby increasing the effective absorption optical path of the system.

4. A gas detection system based on off-axis integrated output cavity enhanced photoacoustic spectroscopy, characterized in that: It includes an off-axis integral output cavity and an H-type photoacoustic cell. The off-axis integral output cavity improves the effective absorption path, and the H-type photoacoustic cell realizes photoacoustic signal detection, reduces the influence of optical noise in the system, and improves the signal-to-noise ratio of the system.

5. The gas detection system according to claim 4, characterized in that: It includes a laser controller, a laser, a collimator, an H-type photoacoustic cell, an off-axis integral output cavity, a microphone module, a phase-locked amplifier, an acquisition card and a host computer; the laser controller controls the laser, and the laser emits laser light that enters the off-axis integral output cavity through the collimator, and the laser forms optical resonance in the off-axis integral output cavity; the H-type photoacoustic cell is placed inside the off-axis integral output cavity, so that the laser light that is reflected back and forth in the off-axis integral cavity to form optical resonance also passes through the H-type photoacoustic cell; The microphone module is placed in the center of the H-type photoacoustic cell to collect photoacoustic signals. The measured signal is transmitted to the phase-locked amplifier, and the phase-locked amplifier transmits the demodulation result to the acquisition card, which is then sent to the host computer; the acquisition card is connected to the laser controller and the phase-locked amplifier respectively, and is responsible for the output of the modulation signal and the reference signal; the gas detection system is used to realize laser control, photoacoustic signal excitation and resonance enhancement, effective absorption path improvement, microphone module detection of photoacoustic signals, phase-locked amplification detection, and display of gas concentration inversion results based on LabVIEW program.

6. The gas detection system according to claim 5, characterized in that: The H-type photoacoustic cell comprises a resonant cavity, two buffer cavities and two optical windows, wherein the buffer cavities are on both sides of the resonant cavity, and the optical windows are arranged on both sides of the buffer cavity; the resonant cavity length, the resonant cavity radius, the buffer cavity length and the buffer cavity radius are 80 mm, 5 mm, 40 mm and 35 mm respectively; the off-axis integral output cavity is composed of two identical high-reflectivity concave mirrors, with a focal length of 2000 mm, a diameter of 25.4 mm, a thickness of 6.35 mm, a reflectivity greater than 99.99%, a polished back surface, and the two concave mirrors are coaxially positioned and 160 mm apart.

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