Ultra-fast and high-sensitivity gas detection device and method based on gas nonlinear optics

Abstract

The invention discloses an ultrafast and high-sensitivity gas detection device and method based on gas nonlinear optics. The device comprises a high-power laser light source module, a light flux cell and a signal processing module. The light flux cell is of an integrated hollow structure, a high-reflectivity mirror face is arranged in the light flux cell to form a closed light path, laser enters the cell through an incidence hole at a certain incidence angle, 50-meter-level long-optical-path accumulation is achieved through multiple times of reflection, and the nonlinear effect excitation threshold value is effectively reduced. The light spot path is changed by adjusting the incident angle, the signal intensity is obviously enhanced by combining the nonlinear effect, and the detection limit reaches ppb level. The signal processing module extracts a target frequency component through phase-locked amplification, separates frequency domain aliasing signals in combination with wavelet transform, and reconstructs a complete nonlinear spectrum through Fourier transform. The device has the advantages of stable structure, high sensitivity, strong interference resistance and the like, and is suitable for environment trace gas monitoring and volatile organic compound early warning.

Description

Technical Field

[0001] The present invention relates to the field of nonlinear spectroscopy technology, and in particular to an ultrafast, highly sensitive gas detection device and method based on gas nonlinear optics. Background Art

[0002] Currently, conventional gas detection technologies, particularly those for ultrafast, high-sensitivity detection, face numerous challenges. For example, conventional absorption spectroscopy typically utilizes a traditional, straight-through gas absorption cell, which has design limitations. The effective optical path length is typically less than 10 meters, limiting spectral sensitivity to the ppm level. This poses the risk of false or missed detections when monitoring trace gases at the ppb level in environmental monitoring and volatile organic compound (VOC) early warning applications.

[0003] In recent years, nonlinear optical effects, such as self-phase modulation, second harmonic generation, and third harmonic generation, have provided new approaches to improve detection sensitivity. By utilizing the nonlinear spectra obtained by these nonlinear effects, higher sensitivity can be achieved in detection.

[0004] Although nonlinear optical effects provide new ideas for breaking through the sensitivity limit, their practical application faces two challenges. First, the intensity of the nonlinear effect is exponentially related to the optical path length, and the short optical path length design of the traditional absorption cell leads to insufficient accumulation of nonlinear signals. Second, research shows that existing technology requires 10 17 W / m 2 The laser threshold of the order of magnitude must be reached to stimulate nonlinear response, which greatly limits the detection of low-concentration gases. When the gas molecule density is insufficient, it is difficult to reach the threshold of nonlinear interaction, causing the detection sensitivity to stagnate in the ppm-ppb range.

[0005] Current technologies attempt to enhance optical path length through resonant cavities, but due to issues such as mode matching difficulty and stability, the gain in practical applications is not ideal. While optical fiber-based solutions can achieve kilometer-level optical path lengths, parasitic effects such as stimulated Raman scattering lead to a reduction in the spectral signal-to-noise ratio. Summary of the Invention

[0006] To address these technical challenges, the present invention proposes a nonlinear optical gas detection method and device based on a conventional annular gas absorption cell. This method fully utilizes the space within the annular gas absorption cell, allowing light to undergo multiple, regular reflections within the cell. This, combined with the nonlinear effect, enhances the intensity and sensitivity of the detection signal. By enhancing or altering the interaction between light and gas, the method enables detection of low-concentration gases and rapidly responds to subtle changes in gas composition.

[0007] A first aspect of the present invention provides an ultrafast, highly sensitive gas detection device based on gas nonlinear optics, comprising:

[0008] A light source module, for generating laser light;

[0009] A light-through cell for multiple reflections after laser incident. The light-through cell is an annular gas absorption cell with a high-reflectivity reflector provided on its inner wall. The entrance and exit holes of the annular gas absorption cell are sealed by optical windows.

[0010] The signal processing module is used to receive light emitted from the luminous flux cell and process the optical signal. After the laser generated by the light source module enters the luminous flux cell, it extends the optical path through multiple reflections, triggering a nonlinear optical effect and generating a nonlinear spectrum. The signal processing module performs signal separation and noise suppression on the nonlinear spectrum to achieve detection of low-concentration gas.

[0011] A second aspect of the present invention provides an ultrafast, highly sensitive gas detection method based on gas nonlinear optics, using the above-mentioned device, comprising the following steps:

[0012] Generate laser light and inject it into an annular gas absorption cell;

[0013] The optical path is extended by multiple reflections in the annular gas absorption cell, triggering the nonlinear optical effect and generating a nonlinear spectrum containing the original incident light frequency and new frequency components;

[0014] Receive the light emitted from the annular gas absorption cell and process the optical signal to detect low-concentration gas;

[0015] The nonlinear optical effect reduces the light intensity threshold of the nonlinear effect by extending the optical path and accumulating nonlinear phase shift, so that the nonlinear effect can also be generated under low light intensity.

[0016] The beneficial effects brought about by the technical solution provided by the embodiments of the present invention are:

[0017] (1) The light-through cell is designed as a ring-shaped gas absorption cell, which is an integrated design. There is no need to adjust the internal structure of the light-through cell. It only needs to adjust the external incident light path, that is, adjust the laser incident angle to achieve the change of the spot position, which simplifies the system structure and improves the system stability.

[0018] (2) The present invention changes the effective absorption path of the gas in the multi-pass cell by changing the laser incident angle, and improves the detection limit to ppb level through a 50m long optical path.

[0019] (3) The present invention effectively reduces the threshold of nonlinear effects by extending the effective optical path, focusing the light beam, and reducing the spot area.

[0020] (4) The present invention utilizes nonlinear effects to not only cause spectral peaks to exhibit changes in intensity but also in their shape, thereby significantly improving the ability to identify spectral peaks. Through the nonlinear effect, the absorption cell can generate new frequency components, amplifying or shifting previously difficult-to-detect absorption lines to more easily observable spectral regions, significantly improving detection sensitivity.

[0021] (6) The present invention uses a combined algorithm of digital lock-in amplification and wavelet transform to efficiently extract weak signals from nonlinear spectra and separate different components. By implementing the functions of a hardware lock-in amplifier at the software level, the simplicity of the hardware system is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 A schematic diagram of an embodiment of the present application;

[0023] Figure 2 A physical cross-sectional view of an embodiment of the present application;

[0024] Figure 3 This is an enlarged view of the light entrance hole and exit hole of the embodiment of the present application;

[0025] Figure 4 This is a schematic diagram of the internal light path of an embodiment of the present application;

[0026] Figure 5 Schematic diagram of the focal area of a partial light beam after mirror reflection;

[0027] Figure 6 This is a curve diagram of light intensity and nonlinear effect threshold. DETAILED DESCRIPTION

[0028] To make the purpose, technical solutions and advantages of the present invention more clear, the present invention will now be described in further detail with reference to the accompanying drawings. These drawings are simplified schematic diagrams, which only illustrate the basic structure of the present invention in a schematic manner, and therefore only show the components related to the present invention.

[0029] like Figure 1 As shown, an ultrafast, highly sensitive gas detection device based on gas nonlinear optics provided in an embodiment of the present application includes a laser 1, an annular gas absorption cell 2, a grating spectrometer 3 and a computer 4.

[0030] In a preferred example, the light source module adopts a central wavelength 1030nm, pulse width 100fs, peak light intensity is 10 15 W / m 2Femtosecond lasers. This wavelength, located in the near-infrared region, can effectively match the nonlinear absorption of various trace gases and possesses excellent gas-selective adaptability. The femtosecond pulse width enables the laser to release high energy in an extremely short time, significantly enhancing the efficiency of nonlinear interactions.

[0031] In a preferred embodiment, the light-through cell is an annular gas absorption cell 2 with an inner diameter of is 60cm, the specific structure is as follows Figure 2 As shown, the annular gas absorption cell 2 is a hollow structure with an arc-shaped groove on the cavity of the annular gas absorption cell. It is mainly composed of a high-precision reflector. The reflector is coated with a mirror reflectivity R>99.5%, which not only achieves efficient light reflection, but also covers the wavelength range from near infrared to mid-infrared. Furthermore, a circular through hole is provided on the circumference of the annular gas absorption cell 2 for light to enter and exit. To simplify the structure, the exit hole coincides with the entrance hole 6. The entrance hole and the exit hole are specifically as follows Figure 3 As shown, specifically, the reflector 5 of the annular gas absorption cell 2 is plated with a reflectivity It is a 99.5% gold film and is sealed with an optical window, which not only ensures the sealing of the light pool but also allows light to enter and exit freely.

[0032] The laser 1 is placed outside the annular gas absorption cell 2 and on the same side as the incident hole 6. The laser beam generated by the laser 1 enters the annular gas absorption cell 2 through the incident hole 6. By adjusting the position of the laser 1, the laser beam is made to have an incident angle of 8°. Incident, by observing the signal collected on the computer 4, the position of the laser 1 is optimized to adjust the incident angle.

[0033] Furthermore, 500ppb acetylene is filled into the annular gas absorption cell 2, and the balance gas is nitrogen. The nonlinear refractive index coefficient of the low concentration acetylene to the mixed gas is The contribution is very low, the nonlinear refractive index coefficient of the mixed gas Mainly depends on nitrogen, the nonlinear refractive index coefficient of nitrogen at 1030nm wavelength 3×10 -23 m 2 / W.

[0034] Laser 1 generates laser light at an angle of 8° The laser enters the luminous flux pool through the incident hole and undergoes multiple reflections in the ring cavity. The single reflection optical path for:

[0035]

[0036] in is the inner diameter of the luminous flux cell, is the laser incident angle.

[0037] like Figure 4 As shown in the figure, after each reflection, the laser beam converges and the divergence angle of the light decreases, forming a relatively stable focal zone with high light intensity and small spot, such as the first focal zone 7 and the second focal zone 8, and extending the focal zone to a certain extent. The long focal zone is not a single focus, but an area where the laser continuously maintains high light intensity in space. It is the main area where most nonlinear optical effects and cumulative effects occur. Because the laser undergoes multiple reflections and focusing, the long focal zone is effectively extended in space and occupies a large proportion of the total optical path, making the nonlinear interaction between the laser and the gas more sufficient, thereby significantly enhancing the generation of nonlinear signals and improving the detection sensitivity of the system.

[0038] In this embodiment, the laser is reflected 90 times in the annular gas absorption cell 2, and the total optical path L is 53.4 m. The optical path of the laser in the annular gas absorption cell 2 is as follows: Figure 5 shown.

[0039] After each reflection, the peak light intensity decays to , the attenuation after the nth reflection is: , the light intensity attenuation curve is as follows Figure 6 As shown, is the specular reflectivity, is the peak light intensity.

[0040] As the number of reflections increases, the optical path increases linearly, and the nonlinear effect threshold decreases with the increase of the optical path. When the peak intensity of a single reflection is ≥Nonlinear effect threshold When , the nonlinear effect is triggered, resulting in spectrum broadening and the generation of new frequencies. The threshold of the nonlinear effect is determined by the following formula:

[0041]

[0042] in is the nonlinear effect threshold, is the wavelength, is the nonlinear refractive index coefficient, is the effective nonlinear optical path;

[0043] Considering the loss factor hour, Corrected to:

[0044]

[0045]

[0046] in The loss coefficient of the gas medium is determined by the mirror reflection loss coefficient and gas absorption coefficient Determine, R is the mirror reflectivity, when the gas absorption coefficient Much smaller than the mirror reflection loss coefficient When Specular reflection loss coefficient Play a leading role.

[0047] The long optical path design ensures that a single light intensity that is insufficient to trigger the nonlinear effect can still trigger the nonlinear effect after multiple reflections, extending the effective optical path and accumulating nonlinear phase shift, thereby lowering the light intensity threshold for the nonlinear effect, allowing the nonlinear effect that originally required high-intensity light conditions to occur to occur at lower light intensities.

[0048] The nonlinear effect threshold attenuation curve is as follows Figure 6 As shown, the light after the nonlinear effect contains the original incident light frequency , as well as new frequency components, the distribution of these frequency components constitutes the nonlinear spectrum.

[0049] The signal processing module consists of a grating spectrometer 3 and a computer 4. The grating spectrometer 3 receives the light emitted from the annular gas absorption cell 2, converts the light intensity into a voltage signal, and transmits it to the computer 4. The computer 4 runs signal processing software to analyze and process the spectral information transmitted from the grating spectrometer 3. The obtained signal can also be used to adjust the position of the light source module and the incident angle of the laser.

[0050] Furthermore, a grating spectrometer detects the outgoing light and converts the light intensity into a voltage signal, which is then transmitted to a computer. Because the signals generated by nonlinear effects are often mixed with noise, and different signal components overlap in the frequency domain, direct detection can be subject to signal aliasing and noise interference, limiting detection sensitivity. Therefore, signal separation and noise suppression are required on the original spectrum.

[0051] Preferably, the phase-locked amplification technology is used to improve the signal-to-noise ratio to extract weak signals, and the wavelet transform is combined to perform signal separation. The amplitude of the target signal and the wavelet reconstructed signal are combined, and the complete nonlinear spectrum can be obtained through Fourier transform.

[0052] Furthermore, assuming the target signal frequency is , the sampling frequency is , Computer 4 generates the orthogonal reference signal through Python as follows:

[0053]

[0054]

[0055] The spectral signal is mixed and low-pass filtered, that is, the voltage signal is mixed with 、 Multiplying together, we get the same phase and orthogonal Quantity,

[0056]

[0057]

[0058] After filtering, we get:

[0059]

[0060]

[0061] Extract the amplitude of the target signal and phase :

[0062]

[0063]

[0064] Subtract the target signal from the original signal to get the residual signal , for the remaining signal Use wavelet transform to separate nonlinear components, select Morlet wavelet as wavelet basis, and calculate wavelet coefficients :

[0065]

[0066] Where a is the scale parameter, b is the translation parameter, is the wavelet basis.

[0067] Wavelet coefficients Perform hard threshold processing to reduce noise interference.

[0068]

[0069] Where N is the number of wavelet coefficients;

[0070] Finally reconstructed signal :

[0071]

[0072] The amplitude of the target signal and wavelet reconstructed signal Through Fourier transform, we get and ,merge and A complete nonlinear spectrum is obtained. This complete nonlinear spectrum, acting as the "optical fingerprint" of gas molecules, can reflect the nonlinear optical behavior of the target gas under specific excitation conditions. Its spectral structure, intensity distribution, and phase change information are directly related to the target gas's molecular structure, polarizability, concentration level, and environmental response characteristics. Therefore, this nonlinear spectrum can be used to identify gas types, quantitatively assess concentrations, and analyze nonlinear effects, enabling rapid and accurate gas detection.

[0073] According to the above design, this gas detection method can not only achieve an effective optical path of 54.3m in a shorter length, but also effectively reduce the cost of the entire gas absorption cell due to its high lens utilization, simple structure and small size, making it very suitable for low-concentration gas detection and analysis.

[0074] In summary, the present invention optimizes the optical path through design, enabling multiple reflections of the laser within the device, thereby significantly enhancing nonlinear effects such as self-phase modulation and second harmonic generation. The cumulative effect of these effects significantly enhances spectral broadening and deformation, enhancing the ability to detect low-concentration gases. This invention is particularly suitable for detecting low-concentration, toxic gases, volatile organic compounds, and trace gases in environmental monitoring.

[0075] Of course, the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. An ultrafast, highly sensitive gas detection device based on gas nonlinear optics, characterized in that: include: A light source module, for generating laser light; A light-through cell for multiple reflections after laser incident. The light-through cell is an annular gas absorption cell with a high-reflectivity reflector provided on its inner wall. The entrance and exit holes of the annular gas absorption cell are sealed by optical windows. The signal processing module is used to receive light emitted from the luminous flux cell and process the optical signal. After the laser generated by the light source module enters the luminous flux cell, it extends the optical path through multiple reflections, triggering a nonlinear optical effect and generating a nonlinear spectrum. The signal processing module performs signal separation and noise suppression on the nonlinear spectrum to achieve detection of low-concentration gas.

2. The gas detection device according to claim 1, characterized in that The light source module includes a high-power laser source with a central wavelength of 1030nm, a pulse width of 100fs, and a peak light intensity of 10 15 W / m 2 .

3. The gas detection device according to claim 1 or 2, characterized in that: The inner diameter of the annular gas absorption cell is 60 cm. The reflector of the annular gas absorption cell is coated, and the mirror reflectivity R>99.5%. The reflectivity of the reflector covers the near-infrared to mid-infrared band.

4. The gas detection device according to claim 3, characterized in that: The entrance hole and the exit hole of the annular gas absorption cell share a through hole, and the through hole is sealed by an optical window. The optical window ensures the sealing of the light absorption cell and allows light to enter and exit freely.

5. The gas detection device according to claim 1, characterized in that: The signal processing module includes a grating spectrometer and a computer. The grating spectrometer is used to receive light emitted from the light flux cell and convert it into a spectral signal; the computer is used to run a signal processing algorithm to analyze and process the spectral signal.

6. The gas detection device according to claim 5, characterized in that: The signal processing module also includes a phase-locked amplifier unit for improving the signal-to-noise ratio and extracting weak signals, and simultaneously performing signal separation in combination with wavelet transform to separate the target signal from the noise signal.

7. The gas detection device according to claim 3, characterized in that: The annular gas absorption cell is filled with low-concentration acetylene gas, the balance gas is nitrogen, and the concentration of the acetylene gas is 500 ppb.

8. An ultrafast, highly sensitive gas detection method based on gas nonlinear optics, using the device according to any one of claims 1 to 7, characterized in that: The following steps are involved: Generate laser light and inject it into an annular gas absorption cell; The optical path is extended by multiple reflections in the annular gas absorption cell, triggering the nonlinear optical effect and generating a nonlinear spectrum containing the original incident light frequency and new frequency components; Receive the light emitted from the annular gas absorption cell and process the optical signal to detect low-concentration gas; The nonlinear optical effect reduces the light intensity threshold of the nonlinear effect by extending the optical path and accumulating nonlinear phase shift, so that the nonlinear effect can also be generated under low light intensity.

9. The gas detection method according to claim 8, characterized in that: The processing of the optical signal to detect low-concentration gas is specifically as follows: generating an orthogonal reference signal, mixing the spectral signal with the orthogonal reference signal and then performing low-pass filtering to extract the amplitude and phase of the target signal; Subtracting the target signal from the original signal to obtain a residual signal, and using wavelet transform to separate nonlinear components from the residual signal; Performing hard threshold processing on the nonlinear component to reduce noise interference; The amplitude of the target signal and the wavelet reconstructed signal are Fourier transformed to obtain a complete nonlinear spectrum.

10. The gas detection method according to claim 9, wherein: The wavelet transform uses Morlet wavelet as a wavelet basis.

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