Ultrafast, high-sensitivity gas detection device and method based on nonlinear optics of gases

Abstract

The application discloses an ultrafast and high-sensitivity gas detection device and method based on gas nonlinear optics. The application comprises a high-power laser light source module, an optical cell and a signal processing module. The optical cell adopts an integrated hollow structure, and a high-reflectivity mirror is arranged in the optical cell to form a closed light path. Laser light enters the optical cell through an incident hole at a certain incident angle, and is reflected for multiple times to realize 50-meter long light path accumulation, thereby effectively reducing the nonlinear effect excitation threshold. By adjusting the incident angle to change the spot path, the signal intensity is significantly enhanced in combination with the nonlinear effect, and the detection limit reaches ppb level. The signal processing module adopts a lock-in amplifier to extract target frequency components, combines wavelet transform to separate frequency domain aliasing signals, and reconstructs complete nonlinear spectrum through Fourier transform. The application has the advantages of stable structure, high sensitivity, strong anti-interference and the like, and is suitable for environmental trace gas monitoring and volatile organic matter early warning.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of nonlinear spectroscopy, and particularly to a device and method for ultrafast and high-sensitivity gas detection based on gas nonlinear optics. BACKGROUND

[0002] Currently, in the field of gas detection, especially ultrafast and high-sensitivity detection, traditional technologies still face many challenges. For example, the traditional straight-through gas absorption cell is usually used in the traditional absorption spectroscopy technology, and the design has limitations. The effective optical path is usually less than 10 meters, which directly limits the spectral detection sensitivity to the ppm level. There is a risk of false detection and missed detection in monitoring ppb-level trace gases in environmental monitoring and volatile organic compound early warning applications.

[0003] In recent years, nonlinear optical effects, such as self-phase modulation, second-harmonic generation, and third-harmonic generation, have provided a new way to improve detection sensitivity. Nonlinear spectroscopy obtained by using these nonlinear effects can achieve higher sensitivity in detection.

[0004] Although nonlinear optical effects provide a new way to break through the sensitivity limit, their practical application faces double challenges. First, the strength of nonlinear effects is exponentially related to the optical path, and the short optical path design of the traditional absorption cell leads to insufficient accumulation of nonlinear signals. Second, studies have shown that the existing technology requires a laser threshold of 10 17 W / m 2 orders of magnitude to excite nonlinear responses, which greatly limits low-concentration gas detection. When the gas molecule density is insufficient, it is difficult to reach the threshold of nonlinear interaction, resulting in detection sensitivity stagnating in the ppm-ppb range.

[0005] Current technologies attempt to enhance the optical path through a resonant cavity, but the gain is not ideal in practical applications due to mode matching difficulties and stability issues. While the optical fiber-based technology solution can achieve a kilometer-level optical path, it suffers from parasitic effects such as stimulated Raman scattering, which reduces the spectral signal-to-noise ratio. SUMMARY

[0006] To address the above technical problems, the present application proposes a nonlinear optical gas detection method and device based on a traditional ring-shaped gas absorption cell. The present application makes full use of the space inside the ring-shaped gas absorption cell, allowing light to be reflected multiple times in a regular manner inside the gas absorption cell. By combining nonlinear effects, the strength and sensitivity of the detection signal are improved. By enhancing or changing the interaction mechanism between light and gas, low-concentration gas detection is achieved, and small changes in gas composition can be quickly responded to.

[0007] The first aspect of the present application provides a device for ultrafast and high-sensitivity gas detection based on gas nonlinear optics, comprising:

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

[0009] A light tunnel for multiple reflections after the laser light is incident, the light tunnel being a ring-shaped gas absorption cell with a high reflectivity mirror on the inner wall, and the incident hole and the exit hole of the ring-shaped gas absorption cell being sealed by an optical window sheet;

[0010] A signal processing module for receiving light emitted from the light tunnel and processing the optical signal; wherein the laser light generated by the light source module is incident on the light tunnel, the optical path is extended by multiple reflections, the nonlinear optical effect is triggered, the nonlinear spectrum is generated, the signal processing module performs signal separation and noise suppression on the nonlinear spectrum to realize detection of low-concentration gas.

[0011] The second aspect of the application provides an ultrafast and high-sensitivity gas detection method based on nonlinear optics of gas, using the above device, comprising the following steps:

[0012] Generating laser light and incident on the ring-shaped gas absorption cell;

[0013] Extending the optical path by multiple reflections in the ring-shaped gas absorption cell, triggering the nonlinear optical effect, and generating a nonlinear spectrum containing the original incident light frequency and new frequency components;

[0014] Receiving light emitted from the ring-shaped gas absorption cell and processing the optical signal to realize detection of low-concentration gas;

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

[0016] The technical scheme provided by the embodiment of the application has the following beneficial effects:

[0017] (1) The light tunnel is designed as a ring-shaped gas absorption cell, which is an integrated design, without the need to adjust the internal structure of the light tunnel, only the external incident light path needs to be adjusted, that is, the laser incidence angle is adjusted to change the position of the light spot, simplifying the system structure and improving the system stability.

[0018] (2) The application changes the effective absorption optical path of the gas in the multi-tunnel by changing the laser incidence angle, and improves the detection limit to the ppb level by 50m long optical path.

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

[0020] (4) This invention utilizes nonlinear effects to not only cause changes in the intensity of spectral peaks, but also to induce changes in their shape, thereby significantly improving the ability to identify spectral peaks. Through nonlinear effects, the absorption cell can generate new frequency components, amplifying or shifting absorption lines that were originally difficult to detect to more easily observed spectral regions, thus greatly improving the sensitivity of detection.

[0021] (6) This invention uses a combined algorithm of digital lock-in amplification and wavelet transform to efficiently extract weak signals and separate different components in nonlinear spectra. By implementing the function of the hardware lock-in amplifier at the software level, the simplicity of the hardware system is improved. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of one embodiment of the present application;

[0023] Figure 2 This is a cross-sectional view of an embodiment of this application;

[0024] Figure 3 This is an enlarged view of the light entrance and exit aperture in an embodiment of this application;

[0025] Figure 4 This is a schematic diagram of the internal optical path in an embodiment of this application;

[0026] Figure 5 This is a schematic diagram of the focal region of a portion of the light beam after specular reflection.

[0027] Figure 6 This is a graph showing the threshold curves for light intensity and nonlinear effects. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will now be described in further detail with reference to the accompanying drawings. These drawings are simplified schematic diagrams, illustrating only the basic structure of the invention, and therefore only show the components relevant to the invention.

[0029] like Figure 1 As shown in the figure, an ultrafast and highly sensitive gas detection device based on gas nonlinear optics provided in this 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 uses a center wavelength. 1030nm, pulse width 100 fs, peak light intensity 10 15 W / m 2This is a femtosecond laser. Its wavelength is located in the near-infrared region, enabling effective matching of various trace gases in nonlinear absorption. It possesses excellent gas selectivity and adaptability. The femtosecond-level pulse width allows the laser to release high energy in an extremely short time, significantly enhancing the efficiency of nonlinear interactions.

[0031] In a preferred example, the light-passing cell is an annular gas absorption cell 2 with an inner diameter of It is 60cm long, and 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 its cavity. It is mainly composed of high-precision reflectors, which are coated to achieve a reflectivity R > 99.5%, thus achieving efficient light reflection and covering the near-infrared to mid-infrared wavelength range. 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 specific details of the entrance and exit holes are as follows... Figure 3 As shown, specifically, the reflector 5 of the annular gas absorption cell 2 is coated with a reflectivity... It is made of 99.5% gold film and uses an optical window seal to ensure the airtightness of the light transmission cell while allowing light to enter and exit freely.

[0032] Laser 1 is positioned outside the annular gas absorption cell 2, on the same side as the entrance aperture 6. Laser 1 generates a laser beam that enters the annular gas absorption cell 2 through the entrance aperture 6. By adjusting the position of laser 1, the laser beam is positioned at an incident angle of 8°. The incident angle is adjusted by observing the signal collected on computer 4 and optimizing the position of laser 1.

[0033] Furthermore, 500 ppb of acetylene is introduced into the annular gas absorption tank 2, with nitrogen as the equilibrium gas. The nonlinear refractive index coefficient of the low-concentration acetylene for the mixed gas is... The contribution is very low; the nonlinear refractive index coefficient of the mixed gas Primarily dependent on nitrogen, the nonlinear refractive index coefficient of nitrogen at a wavelength of 1030 nm... 3×10 -23 m 2 / W.

[0034] The laser generated by laser 1 is at an angle of 8°. The laser light enters the optical cell through the entrance aperture and undergoes multiple reflections within the annular cavity. The optical path length for a single reflection is... for:

[0035]

[0036] in The inner diameter of the light-passing cell. The laser incident angle is denoted as .

[0037] As shown in Figure 4 , after each reflection, the laser beam converges, the light divergence angle decreases, and a relatively stable focal region, such as the first focal region 7 and the second focal region 8, is formed, and the focal region is extended to a certain extent. The long focal region is not a single focal point, but a region in space where the laser maintains a high light intensity, which is the main region where most nonlinear optical effects and cumulative effects occur. Due to the multiple reflections and focusing of the laser, the long focal region is effectively extended in space, occupying a large proportion in 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 53.4m. The optical path of the laser in the annular gas absorption cell 2 is shown in Figure 5 .

[0039] The peak intensity attenuation after each reflection is , and the attenuation after the nth reflection is , and the light intensity attenuation curve is shown in Figure 6 , where is the mirror 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 ≥ nonlinear effect threshold , the nonlinear effect is triggered, resulting in spectral broadening and new frequency generation. The threshold of the nonlinear effect is determined by the following formula:

[0041]

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

[0043] When considering the loss coefficient , the correction is:

[0044]

[0045]

[0046] where the loss coefficient of the gas medium, the mirror reflection loss coefficient and the gas absorption coefficient​ It is determined that R is the specular reflectivity, when the gas absorption coefficient is much smaller than the specular reflection loss coefficient , the coefficient is dominated by the specular reflection loss coefficient .

[0047] The design of long optical path makes the single light intensity insufficient to trigger nonlinear effects. After multiple reflections, the effective optical path is extended and the nonlinear phase shift is accumulated, the triggering of nonlinear effects can still be realized, which reduces the light intensity threshold for the generation of nonlinear effects, and makes the nonlinear effects that originally occur under high intensity light conditions also occur under lower light intensity.

[0048] The nonlinear effect threshold decay curve is shown in Figure 6 , the light after the action of nonlinear effect contains the original incident light frequency , and new frequency components, the distribution of these frequency components constitutes the nonlinear spectrum.

[0049] The signal processing module is composed of a grating spectrometer 3 and a computer 4. The grating spectrometer 3 receives the outgoing light 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 position of the light source module can also be adjusted according to the obtained signal to adjust the incident angle of the laser.

[0050] In addition, the grating spectrometer detects the outgoing light, converts the light intensity into a voltage signal and transmits it to the computer. The signal generated by the nonlinear effect is usually mixed with noise, and different signal components overlap in the frequency domain. Direct detection has problems such as signal aliasing and noise interference, which limits the detection sensitivity, so signal separation and noise suppression need to be performed on the original spectrum.

[0051] Preferably, the lock-in amplification technology is used to improve the signal-to-noise ratio and extract weak signals, and the wavelet transform is used for signal separation. The amplitude of the target signal and the wavelet reconstruction signal are combined, and the complete nonlinear spectrum can be obtained by Fourier transform.

[0052] Further, assuming that the target signal frequency is , the sampling frequency is , the computer 4 generates the orthogonal reference signal by python:

[0053]

[0054]

[0055] After mixing the spectral signal and low-pass filtering, the voltage signals are mixed with , Multiplication, get the same phase and quadrature component,

[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 , the residual signal Separate the nonlinear component using wavelet transform, select Morlet wavelet as the wavelet base, and calculate the wavelet coefficient :

[0065]

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

[0067] Hard threshold processing is performed on the wavelet coefficient to reduce noise interference,

[0068]

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

[0070] Finally, reconstruct the signal :

[0071]

[0072] The amplitude of the target signal and the wavelet reconstructed signal Through Fourier transform, we get and , merge and The complete nonlinear spectrum can be obtained, which can reflect the nonlinear optical behavior of the target gas under specific excitation conditions as the "optical fingerprint" of the gas molecule, and the spectral structure, intensity distribution and phase change are directly related to the molecular structure, polarizability, concentration level and environmental response characteristics of the target gas. Therefore, the identification of the gas type, the quantitative evaluation of the concentration and the analysis of the nonlinear effect can be realized by the nonlinear spectrum, thereby completing the rapid and accurate detection of the gas.

[0073] According to the above design, the gas detection method can realize an effective optical path of 54.3 m in a small length, and can effectively reduce the cost of the whole gas absorption cell due to high utilization rate of the lens, simple structure and small size, and is very suitable for low-concentration gas detection and analysis.

[0074] In summary, the present application optimizes the optical path by design, so that the laser is reflected in the device multiple times, thereby greatly enhancing the nonlinear effects such as self-phase modulation and second harmonic generation. Through the accumulation of these effects, the spectral broadening and shape are significantly enhanced, and the detection ability of low-concentration gas is enhanced. The present application is particularly suitable for the detection of low-concentration, toxic gas, volatile organic compounds and trace gases in environmental monitoring.

[0075] Of course, the above only describes the preferred embodiments of the present application, and is not used to limit the present application, and any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. An ultrafast, high-sensitivity gas detection device based on nonlinear optics of gas, characterized in that, The application relates to a long-path-length laser absorption spectroscopy device and method. The device comprises: a light source module for generating laser light; a light tunnel for multiple reflections of the laser light, each reflection generating a focal region, and the multiple reflections and focusing forming a long focal region, the long focal region being a region in which the laser light continuously maintains high light intensity, and the long focal region being effectively extended in space through multiple reflections and focusing, so that the nonlinear interaction between the laser light and the gas is more sufficient, thereby significantly enhancing the generation of nonlinear signals and improving the detection sensitivity; the light tunnel is a ring-shaped gas absorption cell, the inner wall of the ring-shaped gas absorption cell is provided with a high-reflectivity mirror, and the entrance hole and the exit hole of the ring-shaped gas absorption cell are sealed by an optical window sheet; a signal processing module for receiving the light emitted from the light tunnel and processing the light signal; wherein the laser light generated by the light source module is incident on the light tunnel, the optical path is lengthened through multiple reflections, the nonlinear optical effect is triggered, the nonlinear spectrum is generated, the signal processing module performs signal separation and noise suppression on the nonlinear spectrum to realize the detection of low-concentration gas, and the nonlinear optical effect reduces the light intensity threshold of the nonlinear effect through lengthening the optical path and accumulating the nonlinear phase shift, so that the nonlinear effect can also be generated at low light intensity. The inner diameter of the ring-shaped gas absorption cell is 60 cm, the mirror of the ring-shaped gas absorption cell is subjected to film coating treatment, the mirror surface reflectivity R is greater than 99.5%, and the reflectivity of the mirror covers the near-infrared to mid-infrared wave band. The entrance hole and the exit hole of the ring-shaped gas absorption cell share a through hole, and the through hole is sealed by an optical window sheet, which ensures the sealing of the light tunnel and allows the light to freely enter and exit. The signal processing module comprises a grating spectrometer and a computer, the grating spectrometer is used for receiving the light emitted from the light tunnel and converting the light into a spectrum signal, and the computer is used for running a signal processing algorithm to analyze and process the spectrum signal.

2. The gas detection device of claim 1, wherein, The light source module comprises a high-power laser source, the central wavelength of the laser source is 1030nm, the pulse width is 100fs, the peak light intensity is 10 15 W / m 2 .

3. The gas detection apparatus according to claim 1 or 2, characterized by The signal processing module further comprises a lock-in amplification unit for improving the signal-to-noise ratio and extracting weak signals, and the lock-in amplification unit is combined with wavelet transform to separate the target signal and the noise signal.

4. The gas detection device of claim 3, wherein, The ring-shaped gas absorption cell is filled with low-concentration acetylene gas, and the balance gas is nitrogen, and the concentration of the acetylene gas is 500 ppb.

5. The gas detection device of claim 1, wherein, The method comprises the following steps:

6. The gas detection device of claim 5, wherein, generating laser light and making the laser light incident on the ring-shaped gas absorption cell; 7. The gas detection device of claim 3, wherein lengthening the optical path through multiple reflections in the ring-shaped gas absorption cell, triggering the nonlinear optical effect, and generating a nonlinear spectrum containing the original incident light frequency and a new frequency component; 8. A method for ultrafast, high-sensitivity gas detection based on gas nonlinear optics, using the device of any one of claims 1-7, characterized in that, receiving the light emitted from the ring-shaped gas absorption cell and processing the light signal to realize the detection of low-concentration gas; wherein the nonlinear optical effect reduces the light intensity threshold of the nonlinear effect through lengthening the optical path and accumulating the nonlinear phase shift, so that the nonlinear effect can also be generated at low light intensity. The processing of the light signal to realize the detection of low-concentration gas is specifically: generating a quadrature reference signal, mixing the spectrum signal with the quadrature reference signal, and 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 separating the nonlinear component of the residual signal by using wavelet transform.

9. The gas detection method of claim 8, wherein: ​ ​ ​ Hard threshold processing is performed on the nonlinear component to reduce noise interference; The amplitude of the target signal and the wavelet reconstruction signal are subjected to Fourier transform to obtain complete nonlinear spectrum.

10. The gas detection method of claim 9, wherein: The wavelet transform uses Morlet wavelet as a wavelet base.

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