A spectral detection method based on differential absorption

Abstract

The application belongs to the technical field of optical detection, and particularly relates to a spectrum detection method based on differential absorption, which can detect and analyze the absorption spectrum of most non-single-element molecules. The application is a tunable semiconductor laser absorption spectrum technology detection method based on the principle of differential absorption, which optimizes the defect of insufficient tunable ability of traditional absorption spectrum technology, can detect and analyze the absorption spectrum of most non-single-element molecules, and provides a new path for the development of tunable semiconductor laser absorption spectrum technology.

Description

Technical Field

[0001] This invention belongs to the field of optical detection technology, specifically relating to a spectral detection method based on differential absorption. Background Technology

[0002] Tunable semiconductor laser absorption spectroscopy (TSLAS) is a type of spectroscopic detection technology with advantages such as high selectivity, high sensitivity, and high-speed response, making it a promising detection technology. Tunable SCLAS detects gas concentration by utilizing the absorption of laser light at a specific frequency by the gas. Its basic principle is that when laser light passes through a gas containing the analyte, the gas absorbs a portion of the laser light; by measuring the absorption intensity, the concentration of the analyte can be determined. However, in mixed gases, a considerable number of non-elemental gas components are difficult to detect using TCLAS. This is because their molecular absorption lines are highly concentrated within a small wavelength range, approaching the resolution limit of the spectrum. Furthermore, the broadening of the absorption lines leads to multiple overlapping lines, forming absorption "peak clusters," which the wavelength tuning range of the tunable semiconductor laser cannot cover. Therefore, it is difficult to obtain the effective absorbance area from the laser signal. Summary of the Invention

[0003] In view of this, the present invention provides a spectroscopic detection method based on differential absorption, which can perform absorption spectral detection and analysis on most non-elemental molecules.

[0004] To achieve the objectives of this invention, the following technical solutions are provided.

[0005] The present invention provides a spectral detection method based on differential absorption, comprising the following steps: A tunable semiconductor laser is provided, and a tuning signal is applied to the laser to scan the laser wavelength within the absorption band of the target gas. The detection band is selected, which is located in the absorption spectrum of the target gas, and the selection of the detection band satisfies at least one of the following conditions: the absorption slope changes the most, the absorption intensity changes monotonically, and the laser initial point is located at a non-absorption position. The laser is passed through the gas to be tested, and the change in laser signal intensity over time is detected to obtain the absorption signal; Extract the signal attenuation area caused by gas absorption from the absorption signal; and determine the concentration of the gas to be measured based on the pre-established linear relationship between the signal attenuation area and the gas concentration.

[0006] The tuning signal is either a sawtooth current signal or a square wave signal.

[0007] The selection of the detection band ensures that the entire tuning range of the laser is within the absorption band, and the absorption signal exhibits an overall decreasing trend.

[0008] The signal attenuation area is obtained by subtracting the background signal area from the signal area after absorption by the target gas.

[0009] The target gas is a non-elemental gas molecule, including unsymmetrical dimethylhydrazine gas.

[0010] The tunable range of the laser is smaller than the width at the bottom of the absorption peak of the target gas.

[0011] The laser wavelength scanning range covers a portion of the absorption peak, and the difference in absorption intensity between the initial and final scanning points is used to enhance detection sensitivity.

[0012] The linear relationship between the signal attenuation area and the gas concentration is established through experimental calibration or theoretical modeling.

[0013] Beneficial effects (1) The present invention is a detection method based on the principle of differential absorption using tunable semiconductor laser absorption spectroscopy, which optimizes the shortcomings of insufficient tunability of traditional absorption spectroscopy and enables absorption spectral detection and analysis of most non-elemental molecules, providing a new path for the development of tunable semiconductor laser absorption spectroscopy. The present invention creatively combines the principle of differential absorption with tunable semiconductor laser absorption spectroscopy, effectively solving the detection problem caused by the "peak clusters" formed by the dense, broadened and aliased absorption lines, making it possible to perform highly sensitive and selective spectral analysis of most non-elemental gas molecules (such as unsymmetrical dimethylhydrazine) that are difficult to detect by traditional methods.

[0014] (2) This invention solves the problem of the lack of a single detectable narrow peak due to absorption peak aliasing. It is a technique for improving and optimizing the absorption spectrum of tunable semiconductor lasers, which is applicable to non-elemental gas molecules in most cases. The method of this invention does not require the tunable range of the laser to completely cover the entire absorption peak. It cleverly utilizes the difference in absorption intensity within a local band of the absorption peak to perform concentration inversion. This significantly reduces the dependence on laser performance, enabling narrow-linewidth lasers to still achieve high-precision gas detection even when the spectral lines are broadened or the absorption peaks are aliased, thus broadening the application scenarios of existing equipment.

[0015] (3) The present invention solves the problem of insufficient tunable range of lasers, enabling narrow linewidth lasers to perform high-sensitivity detection even when the spectral lines are broadened or the absorption peaks are mixed.

[0016] (4) This invention is a novel method for tunable semiconductor laser spectroscopy detection of aliased and connected peaks in molecular absorption spectra, which is of great significance for expanding the application of absorption spectroscopy. By establishing clear principles for selecting the detection band (such as selecting the region with the largest change in absorption slope and monotonically changing absorption intensity), this invention provides a universal solution that can systematically and effectively detect aliased and connected peaks in molecular absorption spectra, ensuring a good linear relationship between the detection signal and gas concentration, and improving the accuracy and repeatability of the detection. (5) This method provides a new and effective technical path for tunable semiconductor laser absorption spectroscopy, breaking the application barrier of this technology in the detection of complex gas components, and has great significance for promoting the application of absorption spectroscopy in a wider range of fields such as environmental monitoring, industrial production, and safety early warning. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the tunability characteristics of a semiconductor laser.

[0018] Figure 2 A schematic diagram of the absorption spectrum of water molecules in an example of the method of this invention.

[0019] Figure 3 A schematic diagram of the sawtooth waveform change caused by laser signal absorption in an example of the method of the present invention.

[0020] Figure 4 This is the absorption spectrum of unsymmetrical dimethylhydrazine gas in an example of the method of the present invention.

[0021] Figure 5 This is a schematic diagram of the associated gas concentration utilizing the tunable characteristics of a semiconductor laser in an example of the method of the present invention. Detailed Implementation

[0022] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0023] In the process of determining the concentration of substances using absorption spectroscopy with tunable semiconductor lasers, direct absorption and signal modulation techniques are generally employed. However, this method faces the challenge of wide absorption linewidths and narrow-band laser scanning limitations associated with characteristic gas molecules. This invention provides a spectroscopic detection method based on differential absorption. Addressing the issue in direct absorption methods where the tunable range of the laser is smaller than the peak width of the absorbing substance, this method utilizes the difference in absorption intensity at the absorption peak positions to inversely determine the substance concentration. The principle is as follows: In laser absorption spectroscopy, direct absorption is the most commonly used detection method. It typically utilizes the wavelength tuning capability of a tunable semiconductor laser by applying a sawtooth current signal or a square wave signal to the laser. The stability of the laser under the tuned current signal is then readily apparent. Figure 1 . Figure 1 Due to the tunable characteristic of semiconductor lasers, their center wavelength gradually shifts towards longer wavelengths as the injected current increases. Using a laser with this characteristic as an emission source allows the laser light to pass through a target gas and induce absorption.

[0024] As the current flowing through the laser gradually increases, the center of its wavelength gradually redshifts.

[0025] The center wavelength of a laser needs to scan the entire frequency range of its absorption peak. This means the laser scans from frequencies where absorption is non-absorbent to frequencies where absorption is highest, and then back to the end of the absorption peak, i.e., frequencies where there is no absorption. For example, the absorption spectrum of water molecules... Figure 2 This is the absorption spectrum of water molecules; the initial wavelength of the laser is at 2755.8 cm⁻¹. -1 The position eventually stopped at 2760.2 cm as the scan continued. -1 Location.

[0026] In this scan, the laser actually traveled from its center wavelength of 2755.8 cm⁻¹ -1 Up to 2760.2cm -1 The tuning resulted in a total redshift of approximately 5 cm. -1 This tuning characteristic is inherent to the laser itself; the laser's output light frequency adapts to the position of the absorption peak. Tunable semiconductors, utilizing their fast time response rate, can achieve this scanning process within milliseconds or even microseconds. Here, the laser's tunable range is sufficient to completely cover an absorption peak of a water molecule. This allows the laser to effectively distinguish between absorbing and non-absorbing locations during gas detection, thus highlighting the area of ​​reduced absorbance due to gas absorption. When scanning an absorption peak using a sawtooth laser signal, the intensity "bump" caused by absorption (a "bump" when the signal is inverted, a "depression" when not inverted) can be determined by solving for two areas to establish the correlation between gas concentration and absorbance. Figure 3 To address the sawtooth waveform variation caused by laser signal absorption, direct fitting can be used to fit the non-absorption positions of the sawtooth waveform to obtain the original signal waveform. Then, the absorbance area caused by absorption can be calculated to establish the relationship between absorbance and gas concentration. However, if the width of the absorption peak (peak width) exceeds the tunable range of the semiconductor laser's center wavelength, this method can no longer be used for detection. This invention proposes an absorption spectroscopy detection technique based on the principle of differential absorption.

[0027] However, in most cases, the absorption peaks of a substance do not exist in isolation; they are more likely to overlap and form a "peak cluster." In such cases, it is necessary to optimize the spectral detection method for tunable semiconductor lasers. Taking unsymmetrical dimethylhydrazine (UDMH) gas as an example, its absorption spectrum is as follows: Figure 4 As shown, its absorption spectrum exhibits a broad band of overlapping peaks, making it difficult to find a single narrow peak for measurement. Therefore, a differential absorption method is required. This invention first locates the highest absorption peak in the absorption spectrum, at 2973.3 cm⁻¹. -1 At the wavenumber, the initial laser point is selected based on the tunable range of the laser. When the tunable range of the laser is 5cm... -1 At that time, the initial point was 2968.3 cm. -1 When the tunable range of the laser is 10cm -1 At that time, the initial wavenumber was selected as 2961.3 cm. -1 In this way, during laser scanning, due to the different absorption intensities at different wavenumber positions, the signal attenuation at corresponding frequency positions will also be different. By subtracting the signal area after absorption by the target gas from the area of ​​the background signal, the signal attenuation area caused by absorption can be obtained. This establishes the relationship between laser signal absorbance and gas concentration. In the detection of unsymmetrical dimethylhydrazine gas, this invention selects the absorbance region caused by differential absorption, such as... Figure 5 As shown, specifically Figure 5 To correlate gas concentrations using the principle of differential absorption for absorption peaks with broad absorption peaks or even aliasing across the entire wavelength range, the following method is used: Figure 1 The laser signal mentioned scans from the non-absorption position of the gas absorption peak to the absorption position. Since the entire tuning range of the signal is in the absorption band, the signal generally shows a downward trend. However, since the absorption intensity of the signal is different at different frequencies, the initial point of the sawtooth signal has the least absorption intensity or even no absorption (if it is at the non-absorption position), while the end point of the sawtooth signal corresponds to high absorption intensity. In this way, an absorbance area caused by gas absorption attenuation is obtained. There is a linear relationship between the gas concentration and the absorbance area caused by absorption. This establishes an effective way to detect gas concentration using laser attenuation signal.

[0028] The principles for selecting the detection band are mainly threefold: First, select the position with the greatest change in absorption slope. This makes the absorption difference between the initial and final points of the laser signal scan more obvious, and the change in absorbance area caused by the absorption effect more acute, thereby improving the sensitivity of laser detection. Second, the selected region should be located on one side of the absorption peak, that is, within the band where the absorption intensity increases or decreases monotonically. This ensures a linear relationship between the absorbance area and concentration after subtracting the background signal from the target gas. Third, try to position the initial laser frequency at a location without absorption. This ensures the stability of the initial laser signal intensity, allowing for more accurate calculation of absorbance area in complex environments and effectively reducing detection errors.

[0029] This invention includes, but is not limited to, the above embodiments. Any equivalent substitutions or partial improvements made under the spirit and principles of this invention shall be considered within the scope of protection of this invention.

Claims

1. A spectroscopic detection method based on differential absorption, characterized in that, Includes the following steps: A tunable semiconductor laser is provided, and a tuning signal is applied to the laser to scan the laser wavelength within the absorption band of the target gas. The detection band is selected, which is located in the absorption spectrum of the target gas, and the selection of the detection band satisfies at least one of the following conditions: the absorption slope changes the most, the absorption intensity changes monotonically, and the laser initial point is located at a non-absorption position. The laser is passed through the gas to be tested, and the change in laser signal intensity over time is detected to obtain the absorption signal; Extract the signal attenuation area caused by gas absorption from the absorption signal; and determine the concentration of the gas to be measured based on the pre-established linear relationship between the signal attenuation area and the gas concentration.

2. The method according to claim 1, characterized in that, The tuning signal is a sawtooth current signal or a square wave signal.

3. The method according to claim 1, characterized in that, The selection of the detection band ensures that the entire tuning range of the laser is within the absorption band, and the absorption signal exhibits an overall decreasing trend.

4. The method according to claim 1, characterized in that, The signal attenuation area is obtained by subtracting the background signal area from the signal area after absorption by the target gas.

5. The method according to any one of claims 1-4, characterized in that, The target gas is a non-elemental gas molecule, including unsymmetrical dimethylhydrazine gas.

6. The method according to any one of claims 1-4, characterized in that, The tunable range of the laser is smaller than the width at the bottom of the absorption peak of the target gas.

7. The method according to any one of claims 1-4, characterized in that, The scanning range of the laser wavelength covers a portion of the absorption peak, and the difference in absorption intensity between the initial and final scanning points is used to enhance detection sensitivity.

8. The method according to any one of claims 1-4, characterized in that, The linear relationship between the signal attenuation area and the gas concentration was established through experimental calibration or theoretical modeling.

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