Linear cavity gas concentration measuring device and method based on optical feedback cavity enhancement

Abstract

The invention provides a linear cavity gas concentration measuring device and method based on optical feedback cavity enhancement. Comprising a DFB laser, a first photoelectric detector, a second photoelectric detector, an optical fiber collimator, a polarization beam splitter, a first 1 / 4 wave plate, a second 1 / 4 wave plate, a linear resonant cavity, piezoelectric ceramics, a transflective mirror, a first total-reflection plane mirror, a second total-reflection plane mirror, an optical isolator and a gas filling assembly. Light output by the DFB laser is separated according to a polarization state through the polarization beam splitter, part of output light of the linear resonant cavity is fed back into the DFB laser through the feedback light path by utilizing the transflective mirror, and the light feedback effect is achieved; by arranging the 1 / 4 wave plate between the polarization beam splitter and the linear resonant cavity, transmission light and primary reflection light of a reflector at the front end of the linear resonant cavity are isolated and do not return to the DFB laser any more, the influence of the light on the output mode of the laser is avoided, and the measurement precision of the gas concentration is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of gas concentration measurement, and in particular to a linear cavity gas concentration measurement device and method based on optical feedback cavity enhancement. Background Art

[0002] A gas concentration measuring device is an instrument device that accurately measures gas concentration, and mainly uses chemical or optical methods to detect gas concentration. The detection principles of existing gas concentration measuring devices are mainly based on gas chromatography, electrochemical detection methods, semiconductor sensing methods, and gas tube detection methods. However, these methods all have some defects. For example, gas chromatography requires pretreatment of gas samples; electrochemical detection methods have poor selectivity and are susceptible to cross-interference; semiconductor sensing methods have poor long-term stability due to the easy aging of sensors; and gas tube detection methods are easily interfered by changes in operating conditions, and thus cannot meet the clinical needs for rapid and accurate detection of respiratory gas concentrations.

[0003] The optical feedback cavity enhanced absorption spectroscopy technology based on the Beer-Lambert law measures the gas concentration information by directly measuring the change in the intensity of the output light intensity of the resonant cavity. It has the advantages of fast response, good long-term stability and high selectivity. In the previous gas measurement methods based on the principle of optical feedback cavity enhancement, the commonly used resonant cavity shape is a V-shaped cavity; but due to its simple structure, the linear resonant cavity has lower losses in theory and can achieve higher detection accuracy. However, in the previous linear cavity optical feedback enhancement method, the first reflected light at the incident end of the linear resonant cavity will return to the laser, affecting the normal operation of the laser, and will not produce an optical feedback effect, thus affecting the accuracy of the measurement system. How to avoid the influence of the first reflected light on the laser output mode in the process of measuring gas concentration, while optimizing the overall structure of the optical path and improving the accuracy of the measurement system is an urgent problem to be solved in order to achieve more accurate and efficient gas concentration measurement. Summary of the invention

[0004] To address the defects in the prior art, the present invention provides a linear cavity gas concentration measurement device and method based on optical feedback cavity enhancement.

[0005] In order to achieve the above technical objectives, the specific technical solutions adopted by the present invention are as follows: On the one hand, the present invention provides a linear cavity gas concentration measurement device based on optical feedback cavity enhancement, a DFB laser, a first photodetector, a second photodetector, an optical fiber collimator, a polarization beam splitter, a first 1 / 4 wave plate, a second 1 / 4 wave plate, a linear resonant cavity, a piezoelectric ceramic, a transflective mirror, a first total reflection plane mirror, a second total reflection plane mirror, an optical isolator, and a gas filling component; The laser output by the DFB laser is converted into a spatial Gaussian beam by a fiber collimator and then injected into a polarization beam splitter. The polarization beam splitter separates the incident beam according to the polarization state. The s-polarization state light is isolated by the optical isolator after passing through the second total reflection plane mirror and the second 1 / 4 wave plate. The p-polarization state light is injected into the linear resonant cavity as the detection light through the first 1 / 4 wave plate. The output beam of the linear resonant cavity is divided into two paths by the reflective mirror. One beam is injected into the second photodetector as a light intensity signal. The light intensity signal is converted into an electrical signal and output by the second photodetector; the other beam passes through the first total reflection plane mirror, the optical isolator, the second 1 / 4 wave plate, and the second total reflection plane mirror in sequence and then enters the polarization beam splitter. The beam emitted by the polarization beam splitter is injected into the DFB laser through the fiber collimator to realize optical feedback frequency stabilization. The output end of the DFB laser is also connected to a first photodetector for observing the output power of the DFB laser in real time; The cavity mirror of the linear resonant cavity is provided with a piezoelectric ceramic for adjusting the cavity length of the linear resonant cavity; An air inlet is arranged at one end of the linear resonant cavity, and an air outlet is arranged at the other end. The gas filling component is connected to the linear resonant cavity through the air inlet and the air outlet.

[0006] Furthermore, the gas charging assembly includes a flow meter, a gas filter, a sampling bag, and a vacuum pump; One end of the flowmeter is connected to the air inlet of the linear resonant cavity, and the other end of the flowmeter is connected to a gas filter and a sampling bag in sequence; the vacuum pump is connected to the air outlet of the linear resonant cavity; the gas of the to-be-measured concentration in the sampling bag is continuously filled into the linear resonant cavity through the gas filter and the flowmeter in sequence under the action of the vacuum pump.

[0007] Furthermore, it also includes a laser controller, which is connected to the DFB laser and is used to control the output wavelength of the DFB laser.

[0008] Furthermore, the gas to be measured includes carbon monoxide.

[0009] Furthermore, the first 1 / 4 wave plate is used to isolate the transmitted light and the once reflected light of the reflector at the front end of the linear resonant cavity; the second 1 / 4 wave plate is used to convert the received laser into s-polarized light to ensure that the s-polarized light can be fed back into the DFB laser through the polarization beam splitter.

[0010] On the other hand, the present invention also provides a linear cavity gas concentration measurement method based on optical feedback cavity enhancement applied to the above-mentioned measurement device, comprising the following steps: S1. Build an optical path according to a linear cavity gas concentration measurement device based on optical feedback cavity enhancement; S2. Adjust the optical path setting so that the laser output from the DFB butterfly laser is effectively coupled into the linear resonant cavity and forms a resonance phenomenon in the linear resonant cavity; at the same time, ensure that the light beam output from the linear resonant cavity is fed back to the DFB laser to achieve an optical feedback effect; S3, filling nitrogen into the linear resonant cavity, tuning the output voltage of the DFB laser and the piezoelectric ceramic on the linear resonant cavity mirror at the same time, collecting the light intensity signal without gas absorption through the first photodetector and the second photodetector, and evacuating the linear resonant cavity after the collection is completed; S4, filling a certain concentration of the gas to be tested into the linear resonant cavity, tuning the output voltage of the DFB laser and the piezoelectric ceramics on the mirror of the linear resonant cavity, and collecting the light intensity signal under gas absorption through the first photodetector and the second photodetector; S5. Calculate the absorption coefficient of the gas to be measured according to the light intensity signals collected by S3 and S4, and obtain the absorption curve of the gas to be measured; S6. Perform curve fitting on the absorption curve of the gas to be measured, and calculate the concentration of the gas to be measured.

[0011] Furthermore, the S2 further comprises the following steps: S21, adjusting the fiber collimator so that the laser output from the DFB laser is effectively coupled into the linear resonant cavity; then adjusting the piezoelectric ceramic on the cavity mirror of the linear resonant cavity so that the cavity length of the linear resonant cavity matches the laser output frequency of the DFB laser, thereby causing a resonance peak to appear in the linear resonant cavity; S22. Adjust the first totally-reflective plane mirror and the second totally-reflective plane mirror until the second photodetector detects that the resonance peak appears to be widened, indicating that the optical feedback effect is achieved; continue to adjust the first totally-reflective plane mirror and the second totally-reflective plane mirror until the second photodetector detects that the resonance peak appears to be at a maximum value, and keep the peak value of the resonance peak at a maximum value, so that the optical feedback effect achieves the best effect.

[0012] Further, in S5, the light intensity signals collected in S3 and S4 are used to calculate the absorption coefficient of the gas to be measured, including the following steps: S51, based on the light intensity signal collected by the first photoelectric detector in S3 and S4, the light intensity signal collected by the corresponding second photoelectric detector in S3 and S4 is normalized to obtain a normalized light intensity signal when there is no gas to be measured in the linear resonant cavity and the normalized light intensity signal when there is a gas to be measured in the linear resonant cavity ; S52. Calculate the absorption coefficient α of the gas to be measured according to the following formula:

[0013] in, is the speed of light, is the ring-down time of the linear resonant cavity, is the normalized light intensity signal when there is no gas to be measured in the linear resonant cavity, It is the normalized light intensity signal when there is gas to be measured in the linear resonant cavity.

[0014] Further, in S6, the concentration of the gas to be measured is calculated according to the following formula:

[0015]

[0016] in, is the molecular number density of the gas to be measured, Select the absorption line intensity under the absorption line for the gas to be measured, is the normalized absorption line shape function of the gas.

[0017] Furthermore, the It can be obtained by the fitting area of ​​the Voigt curve and is the integral value of the absorption coefficient α.

[0018] Compared with the prior art, the present invention has the following beneficial technical effects: The linear cavity gas concentration measurement device based on optical feedback cavity enhancement provided by the present invention uses a reflective mirror to split the light beam output by the linear resonant cavity, separates the light output by the DFB laser according to the polarization state through a polarization beam splitter, and uses a reflective mirror to feed back part of the output light of the linear resonant cavity into the DFB laser through a feedback optical path, thereby realizing the optical feedback effect, enhancing the output power of the DFB laser, narrowing the output line width of the DFB laser, and improving the detection accuracy of the device. By arranging a 1 / 4 wave plate between the polarization beam splitter and the linear resonant cavity, the transmitted light and the primary reflected light of the reflector at the front end of the linear resonant cavity are isolated, so that they no longer return to the DFB laser, thereby avoiding the influence of this part of light on the laser output mode.

[0019] Through the above arrangement, the influence of the first reflected light at the incident end of the linear resonant cavity on the operation of the DFB laser is completely eliminated, the coupling efficiency between the laser and the linear resonant cavity is further improved, and the measurement accuracy of the gas concentration is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying creative work.

[0021] Figure 1A schematic diagram of the structure of a linear cavity gas concentration measurement device based on optical feedback cavity enhancement provided by an embodiment; Figure 2 A schematic diagram of the structure of a linear resonant cavity and a gas filling assembly provided in an embodiment; Figure 3 A schematic diagram of the Voigt line shape of the gas absorption coefficient when measuring gas concentration provided by an embodiment; Figure 4 A light intensity signal diagram showing a resonance phenomenon in a linear resonant cavity provided by an embodiment; Figure 5 A diagram of a light intensity signal when the light feedback effect is optimal provided by an embodiment.

[0022] Figure annotation: 1. DFB laser; 2. First photodetector; 3. Fiber collimator; 4. Polarization beam splitter; 5. First 1 / 4 wave plate; 6. Linear resonant cavity; 7. Second photodetector; 8. Computer; 9. First total reflection plane mirror; 10. Optical isolator; 11. Second 1 / 4 wave plate; 12. Second total reflection plane mirror; 13. Transflective mirror; 14. Piezoelectric ceramics; 15. Sampling bag; 16. Gas filter; 17. Flow meter; 18. Vacuum pump. DETAILED DESCRIPTION

[0023] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0024] Reference Figure 1 In one embodiment, a linear cavity gas concentration measurement device based on optical feedback cavity enhancement is provided, comprising a DFB laser 1, a first photodetector 2, a second photodetector 7, a fiber collimator 3, a polarization beam splitter 4, a first 1 / 4 wave plate 5, a second 1 / 4 wave plate 11, a linear resonant cavity 6, a piezoelectric ceramic 14, a transflective mirror 13, a first total reflection plane mirror 9, a second total reflection plane mirror 12, an optical isolator 10, a computer 8, and a gas charging component; The laser output by the DFB laser 1 is converted into a spatial Gaussian beam by the fiber collimator 3 and then incident on the polarization beam splitter 4. The polarization beam splitter 4 separates the incident light beam according to the polarization state. The s-polarized light passes through the second total reflection plane mirror 12 and the second 1 / 4 wave plate 11 and is isolated by the optical isolator 10. The p-polarized light is used as the detection light and is incident on the linear resonant cavity 6 through the first 1 / 4 wave plate 5. The output light beam of the linear resonant cavity 6 is divided into two paths by the reflective mirror 13. One path of the light beam is incident on the second photodetector 7 as a light intensity signal. The light intensity signal is converted into an electrical signal by the second photodetector 7 and output to the computer 8. The other path of the light beam passes through the first total reflection plane mirror 9, the optical isolator 10, the second 1 / 4 wave plate 11, and the second total reflection plane mirror 12 in sequence and then is incident on the polarization beam splitter 4. The light beam emitted by the polarization beam splitter 4 is incident on the DFB laser 1 through the fiber collimator 3 to realize optical feedback frequency stabilization. The output end of the DFB laser 1 is also connected to a first photodetector 2 for real-time observation of the output power of the DFB laser 1. The output power of the DFB laser is used for subsequent data normalization processing. The other end of the DFB laser 1 is connected to a computer 8. A piezoelectric ceramic 14 is provided on the cavity mirror of the linear resonant cavity 6, and the piezoelectric ceramic 14 is connected to the computer 8 for adjusting the cavity length of the linear resonant cavity 6; An air inlet is provided at one end of the linear resonant cavity 6, and an air outlet is provided at the other end. The gas filling component is connected to the linear resonant cavity 6 through the air inlet and the air outlet.

[0025] Reference Figure 2 The gas charging assembly includes a flow meter 17, a gas filter 16, a sampling bag 15, and a vacuum pump 18; One end of the flowmeter 17 is connected to the air inlet of the linear resonant cavity 6, and the other end of the flowmeter 17 is connected to the gas filter 16 and the sampling bag 15 in sequence; the vacuum pump 18 is connected to the air outlet of the linear resonant cavity 6; the gas of the to-be-measured concentration in the sampling bag 15 is continuously filled into the linear resonant cavity 6 through the gas filter 16 and the flowmeter 17 in sequence under the action of the vacuum pump 18.

[0026] The sampling bag 15 is used to collect gas of the concentration to be measured for detection; the gas filter 16 is used to filter small particle pollutants in the gas to prevent small particle pollutants from entering the linear resonant cavity 6 and causing damage to the cavity and inaccurate measurement data; the flow meter 17 is used to monitor the gas flow rate to avoid damage to the linear resonant cavity 6 caused by excessive gas flow; the vacuum pump 18 is used to provide power to enable the gas of the concentration to be measured in the sampling bag 15 to continue to pass through the linear resonant cavity 6.

[0027] The first 1 / 4 wave plate 5 is used to isolate the transmitted light and the once reflected light of the front end reflector of the linear resonant cavity 6; the second 1 / 4 wave plate 11 is used to convert the received laser into s-polarized light to ensure that the s-polarized light can be fed back to the DFB laser 1 through the polarization beam splitter 4.

[0028] The computer 8 is used to collect electrical signal information for data processing, and output drive signals to control the DFB laser 1 and the piezoelectric ceramic 14; the optical isolator 10 is used to prevent the s-polarized light in the incident light from affecting the output signal.

[0029] In one embodiment, the linear cavity gas concentration measurement device based on optical feedback cavity enhancement also includes a laser controller, one end of the laser controller is connected to the DFB laser 1, and the other end of the laser controller is connected to a computer 8 for controlling the output wavelength of the DFB laser 1.

[0030] In one embodiment, a linear cavity gas concentration measurement method based on optical feedback cavity enhancement is provided, which is applied to the above-mentioned device and includes the following steps: S1. Build an optical path according to a linear cavity gas concentration measurement device based on optical feedback cavity enhancement; S2. Adjust the optical path setting so that the laser output from the DFB butterfly laser is effectively coupled into the linear resonant cavity and forms a resonance phenomenon in the linear resonant cavity; at the same time, ensure that the light beam output from the linear resonant cavity is fed back to the DFB laser to achieve an optical feedback effect; S3, filling nitrogen into the linear resonant cavity, and tuning the output voltage of the DFB laser and the piezoelectric ceramic on the linear resonant cavity mirror at the same time, tuning the output voltage of the DFB laser from 40mA to 120mA, continuously tuning the output wavelength of the DFB laser, and continuously applying a triangular wave signal to the piezoelectric ceramic on the linear resonant cavity mirror, and collecting the light intensity signal without gas absorption through the first photodetector and the second photodetector; S4, filling the linear resonant cavity with a certain concentration of the gas to be tested, and tuning the output voltage of the DFB laser and the piezoelectric ceramics on the linear resonant cavity mirror at the same time, tuning the output voltage of the DFB laser from 40mA to 120mA, continuously tuning the output wavelength of the DFB laser, and continuously applying a triangular wave signal to the piezoelectric ceramics on the linear resonant cavity mirror, and collecting the light intensity signal in the case of gas absorption through the first photodetector and the second photodetector; S5. Calculate the absorption coefficient of the gas to be measured according to the light intensity signals collected by S3 and S4, and obtain the absorption curve of the gas to be measured; S6. Perform curve fitting on the absorption curve of the gas to be measured, and calculate the concentration of the gas to be measured.

[0031] The light intensity signal detected by the photoelectric detector is converted into an electrical signal and output to a computer. The computer processes the data to generate a light intensity signal graph, which is used to determine the presence of a resonance peak and whether the resonance peak is at its maximum value.

[0032] The S2 further comprises the following steps: S21, adjust the fiber collimator to effectively couple the laser output from the DFB laser into the linear resonant cavity; then adjust the piezoelectric ceramic on the linear resonant cavity mirror, apply a triangular wave signal with a frequency of 10Hz, an amplitude of 100V, and a bias voltage of 50V to the piezoelectric ceramic, so that the cavity length of the linear resonant cavity matches the laser output frequency of the DFB laser, so that a resonance peak appears in the linear resonant cavity, such as Figure 4 As shown; S22, adjusting the first total reflection plane mirror and the second total reflection plane mirror until the second photoelectric detector detects that the resonance peak appears to be widened, indicating that the light feedback effect is achieved; continuing to adjust the first total reflection plane mirror and the second total reflection plane mirror until the second photoelectric detector detects that the resonance peak appears to be at a maximum value, keeping the resonance peak value at a maximum value, so that the light feedback effect achieves the best effect, such as Figure 5 shown.

[0033] In S5, the light intensity signals collected by S3 and S4 are used to calculate the absorption coefficient of the gas to be measured, including the following steps: S51, based on the light intensity signal collected by the first photoelectric detector in S3 and S4, the light intensity signal collected by the corresponding second photoelectric detector in S3 and S4 is normalized to obtain a normalized light intensity signal when there is no gas to be measured in the linear resonant cavity and the normalized light intensity signal when there is a gas to be measured in the linear resonant cavity ; S52. Calculate the absorption coefficient α of the gas to be measured according to the following formula:

[0034] in, is the speed of light, is the ring-down time of the linear resonant cavity, is the normalized light intensity signal when there is no gas to be measured in the linear resonant cavity, It is the normalized light intensity signal when there is gas to be measured in the linear resonant cavity.

[0035] In S6, the concentration of the gas to be measured is calculated according to the following formula:

[0036]

[0037] in, is the molecular number density of the gas to be measured, Select the absorption line intensity under the absorption line for the gas to be measured, is the normalized absorption line shape function of the gas.

[0038] Said It can be obtained by the fitting area of ​​the Voigt curve and is the integral value of the absorption coefficient α.

[0039] In one embodiment, the laser center wavelength output by the DFB laser is 1564nm, and the gas to be measured is CO. The linear cavity gas concentration measurement device based on optical feedback cavity enhancement adopts the linear cavity gas concentration measurement method based on optical feedback cavity enhancement to measure CO with a standard concentration of 1000ppm. The results are as follows: Figure 3 shown.

[0040] from Figure 3 The curve obtained in the experiment was fitted with the Voigt curve, and the calculated concentration was 1022ppm, which was 22ppm different from the labeled concentration. The minimum detectable absorption coefficient was 1.06×10 -9 cm -1 , the minimum detectable limit is 0.53ppm.

[0041] Matters not covered by the present invention are known technologies.

[0042] The technical features of the above embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0043] The above-described embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention. It should be pointed out that, for a person of ordinary skill in the art, several modifications and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the attached claims.

[0044] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A linear cavity gas concentration measurement device based on optical feedback cavity enhancement, characterized in that: It includes a DFB laser, a first photodetector, a second photodetector, a fiber collimator, a polarization beam splitter, a first 1 / 4 wave plate, a second 1 / 4 wave plate, a linear resonant cavity, a piezoelectric ceramic, a transflective mirror, a first total reflection plane mirror, a second total reflection plane mirror, an optical isolator, and a gas filling component; The laser output by the DFB laser is converted into a spatial Gaussian beam by a fiber collimator and then injected into a polarization beam splitter. The polarization beam splitter separates the incident beam according to the polarization state. The s-polarization state light is isolated by the optical isolator after passing through the second total reflection plane mirror and the second 1 / 4 wave plate. The p-polarization state light is injected into the linear resonant cavity as the detection light through the first 1 / 4 wave plate. The output beam of the linear resonant cavity is divided into two paths by the reflective mirror. One beam is injected into the second photodetector as a light intensity signal. The light intensity signal is converted into an electrical signal and output by the second photodetector; the other beam passes through the first total reflection plane mirror, the optical isolator, the second 1 / 4 wave plate, and the second total reflection plane mirror in sequence and then enters the polarization beam splitter. The beam emitted by the polarization beam splitter is injected into the DFB laser through the fiber collimator to realize optical feedback frequency stabilization. The output end of the DFB laser is also connected to a first photodetector for observing the output power of the DFB laser in real time; The cavity mirror of the linear resonant cavity is provided with a piezoelectric ceramic for adjusting the cavity length of the linear resonant cavity; An air inlet is arranged at one end of the linear resonant cavity, and an air outlet is arranged at the other end. The gas filling component is connected to the linear resonant cavity through the air inlet and the air outlet.

2. The linear cavity gas concentration measurement device based on optical feedback cavity enhancement as claimed in claim 1, characterized in that: The gas charging assembly includes a flow meter, a gas filter, a sampling bag, and a vacuum pump; One end of the flowmeter is connected to the air inlet of the linear resonant cavity, and the other end of the flowmeter is connected to a gas filter and a sampling bag in sequence; the vacuum pump is connected to the air outlet of the linear resonant cavity; the gas of the to-be-measured concentration in the sampling bag is continuously filled into the linear resonant cavity through the gas filter and the flowmeter in sequence under the action of the vacuum pump.

3. The linear cavity gas concentration measurement device based on optical feedback cavity enhancement as claimed in claim 1, characterized in that: It also includes a laser controller, which is connected to the DFB laser and is used to control the output wavelength of the DFB laser.

4. The linear cavity gas concentration measurement device based on optical feedback cavity enhancement as claimed in claim 2, characterized in that: The gas to be measured includes carbon monoxide.

5. The linear cavity gas concentration measurement device based on optical feedback cavity enhancement as claimed in claim 1, characterized in that: The first 1 / 4 wave plate is used to isolate the transmitted light and the once reflected light of the reflector at the front end of the linear resonant cavity; the second 1 / 4 wave plate is used to convert the received laser into s-polarized light to ensure that the s-polarized light can be fed back into the DFB laser through the polarization beam splitter.

6. A linear cavity gas concentration measurement method based on optical feedback cavity enhancement, applied to the device according to any one of claims 1 to 6, characterized in that: The following steps are involved: S1. Build an optical path according to a linear cavity gas concentration measurement device based on optical feedback cavity enhancement; S2. Adjust the optical path setting so that the laser output from the DFB butterfly laser is effectively coupled into the linear resonant cavity and forms a resonance phenomenon in the linear resonant cavity; at the same time, ensure that the light beam output from the linear resonant cavity is fed back to the DFB laser to achieve an optical feedback effect; S3, filling nitrogen into the linear resonant cavity, tuning the output voltage of the DFB laser and the piezoelectric ceramic on the linear resonant cavity mirror at the same time, collecting the light intensity signal without gas absorption through the first photodetector and the second photodetector, and evacuating the linear resonant cavity after the collection is completed; S4, filling a certain concentration of the gas to be tested into the linear resonant cavity, tuning the output voltage of the DFB laser and the piezoelectric ceramics on the mirror of the linear resonant cavity, and collecting the light intensity signal under gas absorption through the first photodetector and the second photodetector; S5. Calculate the absorption coefficient of the gas to be measured according to the light intensity signals collected by S3 and S4, and obtain the absorption curve of the gas to be measured; S6. Perform curve fitting on the absorption curve of the gas to be measured, and calculate the concentration of the gas to be measured.

7. The linear cavity gas concentration measurement method based on optical feedback cavity enhancement as claimed in claim 6, characterized in that: The S2 further comprises the following steps: S21, adjusting the fiber collimator so that the laser output from the DFB laser is effectively coupled into the linear resonant cavity; then adjusting the piezoelectric ceramic on the cavity mirror of the linear resonant cavity so that the cavity length of the linear resonant cavity matches the laser output frequency of the DFB laser, thereby causing a resonance peak to appear in the linear resonant cavity; S22. Adjust the first totally-reflective plane mirror and the second totally-reflective plane mirror until the second photodetector detects that the resonance peak appears to be widened, indicating that the optical feedback effect is achieved; continue to adjust the first totally-reflective plane mirror and the second totally-reflective plane mirror until the second photodetector detects that the resonance peak appears to be at a maximum value, and keep the peak value of the resonance peak at a maximum value, so that the optical feedback effect achieves the best effect.

8. The linear cavity gas concentration measurement method based on optical feedback cavity enhancement as claimed in claim 6, characterized in that: In S5, the light intensity signals collected by S3 and S4 are used to calculate the absorption coefficient of the gas to be measured, including the following steps: S51, based on the light intensity signal collected by the first photoelectric detector in S3 and S4, the light intensity signal collected by the corresponding second photoelectric detector in S3 and S4 is normalized to obtain a normalized light intensity signal when there is no gas to be measured in the linear resonant cavity and the normalized light intensity signal when there is a gas to be measured in the linear resonant cavity ; S52. Calculate the absorption coefficient α of the gas to be measured according to the following formula: in, is the speed of light, is the ring-down time of the linear resonant cavity, is the normalized light intensity signal when there is no gas to be measured in the linear resonant cavity, It is the normalized light intensity signal when there is gas to be measured in the linear resonant cavity.

9. The linear cavity gas concentration measurement method based on optical feedback cavity enhancement as claimed in claim 6, characterized in that: In S6, the concentration of the gas to be measured is calculated according to the following formula: in, is the molecular number density of the gas to be measured, Select the absorption line intensity under the absorption line for the gas to be measured, is the normalized absorption line shape function of the gas.

10. The linear cavity gas concentration measurement method based on optical feedback cavity enhancement as claimed in claim 9, characterized in that: Said It can be obtained by the fitting area of ​​the Voigt curve and is the integral value of the absorption coefficient α.

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