A calibration-free cavity-enhanced absorption spectroscopy method for measuring reflectivity of an optical cavity

Abstract

The present application relates to the technical field of optical gas sensing, and particularly relates to a kind of cavity enhanced absorption spectroscopy methods of calibration-free cavity reflectivity, comprising the following steps: S1: the phase information of reflected light is extracted by detection system and error signal is obtained, and the optical cavity reflectivity at the target absorption line of the gas to be measured is obtained by error signal;Detection system includes laser, electro-optic modulator, three-terminal circulator, collimator, mode matching lens group, optical cavity, photodetector one, function generator, frequency mixer, acquisition card and computer;S2: the gas to be measured is introduced into the optical cavity, and the cavity enhanced absorption spectrum of the gas to be measured is obtained;S3: the concentration of the gas to be measured is obtained by inversion using the determined optical cavity reflectivity and cavity enhanced absorption spectrum.The present application does not need additional standard gas, avoids the error introduced by the uncertainty of standard gas concentration, and also makes up for the deficiency that traditional optical cavity ring-down technique is difficult to measure reflectivity when reflectivity is relatively low.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of optical gas sensing, and in particular to a calibration-free cavity-enhanced absorption spectroscopy method capable of measuring reflectivity of an optical cavity. BACKGROUND

[0002] With the rapid development of sensing technology, cavity-enhanced techniques such as cavity ring-down spectroscopy and cavity-enhanced absorption spectroscopy have also developed rapidly due to the advantages of optical cavities, such as extremely high detection sensitivity and the ability to increase the length of interaction between laser and the gas to be measured. Cavity-enhanced absorption spectroscopy has the characteristics of relatively simple experimental device, high sensitivity, and strong environmental adaptability, and is an important branch of high-sensitivity absorption spectroscopy. In the application process of cavity-enhanced absorption spectroscopy, the reflectivity of the cavity mirror is an important factor affecting the measurement accuracy, and therefore it is crucial to determine the reflectivity of the cavity mirror in cavity-enhanced absorption spectroscopy.

[0003] The current traditional method for measuring the reflectivity of the cavity mirror mainly uses cavity ring-down technology or standard gas samples to measure the reflectivity of the cavity mirror. The existing method mainly has the following problems:

[0004] 1. Although the method of using cavity ring-down technology to determine the reflectivity of the cavity mirror has the advantage of high sensitivity, when the reflectivity is relatively low, i.e. R<0.999, the decay time is too short, and the detector is difficult to detect the accurate decay time, so the reflectivity of the cavity mirror cannot be measured. However, R<0.999 belongs to the reflectivity range of the optical cavity used in cavity-enhanced absorption spectroscopy, and therefore the application of cavity ring-down technology in the cavity-enhanced absorption spectroscopy system with relatively low reflectivity is limited.

[0005] 2. The method of using standard gas to determine the reflectivity of the cavity mirror is easily affected by the inaccuracy of the gas sample concentration and the shift of the laser intensity, and has a large error.

[0006] In summary, how to design a calibration-free cavity-enhanced absorption spectroscopy method that can efficiently and conveniently measure the reflectivity of the optical cavity without using standard gas is a problem that needs to be solved at present. SUMMARY

[0007] To solve the above problems, the present application provides a calibration-free cavity-enhanced absorption spectroscopy method capable of measuring the reflectivity of an optical cavity, which can avoid the error introduced by the inaccuracy of the standard gas concentration, compensate for the deficiency of the traditional cavity ring-down technology in measuring the reflectivity when the reflectivity is relatively low, efficiently and conveniently measure the reflectivity of the optical cavity, and realize calibration-free cavity-enhanced absorption spectroscopy.

[0008] To achieve the above purpose, the present application proposes the following technical solution: a calibration-free cavity-enhanced absorption spectroscopy method capable of measuring the reflectivity of an optical cavity, comprising the following steps:

[0009] S1: phase information of reflected light is extracted by a detection system and an error signal is obtained, and optical cavity reflectivity at a target absorption line of a gas to be measured is obtained through the error signal;

[0010] S2: a gas to be measured is introduced into the optical cavity, and a cavity-enhanced absorption spectrum of the gas to be measured is obtained;

[0011] S3: the concentration of the gas to be measured is obtained by inversion using the measured optical cavity reflectivity and the cavity-enhanced absorption spectrum.

[0012] Further, the detection system in S1 comprises, in sequence along the light path direction, a laser, an electro-optical modulator, a three-port circulator, a collimator, a mode matching lens group and an optical cavity; the three-port circulator is connected with a photoelectric detector one; the electro-optical modulator is driven by a function generator, and the function generator is further connected with a frequency mixer.

[0013] Further, the optical cavity reflectivity in S1 is obtained by the following steps:

[0014] S11: the laser beam emitted by the laser is subjected to electro-optical phase modulation by the electro-optical modulator, and then is incident into the collimator through the three-port circulator, and the collimated beam after collimation by the collimator is coupled into the optical cavity through the mode matching lens group;

[0015] S12, the light beam reflected by the optical cavity passes through the three-port circulator and is received by the photoelectric detector one, so as to obtain the power signal of the reflected light beam and transmit it to the frequency mixer;

[0016] S13, the frequency mixer performs multiplication operation on the power signal of the reflected light beam and the local oscillation signal of the function generator, so as to extract the phase information of the reflected light and obtain the error signal;

[0017] S14, the reflectivity of the optical cavity is obtained according to the error signal.

[0018] Further, the error signal in S13 is represented by formula (1):

[0019]

[0020] Wherein, P c is the optical power of the carrier, P s is the optical power of the sideband introduced by the electro-optical modulator, Ω is the phase modulation frequency, and ω is the frequency of the laser beam;

[0021] Im() is the imaginary part of the complex number;

[0022] F() function is the reflection coefficient of the optical cavity, which is represented by formula (2):

[0023]

[0024] Wherein, L is the length of the optical cavity, c is the speed of light, r is the amplitude reflection coefficient of the cavity mirror, R is the equivalent reflectivity of the optical cavity; F * () is the conjugate of F().

[0025] Further, the three-port circulator comprises an incident end, an outgoing end and a reflection end, the incident end is connected to the electro-optical modulator, the outgoing end is connected to the collimator, and the reflection end is connected to the photoelectric detector one; the light beam modulated by the electro-optical modulator is incident into the collimator through the incident end and the outgoing end, and the light beam reflected back by the optical cavity passes through the outgoing end and the reflection end and is received by the photoelectric detector one.

[0026] Further, the detection system in S1 further comprises a data acquisition card and a computer; in S14, the error signal is normalized and fitted by the least square method through the data acquisition card and the computer, so as to obtain the reflectivity of the optical cavity.

[0027] Further, the mode matching lens group comprises a first mode matching lens and a second mode matching lens arranged in sequence along the optical axis; the collimated light beam collimated by the collimator is transmitted twice through the first mode matching lens and the second mode matching lens in sequence and then is incident into the optical cavity.

[0028] Further, the laser is an external cavity semiconductor laser, the line width of the laser is 5-10 kHz, and the scanning wavelength range of the laser is 1510-1595 nm.

[0029] Further, the collimator is a fiber collimator, the spot diameter of the light beam emitted by the collimator is ≤2.15 mm, and the divergence angle is ≤0.054°.

[0030] Further, the concentration of the to-be-measured gas in S3 is obtained by the following steps:

[0031] S31: the expression of the Beer-Lambert theorem is given:

[0032]

[0033] Wherein, I0 is the initial light intensity, I is the detection light intensity, P is the gas pressure, S(T) is the gas characteristic spectral line intensity, is a linear function, X is the to-be-measured gas concentration, L is the propagation distance of the laser in the gas medium, and a(v) is the absorption coefficient;

[0034] S32: the frequency domain integration and deformation operation are performed on both sides of formula (3), so as to obtain the expression of the absorbance A:

[0035]

[0036] S33: The equivalent absorption distance of the optical cavity without the to-be-detected gas is obtained according to the measured optical cavity reflectivity:

[0037]

[0038] Wherein, L eff is the equivalent absorption distance of the optical cavity, R is the optical cavity reflectivity of the optical cavity, and d is the cavity length of the optical cavity.

[0039] S34: The absorbance of the optical cavity with the cavity length d in a single light transmission is:

[0040] A s = PS(T) Xd (15)

[0041] S35: The equivalent reflectivity with the gas loss is:

[0042] R t = Rexp(-A s ) (16)

[0043] S36: The actual equivalent absorption distance of the optical cavity when the to-be-detected gas is introduced is:

[0044]

[0045] S37: The actual equivalent absorption distance of the optical cavity is substituted into the formula (4) in S32, and the following is obtained:

[0046]

[0047] Wherein, X is the concentration of the to-be-detected gas, and the value of X is solved, that is, the concentration value of the to-be-detected gas is obtained.

[0048] Compared with the prior art, the present application can achieve the following beneficial effects:

[0049] 1. The present application does not need to use a standard gas sample to calibrate the reflectivity of the optical cavity, can directly obtain the error signal for algorithm fitting, and obtains the reflectivity of the optical cavity, thereby avoiding the error introduced by the uncertainty of the standard gas concentration and the offset of the laser intensity.

[0050] 2. The present application has high sensitivity and can quickly measure the reflectivity of the optical cavity; meanwhile, the present application can be applied when the reflectivity of the optical cavity is relatively low or the decay time cannot be detected by building a long optical cavity, thereby making up for the deficiency of the optical cavity decay technology when the reflectivity of the optical cavity is low. DETAILED DESCRIPTION

[0051] Figure 1 is a structure schematic diagram of a calibration-free cavity enhanced absorption spectrum detection system provided by the embodiment of the present application;

[0052] Figure 2 is a fitting result diagram of the error signal when the laser center wavelength is 1531.59 nm according to an embodiment of the application;

[0053] Figure 3 is a cavity enhanced absorption spectrum diagram under different concentrations according to an embodiment of the application;

[0054] Figure 4 is a comparison diagram of the inversion gas concentration and the gas distribution machine gas distribution concentration according to an embodiment of the application.

[0055] The figure marks: laser 1, electro-optic modulator 2, function generator 3, three-terminal circulator 4, incident end 41, outgoing end 42, reflecting end 43, photoelectric detector one 5, frequency mixer 6, collimator 7, mode matching lens group 8, first mode matching lens 81, second mode matching lens 82, optical cavity 9, data acquisition card 10, computer 11, photoelectric detector two 12. DETAILED DESCRIPTION

[0056] In the following, embodiments of the application will be described with reference to the accompanying drawings, in which: Figure 1 - 4. In the following description, the same modules are denoted by the same reference signs. In the case of the same reference signs, their names and functions are also the same. Therefore, their detailed description will not be repeated.

[0057] In order to make the objectives, technical solutions and advantages of the application clearer, the following will further describe the application with reference to the accompanying drawings, 4 and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the application and do not constitute a limitation on the application. Figure 1

[0058] A calibration-free cavity enhanced absorption spectroscopy method for measuring optical cavity reflectivity, comprising the following steps:

[0059] S1: Extracting the phase information of the reflected light by the detection system and obtaining the error signal, and obtaining the optical cavity reflectivity at the target absorption line of the gas to be measured through the error signal. Wherein, the detection system comprises a laser 1, an electro-optic modulator 2, a three-terminal circulator 4, a collimator 7, a mode matching lens group 8, and an optical cavity 9 arranged in sequence along the light path direction; the three-terminal circulator 4 is connected with a photoelectric detector one 5, the electro-optic modulator 2 is driven by a function generator 3, and the function generator 3 is also connected with a frequency mixer 6.

[0060] ​The laser 1 is an external cavity semiconductor laser, the line width of the laser 1 is 5-10 kHz, which is much smaller than the mode line width of the optical cavity 9, and has little effect on the error signal; the scanning wavelength range of the laser 1 is 1510-1595 nm, and the scanning speed is 0.3-0.7 nm / s, and in the embodiment, the scanning speed is 0.5 nm / s; the laser beam emitted by the laser 1 is subjected to electro-optical phase modulation by the electro-optical modulator 2, and the frequency of the local oscillation signal generated by the function generator 3 is 20 MHz.

[0061] The three-port circulator 4 includes an incident end 41, an outgoing end 42 and a reflecting end 43, the incident end 41 is connected to the output end of the electro-optical modulator 2, the outgoing end 42 is connected to the input end of the collimator 7, and the reflecting end 43 is connected to the photoelectric detector 5; the light beam modulated by the electro-optical modulator 2 is incident into the collimator 7 through the incident end 41 and the outgoing end 42, and the light beam reflected back by the optical cavity 9 passes through the outgoing end 42 and the reflecting end 43 and is received by the photoelectric detector 5.

[0062] The collimator 7 is a fiber collimator, the spot diameter of the light beam emitted by the collimator 7 is ≤2.15 mm, and the divergence angle is ≤0.054°; the collimated light beam of the laser beam after collimation by the collimator 7 is incident into the mode matching lens group 8. The mode matching lens group 8 includes a first mode matching lens 81 and a second mode matching lens 82 arranged in sequence along the optical axis, and the first mode matching lens 81 and the second mode matching lens 82 are ultraviolet fused quartz double-convex lenses with focal lengths of 30 mm and 50 mm respectively, and the collimated light beam after collimation by the collimator 7 is transmitted through the first mode matching lens 81 and the second mode matching lens 82 twice in sequence and then is incident into the optical cavity 9, so as to be more accurately matched with the intracavity mode of the optical cavity 9; the optical cavity 9 is a Fabry-Perot resonant cavity, and the cavity length of the optical cavity 9 is 80 mm.

[0063] The optical cavity reflectivity is obtained by the following steps:

[0064] S11: The laser beam emitted by the laser 1 is subjected to electro-optical phase modulation by the electro-optical modulator 2, and then is incident into the collimator 7 through the three-port circulator 4, and the collimated light beam after collimation by the collimator 7 is coupled into the optical cavity 9 through the mode matching lens group 8.

[0065] S12: The light beam reflected back by the optical cavity 9 passes through the three-port circulator 4 and is received by the photoelectric detector 5, so as to obtain the power signal of the reflected light beam and transmit the power signal to the frequency mixer 6.

[0066] S13: The frequency mixer 6 performs multiplication operation on the power signal of the reflected light beam and the local oscillation signal of the function generator 3, so as to extract the phase information of the reflected light and obtain the error signal;

[0067] S14: The optical cavity reflectivity is obtained according to the error signal.

[0068] The detection system further comprises a data acquisition card 10 and a computer 11, and the error signal is normalized and least square fitted by the data acquisition card 10 and the computer 11 to obtain the optical cavity reflectivity; the detection system further comprises a photodetector two 12, the photodetector two 12 is used for detecting the optical power signal of the transmitted light of the optical cavity 9, and the absorption spectrum of the transmitted light is obtained by the data acquisition card 10 and the computer 11, and the concentration of the to-be-detected gas is obtained by inverting the transmission spectrum.

[0069] The error signal in S13 is represented by formula (1):

[0070]

[0071] Wherein, P c is the optical power of the carrier, P s is the optical power of the sideband introduced by the electro-optical modulator 2, Ω is the phase modulation frequency, and ω is the frequency of the laser beam;

[0072] Im() is the imaginary part of the complex number;

[0073] The F() function is the reflection coefficient of the optical cavity 9, which is represented by formula (2):

[0074]

[0075] Wherein, L is the length of the optical cavity 9, c is the speed of light, r is the amplitude reflection coefficient of the cavity mirror, R is the equivalent reflectivity of the optical cavity 9; F * () is the conjugate of the F() function.

[0076] S2: introducing the to-be-detected gas into the optical cavity 9, and obtaining the cavity-enhanced absorption spectrum of the to-be-detected gas cavity.

[0077] S3: using the measured optical cavity reflectivity and the cavity-enhanced absorption spectrum to obtain the concentration of the to-be-detected gas. In this embodiment, acetylene gas is used as the to-be-detected gas to conceptually describe the calibration-free cavity-enhanced absorption spectrum method for measuring the optical cavity reflectivity, and the absorption spectrum line of acetylene at 1531.59 nm is selected as the target absorption line; Figure 2 is the error signal when the central wavelength of the laser is 1531.59 nm and the fitting result of the error signal, wherein the No. 1 line is the original data, and the No. 2 line is the data after fitting.

[0078] Specifically, the laser emitted by the laser 1 scans around 1531.59 nm, and the electro-optical modulator 2 performs electro-optical phase modulation on the scanning laser beam at 20 MHz; the modulated laser beam passes through the three-port circulator 4 and enters the fiber collimator 7, and the collimated beam after passing through the collimator 7 is coupled into the optical cavity 9 through the mode matching lens group 8, and the cavity length of the optical cavity 9 is 80 mm; the reflected beam reflected by the optical cavity 9 is incident into the photodetector 1 5 through the three-port circulator 4, the mixer 6 multiplies the detection signal of the photodetector 1 5 and the 20 MHz oscillation signal of the function generator 3 to obtain an error signal; the data acquisition card 10 and the computer 1 1 receive the error signal and perform normalization processing and least squares fitting on the error signal to obtain the optical cavity reflectivity value at 1531.59 nm; since the laser emitted by the laser 1 is a scanning signal with 1531.59 nm as the center wavelength, the optical cavity reflectivity values at multiple wavelengths can be obtained; specifically, 500 reflectivity values are collected, and the data set is processed to obtain the optical cavity reflectivity at 1531.59 nm as 0.9982, and the variance is 2 x 10 -5 .

[0079] The acetylene and nitrogen gas are proportioned by the gas proportioning machine to obtain acetylene gas with known different concentrations, and the acetylene gas with different concentrations is respectively introduced into the optical cavity 9, the transmitted beam after the laser beam passes through the optical cavity 9 is received by the photodetector 2 1 2, and the cavity enhanced absorption spectrum is obtained by the data acquisition card 1 0 and the computer 1 1, Figure 3 As shown in the figure, the cavity enhanced absorption spectrum under different concentrations is shown, and the concentration of the gas to be measured is obtained by transmission spectrum inversion using the determined optical cavity reflectivity, Figure 3 From top to bottom, the first line is 60 ppm, the second line is 40 ppm, the third line is 20 ppm, and the fourth line is 10 ppm; Figure 4 As shown in the figure, the comparison between the inverted gas concentration and the gas proportioning concentration of the gas proportioning machine is shown, wherein the asterisk represents the gas proportioning concentration of the gas proportioning machine, and the black dot represents the concentration of the gas to be measured obtained by inversion; it can be seen from the figure that the two have good consistency, therefore, the method of determining the optical cavity reflectivity by the error signal has high sensitivity and feasibility, and the calibration-free cavity enhanced absorption spectrum gas detection based on the optical cavity reflectivity also has high sensitivity.

[0080] The concentration of the gas to be measured is obtained by the following steps:

[0081] S31: Give the expression of the Beer-Lambert theorem:

[0082]

[0083] Wherein, I0 is the initial light intensity, I is the detection light intensity, P is the gas pressure, S(T) is the gas characteristic spectral line intensity, is a linear function, X is the concentration of the gas to be measured, L is the propagation distance of the laser in the gas medium, and a(v) is the absorption coefficient;

[0084] S32: Frequency domain integration and deformation operation are performed on both sides of equation (3) to obtain an expression of absorbance A:

[0085]

[0086] S33: The equivalent absorption distance of optical cavity 9 without the introduction of the gas to be measured is obtained according to the measured optical cavity reflectivity:

[0087]

[0088] wherein L eff is the equivalent absorption distance of the empty cavity, R is the optical cavity reflectivity of the empty cavity, and d is the optical cavity length;

[0089] S34: For optical cavity 9 with a cavity length of d, the absorbance of a single light transmission is:

[0090] A s = PS(T) Xd (24)

[0091] S35: The equivalent reflectivity with gas loss is:

[0092] R t = R exp(-A s ) (25)

[0093] S36: When the gas to be measured is introduced, the actual equivalent absorption distance of optical cavity 9 is:

[0094]

[0095] S37: The actual equivalent absorption distance of optical cavity 9 is substituted into equation (4) in S32 to obtain:

[0096]

[0097] wherein X is the concentration of the gas to be measured, and solving the value of X obtains the concentration value of the gas to be measured.

[0098] It should be understood that the various forms of flow shown above can be used to reorder, add or delete steps. For example, the steps described in the present disclosure can be executed in parallel, sequentially or in a different order, as long as the desired results of the technical solutions of the present disclosure can be achieved, which is not limited herein.

[0099] The above detailed description does not limit the scope of the application. Various modifications, combinations, sub-combinations and alternatives can be made to the detailed embodiment within the scope of the application. Any modification, equivalent replacement and improvement made without departing from the spirit and principle of the application shall fall within the scope of the application.

Claims

1. A calibration-free cavity-enhanced absorption spectroscopy method for determining reflectivity of an optical cavity, the method comprising: It comprises the following steps: S1: extracting the phase information of reflected light by a detection system and obtaining an error signal, and obtaining the optical cavity reflectivity at the target absorption line of the measured gas through the error signal; The detection system in S1 comprises a laser (1), an electro-optic modulator (2), a three-port circulator (4), a collimator (7), a mode matching lens group (8) and an optical cavity (9) arranged in sequence along the light path direction; the three-port circulator (4) is connected with a photoelectric detector (5), the electro-optic modulator (2) is driven by a function generator (3), and the function generator (3) is also connected with a frequency mixer (6); The optical cavity reflectivity in S1 is obtained by the following steps: S11: the laser beam emitted by the laser (1) is subjected to electro-optic phase modulation by the electro-optic modulator (2), then is incident into the collimator (7) through the three-port circulator (4), and the collimated beam after collimation by the collimator (7) is coupled into the optical cavity (9) through the mode matching lens group (8); S12: the light beam reflected by the optical cavity (9) passes through the three-port circulator (4) and is received by the photoelectric detector (5), so as to obtain the power signal of the reflected light beam and transmit it to the frequency mixer (6); S13: the frequency mixer (6) performs multiplication operation on the power signal of the reflected light beam and the local oscillation signal of the function generator (3), so as to extract the phase information of the reflected light and obtain the error signal; S14: obtaining the optical cavity reflectivity according to the error signal; The error signal in S13 is represented by formula (1): (1) wherein is the optical power of the carrier, is the optical power of the sideband introduced by the electro-optical modulator (2), is the phase modulation frequency, is the laser beam frequency; is the imaginary part of the complex number; The function is the reflection coefficient of the optical cavity (9) and is expressed by equation (2): (2) wherein is the length of the optical cavity (9), is the speed of light, is the amplitude reflection coefficient of the cavity mirror, , is the equivalent reflectivity of the optical cavity (9); is the conjugate of the function; S2: introducing the measured gas into the optical cavity (9) and obtaining the cavity enhanced absorption spectrum of the measured gas; S3: using the measured optical cavity reflectivity and the cavity enhanced absorption spectrum to obtain the concentration of the measured gas by inversion.

2. The method of claim 1, wherein, The three-port circulator (4) comprises an incident end (41), an exit end (42) and a reflection end (43), the incident end (41) is connected with the electro-optic modulator (2), the exit end (42) is connected with the collimator (7), and the reflection end (43) is connected with the photoelectric detector (5); the light beam modulated by the electro-optic modulator (2) is incident into the collimator (7) through the incident end (41) and the exit end (42), and the light beam reflected by the optical cavity (9) passes through the exit end (42) and the reflection end (43) and is received by the photoelectric detector (5).

3. The method of claim 2, wherein, The detection system in S1 further comprises a data acquisition card (10) and a computer (11); in S14, the error signal is normalized and fitted by the least square method through the data acquisition card (10) and the computer (11), so as to obtain the optical cavity reflectivity.

4. The method of claim 3, wherein, The mode matching lens group (8) comprises a first mode matching lens (81) and a second mode matching lens (82) arranged in sequence along the optical axis direction; the collimated beam after collimation by the collimator (7) is incident into the optical cavity (9) after being transmitted twice through the first mode matching lens (81) and the second mode matching lens (82) in sequence.

5. The method of claim 4, wherein, The laser (1) is an external cavity semiconductor laser, the line width of the laser (1) is 5-10 kHz, and the scanning wavelength range of the laser (1) is 1510-1595 nm.

6. The method of claim 5, wherein the calibration-free cavity-enhanced absorption spectroscopy method of determining reflectivity of an optical cavity is characterized by, The collimator (7) is a fiber collimator (7), and a spot diameter of an exiting light beam of the collimator (7) is less than or equal to 2.15 mm, and a divergence angle is less than or equal to 0.054°.

7. The calibration-free cavity-enhanced absorption spectroscopy method of determining the reflectivity of an optical cavity according to any of claims 1 - 6, characterized in that, The concentration of the to-be-tested gas in S3 is obtained by the following steps: S31: a Beer-Lambert law expression is given; (3) wherein, is the initial light intensity, is the probe light intensity, is the gas pressure, is the gas characteristic spectral line intensity, is a linear function, is the concentration of the gas to be measured, is the propagation distance of the laser in the gas medium, is the absorption coefficient; S32: frequency domain integration and deformation operation are performed on both sides of formula (3) to obtain the expression of absorbance of absorbance (4) S33: an equivalent absorption distance of the optical cavity (9) without the to-be-tested gas is obtained according to the measured optical cavity reflectivity; (5) wherein equivalent absorption distance for a cavity; S34: for the optical cavity (9) with a cavity length d, an absorbance of single light transmission is: (6) S35: an equivalent reflectivity with gas loss is: (7) S36: when the to-be-tested gas is introduced, an actual equivalent absorption distance of the optical cavity (9) is: (8) S37: the actual equivalent absorption distance of the optical cavity (9) is substituted into the formula (4) in S32, and the following formula is obtained: (9) wherein, is the concentration of the gas to be measured, and solving the value of the concentration of the gas to be measured is obtained.

Images