A CO2 optical cavity with long optical path and detection device

Abstract

The present invention discloses a CO2 optical cavity with a long optical path and a detection device, wherein the optical cavity comprises a conical cavity, the longitudinal section of which is trapezoidal, and the inner wall has a silver layer. The detection device comprises the optical cavity, a quantum cascade laser, a collimating lens, an air inlet, an air outlet, a mercury cadmium telluride detector, a digital signal processor (DSP), an air pressure controller, and an air pressure sensor; the air inlet is connected to the small end face of the optical cavity, and the air intake is controlled by a proportional valve and a first solenoid valve, the air outlet is arranged at the large end face of the optical cavity, and is connected to a second solenoid valve and an air pump, the quantum cascade laser emits two laser signals of different wavelengths, which respectively pass through the collimating lens and then enter the optical cavity at a set angle, the signal emitted by the optical cavity is detected by the mercury cadmium telluride detector and then input into the DSP for processing, and the CO2 concentration is calculated in the DSP according to a predetermined model. The present invention can realize intelligent and automatic detection, improve detection efficiency and accuracy, and reduce manual intervention and errors.

Description

Technical Field

[0001] The invention belongs to the technical field of CO2 detection, and relates to a CO2 optical cavity with a long optical path and a detection device. Background Art

[0002] As the global industrialization process accelerates, human activities such as the burning of large amounts of fossil fuels and deforestation have caused the concentration of carbon dioxide (CO2) in the atmosphere to continue to rise, the problem of global climate change is becoming increasingly serious, with rising sea levels and increasing extreme climate events. Accurate detection of CO2 concentration is of great significance for studying climate change trends, evaluating the effectiveness of carbon reduction measures, and environmental monitoring. However, traditional CO2 detection methods, such as chemical absorption and gas chromatography, are complex to operate, slow to respond, and lack precision, making it difficult to meet modern high-precision, real-time detection needs.

[0003] Optical detection technology has obvious advantages in the field of gas detection. It is based on the absorption characteristics of CO2 molecules to light of a specific wavelength. It has the advantages of non-contact measurement, fast response speed, and high precision. By measuring the change in light intensity caused by CO2 molecules absorbing light, its concentration can be determined, avoiding the interference factors of traditional chemical methods. However, existing optical cavities have many defects and shortcomings in CO2 detection. The first is the low sensitivity. From the perspective of fundamental limitations, based on the principle of molecular absorption of light of specific wavelengths in the interaction between light and CO2 molecules, according to the Lambert-Beer law, the absorbance change of low-concentration CO2 is small when the optical path is short. For example, in a simple optical cavity with an optical path of only a few centimeters, trace levels of CO2 molecules (below 100 ppm) have weak absorption, and the detection signal is difficult to distinguish from the noise, because at low concentrations, few photons are absorbed and the intensity change of the electrical or optical signal is not obvious. At the same time, the detectors also have limitations. Existing detectors have limitations in detecting weak light absorption signals. For example, when the noise equivalent power (NEP) of the photodetector is high, the weak light intensity changes caused by the absorption of low-concentration CO2 cannot be effectively detected. When the NEP of a traditional photodiode detector is at the nanowatt level, light intensity changes less than this level will be masked by noise. Secondly, the optical path is short. In terms of structural design, many existing optical cavities have simple structures and are not designed for efficient detection of low-concentration CO2. For example, the early straight-tube optical cavity only had two parallel reflectors, and the limited number of light reflections resulted in a short optical path. Compared with the White pool structure, its optical path is only a fraction or even a few tens of percent; there are also problems with the reflectors. Their reflectivity is not high enough. For example, the reflectivity of ordinary reflectors is around 90% - 95%. After multiple reflections, the light intensity decays rapidly and cannot maintain a sufficient optical path. In addition, the curvature accuracy and surface flatness of the reflectors are not high enough, which will cause the light to scatter and deviate from the ideal optical path during multiple reflections, limiting the effective increase in the optical path. In summary, it can be seen that the existing optical cavity cannot accurately obtain the absorption spectral characteristics of the CO2 gas to be measured due to its short optical path, resulting in large measurement errors and low sensitivity. For some application scenarios that require high-precision spectral analysis, short-path optical cavities are difficult to provide sufficient resolution. Because the optical path is short and the number of round trips of light in the cavity is limited, the spectrum broadening effect is relatively obvious, and the fine structure in the absorption spectrum of CO2 molecules cannot be accurately distinguished.

[0004] Based on this, the present invention provides a newly designed optical cavity and a corresponding detection device for CO2 detection to improve its detection accuracy and reliability. Summary of the invention

[0005] The purpose of the present invention is to provide a CO2 optical cavity and a detection device with a long optical path in view of the deficiencies in the prior art.

[0006] The technical solution adopted by the present invention is:

[0007] The invention discloses a CO2 optical cavity with a long optical path, comprising a conical cavity, the longitudinal section of which is trapezoidal, and the inner wall of the cavity is plated with a silver layer.

[0008] In the above technical solution, further, the incident light is controlled to be incident from the small end face of the conical cavity at an angle as parallel to the end face as possible.

[0009] Furthermore, the angle between the incident light and the small end face does not exceed 0.5°.

[0010] Furthermore, the optical cavity satisfies:

[0011]

[0012] Among them, H is the axial length of the conical chamber, a is the diameter of the small end surface of the conical chamber, is the angle between the incident light and the small end face, and n is the number of reflections of the light beam in the chamber.

[0013] A detection device based on a long optical path CO2 optical cavity, comprising the optical cavity as described in any of the above items, and also comprising a quantum cascade laser, a collimating lens, an air inlet, an air outlet, a mercury cadmium telluride detector, a digital signal processor (DSP), an air pressure controller, and an air pressure sensor; the air inlet is connected to the small end face of the optical cavity, and the air intake is controlled by a proportional valve and a first solenoid valve, the air outlet is arranged at the large end face of the conical optical cavity, and is connected to a second solenoid valve and an air pump, the air pressure controller controls the first solenoid valve, the proportional valve, the air pump, and the second solenoid valve according to a detection signal of the air pressure sensor, and maintains the air pressure in the optical cavity stable during the detection process; the quantum cascade laser emits two laser signals of different wavelengths, which respectively pass through the collimating lens and then enter the optical cavity at a set angle through an adjustable reflector, the signal emitted by the optical cavity is detected by the mercury cadmium telluride detector and then input into the digital signal processor DSP for processing, and the CO2 concentration is calculated in the digital signal processor DSP according to a predetermined model.

[0014] Furthermore, the quantum cascade laser emits two laser signals of different wavelengths, and the wavelength is determined as follows: first, a spectral scan is performed, and a light source with a wide-band emission capability or a tunable laser is used to scan CO2 and possible interfering gases respectively to obtain an absorption spectrum, which can respectively display the absorption peak positions and intensities of CO2 and interfering gases, and then data analysis is performed to find out the area with obvious absorption peaks and relatively weak absorption of interfering gases, and two suitable wavelengths are selected in this area, one is selected near the center of the CO2 absorption peak to obtain the maximum absorbance change, and the other is selected at or near the shoulder of the CO2 absorption peak where the absorption is weaker.

[0015] Furthermore, the model is:

[0016] The two wavelengths are λ1 and λ2. At these two wavelengths, the absorbance of the mixed gas (including CO2 and interfering gas) is measured respectively. Assume that the concentration of CO2 is c CO2 , the concentration of the interfering gas is c 干扰 , the optical path length is l.

[0017]

[0018] Among them, λ1 and λ2 are the two wavelengths emitted by the laser, A1 and A2 are the absorbance of the gas to be measured at λ1 and λ2 respectively. is the molar absorption coefficient of CO2 at λ1, is its molar absorption coefficient at λ2, is the molar absorption coefficient of the interfering gas at λ1, is its molar absorption coefficient at λ2, c CO2 and c 干扰 They are the concentrations of CO2 and other interfering gases in the gas to be measured, respectively.

[0019] Compared with the prior art, the beneficial effects of the present invention are:

[0020] The optical cavity and detection device provided by the present invention have at least the following advantages: First, the detection sensitivity is improved, which overcomes the problem that the absorbance change is small and the signal is easily masked by noise when detecting low-concentration CO2 due to the short optical path and limited detector sensitivity of the traditional optical cavity; second, the optical path is increased to improve the situation that the existing optical cavity has a simple structure, low reflectivity of the reflector and insufficient precision resulting in a short optical path, such as the limited optical path of the traditional straight-tube optical cavity, the rapid attenuation of light after multiple reflections of the ordinary reflector and the unstable optical path, thereby achieving more accurate and efficient detection of CO2. The present invention is committed to realizing intelligent and automated detection, improving detection efficiency and accuracy, and reducing manual intervention and errors. The detection device is not only suitable for traditional detection environments, such as industrial production, environmental monitoring and other fields, but can also be expanded to more fields such as medical treatment and scientific research to meet detection needs in different scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic structural diagram of the long optical path conical optical cavity of the present invention;

[0022] Figure 2 It is a schematic diagram of the reflected light path of a certain cross section in the optical cavity.

[0023] Figure 3 It is a schematic diagram of the optical path analysis results in a certain cross section according to the present invention.

[0024] Figure 4 It is a schematic diagram of the optical path reflection of the longitudinal section of the long optical path conical optical cavity of the present invention.

[0025] Figure 5 It is a schematic diagram of the structure of the detection device of the present invention. DETAILED DESCRIPTION

[0026] In order to make the above-mentioned purpose, features and advantages of the present invention more obvious and easy to understand, the specific implementation mode of the present invention is described in detail below in conjunction with the accompanying drawings. In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below. The technical features in each embodiment of the present invention can be combined accordingly without conflicting with each other.

[0027] The present invention provides a CO2 optical cavity and a detection device with a long optical path, such as Figure 1 As shown in the figure, the long optical path CO2 optical cavity uses a silver plating process inside to better reflect light, and the long optical path optical cavity greatly improves the detection sensitivity. The long optical path of the conical long optical path silver-plated optical cavity is achieved by multiple reflections of light on the silver-plated inner wall. The light entering from the small end of the cone is reflected on the gradually expanding cavity wall. The length of the reflection path increases each time. If the angle between the cone generatrix and the axis is , the number of reflections is , each reflection axial distance The total optical path is the sum of the lengths of each reflection path; the reflectivity of the silver-plated inner wall is close to (Calculated by the Fresnel reflection formula based on the optical constant of silver), but in reality, due to factors such as uneven thickness of the silver coating, surface roughness and impurities, the reflectivity is (If there are defects in the silver plating process, it may drop to In practice, the silver plating process can be optimized to improve stability and value); in terms of energy calculation, when coherent superposition is not considered, the initial energy is set , reflectivity , the energy after one reflection is , After the second reflection (like joule, After the second reflection, the energy is reduced). When considering the coherent superposition (assuming an ideal situation), a standing wave is formed in the cavity. coherent superposition modes, with a total energy of , such as the coherent superposition of two beams The two beams have equal amplitudes and a phase difference of When the energy is increased by 40 times, the energy increases by 40 times. Energy concentration can enhance the interaction strength between light and CO2 molecules. According to the Lambert-Beer law, it can make the absorbance change of the same concentration of CO2 more obvious to improve sensitivity, and can also improve signal strength to distinguish noise and improve accuracy. This energy concentration characteristic is the uniqueness of this optical cavity compared to other long optical path optical cavities. The optical cavity of the traditional reflector combination simply reflects light, and there is no coherent superposition of the conical structure to concentrate energy. Even if the reflectivity is similar, this optical cavity has unique advantages in sensitivity and accuracy.

[0028] Taking CO2 detection as an example, for extremely low concentrations of CO2 gas (such as less than 1 ppm), long optical path optical cavities can allow light to fully interact with CO2 molecules. Due to the long optical path, even low concentrations of CO2 can produce sufficiently strong absorption signals, so that its concentration can be accurately measured. Conical optical cavities have many advantages. In terms of mode selection characteristics, its geometric shape helps to suppress higher-order modes. The loss of higher-order mode light field distribution at the cavity wall is greater than that of the fundamental mode, and it can output high-quality fundamental mode laser beams. The special structure can achieve single-mode output, such as improving signal transmission accuracy and efficiency and reducing mode dispersion in optical fiber communications. In terms of energy focusing characteristics, it can concentrate light energy axially, such as laser cutting and welding, which can increase energy density and enhance the interaction effect with materials; it can also enhance the field strength at specific locations in the cavity, providing suitable conditions for nonlinear optical experiments. In terms of thermal management advantages, it has certain advantages in heat dissipation. Heat is more evenly distributed and conducted on the cavity wall. When high-power lasers are running, it can effectively conduct heat, avoid problems such as optical cavity deformation and refractive index changes, improve thermal stability, and ensure stable and reliable laser output.

[0029] The following is a detailed analysis and explanation of it with specific examples:

[0030] In this example, the CO2 optical cavity adopts a conical optical cavity. First, the conical cavity shell is manufactured by precision machining technology, and then a high-reflectivity silver layer is plated on the inner wall of the cavity by chemical plating. In this example, the bottom radius of the conical cavity is 5 cm, and the height of the conical cavity is 10 cm.

[0031] 1. Relationship between optical path and conical cavity geometry

[0032] In a conical optical cavity, the optical path is closely related to the apex angle of the cone and the incident angle θ. The light is reflected multiple times on the conical cavity wall to increase the optical path. Assume that the apex angle of the cone is , the incident angle of the light in the cavity relative to the normal of the conical cavity wall is θ. According to the geometric relationship and the law of reflection, when the light is reflected once on the conical cavity wall, the angle of change in the propagation direction of the light is .

[0033] 2. Principle and analysis of optical path extension

[0034] A. Derivation of the optical path calculation formula

[0035] Assuming that the light is reflected n times in the circular cavity, the total angle of rotation of the light is For the round pheasant cavity, according to the geometric shape of the round pheasant, Angle of cone When the light goes around the conical cavity, .so ,Right now .

[0036] Optical Path Calculation: Assume that the generatrix length of the conical cavity is According to the geometric relationship, the optical path length increases with each reflection . Then after The total optical path after reflection ,Will Substituting into .

[0037] B. Find the best and

[0038] In order to make the optical path The longest, yes Ask about The derivative of . Let , according to the derivation formula ,in , can be obtained .

[0039] make ,Right now .

[0040] Assume that the conical optical cavity starts from the vertex and the axial distance is The cross-sectional radius at , and the semi-apex angle of the cone is ,but .right Derivation .Depend on , can be obtained . It represents the length of the propagation path of the light in the conical optical cavity. When the light propagates in a certain direction in the cone, the path length is caused by a small radius change along the cone direction With Angle The cosine value of is determined, where the z axis is assumed to be the axis of the optical cavity. is the optical path length, and the component perpendicular to the z-axis is .

[0041] For a certain cross section (in arrive The optical path length between .

[0042] For a certain section of the conical optical cavity, assuming that the single optical path length of light propagating in the section is L, the radius of the cone is , the incident angle is , then the single optical path length L = 2r ;like Figure 2 As shown in the figure, in this section, L1, L2, L3...Ln are the optical paths of the incident and reflected light. It can be seen that in triangles AOB and COB, AO=CO, OB=BO, and because the angle of incidence is equal to the angle of reflection, triangles AOB and COB are congruent, that is, L1=L2. Similarly, L2=L3 can be proved. In summary, the optical paths reflected in the cross section are equal, and the total optical path is NL. Based on the single optical path formula L=2r , from the perspective of the global cumulative path, rather than the path optical path of a single reflection, the total optical path is Proportional, taking r as 12.5 as an example, take The single optical path length is 0.1, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, such as Figure 3 ;

[0043] Depend on Figure 3 As shown, the total optical path L in the same section is Closely related. When it is 0.1°, the optical path is 24.99995, and the optical path effect is the best. Although the light is not reflected in the same cross section in practice, the ideal discussion on the relationship between the optical path and θ in the same cross section is of great significance. This lays a theoretical foundation for understanding the basic relationship between the optical path and the angle of incidence. Through this ideal model analysis, it can be preliminarily clarified that there is a correlation between the two and the optical path effect is better when the incident angle is smaller. This will help to clarify the general direction in the subsequent actual design and optimization of the optical cavity. However, in actual applications, it will be restricted by factors such as the space limitations of the actual device and the production cost. For example, in some portable CO2 detection equipment, due to limited space, It cannot be too small. In some large fixed monitoring stations, factors such as the propagation efficiency of light in the optical cavity, the focusing effect of light at different incident angles, and the matching with the detector must be considered. If the accuracy requirement is very high, it can be appropriately increased. To achieve the best optical performance, increase the number of reflections in the cavity or change the propagation direction to obtain a longer optical path.

[0044] Depend on Figure 4It can be seen that when light is irradiated into the wall cavity, according to the law of reflection of light, the reflection angle is equal to the incident angle. From the perspective of geometric relationship, the incident light and the reflected light emitted from the same point propagate in the same medium. In the same propagation time, their propagation distance (path length) is equal. Define the maximum diameter b, minimum diameter a and height h of the optical cavity, and the incident angle θ of the light. The cross-section of the conical optical cavity is regarded as a trapezoid. The relevant parameters and geometric relationships of the trapezoid are as follows:

[0045] Let the trapezoid be an isosceles trapezoid with a height of (H=h), upper base length , bottom length .

[0046] 1. Calculation of reflection angle

[0047] The initial incident angle of the light is known to be , then the first reflection angle ; Second reflection angle ; The third reflection angle . That is to say, the nth reflection angle is deduced For arrive Reflection angle:

[0048] when hour, .

[0049] when hour, .

[0050] when hour, .

[0051] …

[0052] when hour,

[0053] 2. Calculation of total optical path

[0054] Assuming that the distance from the incident point to the reflection point is l0, and the increment from the upper base a to l0 can be ignored, then l0=a,

[0055] For the first reflection, it can be seen from the figure

[0056] ∠MAE=90°+ ∠EAB=90°- ∠AEB=90°-3

[0057] Then we have:

[0058]

[0059]

[0060] For the second reflection, it can be seen from the figure: ∠EBF=90°- ∠BFE=90°-5 , inferred

[0061] Right now

[0062] By analogy, for the Secondary reflection :

[0063]

[0064]

[0065] Therefore, the total optical path length is

[0066]

[0067] ;

[0068] 3. Calculation of height H

[0069]

[0070] By analogy, for the Secondary reflection

[0071]

[0072] So the total height is

[0073]

[0074] Assume that the upper base of the trapezoid a=21, the lower base b=25, and the angle of incidence .

[0075] When n=1,

[0076] When n=2,

[0077] When n=3,

[0078] …

[0079] When n=30,

[0080] In summary, when the number of reflections n=30, the total height

[0081] Total optical path .

[0082] On the contrary, assuming that the upper base of the trapezoid a=21, the height H=150, and the angle of incidence

[0083] According to the above formula, we can get

[0084]

[0085] So when H=150, a=21,

[0086] The above-mentioned step-by-step recursive calculation method is used to carry out detailed calculation of the reflection and propagation path of light in the structure, and a method for calculating the number of reflections when the upper base and height are known is obtained; in the actual optical system design, the geometric parameters can be determined according to the design requirements, and the number of reflections can be calculated using the formula to ensure that the cumulative effect meets the requirements, and then the parameters can be adjusted according to the results to optimize the performance.

[0087] Based on the above optical cavity, the present invention also provides a corresponding CO2 detection optical device, such as Figure 5 As shown, it includes a conical optical cavity 6, a quantum cascade laser 1, a collimating lens 2, a first solenoid valve 3, a proportional valve 4, an air inlet 5, a second solenoid valve 7, an air pump 8, an air outlet 9, a mercury cadmium telluride detector 10, a digital signal processor (DSP) 11, an air pressure controller 12, and an air pressure sensor 13. The air inlet 5 is connected to the small end of the conical optical cavity 6, and the air intake is controlled by the proportional valve 4 and the first solenoid valve 3. The air outlet 9 is arranged at the large end of the conical optical cavity 6, and is connected to the second solenoid valve 7 and the air pump 8. The optical collimator in the detection device adopts a lens collimator, i.e., a collimating lens 2, which is installed near the top corner of the conical chamber through a high-precision alignment and fixing process, wherein the outgoing light of the collimating lens 2 can enter the cavity at a set angle through an adjustable reflector.

[0088] The specific working process of the device is as follows: after the system is powered on, the air pressure controller opens the air inlet solenoid valve and the air outlet solenoid valve, and turns on the air pump and the control proportional valve, so that the gas in the cavity begins to be replaced by the gas to be tested in the external environment and maintains low pressure stability. The quantum cascade laser emits the required frequency band laser, and then enters the conical optical cavity through the collimating lens. At this time, the CO2 to be tested has been injected into the cavity through the air inlet, and the air outlet is closed after a certain period of time, and then the air inlet is closed. The light is totally reflected in the cavity, and different concentrations of CO2 will correspond to different reflection transmittances, so that the spectrum of the reflected and transmitted light signal has corresponding differences. Finally, the mercury cadmium telluride detector transmits the light signal to the photoelectric sensor and converts it into an electrical signal, which is fed back to the digital signal processor (DSP) for further processing and real-time display of the corresponding results. In the above device, a stable air pressure environment can be effectively maintained by setting an air pressure controller. Because the absorption characteristics of CO2 molecules to light are affected by air pressure, the collision frequency and energy level distribution of CO2 molecules will change under different air pressures, thereby affecting the absorption of light of a specific wavelength. When the air pressure is unstable, the absorption spectrum may broaden or shift, which is not conducive to the accurate detection of CO2 concentration. The air pressure controller can stabilize the air pressure in the cavity at the set value, ensuring the stability of the CO2 absorption characteristics during detection to improve accuracy.

[0089] According to the Lambert-Beer law and the pre-established mathematical model of the absorption characteristics of CO2 and interfering gases at these two wavelengths, the concentration c of CO2 can be calculated.

[0090] First, determine the wavelength: select two specific wavelengths, which should have different absorption characteristics for CO2 and interfering gas. Assume that the two wavelengths are λ1 and λ2. At these two wavelengths, measure the absorbance of the mixed gas (including CO2 and interfering gas) respectively. Let A1 and A2 be the absorbance at λ1 and λ2 respectively. Then establish a mathematical model, assuming that the molar absorption coefficient of CO2 at λ1 is , the molar absorption coefficient at λ2 is Assume that the molar absorption coefficient of the interfering gas at λ1 is , the molar absorption coefficient at λ2 is . Let the concentration of CO2 be c CO2 , the concentration of the interfering gas is c 干扰 , the optical path length is l.

[0091] According to the Lambert-Beer law, we can list the following two equations:

[0092]

[0093] By solving this set of two-variable linear equations, we can find the concentrations of CO2 and interfering gas c CO2 and c 干扰 .

[0094] The quantum cascade laser is used in the scheme of the present invention, which usually adopts advanced semiconductor manufacturing technology to ensure its wavelength accuracy and power stability. And often through precise optical coupling technology, laser sources of different wavelengths are integrated into one module, which can achieve multi-wavelength integration. The optical cavity in the present invention can be prepared as follows: first, an aluminum alloy material is used and a conical chamber shell is manufactured through precision machining technology, and then a high-reflectivity gold layer is plated on the inner wall of the chamber by chemical plating. Ideally, if the optical chamber is long enough, the sides of the cone can be infinitely parallel, so that the light can be reflected multiple times in the chamber, which can greatly increase the distance of light propagation and improve the optical performance. In the embodiment of the present invention, for economic and practical process considerations, a conical cavity with a bottom diameter of 100mm and a height of 100mm is used. The hypotenuse angle is 60° to 82.9°.

[0095] The long optical path CO2 optical cavity and its detection device provided by the present invention have many unique advantages in terms of accuracy and sensitivity compared with traditional non-long optical path detection devices and other long optical path structures (such as White cell). In terms of light coupling efficiency, the internal structure of the long optical path CO2 optical cavity is specially designed, and its optical parameters are optimized for the laser wavelengths related to CO2 gas detection (such as the 4.2μm wavelength emitted by the quantum cascade laser, and the wavelength near the CO2 absorption peak), so that the laser can efficiently enter and propagate in the cavity, while the White cell and the like are less targeted in coupling to specific CO2 detection lasers, and may require additional adjustments or find it difficult to achieve the same high coupling efficiency. In terms of detection sensitivity, the long optical path CO2 optical cavity can increase the optical path through multiple reflections. According to the Lambert-Beer law A = \epsilon cl, the absorption of light by CO2 molecules is proportional to the optical path length. Multiple reflections of light in the cavity give CO2 molecules more opportunities to interact with light. Under the same volume or device scale, it can achieve a more compact and longer effective optical path. For example, when monitoring the concentration changes of trace CO2 in the atmosphere in remote areas, it is more sensitive to low-concentration CO2 detection than the White cell. In terms of anti-interference ability, the long optical path CO2 optical cavity has a frequency selection characteristic, which can select and enhance light of a specific wavelength. In the actual environment where there are multiple interfering gases and background light noise, the specific wavelength laser related to CO2 absorption can be enhanced in the cavity through the resonance characteristic, and other interfering light wavelengths can be suppressed. For example, when there is water vapor interfering gas, it can accurately design the resonant frequency so that the laser wavelength falls on the CO2 absorption peak and avoids the band with strong water vapor absorption. The White cell has a relatively weak ability to distinguish interference and is easily affected by the absorption of interfering gases, thereby affecting the accurate measurement of CO2 concentration. In terms of stability, the long optical path CO2 optical cavity structure is relatively compact, and the relative position of optical elements is more fixed, which reduces the impact of external vibrations, temperature changes, etc. on the optical path and optical performance. In portable CO2 detection equipment, it can maintain good measurement accuracy under different environmental conditions such as temperature fluctuations and slight vibrations in the field. The White cell structure is complex and large, and is more susceptible to interference from external factors, causing fluctuations in measurement results. Specifically: In a laboratory experiment compared with a traditional non-long optical path detection device, a long optical path CO2 optical cavity detection device (optical path 50 meters, using a quantum cascade laser with a wavelength precisely set at the wavelength corresponding to the CO2 absorption peak) and a traditional single-point CO2 sensor (such as a sensor based on the non-dispersive infrared principle) were placed in a closed environmental chamber with a known initial CO2 concentration (500 ppm), and a small amount of CO2 was gradually injected to change the concentration. The results showed that the long optical path CO2 optical cavity detection device can detect CO2 increases as low as 0.1 ppm, with a relative standard deviation (RSD) within ±0.5%, while the traditional single-point sensor can only detect a concentration change as low as 1 ppm, with an RSD of approximately ±2%. This is because the long optical path increases the probability of CO2 molecules interacting with light, which improves the sensitivity according to the Lambert-Beer law.In comparison with the White Pool, in the waste gas emission monitoring of large chemical enterprises, when the long optical path CO2 optical cavity detection device (optical path 80 meters, reflector reflectivity 99.9%) and the White Pool detection system (optical path about 80 meters but large light loss) monitor low concentration CO2 of 100-1000 ppm, the detection limit of the former is 0.5 ppm, and the measurement accuracy is within ±0.3%, while the detection limit of the latter is 2 ppm, and the accuracy is about ±1%. The reason is that the long optical path CO2 optical cavity detection device has a compact structure, high coupling efficiency and good frequency selection characteristics. At urban atmospheric environment monitoring sites, when the long-path CO2 optical cavity detection device (optical path 100 meters) and the White pool detection device (optical path 100 meters) monitor the atmospheric CO2 concentration of about 400 ppm with small fluctuations, the former can detect concentration changes of less than 0.2 ppm per hour, while the latter can only detect concentration changes of more than 0.5 ppm per hour. This is because the long-path CO2 optical cavity design allows light to reflect multiple times to increase the effective optical path, thereby more sensitively capturing slight changes in concentration.

[0096] The embodiments described above provide a detailed description of the technical solutions and beneficial effects of the present invention. It should be understood that the above are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, supplements and equivalent substitutions made within the scope of the principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A CO2 optical cavity with a long optical path, characterized in that: It includes a conical chamber, the longitudinal section of which is trapezoidal, and the inner wall of the chamber is plated with a silver layer; The incident light is controlled to be incident from the small end face of the conical chamber at an angle as parallel to the end face as possible; The angle between the incident light and the small end face does not exceed 0.5°; The optical cavity satisfies: ， Among them, H is the axial length of the conical chamber, a is the diameter of the small end surface of the conical chamber, is the angle between the incident light and the small end face, and n is the number of reflections of the light beam in the chamber.

2. A detection device based on a long optical path CO2 optical cavity, characterized in that: The optical cavity comprises the optical cavity as claimed in claim 1, and further comprises a quantum cascade laser, a collimating lens, an air inlet, an air outlet, a mercury cadmium telluride detector, a digital signal processor DSP, an air pressure controller, and an air pressure sensor; the air inlet is connected to the small end face of the optical cavity, and the air intake is controlled by a proportional valve and a first solenoid valve, the air outlet is arranged at the large end face of the conical optical cavity, and is connected to a second solenoid valve and an air pump, the air pressure controller controls the first solenoid valve, the proportional valve, the air pump, and the second solenoid valve according to the detection signal of the air pressure sensor, and maintains the air pressure in the optical cavity stable during the detection process; the quantum cascade laser emits two laser signals of different wavelengths, which respectively pass through the collimating lens and then enter the optical cavity at a set angle through an adjustable reflector, the signal emitted by the optical cavity is detected by the mercury cadmium telluride detector and then input into the digital signal processor DSP for processing, and the CO2 concentration is calculated in the digital signal processor DSP according to a predetermined model; The quantum cascade laser emits two laser signals of different wavelengths, and the wavelength is determined as follows: first, a spectrum scan is performed, and a light source with a wide-band emission capability or a tunable laser is used to scan CO2 and possible interfering gases respectively to obtain an absorption spectrum, which can respectively display the absorption peak positions and intensities of CO2 and interfering gases, and then data analysis is performed to find out the area with obvious absorption peaks and relatively weak absorption of interfering gases, and two suitable wavelengths are selected in this area, one is selected near the center of the CO2 absorption peak to obtain the maximum absorbance change, and the other is selected at or near the shoulder of the CO2 absorption peak where the absorption is weaker; The model described is: The two wavelengths are λ1 and λ2. At these two wavelengths, the absorbance of the mixed gas is measured respectively. The mixed gas contains CO2 and interfering gas. The concentration of CO2 is c CO2 , the concentration of the interfering gas is c 干扰 , the optical path length is l, ， Among them, λ1 and λ2 are the two wavelengths emitted by the laser, A1 and A2 are the absorbance of the gas to be measured at λ1 and λ2 respectively. is the molar absorption coefficient of CO2 at λ1, is its molar absorption coefficient at λ2, is the molar absorption coefficient of the interfering gas at λ1, is its molar absorption coefficient at λ2, c CO2 and c 干扰 They are the concentrations of CO2 and other interfering gases in the gas to be measured, respectively.

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