Parabolic mirror multi-pass pool design method for TDLAS (Tunable Diode Laser Absorption Spectroscopy) system

Abstract

The invention relates to the technical field of optical detection, in particular to a design method of a parabolic mirror multi-pass cell for a TDLAS (tunable diode laser absorption spectroscopy) system, and the multi-pass cell is composed of two same and coaxially opposite parabolic mirrors; the design method comprises the following steps: calculating a reflected light parameter of an incident light passing through the mirror surface of the multi-pass cell every time through a light tracing method between the two parabolic mirrors; projecting light spots of all intersection points of the light and the two parabolic mirrors to an x-y plane to generate a light spot pattern; the parameters of the parabolic mirror, the center distance of the two mirror surfaces, the position and direction cosine of the incident light and the reflection times of the light are changed until a required light spot pattern is generated; and after the required light spot pattern is generated, keeping the direction cosine and the reflection times of the incident light unchanged, and adjusting the density and the size of the light spot pattern by changing the parameters of the mirror surfaces, the center distance of the two mirror surfaces and the position of the incident light to obtain a final multi-pass cell. According to the invention, the optical path length and the volume of the multi-pass cell are adjusted by adjusting the related parameters of the parabolic mirror and the incident light.

Description

Technical Field

[0001] The invention relates to the technical field of optical detection, and in particular to a design method of a parabolic mirror multi-pass cell for a TDLAS system. Background Art

[0002] Tunable semiconductor laser absorption spectroscopy (TDLAS) is a non-destructive detection technology for gas concentration with fast response speed, high sensitivity and high resolution. It uses narrow bandwidth and tunable wavelength laser to measure the selective absorption of gas molecules and can accurately obtain the concentration, temperature and pressure of the measured gas. The multi-pass cell is an important part of the TDLAS system. Especially for the detection of weak absorption of trace gases, a gas absorption cell is needed to provide the optical path of light propagation in the room to improve the detection sensitivity.

[0003] The early multi-pass cell based on a spherical mirror developed by White and Herriot is still used in laser-based spectral trace gas sensors due to its simplicity, reliability and operability. However, the White and Herriot multi-pass cell cannot produce a dense spot pattern on the mirror surface and is large in size, which limits its use.

[0004] In 2017, S. Ozharar et al. introduced a mirror whose focal length is inversely proportional to the off-axis distance into the multi-pass cell, applied the paraxial approximation theory, gave a numerical expression for ray tracing, and obtained a variety of spot patterns in the mirror simulation. However, for non-paraxial light, as the curvature of the reflecting mirror and the number of reflections increase, the deviation between the angle and optical path length of the light obtained by the paraxial approximation theory and the angle and optical path length of the actual light also increases. However, these errors are not considered in the paraxial approximation theory, and these errors will be accumulated and amplified, so that the actual light trajectory cannot be calculated, and the actual spot pattern deviates greatly from the spot pattern obtained by the paraxial approximation theory.

[0005] In view of the problems existing in the existing methods based on the paraxial approximation theory, the Chinese patent with the publication number CN114065321A discloses "A design method for a spherical mirror multi-pass pool with dense spot patterns", which includes: determining the initial incident point coordinates and incident direction vector of the light beam; obtaining different light parameters for each time passing through the multi-pass pool through the reflected light equation algorithm, the light parameters include the reflection point coordinates, the reflected light direction vector and the number of reflections; projecting all the spots of the light beam passing through the two mirrors onto the xy plane to observe the spot pattern; changing the number of reflections, the distance between the spherical mirrors, and the initial incident point coordinates and incident direction vector of the light beam, and repeating the calculation until the required spot pattern is obtained. This scheme can generate rich spot patterns by numerically simulating the reflection and free transmission of light in a multi-pass pool composed of a pair of spherical mirrors, thereby improving the utilization efficiency of the mirror surface.

[0006] However, the applicant has found that the existing scheme of the multi-pass cell composed of spherical mirrors still has the following problems: 1) The existing scheme does not provide a method for calculating the optical path length and volume of the multi-pass cell, and it is impossible to evaluate the sensitivity and structural compactness of the designed multi-pass cell, and it is impossible to obtain a multi-pass cell with high sensitivity and compact structure, resulting in insufficient performance of the multi-pass cell. 2) After the existing scheme obtains the spot pattern, it is impossible to adjust the spot density and pattern size of the spot image according to actual needs, that is, it is impossible to effectively adjust the performance of the multi-pass cell, resulting in a single application scenario of the multi-pass cell. Therefore, how to provide a multi-pass cell design method that can improve the application scenarios and performance of the multi-pass cell is a technical problem that needs to be solved urgently. Summary of the invention

[0007] In view of the deficiencies of the above-mentioned prior art, the technical problem to be solved by the present invention is: how to provide a design method for a parabolic mirror multi-pass cell for a TDLAS system, by adjusting the relevant parameters of the parabolic mirror and the incident light to adjust the optical path length and volume of the multi-pass cell, so as to obtain a multi-pass cell with high sensitivity and compact structure, thereby improving the performance of the multi-pass cell; at the same time, a method for adjusting the spot density and pattern size of the spot pattern is provided, so that the multi-pass cell can adapt to different measurement requirements and gas sample characteristics, thereby improving the versatility and flexibility of the multi-pass cell.

[0008] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0009] A method for designing a parabolic mirror multi-pass cell for a TDLAS system, wherein the multi-pass cell is composed of two identical and coaxially opposite parabolic mirrors, and the design method comprises the following steps:

[0010] S1: Determine the parameter p of the parabolic mirror, the center distance L between the two mirrors, and the position of the incident light entering the multi-pass cell (x 1 ,y 1 ,z 1 ) and direction cosines (sx 1 ,sy 1 ,sz 1 );

[0011] S2: Calculate the reflected light parameters of the incident light each time it passes through the multi-pass pool mirror by using the ray tracing method between the two parabolic mirrors; the reflected light parameters include the three-dimensional coordinates of the intersection of the incident light and the parabolic mirror and the direction cosine of the reflected light;

[0012] S3: Project the light spots of all intersection points of the light rays and the two parabolic mirrors onto the xy plane to generate a light spot pattern;

[0013] S4: changing the parameter p of the parabolic mirror, the center distance L between the two mirror surfaces, the position and direction cosine of the incident light, and the number of reflections n of the light, and repeating steps S2 and S3 until the desired light spot pattern is generated;

[0014] S5: After the required spot pattern is generated, the direction cosine and the number of reflections of the incident light are kept unchanged, and the spot density and pattern size of the spot pattern are adjusted by adjusting the parameters of the parabolic mirror, the center distance L between the two mirrors, and the position of the incident light to obtain the final multi-pass cell.

[0015] Preferably, in step S1, in a Cartesian coordinate system, the multi-pass cell is composed of two identical and coaxially opposed parabolic mirrors M 1 and M 2 The composition of the two mirror centers O 1 and O 2 The distance between them is L;

[0016] Formula F for two parabolic mirrors 1 (x,y,z) and F 2 (x,y,z) is represented as:

[0017] M 1 :F 1 (x,y,z)=x 2 +y 2 -2pz=0;

[0018] M 2 :F 2 (x,y,z)=x 2 +y 2 +2p(zL)=0;

[0019] Where: M 1 、M 2 Represents two parabolic mirrors.

[0020] Preferably, in step S2, the incident light is incident from the parabolic mirror M 1 The position and direction cosine of the incident light are (x 1 ,y 1 ,z 1 ) and (sx 1 ,sy 1 ,sz 1 ); the incident light is transmitted and reflected in the multi-pass cell, and the position and direction cosine of the nth incident light are (x n ,y n ,z n ) and (sx n ,sy n ,sz n );

[0021] The formula for the nth incident ray is expressed as:

[0022] x=x n +sx n ·d;

[0023] y=y n +sy n ·d;

[0024] z=z n +sz n ·d;

[0025] Where: d represents the length of the incident light.

[0026] Preferably, in step S2, the formula for the reflective mirror surface of the nth incident light is expressed as:

[0027]

[0028] Preferably, in step S2, the optical path length d of the nth incident light is calculated by the following formula: n :

[0029]

[0030] in:

[0031] A n =sx n 2 +sy n 2 ;

[0032] B n =x n ·sx n +y n ·sy n +(-1) n+1 p·sz n ;

[0033]

[0034] Preferably, in step S2, after calculating the length of the incident light, the three-dimensional coordinates (x n+1 ,y n+1 ,z n+1 ), the calculation formula is expressed as:

[0035] x n+1 =x n +sx n ·d n ;

[0036] y n+1 =y n +sy n ·d n ;

[0037] z n+1 =z n +sz n ·d n .

[0038] Preferably, in step S2, according to the vector reflection theorem:

[0039]

[0040] Where: and They represent the direction cosine vectors of the incident and reflected light of the parabolic mirror respectively; Represents the normal vector of the reflective mirror surface, and the formula is expressed as:

[0041]

[0042] Preferably, in step S2, the direction cosine vector of the reflected light is calculated

[0043] Preferably, in step S4, the parameter p of the parabolic mirror, the distance L between the centers of the two mirror surfaces, the position and direction cosine of the incident light, and the number of reflections of the light are adjusted, and ray tracing is performed according to the current conditions of the light to generate the desired spot pattern.

[0044] Preferably, in step S5, after generating the required spot pattern, the light beam initially incident on the parabolic mirror M is kept 1 The direction cosine of the incident light ray on 1 ,sy 1 ,sz 1 ) and the number of reflections n remain unchanged; the parameters p of the parabolic mirror, the distance L between the two mirror centers, and the initial incident light incident on the parabolic mirror M 1 The position of the incident light ray on 1 ,y 1 ,z 1 ) are multiplied by the desired gain factor to adjust the size and density of the spot pattern.

[0045] Compared with the prior art, the parabolic mirror multi-pass cell design method for the TDLAS system in the present invention has the following beneficial effects:

[0046] The present invention forms a multi-pass pool by two identical and coaxially opposite parabolic mirrors. Compared with the existing multi-pass pool formed by a spherical mirror: first, the parabolic mirror has a unique focusing characteristic and can converge parallel light to its focus, while the spherical mirror has aberrations, especially when the light is reflected at a large angle. Therefore, the multi-pass pool formed by the parabolic mirrors of the present invention can more effectively focus the incident light and reflect it multiple times in the pool, reduce the divergence and energy loss of the light, and improve the light path utilization rate and light intensity distribution uniformity of the multi-pass pool. Secondly, the focusing effect of the parabolic mirror makes the reflection path of the light in the pool more compact and efficient, and can achieve more reflection times in the same physical space, thereby increasing the interaction time between the light and the gas to be measured and improving the detection sensitivity of the TDLAS system. Finally, the spherical mirror may produce spot distortion during the reflection process, affecting the uniformity of the spot pattern and the light intensity distribution, while the parabolic mirror of the present invention can effectively reduce such distortion, keep the spot circular or approximately circular, make the energy distribution more uniform, and improve the accuracy and reliability of the multi-pass pool measurement results.

[0047] The present invention adjusts the optical path length and volume of the multi-pass cell by changing the parameters of the parabolic mirror, the incident position and direction cosine of the incident light, and the number of reflections of the light. First, by increasing the optical path length of the multi-pass cell, the interaction time of the light with the gas to be measured in the cell is made longer, thereby improving the detection sensitivity of the system (this is particularly important for the detection of low-concentration gases). Secondly, by optimizing the parameters and mirror distance of the parabolic mirror (i.e., the center distance between the two mirrors), the volume of the multi-pass cell can be reduced while increasing the optical path length, making the multi-pass cell structure more compact, which is particularly important for application scenarios with limited space. The compact structure helps to reduce the impact of external factors on system performance (such as vibration, temperature changes, etc.), and also helps to reduce production costs and transportation costs. The patented solution can easily obtain a small-volume multi-pass cell, and the small volume also means reducing the amount of material used and potential failure points, thereby improving the stability and reliability of the system.

[0048] Using the method of the present invention, the spot density and pattern size of the spot pattern can be further adjusted by adjusting the parameters of the parabolic mirror, the mirror distance, and the incident position of the incident light. First, increasing the spot density means that more light interacts with the gas to be measured in a unit volume, thereby improving the detection sensitivity of the system; at the same time, by optimizing the spot pattern, it can be ensured that more light paths pass through the gas sample, thereby improving the signal strength. Secondly, adjusting the size and shape of the spot pattern helps to optimize the mixing uniformity of the gas in the cell and reduce the measurement error caused by uneven gas concentration (especially when using multi-component gas analysis, a uniform spot pattern can ensure that each gas can be effectively detected), thereby improving the performance of the multi-pass cell. Finally, different application scenarios may require different spot patterns. By flexibly adjusting the spot density and pattern size, the multi-pass cell can be adapted to different measurement requirements and gas sample characteristics, thereby improving the versatility and flexibility of the multi-pass cell. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] In order to make the purpose, technical solution and advantages of the invention more clear, the present invention will be further described in detail below with reference to the accompanying drawings, in which:

[0050] Figure 1 Schematic diagram of light transmission in a multi-pass cell in a Cartesian coordinate system;

[0051] Figure 2 (a) is a simulation diagram of a three-dimensional ray tracing diagram of light in two parabolic mirrors obtained by programming with the method of the present application and Matlab software; Figure 2 (b) and Figure 2 (c) and (d) are the light spot diagrams formed by the projection of the intersection of the light ray and the two mirror surfaces on the xy plane;

[0052] Figure 3 The light spot diagram formed by the intersection of the light and the two mirrors in the xy plane when the gain factors are 0.5 and 2 respectively. DETAILED DESCRIPTION

[0053] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. The components of the embodiments of the present invention generally described and shown in the drawings here can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but only represents selected embodiments of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work belong to the scope of protection of the present invention.

[0054] The following is a further detailed description through specific implementation methods:

[0055] Example:

[0056] This embodiment discloses a method for designing a parabolic mirror multi-pass cell for a TDLAS system.

[0057] Parabolic mirror multi-pass cell design method for TDLAS system, such as Figure 1 As shown, the multi-pass pool is composed of two identical and coaxially opposite parabolic mirrors, and the design method includes the following steps:

[0058] S1: Determine the parameter p of the parabolic mirror, the center distance L between the two mirrors, and the position of the incident light entering the multi-pass cell (x 1 ,y 1 ,z 1 ) and direction cosines (sx 1 ,sy 1 ,sz 1 );

[0059] In this embodiment, the parameter p of the parabolic mirror is the parabola formula y 2 =p in 2px, that is, the semi-diameter.

[0060] S2: Calculate the reflected light parameters of the incident light each time it passes through the multi-pass pool mirror through the precise ray tracing method between the two parabolic mirrors; the reflected light parameters include the three-dimensional coordinates of the intersection of the incident light and the parabolic mirror and the direction cosine of the reflected light;

[0061] S3: Project the light spots of all intersection points of the light rays and the two parabolic mirrors onto the xy plane to generate a light spot pattern;

[0062] S4: changing the parameter p of the parabolic mirror, the center distance L between the two mirror surfaces, the position and direction cosine of the incident light, and the number of reflections n of the light, and repeating steps S2 and S3 until the desired light spot pattern is generated;

[0063] S5: After the required spot pattern is generated, the direction cosine and the number of reflections of the incident light are kept unchanged, and the spot density and pattern size of the spot pattern are adjusted by adjusting the parameters of the parabolic mirror, the center distance L between the two mirrors, and the position of the incident light to obtain the final multi-pass cell.

[0064] In this embodiment, the multi-pass cell composed of two identical and coaxially opposite parabolic mirrors satisfies the self-existence condition, that is, after a cycle of n passes, the incident light can continue to be transmitted and reflected again and produce the same spot pattern again.

[0065] The present invention forms a multi-pass pool by two identical and coaxially opposite parabolic mirrors. Compared with the existing multi-pass pool formed by a spherical mirror: first, the parabolic mirror has a unique focusing characteristic and can converge parallel light to its focus, while the spherical mirror has aberrations, especially when the light is reflected at a large angle. Therefore, the multi-pass pool formed by the parabolic mirrors of the present invention can more effectively focus the incident light and reflect it multiple times in the pool, reduce the divergence and energy loss of the light, and improve the light path utilization rate and light intensity distribution uniformity of the multi-pass pool. Secondly, the focusing effect of the parabolic mirror makes the reflection path of the light in the pool more compact and efficient, and can achieve more reflection times in the same physical space, thereby increasing the interaction time between the light and the gas to be measured and improving the detection sensitivity of the TDLAS system. Finally, the spherical mirror may produce spot distortion during the reflection process, affecting the uniformity of the spot pattern and the light intensity distribution, while the parabolic mirror of the present invention can effectively reduce such distortion, keep the spot circular or approximately circular, make the energy distribution more uniform, and improve the accuracy and reliability of the multi-pass pool measurement results.

[0066] The present invention adjusts the optical path length and volume of the multi-pass cell by changing the parameters of the parabolic mirror, the incident position and direction cosine of the incident light, and the number of reflections of the light. First, by increasing the optical path length of the multi-pass cell, the interaction time of the light with the gas to be measured in the cell is made longer, thereby improving the detection sensitivity of the system (this is particularly important for the detection of low-concentration gases). Secondly, by optimizing the parameters and mirror distance of the parabolic mirror (i.e., the center distance between the two mirrors), the volume of the multi-pass cell can be reduced while increasing the optical path length, making the multi-pass cell structure more compact, which is particularly important for application scenarios with limited space. The compact structure helps to reduce the impact of external factors on system performance (such as vibration, temperature changes, etc.), and also helps to reduce production costs and transportation costs. The patented solution can easily obtain a small-volume multi-pass cell, and the small volume also means reducing the amount of material used and potential failure points, thereby improving the stability and reliability of the system.

[0067] Using the method of the present invention, the spot density and pattern size of the spot pattern can be further adjusted by adjusting the parameters of the parabolic mirror, the mirror distance, and the incident position of the incident light. First, increasing the spot density means that more light interacts with the gas to be measured in a unit volume, thereby improving the detection sensitivity of the system; at the same time, by optimizing the spot pattern, it can be ensured that more light paths pass through the gas sample, thereby improving the signal strength. Secondly, adjusting the size and shape of the spot pattern helps to optimize the mixing uniformity of the gas in the cell and reduce the measurement error caused by uneven gas concentration (especially when using multi-component gas analysis, a uniform spot pattern can ensure that each gas can be effectively detected), thereby improving the performance of the multi-pass cell. Finally, different application scenarios may require different spot patterns. By flexibly adjusting the spot density and pattern size, the multi-pass cell can be adapted to different measurement requirements and gas sample characteristics, thereby improving the versatility and flexibility of the multi-pass cell.

[0068] In the specific implementation process, in the Cartesian coordinate system, the multi-pass cell consists of two identical and coaxially opposite parabolic mirrors M 1 and M 2 The composition of the two mirror centers O 1 and O 2 The distance between them is L;

[0069] Formula F for two parabolic mirrors 1 (x,y,z) and F 2 (x,y,z) is represented as:

[0070] M 1 :F 1 (x,y,z)=x 2 +y 2 -2pz=0;

[0071] M 2 :F 2 (x,y,z)=x 2 +y 2 +2p(zL)=0;

[0072] Where: M 1 、M 2 Represents two parabolic mirrors.

[0073] In the specific implementation process, the incident light is incident from the parabolic mirror M 1 The position and direction cosine of the incident light are (x 1 ,y 1 ,z 1 ) and (sx 1 ,sy 1 ,sz 1); the incident light is transmitted and reflected in the multi-pass cell, and the position and direction cosine of the nth incident light are (x n ,y n ,z n ) and (sx n ,sy n ,sz n );

[0074] The formula for the nth incident ray is expressed as:

[0075] x=x n +sx n ·d

[0076] y=y n +sy n ·d;

[0077] z=z n +sz n ·d;

[0078] Where: d represents the length of the incident light.

[0079] In the specific implementation process, the formula of the reflective mirror surface (i.e., parabolic mirror) of the nth incident light is expressed as:

[0080]

[0081] In the specific implementation process, the optical path length d of the incident light transmitted for the nth time is calculated by the following formula: n :

[0082]

[0083] in:

[0084] A n =sx n 2 +sy n 2 ;

[0085] B n =x n ·sx n +y n ·sy n +(-1) n+1 p·sz n ;

[0086]

[0087] In the specific implementation process, after calculating the length of the incident light, the three-dimensional coordinates (x n+1 ,yn+1 ,z n+1 ), the calculation formula is expressed as:

[0088] x n+1 =x n +sx n ·d n ;

[0089] y n+1 =y n +sy n ·d n ;

[0090] z n+1 =z n +sz n ·d n .

[0091] In the specific implementation process, according to the vector reflection theorem:

[0092]

[0093] Where: and They represent the direction cosine vectors of the incident and reflected light of the parabolic mirror respectively; Represents the normal vector of the reflective mirror surface, and the formula is expressed as:

[0094]

[0095] In the specific implementation process, the direction cosine vector of the reflected light can be further calculated by the above vector formula:

[0096]

[0097]

[0098] In the specific implementation process, the parameter p of the parabolic mirror, the center distance L of the two mirror surfaces, the position and direction cosine of the incident light, and the number of reflections of the light are adjusted, and the method proposed in the present invention is used to trace the light according to the existing conditions of the light to generate the required light spot pattern.

[0099] In the specific implementation process, after the required spot pattern is generated, the initial incident light to the parabolic mirror M is maintained. 1 The direction cosine of the incident light ray on 1 ,sy 1 ,sz 1 ) and the number of reflections n remain unchanged; the parameters p of the parabolic mirror, the distance L between the two mirror centers, and the initial incident light incident on the parabolic mirror M 1 The position of the incident light ray on 1,y 1 ,z 1 ) are multiplied by the expected gain factor to adjust the size and density of the light spot pattern. Or the parameters p of the parabolic mirror, the distance L between the two mirror centers and the initial incident light on the parabolic mirror M are 1 The incident position of the incident light on 1 ,y 1 ) are multiplied by the same gain factor to adjust the pattern size of the spot pattern.

[0100] In order to better illustrate the advantages of the technical solution of the present invention, this embodiment discloses the following simulation analysis.

[0101] This simulation can calculate the three-dimensional ray tracing diagram of light in the multi-pass pool using the method proposed by the present invention and Matlab software programming. The double parabolic mirror multi-pass pool needs to meet the self-existing condition, that is, after a cycle of n reflections, the incident light can continue to transmit and reflect again and produce the same spot pattern again.

[0102] During the simulation, the parameters of the parabolic mirror p = 70 mm and the direction cosine of the incident light are kept unchanged at (0.1045, 0.1045, 0.9890), and the initial position (x 1 ,y 1 ,z 1 ) and the distance between the two mirror centers L, so that the final light (x n+1 ,y n+1 ,z n+1 )、(sx n+1 ,sy n+1 ,sz n+1 ) and the initial ray (x 1 ,y 1 ,z 1 )、(sx 1 ,sy 1 ,sz 1 ) satisfies the present condition. When (x 1 ,y 1 ,z 1 )=(6.00,-6.99,0.6061)mm,L=73.38mm,n=490,the parabolic mirror multi-pass pool meets the current conditions. Table 1 is the light parameter table of multiple iterative operations.

[0103] Table 1 is a table of light parameters for multiple iterative calculations

[0104]

[0105] Figure 2(a) is a three-dimensional ray tracing diagram obtained using the method proposed in this patent and Matlab software programming, where the red line represents the light. Figure 2 (b) and (c) are the light and the parabolic mirror M. 1 、M 2 The spot pattern formed by the projection of the intersection point on the xy plane. Figure 2 The spot pattern in is a spot pattern that has been adjusted and screened by parameters, and its corresponding multi-pass cell meets the current conditions.

[0106] When the desired spot pattern is obtained, keep the incident light direction cosine (sx 1 ,sy 1 ,sz 1 ) and the number of reflections n remain unchanged, and the parameters p of the parabolic mirror and the initial incident light incident on the parabolic mirror M 1 The beam position (x 1 ,y 1 ), the center distance L of the two mirrors is multiplied by the expected gain factor k to adjust the spot density; or the parameter p of the parabolic mirror, the initial incident light to the parabolic mirror M 1 The beam position (x 1 ,y 1 ) and the center distance L of the two mirrors are multiplied by the same gain factor k to adjust the size of the pattern in the light spot.

[0107] Figure 3 (a) and (b) are the spot diagrams of the intersection of the light and the mirror in the xy plane when the gain factor k is 0.5 and 2 respectively, obtained by programming with Matlab software. The blue and green points are the intersection points of the light and the mirror M respectively. 1 、M 2 The gray line represents the projection of the light ray on the xy plane.

[0108] Depend on Figure 3 It can be seen that when the gain factor k is equal to 0.5, the size of the light spot pattern becomes half of the original size; when the gain factor k is equal to 2, the size of the light spot pattern becomes twice of the original size.

[0109] A longer total optical path length can improve the detection sensitivity of the multi-pass cell, while a small multi-pass cell can not only improve the gas exchange rate and shorten the response time, but also make the structure more compact. Figure 2 In the , the distance from the farthest spot to the origin of the pattern is selected as their mirror radius, and the volume V is defined as the product of the mirror area and the distance between the two mirror centers L. The ratio of the total optical path length to the volume RLV can better reflect the spatial utilization of the light trajectories in the multi-pass cell.

[0110] Table 2 lists the multi-pass cells obtained by simulation when the gain factor k is equal to 0.5, 1 and 2 respectively.

[0111]

[0112] As shown in Table 2, when the gain factor becomes k times of the original, the total optical path length becomes k times of the original, and the volume becomes k times of the original. 3 times, the ratio of total optical path length to volume RLV becomes 1 / k of the original 2 When designing a parabolic multi-pass pool, the gain factor can be adjusted according to actual needs to obtain the required multi-pass pool.

[0113] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the technical solution. Those skilled in the art should understand that those modifications or equivalent substitutions of the technical solution of the present invention that do not depart from the purpose and scope of the technical solution should be included in the scope of the claims of the present invention.

Claims

1. A method for designing a parabolic mirror multi-pass cell for a TDLAS system, characterized in that: The multi-pass pool is composed of two identical and coaxially opposite parabolic mirrors, and the design method comprises the following steps: S1: Determine the parameters p of the parabolic mirror, the distance L between the centers of the two mirrors, and the position (x1, y1, z1) and direction cosines (sx1, sy1, sz1) of the incident light entering the multi-pass cell; S2: Calculate the reflected light parameters of the incident light each time it passes through the multi-pass pool mirror by using the ray tracing method between the two parabolic mirrors; the reflected light parameters include the three-dimensional coordinates of the intersection of the incident light and the parabolic mirror and the direction cosine of the reflected light; S3: Project the light spots of all intersection points of the light rays and the two parabolic mirrors onto the xy plane to generate a light spot pattern; S4: changing the parameter p of the parabolic mirror, the center distance L between the two mirror surfaces, the position and direction cosine of the incident light, and the number of reflections n of the light, and repeating steps S2 and S3 until the desired light spot pattern is generated; S5: After the required spot pattern is generated, the direction cosine and the number of reflections of the incident light are kept unchanged, and the spot density and pattern size of the spot pattern are adjusted by adjusting the parameters of the parabolic mirror, the center distance L between the two mirrors, and the position of the incident light to obtain the final multi-pass cell.

2. The parabolic mirror multi-pass cell design method for a TDLAS system as claimed in claim 1, characterized in that: In step S1, in a Cartesian coordinate system, the multi-pass cell is composed of two identical and coaxially opposite parabolic mirrors M1 and M2, and the distance between the centers of the two mirrors O1 and O2 is L; The formulas for two parabolic mirrors F1(x,y,z) and F2(x,y,z) are expressed as: M1:F1(x,y,z)=x 2 +y 2 -2pz=0; M2:F2(x,y,z)=x 2 +y 2 +2p(z-L)=0; Where: M1 and M2 represent two parabolic mirrors.

3. The parabolic mirror multi-pass cell design method for a TDLAS system as claimed in claim 2, characterized in that: In step S2, the incident light is incident from the parabolic mirror M1, and the position and direction cosine of the incident light are (x1, y1, z1) and (sx1, sy1, sz1) respectively; the incident light is transmitted and reflected in the multi-pass cell, and the position and direction cosine of the nth incident light are (x n ,y n ,z n ) and (sx n ,sy n ,sz n ); The formula for the nth incident ray is expressed as: x=x n +sx n ·d; y=y n +and n ·d; z=z n +sh n ·d; Where: d represents the length of the incident light.

4. The parabolic mirror multi-pass cell design method for a TDLAS system as claimed in claim 3, characterized in that: In step S2, the formula of the reflective mirror surface of the nth incident light is expressed as:

5. The parabolic mirror multi-pass cell design method for TDLAS system as claimed in claim 4, characterized in that: In step S2, the optical path length d of the nth incident light is calculated by the following formula: n : in: A n =sx n 2 +and n 2 ; B n =x n ·sx n +y n ·in n +(-1) n+1 p·sz n 4 6. The parabolic mirror multi-pass cell design method for a TDLAS system as claimed in claim 5, characterized in that: In step S2, after calculating the length of the incident light, the three-dimensional coordinates (x n+1 ,y n+1 ,z n+1 ), the calculation formula is expressed as: x n+1 =x n +sx n ·d n ; the n+1 =y n +and n ·d n ; With n+1 =from n +sh n ·d n 。 7. The parabolic mirror multi-pass cell design method for a TDLAS system as claimed in claim 6, characterized in that: In step S2, according to the vector reflection theorem: Where: and They represent the direction cosine vectors of the incident and reflected light of the parabolic mirror respectively; Represents the normal vector of the reflective mirror surface, and the formula is expressed as:

8. The parabolic mirror multi-pass cell design method for a TDLAS system as claimed in claim 7, characterized in that: In step S2, the direction cosine vector of the reflected light is calculated 9. The parabolic mirror multi-pass cell design method for a TDLAS system as claimed in claim 1, characterized in that: In step S4, the parameter p of the parabolic mirror, the distance L between the two mirror centers, the position and direction cosine of the incident light, and the number of reflections of the light are adjusted, and ray tracing is performed according to the current conditions of the light to generate the required spot pattern.

10. The parabolic mirror multi-pass cell design method for a TDLAS system according to claim 1, characterized in that: In step S5, after the required spot pattern is generated, the direction cosines (sx1, sy1, sz1) and the number of reflections n of the incident light initially incident on the parabolic mirror M1 are kept unchanged; the parameter p of the parabolic mirror, the center distance L between the two mirror surfaces, and the position (x1, y1, z1) of the incident light initially incident on the parabolic mirror M1 are multiplied by the expected gain factor to adjust the size and density of the spot pattern.

Images