CN111982817B - A variable optical path multiple reflection pool and optical path adjustment method - Google Patents

Abstract

The application provides a variable optical path multiple reflection pool and an optical path adjusting method. The reflection cell includes: an absorption air chamber, a light window and a reflecting mirror; the absorption air chamber comprises a quartz cavity, a stepped spiral cavity and a differential spiral cavity which are connected in sequence; the ladder spiral cavity is formed by a ladder spiral structure, and the ladder spiral structure comprises a lower ladder spiral structure, an upper ladder spiral structure and an intermediate ladder spiral structure which are connected in sequence; the differential spiral cavity is formed by a differential spiral structure, and the differential spiral structure comprises an upper differential spiral structure, a middle differential spiral structure and a lower differential spiral structure which are connected in sequence; the reflecting mirror comprises two concave reflecting mirrors with the same focal length and confocal, and the concave reflecting mirrors are fixed at the front end and the rear end of the absorption air chamber. The distance between the two reflectors is adjusted through the ladder spiral structure and the differential spiral structure, so that the optical path is adjusted. The application has the characteristics of simple structure, accurate and adjustable optical path and high stability.

Description

A variable optical path multiple reflection pool and optical path adjustment method

technical field

The invention relates to the technical field of gas detection, in particular to a variable optical path multiple reflection pool and an optical path adjustment method.

Background technique

In many research fields and monitoring applications, low-concentration gases can be accurately measured using laser spectroscopy, among which tunable laser absorption spectroscopy (TDLAS) based on gas absorption cells has the advantages of high sensitivity and high precision in measuring gases. The commonly used absorption cells in the TDLAS system are White type, matrix type and Herriott type. The characteristics of White type and matrix type are that the aperture angle is relatively large, and more reflection times can be realized, but more reflectors are used; Herriott The type absorption cell is based on the principle of confining the incident light to two or more reflective mirrors coated with high-reflectivity films, thereby increasing the effective path length of the interaction between light and matter, that is, by increasing the detection of molecules to the incident light. The absorption amount can obtain the absorption spectrum signal with high signal-to-noise ratio. Compared with the former two Herriott types, the structure is simple, the volume is small, the optical path is relatively easy to adjust, and it is suitable for laser light sources.

In practical applications, due to the different detection environments, higher requirements are placed on the gas absorption cell. For the detection of low-concentration gases, the absorption cell is required to have high sensitivity, and the absorption cell is required to be suitable for the detection of high-concentration gases. This requires the reflection cell gas chamber to be able to adjust the range of the absorption cell according to different needs, so that the absorption cell can meet different detection requirements. Sensitivity and range requirements. For low-concentration gases, in order to obtain a smaller gas detection limit, we usually start from two aspects: enhancing the absorption signal intensity and suppressing noise. The main method to enhance the intensity of the absorption signal is to increase the optical path. According to the Lambert-Beer law, the longer the optical path means the stronger the absorption and the higher the sensitivity of the detector. In a multiple reflection cell, noise is almost impossible to completely avoid. The beam path will be changed during the optical path adjustment process. Under different beam paths, the noise has different effects on the absorption measurement. However, the noise in the Herriott type multiple reflection cell is currently , and no exact model. Therefore, in order to meet the requirements of different detection sensitivities and ranges and to explore the influence of the optical length on the noise in the absorption cell, the optical length of the absorption cell needs to be adjustable.

Contents of the invention

The embodiment of the present invention provides a variable optical path multiple reflection cell and an optical path adjustment method to meet the requirements of different detection sensitivities and ranges and to explore the technical problem of the influence of the optical path on the noise in the absorption cell.

According to the first aspect of the embodiments of the present invention, a variable optical path multiple reflection pool is provided, and the reflection pool includes: an absorption gas chamber, a light window, and a reflection mirror;

The absorption gas chamber includes a sequentially connected quartz cavity, a stepped spiral cavity, and a differential spiral cavity, the quartz cavity is fixedly connected to the stepped spiral cavity through an intermediate plate, and the stepped spiral cavity and the Differential helical cavity screw connection;

An air inlet and an air outlet are provided on the absorption air chamber;

The stepped spiral cavity is composed of a stepped spiral structure, and the stepped spiral structure includes a sequentially connected lower stepped helical structure, an upper stepped helical structure, and a middle stepped helical structure;

The differential helical cavity is composed of a differential helical structure, and the differential helical structure includes an upper differential helical structure, a middle differential helical structure, and a lower differential helical structure connected in sequence;

The reflector includes two concave reflectors with the same focal length and confocal, which are fixed at the front and rear ends of the absorption gas chamber;

The reflector at the front end of the absorption chamber is provided with an incident hole;

The light window is arranged at the front end of the reflector at the front end of the absorption gas chamber.

Preferably, the intermediate plate, the stepped spiral chamber, and the differential spiral chamber are all made of aluminum material, and the surface of the inner chamber is blackened.

Preferably, the diameter of the reflection hole is 1-3mm.

Preferably, the reflector is made of quartz material.

Preferably, the reflective surface of the reflective mirror is coated with gold.

Preferably, the gold film is coated with a SiO ₂ layer.

Preferably, O-rings are used to seal the connection between the quartz cavity, the intermediate plate and the stepped spiral cavity.

Preferably, the incident hole is arranged close to the edge of the reflector.

Preferably, a laser adjustment frame is provided above the reflection pool, and the laser adjustment frame is used to install and adjust the laser, and a plane reflector bracket is also provided at the front end of the reflection pool, and the plane reflector bracket is used to install a plane reflector mirror, and the plane reflector is used to reflect the laser light emitted by the laser into the light window.

According to the second aspect of the embodiment of the present application, a method for adjusting the optical path of a variable optical path multiple reflection pool is provided, using the variable optical path multiple reflection pool provided by the first aspect of the embodiment of the present invention, the method includes :

According to the preset optical path, the mirror spacing is obtained, wherein the relationship between the optical path and the mirror spacing is:

In the formula: L is the optical path, P is a positive integer, d is the distance between the mirrors, the value range of d is 0 ~ 4f (f is the focal length), R is the radius of curvature of the mirror, and r is the distance from the center of the entrance hole to the optical axis Distance, A=R ² +r ² , B=t-2R;

The number of reflections is determined according to the distance between the mirrors, and the relationship between the distance between the mirrors and the number of reflections is:

In the formula: n mirror spacing, P is a positive integer, d is the mirror spacing, the value range of d is 0 ~ 4f (f is the focal length), R is the radius of curvature of the mirror;

Determine the direction of the incident light according to the distance between the mirrors, wherein the relationship between the distance between the mirrors and the direction of the incident light is:

In the formula: (x ₀ , y ₀ ) is the coordinates of the incident point, f is the focal length, d is the distance between mirrors, and the value range of d is 0 to 4f, (x' ₀ , y' ₀ ) is the incident angle of light;

Determine the position of each light spot on the reflector according to the direction of the incident light and the distance between the reflectors, wherein the relationship between the direction of the incident light and the distance between the reflectors and the position of each light spot on the reflector is:

In the formula: (x _n , y _n ) is the position coordinates of each light spot on the mirror, (x ₀ , y ₀ ) is the position coordinates of the incident point, f is the focal length, d is the distance between the mirrors, and the value range of d is 0～4f, (x' ₀ , y' ₀ ) is the incident angle of light, θ is the angle between two adjacent spots that satisfy

Determine the position of the exit hole according to the number of reflections;

According to the position of the exit hole, the exit hole is set on the reflector of the reflection pool;

The optical path is obtained by adjusting the stepped helical structure and the differential helical structure of the reflecting pool according to the distance between the reflecting mirrors.

It can be seen from the above technical solutions that the variable optical path multiple reflection pool and the optical path adjustment method provided by the embodiment of the present invention, the reflection pool includes: an absorption gas chamber, a light window, and a reflection mirror; the absorption gas chamber includes sequentially A connected quartz cavity, a stepped spiral cavity, and a differential spiral cavity, the quartz cavity is fixedly connected to the stepped spiral cavity through an intermediate plate, and the stepped spiral cavity is screwed to the differential spiral cavity; The middle plate is provided with an air inlet and an air outlet; the stepped spiral cavity is composed of a stepped spiral structure, and the stepped spiral structure includes a sequentially connected lower stepped spiral structure, an upper stepped spiral structure, and a middle stepped spiral structure The differential helical cavity is composed of a differential helical structure, and the differential helical structure includes an upper differential helical structure, a middle differential helical structure, and a lower differential helical structure connected in sequence; the reflector includes two Concave mirrors with the same focal length and confocal are located at the front and rear ends of the absorption gas chamber; the light window is arranged at the front end of the reflection mirror at the front end of the absorption gas chamber. Coarsely adjust the length of the cavity through the wide thread fit of the stepped helical structure, and fine-tune the length of the cavity through the narrow thread fit of the differential helical structure, so as to realize the precise adjustment of the distance between the mirrors, and because the optical path There is a clear corresponding relationship with the reflector distance, so the purpose of precisely adjusting the optical path can be achieved.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the application.

Description of drawings

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the invention and together with the description serve to explain the principles of the invention.

In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings that need to be used in the description of the embodiments or the prior art. Obviously, for those of ordinary skill in the art, In other words, other drawings can also be obtained from these drawings without paying creative labor.

Fig. 1 is a schematic cross-sectional structure diagram of a variable optical path multiple reflection pool provided by an embodiment of the present invention;

Fig. 2 is a schematic diagram of a three-dimensional structure of a variable optical path multiple reflection pool provided by an embodiment of the present invention;

Fig. 3 is a schematic diagram of a stepped helical structure assembly provided by an embodiment of the present invention;

Fig. 4 is a schematic diagram of a differential helical structure assembly provided by an embodiment of the present invention;

Fig. 5 is a schematic diagram of another three-dimensional structure of a variable optical path multiple reflection pool provided by an embodiment of the present invention;

Fig. 6 is a schematic flow chart of a method for adjusting the optical path of a variable optical path multiple reflection pool provided by an embodiment of the present invention;

Fig. 7 is a light spot distribution diagram on the B mirror when the incident direction provided by the embodiment of the present invention is x' ₀ =0.06723, y' ₀ =-0.06802;

Fig. 8 is the spot distribution diagram of mirror A provided by the embodiment of the present invention, when (a) reflects 58 times, (b) when reflects 118 times;

Fig. 9 is a schematic diagram of the spot size provided by the embodiment of the present invention, (a) theoretical calculation, (b) simulation software;

Fig. 10 is a relationship diagram (f=100mm) between the number of reflections and the mirror spacing provided by the embodiment of the present invention;

Fig. 11 is the figure of the relationship between the number of reflections and the mirror distance (f=100mm) when k=±2 provided by the embodiment of the present invention;

Fig. 12 is a diagram of the relationship between the optical path and the mirror spacing provided by the embodiment of the present invention;

Figure 13 is a spot distribution diagram when the distance between mirrors provided by the embodiment of the present invention is 193.8mm, (a) mirror A, (b) mirror B;

Figure 14 is a spot distribution diagram when the distance between mirrors provided by the embodiment of the present invention is 206.5 mm, (a) mirror A, (b) mirror B;

Fig. 15 is a spot distribution diagram when the distance between mirrors is 398.4mm provided by the embodiment of the present invention, (a) A mirror, (b) B mirror.

Label description:

1-absorbing gas chamber; 11-quartz cavity; 12-step helix cavity; 120-step helix structure; 121-down step helix structure; 122-middle step helix structure; 123-up step helix structure; 13-differential helix Cavity; 130-differential helical structure; 131-upper differential helical structure; 132-middle differential helical structure; 133-lower differential helical structure; 2-light window; 3-mirror; 4-middle plate; 5 - air inlet; 6 - air outlet; 7 - entrance hole; 8 - laser adjustment frame; 9 - plane mirror bracket; 10 - cage rod.

Detailed ways

In order to enable those skilled in the art to better understand the technical solutions in the present invention, 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 The embodiments are only some of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art on the premise of making creative efforts shall fall within the protection scope of the present invention.

Fig. 1 is a schematic cross-sectional structure diagram of a variable optical path multiple reflection pool provided by an embodiment of the present invention, Fig. 2 is a schematic diagram of a three-dimensional structure of a variable optical path multiple reflection pool provided by an embodiment of the present invention, and Fig. 3 is the present invention A schematic diagram of a stepped helical structure assembly provided by an embodiment of the present invention, FIG. 4 is a schematic diagram of a differential helical structure assembly provided by an embodiment of the present invention, and FIG. 5 is another variable optical path multiplex provided by an embodiment of the present invention. A schematic diagram of the three-dimensional structure of the sub-reflecting pool. A variable optical path multi-reflecting pool provided by the embodiment of the present invention will be further described below in conjunction with the accompanying drawings 1-5.

As shown in FIGS. 1-2 , the variable optical path multiple reflection pool includes: an absorption gas chamber 1 , a light window 2 , and a reflection mirror 3 . The absorption gas chamber 1 includes a quartz chamber 11, a stepped spiral chamber 12, and a differential spiral chamber 13 connected in sequence, the quartz chamber 11 and the stepped spiral chamber 12 are fixedly connected through the middle plate 4, the stepped spiral chamber 12 and the differential The spiral cavity 13 is screwed. An air inlet 5 and an air outlet 6 are provided on the absorption chamber 1 , preferably, an air inlet 5 and an air outlet 6 are provided on the middle plate 4 . As shown in Figure 3, the stepped helical cavity 12 is composed of a stepped helical structure 120, and the stepped helical structure includes a sequentially connected lower stepped helical structure 121 stepped helical structure 120, an upper stepped helical structure 123 stepped helical structure 120, a middle stepped helical structure 122 stepped helical structures 120 . As shown in Figure 4, the differential helical cavity 13 is composed of a differential helical structure 130, and the differential helical structure 130 includes an upper differential helical structure 131, a differential helical structure 130, a middle differential helical structure 132, and a differential helical structure 130 connected in sequence. The helical structure 130 , the lower differential helical structure 133 and the differential helical structure 130 . The reflector 3 includes two concave reflectors 3 with the same focal length and confocal. The reflector 3 is fixed at the front and rear ends of the absorber chamber 1. For the convenience of subsequent description, the reflector 3 at the front end of the absorber chamber 1 is defined as A mirror, absorber The reflector 3 at the rear end of the gas chamber 1 is a B mirror. The light window 2 is arranged at the front end of the reflector 3 (mirror A) at the front end of the absorption gas cell 1 . During the assembly process, the optical axes of the two concave mirrors 3 are required to coincide with each other, but in the machining process, certain errors will inevitably occur and cause the optical axes of the two concave mirrors 3 to deviate. At this time, the position of the concave mirrors needs to be fine-tuned.

As a preferred implementation mode provided by the embodiment of this application, the middle plate 4, the stepped spiral cavity 12, and the differential spiral cavity 13 are all made of aluminum materials, and the surface of the inner cavity is blackened, which has good corrosion resistance And the adsorption of gas is small.

The aperture of the incident hole 7 also affects the number of reflections. If the incident hole 7 is too large, it will cause light leakage and other phenomena. In a preferred implementation mode provided by the embodiment of the present application, the aperture size of the reflection hole is 1-3 mm, which is more preferably implemented. The method is that the aperture size of the reflection hole is 2mm.

The lower limit of the optical path is a single optical path, and the upper limit of the optical path is closely related to the power of the incident light and the mirror reflectivity. In a preferred implementation mode provided by the embodiment of the present application, the reflector 3 is made of quartz material. And the reflective surface is plated with gold film to improve the reflectivity, and the SiO ₂ layer on the gold film can prevent the damage to the gold film by corrosive gases such as acid or alkali. In the design, the reflector 3 with a gold-plated mirror surface is selected, and the reflectivity can generally reach more than 99%.

Before measuring the particle concentration, it is necessary to measure the light intensity of the laser in vacuum (no particle secondary state), which requires the multi-path absorption cell to have good sealing. Gas detection experiments for specific gases require a process of vacuuming and ventilation, so there are certain gas-tightness requirements for the multiple reflection pool device. In a preferred implementation mode provided by the embodiment of this application, the connection between the quartz chamber 11, the intermediate plate 4 and the stepped spiral chamber 12 is sealed with an O-ring to increase the airtightness of the gas chamber, and the O-ring is made of corrosion-resistant materials. . The device adopts O-ring sealing, and the radial sealing feature is to fill the radial gap and hinder the axial flow. The axial seal is characterized by filling the axial gap and hindering radial flow. Since the radial seal does not require pre-tightening force, it is more stable, and the structure is simple and the seal is convenient, so the radial seal structure is selected in the form of the O-ring seal structure. In the actual structure, there are grooves at the joints at both ends of the cavity to place the sealing ring to meet the airtightness requirements of the air chamber.

In order to make more effective use of the mirror surface and avoid mutual interference of adjacent light spots, the position of the entrance hole 7 should be as close as possible to the edge of the reflector 3 to make the arrangement of the light spots more dispersed.

In order to improve the integration of the device, a preferred implementation mode provided by the embodiment of the present application, as shown in Figure 5, is provided with a laser adjustment frame 8 above the reflection pool, the laser adjustment frame 8 is used to install and adjust the laser, and the front end of the reflection pool A plane reflector bracket 9 is also provided, and the plane reflector bracket 9 is used for installing a plane reflector, and the plane reflector is used for reflecting the laser light emitted by the laser into the light window 2 . In addition, in order to avoid light spot interference when obtaining more reflection times, a suitable light source should be selected as much as possible to make the light spot size as small as possible. In the embodiment of the present application, the light source type is Gaussian light source.

As a preferred implementation mode provided in the embodiment of the present application, cage rods 10 are used to fasten the absorption chamber 1 , the light window 2 and the reflector 3 of the multiple reflection pool. The size of the portable multiple reflection pool is required to be as small as possible. During the design process, the size of the mechanical fixing bracket should be reduced as much as possible according to the size of the concave mirror.

Combining theory with practice, it is necessary to repeatedly modify the specific dimensions in the design process to meet the actual processing requirements, so that the processing of the mechanical fixing bracket can meet the requirements. This design uses CAD software for mechanical design, which can generate visible three-dimensional stereograms and engineering drawings, and simulate the assembly of various parts to test the feasibility of the device. The fixing bracket has 44 parts in total, mainly including quartz cavity 11, incident structure assembly, laser adjustment frame 8, cavity length adjustment assembly, cage rod 10 and other parts.

The working principle of the variable optical path multiple reflection pool is as follows:

By adjusting the length of the reflecting pool, changing the distance between the reflectors 3 at the front and rear ends of the absorbing gas chamber 1, and then changing the number of reflections of the light beam back and forth between the two reflectors 3, so as to achieve the purpose of adjusting the optical path. As shown in Figure 1, the adjustment structure, as a part of the cavity, is divided into two parts: the stepped helical structure 120 and the differential helical structure 130. The stepped helical structure 120 includes three parts: the upper, middle and lower stepped helical structures. The thread with wider pitch can adjust the cavity length roughly; the differential screw includes upper, middle and lower differential screw structures. By rotating the middle stepped helical structure 122 and the middle differential helical structure 132, the cavity length of the absorption gas chamber 1 can be increased or shortened step by step, and the relative distance of the reflector 3 can be changed to realize the adjustment of the optical path of the absorption gas chamber 1.

Analysis of the working principle of the helical structure:

Functional design goal: to achieve precise adjustment of the cavity length of the multi-reflection pool within a large range of centimeters.

This device comprehensively considers factors such as adjustment convenience and accuracy, and adopts a rough and fine two-stage adjustment method to design the cavity length adjustment structure.

(1) Coarse adjustment structure

As shown in FIG. 3 , a stepped helical structure 120 is designed, which is composed of an upper stepped helical structure 123 , a middle stepped helical structure 122 , and a lower stepped helical structure 121 . According to the meshing principle of thread rotation, the same group of meshing threads has the same rotation direction. The upper stepped helical structure 123 is a right-handed internal thread that engages with the right-handed external thread of the middle stepped helical structure 122 , and the left-handed external thread of the lower stepped helical structure 121 engages with the left-handed internal thread of the middle stepped helical structure 122 .

The two sets of thread pitches are equal, and both are single thread threads, fixing the lower stepped helical structure 121 . Assuming that the moment applied by the outside when rotating the upper stepped helical structure 123 is M, the rotational frictional moment that the upper stepped helical structure 123 acts on the middle stepped helical structure 122 is M(f ₁ ), and the actual frictional moment is M(f); The rotational friction torque of the structure 122 acting on the lower stepped spiral structure 121 is M(f ₂ ), the actual friction torque is M(f'), and M(f ₁ )>M(f ₂ ). Rotation is uniform motion with constant angular velocity.

The movement of the stepped helical structure 120 will be briefly introduced step by step below.

Step 1, fix the middle stepped helical structure 122, take the clockwise direction of the viewing direction as positive, and rotate the upper stepped helical structure 123 clockwise. The upper stepped helical structure 123 is subjected to an externally applied moment M, and is also subjected to the frictional moment -M(f) of the middle stepped helical structure 122 . M(f)=M(f ₁ ), M=M(f ₁ ), then MM(f)=0, so the upward stepped helical structure 123 rotates clockwise. Since the upper stepped helical structure 123 and the middle stepped helical structure 122 are engaged with right-handed threads, the upper stepped helical structure 123 will be fed in a clockwise direction from the thread starting end.

Step 2, unfix the middle stepped helical structure 122, take the counterclockwise direction of the viewing direction as positive, and rotate the upper stepped helical structure 123 counterclockwise. The upper stepped helical structure 123 is subjected to an externally applied moment M, and at the same time is subjected to the frictional moment -M(f) of the middle stepped helical structure 122; the middle stepped helical structure 122 is subjected to the frictional moment M(f) and Frictional moment -M(f') acting on the lower stepped helical structure 121 . Because M(f ₁ )>M(f ₂ ), then M(f)=M(f')=M(f ₂ ), the middle stepped helical structure 122 rotates counterclockwise with respect to the lower stepped helical structure 121; M=M (f), the upper stepped helical structure 123 and the middle stepped helical structure 122 are relatively static. Since the middle stepped helical structure 122 and the lower stepped helical structure 121 are engaged with left-handed threads, the upper stepped helical structure 123 will feed clockwise along with the middle stepped helical structure 122 in the direction of the starting end of the thread.

Step 3, fix the middle stepped helical structure 122 again, take the counterclockwise direction of the viewing direction as positive, and rotate the upper stepped helical structure 123 counterclockwise. Similar to the steps, the upper stepped helical structure 123 is subjected to the externally applied moment M, and is also subjected to the frictional moment -M(f) of the middle stepped helical structure 122 . M(f)=M(f ₁ ), M=M(f ₁ ), then MM(f)=0, so the upward stepped helical structure 123 rotates counterclockwise. Since the upper stepped helical structure 123 and the middle stepped helical structure 122 are engaged with right-handed threads, the upper stepped helical structure 123 will return in the counterclockwise direction of the thread starting end.

Step 4, unfix the middle stepped helical structure 122 again, take the clockwise direction of the viewing direction as positive, and rotate the upper stepped helical structure 123 clockwise. In the same way as step 2, the upper stepped helical structure 123 is subjected to the externally applied moment M, and at the same time is subjected to the frictional moment -M(f) of the middle stepped helical structure 122; the middle stepped helical structure 122 is subjected to the friction of the upper stepped helical structure 123 The moment M(f) and the friction moment −M(f′) acting on the lower stepped helical structure 121 . Because M(f ₁ )>M(f ₂ ), then M(f)=M(f')=M(f ₂ ), the middle stepped helical structure 122 rotates clockwise with respect to the lower stepped helical structure 121; M=M (f), the upper stepped helical structure 123 and the middle stepped helical structure 122 are relatively static. Since the middle stepped helical structure 122 and the lower stepped helical structure 121 are engaged with left-handed threads, the upper stepped helical structure 123 returns counterclockwise with the middle stepped helical structure 122 in the counterclockwise direction of the thread starting end.

(2) Fine-tuning structure:

As shown in FIG. 4 , the differential helical structure 130 is adopted for design, and the structure is composed of an upper differential helical structure 131 , a middle differential helical structure 132 , and a lower differential helical structure 133 . The upper differential helical structure 131 and the middle differential helical structure 132 are engaged with right-handed threads with a pitch of P ₁ ; the middle differential helical structure 132 and the lower differential helical structure 133 are also engaged with right-handed threads with a pitch of P ₂ and P ₁ > P ₂ . The upper differential helical structure 131 is fixed, the middle stepped helical structure 132 can rotate and translate, and the lower differential helical structure 133 can only translate.

The movement of the differential helical structure 130 will be briefly introduced step by step below.

Step 1: Rotate the middle differential helical structure 132 clockwise according to the viewing direction for one week, the middle differential helical structure 132 advances a pitch P ₁ clockwise from the starting end of the thread relative to the upper differential helical structure 131 , and advances a pitch P 1 relative to the lower differential helical structure 133 advances one thread pitch P ₂ in the clockwise direction of the thread starting end. Since the upper differential helical structure 131 is fixed and the lower differential helical structure 133 can move in translation, the advancing distance of the lower differential helical structure 133 relative to the upper differential helical structure 131 in the clockwise direction of the thread starting end is a pitch difference P ₁ - _P2 .

Step 2, the same as step 2, rotate the middle differential helical structure 132 counterclockwise in the direction of observation for one week, and the middle differential helical structure 132 is opposite to the upper differential helical structure ₁₃₁ . The lower differential helical structure 133 returns a pitch P ₂ in the counterclockwise direction from the thread starting end. Since the upper differential helical structure 131 is fixed, the lower differential helical structure 133 can move in translation, so the return distance of the lower differential helical structure 133 relative to the upper differential helical structure 131 in the counterclockwise direction of the thread starting end is a pitch difference P ₁ - _P2 .

Based on the above realization principles, the method for adjusting the optical path of the variable optical path multiple reflection pool provided by this embodiment will be described in detail below in conjunction with the accompanying drawings. Fig. 6 is a schematic flowchart of a method for adjusting the optical path of a variable optical path multiple reflection pool provided by an embodiment of the present invention. As shown in Figure 6, the method specifically includes the following steps:

S1: Obtain the mirror spacing according to the preset optical path, where the relationship between the optical path and the mirror spacing is:

In the formula: L is the optical path, P is a positive integer, d is the distance between the mirrors, the value range of d is 0 ~ 4f (f is the focal length), R is the radius of curvature of the mirror, and r is the distance from the center of the entrance hole to the optical axis Distance, A=R ² +r ² , B=t-2R;

S2: Determine the number of reflections according to the distance between the mirrors. The relationship between the distance between the mirrors and the number of reflections is:

In the formula: n mirror spacing, P is a positive integer, d is the mirror spacing, the value range of d is 0 ~ 4f (f is the focal length), R is the radius of curvature of the mirror;

S3: Determine the direction of the incident light according to the distance between the mirrors, where the relationship between the distance between the mirrors and the direction of the incident light is:

In the formula: (x ₀ , y ₀ ) is the coordinates of the incident point, f is the focal length, d is the distance between mirrors, and the value range of d is 0 to 4f, (x' ₀ , y' ₀ ) is the incident angle of light;

S4: Determine the position of each spot on the mirror according to the direction of the incident light and the distance between the mirrors, where the relationship between the direction of the incident light and the distance between the mirrors and the position of each spot on the mirror is:

In the formula: (x _n , y _n ) is the position coordinates of each light spot on the mirror, (x ₀ , y ₀ ) is the position coordinates of the incident point, f is the focal length, d is the distance between the mirrors, and the value range of d is 0～4f, (x' ₀ , y' ₀ ) is the incident angle of light, θ is the angle between two adjacent spots that satisfy S5: Determine the position of the exit perforation according to the number of reflections;

S5: Determine the position of the exit perforation according to the number of reflections;

S6: setting the exit hole on the reflector of the reflection pool according to the position of the exit hole;

S7: adjusting the stepped helical structure and the differential helical structure of the reflecting pool according to the distance between the reflecting mirrors to obtain the optical path.

The key to the method for adjusting the optical path of a variable optical path multiple reflection pool provided in the embodiment of the present application is the relational expression satisfied between the optical path and the distance between the mirrors and the relational expression satisfied between the distance between the mirrors and the number of reflections. According to the satisfying relationship between the optical path and the distance between the mirrors, it can be obtained from the preset optical distance. In order to realize the optical distance, it is actually necessary to adjust the distance between the mirrors in the reflection pool; direction, and then determine the position of the exit hole. The process of determining the relationship between the optical path and the distance between the mirrors will be described in detail below by analyzing and calculating the spot position distribution and the spot size.

(1) Spot distribution into a circle condition

For the Herriott type multiple reflection pool, the two concave mirrors of the gas pool are parallel to each other. In order to make full use of the circumference of the concave mirror, the light spots on the concave mirror are designed to be arranged in a circle, so that the distance between two adjacent light spots is as large as possible. , to prevent the light from overflowing in advance during the reflection process, and there is a certain gap between the spots, so that there will be no interference between the spots. As shown in Figure 1, the reflector is placed on the xy plane, and the direction vector of the incident light is set as (x′ ₀ , y′ ₀ , z′ ₀ ), for the convenience of calculation, the z′ ₀ in the direction vector is normalized is 1. In order to obtain a circularly distributed reflection spot, assuming that the incident point of the laser is (x ₀ , y ₀ ) and the slope of the incident light is x′ ₀ , y′ ₀ , after n times of reflection, the intersection point between the light and the concave mirror is ( x _n , y _n ), two concave mirrors with focal length f, and the distance between their centers is d.

According to Pierce's theorem, Herriott et al. obtained formula (1):

The angle θ between two adjacent spots depends on the focal length f and distance d of the concave mirror, as shown in formula (2):

When 0<d<4f, the light can be infinitely reflected between the two mirrors without overflowing.

Transform (1) into the form of (3)

x _n ＝Asin(nθ+α) (3) then

Similarly, the y-direction coordinate y _n of the intersection point after the light is reflected n times

Transform (6) into the form of (3)

y _n ＝Bsin(nθ+β) (7)

Among them (7) formula

According to the formulas (3) to (9), it can be found that the intersection point (x _n , y _n ) and the incident point (x ₀ , y ₀ ) after n times of reflection, the incident rate (x′ ₀ , y′ ₀ ) is given by d, f related. Normally, the light spots are distributed in an ellipse, but under certain conditions, the light spots will be distributed in a circle on the concave mirror, that is

A=B (10)

That is, in the case of satisfying (12) and (13), the light spots will be arranged in a circle on the concave mirror.

Therefore, when the concave mirror is selected, that is, the distance f is fixed, and the incident point (x ₀ , y ₀ ) is fixed, it can be seen from equations (12) and (13) that the light incident angle required for the spot to be arranged in a circle on the mirror (x′ ₀ , y′ ₀ ) is only related to the distance d between the two concave mirrors, that is, when the distance d is constant, the light is incident at a specific angle, and the light spots will be arranged in a circle on the concave mirrors.

Select a concave mirror with a focal length of 100mm, set the distance between the mirrors to 170mm, and the light incident point is at (1, 12.4). The incident direction can be calculated as: x′ ₀ =0.06723, y′ ₀ =-0.06802. Fig. 7 is the light spot distribution diagram on the B mirror when the incident direction provided by the embodiment of the present invention is x' ₀ =0.06723, y' ₁₀ =-0.06802. It can be found from Fig. 7 that when the incident light is at a specific angle, the light spot is on the mirror Arranged in a circle, and at this time, the specific position of each spot can be obtained according to the formula (1).

(2) Spot size calculation

If the size of the reflected light spot is known and combined with the position of the reflected light spot, it can be determined whether adjacent light spots on the mirror overlap. If they overlap, light spot interference will occur, resulting in a deviation in the detection signal. At the same time, by knowing the size of the light spot, we can better judge whether there is light leakage during the transmission of the light. For example, the center point of the light has overflowed from the entrance hole, but the entire light spot has not completely exited, or the light at the edge of the light spot has leaked from the entrance. The entrance hole overflows, while the center of the spot is still reflecting, which will lead to errors in the detection results. Knowing the size of each spot on the mirror, you can adjust the tightness of the spot arrangement on the mirror and select the appropriate entrance and exit holes to avoid spot interference and light leakage, thereby reducing the small detection error. As shown in the figure, the spot arrangement on the concave mirror is obtained under certain conditions. The total and reflection times are 58 and 118 respectively. In Figure 8(a), the light is incident from the entrance hole. By observing the distribution of the light spots, it can be seen that there is no overlap between the light spots, but part of the light from the second light spot leaks from the entrance hole. In Figure 8(b), it can be observed that the light spots overlap with each other. Obviously, when the number of reflections is large, the phenomenon of overlapping light spots is easy to occur. Both of these situations will lead to errors in the final detection results, and it is necessary to reset the relevant parameters to avoid overlapping of light spots and light leakage.

In actual situations, since the incident ray is a bundle of incident rays, that is, a collection of countless incident rays in the same direction, and the beam cross section is shaped into a circle, it is possible to trace the edge rays of a beam with a certain divergence angle to realize the tracking of the beam . When the incident light is continuously reflected between the two mirrors, the cross-sectional shape of the beam will continuously change in size due to the converging effect of the spherical mirror, and finally the size of each reflected spot on the mirror is different, and its radius is according to certain regular changes. In the z-axis direction, we assume that the contour lines of the incident light are L1 and L2, the incident position coordinates of the two lines in the x-axis direction are x ₀₁ , x ₀₂ , and the slopes are x′ ₀₁ , x′ ₀₂ , then The diameter of the spot after n reflections is:

R _L ＝|x _n1 -x _n2 |＝|A ₁ sin(nθ+α ₁ )-A ₂ sin(nθ+α ₂ )| (14)

Similarly, the direction of the y-axis is the same as the direction of the x-axis.

As shown in (a) of Figure 9, we applied this theory to calculate the size and position distribution of the light spots on the mirror when the number of reflections is 30, and the light spots are equally spaced on the concentric circles of the entrance hole. According to the same parameters, import it into the simulation software, perform ray tracing, and analyze the light intensity of the reflective surface of the mirror, as shown in (b) in Figure 9. It can be seen that the spot distribution pattern obtained by this theory is basically consistent with the pattern obtained in the simulation software, so we can calculate the spot size.

(3) Calculation of the number of reflections

It can be known from formula (2) that the angle θ between two adjacent spots is determined by the distance d between the mirrors and the focal length f of the mirrors.

Heriott et al proposed that if the light satisfies the following formula:

nθ=2μπ (15)

Then the light will pass through the incident point again after n times of reflection, namely (x ₀ , y ₀ )=(x _n , y _n ), where θ

The number of reflections n starts from 1 and increases in steps of 2, the integer μ increases in steps of 1, and μ represents the number of complete circles that the reflection point walks around the mirror in steps of angle θ. If a number of reflections n is given, under a certain radius of curvature R, there will be multiple sets of solutions satisfying (15) and (16) at the same time, and the reflection modes corresponding to each set of solutions are slightly different, in order to describe different In reflection mode, variables k and p are introduced. In order to avoid confusion, formula (17) is used to express unclustering and each particular solution in unclustering is denoted by {n, μ, k, p}:

n=2pμ+k (17)

In the formula, k=±2,±4,±6..., p is a positive integer.

From equations (14) to (16), we can see that the number of reflections n is a function of d, k, and p, that is

Fig. 10 is the relationship diagram (f=100mm) between the number of reflections n and the mirror distance d provided by the embodiment of the present application. As shown in Fig. 10, redundant solutions have been excluded, including n=2pμ+k, k=±4 . Divisible evenly, there is no restriction on μ when k=±2.

When the incident point and the exit point are in the same hole, when p=2, k=-2, the corresponding relationship between the number of turns μ and the number of reflections n is shown in Table 1.

Table 1 Number of turns and number of reflections

The integer μ in the table is the number of complete turns of the reflection point on the concave mirror before the beam leaves the cavity. We find that for the unclustering of n=4μ-2, when the number of turns μ is incremented by 1, the realized number of reflections n (even numbers) are not continuous on even numbers. If μ can take the value of 1.5, 2.5..., then the number of reflections will be continuous on even numbers, so that any number of reflections can be realized in the unclustering of n=4μ-2. For example, to achieve a value of μ of 1.5, that is, the light beam has a complete orbit in the cavity and leaves the reflection cavity in the middle of the second orbit, since we can determine the specific position of each reflection point, then we can control the exit point position to achieve the purpose of changing the number of reflections. Similarly, for all solutions, when p=1, 2, 3, 4..., we only take k=±2, and by controlling the position of the exit point, the light beam leaves the reflection pool at the corresponding number of reflections, then the number of reflections The relationship between n and mirror spacing d will change as shown in Figure 11.

The mirror distance d can be changed continuously, but the number of reflections n can only be taken as an integer, for example, d=170mm, the number of reflections n is 20.86 in n=4p+2 clustering, here we round down to get n=20. It can be seen from Figure 9 that when the focal length is constant, the number of reflections n is only related to the distance d of the mirrors, and we realize that the number of reflections is continuous on n=2pμ±2 (n is an integer) by controlling the position of the exit point, then We can obtain different reflection times by changing the mirror spacing d, and the corresponding relationship between them has been clarified.

(4) Optical path calculation

a)d>R

In order to calculate the optical path more accurately, the geometric analysis of the two mirrors and the light is performed in a three-dimensional form, where Q is the center of curvature, P is the projection point of the normal on the distribution surface, and d is the center of the two mirrors Distance, t is the distance from the center of curvature to the spot distribution surface of the far mirror, v is the distance from the normal projection to the axis, s is the distance from the center of curvature to the distribution surface of the near mirror, and the angle between the normal and the axis is δ.

The distance t from the center of curvature to the distribution surface

The angle between the normal and the axis is δ

Center of curvature to another distribution surface s

s=d+t-2R (21)

Normal projection to axis distance v

v=s tan(δ) (22)

The distance w from the center of curvature Q to the projection point P

Then, when d>R, the single optical path OD

b)d<R

The distance t from the center of curvature to the distribution surface

The angle between the normal and the axis is δ

Center of curvature to another distribution surface s

s=2R-d-t (27)

Normal projection to axis distance v

v=s tan(δ) (28)

The distance w from the center of curvature Q to the projection point P

Then, when d<R, the single optical path OD

It can be seen from (19) to (30) that when 0<d<R

Through formula (31), it can be found that OD is a function of d, R, r, and when the concave mirror is selected and the incident hole is determined, the single optical path OD is a function of d.

Then the total optical path length L

L=n·OD (32)

Since the number of reflections n is only related to d when the reflection mode is certain, and the single optical path OD is a function of d, it can be seen that the optical path L is a function of d:

where A=R ² +r ² , B=t-2R.

The following is a further explanation based on the above methods and principles with an example:

(1) Parameter design of multiple reflection pools

Table 2 Parameter design of multiple reflection pools

Select the light source type as Gaussian light source.

Table 3 Light source parameter design

(2) Determine the distance between the mirrors and the position of the exit point

After the parameters of the reflective pool system are set, assuming that the optical path L we need is 19.82m, according to formula (33), the relationship between the optical path L and the distance d of the reflectors can be obtained, as shown in Figure 12.

When the optical distance L=19.82m, it can be found that there are multiple ds that can satisfy the optical distance. Obviously, the smaller d is, the more reflections are required, and the larger d is, the less reflections are required. Therefore choose d = 193.9mm, 206.5mm and 398.3mm. According to FIG. 11 , d=193.9mm corresponds to n=102 reflections, d=206.5mm corresponds to n=96 reflections, and d=398.3mm corresponds to n=48 reflections.

Since the position of the incident point (x ₀ , y ₀ ) and the focal length f are known, when d=193.9mm, the direction of the incident light can be obtained from formulas (12) and (13): x′ ₀ =0.02251, y′ ₀ ＝-0.1419; when d＝206.5mm, the direction of incident light is: x′ ₀ ＝0.01744, y′ ₀ ＝-0.1381; when d＝398.3mm, the direction of incident light is: x′ ₀ ＝-0.05488, y ' ₀ = -0.08372. According to formulas (1), (6) and (14), the position and size of each spot on the reflector under the three conditions can be obtained. The spots are distributed on the B mirror.

Table 4 Spot distribution when d=193.9mm

Table 5 Spot distribution when d=206.5mm

Table 6 Spot distribution when d=398.3mm

According to the specific position and size of the light spot, a suitable exit hole can be set to ensure that the reflection times meet the requirements. For example, if the number of reflections required is 102, then an exit hole with an aperture of 1 mm should be set on the concave mirror A placed on the x-y plane (1.8461, 19.9995).

(3) Design results and analysis

In the simulation software, the spherical reflector is used as the concave mirror of the multiple reflection pool, and the Herriott type reflection pool model is built. The light source is set according to the laser light source of the semiconductor laser, and the optical path is simulated. In the case of different d, the spot distribution The figure is shown in Figures 13-15.

It can be seen from Figures 13 to 15 that, in the case of different mirror spacing d, the light spots can be arranged in a circle on the mirror by setting the corresponding incident angle, and by setting the appropriate exit hole position, the required The number of reflections n. In Figure 13, it can be found that although the number of reflections can meet the requirements, there is a phenomenon of spot overlap, which will cause spot interference. It can be seen from Figure 14 and Figure 15 that there is no overlap and light leakage between adjacent light spots, but d=398.4mm compared to d=206.5mm, the volume of the absorption pool is larger, which is not conducive to miniaturization and improvement of sensitivity, so d =206.5mm is the best choice. At this time, it can be known from the optical path analysis of the simulation software that the OPL is 19.823m. That is to say, when f=100mm, d=206.5mm, and the number of reflections is 96 to realize the optical path L, the error between the optical path L and the actual optical path OPL |ΔL|=3mm, is the optical path L=19.82m 0.015%. According to the design method in this paper, a Herriott-type multiple reflection times with the required optical path can be conveniently designed.

The embodiments in this specification are described in a progressive manner. The same and similar parts of the various embodiments can be referred to each other, and each embodiment focuses on the differences from other embodiments.

Other embodiments of the invention will be readily apparent to those skilled in the art from consideration of the specification and practice of the invention invented herein. This application is intended to cover any modification, use or adaptation of the present invention, these modifications, uses or adaptations follow the general principles of the present invention and include common knowledge or conventional technical means in the technical field not invented by the present invention . The specification and examples are to be considered exemplary only, with the true scope and spirit of the invention indicated by the appended claims.

It should be noted that, unless otherwise specified and limited, the terms "connected" and "connected" should be understood in a broad sense, for example, it can be a mechanical connection or an electrical connection, it can also be the internal communication of two elements, and it can be a direct connection. It can also be an indirect connection through an intermediary, and those of ordinary skill in the related art can understand the specific meanings of the above terms according to specific situations. The terms "comprising", "comprising" or any other variation thereof are intended to cover a non-exclusive inclusion such that a circuit arrangement, article or apparatus comprising a set of elements includes not only those elements but also other elements not expressly listed , or also include elements inherent in such articles or equipment. Without further limitations, the presence of an element qualified by the phrase "comprising a ..." does not exclude the presence of additional identical elements in the article or device comprising said element. Additionally, as used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

It should be understood that the present invention is not limited to the precise constructions which have been described above and shown in the accompanying drawings, and various modifications and changes may be made without departing from the scope thereof. The scope of the invention is limited only by the appended claims.

Claims (10)

1. A variable optical path multiple reflection cell, the reflection cell comprising: an absorption air chamber (1), a light window (2) and a reflecting mirror (3);

the absorption air chamber (1) comprises a quartz cavity (11), a step spiral cavity (12) and a differential spiral cavity (13) which are connected in sequence, wherein the quartz cavity (11) is fixedly connected with the step spiral cavity (12) through an intermediate plate (4), and the step spiral cavity (12) is in spiral connection with the differential spiral cavity (13);

an air inlet (5) and an air outlet (6) are arranged on the absorption air chamber (1);

the step spiral cavity (12) is formed by a step spiral structure (120), the step spiral structure (120) comprises a lower step spiral structure (121), an upper step spiral structure (123) and an intermediate step spiral structure (122) which are sequentially connected, the upper step spiral structure (123) is a right-handed internal thread and is meshed with a right-handed external thread of the intermediate step spiral structure (122), and a left-handed external thread of the lower step spiral structure (121) is meshed with a left-handed internal thread of the intermediate step spiral structure (122);

The thread pitches of the two groups of threads are equal and are both single-thread threads, the lower step spiral structure (121) is fixed, when the upper step spiral structure (123) is rotated, the externally applied moment is M, the rotation friction moment acting on the middle step spiral structure (122) by the upper step spiral structure (123) is M (f 1), and the actual friction moment is M (f); the rotating friction moment acting on the lower step spiral structure (121) by the middle step spiral structure (122) is M (f 2), the actual friction moment is M (f'), M (f 1) > M (f 2), and the rotation is uniform speed change motion with constant angular speed;

the differential spiral cavity (13) is composed of a differential spiral structure (130), the differential spiral structure (130) comprises an upper differential spiral structure (131), a middle differential spiral structure (132) and a lower differential spiral structure (133) which are sequentially connected, the upper differential spiral structure (131) and the middle differential spiral structure (132) are in right-handed threaded engagement, and the screw pitches of the upper differential spiral structure (131) and the middle differential spiral structure (132) are P ₁ The method comprises the steps of carrying out a first treatment on the surface of the The middle differential screw structure (132) and the lower differential screw structure (133) are also engaged by right-handed screw, and the screw pitches of the middle differential screw structure (132) and the lower differential screw structure (133) are P ₂ ，P ₁ ＞P ₂ The upper differential screw structure (131) is fixed, the middle differential screw structure (132) can rotate and translate, and the lower differential screw structure (133) can only translate;

the reflecting mirror (3) comprises two concave reflecting mirrors with the same focal length and coaxial, and the two concave reflecting mirrors are fixed at the front end and the rear end of the absorption air chamber (1);

the reflector (3) at the front end of the absorption air chamber (1) is provided with an inlet hole (7);

the light window (2) is arranged at the front end of the front end reflector (3) of the absorption air chamber (1).

2. The variable optical path multiple reflection cell according to claim 1, wherein the intermediate plate (4), the step spiral cavity (12) and the differential spiral cavity (13) are made of aluminum materials, and the surfaces of the inner cavities are blackened.

3. Variable optical path multiple reflection cell according to claim 1, characterized in that the entrance aperture (7) has an aperture size of 1-3mm.

4. Variable optical path multiple reflection cell according to claim 1, characterized in that the mirror (3) is made of quartz material.

5. The variable optical path multiple reflection cell according to claim 4, wherein the reflecting surface of the reflecting mirror (3) is gold-plated.

6. The variable optical path multiple reflection cell according to claim 5, wherein the gold film is coated with SiO ₂ A layer.

7. Variable optical path multiple reflection cell according to claim 1, characterized in that the junction of the quartz cavity (11) with the intermediate plate (4) and the stepped spiral cavity (12) is sealed with an O-ring.

8. Variable optical path multiple reflection cell according to claim 1, characterized in that the entrance aperture (7) is arranged close to the edge of the mirror (3).

9. The variable optical path multiple reflection cell according to claim 1, wherein a laser adjusting frame (8) is arranged above the reflection cell, the laser adjusting frame (8) is used for installing and adjusting a laser, a plane mirror support (9) is further arranged at the front end of the reflection cell, the plane mirror support (9) is used for installing a plane mirror, and the plane mirror is used for reflecting laser emitted by the laser into the optical window (2).

10. A method for adjusting the optical path length of a variable optical path multiple reflection cell, characterized in that the variable optical path multiple reflection cell according to any one of claims 1 to 9 is used, the method comprising:

obtaining a reflector distance according to a preset optical path, wherein the relation between the optical path and the reflector distance is as follows:

wherein: l is the optical path, P is a positive integer, d is the distance between the reflectors, and d is 0-4 f Is the focal length, R is the radius of curvature of the reflector, R is the distance from the center of the entrance aperture to the optical axis, a=r ² +r ² B=t-2 r, t is the distance from the center of curvature to the distribution plane,

the reflection times are determined according to the reflector spacing, and the relation between the reflector spacing and the reflection times is as follows:

wherein: n reflector spacing is P, d is the reflector spacing, d has a value range of 0-4 f, f is a focal length, and R is the radius of curvature of the reflector;

determining the direction of the incident light according to the reflector spacing, wherein the relation between the reflector spacing and the direction of the incident light is as follows:

wherein: (x) ₀ ，y ₀ ) The coordinate of the incident point is that f is the focal length, d is the distance between the reflectors, and the value range of d is 0-4 f, (x ')' ₀ ，y’ ₀ ) Is the incident angle of light;

determining the position of each light spot on the reflector according to the direction of the incident light and the reflector spacing, wherein the relationship between the direction of the incident light and the reflector spacing and the position of each light spot on the reflector is as follows:

wherein: (x) _n ，y _n ) For each spot position coordinates on the mirror, (x) ₀ ，y ₀ ) The coordinate of the incident point is that f is the focal length, d is the distance between the reflectors, and the value range of d is 0-4 f, (x ')' ₀ ，y’ ₀ ) For the incident angle of light, theta is the included angle between two adjacent light spots to satisfy

Determining perforation positions according to the reflection times;

setting an exit hole on a reflecting mirror of the reflecting pool according to the exit hole position;

and adjusting the stepped spiral structure and the differential spiral structure of the reflection pool according to the distance between the reflectors to obtain the optical path.