CN111982817B - Variable optical path multiple reflection pool and optical path adjusting method - Google Patents
Variable optical path multiple reflection pool and optical path adjusting method Download PDFInfo
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- CN111982817B CN111982817B CN202010878267.6A CN202010878267A CN111982817B CN 111982817 B CN111982817 B CN 111982817B CN 202010878267 A CN202010878267 A CN 202010878267A CN 111982817 B CN111982817 B CN 111982817B
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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
Technical Field
The application relates to the technical field of gas detection, in particular to a variable optical path multiple reflection pool and an optical path adjusting method.
Background
In many research fields and monitoring applications, low concentration gases can be accurately measured using laser spectroscopy techniques, where tunable laser absorption spectroscopy (TDLAS) based gas absorption cells have the advantage of high sensitivity, high accuracy, etc. in measuring gases. Several absorption cells commonly used in the TDLAS system are White type, matrix type and Herriott type, and the White type and the matrix type are characterized by larger aperture angle, can realize more reflection times, but use more reflectors; the Herriott type absorption cell is based on the principle that incident light is limited to be reflected back and forth between two or more reflecting mirrors coated with high-reflectivity films, so that the effective optical path length of interaction between the light and a substance is increased, namely, an absorption spectrum signal with high signal-to-noise ratio is obtained by increasing the absorption capacity of detected molecules on the incident light. Compared with the former two Herriott types, the light path is simple in structure, small in size, easy to adjust and suitable for a laser light source.
In practical application, because of different detection environments, higher requirements are put on the gas absorption cell. For low-concentration gas detection, the absorption tank is required to have high sensitivity, and the absorption tank is required to be suitable for detecting high-concentration gas, so that the reflection tank air chamber can be required to adjust the measuring range of the absorption tank according to different requirements, and the absorption tank can meet the requirements of different detection sensitivities and measuring ranges. For low concentration gases, to achieve a smaller gas detection limit, it is common to start with both enhancement of the absorption signal strength and suppression of noise. The main method of enhancing the absorption signal intensity is to increase the optical path, and according to lambert-beer's law, the longer the optical path means the stronger the absorption, the higher the sensitivity of the detector. In the multi-reflection cell, noise is almost impossible to completely avoid, the light beam path is changed in the light path adjusting process, and the influence of noise on absorption measurement is different under different light beam paths, but no exact model exists for the noise in the Herriott multi-reflection cell at present. Therefore, in order to meet the requirements of different detection sensitivity and measuring range and to explore the influence of the optical path on the noise in the absorption cell, the optical path of the absorption cell needs to be adjustable.
Disclosure of Invention
The embodiment of the invention provides a variable optical path multiple reflection pool and an optical path adjusting method, which are used for meeting the requirements of different detection sensitivities and measuring ranges and exploring the technical problem of influence of optical paths on noise in an absorption pool.
According to a first aspect of embodiments of the present invention, there is provided a variable optical path multiple reflection cell, the reflection cell comprising: : 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, wherein the quartz cavity is fixedly connected with the stepped spiral cavity through an intermediate plate, and the stepped spiral cavity is in threaded connection with the differential spiral cavity;
the absorption air chamber is provided with an air inlet and an air outlet;
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 two concave reflecting mirrors are fixed at the front end and the rear end of the absorption air chamber;
The reflector at the front end of the absorption air chamber is provided with an inlet hole;
the light window is arranged at the front end of the reflecting mirror at the front end of the absorption air chamber.
Preferably, the middle plate, the step spiral cavity and the differential spiral cavity are all made of aluminum materials, and the surfaces of the inner cavities are blackened.
Preferably, the aperture size of the reflecting hole is 1-3mm.
Preferably, the mirror is made of quartz material.
Preferably, the reflecting surface of the reflecting mirror is coated with gold.
Preferably, the gold film is coated with SiO 2 A layer.
Preferably, the joint of the quartz cavity and the middle plate and the stepped spiral cavity is sealed by an O-shaped ring.
Preferably, the entrance aperture is disposed proximate an edge of the mirror.
Preferably, a laser adjusting frame is arranged above the reflecting pool, the laser adjusting frame is used for installing and adjusting a laser, a plane reflecting mirror support is further arranged at the front end of the reflecting pool, the plane reflecting mirror support is used for installing a plane reflecting mirror, and the plane reflecting mirror is used for reflecting laser emitted by the laser into the light window.
According to a second aspect of the embodiment of the present application, there is provided a variable optical path multiple reflection cell optical path adjustment method, using the variable optical path multiple reflection cell provided in the first aspect of the embodiment of the present application, the method including:
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 optical path, P is positive integer, d is distance between reflectors, d is 0-4 f (f is focal length), R is radius of curvature of reflector, R is distance from center of incident hole to optical axis, and A=R 2 +r 2 ，B＝t-2R；
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 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.
As can be seen from the above technical solutions, the variable optical path multiple reflection cell and the optical path adjusting method provided by the embodiments of the present invention, 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, wherein the quartz cavity is fixedly connected with the stepped spiral cavity through an intermediate plate, and the stepped spiral cavity is in threaded connection with the differential spiral cavity; the middle plate is provided with an air inlet and an air outlet; 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 positioned at the front end and the rear end of the absorption air chamber; the light window is arranged at the front end of the reflecting mirror at the front end of the absorption air chamber. The length of the cavity is roughly adjusted through the wider thread fit of the stepped spiral structure, and the length of the cavity is finely adjusted through the narrower thread fit of the differential spiral structure, so that the accurate adjustment of the distance between the reflectors is realized, and the purpose of accurately adjusting the optical path is realized because the optical path and the distance between the reflectors have a clear corresponding relation.
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 as claimed.
Drawings
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the application and together with the description, serve to explain the principles of the application.
In order to more clearly illustrate the embodiments of the application or the technical solutions of the prior art, the drawings which are used in the description of the embodiments or the prior art will be briefly described, and it will be obvious to a person skilled in the art that other drawings can be obtained from these drawings without inventive effort.
FIG. 1 is a schematic diagram of a cross-sectional structure of a variable optical path multiple reflection cell according to an embodiment of the present application;
fig. 2 is a schematic perspective view of a variable optical path multiple reflection cell according to an embodiment of the present application;
FIG. 3 is a schematic diagram of an assembly of a step helix structure according to an embodiment of the present application;
FIG. 4 is a schematic diagram of an assembled differential spiral structure according to an embodiment of the present application;
FIG. 5 is a schematic diagram of a three-dimensional structure of another variable optical path multiple reflection cell according to an embodiment of the present application;
Fig. 6 is a basic flow chart of an optical path adjusting method of a variable optical path multiple reflection cell according to an embodiment of the present invention;
FIG. 7 shows an embodiment of the present invention providing an incident direction of x' 0 ＝0.06723，y′ 0 Light spot distribution diagram on mirror B at = -0.06802;
FIG. 8 shows the spot profile of the mirror A provided by the embodiment of the invention, (a) for 58 reflections and (b) for 118 reflections;
FIG. 9 is a schematic view of spot size provided by an embodiment of the present invention, (a) theoretical calculation, (b) simulation software;
fig. 10 is a graph of the relationship between the number of reflections and the distance between the reflectors (f=100 mm) according to an embodiment of the present invention;
fig. 11 is a graph of the relationship between the number of reflections and the distance between the mirrors at k= ±2 (f=100 mm) provided in the embodiment of the present invention;
FIG. 12 is a graph of optical path and mirror spacing relationship provided by an embodiment of the present invention;
FIG. 13 shows a plot of the speckle pattern for a mirror spacing of 193.8mm for an embodiment of the invention, (a) mirror A, (B) mirror B;
FIG. 14 shows the distribution of speckles at a mirror pitch of 206.5mm for an embodiment of the invention, (a) A mirror, (B) B mirror;
FIG. 15 shows the distribution of speckles at a mirror pitch of 398.4mm for the example of the present invention, (a) mirror A and (B) mirror B.
Description of the reference numerals:
1-an absorption air chamber; 11-a quartz chamber; 12-step helical cavity; 120-step helix structure; 121-a lower step helix; 122-an echelon spiral structure; 123-an up-step helix; 13-differential helical cavity; 130-differential screw structure; 131-upper differential screw structure; 132-medium differential screw structure; 133-lower differential screw structure; 2-a light window; a 3-mirror; 4-an intermediate plate; 5-air inlet; 6-an air outlet; 7-entering the hole; 8-a laser adjusting frame; 9-plane mirror support; 10-cage bars.
Detailed Description
In order to make the technical solution of the present invention better understood by those skilled in the art, the technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention, and it is apparent that the described embodiments are only some embodiments of the present invention, not all embodiments. All other embodiments, which can be made by one of ordinary skill in the art based on the embodiments of the invention given the inventive faculty, shall fall within the scope of the invention.
Fig. 1 is a schematic cross-sectional structure of a variable optical path multiple reflection cell according to an embodiment of the present invention, fig. 2 is a schematic perspective structure of a variable optical path multiple reflection cell according to an embodiment of the present invention, fig. 3 is a schematic diagram of a step spiral structure assembly according to an embodiment of the present invention, fig. 4 is a schematic diagram of a differential spiral structure assembly according to an embodiment of the present invention, fig. 5 is a schematic perspective structure of another variable optical path multiple reflection cell according to an embodiment of the present invention, and a variable optical path multiple reflection cell according to an embodiment of the present invention is further described below with reference to fig. 1 to 5.
As shown in fig. 1 to 2, the variable optical path multiple reflection cell includes: an absorption air chamber 1, an optical 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 sequentially connected, 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 threaded connection with the differential spiral cavity 13. The absorption air chamber 1 is provided with an air inlet 5 and an air outlet 6, and preferably the intermediate plate 4 is provided with the air inlet 5 and the air outlet 6. As shown in fig. 3, the stepped spiral cavity 12 is formed of a stepped spiral structure 120 including a lower stepped spiral structure 121, an upper stepped spiral structure 123, an intermediate stepped spiral structure 122, and a stepped spiral structure 120 connected in sequence. As shown in fig. 4, the differential screw cavity 13 is formed by a differential screw structure 130, and the differential screw structure 130 includes an upper differential screw structure 131, a middle differential screw structure 132, a lower differential screw structure 133, and a differential screw structure 130 connected in sequence. The reflecting mirror 3 comprises two concave reflecting mirrors 3 with the same focal length and confocal, the reflecting mirrors 3 are fixed at the front end and the rear end of the absorption air chamber 1, and for convenience of subsequent description, the reflecting mirror 3 at the front end of the absorption air chamber 1 is defined as an A mirror, and the reflecting mirror 3 at the rear end of the absorption air chamber 1 is defined as a B mirror. The optical window 2 is provided at the front end of the front end mirror 3 (a mirror) of the absorption cell 1. In the assembly process, the optical axes of the two concave mirrors 3 are required to be mutually overlapped, but in the machining process, certain errors can not be avoided, so that the optical axes of the two concave mirrors 3 deviate, and the positions of the concave mirrors need to be finely adjusted.
As a preferred implementation manner provided by the embodiment of the application, the middle plate 4, the stepped spiral cavity 12 and the differential spiral cavity 13 are all made of aluminum materials, and the surfaces of the inner cavities are blackened, so that the corrosion resistance is good and the adsorptivity to gas is low.
The aperture of the incident hole 7 also has an influence on the reflection times, and the incident hole 7 is too large, so that light leakage and other phenomena can be caused.
The lower limit of the optical path is a single optical path, the upper limit of the optical path has close relation with the power and the specular reflectivity of the incident light, the reflector 3 is made of quartz material, the reflecting surface is coated with a gold film for improving the reflectivity, and the gold film is provided with SiO 2 The layer can prevent the damage of acidic or alkaline corrosive gases to the gold film. The reflector 3 with a gold-plated mirror surface is selected in the design, and the reflectivity can reach more than 99 percent.
The intensity of the laser light in vacuum (without particle order) needs to be measured before the concentration of the particles is measured, which requires good sealing performance of the multi-path absorption cell. The specific gas is subjected to a gas detection experiment, and a vacuumizing and ventilation process is needed, so that the multi-reflection tank device has certain gas tightness requirements. In a preferred embodiment provided by the embodiment of the application, the connection part of the quartz cavity 11, the middle plate 4 and the stepped spiral cavity 12 is sealed by adopting an O-shaped ring, so that the tightness of the air chamber is improved, and the O-shaped ring is made of a corrosion-resistant material. The O-shaped sealing ring adopted by the device is used for sealing, and the radial sealing characteristic is that radial gaps are filled and axial circulation is blocked. The axial sealing feature fills the axial gap and impedes radial flow. Because the radial seal does not need pretightening force, the structure is more stable, and the structure is simple and convenient, so the radial seal structure is selected on the O-shaped ring seal structure form. In the practical structure, grooves are reserved at the joints of the two ends of the cavity so as to place sealing rings, thereby meeting the requirement of air chamber tightness.
In order to more effectively utilize the mirror surface and avoid the mutual interference of adjacent light spots, the position of the incident hole 7 should be as close to the edge of the reflecting mirror 3 as possible, so that the arrangement of the light spots is more dispersed.
In order to improve the integration of the device, as shown in fig. 5, a laser adjusting frame 8 is disposed above the reflecting pool, the laser adjusting frame 8 is used for installing and adjusting a laser, a plane mirror support 9 is further disposed at the front end of the reflecting pool, 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. In addition, in order to avoid the light spot interference phenomenon when obtaining more reflection times, a proper light source should be selected as much as possible, so that the light spot size is as small as possible.
As a preferred implementation manner provided by the embodiment of the application, the cage bars 10 are used for fastening the parts of the absorption air chamber 1, the optical window 2 and the reflecting mirror 3 of the multi-reflecting pool. The size of the portable multi-reflection pool is required to be as small as possible, and the size of the mechanical fixing support is reduced as much as possible according to the size of the concave mirror in the design process.
In combination with actual theory, the specific dimension needs to be repeatedly modified in the design process so as to meet the actual processing requirement, so that the processing of the mechanical fixing bracket can meet the requirement. The design adopts CAD software to carry out mechanical design, can generate a three-dimensional stereogram and engineering drawing with visibility, and can carry out simulation assembly on each part so as to check the feasibility of the device. The fixed support has 44 parts, and mainly comprises a quartz cavity 11, an incident structure assembly, a laser adjusting frame 8, a cavity length adjusting assembly, a cage bar 10 and other parts.
The working principle of the variable optical path multiple reflection pool is as follows:
the distance between the reflectors 3 at the front end and the rear end of the absorption air chamber 1 is changed by adjusting the length of the reflection pool, so that the number of times of back and forth reflection of the light beam between the two reflectors 3 is changed, and the purpose of adjusting the optical path is achieved. As shown in fig. 1, the adjusting structure is taken as a part of the cavity and is divided into two parts of a step spiral structure 120 and a differential spiral structure 130, wherein the step spiral structure 120 comprises an upper step spiral structure, a middle step spiral structure and a lower step spiral structure, and the main function is to roughly adjust the length of the cavity through the matching of threads with wider pitches; the differential screw comprises an upper differential screw structure, a middle differential screw structure and a lower differential screw structure, and the main function is to finely adjust the length of the cavity through the matching of threads with narrower pitches. By rotating the middle step spiral structure 122 and the middle differential spiral structure 132, the step increase or the step decrease of the cavity length of the absorption air chamber 1 is realized, and the relative distance of the reflecting mirror 3 is further changed, so that the optical path of the absorption air chamber 1 is adjustable.
Working principle analysis of the spiral structure:
functional design targets: the precise and adjustable cavity length of the multi-reflection pool in a larger range of centimeter level is realized.
The device comprehensively considers factors such as adjustment convenience and accuracy, adopts a coarse-fine two-stage adjustment mode, and designs a cavity length adjustment structure.
(1) Coarse adjustment structure
As shown in fig. 3, a stepped spiral structure 120 is designed, which is composed of an upper stepped spiral structure 123, a middle stepped spiral structure 122, and a lower stepped spiral structure 121. According to the meshing principle of the screw thread screwing direction, the same group of meshing screw threads are consistent in screwing direction. The upper step screw structure 123 is a right-handed internal thread, which is engaged with the right-handed external thread of the middle step screw structure 122, and the left-handed external thread of the lower step screw structure 121 is engaged with the left-handed internal thread of the middle step screw structure 122.
The thread pitches of the two sets of threads are equal and are all single-line threads, and the lower step spiral structure 121 is fixed. When the upper step screw structure 123 is rotated, the externally applied torque is set to be M, and the rotation friction torque acting on the upper step screw structure 123 against the center step screw structure 122 is set to be M (f) 1 ) The actual friction torque is M (f); the rotation friction torque of the middle step helical structure 122 acting on the lower step helical structure 121 is M (f) 2 ) The actual friction moment is M (f'), M (f) 1 )＞M(f 2 ). The rotation is uniform speed change motion with constant angular velocity.
The motion of the stepped spiral structure 120 will be briefly described in steps.
Step one, the middle step helical structure 122 is fixed and rotated clockwise with the clockwise direction of the observation direction as the positive direction Moving up the stepped spiral structure 123. The upper step helix 123 is subjected to an externally applied torque M and to a friction torque-M (f) of the middle step helix 122. M (f) =m (f 1 )，M＝M(f 1 ) Then M-M (f) =0, so the upper step helix 123 rotates clockwise. Because of the right-hand thread engagement between the upper step helix 123 and the middle step helix 122, the upper step helix 123 will feed in a clockwise direction from the beginning of the thread.
Step two, the middle step spiral structure 122 is not fixed, and the upper step spiral structure 123 is rotated counterclockwise with the counterclockwise direction of the observation direction as positive. The upper step helix 123 is subjected to an externally applied moment M, while being subjected to a friction moment-M (f) of the middle step helix 122; the middle step helix 122 is subjected to a friction torque M (f) exerted by the upper step helix 123 and a friction torque M (f') exerted by the lower step helix 121. Because M (f) 1 )＞M(f 2 ) Then M (f) =m (f')=m (f) 2 ) The middle step helix 122 rotates counterclockwise relative to the lower step helix 121; m=m (f), the upper step helix 123 is relatively stationary with respect to the middle step helix 122. Because of the left-handed thread engagement between the middle step helix 122 and the lower step helix 121, the upper step helix 123 will feed clockwise with the middle step helix 122 at the beginning of the thread.
Step three, the middle step helical structure 122 is fixed again, and the upper step helical structure 123 is rotated counterclockwise with the counterclockwise direction of the observation direction as positive. In the same manner as the step, the upper step helix 123 is subjected to an externally applied torque M and to a friction torque-M (f) of the middle step helix 122. M (f) =m (f 1 )，M＝M(f 1 ) Then M-M (f) =0, so the upper step spiral structure 123 rotates counterclockwise. Because of the right-hand thread engagement between the upper step helix 123 and the middle step helix 122, the upper step helix 123 will return in a counter-clockwise direction from the beginning of the thread.
Step four, the fixing of the middle step screw structure 122 is canceled again, and the upper step screw structure 123 is rotated clockwise with the clockwise direction of the observation direction as the normal direction. Is identical with the second stepIn other words, the upper step helix 123 receives an externally applied moment M, while receiving the friction moment-M (f) of the middle step helix 122; the middle step helix 122 is subjected to a friction torque M (f) exerted by the upper step helix 123 and a friction torque M (f') exerted by the lower step helix 121. Because M (f) 1 )＞M(f 2 ) Then M (f) =m (f')=m (f) 2 ) The middle step helix 122 rotates clockwise relative to the lower step helix 121; m=m (f), the upper step helix 123 is relatively stationary with respect to the middle step helix 122. Since the middle step screw structure 122 is engaged with the lower step screw structure 121 by left-hand screw, the upper step screw structure 123 is returned along with the middle step screw structure 122 in the counterclockwise direction of the screw start end.
(2) Fine tuning structure:
as shown in fig. 4, the differential screw structure 130 is designed by adopting the principle, and the structure is composed of an upper differential screw structure 131, a middle differential screw structure 132 and a lower differential screw structure 133. The upper differential screw structure 131 and the middle differential screw structure 132 are engaged by right-handed screw, and the pitch is 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 threads, and the pitch is P 2 ，P 1 ＞P 2 . The upper differential screw structure 131 is fixed, the middle step screw structure 132 can rotate and translate, and the lower differential screw structure 133 can only translate.
The motion of the differential screw structure 130 is briefly described in steps.
Step one, the middle differential screw structure 132 is rotated clockwise by one circle according to the observation direction, and the middle differential screw structure 132 advances by one pitch P clockwise relative to the upper differential screw structure 131 at the beginning of the screw thread 1 Advancing a pitch P clockwise relative to the start of the thread by a pitch P relative to the lower differential screw structure 133 2 . Because the upper differential screw structure 131 is fixed and the lower differential screw structure 133 can translate, the lower differential screw structure 133 advances a pitch difference P clockwise relative to the upper differential screw structure 131 at the beginning of the screw thread 1 -P 2 。
Step two, the same as the step two, the time is reversed according to the observation directionThe middle differential screw structure 132 rotates one round, and the middle differential screw structure 132 returns a pitch P in the counterclockwise direction of the thread start end relative to the upper differential screw structure 131 1 The lower differential screw structure 133 is returned by a pitch P in a counterclockwise direction relative to the start of the screw 2 . Because the upper differential screw structure 131 is fixed and the lower differential screw structure 133 can translate, the return distance of the lower differential screw structure 133 relative to the upper differential screw structure 131 in the anticlockwise direction of the thread start end is a pitch difference P 1 -P 2 。
Based on the above implementation principle, the method for adjusting the optical path length of the variable optical path multiple reflection cell according to this embodiment will be described in detail with reference to the accompanying drawings. Fig. 6 is a basic flow chart of an optical path adjusting method of a variable optical path multiple reflection cell according to an embodiment of the present invention. As shown in fig. 6, the method specifically includes the following steps:
s1: 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 optical path, P is positive integer, d is distance between reflectors, d is 0-4 f (f is focal length), R is radius of curvature of reflector, R is distance from center of incident hole to optical axis, and A=R 2 +r 2 ，B＝t-2R；
S2: 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 focal length), and R is the radius of curvature of the reflector;
s3: determining the direction of the incident light according to the reflector spacing, wherein the relationship between the reflector spacing and the direction of the incident light is:
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;
s4: determining the position of each light spot on the reflector according to the direction of the incident light and the reflector spacing, wherein the relation 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 satisfyS5: determining perforation positions according to the reflection times;
s5: determining perforation positions according to the reflection times;
s6: setting an exit hole on a reflecting mirror of the reflecting pool according to the exit hole position;
S7: and adjusting the stepped spiral structure and the differential spiral structure of the reflecting pool according to the distance between the reflecting mirrors to obtain the optical path.
The key of the optical path adjusting method of the variable optical path multiple reflection pool provided by the embodiment of the application is a relation formula which is satisfied between the optical path and the distance between the reflectors and a relation formula which is satisfied between the distance between the reflectors and the reflection times. According to the relation between the optical path and the distance between the reflectors, the distance between the reflectors in the reflecting pool actually needs to be adjusted to realize the optical path can be obtained from the preset optical path; and determining the reflection times and the direction of incident light according to the distance between the reflectors, and further determining the perforation position. The following describes the relation determination process of the optical path and the reflector distance in detail through analysis and calculation of the position distribution of the optical spot and the size of the optical spot.
(1) Light spot distribution into round condition
For the Herriott type multiple reflection pool, the two concave reflectors of the air pool are parallel to each other, so that the perimeter of the concave reflectors is fully utilized, light spots on the concave reflectors are designed to be circularly arranged, the interval between two adjacent light spots is as large as possible, so that light rays are prevented from overflowing in advance in the reflection process, a certain gap is reserved between the light spots, and interference between the light spots cannot be generated. As shown in FIG. 1, the mirror is placed in the x-y plane, and the direction vector of the incident light is set as (x' 0 ，y′ 0 ，z′ 0 ) For simple operation, z 'in the direction vector is calculated' 0 Normalized to 1. In order to obtain a circularly distributed reflected spot, it is assumed that the laser incident point is (x 0 ，y 0 ) The slope of the incident ray is x' 0 、y′ 0 After n times of reflection, the intersection point of the light ray and the concave reflector is (x) n ，y n ) Two concave mirrors with focal length f have a center-to-center spacing d.
Herriott et al, according to the Pierce theorem, find formula (1):
the angle θ between two adjacent spots depends on the concave mirror focal length f and the spacing d, as shown in equation (2):
when 0 < d < 4f, light can be infinitely reflected between the two reflectors without overflowing.
Converting the form of formula (1) into the form of formula (3)
x n As much as =asin (nθ+α) (3)
Similarly, the y-direction coordinate y of the intersection point after the light rays are reflected n times n
Converting the form (6) into the form (3)
y n ＝Bsin(nθ+β) (7)
Wherein (7) formula
As can be seen from equations (3) to (9), the intersection point (x) after n reflections n 、y n ) With the incident point (x) 0 、y 0 ) Incidence (x' 0 、y′ 0 ) And d and f are related. Typically, the spots will be distributed in an elliptical shape, but under certain conditions the spots will be distributed in a circle on a concave mirror, i.e
A＝B (10)
That is, in the case where the equations (12) and (13) are satisfied, the spots will be arranged in a circular shape on the concave mirror.
Thus, the concave mirror is selected, i.e. the pitch f is determined, and the point of incidence (x 0 ，y 0 ) When fixed, as can be seen from (12) and (13), the light spots are arranged on the mirror at the required light incidence angle (x '' 0 、y′ 0 ) Only in relation to the distance d between the two concave mirrors, i.e. at a certain distance d, the light is incident at a specific angle, and the light spots are arranged in a circle on the concave mirrors.
A concave reflector with a focal length of 100mm is selected, the distance between the reflectors is set to 170mm, the incidence point of the light is at (1, 12.4), and the incidence direction can be calculated as follows: x's' 0 ＝0.06723，y′ 0 = -0.06802. FIG. 7 shows an embodiment of the present invention providing an incident direction of x' 0 ＝0.06723，y′ 10 As can be seen from fig. 7, the beam spot distribution diagram on the B mirror at = -0.06802 is that the beam spots are arranged in a circle on the mirror when the incident light is at a specific angle, and the specific position of each beam spot can be found according to formula (1).
(2) Spot size calculation
If the size of the reflected light spot is known, and the position of the reflected light spot is combined, whether adjacent light spots on the reflecting mirror are overlapped or not can be determined, and if the adjacent light spots are overlapped, light spot interference occurs, so that deviation occurs to a detection signal. Meanwhile, by knowing the size of the light spot, whether light leakage exists in the transmission process of the light ray or not can be better judged, for example, the center point of the light ray overflows from the incident hole, but the whole light spot does not completely exit, or the edge light ray of the light spot overflows from the incident hole, and the center of the light spot is still reflected, so that errors occur in the detection result. Under the condition that the size of each light spot on the reflecting mirror is known, the arrangement compactness of the light spots on the reflecting mirror can be adjusted, and proper incident holes and emergent holes are selected to avoid light spot interference and light leakage, so that detection errors are reduced. As shown in the figure, the arrangement of the light spots on the concave mirror is obtained under certain conditions, and the total reflection times are 58 times and 118 times respectively. In fig. 8 (a), light is incident from the incidence hole, and by observing the distribution of the spots, it can be seen that there is no overlap between the spots, but that part of the light of the 2 nd spot leaks out from the incidence hole. In fig. 8 (b), it is observed that the spots overlap each other, and it is apparent that the spot overlap easily occurs when the number of reflections is large. Both conditions can cause errors of the final detection result, and related parameters need to be reset, so that the light spot overlapping and light leakage conditions are avoided.
In practical situations, since the incident light is a set of a beam of incident light, i.e. innumerable incident light beams in the same direction, and the beam section is shaped into a circle, the light beam edge light beam with a certain divergence angle can be tracked, and the tracking of the light beam is realized. When the incident light is continuously reflected between the two mirrors, the cross section of the light beam is continuously changed in size due to the converging action of the spherical mirrors, so that the sizes of the reflection light spots on the mirrors are different, and the radius of the reflection light spots is changed according to a certain rule. In the z-axis direction, we assume that the incident ray has contours L1 and L2, and that the incident bit coordinates of the two lines in the x-axis direction are x 01 ，x 02 The slopes are x 'respectively' 01 ，x′ 02 The diameter of the light spot after n times of reflection is:
R L ＝|x n1 -x n2 |＝|A 1 sin(nθ+α 1 )-A 2 sin(nθ+α 2 )| (14)
and similarly in the y-axis direction as in the x-axis direction.
As shown in fig. 9 (a), when we calculated the number of reflections to be 30, the size and position distribution of the light spots on the mirror were equally spaced on the concentric circle of the entrance aperture. According to the same parameters, the light trace is performed by introducing the same parameters into simulation software, and the light intensity analysis is performed on the reflecting surface of the reflecting mirror, as shown in fig. 9 (b). It can be seen that the light spot distribution pattern obtained by the theory is basically consistent with the pattern obtained in the simulation software, so that the light spot size can be calculated.
(3) Reflection count calculation
From the equation (2), the included angle θ between two adjacent light spots is determined by the mirror distance d and the mirror focal length f.
Herriott et al propose that if the light satisfies the following formula:
nθ＝2μπ (15)
the light ray will pass through the incident point again after n times of reflection (x) 0 ，y 0 )＝(x n ，y n ) In theta
The number of reflections n increases from 1 in steps of 2 and the integer μ increases in steps of 1, μ representing the complete number of revolutions the reflection point has around the mirror in steps of angle θ. Given a number of reflections n, under a certain radius of curvature R, there are multiple sets of solutions that satisfy both equations (15) and (16), and the reflection modes corresponding to each set of solutions are slightly different, so that variables k and p are introduced to describe different reflection modes. To avoid confusion, the solution clusters are represented using the formula (17) and each special solution in the solution clusters is represented by { n, μ, k, p }:
n＝2pμ+k (17)
wherein k= ±2, ±4, ±6..p is a positive integer.
As can be seen from equations (14) through (16), the number of reflections n is a function of d, k, p, i.e
Fig. 10 is a graph of the relationship between the number of reflections n and the distance d between the reflectors (f=100 mm), and as shown in fig. 10, the redundant solution is eliminated, where n=2pμ+k, k= ±4, ±6, ±8, ±10 are included, the number of turns μ in the formula is strictly limited, and μ is not divided by 2, 3, 2, 5 when k= ±4, ±6, ±8, ±10 is not limited, respectively.
When the incident point and the exit point are in the same hole, the corresponding relationship between the number of turns μ and the number of reflections n is shown in table 1 when p=2 and k= -2.
Table 1 number of turns and number of reflections
The integer μ in the table is the complete number of reflection points on the concave mirror before the beam exits the cavity, and we find that for this solution cluster n=4μ -2, the number of reflections achieved n (even) is not continuous even when the number of turns μ is incremented by 1, and if μ can take the value 1.5,2.5. For example, to achieve a μ value of 1.5, i.e., a complete track of the beam within the cavity, leaving the reflective cavity midway along the second track, we can change the number of reflections by controlling the position of the exit point since we can determine the specific position of each reflection point. Similarly, for all solutions, p=1, 2,3, 4..when we take k= ±2 only, by controlling the position of the exit point such that the beam leaves the reflecting pool at the corresponding number of reflections, the graph of the number of reflections n versus the mirror spacing d will change as shown in fig. 11.
The mirror spacing d is continuously variable, and the number of reflections n can only be an integer, e.g. d=170 mm, which is 20.86 in the n=4p+2 solution cluster, we here have n=20 by rounding down. As can be seen from fig. 9, at a certain focal length, the number of reflections n is only related to the mirror spacing d, and by controlling the position of the exit point, we realize that the number of reflections is continuous at n=2pμ±2 (n is an integer), so we can obtain different numbers of reflections by changing the mirror spacing d, and the correspondence between them is already clear.
(4) Optical path computation
a)d＞R
In order to calculate the optical path more accurately, geometric analysis is carried out on the two reflectors and the light rays in a three-dimensional form, wherein Q is a curvature center, P is a projection point of a normal on a distribution surface, d is a center distance between the two reflectors, t is a distance from the curvature center to a far-mirror light spot distribution surface, v is a distance from the normal projected to the axis, s is a distance from the curvature center to a near-mirror distribution surface, and an included angle between the normal and the axis is delta.
Distance t from the center of curvature to the distribution surface
The angle between the normal line and the axis is delta
Center of curvature to another distribution surface s
s＝d+t-2R (21)
Normal projection to axis distance v
v＝s·tan(δ) (22)
Distance w from center of curvature Q to projection point P
Then, when d > R, the single optical path OD
b)d＜R
Distance t from the center of curvature to the distribution surface
The angle between the normal line and the axis is delta
Center of curvature to another distribution surface s
s＝2R-d-t (27)
Normal projection to axis distance v
v＝s·tan(δ) (28)
Distance w from center of curvature Q to projection point P
Then, when d < R, the single optical path OD
As can be seen from formulas (19) to (30), when 0 < d < R
From the equation (31), it can be found that OD is a function of d, R, and that the single optical path OD is a function of d with the concave mirror selected and the entrance aperture determined.
Then the total optical path L
L＝n·OD (32)
Since the number of reflections n is related to d only in the case of a certain reflection mode, and the single optical path OD is a function of d, it is known that the optical path L is a function of d:
Wherein a=r 2 +r 2 ，B＝t-2R。
The following is further illustrated by way of example in accordance with the methods and principles set forth above:
(1) Multiple reflection pool parameter design
TABLE 2 multiple reflection cell parameter design
The light source type is selected to be a gaussian light source.
TABLE 3 light source parameter design
(2) Determining mirror spacing and exit point position
After the completion of the setting of the parameters of the reflection cell system, assuming that the required optical path L is 19.82m, a graph of the relationship between the optical path L and the mirror spacing d can be obtained according to the equation (33), as shown in FIG. 12.
When the optical path l=19.82 m, it can be found that a plurality of d can satisfy the optical path, and it is apparent that the smaller d is, the more the number of reflections is required, the larger d is, and the fewer the number of reflections is required. D=193.9 mm,206.5mm and 398.3mm were therefore chosen. As can be seen from fig. 11, the number of reflections n=102 times corresponding to d= 193.9mm, the number of reflections n=96 times corresponding to d= 206.5mm, and the number of reflections n=48 times corresponding to d= 398.3mm.
Since the incident point position (x 0 、y 0 ) And focal length f, at d= 193.9mm, the direction of the incident light can be determined from formulas (12) and (13) as: x's' 0 ＝0.02251，y′ 0 -0.1419; d= 206.5mm, the direction of the incident light is: x's' 0 ＝0.01744，y′ 0 ＝-0.1381, a step of; d= 398.3mm, the direction of the incident light is: x's' 0 ＝-0.05488，y′ 0 = -0.08372. According to formulas (1), (6) and (14), the positions of each light spot on the reflecting mirror and the sizes of each light spot in three cases can be obtained, and as shown in tables 4-6, even light spots are distributed on the A mirror, and odd light spots are distributed on the B mirror.
Table 4 spot distribution at 4 d = 193.9mm
Table 5 spot distribution at 5 d = 206.5mm
Table 6 spot distribution at 6 d = 398.3mm
Proper exit holes can be arranged according to the specific positions and the sizes of the light spots so as to ensure that the reflection times meet the requirements. For example, a number of reflections of 102 is required, and then an exit aperture of 1mm is provided at a concave mirror a (1.8461, 19.9995) placed in the x-y plane.
(3) Design results and analysis
In simulation software, a spherical reflector is used as a concave reflector of a multiple reflection pool, a Herriott reflection pool model is built, a light source is arranged according to a laser light source of a semiconductor laser, a light path is simulated, and under the condition that d is different, a light spot distribution diagram is shown in figures 13-15.
As can be seen from fig. 13 to 15, in the case of different mirror pitches d, the spots can be arranged in a circular shape on the mirrors by setting the corresponding incidence angles, and the required number of reflections n can be achieved by setting appropriate exit holes. In fig. 13, it is found that although the number of reflections can be satisfied, there is a spot overlapping phenomenon, which causes spot interference. As can be seen from fig. 14 and 15, there is no overlapping and light leakage phenomenon of adjacent spots, but d= 398.4mm is larger than d= 206.5mm, so that miniaturization and sensitivity improvement are not facilitated, and d= 206.5mm is the best choice. At this time, the OPL was 19.823m as known from the simulation software optical path analysis. That is, when the optical path L is realized with the number of reflections of 96 at f=100 mm and d= 206.5mm, the error |Δl|=3 mm of the optical path L from the actual optical path OPL is 0.015% of the optical path l=19.82 m. According to the design method, herriott multiple reflection times of a 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 embodiments are mutually referred to, and each embodiment focuses on the differences from the other embodiments.
Other embodiments of the application will be apparent to those skilled in the art from consideration of the specification and practice of the application disclosed herein. This application is intended to cover any variations, uses, or adaptations of the application following, in general, the principles of the application and including such departures from the present disclosure as come within known or customary practice within the art to which the application pertains. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the application being indicated by the following claims.
It should be noted that, unless otherwise specified and defined, the terms "connected" and "connected" are to be construed broadly, and may be, for example, mechanical or electrical, or may be a direct connection between two elements, or may be an indirect connection via an intermediary, and the specific meaning of the terms may be understood by those of ordinary skill in the relevant art in view of the specific circumstances. The terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a circuit structure, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such article or apparatus. Without further limitation, the statement "comprises one … …" does not exclude that an additional identical element is present in an article or device that comprises the element. In addition, the term "and/or" as used herein includes any and all combinations of one or more of the associated listed items.
It is to be understood that the invention is not limited to the precise arrangements and instrumentalities shown in the drawings, which have been described above, and that various modifications and changes may be effected 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.
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