A differential laser velocimeter for cooperative gratings with multiple grating pitches

Abstract

The present invention discloses a differential laser velocimeter for cooperative gratings with multiple grating pitches, belonging to the field of impact measurement. The present invention includes a laser light source, a polarizer, a first biconvex lens, a first parallel plate, a second parallel plate, a plano-convex lens, a cooperative grating, a first reflector, a second reflector, a second biconvex lens, a plano-concave lens, and a photodetector; and also includes an adjustable beam splitter prism. The present invention changes the beam splitting spacing of the beam splitter prism of the differential grating laser velocimeter, and only needs to adjust the small triangular prism in the middle of the beam splitter prism by moving back and forth to adjust the beam splitting spacing to the required size, so that the velocimeter can be simultaneously applied to cooperative gratings with multiple different grating constants without changing the overall structure. The present invention has the advantage of being able to replace cooperative gratings with different grating pitches without changing the overall structure of the velocimeter, and can flexibly select a suitable grating according to measurement requirements, thereby improving the applicability of the laser velocimeter and saving costs.

Description

Technical Field

[0001] The invention relates to a differential laser velocimeter for cooperative gratings with multiple grating pitches, and belongs to the field of impact measurement. Background Art

[0002] The laser absolute method is generally used in the field of impact measurement, that is, the acceleration is calibrated using a laser Doppler velocimeter. There are two main types of laser velocimeters that are most commonly used. One is a heterodyne (or zero-difference) laser vibrometer based on the Mach-Zehnder interferometer structure. Its working mode is axial measurement. The laser beam is divided into two beams, reference light and measurement light, by a beam splitter prism. The measurement light hits the target in the direction of the impact velocity. The reflected light carries information such as the velocity of the impact target and interferes with the reference light to form a Doppler signal. The relationship between the Doppler frequency shift Δf and the velocity v is: The impact velocity can be demodulated. Another most commonly used laser velocimeter is the differential laser velocimeter with lateral measurement mode. Its optical principle is shown in Figure 1(a). It needs to work with the help of a cooperative target - a reflection grating. The reflection grating is fixed on the side of the anvil of the impact device. The main principle is to use the Doppler frequency shift generated by the double incident beam grating. The light emitted by the laser is divided into two beams and then symmetrically illuminated on the surface of the reflection grating to diffract. By adjusting the incident angle of the grating through optical design, the diffracted light of the two beams can be returned along the optical axis and interfered. The interference signal is received by the photoelectric detector. Under the action of the impact, the anvil (and the grating) is driven to move axially, "cutting" the interference area, causing the movement of the interference fringes. The impact displacement / velocity can be measured by measuring the movement of the fringes. In order to ensure that the grating diffracted light waves form constructive interference, the optical path lengths passing through adjacent grating lines must differ by an integer multiple of half the wavelength. Let the grating constant be d, the incident angles of the two incident light beams of the grating be ψ and -ψ, and the diffraction angles be θ1 and θ2. When the grating moves, each level of diffracted light undergoes a Doppler frequency shift. The expression for the optical path difference can be used to deduce: The Doppler frequency shift According to the interference condition and grating equation, we can get (m, k are diffraction orders) The two diffracted lights coincide and the diffraction direction is along the optical axis, that is, θ1 = θ2 = 0, then Usually the +1 and -1 levels of the two beams are selected to make the interferometer lens aperture smaller. By comparing the working principles of the two types of laser velocimeters, it can be seen that the biggest advantage of the differential grating laser velocimeter is that it can reduce the frequency shift corresponding to the impact velocity, making it easier to measure high-speed impact velocity. Taking the differential laser Doppler velocimeter based on the grating constant d=150, that is, the 150 lines / mm cooperative grating as an example, according to the measurement formula, the impact velocity of 1m / s corresponds to a frequency shift of 0.316MHz, while the heterodyne laser vibrometer corresponds to a frequency shift of 3.16MHz. The former can greatly reduce the pressure on subsequent signal acquisition and processing circuits, making it easier to achieve high-speed measurement. However, the main problem with the traditional grating differential laser velocimeter is that once the structure is established, it can only work for a cooperative grating of a certain grating pitch. If the grating is replaced, the structural design and interferometer construction need to be re-performed, resulting in application limitations and increased use costs. Summary of the invention

[0003] The main purpose of the present invention is to provide a differential laser velocimeter for cooperative gratings with multiple grating pitches, and by changing the splitting spacing of the splitting prism of the differential grating laser velocimeter, the velocimeter can be simultaneously applied to cooperative gratings with multiple different grating constants without changing the overall structure. The present invention has the advantages of being able to replace cooperative gratings with different grating pitches without changing the overall structure of the velocimeter, being able to flexibly select a suitable grating according to measurement requirements, improving the applicability of the laser velocimeter, and saving costs.

[0004] The purpose of the present invention is achieved through the following technical solutions.

[0005] The invention discloses a differential laser velocimeter for cooperative gratings with various grating pitches, comprising a laser light source, a polarizing plate, a first biconvex lens, a first parallel plate, a second parallel plate, a plano-convex lens, a cooperative grating, a first reflector, a second reflector, a second biconvex lens, a plano-concave lens, and a photoelectric detector; and also comprises an adjustable beam splitter prism.

[0006] The adjustable beam splitter prism is composed of two upper and lower reflecting prisms and a small triangular prism; by adjusting the position of the small triangular prism, the laser beam splitting spacing is changed to adapt to the measurement of lateral motion speed under different cooperative gratings, that is, differential laser speed measurement of cooperative gratings with multiple grating pitches.

[0007] Furthermore, the adjustment of the position of the small triangular prism is achieved by:

[0008] According to the working principle of differential grating laser velocimeter, it is deduced that the splitting spacing of two parallel beams, the grating pitch and the wavelength satisfy the relationship: Where D is the splitting spacing of two parallel light beams, f2 is the focal length of the plano-convex lens, λ is the wavelength of the laser light source, and d is the grating constant, that is, the distance between every two adjacent lines of the grating. According to this formula, when the grating pitch of the cooperative grating (that is, the grating constant d) changes, it is only necessary to adjust the splitting spacing to the desired size by moving the small triangular prism in the middle of the dichroic prism back and forth. As the grating pitch increases (the grating constant d is large), the smaller the spacing between the two parallel light beams, the small triangular prism needs to be translated to the left. As the grating pitch decreases (the grating constant d is small), the larger the spacing between the two parallel light beams, the small triangular prism needs to be translated to the right. The upper and lower reflecting prism components of the dichroic prism are both 45-degree reflecting surfaces. According to the geometric relationship, the interval change ΔD of the parallel light beams and the displacement Δx of the triangular prism satisfy a two-fold relationship, that is: ΔD = 2Δx.

[0009] Preferably, the first parallel flat plate and the second parallel flat plate are a first parallel glass flat plate and a second parallel glass flat plate.

[0010] The present invention discloses a differential laser velocimeter for cooperative gratings with multiple grating pitches. The working method is as follows: the laser light emitted by the laser light source is converted into linear polarized light of a certain polarization direction through a polarizing plate, a double convex lens and a plano-convex lens form a beam collimating lens group, the linear polarized light first passes through the double convex lens and then is divided into two parallel light beams with a certain interval by an adjustable beam splitter prism, and passes through a first parallel plate and a second parallel plate respectively, and the property of the parallel plates that does not change the direction of the incident light but causes the incident light beam to be translated is utilized, and the light splitting interval of the two separated parallel light beams is finely adjusted by rotating the first parallel plate and the second parallel plate, and the adjusted two parallel light beams are converged to the same point on the cooperative grating through the plano-convex lens, and then diffracted respectively. , generating multi-level diffraction light, according to the grating parameters of the cooperative grating, using the diffraction and interference theory of double incident light beams, by selecting a lens with a suitable focal length, controlling the incident angle of the two light beams on the grating (the two light beams are symmetrical relative to the optical axis, and the incident angles of the two gratings are equal), so that the direction of the diffracted light of a certain order of the two light beams is just along the direction of the optical axis, so that the two light beams interfere with each other, and the interference light beams pass through the first reflector and the second reflector in turn, and enter the magnifying glass group composed of a double convex lens and a plano-concave lens, and the formed interference fringes are amplified and then enter the detector, in order to deal with the interference signal when the light beam deflects or the interference fringes jitter caused by external interference, the receiving aperture of the photocell detector can still receive it. When the interferometer is working, the cooperative target will be fixed on the surface of the target to be measured. When the grating (the object to be measured) moves horizontally perpendicular to the optical axis, the interference fringes will move. By measuring the change in the fringes, the Doppler frequency shift signal generated by the grating movement can be obtained. According to the laser Doppler principle, the linear relationship between Doppler frequency shift Δf and velocity v, Δf=2v / λ, where λ is the laser wavelength, can be used to calculate the moving speed of a moving object.

[0011] According to the working principle of the differential grating laser velocimeter, when the grating pitch of the cooperative grating and the selected interference diffraction light order are determined, the grating incident angle of the two light beams is determined, which means that the spacing between the two parallel light beams when the laser beam is divided into two is fixed. In order to ensure that the structure and component parameters can be simultaneously applicable to multiple cooperative gratings with different grating pitches, it is necessary to adjust the spacing of the two parallel light beams separated by the original laser beam. The core component to achieve this function is an adjustable beam splitter prism, which is composed of two irregular upper and lower reflection prism modules and a movable right-angle prism in the middle. The front light-transmitting surface is coated with a semi-transparent and semi-reflective film that matches the working wavelength, so that the light is divided into two, half of which is reflected into the upper prism, and the other half is transmitted to the rear light-transmitting surface of the right-angle prism. The rear light-transmitting surface is coated with a total reflection film that matches the working wavelength to ensure that the light is completely reflected and enters the lower reflection prism through the bottom surface of the right-angle prism. The reduction and increase of the beam splitting spacing are achieved by moving the middle triangular prism forward and backward along the optical axis, and the spacing between the two light beams required by different cooperative gratings is adjusted according to the calculation results of the spacing between the two light beams, and the optical path is compensated and adjusted through the first parallel plate and the second parallel plate.

[0012] Beneficial Effects

[0013] 1. The present invention discloses a differential laser velocimeter for cooperative gratings with multiple grating pitches. By changing the splitting spacing of the splitting prism of the differential grating laser velocimeter, the velocimeter can be simultaneously applied to cooperative gratings with multiple different grating constants without changing the overall structure. Compared with the existing differential laser velocimeter, the present invention has the advantages of being able to replace cooperative gratings with different grating pitches without changing the overall structure of the velocimeter, being able to flexibly select a suitable grating according to measurement requirements, improving the applicability of the laser velocimeter without requiring testing, and saving costs.

[0014] 2. The present invention discloses a differential laser velocimeter for cooperative gratings with various grating pitches. According to the working principle of the differential grating laser velocimeter, it is deduced that the splitting spacing of two parallel light beams satisfies the grating pitch and the wavelength. According to the formula, when the grating pitch of the cooperative grating changes, it is only necessary to adjust the splitting spacing to the required size by moving the small triangular prism in the middle of the splitting prism back and forth, so as to realize the change of the laser beam splitting spacing to adapt to the measurement of the lateral motion speed under different cooperative gratings, that is, differential laser velocimeter for cooperative gratings with various grating pitches. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] FIG. 1( a ) is a schematic structural diagram of a differential laser velocimeter for cooperative gratings with multiple grating pitches according to the present invention;

[0016] FIG1( b ) is a schematic diagram of the structure of the core structure adjustment beam splitter prism of the present invention;

[0017] FIG1(c) is a schematic diagram showing an application example of the present invention, in which the adjustable module of the beam splitter prism uses a pulley motion to adjust the beam splitting spacing;

[0018] Among them, 1 is a laser light source, 2 is a polarizer, 3 is a double convex lens, 4 is an adjustable beam splitter prism, 5 is a first parallel plate, 6 is a second parallel plate, 7 is a plano-convex lens, 8 is a cooperative grating, 9 is a first reflector, 10 is a second reflector, 11 is a double convex lens, 12 is a plano-concave lens, and 13 is a photodetector. DETAILED DESCRIPTION

[0019] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0020] Embodiment 1:

[0021] As shown in Figures 1(a) and (b), the present embodiment discloses a differential laser velocimeter for cooperative gratings with various grating pitches, comprising a laser light source 1, a polarizer 2, a biconvex lens 3, an adjustable beam splitter prism 4, a first parallel plate 5, a second parallel plate 6, a plano-convex lens 7, a cooperative grating 8, a first reflector 9, a second reflector 10, a biconvex lens 11, a plano-concave lens 12, and a photodetector 13.

[0022] The laser light source 1 is emitted by a laser, and is converted into linear polarized light of a predetermined polarization direction by a polarizing plate 2. A convex lens 3 and a plano-convex lens 7 form a beam collimating lens group. The linear polarized light first passes through the convex lens 3 and is then divided into two parallel beams of a predetermined interval by an adjustable beam splitter prism 4, and passes through a first parallel plate 5 and a second parallel plate 6 respectively. By utilizing the property that the parallel plates do not change the direction of the incident light but cause the incident light beam to shift, the beam splitting interval of the two separated parallel light beams can be fine-tuned by rotating the parallel plates. After the two adjusted parallel light beams converge to the same point on a cooperative grating 8 by a plano-convex lens 7, they are diffracted respectively to generate multi-level diffracted light. According to the parameters such as the grating pitch of the cooperative grating used, the diffraction and interference of the double incident light beams are utilized. According to theoretical calculation, by selecting a lens with a suitable focal length, the incident angle of the two light beams on the grating can be controlled (the two light beams are symmetrical relative to the optical axis, and the incident angles of the two gratings are equal), so that the direction of the diffracted light of a certain order of the two light beams is just along the direction of the optical axis. For example, the +1st order and -1st order diffracted light of the two light beams can be selected respectively, and the directions of both coincide with the direction of the optical axis, and interference occurs. The interference light beam passes through the first reflector 9 and the second reflector 10 in turn, and enters the magnifying glass group composed of the double convex lens 11 and the plano-concave lens 12. The formed interference fringes are magnified and then enter the detector 13. The purpose is to deal with the interference signal when the light beam deflects or the interference fringes jitter caused by external interference, and the interference signal can still be received by the receiving aperture of the photocell detector. When the interferometer is working, the cooperative target 8 will be fixed on the surface of the target to be measured. When the grating (the object to be measured) moves horizontally perpendicular to the direction of the optical axis, the interference fringes will move. By measuring the change in the fringes, the Doppler frequency shift signal generated by the grating movement can be obtained. According to the laser Doppler principle, the linear relationship between Doppler frequency shift Δf and velocity v, Δf=2v / λ, where λ is the laser wavelength, can be used to calculate the moving speed of a moving object.

[0023] According to the working principle of the differential grating laser velocimeter, when the grating pitch of the cooperative grating 8 and the selected interference diffraction light order are determined, the grating incident angle of the two light beams is determined, which means that the spacing between the two parallel light beams when the laser beam is divided into two is fixed. In order to ensure that the structure and component parameters can be simultaneously applied to multiple cooperative gratings with different grating pitches, it is necessary to adjust the spacing of the two parallel light beams separated by the original laser beam. The core component to achieve this function is the adjustable beam splitter prism 4, which is composed of two irregular upper and lower reflecting prism modules and a movable right-angle prism in the middle, wherein the front and rear light-transmitting surfaces of the triangular prism are both 45-degree inclined surfaces, and the front light-transmitting surface is coated with a semi-transparent and semi-reflective film that matches the working wavelength, so that the light is divided into two, half of which is reflected into the upper prism, and the other half is transmitted to the rear light-transmitting surface of the right-angle prism, and the rear light-transmitting surface is coated with a total reflection film that matches the working wavelength, ensuring that the light is completely reflected and enters the lower reflecting prism through the bottom surface of the right-angle prism. By moving the middle triangular prism forward and backward along the optical axis, the reduction and increase of the beam splitting spacing can be achieved, and it is adjusted according to the calculation results of the spacing between the two light beams required by different cooperative gratings, and the optical path is compensated and adjusted through parallel glass plates 5 and 6.

[0024] The structure of the adjustable beam splitter prism is mainly composed of two upper and lower 45° reflecting prisms and a movable small triangular prism in the middle. It can be but is not limited to this structure. For example, the movable small triangular prism can be replaced by two cubic prisms.

[0025] Embodiment 2:

[0026] As shown in Figures 1(a) and (b), the present embodiment discloses a differential laser velocimeter for cooperative gratings with various grating pitches, comprising a laser light source 1, a polarizer 2, a biconvex lens 3, an adjustable beam splitter prism 4, a first parallel plate 5, a second parallel plate 6, a plano-convex lens 7, a cooperative grating 8, a first reflector 9, a second reflector 10, a biconvex lens 11, a plano-concave lens 12, and a photodetector 13.

[0027] According to the parameters of the main components in the initial structure of the laser velocimeter, the moving distance formula and working conditions of the triangular prism when replacing different cooperative gratings 8 are specifically introduced: assuming that the wavelength of the laser light source 1 is λ, the laser wavelength is , the focal length of the double convex lens 3 is f1, the focal length of the plano-convex lens 7 is f2, and the grating constant of the cooperative grating 8 is d, the corresponding grating pitch is 1 / dmm. According to the working principle of the velocimeter, assuming that the ±1-order diffraction light of the cooperative grating is selected (relative to the higher-order diffraction light, the diffraction angle of the ±1-order diffraction light is small, which can effectively reduce the size of the optical element), according to the grating equation: dsinθ=kλ (where k is the diffraction order and θ is the diffraction angle), the corresponding diffraction angle can be obtained to satisfy According to the optical path principle and structure of the velocimeter, if it is to be ensured that the ±1-order diffracted light overlaps and interferes along the optical axis and then passes through the reflector 9 and other components to the photodetector 13, the diffraction angle θ should also satisfy: Where D is the distance between the two parallel beams incident on the cooperative grating. According to the two relations of the diffraction angle, the distance between the two parallel beams can be obtained: It can be seen that when the grating pitch (i.e. d) of the cooperative grating changes, it is only necessary to adjust the dichroic interval to the desired size by moving the triangular prism in the middle of the dichroic prism 4 back and forth. As the grating pitch increases (the grating constant d is large), the spacing between the two parallel light beams becomes smaller, and the small triangular prism needs to be translated to the left. As the grating pitch decreases (the grating constant d is small), the spacing between the two parallel light beams becomes larger, and the small triangular prism needs to be translated to the right. The reflective surfaces of the upper and lower reflecting prism components of the dichroic prism 4 are both 45-degree reflective surfaces. According to the geometric relationship, the interval change ΔD of the parallel light beams and the displacement Δx of the triangular prism satisfy a two-fold relationship, that is: ΔD=2Δx. In the actual structure, there is a certain air gap between the upper and lower reflecting components of the dichroic prism 4 and the small triangular prism in the middle. Assuming that the light beam is incident at the middle height of the small triangular prism in the middle, the initial intervals between the light beam incident point and the upper and lower reflecting prisms are both The heights of the upper and lower prisms and other dimensions can be calculated, thereby flexibly designing and adjusting the size of the beam splitter prism.

[0028] A specific parameterized example is used to illustrate: suppose the optical axis passes through the geometric center of the right-angle prism, and assume that the wavelength λ of the laser light source 1 of the initial interferometer is 633nnm, the focal length of the double convex lens 3 is 100mm, the focal length of the plano-convex lens 7 is 300mm, and the grating constant of the cooperative grating 8 is 150 lines / mm, that is, the splitting distance at this time is about 57mm, and the air gap between the upper and lower light prisms of the splitter prism 4 and the middle right-angle prism is 5mm. The right-angle prism adopts a conventional model with a side length of 25.4mm. If the cooperative grating is replaced with 100 lines / mm, the required splitting distance is shortened to about 38mm. After geometric deduction and calculation, it is only necessary to move the middle right-angle prism of the splitter prism 4 to the left by (57-38) / 2=8.5mm, and adjust the two incident light beams of the grating to the best working state in combination with the parallel glass plates 5 and 6. Similarly, if a 200-line / mm reflection grating is used, the calculated splitting distance of the beam splitter prism is 85 mm. At this time, the middle movable prism of the beam splitter prism 4 needs to be moved rightward by (85-57) / 2=14 mm to increase the splitting distance to 85 mm, and the parallel glass plate is adjusted to achieve complete overlap of the light spots of the two beams of the grating. At this time, it is only necessary to ensure that the bottom surface of the beam splitter prism is greater than 17 mm, the lowest height of the reflection surface is less than 5 mm, and the highest point is greater than 29 mm.

[0029] Embodiment 3:

[0030] As shown in Figures 1(a) and (b), the present embodiment discloses a differential laser velocimeter for cooperative gratings with various grating pitches, comprising a laser light source 1, a polarizer 2, a biconvex lens 3, an adjustable beam splitter prism 4, a first parallel plate 5, a second parallel plate 6, a plano-convex lens 7, a cooperative grating 8, a first reflector 9, a second reflector 10, a biconvex lens 11, a plano-concave lens 12, and a photodetector 13.

[0031] The adjustable dichroic prism 4 can adjust the dichroic spacing according to the selected cooperative grating, and change the grating incident angle to ensure that after the grating is replaced, the diffracted light can still form interference, thereby improving the applicability of the laser speed meter. The adjustable dichroic prism is composed of three parts. The upper and lower irregular reflecting prisms are fixed, and the middle prism is a semi-transparent and semi-reflective mirror, which can slide back and forth to change the dichroic spacing of the two beams of light. The laser speed meter based on the cooperative grating with a certain initial state is described as follows: the light source is a He-Ne laser with a wavelength of 632.8nm, the lens 3 is a biconvex lens with a focal length of 100mm, and the lens 7 is a plano-convex lens with a focal length of 300mm, that is, the working distance of the grating from the laser speed meter lens is 300mm, the biconvex lens 11 is a biconvex lens with a focal length of 120mm, and the plano-concave lens 12 is a plano-concave lens with a focal length of 15mm. Due to the grating constant of the cooperative grating According to the grating equation, the incident angles of the two incident light beams of the grating can be obtained satisfy: When , the positive and negative first-order diffraction angles of the two light beams are 0, that is, the positive and negative first-order diffracted light returns along the optical axis. At this time, according to the triangular geometric relationship, it can be calculated that the splitting distance of the beam splitter is about 57mm. This distance can be compensated and corrected by parallel glass plates 5 and 6 to ensure that the light spots of the two light beams completely overlap on the grating surface. If the cooperative grating is replaced with a reflection grating with a line / mm of 100, that is, the grating constant It can be calculated that the splitting distance of the beam splitter should be 38mm. According to the geometric relationship, it can be deduced that the change in the interval between the two outgoing light beams of the beam splitter satisfies a 2-fold relationship with the moving distance of the right-angle prism.

[0032] The specific description above further illustrates the purpose, technical solutions and beneficial effects of the invention in detail. It should be understood that the above is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A differential laser velocimeter for cooperative gratings with multiple grating pitches, characterized in that: It includes a laser light source, a polarizer, a first biconvex lens, a first parallel plate, a second parallel plate, a plano-convex lens, a cooperative grating, a first reflector, a second reflector, a second biconvex lens, a plano-concave lens, and a photoelectric detector; and also includes an adjustable beam splitter prism; The adjustable beam splitter prism is composed of two upper and lower reflecting prisms and a small triangular prism; by adjusting the position of the small triangular prism, the laser beam splitting spacing is changed to adapt to the measurement of lateral motion speed under different cooperative gratings, that is, differential laser speed measurement of cooperative gratings with multiple grating pitches.

2. A differential laser velocimeter for cooperative gratings with multiple grating pitches as claimed in claim 1, characterized in that: The method for adjusting the position of the small triangular prism is as follows: According to the working principle of differential grating laser velocimeter, it is deduced that the splitting spacing of two parallel beams, the grating pitch and the wavelength satisfy the relationship: Wherein D is the splitting spacing of two parallel light beams, f2 is the focal length of the plano-convex lens, λ is the wavelength of the laser light source, and d is the grating constant, i.e., the distance between every two adjacent lines of the grating. According to the formula, when the grating pitch of the cooperative grating changes, it is only necessary to adjust the splitting spacing to the desired size by moving the small triangular prism in the middle of the splitting prism back and forth. As the grating pitch increases, the spacing between the two parallel light beams becomes smaller, and the small triangular prism needs to be translated to the left. As the grating pitch decreases, the spacing between the two parallel light beams becomes larger, and the small triangular prism needs to be translated to the right. The reflective surfaces of the upper and lower reflective prism assemblies of the adjusted splitting prism are both 45-degree reflective surfaces. According to the geometric relationship, the interval change ΔD of the parallel light beams and the displacement Δx of the triangular prism satisfy a two-fold relationship, i.e., ΔD=2Δx.

3. A differential laser velocimeter for cooperative gratings with multiple grating pitches as claimed in claim 2, characterized in that: The first parallel flat plate and the second parallel flat plate are a first parallel glass flat plate and a second parallel glass flat plate.

4. A differential laser velocimeter for cooperative gratings with multiple grating pitches as claimed in claim 1, 2 or 3, characterized in that: The working method is as follows: the laser light emitted by the laser light source is converted into linear polarized light of a certain polarization direction through a polarizing plate; a double convex lens and a plano-convex lens constitute a beam collimating lens group; the linear polarized light first passes through the double convex lens and is then divided into two parallel light beams with a certain interval by an adjustable beam splitter prism, and passes through a first parallel plate and a second parallel plate respectively; by utilizing the property that the parallel plates do not change the direction of the incident light but cause the incident light beam to shift in a translational manner, the light splitting interval of the two separated parallel light beams is finely adjusted by rotating the first parallel plate and the second parallel plate; the adjusted two parallel light beams are converged to the same point on the cooperative grating through the plano-convex lens, and then diffracted respectively to generate multi-level diffracted light; according to the grating parameters of the cooperative grating adopted, the diffraction and interference theory of the double incident light beams is used for calculation, and by selecting a lens with a suitable focal length, the incident angles of the two light beams on the grating are controlled so that the two light beams are The direction of a certain order of diffraction light of the beam is just along the optical axis, so that the two beams interfere with each other. The interference light beam passes through the first reflector and the second reflector in turn, and enters the magnifying glass group composed of a double convex lens and a plano-concave lens. The formed interference fringes are amplified and then enter the detector. The purpose is to deal with the interference signal when the light beam deflects or the interference fringes shake due to external interference, and the interference signal can still be received by the receiving aperture of the photocell detector; when the interferometer is working, the cooperative target will be fixed on the surface of the target to be measured. When the grating moves horizontally perpendicular to the optical axis, the interference fringes will move. The Doppler frequency shift signal generated by the grating movement can be obtained by measuring the change in the fringes; according to the laser Doppler principle, the linear relationship between the Doppler frequency shift Δf and the speed v Δf=2v / λ, where λ is the laser wavelength, can be used to solve the moving speed of the moving object.

5. A differential laser velocimeter for cooperative gratings with multiple grating pitches as claimed in claim 4, characterized in that: According to the working principle of the differential grating laser velocimeter, when the grating pitch of the cooperative grating and the selected interference diffraction light order are determined, the grating incident angle of the two light beams is determined, which means that the spacing between the two parallel light beams when the laser beam is divided into two is fixed; in order to ensure that the structure and component parameters can be simultaneously applicable to multiple cooperative gratings with different grating pitches, it is necessary to adjust the spacing between the two parallel light beams separated by the original laser beam. The core component to achieve this function is the adjustable beam splitter prism, which is composed of two irregular upper and lower reflecting prism modules and a movable right-angle prism in the middle, in which the front and rear of the triangular prism are Both light-transmitting surfaces are 45-degree inclined surfaces. The front light-transmitting surface is coated with a semi-transparent and semi-reflective film that matches the working wavelength, so that the light is divided into two, half of which is reflected into the upper prism, and the other half is transmitted to the rear light-transmitting surface of the right-angle prism. The rear light-transmitting surface is coated with a total reflection film that matches the working wavelength to ensure that the light is completely reflected and enters the lower reflection prism through the bottom surface of the right-angle prism. The reduction and increase of the splitting spacing are achieved by moving the middle triangular prism forward and backward along the optical axis, and the spacing between the two light beams required by different cooperative gratings is adjusted. The optical path is compensated and adjusted through the first parallel plate and the second parallel plate to improve the speed measurement accuracy.

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