Two-dimensional refocusing laser differential interferometer and measurement method thereof

Abstract

The application discloses a two-dimensional refocusing laser differential interferometer and a measuring method thereof, and belongs to the field of flow field measurement. The two-dimensional refocusing laser differential interferometer comprises an optical signal emitting unit, a cylindrical lens, a first polarizer, a first light splitting prism, a first cylindrical convex lens, a second cylindrical convex lens, a second light splitting prism, a second polarizer, a photoelectric detector, a first spherical lens, a second spherical lens, a first spherical convex lens and a second spherical convex lens. The optical signal of the optical signal emitting unit sequentially passes through the first spherical lens, the second spherical lens, the cylindrical lens, the first polarizer, the first light splitting prism, the first cylindrical convex lens, the first spherical convex lens, the second spherical convex lens, the second cylindrical convex lens, the second light splitting prism and the second polarizer, and then reaches the photoelectric detector. The application solves the problems that the spatial resolution is reduced when a two-dimensional focusing laser differential interferometer measures the density pulsation of a near-wall region of a spanwise stretching model, and the non-measurement region has a great extra influence on the measurement region.

Description

Technical Field

[0001] This invention belongs to the field of flow field measurement, and more specifically, relates to a two-dimensional refocusing laser differential interferometer and its measurement method. Background Technology

[0002] Focused laser differential interferometry (FDI) is a non-invasive flow field density pulsation measurement technique developed in recent years, possessing excellent spatial resolution and time response frequency. (See traditional focused laser differential interferometry...) Figure 1 The density pulsation information of the focusing area is reflected by the interference change of the phase difference of the optical path. Due to the refraction of the side-view beam through the spherical lenses S0 S1 S2, the laser beam forms a cone-shaped beam between lenses S1 and S2 (see...). Figure 4 For measurements of the near-wall region of a spanwise stretched model, the conical beam can be blocked by the model's sides, affecting the measurement. Chinese Utility Model Patent CN110987357B discloses a two-dimensional focused laser differential interferometer and a method for measuring the density pulsation of a flat plate boundary layer (see...). Figure 2 This measurement method replaces the S0 S1 S2 spherical lenses of a traditional focused laser differential interferometer with cylindrical lenses C0 C1 C2. Since light passing through a cylindrical lens only refracts in the spanwise direction and remains unchanged in the normal direction, the laser beam forms a square beam between lenses C1 and C2 (see...). Figure 5 This allows for the measurement of the near-wall region of a flat plate. However, since the initial beam itself has a certain diameter, its focusing area is a vertical line with the same beam diameter. The measured density perturbation will be averaged along the entire vertical line, resulting in a lower spatial resolution compared to traditional focused laser differential interferometers. Furthermore, due to the smaller beam diameter and the smaller integration volume of the non-measurement area, it is easy to cause additional effects on the measurement area.

[0003] Therefore, the reduced spatial resolution when measuring density fluctuations in the near-wall region of a spanwise stretched model using a two-dimensional focused laser differential interferometer, and the significant additional impact of non-measurement regions on the measurement region, have become technical challenges in this field. Summary of the Invention

[0004] To address the aforementioned deficiencies or improvement needs of existing technologies, this invention provides a two-dimensional refocusing laser differential interferometer and its measurement method. The purpose is to optimize the spatial resolution of measurement points when measuring density pulsations in the near-wall region of a spanwise stretched model by improving the structure of the two-dimensional refocusing laser differential interferometer, thereby reducing the influence of non-measurement areas on the measurement results in the measurement area. This solves the technical problems of reduced spatial resolution and significant additional influence of non-measurement areas on the measurement area when measuring density pulsations in the near-wall region of a spanwise stretched model using a two-dimensional refocusing laser differential interferometer.

[0005] To achieve the above objectives, according to one aspect of the present invention, the following technical solution is provided:

[0006] A two-dimensional refocusing laser differential interferometer includes: an optical signal transmitting unit, a cylindrical lens, a first polarizer, a first beam splitter, a first cylindrical convex lens, a second cylindrical convex lens, a second beam splitter, a second polarizer, and a photodetector; it also includes: a first spherical lens, a second spherical lens, a first spherical convex lens, and a second spherical convex lens, wherein...

[0007] The optical signal from the optical signal transmitting unit passes sequentially through the first spherical lens, the second spherical lens, the cylindrical lens, the first polarizer, the first beam splitter, the first cylindrical convex lens, the first spherical convex lens, the second spherical convex lens, the second cylindrical convex lens, the second beam splitter, and the second polarizer before reaching the photodetector.

[0008] The optical signal transmitting unit is used to generate a parallel coherent beam; the first spherical lens is used to diverge the coherent beam in all directions; the second spherical lens is used to convert the diverging beam into a parallel beam; the cylindrical lens is used to diverge the parallel beam in a single direction; the first polarizer is used to filter out noise rays in the diverging coherent beam; the first beam splitter is used to separate the beam diverging in a single direction into two unidirectional diverging beams with equal intensity and mutually perpendicular polarization directions; the first cylindrical convex lens is used to convert the two unidirectional diverging beams back into parallel beams; the first spherical convex lens is used to focus the two parallel beams in all directions; the second spherical convex lens is used to convert the two diverging beams after the focal point back into parallel beams; the second cylindrical convex lens is used to convert the two parallel beams into unidirectional focused beams; the second beam splitter is used to merge the two unidirectional focused beams; the second polarizer is used to perform interference filtering on the merged coherent beam; the photodetector is used to linearly convert the intensity of the merged interference focused beam into a voltage signal.

[0009] Preferably, the first spherical lens is a concave spherical lens or a convex spherical lens, and the second spherical lens is a convex spherical lens; the absolute value of the focal length of the first spherical lens is smaller than the absolute value of the focal length of the second spherical lens.

[0010] Preferably, the distance L0 between the first spherical lens and the second spherical lens is f. S01 +f S02 The distance L1 between the cylindrical lens and the first cylindrical convex lens is f. C0 +f C1 The distance between the first spherical convex lens and the second spherical convex lens is 2L² = f. S1 +fS2 The distance between the second cylindrical convex lens and the photodetector is L3 = f C2 ; where f S01 It is the focal length of the first spherical lens, f S02 It is the focal length of the second spherical lens, f S1 It is the focal length of the first spherical convex lens, f S2 It is the focal length of the second spherical convex lens, f C0 It is the focal length of the cylindrical lens, f. C1 It is the focal length of the first cylindrical convex lens, f C2 It is the focal length of the second cylindrical convex lens.

[0011] Preferably, the optical signal emitting unit, the first spherical lens, and the second spherical lens constitute an incident light diameter adjustment system for adjusting the diameter of the light source incident on the cylindrical lens.

[0012] Preferably, the diameter d of the light source incident on the cylindrical lens is... S =d L *f S02 / f S01 , where d L It is the diameter of the light beam emitted by the optical signal transmitting unit.

[0013] Preferably, the first polarizer and the second polarizer are of the same size and are arranged symmetrically about the beam convergence point F; the first beam splitter and the second beam splitter have the same beam splitting angle and size and are arranged symmetrically about the beam convergence point F; the first cylindrical convex lens and the second cylindrical convex lens have the same focal length and size and are arranged symmetrically about the beam convergence point F; the first spherical convex lens and the second spherical convex lens have the same focal length and size and are arranged symmetrically about the beam convergence point F.

[0014] Preferably, the cylindrical lens is a concave cylindrical lens or a convex cylindrical lens; the absolute value of the focal length of the cylindrical lens is less than the absolute value of the focal length of the first convex cylindrical lens.

[0015] According to another aspect of the present invention, the following technical solution is also provided:

[0016] The method for measuring density fluctuations in the near-wall region of a spanwise tensile model using the aforementioned two-dimensional refocusing laser differential interferometer includes the following steps:

[0017] (S1) Place the beam convergence point F of the two-dimensional refocusing laser differential interferometer on the spatial point that needs to be measured in the near-wall region of the spanwise stretching model;

[0018] (S2) Start the wind tunnel and record the voltage signal output by the two-dimensional refocusing laser differential interferometer;

[0019] (S3) Obtain the density fluctuation information of the measured flow field based on the voltage signal.

[0020] Preferably, in step (S1), the diameter of the light source incident on the cylindrical lens is adjusted by adjusting the focal length of the first spherical lens and the focal length of the second spherical lens.

[0021] In summary, compared with the prior art, the above-described technical solutions conceived by this invention can achieve the following beneficial effects:

[0022] 1. The two-dimensional refocusing laser differential interferometer provided by the present invention improves the structure of the two-dimensional focusing laser differential interferometer, optimizes the spatial resolution of the measuring points when measuring the density pulsation in the near-wall region of the spanwise stretched model, and reduces the influence of the non-measuring region on the measurement results of the measuring region, thereby enabling the acquisition of high-precision density pulsation information of the flow field closer to the wall of the spanwise stretched model. Specifically, by adding a first spherical lens and a second spherical lens after the optical signal transmitting unit L, the diameter of the light source incident on the cylindrical lens is adjustable, saving the cost required to purchase a large-diameter optical signal transmitting unit and avoiding the power loss of the light source caused by the circular grating setting, thus making full use of the full power of the optical signal transmitting unit. In addition, by adding a first spherical convex lens and a second spherical convex lens between the first cylindrical convex lens and the second cylindrical convex lens, the light path converges along the optical axis after refraction by the first spherical convex lens and the second spherical convex lens, forming a conical compressed beam. The light path is focused into a point rather than a line at the measurement position, and the density pulsation along the entire focal line is not averaged due to the formation of the focal line. Moreover, it can be measured close to the near-wall region of the spanwise tensile model. Its beam range is smaller in the measurement area and larger in the non-measurement area, effectively reducing the influence of the non-measurement area on the measurement results of the measurement area, and has excellent spatial resolution and accuracy.

[0023] 2. In the two-dimensional refocusing laser differential interferometer provided by this invention, the first spherical lens can be a concave spherical lens or a convex spherical lens, and the second spherical lens must be a convex spherical lens, and the absolute value of the focal length of the first spherical lens |f S01 |less than the absolute value of the focal length of the second spherical lens|f S02 This design is necessary because the beam emitted by conventional optical signal transmitting units is often very small in diameter. If both the first and second spherical lenses are concave lenses, it will be impossible to obtain the parallel incident light source required for measurement; if the first spherical lens is a convex lens and the second spherical lens is a concave lens, or |f S01 |≥|f S02 It will be impossible to obtain a parallel incident light source with a large diameter, resulting in inaccurate measurements.

[0024] 3. The two-dimensional refocusing laser differential interferometer provided by this invention is designed with the following distances: L0 between the first spherical lens and the second spherical lens, L1 between the cylindrical lens and the first cylindrical convex lens, 2L2 between the first spherical convex lens and the second spherical convex lens, and L3 between the second cylindrical convex lens and the photodetector. These distances conform to the imaging theorem. The distance L2 needs to consider the stretching width of the spanwise stretched model (in an open measurement environment) or the size of the wind tunnel test section (in a closed high-speed wind tunnel measurement). 2L2 must be greater than one of these two dimensions depending on the measurement conditions. Through the design of these distances, the two-dimensional refocusing laser differential interferometer of this invention can obtain high-precision density pulsation information of the near-wall flow field of the spanwise stretched model under wind tunnel measurement conditions.

[0025] 4. The two-dimensional refocusing laser differential interferometer provided by this invention comprises an incident light diameter adjustment system consisting of an optical signal transmitting unit L, a first spherical lens, and a second spherical lens. By adjusting the focal lengths of the first and second spherical lenses, the diameter of the light source incident on the cylindrical lens can be adjusted, thereby providing a parallel light source of a certain diameter. This light source diameter can be adjusted by replacing the lenses (the first and / or the second spherical lens) and adjusting the lens spacing. Compared to directly purchasing a large-size parallel light source device, this significantly reduces costs, provides greater flexibility, and fully utilizes the power of the optical signal transmitting unit. This is because if a parallel light source device is used directly, a circular grating often needs to be added behind the light source to block and adjust the light source diameter of the incident cylindrical lens, resulting in power loss. In this invention, the maximum height of the conical compressed beam depends on the diameter of the incident light source. During implementation, selecting a larger diameter incident light source can obtain a larger conical compressed beam in the non-measurement area, making the measurement results less affected by the non-measurement area and more accurate.

[0026] 5. In the two-dimensional refocusing laser differential interferometer provided by this invention, the focal length of the first cylindrical convex lens is greater than the absolute value of the focal length of the cylindrical lens. This is because, to improve measurement accuracy, it is necessary to maximize the volume of the conical compressed beam in the non-sensitive region. The maximum height of the conical compressed beam depends on the diameter of the light source incident on the cylindrical lens, and the maximum width depends on the beam width between the first cylindrical convex lens and the first spherical convex lens, i.e., the degree of beam divergence between the cylindrical lens and the first cylindrical convex lens. To increase the maximum width of the conical compressed beam, the absolute value of the focal length of the cylindrical lens must be smaller than the absolute value of the focal length of the first cylindrical convex lens.

[0027] 6. The method for measuring density pulsations in the near-wall region of a spanwise tensile model provided by the present invention utilizes the two-dimensional refocusing laser differential interferometer provided by the present invention. When measuring the spanwise tensile model, the measurement can be performed close to the wall region. The beam range in the measurement area is small while the beam range in the non-measurement area is large, which has excellent spatial resolution and accuracy. At the same time, it can make full use of the full power of the optical signal transmitting unit. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the principle and structure of a traditional focused laser differential interferometer (top view).

[0029] Figure 2 This is a schematic diagram (top view) of the principle and structure of an existing two-dimensional focused laser differential interferometer.

[0030] Figure 3 This is a schematic diagram (top view) of the principle structure of a two-dimensional refocusing laser differential interferometer in a preferred embodiment of the present invention.

[0031] Figure 4 This is a side view of the conical beam of a traditional focused laser differential interferometer.

[0032] Figure 5 This is a side view of the square beam of an existing two-dimensional focused laser differential interferometer.

[0033] Figure 6 This is a side view of the conical compressed beam of a two-dimensional refocusing laser differential interferometer in a preferred embodiment of the present invention. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0035] The terms "first," "second," "third," "fourth," etc., used in the specification of this invention are used to distinguish different objects, rather than to describe a specific order.

[0036] To address the limitations of traditional focused laser differential interferometers and two-dimensional focused laser differential interferometers in measuring the near-wall region of spanwise stretched models, this invention provides a two-dimensional refocusing laser differential interferometer with high spatial resolution and high sensitivity. Its purpose is to optimize the spatial resolution of the instrument's measurement points, reduce the influence of non-measurement areas on the measurement results of the measurement area, and obtain high-precision density pulsation information of the flow field closer to the wall of the spanwise stretched model.

[0037] This invention adds two spherical lenses, namely the first spherical lens S, after the optical signal transmitting unit L. 01 Second spherical lens S 02 This allows for adjustable incident light source diameter, avoiding power loss due to the circular grating setup. Furthermore, by adding two spherical convex lenses (first spherical convex lens S1 and second spherical convex lens S2) between the cylindrical lenses C1 and C2 in a traditional two-dimensional focusing laser differential interferometer, the light path converges along the optical axis after refraction by the spherical lenses, forming a conical compressed beam (see...). Figure 6 The optical path is focused into a point rather than a line at the measurement position, so the density fluctuations are not averaged due to the formation of the focal line. It can also be placed in the near-wall region of the spanwise stretching model for measurement. Its beam range is smaller in the measurement area and larger in the non-measurement area, which effectively reduces the influence of the non-measurement area on the measurement results in the measurement area, and has excellent spatial resolution and accuracy.

[0038] like Figure 3 As shown, the two-dimensional refocusing laser differential interferometer provided in this embodiment of the invention includes: an optical signal transmitting unit L and a first spherical lens S. 01 Second spherical lens S 02 Cylindrical lens C0, first polarizer P1, first beam splitter W1, first cylindrical convex lens C1, first spherical convex lens S1, second spherical convex lens S2, second cylindrical convex lens C2, second beam splitter W2, second polarizer P2, photodetector D.

[0039] The optical signal from the optical signal transmitting unit L passes sequentially through the first spherical lens S. 01 Second spherical lens S 02 The first polarizer P1, the first beam splitter W1, the first cylindrical convex lens C1, the first spherical convex lens S1, the second spherical convex lens S2, the second cylindrical convex lens C2, the second beam splitter W2, and the second polarizer P2 then reach the photodetector D.

[0040] The optical signal transmitting unit L is used to generate a parallel coherent beam; the first spherical lens S 01 Used to diverge the coherent beam in various directions; second spherical lens S 02The first polarizer is used to convert a diverging beam into a parallel beam; the cylindrical lens C0 is used to diverge the parallel beam in a single direction; the first polarizer P1 is used to filter out noise rays in the diverging coherent beam; the first beam splitter W1 is used to separate the beam diverging in a single direction into two unidirectional diverging beams with equal intensity and perpendicular polarization directions; the first cylindrical convex lens C1 is used to convert the two unidirectional diverging beams back into parallel beams; the first spherical convex lens S1 is used to focus the two parallel beams in various directions; the second spherical convex lens S2 is used to convert the two diverging beams after the focal point back into parallel beams; the second cylindrical convex lens C2 is used to convert the two parallel beams into a unidirectional focused beam; the second beam splitter W2 is used to re-merge the two beams into one beam; the second polarizer P2 is used to perform interference filtering on the merged coherent beam; and the photodetector D is used to linearly convert the intensity of the merged interference focused beam into a voltage signal.

[0041] As an alternative implementation, the optical signal transmitting unit L can be implemented using a laser.

[0042] As an optional implementation, the first spherical lens S 01 It is a concave spherical lens or a convex spherical lens, the second spherical lens S 02 It is a spherical convex lens; the absolute value of the focal length of the first spherical lens is |f S01 |less than the absolute value of the focal length of the second spherical lens|f S02 |

[0043] Cylindrical lens C0 is either a concave cylindrical lens or a convex cylindrical lens; the absolute value of the focal length of cylindrical lens C0 is |f C0 |less than the absolute value of the focal length of the first cylindrical convex lens|f C1 |

[0044] As an optional implementation, the first spherical lens S 01 Second spherical lens S 02 The distance L0 between them conforms to the imaging theorem L0 = f S01 +f S02 The distance L1 between the cylindrical lens C0 and the first cylindrical convex lens C1 conforms to the imaging theorem L1 = f C0 +f C1 The distance between the first spherical convex lens S1 and the second spherical convex lens S2 is 2L2, which conforms to the imaging theorem 2L2 = f. S1 +f S2 The distance between the second cylindrical convex lens and the photodetector is L3 = f C2 ; where f S01 It is the first spherical lens S 01 focal length, f S02 It is the second spherical lens S 02focal length, f S1 It is the focal length of the first spherical convex lens S1, f S2 It is the focal length of the first spherical convex lens S2, f C0 It is the focal length of the cylindrical lens C0, f C1 It is the focal length of the first cylindrical convex lens C1, f C2 It is the focal length of the second cylindrical convex lens C2.

[0045] As an optional implementation, the optical signal transmitting unit L and the first spherical lens S 01 Second spherical lens S 02 This constitutes an incident light diameter adjustment system, used to adjust the diameter of the light source incident on the cylindrical lens C0.

[0046] The diameter d of the light source incident on the cylindrical lens C0 S Depending on the diameter of the optical signal transmitting unit L and the first spherical lens S 01 focal length f S01 Second spherical lens S 02 focal length f S02 d S =d L *f S02 / f S01 , where d L It is the diameter of the light beam emitted by the optical signal transmitting unit L.

[0047] The maximum height of the conical compressed beam depends on the diameter of the incident light source. In the implementation process, using an incident light source with a larger diameter can obtain a conical compressed beam with a larger non-measurement area, making the measurement results less affected by the non-measurement area and more accurate.

[0048] As an optional implementation, the first polarizer P1 and the second polarizer P2 are of the same size and are symmetrically arranged about the beam convergence point F; the first beam splitter W1 and the second beam splitter W2 have the same splitting angle and size and are symmetrically arranged about the beam convergence point F; the first cylindrical convex lens C1 and the second cylindrical convex lens C2 have the same focal length and size and are symmetrically arranged about the beam convergence point F; the first spherical convex lens S1 and the second spherical convex lens S2 have the same focal length and size and are symmetrically arranged about the beam convergence point F (i.e., the measurement area). Due to the reversible principle of the optical path, the optical elements centered on the beam convergence point F only need to be symmetrically arranged to re-obtain the converged single beam, which is ultimately received by the photodetector D. The two-dimensional refocusing laser differential interferometer built according to this invention can perform measurements close to the wall region when measuring the spanwise stretched model. Its beam range is small in the measurement area but large in the non-measurement area, exhibiting excellent spatial resolution and accuracy, while fully utilizing the total power of the optical signal transmitting unit.

[0049] The method for measuring density fluctuations in the near-wall region of a spanwise tensile model using a two-dimensional refocusing laser differential interferometer constructed according to this invention is as follows:

[0050] (S1) Place the beam convergence point F of the two-dimensional refocusing laser differential interferometer on the spatial point that needs to be measured in the near-wall region of the spanwise stretching model;

[0051] (S2) Start the wind tunnel and record the voltage signal output by the two-dimensional refocusing laser differential interferometer;

[0052] (S3) Obtain the density fluctuation information of the measured flow field based on the voltage signal.

[0053] The present invention will be described in detail below with reference to the accompanying drawings and embodiments. It should be noted that the embodiments are only one optional implementation method, and other parameters can be used for each optical device. The embodiments of the present invention are not limited to one uniqueness.

[0054] A suitable optical path measurement system was constructed according to the actual measurement requirements and optical path space dimensions. The specific construction method is as follows:

[0055] Based on the structural diagram of the two-dimensional refocusing laser differential interferometer (see...) Figure 3 The measurement system is constructed by selecting the corresponding optical components. The optical signal transmitting unit L can be a polarized laser with a beam diameter of 0.63 mm; the first spherical lens S... 01 You can choose a spherical concave lens with a focal length of -30mm, or a second spherical lens S. 02 A spherical convex lens with a focal length of 125mm can be selected; a cylindrical lens C0 can be selected with a focal length of 25mm; the first and second beam splitters W1 and W2 can be selected with a Wollaston prism with a separation angle of 1 arcminute; the first and second cylindrical convex lenses C1 and C2 can be selected with a focal length of 300mm; and the first and second spherical convex lenses S1 and S2 can be selected with a focal length of 400mm. Based on the imaging principle, L0 = 95mm, L1 = 325mm, L2 = 400mm, and L3 = 300mm.

[0056] Specific measurement methods: such as Figure 6 As shown, the conical compressed beam generated between the first and second spherical convex lenses S1 and S2 by the two-dimensional refocusing laser differential interferometer provided in this embodiment of the invention is the measurement position of the instrument. This point needs to be placed at the position to be measured in the flow field. After checking that there are no errors, the wind tunnel can be started and the voltage signal of the photodetector can be recorded to obtain the original measurement value. The original signal is then processed using the same processing method as the two-dimensional focusing laser differential interferometer (see Chinese Utility Model Patent Specification CN110987357B) to obtain the density pulsation of the point to be measured.

[0057] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A two-dimensional refocusing laser differential interferometer, comprising: The optical signal transmitting unit, cylindrical lens, first polarizer, first beam splitter, first cylindrical convex lens, second cylindrical convex lens, second beam splitter, second polarizer, and photodetector are characterized in that they further include: a first spherical lens, a second spherical lens, a first spherical convex lens, and a second spherical convex lens, wherein... The optical signal from the optical signal transmitting unit passes sequentially through the first spherical lens, the second spherical lens, the cylindrical lens, the first polarizer, the first beam splitter, the first cylindrical convex lens, the first spherical convex lens, the second spherical convex lens, the second cylindrical convex lens, the second beam splitter, and the second polarizer before reaching the photodetector. The optical signal transmitting unit is used to generate a parallel coherent beam; the first spherical lens is used to diverge the coherent beam in all directions; the second spherical lens is used to convert the diverging beam into a parallel beam; the cylindrical lens is used to diverge the parallel beam in a single direction; the first polarizer is used to filter out noise rays in the diverging coherent beam; the first beam splitter is used to separate the beam diverging in a single direction into two unidirectional diverging beams with equal intensity and mutually perpendicular polarization directions; the first cylindrical convex lens is used to convert the two unidirectional diverging beams back into parallel beams; the first spherical convex lens is used to focus the two parallel beams in all directions; the second spherical convex lens is used to convert the two diverging beams after the focal point back into parallel beams; the second cylindrical convex lens is used to convert the two parallel beams into unidirectional focused beams; the second beam splitter is used to merge the two unidirectional focused beams; the second polarizer is used to perform interference filtering on the merged coherent beam; the photodetector is used to linearly convert the intensity of the merged interference focused beam into a voltage signal.

2. The two-dimensional refocusing laser differential interferometer as described in claim 1, characterized in that, The first spherical lens is a concave spherical lens or a convex spherical lens, and the second spherical lens is a convex spherical lens; the absolute value of the focal length of the first spherical lens is smaller than the absolute value of the focal length of the second spherical lens.

3. A two-dimensional refocusing laser differential interferometer as described in claim 1, characterized in that, The distance L0 between the first spherical lens and the second spherical lens is f S01 +f S02 The distance L1 between the cylindrical lens and the first cylindrical convex lens is f. C0 +f C1 The distance between the first spherical convex lens and the second spherical convex lens is 2L² = f. S1 +f S2 The distance between the second cylindrical convex lens and the photodetector is L3 = f C2 ; where f S01 It is the focal length of the first spherical lens, f S02 It is the focal length of the second spherical lens, f S1 It is the focal length of the first spherical convex lens, f S2 It is the focal length of the second spherical convex lens, f C0 It is the focal length of the cylindrical lens, f. C1 It is the focal length of the first cylindrical convex lens, f C2 It is the focal length of the second cylindrical convex lens.

4. A two-dimensional refocusing laser differential interferometer as described in claim 3, characterized in that, The optical signal emitting unit, the first spherical lens, and the second spherical lens constitute an incident light diameter adjustment system, used to adjust the diameter of the light source incident on the cylindrical lens.

5. A two-dimensional refocusing laser differential interferometer as described in claim 4, characterized in that, The diameter d of the light source incident on the cylindrical lens S =d L *f S02 / f S01 , where d L It is the diameter of the light beam emitted by the optical signal transmitting unit.

6. A two-dimensional refocusing laser differential interferometer as described in claim 1, characterized in that, The first polarizer and the second polarizer have the same size and are arranged symmetrically about the beam convergence point F; the first beam splitter and the second beam splitter have the same splitting angle and size and are arranged symmetrically about the beam convergence point F; the first cylindrical convex lens and the second cylindrical convex lens have the same focal length and size and are arranged symmetrically about the beam convergence point F; the first spherical convex lens and the second spherical convex lens have the same focal length and size and are arranged symmetrically about the beam convergence point F.

7. A two-dimensional refocusing laser differential interferometer as described in claim 2, characterized in that, The cylindrical lens is a concave cylindrical lens or a convex cylindrical lens; the absolute value of the focal length of the cylindrical lens is less than the absolute value of the focal length of the first convex cylindrical lens.

8. A method for measuring density fluctuations in the near-wall region of a spanwise stretched model using a two-dimensional refocusing laser differential interferometer as described in any one of claims 1-7, characterized in that, Includes the following steps: (S1) Place the beam convergence point F of the two-dimensional refocusing laser differential interferometer on the spatial point that needs to be measured in the near-wall region of the spanwise stretching model; (S2) Start the wind tunnel and record the voltage signal output by the two-dimensional refocusing laser differential interferometer; (S3) Obtain the density fluctuation information of the measured flow field based on the voltage signal.

9. The method as described in claim 8, characterized in that, In step (S1), the diameter of the light source incident on the cylindrical lens is adjusted by adjusting the focal length of the first spherical lens and the focal length of the second spherical lens.

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