Self-referencing speckle interferometry system

Abstract

The present invention provides a self-referenced speckle interferometry measurement system, which relates to an optical measurement system, comprising a laser beam generating device, an imaging lens, a first lens, a second lens, a first beam splitter prism, a second beam splitter prism, a first reflector, a second reflector, an aperture, a pinhole, and an imaging device. The laser beam generating device generates a laser beam and irradiates the laser beam on an object to be measured to form reflected light; part of the reflected light sequentially passes through the imaging lens and the first lens and then irradiates the first beam splitter prism, and is divided by the first beam splitter prism into reflected light and transmitted light; part of the reflected light is reflected by the first reflector and passes through the aperture, then reflected by the second beam splitter prism and passes through the second lens and irradiates an imaging surface of the imaging device; part of the transmitted light passes through the pinhole and is reflected by the second reflector, then sequentially passes through the second beam splitter prism and the second lens and irradiates the imaging surface of the imaging device. The present invention can effectively solve the technical problem of air disturbance on the propagation path of object light interfering with measurement results.

Description

Technical Field

[0001] The present invention relates to an optical measurement system, in particular to a self-reference speckle interferometry measurement system. Background Art

[0002] Speckle interferometry technology uses laser speckle as a carrier of field change information of the object being measured. It uses the related fringes of the interference speckle field generated by the object being measured after being irradiated by laser to detect tiny displacements and deformations on the surface of the object being measured. It has a wide range of applications in many fields such as non-destructive testing, material mechanical properties testing, and biomedicine.

[0003] Speckle interferometry uses an interference method to illuminate the object under test with coherent laser light. After being scattered by the object, it is collected by the imaging lens and imaged onto the imaging device. This object light, known as the object light, has a speckle-like light field with a randomly distributed intensity. The object light interferes with the reference light, forming interference fringes that are captured by the imaging device. Phase shifting technology is used to extract phase-difference images from the interference fringes. The first phase-difference image is obtained before the object is deformed, and the second phase-difference image is obtained after the object is deformed. The two phase-difference images are subtracted to obtain deformation information about the object under test.

[0004] The existing conventional digital speckle interferometry optical path structure is as follows Figure 1 The light emitted by the laser is divided into two beams. One beam is expanded and directly illuminates the object being measured. After being scattered by the object, it is collected by the imaging lens and imaged onto the imaging device, forming the object light. The other beam is directly illuminated by the imaging device, forming the reference light, which causes the object light and the reference light to interfere.

[0005] However, in this optical path structure, the object light travels a longer path, which is much longer than the path experienced by the reference light. In addition, the air disturbance in the propagation path of the object light will affect the wavefront of the object light, but this air disturbance has no effect on the reference light. As a result, the phase difference between the object light and the reference light is not only affected by the deformation of the object being measured, but also by the air disturbance, which affects the final measurement result and measurement accuracy. Summary of the Invention

[0006] (1) Technical problems solved

[0007] In view of the shortcomings of the prior art, the present invention provides a self-referenced speckle interferometry measurement system to solve the technical problem in the prior art that air disturbances on the object light propagation path interfere with the measurement results.

[0008] (2) Technical solution

[0009] To achieve the above objectives, the present invention is implemented through the following technical solutions:

[0010] The present invention provides a self-referenced speckle interferometry measurement system, comprising a laser beam generating device, wherein the laser beam generated by the laser beam is irradiated on the object to be measured to form reflected light;

[0011] Part of the reflected light passes through the imaging lens and lens 1 in sequence and then illuminates the beam splitter prism 1, where it is split into reflected light and transmitted light.

[0012] Part of the reflected light is reflected by the reflector 1 and passes through the aperture, then reflected by the beam splitter prism 2 and passes through the lens 2, and is irradiated on the imaging surface of the imaging device;

[0013] Part of the penetrating light passes through the pinhole and is reflected by the second reflector, then passes through the second beam splitter prism and the second lens in sequence, and is irradiated on the imaging surface of the imaging device;

[0014] Lens 1 and 2 form a 4f system;

[0015] The aperture and the pinhole both coincide with the optical axis and are located on the middle spectrum plane of the 4f system;

[0016] Aperture D of the diaphragm 光阑 satisfy: λ is the wavelength of the laser beam, f is the focal length of lens 1, p is the spacing between pixels in the imaging device, and the diameter of the pinhole D is 小孔 Less than 2.44λf / D, where D is the clear aperture diameter of lens one.

[0017] Furthermore, the laser beam generating device includes a laser and a beam expander. The laser light emitted by the laser is expanded by the beam expander to form a laser beam and irradiated on the object to be measured to form reflected light.

[0018] Furthermore, an attenuation plate is provided between the aperture and the second beam splitter prism.

[0019] Furthermore, it also includes: a driving structure 1, used to drive the reflecting mirror 2.

[0020] Furthermore, the beam splitter prism 1 and the beam splitter prism 2 are replaced by polarization beam splitter prism 1 and polarization beam splitter prism 2 respectively, and a polarizer 1 and a half-wave plate are further arranged between the front focal plane of lens 1 and lens 1, and the half-wave plate is between polarizer 1 and lens 1, and a polarizer 2 is further arranged between the back focal plane of lens 2 and lens 2.

[0021] Furthermore, it also includes: a second driving structure for driving the half-wave plate to rotate.

[0022] (3) Beneficial effects

[0023] The present invention provides a self-referencing speckle interferometry system. Compared with the prior art, the reference light is obtained by filtering a portion of the object light. Based on this, the system has the following beneficial effects:

[0024] 1. Before lens 1, the object light and the reference light are transmitted in the same optical path, and the difference in optical path length between the two is small, so a laser with a short coherence length can be used.

[0025] 2. Before the lens, the object light and the reference light are transmitted in the same optical path. Air disturbance or environmental vibration has the same effect on the optical path of both, and cancels each other out in mutual interference.

[0026] 3. The phase can be calculated using a single interferogram, which improves the time resolution of the measurement compared to calculating the phase using multiple interferograms.

[0027] 4. By adjusting the positional relationship between the reference light pinhole and the object light diaphragm so that their centers are aligned, the influence of speckle on the phase shift of the reference light can be eliminated. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0029] Figure 1 The figure shows a schematic diagram of the conventional digital speckle interferometry optical path structure in the prior art;

[0030] Figure 2 FIG2 shows a schematic structural diagram of the self-referenced speckle interferometry system in Example 1;

[0031] Figure 3 Schematic diagram of spatial carrier phase-shift speckle interferometry spectrum;

[0032] Figure 4 Schematic diagram of the structure of the self-referenced speckle interferometry measurement system in Example 2.

[0033] In the picture:

[0034] 1. Laser beam generating device; 1-1. Laser; 1-2. Beam expander; 2. Imaging lens; 3. Lens 1; 4. Lens 2; 5. Beam splitter prism 1; 6. Beam splitter prism 2; 7. Reflector 1; 8. Reflector 2; 9. Aperture; 10. Pinhole; 11. Imaging device; 12. Attenuation plate; 13. Driving structure 1; 14. Polarization beam splitter prism 1; 15. Polarization beam splitter prism 2; 16. Polarizer 1; 17. Half-wave plate; 18. Polarizer 2; 19. Driving structure 2. DETAILED DESCRIPTION

[0035] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention are clearly and completely described. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0036] Example 1

[0037] Combined with attachment Figure 2 This embodiment provides a self-referenced speckle interferometry measurement system, which includes a laser beam generating device 1, an imaging lens 2, a lens 1 3, a lens 2 4, a beam splitter prism 1 5, a beam splitter prism 2 6, a reflector 1 7, a reflector 2 8, an aperture 9, a pinhole 10 and an imaging device 11.

[0038] in:

[0039] The laser beam generating device 1 is composed of a laser 1-1 and a beam expander 1-2. The laser generated by the laser 1-1 is expanded by the beam expander 1-2 to form a laser beam. The laser beam is irradiated on the object to be measured to form reflected light.

[0040] Part of the reflected light passes through the coaxially distributed imaging lens 2 and lens 3 in sequence and then illuminates the beam splitter prism 5, where it is divided into the reflected light reflected by the prism itself and the penetrating light passing through the prism itself.

[0041] Part of the reflected light is reflected by the reflector 1 7, passes through the aperture 9, is reflected by the dichroic prism 2 6, and passes through the lens 2 4, and is irradiated on the imaging surface of the imaging device 11 as object light;

[0042] Part of the penetrating light passes through the pinhole 10 and is reflected by the second reflector 8, then passes through the second beam splitter 6 and the second lens 4 in sequence, and is irradiated onto the imaging surface of the imaging device 11 as reference light;

[0043] That is, the object light and the reference light are combined by the beam splitter prism and interfere with each other on the imaging surface of the imaging device 11 .

[0044] In order to meet the measurement requirements, lens 1 3 and lens 2 4 in the above system constitute a 4f system, and the aperture 9 and the pinhole 10 are both located on the intermediate spectrum plane of the 4f system; correspondingly, the focus of the imaging lens 2 is on the front focal plane of lens 1 3, and the imaging surface of the imaging device 11 is on the back focal plane of lens 2 4.

[0045] In order to obtain a plane wave with uniform phase on the back focal plane of the lens 2 4, the diameter D of the aperture 10 is set to 小孔 Less than 2.44λf / D, λ is the wavelength of the laser beam, f is the focal length of lens 3, and D is the diameter of the clear aperture of lens 1.

[0046] In order to facilitate the adjustment of the relative intensity of the object light and the reference light, thereby ensuring that the interference pattern has a high contrast, in some embodiments, an attenuation plate 12 is further provided between the aperture 9 and the second beam splitter prism 6 to adjust the light intensity of the reference light and the object light.

[0047] In order to facilitate changing the propagation direction of the reference light and thereby superimposing the carrier frequency on the imaging plane of the imaging device 11 , in some embodiments, the second reflector 8 is further connected to the driving structure 13 to adjust the propagation direction of the reference light via the second reflector 8 .

[0048] The working principle of the self-referenced speckle interferometry system provided in this embodiment is as follows:

[0049] 1. In this embodiment, part of the transmitted light passes through the pinhole 10, is reflected by the second reflector 8, and then passes through the second beam splitter 6 and the second lens 4 in sequence, becoming parallel light (i.e., a plane wave). This light is then irradiated onto the imaging surface of the imaging device 11 as reference light. The complex amplitude of the reference light on the secondary image surface (the imaging surface of the imaging device 11) in the 4f system is then R(x, y), where (x, y) is the spatial coordinate on the imaging surface of the imaging device 11 (the center of this coordinate system is located on the optical axis).

[0050] 2. In this embodiment, part of the reflected light is reflected by the reflector 1 7 and passes through the aperture 9, then reflected by the beam splitter 2 6 and passes through the lens 2 4, irradiating the imaging surface of the imaging device 11 as object light. The complex amplitude of the object light imaged on the secondary image surface (the imaging surface of the imaging device 11) in the 4f system is O(x, y). Then, the complex amplitude of the light field F(ξ, η) on the spectrum surface in the 4f system is:

[0051] F(ξ,η)=∫∫ x,y O(x,y)e i2π(ξx+ηy) dxdy (1)

[0052] Among them, (ξ,η) is the spatial coordinate on the spectrum plane (the coordinate center of the spectrum plane is on the optical axis), is the phase space distribution of the object light, and i is the imaginary unit.

[0053] 3. Let the coordinate of the center of the small hole 10 on the spectrum plane be (ξ p ,η p ), then:

[0054]

[0055] Among them, (f x x+f yy) is the phase distribution of the reference light on the imaging surface of the imaging device 11 caused by the non-perpendicularity of the incident angle of the reference light to the imaging surface of the imaging device 11, and the phase distribution will introduce a frequency f into the interference pattern. x and f y carrier.

[0056] 4. Since the center of the pinhole 10 and the center of the aperture 9 in this embodiment coincide with the optical axis, the coordinate of the center of the pinhole 10 on the spectrum plane (ξ p ,η p )=(0,0), then based on this we can get:

[0057]

[0058] Then when the object under test produces a vibration of (Δx, Δy, Δz), the complex amplitude O′(x, y) of the object light on the secondary image plane of the 4f system and the complex amplitude R′(x, y) of the reference light on the secondary image plane of the 4f system are respectively:

[0059] O′(x,y)=O(x+Δx,y+Δy)e i4πΔz / λ ;

[0060]

[0061] From the above formula, we can see that R′(x,y) consists of three parts:

[0062] The first part is (∫∫ x,y O(x,y)dxdy), this part is only affected by the randomness of the object light speckle field and is not affected by the vibration of the object;

[0063] The second part is e i4πΔz / λ , which is affected only by the axial vibration of the object being measured. However, as shown in the aforementioned expression for O′(x,y), the phase of the object beam is also affected by the axial vibration of the object being measured. Therefore, in interference, the phase changes between the reference beam and the object beam caused by the axial vibration of the object being measured cancel each other out, making the phase difference between the object beam and the reference beam constant during the exposure time and unaffected by the axial vibration of the object being measured. This ensures that the single interferogram has a high contrast. Therefore, in the following derivation, this part is not considered for both the reference beam and the object beam.

[0064] The third part is Corresponding to the space carrier.

[0065] In the subsequent derivation process, let R b =∫∫ x,y O(x,y)dxdy, then based on the above analysis, R′(x,y) can be simplified to:

[0066]

[0067] 5. Since the interference pattern I(x,y) recorded at the secondary image plane in the 4f system is the square of the modulus of the sum of the complex amplitudes of the reference light and the object light, we can get:

[0068]

[0069] in,(·) * Represents the conjugate of a complex number.

[0070] 6. The carrier spectrum distribution diagram obtained by Fourier transforming the obtained interference pattern I(x,y) is as follows: Figure 3 shown by Figure 3 It can be seen that the spectrum contains a low-frequency part |O(x,y)| 2 +|R(x,y)| 2 and two high-frequency parts and

[0071] 7. In order to avoid spectrum aliasing, in this embodiment, the spectrum width of the object light is constrained by setting the size of the aperture 9; that is, Among them, ρ o is the object light cutoff frequency, and D 光阑 is the diameter of the aperture 9, ρ s is the sampling cutoff frequency of the image device, and p is the pixel pitch in the imaging device 11;

[0072] That is to say

[0073] 8. In order to calculate the object light phase from the single interference patterns collected before and after deformation, in this embodiment, the spectrum obtained by Fourier transforming the obtained interference pattern I(x,y) is multiplied by So that the high frequency part of the spectrum Move to the center of the frequency domain, then use a low-pass filter to filter out other frequency components, and then perform inverse Fourier transform to obtain the phase amplitude vector A(x,y) (i.e. O(x,y)R) containing the object light phase information. b * ), the above process is expressed as follows:

[0074]

[0075] Among them, LPF is a low-pass filter, FFT and IFFT represent Fourier transform and inverse Fourier transform, respectively.

[0076] 9. Let the complex amplitudes of the object light before and after deformation be:

[0077]

[0078] Among them, O1(x,y) is the object-light complex amplitude before deformation, O2(x,y) is the object-light complex amplitude after deformation, is the phase of the complex amplitude of the object light before deformation, It is the difference between the phase of the complex amplitude of the object light after deformation caused by the deformation of the measured object and the phase of the complex amplitude of the object light before deformation.

[0079] 10. In this embodiment, the laser beam irradiating the object is approximately parallel to the optical axis of the imaging optical path (the angle formed by the laser beam and the imaging optical path in the figure is only for illustration; in practice, the two are approximately parallel). Let w(x, y) be the out-of-plane deformation of the surface of the object, then The relationship with w(x,y) is:

[0080]

[0081] 11. Let the phase and amplitude vectors calculated by collecting the interference patterns before and after deformation be:

[0082] A1(x,y)=O1(x,y)R b1 * , A1(x,y) is the phase-amplitude vector calculated by collecting the interference pattern before deformation;

[0083] A2(x,y)=O2(x,y)R b2 * , A2(x,y) is the phase-amplitude vector calculated by collecting the interference pattern after deformation;

[0084] Then we have:

[0085]

[0086] Where ∠(·) represents the argument of the complex number. Δφ=∠(R b1 R b2 * ), is the phase difference of the reference light before and after deformation, which does not change with spatial position. Therefore, the out-of-plane deformation w(x,y) can be calculated by formula (7):

[0087]

[0088] in is a constant.

[0089] Example 2

[0090] Combined with attachment Figure 4To further enhance the system's adjustability, in this embodiment, the beam splitter prism 15 and the beam splitter prism 26 are replaced with polarization beam splitter prism 14 and polarization beam splitter prism 2 15, respectively, to achieve polarization beam splitting. Both polarization beam splitters reflect s-light and transmit p-light. A polarizer 16 and a half-wave plate 17 are also provided between the front focal plane of lens 13 and lens 13, with the half-wave plate 17 located between polarizer 16 and lens 13. Polarizer 16 is used for polarization, and the half-wave plate 17 is used to change the direction of polarized light, thereby altering the energy ratio of s-light and p-light incident on polarization beam splitter prism 14. A polarizer 2 18 is also provided between the back focal plane of lens 24 and lens 24. After passing through polarizer 2 18, the reference light and the object light become of the same polarization state, thereby interfering with each other.

[0091] In the system provided in this embodiment, the relative intensity of the object light and the reference light can be adjusted by rotating the half-wave plate 17, thereby ensuring that the obtained interference pattern has a high contrast; wherein, in order to facilitate the adjustment of the half-wave plate 17, in some embodiments, the half-wave plate 17 is also connected to the driving structure 19, and the relative intensity of the object light and the reference light can be adjusted by rotating the half-wave plate 17.

[0092] It should be noted that, in this document, the terms "comprises," "includes," or any other variations thereof are intended to encompass non-exclusive inclusion, such that a process, method, article, or apparatus comprising a series of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus. In the absence of further limitations, an element defined by the phrase "comprising a ..." does not exclude the presence of other identical elements in the process, method, article, or apparatus comprising the element.

[0093] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A self-referencing speckle interferometry system, characterized in that: It includes a laser beam generating device, which generates a laser beam that irradiates the object to be measured to form reflected light; Part of the reflected light passes through the imaging lens and lens 1 in sequence and then illuminates the beam splitter prism 1, where it is split into reflected light and transmitted light. Part of the reflected light is reflected by the reflector 1 and passes through the aperture, then reflected by the beam splitter prism 2 and passes through the lens 2, and is irradiated on the imaging surface of the imaging device; Part of the penetrating light passes through the pinhole and is reflected by the second reflector, then passes through the second beam splitter prism and the second lens in sequence, and is irradiated on the imaging surface of the imaging device; Lens 1 and Lens 2 system; The aperture and the pinhole are both coincident with the optical axis and are located The middle spectrum of the system; Aperture of the iris satisfy: , is the wavelength of the laser beam, is the focal length of lens 1, is the spacing between pixels in the imaging device, the diameter of the small hole Less than 2.44 , D is the diameter of the clear aperture of lens 1; The method for measuring the out-of-plane deformation of the object under test using the self-referenced speckle interferometry system includes: The following formula is used to calculate the out-of-plane deformation of the object being measured: Obtaining: in is the spatial coordinate on the imaging surface of the imaging device, c is a constant, and c= , ∠(•) means finding the argument of a complex number, is the phase difference of the reference light before and after deformation, is the phase amplitude vector calculated by collecting the interference pattern before deformation, represents the conjugate of a complex number, is the phase amplitude vector calculated from the collected interference pattern after deformation.

2. A self-referenced speckle interferometry system according to claim 1, characterized in that: The laser beam generating device comprises a laser and a beam expander. The laser light emitted by the laser is expanded by the beam expander to form a laser beam and irradiated on the object to be measured to form reflected light.

3. The self-referencing speckle interferometry system according to claim 1, wherein: An attenuation plate is further provided between the aperture and the second beam splitter prism.

4. The self-referencing speckle interferometry system according to claim 1, wherein: Also includes: The driving structure 1 is used to drive the reflecting mirror 2.

5. The self-referencing speckle interferometry system according to claim 1, characterized in that: The beam splitter prism 1 and the beam splitter prism 2 are replaced with the polarization beam splitter prism 1 and the polarization beam splitter prism 2 respectively. A polarizer 1 and a half-wave plate are further arranged between the front focal plane of lens 1 and lens 1, and the half-wave plate is between polarizer 1 and lens 1. A polarizer 2 is also arranged between the back focal plane of lens 2 and lens 2.

6. The self-referencing speckle interferometry system according to claim 5, characterized in that: Also includes: The second driving structure is used to drive the half-wave plate to rotate.

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