Self-reference speckle interferometry system

Abstract

The invention provides a self-reference speckle interference measuring system, and relates to an optical measuring system, the self-reference speckle interference measuring system comprises 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, a diaphragm, a small hole and an imaging device, the laser beam generating device generates a laser beam and irradiates a measured object to form reflected light; part of the reflected light sequentially penetrates through the imaging lens and the first lens, then irradiates the first beam splitter prism and is divided into reflected light and penetrating light by the first beam splitter prism. Part of the reflected light is reflected by the reflector I, passes through the diaphragm, is reflected by the beam splitter prism II, passes through the lens II and is irradiated on an imaging surface of the imaging device; part of the penetrating light penetrates through the small hole, is reflected by the second reflector, then sequentially penetrates through the second beam splitter prism and the second lens and irradiates the imaging face of the imaging device. The technical problem that the air disturbance on the object light propagation path interferes the measurement result can be effectively solved.

Description

Technical Field

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

[0002] Speckle interferometry uses laser speckles as carriers of information on the changes in the measured object field, and uses the correlation fringes of the interference speckle field generated after the measured object is irradiated by laser to detect the minute displacements and deformations on the surface of the measured object, and has extensive applications in many fields such as non-destructive testing, material mechanics performance testing, and biomedicine.

[0003] Speckle interferometry adopts an interference method, where coherent laser light is made to irradiate the measured object, and after being scattered by the measured object, it is collected by an imaging lens and imaged onto an imaging device, which is the object light. The light field of the object light has speckle characteristics, and the light intensity is randomly distributed. The object light interferes with the reference light to form interference fringes, which are collected by the imaging device. The phase-shifting technique is used to obtain the phase difference image between the object light and the reference light from the interference fringes. A first phase difference image is obtained before the object deforms, and a second phase difference image is obtained after the object deforms. The two phase difference images are subtracted from each other, and the deformation information of the measured object can be obtained therefrom.

[0004] The existing conventional digital speckle interferometry optical path structure is as Figure 1 , the light emitted by the laser is divided into two beams. One beam is expanded and then directly irradiates the measured object, and after being scattered by the measured object, it is collected by an imaging lens and imaged onto an imaging device, constituting the object light. The other beam is directly irradiated onto the imaging device to form the reference light, so that the object light interferes with the reference light.

[0005] However, in this optical path structure, the object light passes through a relatively long path, which is much greater than the path experienced by the reference light, and 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, resulting in the phase difference between the object light and the reference light being affected not only by the deformation of the measured object but also by the air disturbance, and affecting the final measurement result and measurement accuracy. Summary of the Invention

[0006] (1) Technical Problems to be Solved

[0007] Aiming at the deficiencies of the prior art, the present invention provides a self-reference speckle interferometry measurement system to solve the technical problem that the air disturbance on the propagation path of the object light in the prior art interferes with the measurement result.

[0008] (2) Technical Solutions

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

[0010] The present invention provides a self-reference speckle interferometry system, which includes a laser beam generating device. The laser beam generated by the device irradiates on the object to be measured, forming a reflected light.

[0011] Part of the reflected light sequentially passes through an imaging lens and a first lens and then irradiates on a first beam splitter prism, and is split by the first beam splitter prism into a reflected light and a transmitted light.

[0012] Part of the reflected light is reflected by a first mirror and passes through a diaphragm, and then is reflected by a second beam splitter prism and passes through a second lens, and irradiates on the imaging surface of an imaging device.

[0013] Part of the transmitted light passes through a small hole and is reflected by a second mirror, and then sequentially passes through the second beam splitter prism and the second lens, and irradiates on the imaging surface of the imaging device.

[0014] The first lens and the second lens form a 4f system.

[0015] The diaphragm and the small hole are both coincident with the optical axis and are both located on the intermediate frequency spectrum plane of the 4f system.

[0016] The aperture D of the diaphragm 光阑 satisfies: λ is the wavelength of the laser beam, f is the focal length of the first lens, p is the pitch of the pixels in the imaging device, and the diameter D of the small hole 小孔 is less than 2.44λf / D, and D is the diameter of the light passing aperture of the first lens.

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

[0018] Furthermore, an attenuation sheet is further arranged between the diaphragm and the second beam splitter prism.

[0019] Furthermore, it further includes: a first driving structure for driving the second mirror.

[0020] Furthermore, the first beam splitter prism and the second beam splitter prism are respectively replaced by a polarization beam splitter prism one and a polarization beam splitter prism two. A polarizer one and a half-wave plate are further arranged between the front focal plane of the first lens and the first lens, and the half-wave plate is located between the polarizer one and the first lens. A polarizer two is further arranged between the rear focal plane of the second lens and the second lens.

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

[0022] (III) Advantageous Effects

[0023] A self-reference speckle interferometry system provided by the present invention, compared with the prior art, the reference light is obtained by filtering a part of the object light. Based on this, the system has the following advantageous effects:

[0024] 1. The object light and the reference light are transmitted in a common optical path in front of a lens. The difference in the optical path lengths of the two is relatively small. Therefore, a laser with a short coherence length can be used.

[0025] 2. The object light and the reference light are transmitted in a common optical path in front of a lens. The influence of air disturbance or environmental vibration on the optical paths of the two is the same and cancels each other out during mutual interference.

[0026] 3. The phase can be calculated using a single interference pattern. Compared with calculating the phase through multiple interference patterns, the time resolution of the measurement is improved.

[0027] 4. By adjusting the positional relationship between the reference light small hole 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. Description of the Drawings

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the following-described drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

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

[0030] Figure 2 Shows a schematic diagram of the structure of a self-reference speckle interferometry measurement system in Embodiment 1;

[0031] Figure 3 Is a schematic diagram of the spectrum of spatial carrier phase-shifting speckle interferometry;

[0032] Figure 4 Is a schematic diagram of the structure of a self-reference speckle interferometry measurement system in Embodiment 2.

[0033] In the figure:

[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. Mirror 1; 8. Mirror 2; 9. Diaphragm; 10. Small hole; 11. Imaging device; 12. Attenuator; 13. Driving structure 1; 14. Polarizing beam splitter prism 1; 15. Polarizing beam splitter prism 2; 16. Polarizer 1; 17. Half-wave plate; 18. Polarizer 2; 19. Driving structure 2. Detailed Embodiments

[0035] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are clearly and completely described. Apparently, the described embodiments are some, rather than all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

[0036] Embodiment 1

[0037] Combined with the attached Figure 2 , this embodiment provides a self-referencing speckle interferometry measurement system, which consists of a laser beam generating device 1, an imaging lens 2, a first lens 3, a second lens 4, a first beam splitter prism 5, a second beam splitter prism 6, a first mirror 7, a second mirror 8, a diaphragm 9, a small hole 10, and an imaging device 11.

[0038] Among them:

[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, and the laser beam irradiates on the object to be measured, forming a reflected light.

[0040] And part of the reflected light sequentially passes through the coaxially distributed imaging lens 2 and the first lens 3 and then irradiates on the first beam splitter prism 5, and is split by the first beam splitter prism 5 into a reflected light reflected by itself and a transmitted light passing through itself;

[0041] And part of the reflected light is reflected by the first mirror 7, passes through the diaphragm 9, and then is reflected by the second beam splitter prism 6 and passes through the second lens 4, serving as object light and irradiating on the imaging surface of the imaging device 11;

[0042] And part of the transmitted light passes through the small hole 10, is reflected by the second mirror 8, and then sequentially passes through the second beam splitter prism 6 and the second lens 4, serving as reference light and irradiating on the imaging surface of the imaging device 11;

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

[0044] To meet the measurement requirements, in the above system, the first lens 3 and the second lens 4 form a 4f system, and both the diaphragm 9 and the small hole 10 are located on the intermediate frequency spectrum plane of the 4f system; correspondingly, the focal point of the imaging lens 2 is on the front focal plane of the first lens 3, and the imaging surface of the imaging device 11 is on the rear focal plane of the second lens 4.

[0045] And to obtain a plane wave with uniform phase on the rear focal plane of the second lens 4, in this embodiment, the diameter D of the small hole 10 小孔 is less than 2.44λf / D, where λ is the wavelength of the laser beam, f is the focal length of the first lens 3, and D is the diameter of the light passing aperture of the first lens.

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

[0047] In order to facilitate changing the propagation direction of the reference light, so as to superimpose a carrier frequency on the imaging plane of the imaging device 11, in some embodiments, the second mirror 8 is further connected to the first driving structure 13 for adjusting the propagation direction of the reference light through the second mirror 8.

[0048] The working principle of a self-reference speckle interference measurement system provided in this embodiment is as follows:

[0049] 1. In this embodiment, part of the transmitted light passes through the small hole 10, is reflected by the second mirror 8, and then passes through the second beam splitter prism 6 and the second lens 4 in sequence to become parallel light (i.e., a plane wave), and is used as the reference light to irradiate on the imaging surface of the imaging device 11; let the complex amplitude of the reference light on the secondary image plane (the imaging surface of the imaging device 11) in the 4f system be R(x, y), where (x, y) are the spatial coordinates 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 first mirror 7, passes through the diaphragm 9, and then is reflected by the second beam splitter prism 6 and passes through the second lens 4, and is used as the object light to irradiate on the imaging surface of the imaging device 11; let the complex amplitude of the object light imaging on the secondary image plane (the imaging surface of the imaging device 11) in the 4f system be O(x, y), then the light field complex amplitude F(ξ, η) on the spectrum plane in the 4f system is:

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

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

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

[0054]

[0055] where (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 between the incident angle of the reference light and the imaging surface of the imaging device 11, and this phase distribution will introduce carriers with frequencies of f x and f y in the interference pattern.

[0056] 4. Since the centers of the small hole 10 and the aperture 9 in this embodiment both coincide with the optical axis, the coordinates (ξ p , η p ) of the center of the small hole 10 on the spectral plane are (0, 0). Based on this, it can be obtained that:

[0057]

[0058] Then when the measured object generates vibrations of magnitudes (Δx, Δy, Δz), the complex amplitude O′(x, y) of the object light on the secondary image plane in the 4f system and the complex amplitude R′(x, y) of the reference light on the secondary image plane in the 4f system are respectively:

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

[0060]

[0061] As can be seen from the above formula, R′(x, y) contains 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 . This part is only affected by the axial vibration of the measured object. However, from the above expression of O′(x, y), it can be seen that the phase of the object light is also affected by the axial vibration of the measured object. Then, in the interference, the phase change between the reference light and the object light caused by the axial vibration of the measured object will cancel each other out, so that within the exposure time, the phase difference between the object light and the reference light is constant and not affected by the axial vibration of the measured object. Therefore, a single interference pattern has a high contrast. So in the following derivation process, this part is not considered for both the reference light and the object light.

[0064] The third part is corresponding to the spatial 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 as:

[0066]

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

[0068]

[0069] where (·) * denotes the conjugate of a complex number.

[0070] 6. The carrier frequency spectrum distribution diagram obtained by performing a Fourier transform on the resulting interference pattern I(x, y) is as Figure 3 shown; from 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. To avoid spectral aliasing, in this embodiment, the spectral width of the object light is constrained by setting the size of the aperture 9; that is, let where ρ o is the cut-off frequency of the object light, and D 光阑 is the diameter of the aperture 9, ρ s is the sampling cut-off frequency of the image device, and p is the pixel pitch in the imaging device 11;

[0072] That is

[0073] 8. To calculate the object light phase from the single interference patterns collected before and after deformation, in this embodiment, the frequency spectrum diagram obtained by Fourier-transforming the resulting interference pattern I(x, y) is multiplied by to move the high-frequency parts in the spectrum to the center of the frequency domain, and then a low-pass filter is used to filter out other frequency components, and then an inverse Fourier transform is performed to obtain the phase-amplitude vector A(x, y) containing the object light phase information (i.e., O(x, y)R b * ). When the above process is expressed by a formula, it is as follows:

[0074]

[0075] where LPF is the low-pass filter, and FFT and IFFT respectively represent the Fourier transform and the inverse Fourier transform.

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

[0077]

[0078] Among them, O 1 (x, y) is the complex amplitude of the object light before deformation, O 2 (x, y) is the complex amplitude of the object light 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 to be measured 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, and the two are approximately parallel in practice), and w(x, y) is the out-of-plane deformation of the surface of the object to be measured, then The relationship with w(x,y) is:

[0080]

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

[0082] A 1 (x,y)=O 1 (x,y)R b1 * , A 1 (x, y) is the phase amplitude vector calculated by collecting the interference pattern before deformation;

[0083] A 2 (x,y)=O 2 (x,y)R b2 * , A 2 (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 a complex number. Δφ=∠(R b1 R b2 * ), is the phase difference of the reference light before and after deformation, and this phase difference does not change with the 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 Figure 4, in order to further improve the adjustability of the system, in this embodiment, the beam splitting prism 1 and the beam splitting prism 2 are respectively replaced by a polarization beam splitting prism 14 and a polarization beam splitting prism 15 to achieve polarization beam splitting. Both polarization beam splitting prisms reflect the s light and transmit the p light. A polarizer 16 and a half-wave plate 17 are further arranged between the front focal plane of the lens 1 and the lens 1, and the half-wave plate 17 is located between the polarizer 16 and the lens 1. The polarizer 16 is used for polarization, and the half-wave plate 17 is used for changing the direction of the polarized light, so as to change the energy ratio of the s light and the p light incident on the polarization beam splitting prism 14. A polarizer 18 is further arranged between the rear focal plane of the lens 2 and the lens 2. The reference light and the object light become the same polarization state after passing through the polarizer 18, so as to interfere 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, so as to ensure that the obtained interference pattern has a high contrast. Among them, in order to facilitate the adjustment of the half-wave plate 17, in some embodiments, the half-wave plate 17 is further connected to the driving structure 2, 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 article, the term "including", "comprising" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not expressly listed, or also includes elements inherent to such process, method, article or device. Without further limitation, an element defined by the statement "including a..." does not exclude the existence of additional identical elements in the process, method, article or device including the said element.

[0093] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A self-referenced 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 irradiates the beam splitter prism 1, where it is divided into reflected light and transmitted light. Part of the reflected light is reflected by the reflector 1 and passes through the aperture, and then is reflected by the beam splitter prism 2 and passes through the lens 2 to illuminate the imaging surface of the imaging device; After part of the penetrating light passes through the small hole and is reflected by the second reflector, it passes through the second beam splitter prism and the second lens in sequence and irradiates the imaging surface of the imaging device; Lens 1 and lens 2 form a 4f system; The aperture and the pinhole both coincide with the optical axis and are located on the middle spectrum plane of the 4f system; Aperture of the diaphragm D 光阑 satisfy: λ is the wavelength of the laser beam, f is the focal length of lens 1, p is the spacing of the pixels in the imaging device, and the diameter of the small hole D is 小孔 Less than 2.44λf / D, where D is the aperture diameter of lens one.

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 irradiates the object to be measured to form reflected light.

3. A self-referenced speckle interferometry system according to claim 1, characterized in that: An attenuation plate is also arranged between the aperture and the second beam splitter prism.

4. A self-referenced speckle interferometry system according to claim 1, characterized in that: Also includes: The driving structure 1 is used to drive the reflecting mirror 2.

5. The self-referenced speckle interferometry system according to claim 1, characterized in that: 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 polarizer 1 and half-wave plate are 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 polarizer 2 is arranged between the back focal plane of lens 2 and lens 2.

6. A self-referenced 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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