Light deflection symmetrical elimination of zero-order term in reconstruction of light-off-axis digital holography

Abstract

The application discloses a light-off-axis digital holography zero-order term elimination reconstruction method based on symmetrical deflection of reference light. Two object parameter angles are calculated, which make the +1 order terms of two holographic spectrum diagrams separate from each other and do not interfere with each other; the holographic diagram of the measured object is collected by twice adjusting the angle of the reference light without changing the angle of the object light; a composite spectrum diagram and a binary mask are obtained through holographic diagram processing; the target spectrum diagram is obtained according to the binary mask combined with the composite spectrum diagram, and the +1 order spectrum information is extracted to reconstruct the image of the measured object. The application utilizes the light intensity symmetry of the Gaussian laser beam, and the two light-off-axis holographic diagrams obtained by symmetrical deflection of the reference light beam are subtracted, so that the holographic diagram eliminating the zero-order spectrum can be obtained, and the imaging artifacts caused by the spectrum aliasing of the zero-order term can be suppressed while improving the frequency domain bandwidth utilization rate of the holographic optical system.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of digital holography, and particularly relates to a light-off-axis digital holography zero-order term elimination reconstruction method based on a holographic optical system and symmetrical deflection of reference light. BACKGROUND

[0002] Digital holography records a hologram through the interference measurement principle of object light and reference light, and can rely on numerical calculation to reconstruct the image of the measured object. In the frequency domain of the hologram, there are +1 order term, -1 order term and zero order term. The higher the frequency domain bandwidth utilization of the +1 order term is, the more high-frequency information of the object it contains, and the higher the quality of the reconstructed image is. In light-off-axis holography, the +1 order term spectrum carrying object information is separated, but the zero order term partially overlaps with the +1 order term. This structure makes full use of the sensor bandwidth, can significantly improve the frequency domain bandwidth utilization of the +1 order term, and can obtain a reconstructed image through at most two holograms. Compared with on-axis holography and off-axis holography, a good balance between acquisition rate and frequency domain bandwidth utilization is achieved. However, the partial spectrum information overlap between the zero order term and the +1 order term will cause artifacts in the reconstructed image, and reduce the holographic imaging quality and phase measurement accuracy. In recent years, many methods for eliminating the zero order term have been proposed by domestic and foreign scholars. However, the method of subtracting the average intensity from the hologram or the double hologram subtraction method based on reference wave polarization adjustment will cause incomplete elimination of the zero order term. The subtraction of two phase-shifted holograms can completely eliminate the zero order term, but different phase shift methods based on PZT, half-wave plate and grating will produce phase shift errors, and reduce the accuracy of holographic measurement. In summary, the research goal in the field of zero order term elimination of light-off-axis digital holography is to accurately and completely eliminate the zero order term, and to ensure the measurement accuracy without disturbing the +1 order term spectrum information carrying object information. SUMMARY

[0003] In order to solve the above technical problems, the present application provides a light-off-axis digital holography zero-order term elimination reconstruction method based on a holographic optical system and symmetrical deflection of reference light, which can accurately and completely eliminate the zero order term, solve the spectrum overlap problem while ensuring the high frequency domain bandwidth utilization of the +1 order term, and realize high-resolution and artifact-free phase quantitative measurement.

[0004] The present application utilizes the symmetry of the light intensity of a Gaussian laser beam, and can obtain a hologram eliminating the zero-order spectrum by subtracting two light-off-axis holograms obtained by symmetrical deflection of the reference beam, so as to improve the frequency domain bandwidth utilization of the holographic optical system while suppressing the imaging artifacts caused by the zero-order spectrum overlap.

[0005] The present application is implemented by the following technical solutions:

[0006] Step one: for the holographic optical system, before the experiment, according to the parameters of the laser wavelength in the holographic optical system, the pixel number and the pixel size of the CCD(charge-coupled device) camera and the frequency domain bandwidth of the +1 order term, through the formula relationship between the object parameter angle and the +1 order center point coordinate in the holographic optical system, under the condition that the spacing between the +1 order center points of the two holograms obtained by the CCD in sequence meets the restriction condition that it is greater than or equal to the frequency domain bandwidth of the +1 order term, the feasible range of the two object parameter angles required by the holographic optical system is calculated;

[0007] Step two: in the experiment, the incident angle of the object light of the holographic optical system to the CCD is unchanged, the two incident angles of the reference light to the CCD are adjusted according to the feasible range of the two object parameter angles calculated in step one, and then the two holograms obtained by interference of the measured object at different object parameter angles are collected by the CCD in sequence;

[0008] Step three: the composite frequency spectrum F3 and the binary mask B1 are obtained by processing the two holograms obtained in step two;

[0009] Step four: another binary mask B2 is extracted from the binary mask B1 of the composite frequency spectrum F3 obtained in step three, and the target frequency spectrum F4 is obtained by combining the composite frequency spectrum F3, the +1 order spectrum information is extracted in the target frequency spectrum F4, and the image of the measured object is reconstructed.

[0010] The measured object is a micro-nano structure object, such as an ultra-precision part, a MEMS chip and a biological cell.

[0011] The resolution test target is used as the measured object in the embodiment, and the hologram of the surface of the measured object is collected.

[0012] The step one is specifically:

[0013] For the holographic optical system, the laser emitted by the laser is dispersed into two paths of laser by the beam splitter, which are the reference light R and the object light O, and they interfere to form the hologram H:

[0014]

[0015] Wherein, j represents the imaginary unit; exp() represents the exponential function with natural constant e as the base; Represents the phase difference between the reference light R and the object light O; Represents the tilt phase;

[0016] The tilt phase Is set to:

[0017]

[0018] Where λ represents the laser wavelength; θ x θ represents the angle of the object parameter relative to the x-axis. y The x-axis represents the angle of the object parameter relative to the y-axis. The object parameter angle refers to the interference angle between the object beam and the reference beam on the CCD surface. The x-axis represents the horizontal direction of the CCD surface, and the y-axis represents the vertical direction of the CCD surface.

[0019] After undergoing a two-dimensional Fourier transform, the hologram H yields a holographic spectrum F containing three spectral components: +1-level terms, -1-level terms, and 0-level terms. The aforementioned angle θ... x The x-coordinate f of the center point of the +1 level term in F x The following formulas relate to each other:

[0020] θ x =sin -1 ((f x0 -f x )·λ / (M·Δx))

[0021] Among them, f x0 Let f be the x-coordinate of the center point of the holographic spectrum F. x Δx represents the x-coordinate of the center point of the +1 level term in F; M is the number of horizontal pixels of the CCD in the holographic optical path system; Δx is the horizontal pixel size of the CCD camera in the holographic optical path system.

[0022] In the formula, the angle θ of the object parameter x The x-coordinate f of the center point of the +1 level term in the holographic spectrogram is determined. x When the CCD sequentially acquires two holograms, the object parameter angles are θ. x1 and θ x2 At that time, the x-coordinates of the center points of the +1 level terms in the corresponding holographic spectrum are f x1 and f x2 .

[0023] Therefore, the distance between the center points of the two +1 level terms, |f, can be calculated using the above formula. x2 -f x1 |θ when they are greater than or equal to the frequency domain bandwidth of the +1 level term respectively x1 and θ x2 The feasible range.

[0024] Specifically, the frequency domain bandwidth of a CCD is represented by B. CCD This indicates that, considering the frequency domain bandwidth of the +1 order term in a light off-axis hologram, it can reach up to B. CCD / 2, then the center-to-center distance of the +1 level terms of the holographic spectra F1 and F2 corresponding to the two holograms acquired by the CCD in sequence needs to be greater than or equal to B. CCD / 2, calculate the required object parameter angle θ under this condition using the formula. x1 and θ x2The feasible range can ensure that the +1 order terms of the two holographic spectrum diagrams obtained in the experiment are separated from each other and do not interfere with each other.

[0025] The step two specifically comprises the following steps: first, a holographic optical path system is built, so that the reference light is vertically incident into the CCD, and the object light is obliquely incident into the CCD; then, the angle of incidence of the object light is unchanged, the angle of the mirror is controlled and adjusted by the piezoelectric driver loaded on the mirror in the optical path of the reference light, so that the angle of incidence of the reference light is changed, the reference light is symmetrically rotated counterclockwise and clockwise around the positive propagation direction of the optical axis by a fixed angle, so that the object parameter angles θ x1 and θ x2 after the two adjustments are both within the feasible range calculated in step one; finally, the CCD is used to sequentially collect two holograms obtained by interference of the measured object with the object parameter angles θ x1 and θ x2 .

[0026] The step three specifically comprises the following steps: the two holograms obtained in step two are converted into holographic spectrum diagrams F1 and F2 respectively, the complex spectrum diagram F3 in which most of the zero-order terms are eliminated is obtained by subtracting the two holographic spectrum diagrams F1 and F2, and the binary mask B1 is obtained by performing global threshold segmentation processing on the complex spectrum diagram F3.

[0027] The step three specifically comprises the following steps:

[0028] 3.1) the two holograms are converted into holographic spectrum diagrams F1 and F2 respectively by two-dimensional Fourier transform, and the complex spectrum diagram F3 in which most of the zero-order terms are eliminated is obtained by subtracting the two holographic spectrum diagrams F1 and F2;

[0029]

[0030] Wherein, FFT{} represents two-dimensional Fourier transform; O represents the object light signal of the measured object; R1 and R2 represent the reference light signals incident on the CCD surface after the angle adjustment for two times respectively;

[0031] According to the intensity symmetry of the Gaussian laser beam, the intensities of the reference light recorded by the angle symmetric deflection are the same, so |R1| 2 = |R2| 2 ; the two object parameter angles relative to the x-axis formed by the reference light R1 and R2 with the same object light are θ x1 and θ x2 , respectively. It can be seen that theoretically, the zero-order terms are completely eliminated by subtracting the holographic spectrum diagrams F1 and F2.

[0032] However, due to the error caused by the real-time environment of multi-frame measurement, the intensities of the two reference lights recorded by the angle symmetric deflection actually have a slight difference, but there are still a small amount of residual zero-order terms in the complex spectrum diagram F3, which still need to be corrected.

[0033] 3.2) In MATLAB software, the intensity segmentation threshold T corresponding to the composite spectrum F3 is obtained by using the maximum inter-class variance method adaptive threshold acquisition function graythresh(F3), and then all the pixel points in the composite spectrum F3 are traversed, the intensity value of the pixel point with the intensity less than the intensity segmentation threshold T is changed to 0, and the intensity value of the remaining pixel points is changed to 1, so that the binary mask B1 of the composite spectrum F3 is obtained, and the white area with the intensity value of 1 in the binary mask B1 is the foreground area.

[0034] The step four is specifically to extract the binary mask B2 of the residual zero-order term spectrum in the binary mask B1 by combining area recognition positioning, and obtain the target spectrum F4 by eliminating the residual zero-order term in the composite spectrum F3 according to the binary mask B2, and extract the +1 order spectrum information on the target spectrum F4 which has accurately and completely eliminated the zero-order term, and reconstruct the image of the measured object.

[0035] The step four is specifically:

[0036] 4.1) Taking the white area with the intensity value of 1 in the binary mask B1 as the foreground area, the distance L i between the center of each foreground area in the binary mask B1 and the spectrum center is compared. i The foreground area with the shortest distance L i is extracted as the binary mask B2 of the residual zero-order term spectrum.

[0037] 4.2) The intensity value of the pixel point corresponding to the foreground area range of the binary mask B2 of the residual zero-order term spectrum on the composite spectrum F3 is changed to 0 according to the following formula, so as to directly eliminate the spectrum information of the residual zero-order term, and obtain the target spectrum F4:

[0038] F4=F3·(1-B2)

[0039] In the formula, F4 is the target spectrum which has accurately and completely eliminated the zero-order term, and only has four spectrum components which are separated from each other and do not interfere with each other in the target spectrum;

[0040] 4.3) Finally, the +1 order spectrum information is extracted on the target spectrum F4 through a rectangular filter window, and the image of the measured object is reconstructed.

[0041] The distance L i in 4.1) is calculated according to the following formula:

[0042]

[0043] Wherein, (f 0x , f 0y ) is the coordinate of the center point of the binary mask B1 image; (f ix , fiy ) is the centroid coordinate of each foreground region, i represents the ordinal number of the foreground region.

[0044] In the present application, the +1 order term and the -1 order term do not overlap, but both the +1 order term and the -1 order term overlap with the 0 order term, and the holographic system with the spectral characteristics is called light off-axis holography.

[0045] The present application is based on the intensity symmetry of the laser Gaussian beam, and the reference light intensity is the same when the reference beam is symmetrically deflected. The relationship between the object parameter angle and the center position of the +1 order spectrum is obtained by formula derivation. According to the frequency domain bandwidth requirement of the +1 order spectrum, the reference light is symmetrically deflected by the same angle by using a piezoelectric driver, so as to obtain two holographic spectrum graphs in which the +1 order terms are separated from each other. The two spectrum graphs are subtracted to eliminate most of the zero order terms. The residual zero order terms are automatically positioned and eliminated based on region identification. The +1 order spectrum information is extracted to reconstruct the image of the measured object.

[0046] The principle of the present application is based on the intensity symmetry of the Gaussian laser beam, and the intensity of the reference light recorded twice by angle symmetric deflection is the same, and the object light in the experiment remains unchanged, so the zero order terms of the two holographic spectrum graphs obtained twice are the same. The accurate and complete elimination of the zero order terms can be realized by double spectrum graph subtraction and residual zero order term elimination algorithm, which can solve the spectral aliasing problem while ensuring the utilization rate of the +1 order term high frequency domain bandwidth product, and realize high resolution and artifact-free phase quantitative measurement.

[0047] Compared with the prior art, the present application has the following beneficial effects:

[0048] The present application utilizes the light intensity symmetry of the Gaussian laser beam, and the holographic graph eliminating the zero order spectrum can be obtained by subtracting two off-axis holographic graphs obtained by symmetric deflection of the reference beam. The zero order terms can be accurately and completely eliminated to improve the frequency domain bandwidth utilization rate of the holographic optical system, and the +1 order term spectrum information carrying the object information is not disturbed, which ensures the accuracy of the holographic measurement and has good artifact suppression and high resolution imaging capability. BRIEF DESCRIPTION OF DRAWINGS

[0049] Figure 1 The flow chart of the light off-axis digital holographic zero order term elimination and reconstruction method for symmetric deflection of the reference light;

[0050] Figure 2 The schematic diagram of three object parameter angles and their corresponding spectra;

[0051] Figure 3 The acquisition method of the composite spectrum eliminating the 0 order term in the ideal state

[0052] Figure 4 The holographic optical system used in the experiment;

[0053] Figure 5 Process diagram for obtaining binary mask B1 of composite holographic spectrum for the example;

[0054] Figure 6 Binary mask B2 of residual zero-order spectrum extracted from B1 for the example;

[0055] Figure 7 Target spectrum F4 with zero-order completely eliminated for the example;

[0056] Figure 8 Extracting +1 order spectrum information on F4 through a rectangular filtering window for the example;

[0057] Figure 9 Reconstructed intensity map of the object and its local magnified image for the example. DETAILED DESCRIPTION

[0058] The application will be further described below in conjunction with the drawings and examples.

[0059] The application is implemented as shown in the flowchart of Figure 1 , and the specific steps are as follows:

[0060] Step one: for the holographic optical system, before the experiment, according to the parameters in the system such as the wavelength of the laser, the number of pixels and the pixel size of the CCD camera, and the frequency domain bandwidth of the +1 order, through the formula relationship between the object parameter angle and the coordinate of the center point of the +1 order in the holographic optical system, the feasible range of the two object parameter angles required by the holographic optical system is calculated under the condition that the distance between the spectrum +1 order center points of the two holograms obtained by the CCD in sequence needs to meet the restriction condition of being greater than or equal to the frequency domain bandwidth of the +1 order. Specifically:

[0061] For the holographic optical system, the laser emitted by the laser is dispersed into two paths of laser by the beam splitter, which are the reference light R and the object light O, which interfere to form a hologram H. The hologram H can obtain a holographic spectrum F after two-dimensional Fourier transform:

[0062]

[0063] Wherein, j represents the imaginary unit; exp() represents the exponential function with natural constant e as the base; represents the phase difference between the reference light R and the object light O; represents the tilt phase;

[0064] The above tilt phase is set as:

[0065]

[0066] wherein λ represents the laser wavelength; θ x represents the angle of the object parameter angle relative to the x-axis, θ y represents the angle of the object parameter angle relative to the y-axis, the object parameter angle refers to the interference angle of the object light and the reference light on the surface of the CCD, the x-axis represents the horizontal direction of the surface of the CCD, and the y-axis represents the vertical direction of the surface of the CCD;

[0067] After the hologram H is subjected to two-dimensional Fourier transform, a holographic spectrum F containing three spectral components of +1 order, -1 order and 0 order can be obtained, and the angle θ x between the center point horizontal coordinate f x of the +1 order in F and the center point horizontal coordinate f x of the -1 order in F satisfies the following formula:

[0068] θ -1 = sin x0 ((f x -f x0 )·λ / (M·Δx))

[0069] wherein f x is the center point horizontal coordinate of the holographic spectrum F, f x represents the center point horizontal coordinate of the +1 order region in F, M is the horizontal pixel number of the CCD in the holographic optical system, and Δx is the horizontal pixel size of the CCD camera in the holographic optical system;

[0070] As can be seen from the formula, the angle θ x of the object parameter angle determines the center point horizontal coordinate f x1 of the +1 order of the holographic spectrum, and therefore when the object parameter angles of the CCD when collecting two holograms in sequence are θ x2 and θ x1 , the center point horizontal coordinates of the +1 orders of the corresponding holographic spectrum are f x2 and f x2 , respectively. The feasible range of θ x1 and θ x1 can be obtained by calculating the center point distance |f x2 | between the two +1 orders, which is greater than or equal to the frequency domain bandwidth of the +1 order.

[0071] The specific process adopted in this embodiment is: according to the frequency domain bandwidth requirement of the +1 order, the center point coordinates of the +1 orders of two light off-axis holographic spectrum diagrams satisfying the condition of not overlapping each other are determined. For example Figure 2 (a) is a holographic spectrum diagram F0 in the initial state, and it can be seen that the frequency domain bandwidth of the +1 order is half of the complete bandwidth of the spectrum diagram. Figure 2 (b) and 2(c) are two light off-axis holographic spectrum diagrams F 01 and F 02 that are expected to be obtained, that is, it can be known that under the current condition, F01 and F 02 +1 order term region center point coordinate data, assuming that the size of the spectral diagram is MxN, and the coordinate axis is established with the center point of the spectral diagram as the coordinate origin, then the center point coordinates of F 01 and F 02 +1 order terms are (0, N / 4) and (M / 2, N / 4) respectively, and the center point distance between the two +1 order terms satisfies the condition of being greater than or equal to the frequency domain bandwidth M / 2 of the +1 order term, so according to the center point coordinates of F 01 and F 02 +1 order terms, the laser wavelength, and the pixel number and pixel size data of the CCD camera, the required deflection angle θ value of the reference light can be calculated through the formula relationship between the object parameter angle and the +1 order term region center point coordinate and by inputting the known parameters. This makes it satisfy that on the basis of the initial spectral state F0, if the reference light is counterclockwise rotated by θ degrees, F 01 can be obtained, and if the reference light is clockwise rotated by θ degrees, F 02 can be obtained. In theory, the spectral diagram F 01 minus F 02 can obtain a spectral diagram in which the 0 order term is eliminated and the remaining spectral components are separated from each other without aliasing, as shown in Figure 3 .

[0072] Step two: first, build a holographic optical system so that the reference light is vertically incident into the CCD, and the object light is obliquely incident into the CCD; then the incident angle of the object light is unchanged, and the angle of the mirror loaded on the reference light path is controlled and adjusted by the piezoelectric driver to change the incident angle of the reference light, so that the reference light is symmetrically rotated counterclockwise and clockwise around the optical axis in the positive propagation direction by a fixed angle, so that the object parameter angles θ x1 and θ x2 after adjustment are within the feasible range calculated in step one; finally, two holograms obtained by interference of the measured object with the object parameter angles θ x1 and θ x2 are sequentially collected by the CCD.

[0073] The specific process adopted in this embodiment is: first, build a holographic optical system as shown in Figure 4 , the light beam emitted by the laser passes through the attenuator and the beam expander in sequence, is transmitted and reflected by the first beam splitter, the reflected light by the first beam splitter is used as the object light, passes through the first mirror, and is incident into the measured object, is transmitted through the resolution test target of the measured object, and is reflected by the second beam splitter, the transmitted light by the first beam splitter is used as the reference light, passes through the beam expander and the second mirror in sequence, and is transmitted by the second beam splitter, and the reflected light and the transmitted light by the second beam splitter are received and detected by the CCD camera. At the same time, the piezoelectric driver is installed on the back of the second mirror, and the piezoelectric driver is used to drive the second mirror to rotate.

[0074] The measured object in this example is a resolution test target, which can be a micro-nano structure object, such as an ultra-precision part, a MEMS chip, or a biological cell. In the experiment, the position of the measured object, the angle of the object light, and the like are unchanged, and the only variable is the angle of the reference light, which is changed by changing the angle of the mirror in the reference light path.

[0075] The object parameter angle is first set to an intermediate state between the two required object parameter angles. Then, the reference light is controlled to rotate by θ degrees in the counterclockwise and clockwise directions, respectively, by a piezoelectric driver mounted on the mirror. The angle θ is calculated from step one. Finally, two holograms of the measured object at the object parameter angles θ x1 and θ x2 are obtained by using a CCD to sequentially collect the reference light.

[0076] Step three: The composite spectrum F3 and its binary mask B1 are obtained by processing the two holograms obtained in step two. Specifically:

[0077] 3.1) The two holograms are converted into holographic spectrum F1 and F2, respectively, by two-dimensional Fourier transform. The composite spectrum F3, which eliminates most zero-order terms, is obtained by subtracting the two holographic spectrum F1 and F2.

[0078]

[0079] where FFT{} represents two-dimensional Fourier transform; O represents the object light signal of the measured object; R1 and R2 represent the reference light signals incident on the CCD surface after adjusting the angles twice, respectively;

[0080] According to the intensity symmetry of the Gaussian laser beam, the reference light intensities recorded by the angle symmetric deflection are the same, so |R1| 2 = |R2| 2 ; The two object parameter angles formed by the reference light R1 and R2 with the same object light relative to the x-axis are θ x1 and θ x2 , respectively. It can be seen that theoretically, the holographic spectrum F1 and F2 are subtracted to completely eliminate the zero-order terms.

[0081] However, due to the error caused by the real-time environment of multiple frame measurements, the reference light intensities recorded by the angle symmetric deflection actually have a slight difference, but there are still a small number of residual zero-order terms in the composite spectrum F3, which still need to be corrected.

[0082] 3.2) In MATLAB software, the intensity segmentation threshold T corresponding to the composite spectrum F3 is obtained by using the maximum inter-class variance method adaptive threshold acquisition function graythresh(F3), and then all the pixel points in the composite spectrum F3 are traversed, the intensity value of the pixel point whose pixel intensity is less than the intensity segmentation threshold T is changed to 0, and the intensity value of the remaining pixel points is changed to 1, so that the binary mask B1 of the composite spectrum F3 is obtained, and the white area with an intensity value of 1 in the binary mask B1 is the foreground area.

[0083] The specific process adopted in this embodiment is: first, two-dimensional Fourier transform is performed on the two holograms obtained in step two to obtain holographic spectrum F1 and F2, as shown in Figs. Figure 5 (a) and 5(b). Then, the composite spectrum F3 with residual zero-order terms is obtained by F1-F2, as shown in Fig. Figure 5 (c). Finally, the binary mask B1 is obtained by using the maximum inter-class variance method for global threshold segmentation processing of the spectrum F3, as shown in Fig. Figure 5 (d).

[0084] Step four: the binary mask B2 of the residual zero-order spectrum extracted from B1 obtained in step three is processed, and the target spectrum F4 is obtained in combination with the composite spectrum F3, and the +1 order spectrum information in the target spectrum F4 is extracted to reconstruct the image of the measured object. Specifically:

[0085] 4.1) Taking the white area with an intensity value of 1 in the binary mask B1 as the foreground area, the distance L i of the centroid of each foreground area in the binary mask B1 to the spectrum center is compared. i The foreground area with the shortest distance L i is extracted as the binary mask B2 of the residual zero-order spectrum;

[0086] 4.2) The intensity value of the pixel point corresponding to the composite spectrum F3 in the foreground area range of the binary mask B2 of the residual zero-order spectrum is changed to 0 according to the following formula, so as to directly eliminate the spectrum information of the residual zero-order term, and obtain the target spectrum F4:

[0087] F4=F3·(1-B2)

[0088] In the formula, F4 is the target spectrum in which the zero-order term is accurately and completely eliminated, and only four spectrum components separated from each other and not interfering with each other exist in the target spectrum;

[0089] 4.3) Finally, the +1 order spectrum information is extracted from the target spectrum F4 through a rectangular filter window, and the image of the measured object is reconstructed.

[0090] The distance L i in 4.1) is calculated according to the following formula:

[0091]

[0092] Among them, (f 0x f 0y (f) represents the coordinates of the center point of the binary mask B1 image; ix f iy ) are the centroid coordinates of each foreground region, and i represents the ordinal number of the foreground region.

[0093] Let the size of the hologram be M*N, then the size of its corresponding spectrogram is also M*N, and the size of the binary mask B1 of the spectrogram is also M*N. The center point coordinates of these three graphs are (M / 2, N / 2).

[0094] The specific process used in this embodiment is as follows: First, the centroid coordinate data of each foreground region in the binary mask B1 is automatically obtained through region recognition technology. The distance from each centroid to the center point of the spectrum is calculated, and the foreground region with the shortest distance is extracted to obtain the binary mask B2 of the residual zero-order term spectrum. Figure 6 As shown. Then, the residual zero-order terms in the composite spectrum F3 are eliminated using the formula F4 = F3·(1-B2) to obtain the target spectrum F4, as shown. Figure 7 As shown. Finally, the +1 level spectrum information is extracted on F4 using a rectangular filter window, as shown. Figure 8 As shown, the image of the object under test is reconstructed based on this information.

[0095] The reconstructed image in this embodiment is as follows: Figure 9 As shown, the reconstructed intensity image obtained by the present invention has the advantages of clear contours, no artifacts, and high resolution, which confirms the effectiveness of the present invention.

[0096] This invention addresses the problem of accurately and completely eliminating the zero-order term spectrum in light off-axis holography without interfering with the +1-order term spectral information. Based on the intensity symmetry of the Gaussian laser beam, the reference light intensity recorded by symmetrical angle deflection is the same, and the object light in the experiment remains unchanged; therefore, the zero-order terms in the two acquired holographic spectra are identical. Theoretically, subtracting the two holographic spectra can achieve complete elimination of the zero-order terms.

[0097] Considering that experimental and environmental errors may lead to some residual zero-order spectra in the composite spectrogram, we further automatically locate and eliminate residual zero-order spectra through region identification, ultimately achieving accurate and complete elimination of zero-order terms. While ensuring the utilization rate of the high-frequency domain bandwidth product of +1-order terms, we solve the problem of spectral aliasing and improve the holographic imaging quality and phase measurement accuracy.

Claims

1. A light reference symmetrical deflection light off-axis digital holographic zero-order term elimination reconstruction method, characterized by comprising the following steps: Step 1: for a holographic optical system, before the experiment, according to the wavelength of the laser in the holographic optical system, the pixel number and pixel size of the CCD camera and the frequency domain bandwidth of the +1 order term, the feasible range of the two object parameters angles required by the holographic optical system is calculated through the formula relationship between the object parameter angle and the +1 order term center point coordinate in the holographic optical system, under the condition that the center point distance of the +1 order term in the frequency spectrum of the two holograms obtained by the CCD in sequence meets the limitation condition of being greater than or equal to the frequency domain bandwidth of the +1 order term; Step 2: in the experiment, the incident angle of the object light of the holographic optical system to the CCD is unchanged, and the two incident angles of the reference light to the CCD are adjusted according to the feasible range of the two object parameter angles calculated in step 1, and then two holograms of the measured object obtained by interference at different object parameter angles are collected by the CCD in sequence; Step 3: the composite frequency spectrum F3 and the binary mask B1 thereof are obtained by processing the two holograms obtained in step 2; Step 4: another binary mask B2 is extracted from the binary mask B1 of the composite frequency spectrum F3 obtained in step 3, and the target frequency spectrum F4 is obtained by combining the composite frequency spectrum F3, and the +1 order spectrum information is extracted in the target frequency spectrum F4 to reconstruct the image of the measured object; The step 4 is specifically that the binary mask B2 of the residual zero-order term spectrum is extracted in the binary mask B1 combined with the region recognition positioning, the target frequency spectrum F4 is obtained by eliminating the residual zero-order term in the composite frequency spectrum F3 according to the binary mask B2, and the +1 order spectrum information is extracted in the target frequency spectrum F4 to reconstruct the image of the measured object; The step 4 is specifically: 4.1) Take the white region with intensity value 1 of the pixel point in the binary mask B1 as the foreground region, and compare the distance L of the center of mass of each foreground region in the binary mask B1 to the spectrum center i The distance L i The shortest foreground region is extracted as the binary mask B2 of the residual zero-order term spectrum; 4.2) the intensity values of the pixel points corresponding to the composite frequency spectrum F3 in the foreground area range of the binary mask B2 of the residual zero-order term spectrum are all set to 0 according to the following formula to obtain the target frequency spectrum F4: F4=F3·(1-B2) In the formula, F4 is the target frequency spectrum in which the zero-order term is accurately and completely eliminated; 4.3) finally, the +1 order spectrum information is extracted in the target frequency spectrum F4 to reconstruct the image of the measured object; The distance L in 4.1) i is calculated according to the following formula: wherein (f 0x , f 0y ) are the coordinates of the center point of the binary mask B1 image; (f ix , f iy ) are the coordinates of the centroid of each foreground region, and i denotes the ordinal number of the foreground region.

2. The method of claim 1, wherein the reference light is symmetrically deflected. The step 1 is specifically: for the holographic optical system, the laser emitted by the laser is dispersed into two paths of laser by the beam splitter, which are the reference light R and the object light O, and they interfere to form the hologram H: where j represents an imaginary unit; exp() represents an exponential function with a natural constant e as a base; represents a phase difference between the reference light R and the object light O; represents a tilt phase; The above-mentioned tilted phase is set to: where λ represents the laser wavelength; θ x represents the angle of the object angle with respect to the x-axis, θ y represents the angle of the object angle with respect to the y-axis; After the hologram H is subjected to two-dimensional Fourier transform, a holographic spectrum F containing three kinds of spectral components of +1 order, -1 order and 0 order is obtained, and the above angle θ x There is the following formula relationship between the center point horizontal coordinate f of the +1 order in F x and the center point horizontal coordinate f of the -1 order in F. θ x = sin -1 ((f x0 -f x )·λ / (M·Δx)) wherein f x0 is the center point horizontal coordinate of the holographic spectrum F, f x represents the center point horizontal coordinate of the +1 order term in F; M is the number of horizontal pixels of the CCD in the holographic optical system; Δx is the horizontal pixel size of the CCD camera in the holographic optical system; The distance between the center points of the two +1 level terms, |f, can be calculated using the above formula. x2 -f x1 |θ when they are greater than or equal to the frequency domain bandwidth of the +1 level term respectively x1 and θ x2 The feasible range.

3. The method for eliminating and reconstructing zero-order terms in a lightly off-axis digital hologram with reference light symmetric deflection according to claim 1, characterized in that: The step two is specifically: first, a holographic optical system is built, so that the reference light is perpendicularly incident into the CCD, and the object light is obliquely incident into the CCD; then the incident angle of the object light is unchanged, the angle of the mirror loaded on the light path of the reference light is controlled by the piezoelectric driver to change the incident angle of the reference light, the reference light is symmetrically rotated counterclockwise and clockwise around the positive propagation direction of the optical axis by a fixed angle, so that the object parameter angles θ x1 and θ x2 after the two adjustments are both within the feasible range calculated in step one; finally, two holograms obtained by interference of the measured object with the object parameter angles θ x1 and θ x2 are collected by the CCD in sequence.

4. The method of claim 2, wherein the reference light is symmetrically deflected. The step 3 is specifically: 3.1) the two holograms are converted into holographic frequency spectrum F1 and F2 by two-dimensional Fourier transform, and the composite frequency spectrum F3 is obtained by subtracting the two holographic frequency spectrum F1 and F2; wherein, FFT{} represents two-dimensional Fourier transform; O represents the object light signal of the measured object; R1 and R2 represent the reference light signals incident to the surface of the CCD after the two adjustment angles, respectively; 3.2) The intensity segmentation threshold T corresponding to the composite spectrogram F3 is obtained by using the maximum inter-class variance method adaptive threshold acquisition function graythresh(F3) in MATLAB software, and then all the pixel intensities in the composite spectrogram F3 are traversed, the intensity value of the pixel point with the pixel intensity less than the intensity segmentation threshold T is set to 0, and the intensity value of the remaining pixel points is changed to 1, to obtain the binary mask B1 of the composite spectrogram F3.

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