Micro-off-axis digital holography method and device based on Mach-Zehnder interference structure

Abstract

A micro-off-axis digital holography method and device based on a Mach-Zehnder interference structure belongs to the field of holographic optical imaging technology. It is characterized by comprising the following steps: step a, obtaining linearly polarized light with an amplitude of A0, and injecting the linearly polarized light into a Mach-Zehnder interference system; step b, obtaining a virtual light source of an object light point source; step c, bringing the object light and reference light point sources into a coaxial state; step d, bringing the object light point source and the reference light point source into a state of equal optical path; step e, causing the reference light to undergo a quantitative horizontal translation and allowing the reference light to continuously pass through the center of the reference light path again; step f, obtaining a background hologram, an object hologram, and an object intensity map; and step g, performing numerical reconstruction. This micro-off-axis digital holography method and device based on a Mach-Zehnder interference structure can achieve non-coplanarity between the object light filter aperture and the reference light point source, as well as independent control of the aperture size and point light source position.

Description

Technical Field

[0001] A micro-off-axis digital holographic method and device based on a Mach-Zehnder interference structure belong to the technical field of holographic-based optical imaging. Background Art

[0002] Digital holography, as an important quantitative phase imaging technique, has attracted widespread attention in recent years due to its ability to quantitatively measure the complex amplitude of an object's wavefront (especially phase information). It has been widely applied in fields such as physics, biology, and chemistry. Traditional digital holography can be divided into two categories, depending on the angle between the object beam and the reference beam in the recording setup: on-axis digital holography and off-axis digital holography. In on-axis digital holography, the angle between the object beam and the reference beam is close to zero. Such a recording setup fully utilizes the resolution of the image sensor, enabling high-spatial-resolution image reconstruction. A major challenge is that the reconstructed image of an on-axis hologram is often affected by autocorrelation and conjugate noise, which can be eliminated or suppressed through phase shifting or other time-consuming algorithms. In off-axis digital holography, the autocorrelation and conjugate terms can be well separated in the spatial frequency domain of the hologram. Therefore, spatial filtering of the off-axis digital hologram can remove the autocorrelation and conjugate terms during image reconstruction, but this comes at the cost of high image sensor bandwidth requirements. Generally speaking, in order to separate the spatial frequency of the object light from the spatial frequencies of the autocorrelation and conjugate terms, the bandwidth of the image sensor is required to be four times that of the object light to be recorded.

[0003] In recent years, a new solution, called micro-off-axis digital holography, has been proposed, which lies between traditional on-axis and off-axis digital holography. Micro-off-axis digital holography does not require a complete separation of the spatial frequency of the object light from the spatial frequency of the autocorrelation term. Instead, it only requires a separation of the spatial frequency of the object light from the spatial frequency of its conjugate term. This means setting the spatial frequency of the reference light equal to or slightly greater than the maximum spatial frequency of the recorded object light wave. Therefore, compared to off-axis digital holography, the lateral resolution of reconstructed images based on micro-off-axis digital holography can be significantly improved. Furthermore, micro-off-axis digital holography is superior to on-axis digital holography in observing dynamic processes due to its simpler phase reconstruction algorithms and processes, such as those based on Hilbert transforms, nonlinear filtering, and phase derivatives. In summary, compared to off-axis digital holography, micro-off-axis digital holography has the advantage of lower image sensor bandwidth requirements and a simpler information recovery process compared to on-axis digital holography.

[0004] While most phase reconstruction algorithms for micro-off-axis digital holography are also applicable to conventional off-axis holography, to achieve optimal resolution for the reconstructed image, the carrier frequency of the reference beam should be calibrated as accurately as possible to the maximum spatial frequency of the recorded object beam when designing the recording device. However, most designed micro-off-axis digital holography devices are directly based on existing off-axis digital holography recording devices, where the maximum spatial frequency of the recorded object beam and the carrier frequency of the reference beam are difficult to quantitatively determine and control. Consequently, achieving the optimal frequency conditions for micro-off-axis digital holography presents certain difficulties.

[0005] In recent years, existing technologies have documented an optical system based on a micro-off-axis digital holography algorithm that can effectively meet optimal frequency conditions. This system uses a converging spherical beam to illuminate the object, placing a specially designed aperture filter in the plane of the object light's spatial frequency. A point light source emitted from the edge of the aperture serves as a reference beam, ensuring the optimal frequency conditions for micro-off-axis digital holography. Furthermore, by varying the distance between the aperture filter and the object, the magnification of the field of view of the object under test can be quantitatively controlled. However, because this system is based on a 1×2 single-mode fiber splitter, it requires higher external stability. Furthermore, the aperture filter is a mechanical design and cannot be flexibly adjusted. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to overcome the shortcomings of the existing technology and provide a micro-off-axis digital holography method and device based on a Mach-Zehnder interference structure, which can achieve the non-coplanarity of the object light filter aperture and the reference light point light source and the independent regulation of the aperture size and the point light source position.

[0007] The technical solution adopted by the present invention to solve the technical problem is: the micro-off-axis digital holography method based on the Mach-Zehnder interference structure is characterized by comprising the following steps:

[0008] Step a: turning on the laser to obtain linearly polarized light with an amplitude of A0, and the linearly polarized light is incident on a Mach-Zehnder interferometer system including an object light path and a reference light path;

[0009] Step b, obtaining a virtual light source S1" of the object light point source S1 according to the reflection imaging principle;

[0010] Step c, according to the Michelson interference principle, using the change of interference fringes to adjust the object light path so that the object light and the reference light are coaxial;

[0011] Step d, adjusting the axial position of the lens of the reference light path so that the object light point source and the reference light point source reach a state of equal optical path;

[0012] Step e, moving the reference light path in a lateral direction perpendicular to the optical axis, causing the reference light to undergo a quantitative translation in the lateral direction and allowing the reference light to continuously pass through the center of the reference light path again;

[0013] Step f, obtaining a background hologram through an image sensor, and after placing the object to be measured in the object light path, obtaining an object hologram and an object intensity map through the image sensor;

[0014] Step g: perform numerical reproduction.

[0015] Preferably, in step b, according to the reflective imaging principle, the object light point source S1 is mirror-symmetrically made about the reflector M1 to obtain a virtual light source S1', and then the virtual light source S1' is mirror-symmetrically made about NPBS2 to obtain a virtual light source S1".

[0016] Preferably, in step c, according to the Michelson interference principle, the reflector M1 is adjusted so that the inclined interference fringes on the image sensor gradually become vertical, and then the reflector M1 is adjusted so that the fringes gradually become thicker until concentric rings appear, and M1 is further adjusted so that the concentric rings are in the center of the image sensor's field of view, so that the object light and the reference light point light source reach a state of equal optical path.

[0017] Preferably, in step d, the front-rear axial position of lens L2 on the reference light path is adjusted so that the stripes are gradually inward-engulfed and gradually become thicker. When the stripes are the thickest, the object light and the reference light point light source reach a coaxial equal optical path state.

[0018] Preferably, in step e, the lens L2 and the aperture stop FA2 in the reference light path are moved in the same transverse direction by the same distance x r Then, the parallel plate in the reference light path is rotated to make the reference light source S2 produce a lateral displacement x r and make the reference light pass through lens L2 and the center of aperture diaphragm FA2 again.

[0019] Preferably, in step g, the intensity map of the object is first subtracted from the object hologram, and then Fourier transform is performed. Subsequently, the correct spectrum is selected, and inverse Fourier transform and numerical diffraction are performed to obtain the complex amplitude of the object to be measured. Finally, the background noise of the system is recovered from the background hologram, and its complex conjugate is multiplied by the complex amplitude of the reconstructed object to further eliminate the background noise, and finally the amplitude and phase distribution map of the reconstructed object after noise elimination is obtained.

[0020] A micro-off-axis digital holographic system based on a Mach-Zehnder interference structure is characterized by comprising a laser for emitting a laser light source, and a Mach-Zehnder interference system consisting of two non-polarizing beam splitting prisms: non-polarizing beam splitting prisms NPBS1 and NPBS2, and two reflectors: reflectors M1 and M2; an attenuation plate for adjusting the intensity of the input laser and a conversion mechanism for converting the laser light source into linearly polarized light are provided between the laser and the Mach-Zehnder interference system;

[0021] An object light path is set between the non-polarizing beam splitter prism NPBS1 and the reflector M1, and a reference light path is set between the reflector M2 and the non-polarizing beam splitter prism NPBS2. The reflector M2 is set on the propagation path of the light beam reflected by the non-polarizing beam splitter prism NPBS1, and the reflector M1 is set on the propagation path of the light beam transmitted by the non-polarizing beam splitter prism NPBS1. The light reflected by M2 and M1 is simultaneously incident on the non-polarizing beam splitter prism NPBS2, and the image sensor is set on the output light path of the non-polarizing beam splitter prism NPBS2.

[0022] Preferably, an attenuation plate ND for adjusting the intensity of the input laser and a polarizer P for the laser polarization state conversion mechanism are sequentially placed between the laser and the Mach-Zehnder interferometer system.

[0023] Preferably, the object light path includes a pinhole filter SF, a lens L1, and an aperture stop FA1 arranged in sequence; the reference light path includes a parallel plate PL, a lens L2, and an aperture stop FA2 arranged in sequence.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] Through the present micro-off-axis digital holographic method and device based on the Mach-Zehnder interference structure, the non-coplanarity of the object light aperture diaphragm and the reference light point light source can be achieved, and the aperture size and the point light source position can be independently regulated, which can better determine and achieve the micro-off-axis state.

[0026] In this micro-off-axis digital holography device based on the Mach-Zehnder interference structure, the aperture of the object light path and the point light source of the reference light path are actually not coplanar, and the aperture size and the position of the point light source can be adjusted separately, which improves the practicality of the system. Moreover, the interference pattern of the point light source can also realize imaging of different magnifications of the object to be measured, which will further expand the application of micro-off-axis digital holography in the field of high-resolution quantitative phase imaging of biological samples.

[0027] The present micro-off-axis digital holography method and device based on the Mach-Zehnder interference structure introduces a lens into each of the two light paths of a Mach-Zehnder interference system composed of two non-polarizing beam splitters and two reflectors to generate interference of two point light sources, and inserts the object to be measured between the lens and the aperture diaphragm in the object light path; a parallel plate and an aperture diaphragm are respectively introduced on both sides of the lens in the reference light path, and the lateral displacement of the light path perpendicular to the optical axis is achieved by rotating the parallel plates, thereby realizing micro-off-axis digital holography. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 Schematic diagram of a micro-off-axis digital holographic device based on the Mach-Zehnder interference structure.

[0029] Figure 2 This is the distribution diagram of the spatial spectrum corresponding to the theoretical slightly off-axis digital hologram.

[0030] Figure 3 This is a flow chart of the micro-off-axis digital holography method based on the Mach-Zehnder interference structure.

[0031] Figure 4 In theory, the object light and reference light point light sources are equivalent to coaxial and equal optical path state diagrams by using the principle of reflection imaging.

[0032] Figures 5 to 12 Schematic diagram of the changes in interference fringes corresponding to different interference states of the two point light sources of object light and reference light.

[0033] Figure 13 Schematic diagram of the spatial distribution of the mirror image of the object light diaphragm FA1 and the reference light point light source S2 on the plane where the point light source S2 is located in the coaxial equal optical path state.

[0034] Figure 14 It is the mirror image of the object light diaphragm FA1 in a slightly off-axis state and the spatial distribution of the reference light point source S2.

[0035] Figure 15 This is the background hologram captured by the image sensor under the recording parameters z1 = 262.4 mm, z2 = 133.4 mm, and object magnification M = 0.5084.

[0036] Figure 16 This is the object hologram captured by the image sensor under the recording parameters z1 = 262.4 mm, z2 = 133.4 mm, and object magnification M = 0.5084.

[0037] Figure 17 This is the object intensity map captured by the image sensor under the recording parameters z1 = 262.4 mm, z2 = 133.4 mm, and object magnification M = 0.5084.

[0038] Figure 18It is the spectrum diagram of the object hologram after Fourier transformation.

[0039] Figure 19 This is the amplitude distribution diagram of the object to be measured reproduced by computer under the reproduction effect of micro-off-axis digital holography.

[0040] Figure 20 This is the phase distribution diagram of the object to be measured reproduced by computer under the reproduction effect of micro-off-axis digital holography.

[0041] Figures 21-22 This is a diagram showing the effect of reproducing the amplitude and phase of the object to be measured with different distance parameters and object magnifications when recording distance parameters z1 = 121.8 mm, z2 = 133.4 mm and object magnification M = 1.0952. DETAILED DESCRIPTION

[0042] Figures 1 to 22 The best embodiment of the present invention is shown below in conjunction with the attached Figures 1 to 22 The present invention is further described.

[0043] like Figure 1 As shown, a micro-off-axis digital holographic device based on a Mach-Zehnder interference structure includes a laser, a parallel plate PL, an attenuation plate ND, a polarizer P, a pinhole filter SF, two non-polarizing beam splitter prisms: non-polarizing beam splitter prism NPBS1 and non-polarizing beam splitter prism NPBS2, two Fourier lenses: lens L1 and lens L2, two aperture diaphragms: aperture diaphragm FA1 and aperture diaphragm FA2, two reflectors: reflector M1 and reflector M2, and an image sensor CCD.

[0044] The attenuation plate ND, polarizer P, non-polarizing beam splitter prism NPBS1, non-polarizing beam splitter prism NPBS2, pinhole filter SF, lens L1, aperture diaphragm FA1, and reflector M1 are arranged in sequence, wherein the pinhole filter SF, lens L1, aperture diaphragm FA1, reflector M1 and non-polarizing beam splitter prism NPBS2 are placed in sequence in the object light path corresponding to the transmitted light beam after splitting by the non-polarizing beam splitter prism NPBS1, and the reflector M2 is arranged in the reference light path corresponding to the reflected light beam after splitting by the non-polarizing beam splitter prism NPBS1. In the reference light path, the reflector M2, parallel plate PL, lens L2, aperture diaphragm FA2 and non-polarizing beam splitter prism NPBS2 are arranged in sequence, and the object light path and the reference light path are non-co-path and spatially separated.

[0045] The light emitted from the reflector M2 passes through the parallel plate PL, lens L2, and aperture FA2 in sequence and enters the non-polarizing beam splitter prism NPBS2. The light reflected by the reflector M1 in the object light path also enters the non-polarizing beam splitter prism NPBS2. The image sensor CCD is set on the output light path of the non-polarizing beam splitter prism NPBS2.

[0046] The laser light emitted by the laser passes through the attenuator ND and polarizer P before entering the Mach-Zehnder interferometer system consisting of two non-polarizing beam splitters NPBS1 and NPBS2 and two mirrors M1 and M2. The object light in the object light path passes sequentially through the pinhole filter SF, lens L1, and aperture diaphragm FA1 (lens L1 converges the light at aperture diaphragm FA1 to form a point light source). The reference light in the reference light path passes sequentially through the parallel plate PL, lens L2, and aperture diaphragm FA2 (lens L2 converges the light near aperture diaphragm FA2 to form a point light source).

[0047] Move the lens L2 and the aperture stop FA2 in the reference light path in a lateral direction perpendicular to the optical axis by the same distance x r , and then rotate the parallel plate PL to make the reference light produce a quantitative translation x in the lateral direction r The reference light is then passed through the center of lens L2 and aperture FA2 in the reference light path. Finally, the object OBJ is inserted between lens L1 and aperture FA1 (FA1 radius is R) in the object light path, and a slightly off-axis digital hologram of the object OBJ is obtained on the CCD surface of the image sensor.

[0048] based on Figure 1 The principle of the micro-off-axis digital holographic device based on the Mach-Zehnder interference structure is as follows:

[0049] First, assume that the laser illumination wavelength is λ, the distance from the object to be measured OBJ to the plane of aperture diaphragm FA1 in the object light path is z1, and the distance from aperture diaphragm FA1 and aperture diaphragm FA2 in the reference light path to the image sensor CCD is z2. Let t(x1, y1) represent the complex transmittance of the object to be measured. The coordinates of the planes FA1 and FA2 are defined as (x2, y2), and the coordinates of the recording plane of the image sensor CCD are defined as (x3, y3).

[0050] exist Figure 1 In the micro-off-axis digital holographic device based on the Mach-Zehnder interference structure shown in FIG, if the reference light has undergone a lateral displacement (assuming that the displacement occurs on the x-axis) x r , and assume that the reference light amplitude constant after passing through the non-polarizing beam splitter NPBS1, the reflector M2 and the parallel plate PL is A r , then the complex amplitude of the reference light point light source reaching the CCD surface can theoretically be expressed as:

[0051]

[0052] Among them, A r is the reference light amplitude constant, (x3, y3) is the coordinate of the image sensor CCD recording surface, λ represents the laser illumination wavelength, z2 is the distance from the aperture stop FA1 and the aperture stop FA2 in the reference light path to the image sensor CCD, x r is the lateral displacement of the reference light, and i represents the imaginary unit.

[0053] The complex amplitude of the corresponding object light wave reaching the CCD surface can be expressed as:

[0054]

[0055] in, Represents the spatial spectrum corresponding to t(x1,y1) after Fourier transformation; A represents the transmittance function of the aperture stop (aperture radius is R) on the FA1 plane; i is the amplitude constant of the object light after passing through the non-polarizing beam splitter NPBS1 and the pinhole filter SF; i represents the imaginary unit, k represents the wave number, λ represents the laser illumination wavelength, z1 is the distance from the object to be measured OBJ to the aperture diaphragm FA1 plane in the object light path, z2 is the distance from the aperture diaphragm FA1 and the reference light path to the image sensor CCD, (x2, y2) are the coordinates of the FA1 and FA2 surfaces, (x3, y3) are the coordinates of the CCD recording surface, and in micro-off-axis digital holography, x is required. r Equal to or slightly greater than R;

[0056] Further through variable substitution, use and Represents the spatial frequency domain coordinates, so the complex amplitude of the object light on the CCD surface of the image sensor can be expressed as:

[0057]

[0058] Where, Represents the Fourier transform, C is a constant, and defines a magnification coefficient that depends on z1 and z2 R represents the aperture radius, z1 is the distance from the object to be measured OBJ to the aperture aperture FA1 plane in the object light path, z2 is the distance from the aperture aperture FA1 and the aperture aperture FA2 in the reference light path to the image sensor CCD, (x2, y2) are the coordinates of the plane where FA1 and FA2 are located, (x3, y3) are the coordinates of the CCD recording surface, λ represents the laser illumination wavelength, and i represents the imaginary unit. In particular, after the above variable substitution, Represents the spatial spectrum corresponding to t(x1,y1) after Fourier transformation, Represents the transmittance function of the aperture stop (aperture radius is R) on the FA1 plane.

[0059] Then the intensity of the interference between the object light and the reference light on the CCD surface of the image sensor can be expressed as:

[0060] I H (x3,y3)=|O(x3,y3)+R(x3,y3) 2

[0061] =|O(x3,y3) 2 +|R(x3,y3) 2 +O(x3,y3)R * (x3,y3)+O * (x3,y3)R(x3,y3)

[0062] Where the asterisk * represents the complex conjugate operator; O(x3, y3) represents the complex amplitude of the object light wave reaching the CCD surface, and R(x3, y3) represents the complex amplitude of the reference light point light source reaching the CCD surface of the image sensor.

[0063] Next, the interference intensity is Fourier transformed, and the object wave of the object to be measured in the spatial frequency domain (the third term of the above formula O(x3,y3)R * (x3,y3)) and its conjugate term (the fourth term O * The spatial spectrum formula of (x3,y3)R(x3,y3)) can be simplified as follows:

[0064]

[0065] In the formula, C' represents a constant, and Respectively represent |O(x3,y3)| 2 and |R(x3,y3)| 2 The spatial spectrum of M is the magnification factor that depends on z1 and z2. R represents the aperture radius, z1 is the distance from the object to be measured OBJ to the aperture aperture FA1 plane in the object light path, z2 is the distance from the aperture aperture FA1 and the aperture aperture FA2 in the reference light path to the image sensor CCD, and represents the spatial frequency domain coordinates, (x2, y2) are the coordinates of the FA1 and FA2 planes, λ represents the laser illumination wavelength, the asterisk * represents the complex conjugate operator, and i represents the imaginary unit.

[0066] From the above formula, we can find that as long as x r Greater than or equal to R, the spatial spectrum of the object light wave and its conjugate term can be clearly separated. For specific spectrum distribution, refer to Figure 2In summary, the micro off-axis digital hologram captured by the micro off-axis digital holographic optical system based on the Mach-Zehnder interference structure can always meet the best frequency conditions required for micro off-axis.

[0067] Through the above analysis, it can also be determined that the magnification coefficient of the micro off-axis digital holographic device based on the Mach-Zehnder interference structure for the object to be measured is determined by z1 and z2. When z1 < z2, the magnification coefficient is greater than 1. At this time, the micro off-axis system can achieve a higher spatial resolution and a smaller field of view for the object, and is particularly suitable for recording objects smaller than the size of the image sensor CCD. When z1 > z2, the micro off-axis system at this time is particularly suitable for recording object information larger than the size of the image sensor CCD.

[0068] Based on Figure 1 the micro off-axis digital holographic device based on the Mach-Zehnder interference structure shown above and its principle, the micro off-axis digital holographic method based on the Mach-Zehnder interference structure shown in Figure 3 is obtained, which specifically includes the following steps:

[0069] Step 1, start the laser to obtain linearly polarized light;

[0070] Turn on the laser Laser, and make the laser emitted by the laser Laser pass through the attenuator ND and the polarizer P in sequence to obtain linearly polarized light with an amplitude of A0, ensuring a good interference effect.

[0071] Step ②, construct a Mach-Zehnder interference system;

[0072] Use two non-polarizing beam splitters NPBS1, NPBS2 and two mirrors M1, M2 to construct the Mach-Zehnder interference system shown in Figure 1 , and insert a pinhole filter SF, a lens L1 and an aperture stop FA1 into the object light optical path in sequence to obtain object light with an amplitude of A i , and at the same time insert a parallel plate PL, a lens L2 and an aperture stop FA2 into the reference light optical path in sequence to obtain reference light with an amplitude of A r .

[0073] Step 3, obtain the virtual source linearly polarized light corresponding to the object point light source;

[0074] According to the principle of reflection imaging, the object point light source S1 is mirror-symmetrical about the mirror M1 to obtain the virtual light source S1', and then the virtual light source S1' is mirror-symmetrical about NPBS2 to obtain the virtual light source S1'', as shown in Figure 4 .

[0075] Step 4, adjust the mirror to make the two point light sources on the object light optical path and the reference light optical path coaxial;

[0076] According to the Michelson interference principle, the reflector M1 is adjusted to make the inclined interference fringes on the image sensor CCD gradually become vertical, such as Figure 5 Then adjust the reflector M1 to make the stripes gradually thicker until concentric rings appear, as shown in Figures 6-8 As shown, M1 is further adjusted so that the concentric rings are in the center of the image sensor CCD field of view. At this time, the object light and reference light point light sources are coaxial.

[0077] Step 5: Adjust the lens so that the object light path and the reference light path reach a coaxial equal optical path state;

[0078] Adjust the front and rear axial positions of lens L2 on the reference light path to make the fringes gradually swallowed and thickened, such as Figure 9-12 As shown in the figure, when the stripes are the thickest, it means that the optical path between the two point light sources and the image sensor CCD is almost equal. At this time, the object light and the reference light point light source reach the coaxial equal optical path state, that is, Figure 4 The state shown in the figure, its cross-section state is as follows Figure 13 As shown, Figure 13 The middle circle represents the mirror image of FA1 on the S2 plane, with a radius of R.

[0079] Step 6, adjusting the reference light path so that the reference light passes through the center of the lens and aperture stop in the reference light path again;

[0080] Move the lens L2 and aperture diaphragm FA2 in the same lateral direction (x direction) by 3 mm, and then rotate the parallel plate to make the reference light source S2 produce a lateral displacement x r And make the reference light pass through the lens L2 and the center of aperture diaphragm FA2 again (x r represents the distance from the reference light source S2 to the center of the mirror hole of FA1), that is, R = x r =3mm, indicating that the slightly off-axis state is achieved in the experiment (the object light aperture and the reference light point source are not actually coplanar in space, and R and x r can be flexibly adjusted according to actual needs), such as Figure 14 As shown, at this time, the distance z2 from FA2 to the image sensor CCD is 133.4 mm.

[0081] Step 7: Keep the light path stable and use the image sensor CCD to shoot a background hologram in a dark environment, such as Figure 15 shown.

[0082] Step 8, using an image sensor CCD to capture a hologram of the object in a dark environment;

[0083] Insert an object to be measured OBJ between the lens L1 and the aperture diaphragm FA1 in the object light path, where the distance z1 from the plane of the object to be measured to the plane of the aperture diaphragm FA1 is 262.4 mm. Then, while keeping the light path stable, use the image sensor CCD to capture a hologram of the object in a dark environment, as shown in Figure 2. Figure 16 shown.

[0084] Step 9: Block the reference light, keep the light path stable, and use the image sensor CCD to take an intensity map of the object in a dark environment, such as Figure 17 shown.

[0085] Step 10, numerical reproduction;

[0086] The numerical reconstruction is performed by computer, specifically: first subtracting the intensity map of the object from the object hologram, then performing Fourier transformation, and then selecting the correct spectrum, such as Figure 18 The dotted circle shown is the filtering area, corresponding to the spectral information of the object The complex amplitude of the object to be measured can be obtained by performing inverse Fourier transform and numerical diffraction. Finally, the background noise of the system is recovered from the background hologram. Its complex conjugate is multiplied by the complex amplitude of the reconstructed object to further eliminate the background noise. Finally, the amplitude and phase distribution diagrams of the reconstructed object after noise elimination are as follows: Figures 19-20 shown.

[0087] Figures 21-22 The amplitude and phase of the reproduced object shown correspond to the recording distance parameters z1 = 121.8 mm, z2 = 133.4 mm, and the object magnification is M = 1.0952. Figures 21-22 The amplitude and phase shown correspond to Figures 19-20 The amplitude and phase of the object in the middle dashed box. Figures 19-20 and Figures 21-22 By controlling z1 and z2, the magnification of the object to be measured can be controlled.

[0088] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any other manner. Any person skilled in the art may utilize the above-disclosed technical content to modify or modify the present invention into equivalent embodiments. However, any simple modifications, equivalent variations, and modifications to the above embodiments that do not depart from the technical content of the present invention and are based on the technical essence of the present invention remain within the scope of protection of the present invention.

Claims

1. A micro-off-axis digital holographic system based on a Mach-Zehnder interferometer structure, characterized by: The invention comprises a laser for emitting a laser light source, and a Mach-Zehnder interferometer system composed of two non-polarizing beam splitter prisms: non-polarizing beam splitter prisms NPBS1 and NPBS2 and two reflectors: reflectors M1 and M2. An attenuation plate for adjusting the intensity of the input laser and a conversion mechanism for converting the laser light source into linearly polarized light are provided between the laser and the Mach-Zehnder interferometer system. An object light path is provided between the non-polarizing beam splitter prism NPBS1 and the reflector M1, a reference light path is provided between the reflector M2 and the non-polarizing beam splitter prism NPBS2, the reflector M2 is provided on the propagation path of the light beam reflected by the non-polarizing beam splitter prism NPBS1, and the reflector M1 is provided on the propagation path of the light beam transmitted by the non-polarizing beam splitter prism NPBS1, the light reflected by M2 and M1 is simultaneously incident on the non-polarizing beam splitter prism NPBS2, and the image sensor is provided on the output light path of the non-polarizing beam splitter prism NPBS2; An attenuator ND for adjusting the intensity of the input laser and a polarizer P for laser polarization state conversion are placed between the laser and the Mach-Zehnder interferometer system. The object light path includes a pinhole filter SF, a lens L1, and an aperture stop FA1 arranged in sequence; the reference light path includes a parallel plate PL, a lens L2, and an aperture stop FA2 arranged in sequence; The parallel plate PL is used to produce a quantitative translation of the reference light in the lateral direction. x r And the reference light is made to continuously pass through the center of the lens L2 and the aperture stop FA2 in the reference light optical path again.

2. A micro-off-axis digital holography method based on a Mach-Zehnder interferometer structure implemented by the micro-off-axis digital holography system based on a Mach-Zehnder interferometer structure according to claim 1, characterized in that: The steps include: Step a, turn on the laser and obtain an amplitude of A The linearly polarized light of 0 is incident on the Mach-Zehnder interferometer system including the object light path and the reference light path; Step b: According to the reflection imaging principle, obtain the virtual light source of the object light point source S1 ; Step c, according to the Michelson interference principle, using the change of interference fringes to adjust the object light path so that the object light and the reference light are coaxial; Step d, adjusting the axial position of the lens of the reference light path so that the object light point source and the reference light point source reach a state of equal optical path; Step e, moving the reference light path in a lateral direction perpendicular to the optical axis, causing the reference light to undergo a quantitative translation in the lateral direction and allowing the reference light to continuously pass through the center of the reference light path again; Step f, obtaining a background hologram through an image sensor, and after placing the object to be measured in the object light path, obtaining an object hologram and an object intensity map through the image sensor; Step g: perform numerical reproduction.

3. The micro-off-axis digital holography method based on the Mach-Zehnder interference structure according to claim 2, characterized in that: In step b, according to the principle of reflection imaging, the object light point source S1 is mirror-symmetrical about the reflector M1 to obtain a virtual light source , and then the virtual light source About NPBS2 to make mirror-symmetric virtual light source .

4. The micro-off-axis digital holography method based on the Mach-Zehnder interference structure according to claim 2, characterized in that: In step c, according to the Michelson interference principle, adjust the reflector M1 so that the inclined interference fringes on the image sensor gradually become vertical, then adjust the reflector M1 so that the fringes gradually become thicker until concentric rings appear, and further adjust M1 so that the concentric rings are in the center of the image sensor's field of view, so that the object light and the reference light point light source reach a state of equal optical path.

5. The micro-off-axis digital holography method based on the Mach-Zehnder interference structure according to claim 2, characterized in that: In step d, the front-back axial position of lens L2 on the reference light path is adjusted to make the stripes gradually inward and thicker. When the stripes are the thickest, the object light and the reference light point light source reach a coaxial equal optical path state.

6. The micro-off-axis digital holography method based on the Mach-Zehnder interference structure according to claim 2, characterized in that: In step e, the lens L2 and the aperture stop FA2 in the reference light path are moved in the same lateral direction by the same distance. x r , and then rotate the parallel plate in the reference light path to make the reference light source S2 produce a lateral displacement x r and make the reference light pass through lens L2 and the center of aperture diaphragm FA2 again.

7. The micro-off-axis digital holography method based on the Mach-Zehnder interference structure according to claim 2, characterized in that: In step g, the intensity map of the object is first subtracted from the object hologram, and then a Fourier transform is performed. Subsequently, the correct spectrum is selected, and an inverse Fourier transform and numerical diffraction are performed to obtain the complex amplitude of the object to be measured. Finally, the background noise of the system is recovered from the background hologram, and its complex conjugate is multiplied by the complex amplitude of the reconstructed object to further eliminate the background noise, and finally the amplitude and phase distribution map of the reconstructed object after noise elimination is obtained.

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