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

Abstract

The invention discloses a micro-off-axis digital holographic method and device based on a Mach-Zehnder interference structure, and belongs to the technical field of holographic optical imaging. The method is characterized by comprising the following steps: step a, obtaining linearly polarized light with the amplitude of A0, and enabling the linearly polarized light to enter a Mach-Zehnder interference system; b, obtaining a virtual light source of the object light spot light source; c, enabling two-point light sources of the object light and the reference light to be in a coaxial state; step d, enabling the object light spot light source and the reference light spot light source to reach an aplanatism state; e, enabling the reference light to generate quantitative translation in the transverse direction and enabling the reference light to continuously pass through the center of the light path of the reference light again; step f, obtaining a background hologram, an object hologram and an object intensity graph; and g, performing numerical value reproduction. According to the micro off-axis digital holographic method and device based on the Mach-Zehnder interference structure, the object light filtering aperture and the reference light spot light source are not coplanar, and the aperture size and the position of the spot light source can be independently regulated and controlled.

Description

Technical Field

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

[0002] As an important quantitative phase imaging technology, digital holography has attracted widespread attention in recent years due to its ability to quantitatively measure the complex amplitude of the object wavefront (especially the phase information), and has been widely used in the research of physics, biology, chemistry and other fields. According to the different angles between the object light and the reference light in the recording device, traditional digital holography can be divided into two categories: on-axis digital holography and off-axis digital holography. In on-axis digital holography, the angle between the object light and the reference light is close to zero. Such a recording device can make full use of the resolution of the image sensor to achieve high spatial resolution image reconstruction. The main problem is that the reconstructed image of the on-axis hologram is often affected by autocorrelation and conjugate noise, which can be eliminated or suppressed by 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, the autocorrelation and conjugate terms can be removed in the image reconstruction process by simply spatial filtering the off-axis digital hologram, but at the cost of high bandwidth requirements for the image sensor. 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 between traditional on-axis and off-axis digital holography has been proposed, called micro-off-axis digital holography. Micro-off-axis digital holography does not need to completely separate the spatial frequency of the object light from the spatial frequency of the autocorrelation term, but only needs to separate the spatial frequency of the object light from the spatial frequency of its conjugate term, that is, to set 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 with off-axis digital holography, the lateral resolution of the reconstructed image based on micro-off-axis digital holography can be greatly improved. At the same time, micro-off-axis digital holography is also superior to on-axis digital holography in observing dynamic processes because its phase reconstruction algorithm and process are simpler, such as the phase reconstruction algorithm based on Hilbert transform, nonlinear filtering and phase derivative. In short, compared with off-axis digital holography, micro-off-axis digital holography has the advantage of low bandwidth requirements for image sensors, and compared with on-axis digital holography, micro-off-axis digital holography has the advantage of a simple information recovery process.

[0004] Although most phase reconstruction algorithms for micro-off-axis digital holography are also applicable to conventional off-axis holography, in order to obtain the best resolution of the reconstructed image, the carrier frequency of the reference light should be calibrated as accurately as possible to the maximum spatial frequency of the recorded object light 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, in which the maximum spatial frequency of the recorded object light and the carrier frequency of the reference light are difficult to quantitatively determine and control, so there are certain difficulties in meeting the optimal frequency conditions for micro-off-axis digital holography.

[0005] In recent years, an optical system based on a micro-off-axis digital holography algorithm and which can better meet the optimal frequency conditions has been documented in the prior art. The system uses a converging spherical light beam to illuminate the object, and places a specially designed aperture filter on the plane of the object light spatial frequency; at the same time, a point light source emitted from the edge of the aperture is used as a reference light to ensure the optimal frequency conditions of the micro-off-axis digital holography. In addition, by changing the distance between the aperture filter and the object, the magnification of the field of view of the object to be measured in the system can be quantitatively controlled. However, since the system is designed based on a 1×2 single-mode fiber splitter, it has higher requirements for external stability, and 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 prior art and provide a micro-off-axis digital holographic method and device based on a Mach-Zehnder interference structure, which can achieve the non-coplanarity of the object light filtering aperture and the reference light point light source and the separate 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 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 light 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 two point light sources in a coaxial state;

[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, so that the reference light is quantitatively translated in the lateral direction and the reference light passes through the center of the reference light path again continuously;

[0013] Step f, obtaining a background hologram through an image sensor, and after placing the object to be tested 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 reflection imaging principle, the object light point source S1 is mirror-symmetric about the reflector M1 to obtain a virtual light source S1', and then the virtual light source S1' is mirror-symmetric 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 inwardly 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 rotate the parallel plate in the reference light path to make the reference light point source S2 produce a lateral displacement x r and make the reference light pass through lens L2 and the center of aperture stop 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 composed of two non-polarizing beam splitting prisms: non-polarizing beam splitting prisms NPBS1-NPBS2 and two reflectors: reflectors M1-M2, an attenuation plate for adjusting the intensity of input laser and a conversion mechanism for converting the laser light source into linearly polarized light are arranged 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, 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, 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 polarizing plate 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 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 and the individual regulation of the diaphragm size and the point light source position can be achieved, 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; and 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] Through the micro-off-axis digital holography method and device based on the Mach-Zehnder interference structure, a lens is introduced into each of the two light paths of the Mach-Zehnder interference system composed of two non-polarized beam splitting prisms and two reflectors to generate interference of two point light sources, and the object to be measured is inserted between the lens and the aperture diaphragm of the object light path; a parallel plate and an aperture diaphragm are introduced on both sides of the lens of 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] Fig.13 It is a schematic diagram of the spatial distribution of the mirror image of the object light diaphragm FA1 on the plane where the point light source S2 is located and the reference light point light source S2 in the coaxial equal optical path state.

[0034] Fig.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] Fig.15 The background hologram captured by the image sensor under the recording parameters z1=262.4mm, z2=133.4mm and object magnification M=0.5084.

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

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

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

[0039] Fig.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] Fig. 20 It 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 of the effect of reproducing amplitude and phase of the object to be tested with different distance parameters and object magnifications when recording distance parameters z1=121.8mm, z2=133.4mm 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 (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, the polarizing plate P, the non-polarizing beam splitter prism NPBS1, the non-polarizing beam splitter prism NPBS2, the pinhole filter SF, the lens L1, the aperture diaphragm FA1, and the reflector M1 are arranged in sequence, wherein the pinhole filter SF, the lens L1, the aperture diaphragm FA1, the reflector M1 and the non-polarizing beam splitter prism NPBS2 are placed in sequence in the object light path corresponding to the transmitted light beam after the non-polarizing beam splitter prism NPBS1 splits the light, the reflector M2 is arranged in the reference light path corresponding to the reflected light beam after the non-polarizing beam splitter prism NPBS1 splits the light, and in the reference light path, the reflector M2, the parallel plate PL, the lens L2, the aperture diaphragm FA2 and the 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, the lens L2, and the 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 emitted by the laser passes through the attenuation plate ND and the polarizer P, and then enters the Mach-Zehnder interferometer system composed of two non-polarized beam splitters NPBS1 and NPBS2 and two reflectors M1 and M2. Among them, the object light in the object light path passes through the pinhole filter SF, lens L1, and aperture diaphragm FA1 in sequence (lens L1 converges the light at the aperture diaphragm FA1 to form a point light source), and the reference light in the reference light path passes through the parallel plate PL, lens L2, and aperture diaphragm FA2 in sequence (lens L2 converges the light near the 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 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 made to continuously pass through the center of the lens L2 and the aperture diaphragm FA2 in the reference light path again. Finally, the object OBJ to be measured is inserted between the lens L1 and the aperture diaphragm FA1 (the radius of FA1 is R) in the object light path, and a micro-off-axis digital hologram of the object OBJ to be measured can be obtained on the CCD surface of the image sensor.

[0048] based on Figure 1 The micro-off-axis digital holographic device based on the Mach-Zehnder interference structure shown in the figure has the following principle:

[0049] First, assume that the wavelength of laser illumination is λ, the distance from the object to be measured OBJ to the plane of aperture stop FA1 in the object light path is z1, and the distance from aperture stop FA1 and aperture stop FA2 in the reference light path to the image sensor CCD is z2. Use t(x1,y1) to represent the complex transmittance of the object to be measured, and define the coordinates of the FA1 and FA2 planes as (x2,y2), and the coordinates of the image sensor CCD recording surface 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 be theoretically 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 plane of the aperture stop FA1 in the object light path, z2 is the distance from the aperture stop FA1 and the aperture stop FA2 in 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 It can be equal to or slightly larger than R;

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

[0057]

[0058] In the formula, represents the Fourier transform, C is a constant, and defines a 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, (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] Wherein, 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 to obtain 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 (the fourth term of the above formula 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 depending on z1 and z2. R represents the aperture radius, z1 is the distance from the object to be measured OBJ to the plane of the aperture aperture FA1 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 is guaranteed 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 optimal 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 neutral density filter ND and the polarizer P in sequence to obtain linearly polarized light with an amplitude of A0, ensuring better interference effects.

[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 in the object light path in sequence to obtain object light with an amplitude of A i At the same time, insert a parallel plate PL, a lens L2 and an aperture stop FA2 in the reference light path in sequence to obtain reference light with an amplitude of A r .

[0073] Step 3, obtain the virtual light 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 path and the reference light 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 to 8 As shown, M1 is further adjusted to make the concentric rings in the center of the image sensor CCD field of view. At this time, the object light and reference light are coaxial.

[0077] Step 5, adjusting 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 position of lens L2 on the reference light path so that the stripes gradually swallow up and become thicker, such as Figure 9-12 As shown in the figure, when the stripes are the thickest, it means that the optical path from the two point light sources to the image sensor CCD is almost equal. At this time, the object light and the reference light point light source reach a coaxial equal optical path state, that is, Figure 4 The state shown in the figure, its cross-sectional state is as follows Fig.13 As shown, Fig.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 lens L2 and aperture stop FA2 in the same lateral direction (x direction) by 3 mm, and then rotate the parallel plate to make the reference light point source S2 produce a lateral displacement x r And make the reference light pass through the lens L2 and the center of the 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 stop 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 Fig.14 As shown, at this time, the distance z2 from FA2 to the image sensor CCD is 133.4 mm.

[0081] Step 7, while keeping the light path stable, use the image sensor CCD to shoot a background hologram in a dark environment, such as Fig.15 shown.

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

[0083] An object OBJ is inserted between the lens L1 and the aperture stop 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 stop FA1 is 262.4 mm. Then, while keeping the light path stable, an object hologram is taken in a dark environment using the image sensor CCD, as shown in FIG. Fig.16 shown.

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

[0085] Step 10, numerical reproduction;

[0086] The numerical reconstruction is performed by computer, specifically: first subtract the intensity map of the object from the object hologram, then perform Fourier transformation, and then select the correct spectrum, such as Fig.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 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 recorded distance parameters corresponding to the amplitude and phase of the reproduced object shown are z1=121.8mm, z2=133.4mm, 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 is only a preferred embodiment of the present invention, and does not limit the present invention in other forms. Any technician familiar with the profession may use the above disclosed technical content to change or modify it into an equivalent embodiment with equivalent changes. However, any simple modification, equivalent change and modification made to the above embodiment according to the technical essence of the present invention without departing from the technical solution of the present invention still belongs to the protection scope of the technical solution of the present invention.

Claims

1. A micro-off-axis digital holography method based on Mach-Zehnder interference structure, characterized in that: The steps include: 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; Step b, obtaining a virtual light source S1" of the object light point light source S1 according to the reflection imaging principle; 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 two point light sources in a coaxial state; 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, so that the reference light is quantitatively translated in the lateral direction and the reference light passes through the center of the reference light path again continuously; Step f, obtaining a background hologram through an image sensor, and after placing the object to be tested in the object light path, obtaining an object hologram and an object intensity map through the image sensor; Step g: perform numerical reproduction.

2. The micro-off-axis digital holography method based on Mach-Zehnder interference structure according to claim 1, characterized in that: In step b, according to the reflection imaging principle, the object light point source S1 is mirror-symmetric with respect to the reflector M1 to obtain a virtual light source S1', and then the virtual light source S1' is mirror-symmetric with respect to NPBS2 to obtain a virtual light source S1".

3. The micro-off-axis digital holography method based on Mach-Zehnder interference structure according to claim 1, characterized in that: In step c, according to the Michelson interference principle, the reflector M1 is adjusted to make the inclined interference fringes on the image sensor gradually become vertical, and then the reflector M1 is adjusted to make the fringes gradually thicker until concentric rings appear. M1 is further adjusted to make the concentric rings 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.

4. The micro-off-axis digital holography method based on Mach-Zehnder interference structure according to claim 1, characterized in that: In step d, the front-rear 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 source reach a coaxial equal optical path state.

5. The micro-off-axis digital holography method based on Mach-Zehnder interference structure according to claim 1, characterized in that: 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 rotate the parallel plate in the reference light path to make the reference light point source S2 produce a lateral displacement x r and make the reference light pass through lens L2 and the center of aperture stop FA2 again.

6. The micro-off-axis digital holography method based on Mach-Zehnder interference structure according to claim 1, characterized in that: 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.

7. A micro-off-axis digital holography system based on a Mach-Zehnder interference structure, used to implement the micro-off-axis digital holography method based on a Mach-Zehnder interference structure according to any one of claims 1 to 6, characterized in that: It includes a laser for emitting a laser light source, and a Mach-Zehnder interferometer system composed of two non-polarization beam splitter prisms: non-polarization beam splitter prisms NPBS1-NPBS2 and two reflectors: reflectors M1-M2, and 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 arranged between the laser and the Mach-Zehnder interferometer system; An object light path is set between the non-polarizing beam splitter prism NPBS1 and the reflector M1, 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, 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.

8. The micro-off-axis digital holographic system based on Mach-Zehnder interference structure according to claim 7, characterized in that: An attenuation plate ND for adjusting the intensity of the input laser and a polarizing plate P for the laser polarization state conversion mechanism are placed in sequence between the laser and the Mach-Zehnder interferometer system.

9. The micro-off-axis digital holographic system based on Mach-Zehnder interference structure according to claim 7, characterized in that: The object light path includes a pinhole filter SF, a lens L1, and an aperture stop FA1 which are arranged in sequence; the reference light path includes a parallel plate PL, a lens L2, and an aperture stop FA2 which are arranged in sequence.

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