An angular spectrum engineering based non-diffracting holographic reconstruction method and system
By introducing angular spectrum engineering into holographic reconstruction and utilizing phase modulation and Fourier domain ring bandpass constraints, the problems of limited depth of field and high system complexity in existing technologies are solved, achieving axially stable high-quality reconstruction and flexible depth of field adjustment.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- NANJING UNIV
- Filing Date
- 2026-03-04
- Publication Date
- 2026-06-09
- Estimated Expiration
- Not applicable · inactive patent
AI Technical Summary
The effective depth of field of existing computational holographic reconstruction methods is limited by diffraction propagation and system numerical aperture, which leads to a decrease in image quality when out of focus. Existing improvement schemes increase system complexity and cost, which is not conducive to engineering integration.
By employing a diffraction-free holographic reconstruction method based on angular spectrum engineering, a random phase distribution is introduced using a phase modulator, and a ring bandpass constraint is implemented in the Fourier domain to restrict the angular spectrum distribution of the light field, forming a confined angular spectrum light field, thus achieving high-quality diffraction-free reconstruction.
The system achieves axially stable reconstructed image quality, simplifies the system structure, reduces implementation difficulty, facilitates integration, and enhances the system's flexibility by adjusting the annular angular spectrum constraint parameters to achieve adjustable effective depth of field.
Smart Images
Figure CN122172524A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of holographic reconstruction technology, and in particular to a diffraction-free holographic reconstruction method and system based on angular spectrum engineering. Background Technology
[0002] Optical holography, by simultaneously recording and reconstructing the amplitude and phase information of a light field, has wide applications in display, data storage, and light field manipulation. With the development of spatial light modulator technology, computational holography methods such as computational Fourier holography and Fresnel holography can achieve wavefront generation without physical targets, providing a foundation for technologies such as augmented reality (AR), head-up displays (HUDs), and wearable displays. However, the effective depth of field of existing computational holographic reconstruction methods is usually limited by factors such as diffraction propagation and the numerical aperture of the system. When the observed light field deviates from the optimal reconstruction position, the reconstructed image suffers from problems such as energy diffusion, detail blurring, and decreased contrast, thus limiting the depth of field of the holographic reconstructed image.
[0003] To improve the axial robustness of holographic images, existing research has proposed methods such as adjustable lenses, active alignment, or dynamic compensation to improve reconstruction performance under defocus conditions. However, these approaches typically increase system complexity and cost, and raise implementation difficulty, hindering engineering integration. On the other hand, structured beams such as Bessel beams, Mathieu beams, and Weber beams are considered approximate transport-invariant solutions to the Helmholtz equations. Their mechanism can be explained by an approximate conical distribution in wave vector space and a ring-shaped bandpass angular spectrum constraint in the Fourier domain. Although these structured beams have applications in fields such as microscopic imaging and fabrication, how to reasonably introduce angular spectrum constraints to achieve axially stable reconstruction while maintaining the degree of freedom in computing holographic pattern encoding still faces challenges such as insufficient configuration flexibility and the introduction of sidelobe energy. Summary of the Invention
[0004] To address the technical problems of limited effective depth of field, image quality degradation when out of focus, and high system complexity and increased cost of existing improvement schemes in existing holographic reconstruction technologies, this invention aims to provide a diffraction-free holographic reconstruction method and system with a simple structure, configurable parameters, and the ability to achieve axially stable reconstruction.
[0005] According to a first aspect of the present invention, a diffraction-free holographic reconstruction method based on angular spectral engineering is provided, comprising: The target pattern is projected onto the phase modulator, and the phase modulator is used to introduce a random phase distribution into the target pattern to form a modulated light field; The modulated light field is subjected to a 4f ring filter module to perform a Fourier transform at the central Fourier surface. A ring bandpass mask is set on the Fourier surface to allow the spatial frequency components located in the preset ring angular spectrum region to pass through, thereby obtaining a restricted angular spectrum light field. The restricted angular spectral light field is relayed to the imaging plane of the detection device via an imaging lens group to form a target image.
[0006] According to a second aspect of the present invention, a diffraction-free holographic reconstruction system based on angular spectral engineering is provided, comprising a light source, a collimating and beam expanding module, a mirror, an aperture stop, a target pattern projection mask, a phase modulator, a 4f ring filter module, an imaging lens group, and a detection device, wherein: The beam of light output from the light source illuminates the target pattern projection mask after passing through the collimation and beam expansion module; The target pattern is projected onto the phase modulator and randomly phase modulated to form a modulated light field; The 4f ring filter module is used to implement angular spectrum ring bandpass limiting of the modulated optical field to obtain a limited angular spectrum optical field; The imaging lens group is used to relay the restricted angular spectral light field onto the plane of the detection device to form a target image.
[0007] This invention applies a ring-shaped bandpass constraint to the holographic light field in the Fourier domain, causing the spatial frequency components of the transmitted light field to form an approximately conical distribution in the wave vector space. This narrows the distribution bandwidth of the longitudinal wave vector components, reduces structural degradation caused by defocus propagation, and enables the reconstructed image to maintain stable quasi-diffraction-free reconstruction within a preset axial range. Compared with existing schemes that require complex active alignment or dynamic compensation, this invention has a simple system structure, does not require complex active alignment or dynamic compensation mechanisms, and facilitates system implementation and integration. By adjusting the relevant parameters of the ring-shaped angular spectrum constraint, the effective depth of field range can be adjusted and configured, enhancing the system's flexibility. This invention can be combined with computational holographic pattern coding technology and is suitable for various application scenarios such as display, data storage, optical encryption, and projection. Attached Figure Description
[0008] Figure 1 A schematic diagram of a diffraction-free holographic reconstruction method based on angular spectrum engineering is provided for an embodiment of the present invention.
[0009] Figure 2 A schematic diagram illustrating the principle of the diffraction-free holographic reconstruction method provided in this embodiment of the invention;
[0010] Figure 3 This is a schematic diagram of the structure of the diffraction-free holographic reconstruction system provided in an embodiment of the present invention;
[0011] Figure 4 This is a schematic diagram illustrating the effect of random phase loading in an embodiment of the present invention.
[0012] Figure 5 This is a schematic diagram showing the actual dimensions of the annular mask provided in an embodiment of the present invention;
[0013] Figure 6 This is a schematic diagram of the diffraction-free holographic reconstruction result provided in an embodiment of the present invention. Detailed Implementation
[0014] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the protection scope of this application.
[0015] It should be noted that, in the description of this application, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. The terms "first," "second," etc., in this application are used to distinguish similar objects and are not used to describe a specific order or sequence.
[0016] The technical problem this invention aims to solve is that the effective depth of field in existing computational holographic reconstructions is typically limited by factors such as diffraction propagation and system numerical aperture. When the observation plane deviates from the optimal reconstruction position, the reconstructed image exhibits energy diffusion, detail blurring, and decreased contrast, resulting in limited depth of field. To improve axial robustness, existing solutions employ adjustable lenses, active alignment, or dynamic compensation to enhance defocus performance. However, these solutions generally increase system complexity and cost, and raise the difficulty of implementation, hindering engineering integration. Maintaining the degree of freedom in computational holographic pattern encoding while rationally introducing angular spectral constraints to achieve axially stable reconstruction still faces challenges such as insufficient configuration flexibility and the introduction of sidelobe energy.
[0017] Example 1 The technical solution adopted by this invention to solve its technical problem is: to provide a diffraction-free holographic reconstruction method based on angular spectrum engineering, which achieves high-quality diffraction-free holographic reconstruction by precisely engineering the angular spectrum of the target pattern. The method utilizes ring filtering technology to limit the angular spectrum distribution of the light field, thereby obtaining a reconstructed image with long-axis stability.
[0018] See Figure 1 A diffraction-free holographic reconstruction method based on angular spectral engineering specifically includes the following steps: S1. Project the target pattern onto the phase modulator, and use the phase modulator to introduce a random phase distribution into the target pattern to form a modulated light field.
[0019] For example, the beam output from the light source is collimated by the collimation and beam expander module to produce parallel light. This parallel light is reflected by a mirror and filtered by an aperture stop before being emitted to illuminate a projection mask, forming a target pattern on the mask. The target pattern is then projected onto the surface of a phase modulator, which introduces a random phase distribution into the target pattern, forming a modulated light field. The collimation and beam expander module ensures the parallelism and uniformity of the beam, providing a stable light source foundation for subsequent phase modulation. The aperture stop controls the numerical aperture of the beam, optimizing the illumination conditions.
[0020] The introduction of random phase distribution is a key technical feature for achieving diffraction-free reconstruction, effectively suppressing coherent noise and improving the quality of the reconstructed image. Specifically, the random phase can be generated by loading a random function onto a programmable spatial light modulator, or it can be introduced through a static diffuser, such as frosted glass, with a phase range of [0, 2π]. When using a programmable spatial light modulator, the statistical characteristics of the random phase distribution can be adjusted in real time; when using a static diffuser, the system structure is simpler and more stable.
[0021] Furthermore, the complex amplitude distribution of the modulated light field is equal to the two-dimensional intensity distribution I of the target pattern. target The square root of (x,y) and the random phase distribution φ diffsuer The product of exponential functions of (x, y) is expressed as follows:
[0022] Where U represents the complex amplitude optical field incident on the subsequent device, which has a target pattern amplitude distribution and a random phase distribution, I target (x,y) represents the two-dimensional intensity distribution of the target pattern, φ diffsuer (x,y) represents a random phase distribution. i The unit is the imaginary unit. This modulation method preserves the intensity information of the target pattern while achieving scattering of the light field through random phase.
[0023] By clarifying the specific implementation and mathematical expression of the random phase, this invention provides concrete guidance for generating complex amplitude light fields with target pattern amplitude distribution and random phase distribution in practice. The introduction of the random phase effectively reduces the spectral concentration of light field information in the Fourier domain, thereby improving image uniformity, reducing diffraction artifacts and speckle noise, and enhancing the quality of the reconstructed image during subsequent reconstruction. Programmable SLMs offer dynamic adjustability and flexibility, while static diffusers provide a lower-cost alternative.
[0024] S2. The modulated light field is subjected to a 4f ring filter module to perform a Fourier transform on the central Fourier surface. A ring bandpass mask is set on the Fourier surface to allow the spatial frequency components located in the preset ring angular spectrum region to pass through, thereby obtaining a restricted angular spectrum light field.
[0025] For example, the modulated light field undergoes a Fourier transform at the central Fourier surface using a 4f ring filter module. A ring-shaped bandpass mask is then placed on the Fourier surface, allowing spatial frequency components located within a preset ring-shaped angular spectrum region to pass through, thus obtaining a restricted angular spectrum light field. The first lens of the 4f ring filter module performs a Fourier transform on the modulated light field, forming the angular spectrum distribution of the light field at the central Fourier surface. The ring-shaped bandpass mask is the core device for achieving diffraction-free characteristics; it only allows angular spectrum components within a specific ring region to pass through, filtering out other frequency components.
[0026] Optionally, the annular bandpass mask can be any one of a physical annular aperture, an amplitude-type spatial light modulator, or a digital micromirror device; the annular mask is disposed on the central Fourier surface of the 4f annular filter module.
[0027] Optionally, the radial transmission profile of the annular bandpass mask is one of either a rect-type or a sinc-type annular bandpass function. The rect-type function has a steep cutoff characteristic, enabling precise frequency selection; the sinc-type function has a smoother transition characteristic, which helps reduce the ringing effect caused by filtering.
[0028] Furthermore, the physical model for obtaining the confined angular spectral light field step is as follows:
[0029] in, S(k) represents the Fourier transform. x , k y This is the expression for the confined angular spectral light field transmitted through the annular mask at this time, with subscripts indicating the specific angles of the light. xy This represents a two-dimensional Fourier transform performed on the horizontal XY plane. This represents the mathematical expression of the added annular mask, where k = 2π / λ represents the wave vector, and... λ The wavelength of the light source used is indicated by k. z0 The setting represents the central longitudinal wave vector component, and 'a' represents the setting of the non-diffraction propagation distance. For a 4f ring filter module with a single lens focal length of f, the theoretical and actual size calculation formula for its ring mask ring width is as follows:
[0030] The formula for calculating the theoretical center radius is as follows:
[0031] Furthermore, due to the ring-shaped spectral constraint, the spatial frequency components (k) of the modulated optical field are... x , k y The longitudinal wave vector component k is concentrated within a narrow ring band, thus narrowing the longitudinal wave vector component k. z The distributed bandwidth; when the annular band is sufficiently narrow, the k corresponding to the transmission component z Approximate consistency is achieved as the longitudinal phase difference decreases during propagation, keeping the reconstructed image approximately stable within a preset axial range (quasi-diffraction-free reconstruction). This preset axial range, i.e., the annular angular spectral region, can be adjusted by changing the physical annular bandwidth of the annular bandpass mask. b (For example, b = r) _out -r _in r _in Represents the physical inner radius, r _out Represents the physical outer radius, and its corresponding spatial frequency width is Configuration: The narrower the ring band, the greater the longitudinal wave vector component k. z The more concentrated the distribution is at k z0 The longer the axial stability range, the shorter the range; conversely, the shorter the range. The theoretical formula for calculating the axial stability range is:
[0032] This adjustable design allows the system to optimize axial stability performance according to different application requirements, thereby adjusting the effective propagation distance for diffraction-free reconstruction.
[0033] Furthermore, the annular bandpass mask defines its transmission region as an annular bandwidth region centered on the spatial frequency origin, i.e., the annular angular spectral region M(k x , k y The region meets the following conditions: , and k r_in < k r_out Wherein, the spatial frequency radius k r The physical radius r satisfies k r = 2πr / λf.
[0034] S3. The restricted angular spectral light field is relayed to the imaging plane of the detection device via an imaging lens group to form a target image.
[0035] For example, the confined angular spectrum light field is relayed to the imaging plane of the detection device via an imaging lens group to form a target image. The second lens of the 4f ring filter module performs an inverse Fourier transform on the filtered angular spectrum to reconstruct the spatial light field distribution. Because the angular spectrum is confined to a specific ring region, the reconstructed light field has quasi-diffraction-free propagation characteristics, maintaining a stable transverse intensity distribution over a long axial distance. The principle of diffraction-free holographic reconstruction is as follows: Figure 2 As shown.
[0036] The method provided in this embodiment achieves diffraction-free holographic reconstruction through angular spectral confinement and control, exhibiting better axial stability and reconstruction quality compared to traditional holographic methods. By adjusting the parameters of the annular mask, an optimal balance can be achieved between axial stability range and lateral resolution, meeting the needs of different application scenarios.
[0037] Example 2 See Figure 3 The present invention also provides a diffraction-free holographic reconstruction system based on angular spectral engineering, comprising a light source, a collimating and beam expanding module, a mirror, an aperture stop, a target pattern projection mask, a phase modulator, a 4f ring filter module, an imaging lens group, and a detection device, wherein: The light beam output by the light source illuminates the target pattern projection mask after passing through the collimation and beam expansion module.
[0038] For example, a laser is used as the light source, capable of outputting a stable, coherent beam. The collimation and beam-expanding module consists of a collimating lens and a beam-expanding lens. The collimating lens converts the diverging beam output from the light source into a parallel beam, while the beam-expanding lens enlarges the diameter of the parallel beam, ensuring that the beam can completely illuminate the subsequent target pattern projection mask. A reflector is used to change the direction of the optical path, achieving a compact layout of the optical system. An aperture stop controls the size of the beam aperture, improving beam quality and reducing the influence of stray light.
[0039] The target pattern is projected onto the phase modulator for random phase modulation to form a modulated light field.
[0040] For example, the target pattern projection mask carries the target pattern information to be reconstructed. When the light beam processed by the collimation and beam expansion module shines on the target pattern projection mask, the beam carries the amplitude information of the target pattern. The phase modulator is located behind the target pattern projection mask. In this embodiment, it is implemented using a spatial light modulator, which can perform random phase modulation on the incident light field. The target pattern is projected onto the phase modulator. By applying a random phase distribution, the phase modulator converts the original amplitude-type target pattern into a modulated light field with a complex amplitude distribution. This random phase modulation pre-realizes the uniformity of the information distribution of the angular spectrum modulation Fourier surface for the subsequent 4f ring filter module, enhancing the image quality of the diffraction-free holographic reconstruction.
[0041] The 4f ring filter module is used to implement angular spectrum ring bandpass limiting of the modulated optical field to obtain a restricted angular spectrum optical field.
[0042] For example, the 4f ring filter module is the core component of the system, consisting of two focusing lenses and a ring bandpass mask located on the Fourier surface between the two lenses. The first focusing lens performs a Fourier transform on the modulated light field from the phase modulator, forming an angular spectral distribution of the light field on the Fourier surface between the two lenses. The ring bandpass mask is precisely placed on this Fourier surface. This mask has a ring-shaped transmission region, which can selectively transmit spectral components within a specific spatial frequency range, filtering out low-frequency and high-frequency components, thus achieving angular spectral ring bandpass confinement. The second focusing lens performs an inverse Fourier transform on the spectral components passing through the ring bandpass mask to obtain a confined angular spectral light field. Through this angular spectral engineering technique, the system can generate a light field distribution with diffraction-free characteristics, which maintains an approximately constant transverse intensity distribution during propagation.
[0043] The imaging lens group is used to relay the restricted angular spectral light field onto the plane of the detection device to form a target image.
[0044] For example, the imaging lens group contains an even number of focusing lenses. These lenses are arranged according to the principle of a 4f system, with the distance between adjacent lenses equal to the sum of their focal lengths. The function of the imaging lens group is to relay the limited angular spectrum light field after passing through the 4f ring filter module, ensuring that the light field information is completely transmitted to the detection device plane.
[0045] Optionally, the 4f ring filter module consists of two focusing lenses and a ring bandpass mask located on the Fourier surface between the two lenses.
[0046] Optionally, the imaging lens group includes an even number of focusing lenses for relaying the light field image through the 4f ring filter module to form a diffraction-free holographic reconstructed image in the space where the detection device is located.
[0047] For example, the detection device can employ a CCD camera or a CMOS image sensor to record the intensity distribution of the reconstructed light field. The detection device is mounted on a movable stage along the optical axis, which has a precision stepper motor drive system capable of achieving micron-level displacement accuracy. By controlling the stage, the detection device can perform axial scanning along the optical axis, recording the intensity distribution of the reconstructed light field at different axial positions. Because the system generates a diffraction-free light field, the image obtained during axial scanning exhibits axial stability; that is, the lateral distribution of the reconstructed image remains essentially unchanged over a large axial range, thus achieving axially stable holographic reconstruction.
[0048] The entire system works as follows: The light beam output from the light source is processed by the collimation and beam expansion module to form a parallel extended beam. This beam is guided by a mirror and passes through an aperture stop to illuminate the target pattern projection mask. The amplitude information of the target pattern is carried by the beam and projected onto the phase modulator. The phase modulator applies random phase modulation to the light field, forming a modulated light field with a complex amplitude distribution. The modulated light field enters the 4f ring filter module and, after being limited by the angular spectrum ring bandpass, obtains a confined angular spectrum light field, which has diffraction-free propagation characteristics. Finally, the imaging lens group relays the confined angular spectrum light field onto the detection device plane, forming a clear and stable target image. Through the axial scanning function, the system can verify the stability of the reconstructed image in the axial direction, achieving high-quality diffraction-free holographic reconstruction.
[0049] Example 3 This embodiment illustrates the method of the present invention through specific experiments: S1. Parallel illumination is obtained by collimating and expanding a 532nm wavelength laser beam and passing it through an aperture stop. The parallel light illuminates the target pattern mask I. target The target pattern (x, y) is projected onto the spatial light modulator. In this example, the target pattern is a tangram pattern. The spatial light modulator is loaded with a random phase distribution φ. diffuser The modulated optical field obtained at (x,y) can be written as:
[0050] in, φ represents the amplitude of the target pattern. diffuser (x,y) is used to reduce energy concentration caused by spectral concentration and improve reconstruction uniformity. Its function is as follows: Figure 4 As shown, in this embodiment, φ diffuser Generated by a random function.
[0051] S2. The modulated light field undergoes a Fourier transform at the central Fourier plane after passing through a 4f ring filter module. Simultaneously, a mathematical expression is placed on this plane. A ring-shaped bandpass mask applies a ring-shaped bandpass constraint to spatial frequency components, allowing only components located within a predetermined ring region to pass through. In this example, a is 0.15m, the focal length f of the 4f ring filter module lens is 0.2m, and k... z0 =0.999996·2π / λ 。
[0052] The theoretical and actual spatial dimensions of the annular bandpass mask are approximately: center radius 0.52 mm, ring width 21.8 μm (see [reference]). Figure 5 The modulation of the incident light field by the annular mask can be written as:
[0053] The annular constraint it brings restricts the spatial frequency domain of the incident light field, causing the longitudinal wave vector component of the transmitted component to be concentrated in a preset interval, thereby maintaining the stability of the reconstructed image within a preset axial range.
[0054] S3. The light field passing through the 4f ring filter module is relayed to the imaging detector plane via the imaging lens group. The imaging detector can be mounted on a displacement stage that can move along the optical axis to perform axial scanning, record the intensity distribution of the reconstructed light field, and obtain an axially stable holographic reconstructed image.
[0055] The imaging detector is positioned at multiple different axial positions along the optical axis. i The reconstructed image is recorded at the focal plane of the lens closest to the detector. Using the reconstruction result at the focal plane as a benchmark, other z-axis lenses are then compared. i The correlation coefficient consistency of the images was evaluated, and the continuous axial interval corresponding to a consistency index of not less than 90% of the peak value was defined as the axial stability range. The axial stability range was found to be approximately 30 cm. The reconstruction results are as follows. Figure 6 As shown.
[0056] Example 4 Without altering the system structure and reconstruction method described in the above embodiments, the axial stability range is configurable by adjusting the physical ring width *b* of the Fourier annular bandpass mask. Specifically, keeping the light source power, lens parameters, mask position, and imaging detector settings unchanged, only at least two sets of annular bandpass masks with different ring widths are used: *b1* = 21.8 μm and *b2* = 32.7 μm. Reconstructed images are recorded at different propagation distances *z*. The reconstruction result at the focal plane of the lens closest to the detector is used as a benchmark to evaluate the consistency of the reconstructed images at different *z* locations. The determined axial stability range is approximately 30 cm and 20 cm.
[0057] In summary, the advantages of this invention compared to existing technologies are as follows: Simplified system structure: The system design is simple, requiring no complex active alignment or dynamic compensation mechanisms, facilitating system implementation and integration; Adjustable annular angular spectrum bandpass constraint parameters: By adjusting the relevant parameters of the annular angular spectrum constraint, the effective depth range can be adjusted and configured, enhancing the system's flexibility; Compatible with computational holographic pattern coding: This invention can be combined with computational holographic coding technology, making it suitable for various application scenarios such as display, data storage, optical encryption, and projection.
Claims
1. A diffraction-free holographic reconstruction method based on angular spectral engineering, characterized in that, include: The target pattern is projected onto the phase modulator, and the phase modulator is used to introduce a random phase distribution into the target pattern to form a modulated light field; The modulated light field is subjected to a 4f ring filter module to perform a Fourier transform at the central Fourier surface. A ring bandpass mask is set on the Fourier surface to allow the spatial frequency components located in the preset ring angular spectrum region to pass through, thereby obtaining a restricted angular spectrum light field. The restricted angular spectral light field is relayed to the imaging plane of the detection device via an imaging lens group to form a target image.
2. The diffraction-free holographic reconstruction method according to claim 1, characterized in that, Before projecting the target pattern onto the phase modulator, the method further includes: the light source outputs parallel light through a collimating and expanding module and illuminates a projection mask after passing through a reflector and an aperture stop, thereby forming the target pattern on the projection mask.
3. The diffraction-free holographic reconstruction method according to claim 1, characterized in that, The random phase can be generated by loading a random function through a programmable spatial light modulator, or it can be introduced through a static diffuser.
4. The diffraction-free holographic reconstruction method according to claim 1, characterized in that, The modulated light field satisfies the following relationship: the complex amplitude distribution of the modulated light field is equal to the two-dimensional intensity distribution I of the target pattern. target The square root of (x,y) and the random phase distribution φ diffsuer The product of exponential functions of (x,y).
5. The diffraction-free holographic reconstruction method according to claim 1, characterized in that, The radial transmission profile of the annular bandpass mask is either a rect-type or a sinc-type annular bandpass function.
6. The diffraction-free holographic reconstruction method according to claim 1, characterized in that, The preset annular angular spectrum region can be configured by adjusting the width of the annular bandpass mask region: the narrower the annular band, the more concentrated the longitudinal wave vector component distribution, and the longer the corresponding axial stability range; conversely, the wider the annular band, the shorter the range.
7. A diffraction-free holographic reconstruction system based on angular spectral engineering, characterized in that, It includes a light source, a collimating and beam expanding module, a mirror, an aperture stop, a target pattern projection mask, a phase modulator, a 4f ring filter module, an imaging lens group, and a detection device, among which: The beam of light output from the light source illuminates the target pattern projection mask after passing through the collimation and beam expansion module; The target pattern is projected onto the phase modulator and randomly phase modulated to form a modulated light field; The 4f ring filter module is used to implement angular spectrum ring bandpass limiting of the modulated optical field to obtain a limited angular spectrum optical field; The imaging lens group is used to relay the restricted angular spectral light field onto the plane of the detection device to form a target image.
8. The diffraction-free holographic reconstruction system according to claim 7, characterized in that, The 4f ring filter module consists of two focusing lenses and a ring bandpass mask located on the Fourier surface between the two lenses.
9. The diffraction-free holographic reconstruction system according to claim 7, characterized in that, The imaging lens group includes an even number of focusing lenses, which are used to relay the light field image after passing through the 4f ring filter module to form a diffraction-free holographic reconstruction image in the space where the detection device is located.
10. The diffraction-free holographic reconstruction system according to claim 7, characterized in that, The detection device is installed on a displacement stage that can move along the optical axis, and is used to perform axial scanning to record the intensity distribution of the reconstructed optical field and obtain an axially stable holographic reconstruction.