An ultrafast holographic imaging system and method

Abstract

The application provides an ultrafast holographic imaging system and method, which comprises a laser emitter, a laser collimation module, an object light path, a reference light path and a camera; the laser emitted by the laser emitter is converted into plane light by the laser collimation module, the plane light is divided into two beams of light by a beam splitter and enters the object light path and the reference light path respectively, the reference light output through the reference light path and the object light output through the object light path form a hologram after interference, and a compressed hologram is obtained after shearing processing by the camera; wherein the reference light path encodes the reference light by using a 4f spatial filtering module, the 4f spatial filtering module comprises a grating array, a first Fourier transform lens, a rectangular spatial filter and a second Fourier transform lens arranged in sequence, and the grating array is obtained by splicing a plurality of one-dimensional gratings with different periods along the vertical direction.

Description

Technical Field

[0001] This invention belongs to the field of imaging technology, and in particular relates to an ultrafast holographic imaging system and method. Background Technology

[0002] The statements in this section are merely background information related to the present invention and do not necessarily constitute prior art.

[0003] Time is a crucial parameter in the light field function, and imaging is essentially the integral of the light field function over a certain time interval. The shorter this time interval, the higher the temporal resolution of the image. Typically, we use frame rate (fps) to describe the image information of ultrafast phenomena captured by high-speed photography. From early film-based frame cameras to frame cameras based on image sensors such as CCD or CMOS, and then to streak cameras, the frame rate has continuously increased. Today, streak cameras have shooting frequencies reaching 10... 13 At frame rates exceeding fps, ultrafast processes such as molecular rotational dynamics and solvation in chemistry can be recorded, which is of great significance for studying various ultrafast phenomena in physics, chemistry, and biology. However, streak cameras can only capture one-dimensional information. If two-dimensional images are captured, these images will be spatially mixed and superimposed, making them difficult to separate.

[0004] To record two-dimensional images using a streak camera, Lihong V. Wang's team invented Compressed Ultrafast Photography (CUP). This technique first encodes a sequence of dynamic images using a Digital Micromirror Device (DMD), then records the encoded image sequence using a streak camera, resulting in a compressed image that is spatially superimposed. Finally, compressed sensing (CS) theory is used to recover all the images from the compressed image. This technique makes it possible for streak cameras to capture ultrafast phenomena. However, the two-dimensional images recorded by the streak camera in the CUP only contain intensity information of the ultrafast scene, while phase information, which is equally important as intensity information, is ignored. Lin et al. proposed a time-resolved imaging method based on "tick-tock" pulse digital holographic microscopy, which encodes the "tick-tock" pulse using two complementary binary coding patterns, achieving the recording of two frames of holographic images in a single capture. Chen et al. proposed an ultrafast holographic imaging method based on wavelength space mapping imaging technology, which uses a series of chirped multi-wavelength sub-pulses generated by a DMD-based quasi-4f system and a dispersive medium as illumination light for the holographic system, achieving the recording of 10 frames of holographic images in an ultrafast process, with a frame rate of up to 6.8 × 10⁻⁶. 10While these methods achieve FPS, compared to CUP technology, they record fewer images per burst and involve more complex recording processes. To address this, patent application CN2022112674090 proposes an ultrafast compressed holographic imaging system and method based on a streak camera. This method utilizes a specially designed pinhole array to "encode" the reference light. The pinhole positions and sizes are at the micrometer level. However, this method requires precise pinhole positions and sizes, which poses significant manufacturing challenges. Furthermore, each pinhole has low light transmittance, resulting in low light source utilization. Summary of the Invention

[0005] To address the aforementioned problems, this invention provides an ultrafast holographic imaging system and method. The scheme utilizes a self-designed 4f spatial filter module to encode the reference light. The encoded reference light interferes with the object light at different times during the ultrafast process on the recording plane. After being sheared by a fringe camera, a compressed hologram is formed. Finally, a compressed sensing algorithm is used to recover the intensity and phase information of the object light at different times from the compressed hologram. This scheme has the advantages of being easy to implement and having high light utilization.

[0006] According to a first aspect of the present invention, an ultrafast holographic imaging system is provided, comprising a laser emitter, a laser collimation module, an object light path, a reference light path, and a camera; the laser emitted by the laser emitter is converted into planar light by the laser collimation module, the planar light is split into two beams by a beam splitter and enters the object light path and the reference light path respectively, the reference light output from the reference light path and the object light output from the object light path interfere to form a hologram, and the compressed hologram is obtained after cropping by the camera;

[0007] The reference light path uses a 4f spatial filtering module to encode the reference light. The 4f spatial filtering module includes a grating array arranged in sequence, a first Fourier transform lens, a rectangular spatial filter, and a second Fourier transform lens. The grating array is a two-dimensional grating array, which can be understood as being obtained by splicing together multiple one-dimensional gratings with different periods in the x-direction along the vertical direction.

[0008] Furthermore, the object light path is as follows: a beam of planar light divided by a beam splitter passes through an ultrafast dynamic object and then sequentially enters the objective lens, the magnifying light path, and the reflecting mirror. The object light reflected by the reflecting mirror enters the beam splitter and interferes with the reference light from the reference light path.

[0009] Furthermore, the reference light path is as follows: another beam of planar light, divided by a beam splitter, is reflected by a mirror and enters a 4f spatial filter module. The reference light processed by the 4f spatial filter module enters the beam splitter and interferes with the object light from the object light path.

[0010] Furthermore, each row of the grating array employs a different grating constant.

[0011] Furthermore, the first Fourier transform lens and the second Fourier transform lens have different focal lengths.

[0012] Furthermore, the grating array is located on the front focal plane of the first Fourier transform lens, and the rectangular spatial filter is located on the rear focal plane of the first Fourier transform lens, wherein the rectangular spatial filter is used to filter out the positive first-order diffraction beam.

[0013] Furthermore, the laser collimation module includes an objective lens, a pinhole filter, and a collimating lens arranged in sequence.

[0014] Furthermore, the camera is a stripe camera.

[0015] According to a second aspect of the present invention, an ultrafast holographic imaging method is provided, which is based on the above-described ultrafast holographic imaging system, the method comprising:

[0016] The laser emitted by the laser emitter is converted into planar light by the laser collimation module;

[0017] The plane light is split into two beams by a beam splitter, which then enter the object light path and the reference light path, respectively.

[0018] The reference light output from the reference light path and the object light output from the object light path interfere to form a hologram;

[0019] Based on the obtained hologram, a compressed hologram is obtained after clipping by a stripe camera.

[0020] Furthermore, for the obtained compressed hologram, the complex amplitude distribution of each frame in the ultrafast dynamic process is obtained by decompression.

[0021] Compared with the prior art, the beneficial effects of the present invention are:

[0022] (1) The present invention provides an ultrafast holographic imaging system and method. The scheme is based on a self-designed 4f spatial filter module to encode the reference light. The encoded reference light interferes with the object light at different times in the ultrafast process on the recording plane. After being sheared by a fringe camera, a compressed hologram is formed. Finally, the intensity and phase information of the object light at different times can be recovered from the compressed hologram using a compressed sensing algorithm. The scheme has the advantages of being easy to implement and having high light utilization.

[0023] (2) The grating array in the 4f spatial filtering module of the present invention is composed of multiple one-dimensional gratings with different periods spliced ​​in the y direction (i.e., the vertical direction). That is, the grating constant of each row of the grating array is different, which can effectively ensure that the diffraction angles between the positive first-order diffraction term and the zero-order diffraction term after the plane light is diffracted by the specially designed grating are different.

[0024] Advantages of additional aspects of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0025] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0026] Figure 1 This is a schematic diagram of the structure of an ultrafast holographic imaging system as described in an embodiment of the present invention;

[0027] Figure 2 This is a schematic diagram of the grating array structure described in an embodiment of the present invention;

[0028] Figure 3 This is a schematic diagram of the rectangular filter structure described in an embodiment of the present invention;

[0029] Figure 4 This is a schematic diagram of the phase distribution of the filtered array reference light as described in an embodiment of the present invention;

[0030] Figure 5 This is a schematic diagram of the 4f spatial filtering module described in an embodiment of the present invention;

[0031] Figure 6 This is a schematic diagram of the intensity distribution of the spatial spectrum before filtering as described in this embodiment of the invention;

[0032] Figure 7 This is a schematic diagram of the intensity distribution of the filtered spatial spectrum as described in an embodiment of the present invention;

[0033] Figure 8 This is a compressed hologram and its spectrum diagram as described in the embodiments of the present invention;

[0034] Figure 9 This is a schematic diagram of the decompression result described in the embodiment of the present invention (where the two columns on the left are intensity maps and the two columns on the right are the corresponding phase maps). Detailed Implementation

[0035] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0036] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0037] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0038] Where there is no conflict, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0039] Terminology Explanation:

[0040] The 4f spatial filtering system is a typical filtering system and a technical term in the field of optical imaging. It is based on the fact that the input and output surfaces are separated by four separate focal lengths, where f is the focal length. Surface P0 is the input surface. One focal length behind the input surface is the first Fourier transform lens, which performs a Fourier transform on the input light source. A rectangular filter is placed on the subsequent focal plane P1 to spatially filter the resulting spectrum. One focal length behind P1 is the second Fourier transform lens, which performs a further Fourier transform on the filtered spectrum. The subsequent focal plane P2 is the output surface.

[0041] Example 1:

[0042] The purpose of this embodiment is to provide an ultrafast holographic imaging system.

[0043] An ultrafast holographic imaging system includes a laser emitter, a laser collimation module, an object light path, a reference light path, and a camera. The laser emitted by the laser emitter is converted into planar light by the laser collimation module. The planar light is split into two beams by a beam splitter, which enter the object light path and the reference light path respectively. The reference light output from the reference light path and the object light output from the object light path interfere to form a hologram. The hologram is then processed by the camera to obtain a compressed hologram.

[0044] The reference light path uses a 4f spatial filtering module to encode the reference light. The 4f spatial filtering module includes a grating array arranged in sequence, a first Fourier transform lens, a rectangular spatial filter, and a second Fourier transform lens. The grating array is obtained by splicing multiple one-dimensional gratings with different periods along the vertical direction.

[0045] In a specific implementation, the object light path is as follows: a beam of plane light divided by a beam splitter passes through an ultrafast dynamic object and then sequentially enters the objective lens, the magnifying light path, and the reflecting mirror. The object light reflected by the reflecting mirror enters the beam splitter and interferes with the reference light from the reference light path.

[0046] In a specific implementation, the reference light path is as follows: another beam of plane light, divided by a beam splitter, is reflected by a mirror and enters a 4f spatial filtering module. The reference light processed by the 4f spatial filtering module enters the beam splitter and interferes with the object light from the object light path.

[0047] In a specific implementation, each row of the grating array uses a different grating constant.

[0048] In practice, the first Fourier transform lens and the second Fourier transform lens use different focal lengths.

[0049] In a specific implementation, the grating array is located on the front focal plane of the first Fourier transform lens, and the rectangular spatial filter is located on the rear focal plane of the first Fourier transform lens. The rectangular spatial filter is used to filter out the positive first-order diffraction beam.

[0050] In a specific implementation, the laser collimation module includes an objective lens, a pinhole filter, and a collimating lens arranged in sequence.

[0051] In practice, the camera used is a stripe camera.

[0052] Specifically, for ease of understanding, the following detailed description of the solution in this embodiment is provided in conjunction with the accompanying drawings:

[0053] This embodiment provides an ultrafast holographic imaging system. The system employs a specially designed 4f spatial filter module to "encode" the reference light. This 4f spatial filter module consists of a specially designed grating array, a rectangular spatial filter, and two convex lenses. The reference light, after being "encoded" by this 4f spatial filter module, is a collection of elongated plane waves with different incident directions, contiguous but not overlapping. The "encoded" reference light interferes with the object light at different times during the ultrafast process on the recording plane. After being sheared by a streak camera, a compressed hologram is formed. Finally, a compressed sensing algorithm is used to recover the intensity and phase information of the object light at different times from the compressed hologram. The holographic imaging system described in this embodiment has the advantages of easy implementation and high light utilization.

[0054] like Figure 1The diagram shows a schematic of the ultrafast holographic imaging system described in this embodiment. In this system, BS1 and BS2 are beam splitters, Obj is a dynamic object, Laser is a laser, MO1 and MO2 are microscope objectives, PF is a pinhole filter, CL is a collimating lens, TL is a tube lens, M1 and M2 are reflecting mirrors, G is a specially designed grating array, L1 and L2 are Fourier transform lenses, F is a rectangular spatial filter, and Streak Camera is a streak camera.

[0055] Combination Figure 1 The principle of the ultrafast holographic imaging system described in this embodiment is explained. Essentially, this holographic imaging system is equivalent to a specially designed Mach-Zehnder interferometer. Compared to traditional holographic recording optical paths, this system uses a 4f spatial filter module based on a specially designed grating array in the reference optical path to "encode" the reference light. The "encoded" reference light becomes a plane wave array with different incident angles, providing the conditions for subsequent recording of compressed holograms. The system is described in detail below:

[0056] The light emitted by the laser is converted into plane light after passing through the laser collimation module consisting of objective lens MO1, pinhole filter PF and collimating lens CL. Then, it is split into two beams by beam splitter BS1 and enters the object beam path and reference beam path respectively.

[0057] In the object-light path, the object-light beam passes through the ultrafast dynamic object Obj and carries information about the object at different times into the objective lens MO2. Then, after being magnified by the magnifying optical path composed of objective lens MO2 and tube lens TL, it passes through the reflecting mirror M1 and beam splitter BS2 in sequence before reaching the recording plane of the streak camera.

[0058] In the reference light path, the reference light is reflected by mirror M2 and then enters the 4f spatial filtering module.

[0059] In its implementation, the 4f spatial filtering module consists of a specially designed grating array G, a rectangular spatial filter F, and two Fourier transform lenses with different focal lengths. The grating array G is composed of multiple one-dimensional gratings with different periods spliced ​​together in the y-direction; that is, the grating constant d of each row in the grating array G is different. Its specific structure is as follows: Figure 2 As shown, the diffraction angles θ between the positive first-order diffraction term and the zeroth-order diffraction term are different after plane light is diffracted by this specially designed grating.

[0060] In specific implementation, the grating array G is located on the front focal plane of the Fourier transform lens L1, and the rectangular filter F (such as...) Figure 3As shown, the rectangular filter F, located at the back focal plane of lens L1, filters out the positive first-order diffracted beam. The filtered light passes sequentially through Fourier transform lens L2 and beam splitter BS2 before reaching the CCD (i.e., the streak camera) plane. Its phase distribution is as follows: Figure 4 As shown, the light interferes with the object light and forms a hologram. These holograms are then cut by a stripe camera to form a compressed hologram.

[0061] For ease of understanding, the 4f spatial filtering module in the holographic imaging system described in this embodiment will be explained in detail below, such as... Figure 5 As shown, P0 is the grating array surface, P1 is the spectral surface, and P2 is the CCD recording surface. Let the focal lengths of Fourier transform lenses L1 and L2 be f1 and f2, respectively, the laser wavelength be λ, and the period of the two-dimensional grating array along the x-direction be d. x =[d x1 ,d x2 ,...,d xn ], seam width The period in the y-direction is d y The transmittance function of the grating is denoted by t(x,y). When a plane wave of unit amplitude is incident perpendicularly on the grating, the amplitude distribution on the grating surface can be expressed as:

[0062]

[0063] Where * denotes convolution operation, L represents the length of the grating array in the x-direction, x0, y0 are the horizontal and vertical coordinates of the P0 plane, N is the number of one-dimensional gratings stitched together in the y-direction, and rect() is the rectangle function, defined as...

[0064]

[0065] comb() is a comb function, defined as follows:

[0066]

[0067] Then the light field distribution on the P1 surface ( Figure 6 Its intensity distribution can be expressed as

[0068]

[0069] Where A is a constant, The physical meaning of m is the diffraction order of a one-dimensional grating, and j is the imaginary unit. A rectangular filter H(ξ) is used to filter out all +1 diffraction orders in the ξ direction, and the light field distribution ( Figure 7 Its intensity distribution is:

[0070]

[0071] The light field distribution on the P2 surface can then be expressed as:

[0072]

[0073] Where B is a constant, The phase distribution of the reference light is as follows Figure 4 As shown.

[0074] Then the reference light R(x2,y2) at the CCD recording plane and t n Momentary light O n The hologram obtained after interference at (x2, y2) is as follows:

[0075] H n (x,y)=|O n (x,y)| 2 +|R(x,y)| 2 +O n (x,y)R * (x,y)+O n * (x,y)R(x,y) (7)

[0076] After all the holograms are cropped by the stripe camera, they become a compressed hologram, which can be represented as:

[0077]

[0078] In the specific implementation, we use a two-step iterative shrinkage / thresholding (TwIST) method based on TV variation to decompress the compressed hologram, thereby reproducing the complex amplitude distribution of each frame of the image in the ultrafast dynamic process. The TwIST method is an existing method, specifically from the paper: Bioucas-Dias JM, Figueiredo MA TA new TwIST: Two-step iterative shrinkage / thresholding algorithms for image restoration[J].IEEE Transactions on Imageprocessing,2007,16(12):2992-3004, so its specific process will not be described in this embodiment.

[0079] To verify the feasibility of the holographic imaging system described in this embodiment, we conducted a simulation experiment using simulation software (Matlab in this embodiment), with the sample being 10 frames of a swimming whale. Figure 8(a) shows the spectrogram of the compressed hologram. Observation reveals that the +1 level spatial spectra of each frame do not completely overlap, but rather exhibit some offset. This offset is highly beneficial for the TwIST recovery algorithm to successfully decompress the hologram. Figure 8 (b) is a compressed hologram recorded by a CCD. Due to the mixing and superposition problem inherent in streak cameras, observing the specific process of dynamic scenes is not easy. We use the TwTST recovery algorithm to decompress it. Figure 9 The results are shown, from which we can clearly see the entire process of the dynamic scene. Based on the ultra-high spatiotemporal resolution of the streak camera, this system is expected to become a powerful tool for observing ultrafast phenomena.

[0080] Example 2:

[0081] The purpose of this embodiment is to provide an ultrafast holographic imaging method.

[0082] A holographic imaging method, based on the aforementioned holographic imaging system, the method comprising:

[0083] The laser emitted by the laser emitter is converted into planar light by the laser collimation module;

[0084] The plane light is split into two beams by a beam splitter, which then enter the object light path and the reference light path, respectively.

[0085] The reference light output from the reference light path and the object light output from the object light path interfere to form a hologram;

[0086] Based on the obtained hologram, a compressed hologram is obtained after clipping by a stripe camera.

[0087] Furthermore, for the obtained compressed hologram, the complex amplitude distribution of each frame in the ultrafast dynamic process is obtained by decompression.

[0088] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. An ultrafast holographic imaging system, characterized in that, The system includes a laser emitter, a laser collimation module, an object light path, a reference light path, and a camera. The laser emitted by the laser emitter is converted into planar light by the laser collimation module. The planar light is split into two beams by a beam splitter, which enter the object light path and the reference light path respectively. The reference light output from the reference light path and the object light output from the object light path interfere to form a hologram. After being cropped by the camera, a compressed hologram is obtained. The object light path is as follows: a beam of plane light divided by a beam splitter passes through an ultrafast dynamic object and then sequentially enters the objective lens, the magnifying optical path, and the reflecting mirror. The object light reflected by the reflecting mirror enters the beam splitter and interferes with the reference light from the reference light path. The reference optical path is as follows: The other beam of plane light, after being split by the beam splitter, is reflected by the mirror and enters the 4f spatial filter module. The reference light processed by the 4f spatial filter module enters the beam splitter and interferes with the object light from the object light path. The reference light path uses a 4f spatial filtering module to encode the reference light. The 4f spatial filtering module includes a grating array arranged in sequence, a first Fourier transform lens, a rectangular spatial filter, and a second Fourier transform lens. The grating array is obtained by splicing multiple one-dimensional gratings with different periods along the vertical direction.

2. An ultrafast holographic imaging system as claimed in claim 1, characterized in that Each row of the grating array uses a different grating constant.

3. The ultrafast holographic imaging system as described in claim 1, characterized in that, The first Fourier transform lens and the second Fourier transform lens have different focal lengths.

4. The ultrafast holographic imaging system as described in claim 1, characterized in that, The grating array is located on the front focal plane of the first Fourier transform lens, and the rectangular spatial filter is located on the rear focal plane of the first Fourier transform lens. The rectangular spatial filter is used to filter out the positive first-order diffraction beam.

5. The ultrafast holographic imaging system as described in claim 1, characterized in that, The laser collimation module includes an objective lens, a pinhole filter, and a collimating lens arranged in sequence.

6. The ultrafast holographic imaging system as described in claim 1, characterized in that, The camera used is a stripe camera.

7. An ultrafast holographic imaging method, characterized in that, It is based on an ultrafast holographic imaging system according to any one of claims 1-6, the method comprising: The laser emitted by the laser emitter is converted into planar light by the laser collimation module; The plane light is split into two beams by a beam splitter, which then enter the object light path and the reference light path, respectively. The reference light output from the reference light path and the object light output from the object light path interfere to form a hologram; Based on the obtained hologram, a compressed hologram is obtained after clipping by a stripe camera.

8. The ultrafast holographic imaging method as described in claim 7, characterized in that, For the obtained compressed hologram, the complex amplitude distribution of each frame in the ultrafast dynamic process is obtained by decompression.

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