Ultrafast coherent modulation imaging device based on chirped pulse illumination

Abstract

The invention discloses an ultrafast coherent modulation imaging device based on chirped pulse illumination, which can collect evolution information of intensity and phase along with time change in an ultrafast dynamic scene under the condition of single exposure. The device encodes an ultrafast scene by using the spectrum-time mapping characteristic of chirped pulses, realizes phase encoding through random phase modulation, then acquires an encoded diffraction pattern through a snapshot hyperspectral camera, and finally recovers space-time complex amplitude information of the ultrafast dynamic scene through a phase retrieval algorithm. The invention provides an imaging device capable of obtaining dozens of ultrafast time resolution complex amplitude images in single exposure, and the imaging dimension and the time resolution capability are remarkably improved. The device and method provided by the invention provide a new technical path for researching the ultrafast dynamic process, and have certain scientific research application potential and engineering popularization value.

Description

Technical Field

[0001] The present invention relates to the technical fields of computational optical imaging and ultrafast optical imaging, and in particular to an ultrafast coherent modulation imaging device based on chirped pulse illumination. Background Art

[0002] Single-shot multi-frame ultrafast optical imaging technology, as an important means to study the microscopic motion laws of substances, has become an important tool for recording non-repeatable or irreversible transient events. The development of this technology not only helps to promote the original innovation of basic science, but also can meet the major national needs in high-end manufacturing, biomedicine and other fields. With the rapid development of frontier fields such as ultrafast dynamics, advanced materials, and laser manufacturing, higher requirements are put forward for the research of ultrafast processes. Especially in the acquisition of multi-dimensional information such as time, space, spectrum, phase, and polarization, there is an urgent need for imaging technologies with high-dimensional joint analysis capabilities. Most of the existing ultrafast imaging methods mainly rely on intensity information, and there are many deficiencies such as limited information dimension and difficulty in completely recording the evolution process of ultrafast dynamic scenes. In contrast, phase imaging technology can not only obtain the structure and refractive index distribution of the target, but also be used to reveal key physical mechanisms such as the unique optical properties of the target and its rapid evolution process. Developing an ultrafast complex amplitude imaging technology that can simultaneously obtain the intensity and phase information of the target has become an important direction to break through the limitations of traditional imaging and deeply analyze ultrafast physical phenomena. Therefore, the research on related technologies is not only of great significance to basic scientific exploration, but also shows broad application prospects in applied science and engineering technology. Summary of the Invention

[0003] To overcome the technical problem that traditional optical systems cannot directly detect phase information, and the problem that the time resolution of existing phase imaging methods is limited by the camera frame rate, the object of the present invention is to propose a spatio-temporal three-dimensional complex amplitude imaging device with the highest sub-picosecond time resolution, which can simultaneously obtain multi-frame target intensity and phase information in a single exposure. This device can capture the dynamic changes of intensity and phase evolving with time in an ultrafast transient scene in real time. The present invention uses a supercontinuum laser generated by femtosecond laser excitation as the illumination light. The supercontinuum laser is temporally broadened after positive chirping through a dispersive medium. This chirped pulse illuminates the ultrafast dynamic scene to be measured and carries the transient intensity and phase modulation information of the scene during the illumination process. First, through temporal broadening, different wavelength components within the pulse correspond one-to-one with specific time points, realizing time-spectral mapping. Subsequently, each wavelength component undergoes phase and intensity modulation at different time points in the dynamic scene, thereby constructing a hyperspectral data cube containing spatio-temporal modulation information. This data cube undergoes phase modulation by a random phase plate during the free-space diffraction transmission process. This phase modulation process introduces additional constraints, which helps to enhance the stability and accuracy of phase recovery in the coherent diffraction imaging (CDI) process and can be regarded as a coherent modulation imaging method (CMI). The modulated light field is filtered by a spectral filter array with a mosaic distribution on the hyperspectral camera sensor. This array can spatially separate different wavelength components at the pixel level to achieve parallel acquisition of multi-wavelength information. Finally, the intensity information of each wavelength channel is collected through a single exposure on the same detection plane to obtain two-dimensional composite detection data. The present invention makes full use of the time-spectral correspondence characteristics of the time-chirped pulse, and on the basis of the fastest sub-picosecond time resolution, realizes the efficient acquisition and analysis of spatio-temporal complex amplitude information, enabling the CDI technology to have the ability to perform multi-frame complex amplitude imaging in a single exposure, which has important scientific research and engineering application values.

[0004] The specific technical solution for achieving the object of the present invention is as follows:

[0005] An ultrafast coherent modulation imaging device based on chirped pulse illumination, comprising a chirped pulse illumination system, a snapshot coherent modulation imaging system, and a data processing and reconstruction system;

[0006] The femtosecond laser of the chirped pulse illumination system is optically connected in sequence with a first lens, a nonlinear medium, a second lens, a glass rod, and a first mirror;

[0007] The second mirror of the snapshot coherent modulation imaging system is optically connected in sequence with a dynamic scene, an objective lens, a third lens, a diaphragm, a phase modulation plate, and a hyperspectral camera;

[0008] The first mirror of the chirped pulse illumination system is optically connected to the second mirror of the snapshot coherent modulation imaging system; the computer of the data processing and reconstruction system is circuit-connected to the hyperspectral camera of the snapshot coherent modulation imaging system;

[0009] The nonlinear medium of the chirped pulse illumination system is placed at the image-side focal plane of the first lens and at the object-side focal plane of the second lens.

[0010] The aperture of the snapshot coherent modulation imaging system is placed at the image-side focal plane of the third lens.

[0011] The phase modulation plate of the snapshot coherent modulation imaging system is placed between the aperture and the hyperspectral camera and is used to perform random phase modulation on the diffracted light.

[0012] The data processing and reconstruction system performs channel separation and phase retrieval reconstruction. The specific process includes: assuming the data acquisition process is:

[0013]

[0014] where I(x′, y′) represents two-dimensional intensity measurement, U(x, y, t) is the original dynamic scene, and the original dynamic scene becomes a hyperspectral image after chirped pulse illumination. Here, M represents the time-spectral transformation operator that transforms U(x, y, t) into U(x, y, λ). Hz1 and Hz2 respectively represent the spatial propagation operators before and after random phase encoding, C is the random phase encoding operator, and F and T respectively correspond to the filtering effect of the hyperspectral camera and the time integration of the camera sensor. The two-dimensional intensity measurement maps of different spectra can be reshaped into a three-dimensional data cube according to the arrangement of spectral channels, and the corresponding images of each spectral channel are represented as I(x”, y”, λ n ) = S -1 I(x′, y′), n = 1...k, where λ n represents the filtering channels of different wavelengths, and S -1 represents the channel separation operation.

[0015] To restore the original dynamic scene, a phase retrieval algorithm is used to reconstruct each channel image I(x”, y”, λ n ) respectively, and then the time-spectral transformation relationship M -1 is used to convert the hyperspectral image into a time-sequence image.

[0016] The present invention develops a single-shot multi-frame broadband chirped pulse illumination coherent modulation imaging method based on time-spectral mapping illumination and snapshot hyperspectral coherent modulation imaging. The system developed based on this method can achieve a time resolution on the order of sub-picoseconds and has the ability of phase imaging. It can be used to detect the spatio-temporal evolution of intensity and phase in ultrafast physical, chemical, and biological processes.

[0017] The advantages of the present invention are as follows:

[0018] The present invention can directly observe the spatio-temporal complex amplitude information (two-dimensional space, phase information, and one-dimensional time) of ultrafast dynamic scenes. The present invention belongs to a single-shot multi-frame imaging measurement device, which is applicable to the measurement of non-repeatable and difficult-to-repeat ultrafast events, and the highest time resolution can reach the order of sub-picoseconds. Brief Description of the Drawings

[0019] Figure 1 is a schematic structural diagram of the present invention;

[0020] Figure 2 is an experimental system diagram of the femtosecond laser ablation of glass scene taken in the embodiment;

[0021] Figure 3 is an ultrafast complex amplitude image reconstructed and restored from the embodiment. Detailed Embodiment

[0022] The present invention will be described in detail below with reference to the drawings and embodiments.

[0023] Refer to Figure 1 , the present invention includes a chirped pulse illumination system 100, a snapshot coherent modulation imaging system 200, and a data processing and reconstruction system 300 composed of a computer;

[0024] The femtosecond laser 101 of the chirped pulse illumination system 100 is optically connected in sequence with a first lens 102, a nonlinear medium 103, a second lens 104, a glass rod 105, and a first mirror 106;

[0025] The second mirror 201 of the snapshot coherent modulation imaging system 200 is optically connected in sequence with a dynamic scene 202, an objective lens 203, a third lens 204, a diaphragm 205, a phase modulation plate 206, and a hyperspectral camera 207;

[0026] The computer of the data processing and reconstruction system 300 is connected to the hyperspectral camera 205;

[0027] Refer to Figure 1 , the nonlinear medium of the chirped pulse illumination system is placed at the image-side focal plane of the first lens, and the nonlinear medium is placed at the object-side focal plane of the second lens.

[0028] Refer toFigure 1 , the aperture of the snapshot coherent modulation imaging system is placed on the image-side focal plane of the third lens.

[0029] Refer to Figure 1 , the phase modulation plate of the snapshot coherent modulation imaging system is placed between the aperture and the hyperspectral camera, and is used to perform random phase modulation on the diffracted light.

[0030] The present invention works as follows:

[0031] Refer to Figure 1 , the femtosecond pulsed laser generated by the femtosecond laser 101 is focused by the first lens 102 in the nonlinear medium 103 to generate a broadband supercontinuum laser, and then collimated by the first lens 102, and then the supercontinuum laser is chirped and broadened by the glass rod 105, and successively passes through the first mirror 106 and the second mirror 201, and illuminates the dynamic scene 202. The microscopic objective lens 203 collects the illumination light passing through the dynamic scene, images it on the aperture 205 after passing through the third lens 204, and after being spatially constrained by the aperture, it diffracts freely in space, and is randomly phase-modulated by the phase modulation plate 206 during the diffraction process, and finally a two-dimensional image is acquired by the hyperspectral camera 207. The data processing and reconstruction system 300 performs channel separation and phase retrieval reconstruction operations. First, the measurement map acquired by the hyperspectral camera is extracted to separate the original images of different wavelengths. To restore the intensity and phase information of the original dynamic scene, a phase retrieval algorithm is used to reconstruct each channel image separately, and then the time-spectral transformation relationship is used to convert the spectrally resolved high image into a time-resolved time series image.

[0032] Embodiment

[0033] The working process of this embodiment:

[0034] Refer to Figure 1 , in this embodiment, the chirped pulse illumination system 100 uses the femtosecond laser 101 to generate a femtosecond pulsed laser. The single pulse duration of the femtosecond light generated by this laser is 35 femtoseconds, the repetition frequency is 1000 Hz, the central wavelength is 800 nanometers, and the single pulse energy is 7 millijoules; the focal length of the first lens is 100 millimeters, which is used to focus the femtosecond light into the nonlinear medium; the nonlinear medium uses a 0.02 mol / L copper sulfate solution to generate a supercontinuum laser. Here, the generated supercontinuum laser wavelength covers 450 - 950 nanometers; the focal length of the second lens is 50 millimeters, which is used to collimate the generated supercontinuum laser; the glass rod 105 is used to chirp the supercontinuum laser, and by adjusting the length of the used glass rod, the chirped supercontinuum laser duration can be adjusted in the range of several hundred femtoseconds to several hundred picoseconds.

[0035] Refer to Figure 1 , Figure 2, the dynamic scene 202 in this embodiment is the kinetic process after femtosecond laser ablation of solid materials. Here, a 400-nanometer femtosecond laser is focused on the surface of a 170-micron-thick glass through a lens with a focal length of 150 millimeters, generating an ablation dynamic scene. After illuminating the ablation scene with the chirped supercontinuum laser output by the chirped pulse illumination system 100, the outgoing light is collected by the objective lens 203.

[0036] Refer to Figure 1 , in the snapshot coherent modulation imaging system 200, the magnification of the objective lens is 20 times, magnifying the dynamic scene generated by ablation for subsequent observation. The dynamic scene passes through an imaging system composed of the objective lens and a lens with a focal length of 200 millimeters, and is magnified 20 times and imaged onto the aperture 205. The aperture 205 plays a role in restricting the spatial size of the imaging here. The phase modulation 206 is formed by irregular etching on a fused silica substrate, and is designed to have a binary modulation of [0, π] for light with a wavelength of 532 nanometers. The modulation regions are randomly distributed, and the size of the modulation unit is 50 microns. The phase modulation plate is placed between the aperture and the hyperspectral camera, and is used to generate a random phase modulation for the diffracted light after passing through the aperture 205. The spectral camera 207 is model MQ022HG-IM-SM5X5-NIR, which has 25 spectral channels, covering a detection range from 659 nanometers to 949 nanometers, and is used to collect the broadband diffracted light spots of the target scene after diffraction and phase modulation.

[0037] Refer to Figure 1 , the data processing and reconstruction system 300 performs channel separation and phase retrieval reconstruction on the images collected by the spectral camera. The specific process includes: Let the data acquisition process be:

[0038]

[0039] Among them, I(x′, y′) represents the two-dimensional intensity measurement, U(x, y, t) is the original dynamic scene. After the original dynamic scene is illuminated by the chirped pulse, it becomes a hyperspectral image. Here, M represents the time-spectral transformation operator, which transforms U(x, y, t) into U(x, y, λ). Hz1 and Hz2 respectively represent the spatial propagation operators before and after random phase encoding, C is the random phase encoding operator, and F and T respectively correspond to the filtering effect of the hyperspectral camera and the time integration of the camera sensor. The two-dimensional intensity measurement maps of different spectra can be reshaped into a three-dimensional data cube according to the arrangement of the spectral channels, and the corresponding images of each spectral channel are represented as I(x”, y”, λ n ) = S -1 I(x′, y′), n = 1...k, where λ n represents the filtering channels of different wavelengths, S -1Indicates the channel separation operation. To restore the original dynamic scene, a phase retrieval algorithm is used to reconstruct each channel image I(x”, y”, λ n ) separately, and then the time-spectral transformation relationship M -1 is used to convert the hyperspectral image into a temporal image.

[0040] Refer to Figure 3 , the dynamic scene of the femtosecond laser ablating the glass surface captured by the device of the present invention is restored through a data processing and reconstruction system, and 25 temporal images of intensity and phase distribution are obtained, with an average time interval of 4 ps. Figure 3 Five intensity images and five phase images are selected from

[0041] for display. Generally speaking, the present invention discloses an ultrafast coherent modulation imaging device based on chirped pulse illumination. By capturing the dynamic scene based on the time-spectral mapping relationship of chirped pulses and combining with a snapshot coherent modulation imaging system to collect broadband diffraction images, the spatio-temporal complex amplitude information of the ultrafast dynamic scene can be observed.

[0042] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. An ultrafast coherent modulation imaging device based on chirped pulse illumination, characterized in that, The device includes: A chirped pulse illumination system (100) composed of a femtosecond laser (101), a first lens (102), a nonlinear medium (103), a second lens (104), a glass rod (105), and a first mirror (106); The femtosecond laser (101) of the chirped pulse illumination system (100) is optically connected to the first lens (102), the nonlinear medium (103), the second lens (104), the glass rod (105), and the first mirror (106) in sequence; A snapshot coherent modulation imaging system (200) composed of a second mirror (201), a dynamic scene (202), an objective lens (203), a third lens (204), a diaphragm (205), a phase modulation plate (206), and a hyperspectral camera (207); The second mirror (201) of the snapshot coherent modulation imaging system (200) is optically connected to the dynamic scene (202), the objective lens (203), the third lens (204), the diaphragm (205), the phase modulation plate (206), and the hyperspectral camera (207) in sequence; A data processing and reconstruction system (300) composed of a computer; The first mirror (106) of the chirped pulse illumination system (100) is optically connected to the second mirror (201) of the snapshot coherent modulation imaging system (200); The computer of the data processing and reconstruction system (300) is electrically connected to the hyperspectral camera (207) of the snapshot coherent modulation imaging system (200); The time-chirped broadband pulse generated by the chirped pulse illumination system (100) illuminates the dynamic scene (202), and after being relayed by the objective lens (203) and the lens (204) to be imaged onto the diaphragm (205) and restricted by the spatial constraint of the diaphragm, then during the free-space diffraction propagation process, through the random phase modulation of the phase modulation plate (206), it is snapshot-collected by the hyperspectral camera (207); The data processing and reconstruction system (300) performs channel separation and phase retrieval reconstruction. The specific process includes: assuming the data acquisition process is: I(x′, y′) = TF|H z2 CH z1 MU(x, y, t)| 2 (1) Among them, I(x′, y′) represents two-dimensional intensity measurement, U(x, y, t) is the original dynamic scene. After being illuminated by a chirped pulse, the original dynamic scene becomes a hyperspectral image. Here, M represents the time-spectral transformation operator, which transforms U(x, y, t) into U(x, y, λ); Hz1 and Hz2 represent the spatial propagation operators before and after random phase encoding respectively, C is the random phase encoding operator, and F and T correspond to the filtering function of the hyperspectral camera and the time integration of the camera sensor respectively; the two-dimensional intensity measurement maps of different spectra can be reshaped into a three-dimensional data cube according to the arrangement of spectral channels, and the corresponding images of each spectral channel are represented as I(x”, y”, λ n ) = S -1 I(x′, y′), n = 1...k, where λ n represents the filtering channels of different wavelengths, S -1 represents the channel separation operation; To restore the original dynamic scene, the phase retrieval algorithm is used to reconstruct each channel image I(x”, y”, λ n ) respectively, and then the time-spectral transformation relationship M -1 is used to convert the hyperspectral image into a temporal image.

2. The ultrafast coherent modulation imaging device according to claim 1, characterized in that, The nonlinear medium (103) of the chirped pulse illumination system (100) is placed on the image-side focal plane of the first lens (102), and the nonlinear medium (103) is placed on the object-side focal plane of the second lens (104).

3. The ultrafast coherent modulation imaging device according to claim 1, wherein The diaphragm (205) of the snapshot coherent modulation imaging system (200) is placed on the image-side focal plane of the third lens (204); The phase modulation plate (206) is placed between the diaphragm (205) and the hyperspectral camera (207) for random phase modulation of the diffracted light.

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