Multi-angle multi-pulse ultrafast x-ray imaging system and method

Abstract

The application discloses a multi-angle multi-pulse ultrafast X-ray imaging system and method. The system comprises a laser light source module for generating superstrong and ultrashort main pulse laser; a laser beam splitting and modulation module comprising an off-axis parabolic mirror and a mirror array for splitting the main pulse into multiple sub-pulses and controlling the time and space parameters of the sub-pulses; a radiation source generation module comprising a gas target array for generating multiple Betatron radiation X-ray pulses through a laser wakefield acceleration mechanism; an electron deflection module for deflecting high-energy electron beams generated by the laser wakefield acceleration; and a multi-dimensional detection module for recording multiple frames of X-ray images transmitted through a sample. The main pulse is focused and split into sub-pulses, which act on the gas target to generate Betatron radiation X-rays, and after electron deflection, the sample is irradiated, and the detector records multiple frames of spatially separated images, thereby achieving single-experiment multi-frame dynamic imaging of an ultrafast process. The application realizes femtosecond to picosecond time resolution and micron-level spatial resolution imaging, and is suitable for inertial confinement fusion and high-energy density physics diagnosis.

Description

Technical Field

[0001] This invention relates to the fields of laser plasma physics and ultrafast diagnostic technology, specifically to an ultrafast X-ray imaging system and method. Background Technology

[0002] In extreme-condition physics research such as inertial confinement fusion (ICF) and high-energy-density physics, key transient processes are often characterized by being single-shot and non-repeatable, evolving at extremely fast speeds (picosecond to femtosecond levels), and having tiny spatial scales (micrometer scale). This creates an urgent need for diagnostic techniques that are "single-shot, high temporal resolution, and strong penetration." However, existing diagnostic methods still face significant limitations: traditional X-ray streak cameras are limited by scintillator response and electronic systems, making it difficult to achieve sub-picosecond temporal resolution; pump-probe methods rely on repeated experiments and cannot achieve continuous dynamic capture of single-shot, non-repeatable processes; while synchrotron radiation sources have excellent overall performance, they are limited by beam structure and operating modes, making them unsuitable for single-shot, multi-frame ultrafast diagnostics. Currently, the Betatron radiation source, driven by a petawatt laser, possesses single-shot imaging potential, but its low repetition frequency (0.1–10 Hz) also restricts its application in continuous dynamic imaging.

[0003] Several patents have explored the need for single-shot multi-frame imaging. For example, patent [CN110455837B] proposes splitting the main laser beam using a multi-stage beam splitter to drive multiple gas nozzles, generating multiple X-ray beams for multi-angle imaging. However, this method relies on multi-stage transmission beam splitting, which easily introduces pulse broadening and energy loss, and places high demands on the robustness of high-power laser devices and system cost. Patent [CN121410012A] uses camera rotation and a detachable scintillator to achieve multi-angle X-ray imaging. While it can acquire information from different perspectives, its imaging relies on mechanical adjustment, making it difficult to meet the diagnostic needs of ultrafast single-shot processes at the femtosecond scale. Patent [CN104132676A] uses a dual-FP cavity structure to achieve time delay and coaxial framing imaging of multiple beams, obtaining multi-frame ultrafast process information. However, it is mainly geared towards the visible light band, and the imaging angle is singular, making it difficult to apply to high-penetration X-ray diagnostic scenarios. In addition, patent [CN115268200B] proposes to achieve multi-frame ultrafast phase imaging under a single exposure by spatiotemporal shaping of femtosecond laser pulses and multi-aperture filtering interference, which has a time resolution capability on the order of Tfps. However, this method relies on an optical interference system and is mainly applicable to the visible / near-infrared bands. It has not yet been applied to imaging under high-energy X-ray conditions.

[0004] In summary, while existing technologies have made some progress in multi-frame acquisition, temporal resolution, and multi-angle imaging, they still struggle to simultaneously meet the requirements of single-shot capability, ultra-high temporal resolution, multi-frame capability, and high-penetration X-ray imaging. Therefore, developing a novel ultrafast imaging method that addresses single, non-repeatable processes and combines multi-frame capability with high-energy X-ray diagnostic capabilities is of significant scientific and practical value. Summary of the Invention

[0005] This invention provides a multi-angle, multi-pulse ultrafast X-ray imaging system and method, aiming to solve the problem of continuous, high spatiotemporal resolution imaging of non-repeatable ultrafast processes in a single experiment.

[0006] On one hand, the present invention provides a multi-angle, multi-pulse ultrafast X-ray imaging system, mainly comprising: Laser source module, used to generate ultra-intense and ultra-short main pulse lasers with peak power of hundreds of terawatts to petawatts or more; The laser beam splitting and modulation module includes an off-axis parabolic mirror and an independently adjustable array of mirrors or an equivalent optical beam splitting structure, used to focus the main pulse and split it into multiple sub-pulses, and to independently control the spatial direction and time delay of each sub-pulse. The radiation source generation module includes a gas target array, with each gas target located on the optical path of the corresponding sub-pulse, used to generate multiple Betatron radiation X-ray pulses through a laser tail field acceleration mechanism under the action of each sub-pulse; An electron deflection module, including a magnet or electromagnet, is used to generate a magnetic field to deflect the high-energy electron beam generated by the laser wake field acceleration. The multi-dimensional detection module is used to record multiple frames of X-ray images after passing through the sample under test.

[0007] The ultra-intense, ultra-short master pulse laser generated by the laser source module is focused by an off-axis parabolic mirror in the laser beam splitting and modulation module, and then split into multiple sub-pulses by a mirror array or an equivalent optical beam splitting structure. Each sub-pulse acts on a corresponding gas target in the radiation source generation module, generating multiple Betatron radiation X-ray pulses through a laser tail field acceleration mechanism. The electron deflection module deflects the high-energy electron beam generated by the laser tail field acceleration, allowing the X-ray pulses to be transmitted to the sample without interference. The multi-dimensional detection module records multiple frames of X-ray images after transmission through the sample. By adjusting the spatial pointing and time delay between each sub-pulse, the X-ray pulses at different times are spatially separated on the detector plane, thereby obtaining multiple frames of images in a single experiment and realizing multi-frame dynamic imaging of ultrafast processes in a single experiment.

[0008] Furthermore, the mirror array or its equivalent optical beam-splitting structure splits the main pulse into 2–20 beams. Sub-pulse.

[0009] Furthermore, the main pulse laser generated by the laser source module has a wavelength range of 700–1100 nm, a pulse width range of 20–80 fs, an energy range of 10–50 J, and a repetition frequency range of 0.1–10 Hz.

[0010] Furthermore, in the laser beam splitting and modulation module, an off-axis parabolic mirror is used to focus the main pulse onto the mirror array; the mirror array is a hexagonal honeycomb structure or a ring structure, composed of multiple independently controllable mirror units, each unit being connected to a piezoelectric actuator; the piezoelectric ceramic actuator drives the mirror units to generate displacement and angle changes, achieving submicron-level precision time delay (femtosecond to picosecond level) and milliradian-level precision spatial pointing control.

[0011] Furthermore, in the radiation source generating module, the gas target is a supersonic gas nozzle array with a gas density of 10. 18 -10 20 cm -3 Adjustable; the generated Betatron radiation X-ray pulses have a pulse width of 20-50 fs, a source size of 0.5-10 μm, and an energy range of 5-30 keV.

[0012] Furthermore, the electron deflection module uses a magnetic field generated by a magnet or electromagnet to deflect the electron beam generated during the laser tail field acceleration process away from the detection direction.

[0013] Furthermore, the multidimensional detection module is a flat panel detector, an imaging plate, or a fluorescent plate; by adjusting the angle difference between each sub-pulse to be greater than the Betatron radiation divergence angle, the projected images at different times are spatially separated on the detector plane, with the angle difference ranging from 10 to 100 mrad.

[0014] Furthermore, the X-ray images recorded by the multi-dimensional detection module are post-processed to achieve phase contrast enhancement, geometric correction, and noise filtering, thereby obtaining imaging effects with micrometer-level spatial resolution and femtosecond to picosecond-level temporal resolution.

[0015] Furthermore, the output end of the radiation source generating module is provided with a light-blocking film to block laser light that does not interact with the gas target, allowing electron beams and X-ray pulses to pass through.

[0016] On the other hand, the present invention also provides a multi-angle, multi-pulse ultrafast X-ray imaging method based on the above system, which mainly includes the following steps: (a) Set the time delay and spatial orientation of each sub-pulse according to experimental requirements; (b) The ultra-intense and ultra-short master pulse laser generated by the laser source module is focused by an off-axis parabolic mirror and then split by a reflector array to generate multiple sub-pulses with a set time delay and spatial orientation; (c) Each sub-pulse acts on the corresponding gas target in the radiation source generation module, generating multiple Betatron radiation X-ray pulses through the laser tail field acceleration mechanism; (d) After the multiple X-ray pulses pass through the sample under test, they are recorded as multiple frames of images at different times by the multi-dimensional detection module; (e) Post-process the recorded multi-frame images, including phase contrast enhancement, geometric correction and noise filtering, to obtain imaging effects with micrometer-level spatial resolution and femtosecond to picosecond-level temporal resolution.

[0017] Furthermore, in step (d), by adjusting the angle difference between each sub-pulse to be greater than the Betatron radiation divergence angle, the projected images at different times are spatially separated on the detector plane, thus avoiding signal aliasing.

[0018] The beneficial technical effects of the present invention are as follows: (1) Ultra-high temporal resolution: The detection accuracy of this invention is determined by the Betatron radiation pulse width (20-50 fs level) and the displacement control accuracy of the mirror array, which can realize continuously adjustable temporal resolution imaging from femtosecond to picosecond level, covering the diagnostic needs of ultrafast processes from femtosecond to 200 ps. (2) Micrometer-level spatial resolution: By utilizing the small source size (0.5–10 μm) of the Betatron radiation source and the high magnification of the imaging system, micrometer-level spatial resolution X-ray images can be obtained, which are suitable for the precise observation of fine structures; (3) Single-shot multi-frame dynamic imaging: By splitting the main pulse and controlling the timing and temperature of the main pulse through a reflector array, multiple X-ray images with precise time intervals and spatial separation can be obtained in a single laser target test, which is suitable for continuous dynamic diagnosis of non-repeatable transient processes; (4) Phase contrast enhancement: The Betatron radiation X-rays generated by this invention have good spatial coherence. Combined with post-processing algorithms, phase contrast enhancement imaging can be achieved. It has extremely high interface detection sensitivity for low Z materials (such as CH ablation layer and DT ice layer in ICF target pellets), significantly improving the imaging contrast of weak absorption structures. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the structure of the multi-angle, multi-pulse ultrafast X-ray imaging system according to an embodiment of the present invention.

[0020] Figure 2 yes Figure 1 A schematic diagram of the structure of the central reflector array.

[0021] In the figure: 1-Main pulse laser, 2-Off-axis parabolic mirror, 3-Mirror array, 4-Gas target, 5-Blocking film, 6-Pulse magnet, 7-Electron beam, 8-Fluorescent plate, 9-Sample, 10-Imaging plate, 301-307-Sub-mirrors, t1, t2, t3-Different times (left) and the corresponding imaging position (right). Detailed Implementation

[0022] The following description, in conjunction with the accompanying drawings and embodiments, further illustrates the multi-angle, multi-pulse, ultrafast X-ray imaging system and method based on ultra-intense laser driving according to the present invention, but the embodiments of the present invention are not limited thereto.

[0023] Example 1 Imaging System: like Figure 1 As shown, the multi-angle, multi-pulse ultrafast X-ray imaging system of this embodiment includes: A laser source module is used to generate ultra-intense, ultra-short main pulse lasers with peak power ranging from hundreds of terawatts to petawatts or more. In this embodiment, the center wavelength is 800 nm, the main pulse laser pulse width is 30 fs, the energy is 30 J, and the repetition frequency is 1 Hz. It should be noted that the above laser parameters are only examples, and those skilled in the art can adjust them according to experimental needs. For example, the laser center wavelength can be 700-1100 nm, the pulse width range can be 20-80 fs, the energy range can be 10-50 J, and the repetition frequency range can be 0.1-10 Hz.

[0024] The laser beam splitting and modulation module includes an off-axis parabolic mirror 2 and an independently adjustable mirror array 3. The main pulse laser is focused by the off-axis parabolic mirror onto the mirror array, which then splits it into multiple sub-pulses. For example... Figure 2As shown, the mirror array adopts a hexagonal honeycomb structure, consisting of seven independently controllable sub-mirrors (301-307). Each sub-mirror has three piezoelectric ceramic actuators connected to its back, arranged in a three-point support layout. The piezoelectric ceramic actuators drive the sub-mirrors to generate Z-direction displacement and X and Y-direction angular deflections, respectively achieving sub-micron precision (better than 0.3 μm) time delay (corresponding to femtosecond to picosecond levels) and milliradian precision (better than 0.1 mrad) spatial pointing control. Those skilled in the art can flexibly adjust the number of sub-pulses, time delay, and pointing angle by independently setting the displacement and angle parameters of each sub-mirror according to experimental requirements, achieving multi-pulse, multi-angle imaging. It should be noted that the beam-splitting structure is not limited to a hexagonal arrangement; it can also be a ring array or other optical beam-splitting structures capable of independent angle and delay control. It should be noted that the independently controllable mirror array used in this embodiment is a preferred solution of the present invention, but not the only solution. For those skilled in the art, other optical beam-splitting structures with equivalent functions can be readily foreseen. The term "equivalent" refers to the ability to split a main laser pulse into multiple sub-pulses and independently control the spatial direction and time delay of each sub-pulse. For example, a combination of diffractive optical elements and microlens arrays can be used. The diffractive optical elements split the beam, and an independent displacement mechanism behind the microlens array adjusts the optical path and direction of each sub-pulse. Alternatively, a spatial light modulator can be used, loading different phase holograms to achieve beam splitting, deflection, and phase delay control of the incident laser. Any optical structure capable of achieving the above-mentioned beam splitting and independent temporal and spatial control functions is considered equivalent to the mirror array of this invention.

[0025] The radiation source generation module includes a gas target array 4 and a light-blocking film 5 disposed behind it. Each gas target is located on the optical path of its corresponding sub-pulse. In this embodiment, the gas targets employ a supersonic helium nozzle array with a back pressure of 5 MPa, resulting in a plasma electron density of approximately 5 × 10⁻⁶ at the focal point. 18 cm -3 Each sub-pulse acts on its corresponding gas target, generating multiple Betatron X-ray pulses through a laser wake field acceleration mechanism. The generated Betatron X-rays have the following characteristics: pulse width of approximately 30 fs, source size of approximately 1 μm, photon energy range of 5-20 keV, and single-pulse photon yield of approximately 10... 6 -10 9Photons / shot. The light-blocking film 5 is used to block residual laser light that has not interacted with the gas target, allowing electron beams and X-ray pulses to pass through. It should be noted that the use of a helium ultrasonic nozzle in this embodiment is a preferred solution and does not constitute a limitation of the invention. Those skilled in the art can select other gas types and supply structures according to the X-ray source quality requirements. The protection scope of the gas target array covers various devices capable of generating high-pressure gas, and is not limited to specific types of valves, gas types, or specific parameters.

[0026] An electron deflection module is used to deflect the accompanying electron beam. In this embodiment, a pulsed magnetic field is generated by a pulsed magnet 6. Through precise timing control, a strong magnetic field is generated at the moment synchronized with the main laser pulse (i.e., the instant the electron beam is generated), deflecting the electron beam 7 away from the detection direction. The deflected electron beam strikes the fluorescent plate 8 and produces fluorescence, which is used to verify that the electron beam has been successfully deflected.

[0027] A multi-dimensional detection module is used to record multiple frames of X-ray images after passing through the sample under test. In this embodiment, an imaging plate 10 is used as the detector, achieving a spatial resolution down to the micrometer level. By adjusting the angular difference between each sub-pulse to be greater than the Betatron radiation divergence angle (e.g., greater than 40 mrad), spatial separation of the projected images at different times is achieved on the detector plane, such as... Figure 1 The imaging positions at different times, t1, t2, and t3, are shown on the right side of the image.

[0028] Example 2 Imaging Method: This embodiment provides a multi-angle, multi-pulse ultrafast X-ray imaging method based on the above system, including the following steps: (a) Parameter setting steps Based on the ultrafast evolution characteristics of the sample under test, the time delay and spatial orientation of each sub-pulse are set. For example, for the implosion process of an inertial confinement fusion target on the picosecond scale, the time delay between sub-pulses can be set to 1 ps, 2 ps, or 3 ps to capture the evolution state at different times; at the same time, the angular difference between sub-pulses is set to be greater than the Betatron radiation divergence angle (for example, set to 15 mrad, which is greater than the typical 10 mrad divergence angle) to ensure that the projected images at different times are spatially separated on the detector.

[0029] (b) Sub-pulse generation steps The ultra-intense, ultra-short master pulse laser (pulse width 30 fs, energy 30 J in this embodiment), generated by the laser source module, is focused by an off-axis parabolic mirror and then incident on a mirror array. The mirror array, according to the parameters set in step (a), splits the master pulse into multiple sub-pulses with set time delays and spatial orientations. In this embodiment, three sub-pulses are generated by the mirror array, with time delays set to 0 ps, ​​1 ps, and 2 ps, respectively, and an angle difference set to 15 mrad.

[0030] (c) X-ray generation steps Each sub-pulse generated in step (b) acts on the corresponding gas target in the radiation source generation module. In this embodiment, the gas target uses a supersonic helium nozzle array with a back pressure of 5 MPa and an electron density of approximately 5 × 10⁻⁶. 18 cm -3 Each sub-pulse generates multiple Betatron radiation X-ray pulses via a laser wakefield acceleration mechanism. Each X-ray pulse has a pulse width of approximately 30 fs, a source size of approximately 1 μm, and a photon energy range of 5-20 keV. The single-pulse photon yield is approximately 10 6 -10 7 photons / shot.

[0031] (d) Image recording steps The multiple Betatron X-ray pulses generated in step (c) pass through the sample at different angles. Since the angle difference between sub-pulses set in step (a) (15 mrad) is greater than the typical divergence angle of Betatron radiation (approximately 10 mrad), the X-ray pulses at different times are spatially separated on the detector plane. In this embodiment, the distance from the sample to the detector is set to 1 m, resulting in a center-to-center spacing of approximately 15 mm between adjacent images, ensuring that the images do not overlap. Figure 1 As shown on the right, t1, t2, and t3 correspond to the imaging positions at 0 ps, ​​1 ps, and 2 ps, respectively. In this embodiment, an imaging plate is used to record these spatially separated projected images. The spatial resolution of the imaging plate can reach 50 μm. Combined with the system's geometric magnification (approximately 6 times), it is possible to observe the micron-level structure of the sample.

[0032] (e) Image post-processing steps Post-processing of the multiple frames of images recorded in step (d) includes: Phase contrast enhancement: Utilizing the good spatial coherence of Betatron radiation, the interface contrast of low-Z materials (such as the CH layer and DT ice layer in ICF pellets) is enhanced through a phase retrieval algorithm. Geometric correction: Based on the system's geometric magnification (approximately 6 times) and angular difference, the image is corrected for geometric distortion; Noise filtering: Use appropriate filtering algorithms to reduce noise and improve the image signal-to-noise ratio.

[0033] After the above post-processing, a multi-frame imaging effect with micrometer-level spatial resolution (better than 10 μm) and femtosecond to picosecond-level temporal resolution is finally obtained.

[0034] It should be noted that the specific parameters in this embodiment (such as the number of sub-pulses of 3, time delay of 0 / 1 / 2 ps, angle difference of 15 mrad, amplification of 6 times, etc.) are only examples. Those skilled in the art can adjust these parameters according to actual experimental needs to adapt to the time scale and spatial scale of different ultrafast processes.

Claims

1. A multi-angle, multi-pulse ultrafast X-ray imaging system, characterized in that, include: Laser source module, used to generate ultra-intense and ultra-short main pulse lasers with peak power of hundreds of terawatts to petawatts or more; The laser beam splitting and modulation module includes an off-axis parabolic mirror and an independently adjustable array of mirrors or an equivalent optical beam splitting structure, used to focus the main pulse and split it into multiple sub-pulses, and to independently control the spatial direction and time delay of each sub-pulse. The radiation source generation module includes a gas target array, with each gas target located on the optical path of the corresponding sub-pulse, used to generate multiple Betatron radiation X-ray pulses through a laser tail field acceleration mechanism under the action of each sub-pulse; An electron deflection module, including a magnet or electromagnet, is used to generate a magnetic field to deflect the high-energy electron beam generated by the laser wake field acceleration. The multi-dimensional detection module is used to record multiple frames of X-ray images after passing through the sample under test. The ultra-intense, ultra-short master pulse laser generated by the laser source module is focused by an off-axis parabolic mirror in the laser beam splitting and modulation module, and then split into multiple sub-pulses by a mirror array or an equivalent optical beam splitting structure. Each sub-pulse acts on a corresponding gas target in the radiation source generation module, generating multiple Betatron radiation X-ray pulses through a laser wake field acceleration mechanism. The electron deflection module deflects the high-energy electron beams generated by the laser wake field acceleration, allowing the X-ray pulses to be transmitted to the sample without interference. The multi-dimensional detection module records multiple frames of X-ray images after passing through the sample. By adjusting the spatial direction and time delay between each sub-pulse, the X-ray pulses at different times are spatially separated on the detector plane, thereby obtaining multiple frames of images in a single experiment and realizing multi-frame dynamic imaging of ultrafast processes in a single experiment.

2. The multi-angle, multi-pulse ultrafast X-ray imaging system according to claim 1, characterized in that, The mirror array or its equivalent optical beam splitter structure splits the main pulse into 2–20 sub-pulses.

3. A multi-angle, multi-pulse ultrafast X-ray imaging system according to claim 1, characterized in that, The main pulse laser generated by the laser source module has a wavelength range of 700–1100 nm, a pulse width range of 20–80 fs, an energy range of 10–50 J, and a repetition frequency range of 0.1–10 Hz.

4. The multi-angle, multi-pulse ultrafast X-ray imaging system according to claim 1, characterized in that, The reflector array is a hexagonal honeycomb structure or a ring structure, composed of multiple independent and controllable reflector units, each of which is connected to a piezoelectric actuator. The piezoelectric ceramic actuator drives the reflector units to generate displacement and angle changes, thereby achieving submicron-level precision time delay (femtosecond to picosecond level) and milliradian-level precision spatial pointing control.

5. A multi-angle, multi-pulse ultrafast X-ray imaging system according to claim 1, characterized in that, The gas target is a supersonic gas nozzle array with a gas density of 10. 18 -10 20 cm -3 Adjustable; the generated Betatron radiation X-ray pulses have a pulse width of 20-50 fs, a source size of 0.5-10 μm, and an energy range of 5-30 keV.

6. The multi-angle, multi-pulse ultrafast X-ray imaging system according to claim 1, characterized in that, The multidimensional detection module is a flat panel detector, an imaging plate, or a fluorescent plate; by adjusting the angle difference between each sub-pulse to be greater than the Betatron radiation divergence angle, the projected images at different times are spatially separated on the detector plane.

7. A multi-angle, multi-pulse ultrafast X-ray imaging system according to claim 1, characterized in that, The X-ray images recorded by the multi-dimensional detection module are post-processed to achieve phase contrast enhancement, geometric correction, and noise filtering, thereby obtaining imaging effects with micrometer-level spatial resolution and femtosecond to picosecond-level temporal resolution.

8. A multi-angle, multi-pulse ultrafast X-ray imaging system according to claim 1, characterized in that, The output end of the radiation source generating module is equipped with a light-blocking film to block laser light that does not interact with the gas target, allowing electron beams and X-ray pulses to pass through.

9. A multi-angle, multi-pulse ultrafast X-ray imaging method based on the system of any one of claims 1-8, comprising the following steps: (a) Set the time delay and spatial orientation of each sub-pulse according to experimental requirements; (b) The ultra-intense and ultra-short master pulse laser generated by the laser source module is focused by an off-axis parabolic mirror and then split by a reflector array to generate multiple sub-pulses with a set time delay and spatial orientation; (c) Each sub-pulse acts on the corresponding gas target in the radiation source generation module, generating multiple Betatron radiation X-ray pulses through the laser tail field acceleration mechanism; (d) After the multiple X-ray pulses pass through the sample under test, they are recorded as multiple frames of images at different times by the multi-dimensional detection module; (e) Post-process the recorded multi-frame images, including phase contrast enhancement, geometric correction and noise filtering, to obtain imaging effects with micrometer-level spatial resolution and femtosecond to picosecond-level temporal resolution.

10. The multi-angle, multi-pulse ultrafast X-ray imaging method according to claim 9, characterized in that, In step (d), by adjusting the angle difference of the sub-pulses to be greater than the Betatron radiation divergence angle, the projected images at different times are spatially separated on the detector plane, thus avoiding signal aliasing.

Images