Self-interference based spatiotemporal focusing femtosecond laser pulse front tilt measurement device and method

Abstract

The application discloses a kind of based on self-interference's spatiotemporal focusing femtosecond laser pulse front skew measurement device and method.For the measurement light path complex problem that traditional method needs to provide reference light without pulse front skew in spatiotemporal focusing pulse front skew measurement by compressor, self-interference method is proposed to simplify light path, by splitting the light to be measured, reference light opposite to the skew direction of the pulse to be measured is directly generated, so that interference measurement is realized.Through the linear relationship between the center position of interference fringe and the delay difference between beams, the pulse front skew can be directly obtained.The device provides a simple and reliable pulse spatiotemporal characteristic analysis scheme for application in large-aperture high-power devices and implementation of spatiotemporal focusing pulse front skew measurement at focal point.

Description

Technical Field

[0001] This invention belongs to the field of ultrafast optics technology, specifically relating to a device and method for measuring the leading edge tilt of a spatiotemporally focused femtosecond laser pulse based on self-interference, which is particularly suitable for diagnosing the pulse spatiotemporal characteristics of spatiotemporally focused spots in large-aperture high-power laser systems. Background Technology

[0002] Pulse Front Tilt (PFT) in spatiotemporal focusing refers to the linear variation in pulse wavefront group delay with lateral position, resulting in differences in pulse arrival times at different spatial locations. This phenomenon originates from the convergence of different frequency components at different angles during the focusing of a spatially chirped beam in front of a lens. It is a result of the coupling effect between the focusing process and spatial dispersion, and is essentially a distortion of the pulse front. PFT not only increases the pulse width and focal spot at the focal point and reduces peak power, but also has important applications in high-energy particle acceleration, terahertz radiation, and other fields. Therefore, accurate measurement of PFT at the focal point is of great significance.

[0003] Currently, for measuring PFT in spatiotemporally focused lasers, only Wang et al. proposed a method using interference between a reference beam without pulse front tilt and the spatiotemporally focused beam under test (see the literature "Interferometric characterization of pulse front tilt of spatiotemporally focused femtosecond laser pulses"). This method utilizes the PFT-free reference beam and the beam under test to generate interference fringes in the spatiotemporally overlapping region. By adjusting the delay difference between the two pulses, the interference region is spatially shifted, and the PFT at the focal point is obtained based on the relationship between the inter-beam delay difference and the corresponding interference fringe positions. However, the generation of the reference beam requires a compressor to compress the positively chirped pulses output by the amplifier to form an ideal reference beam that is optically synchronized and without pulse front tilt. Especially in petawatt-level large-aperture high-power laser devices, the compressor usually needs to be compressed in stages and relies on a large number of high-precision optical components for assembly and adjustment, increasing system complexity and measurement costs. This makes the PFT measurement process difficult and expensive, thus limiting the further application and promotion of this technology in high-end fields such as ultra-precision machining.

[0004] Therefore, there is an urgent need to develop a pulse leading edge tilt measurement technology that is simpler in structure, lower in cost, and easier to integrate, in order to meet the needs of rapid and accurate diagnosis of spatiotemporally focused beams in high-power laser systems. Summary of the Invention

[0005] Traditional interferometric methods for measuring the wavefront tilt of spatiotemporally focused femtosecond laser pulses require a compressor to generate an ideal reference pulse that is optically synchronized and free from wavefront tilt. This compressor typically consists of multiple pairs of precision optical components such as gratings and prisms, and necessitates the introduction of an adaptive optics system to ensure the wavefront quality of the output pulse. This results in a large overall system structure, complex assembly and adjustment procedures, stringent requirements for component positioning accuracy, and high implementation costs. These issues limit the application of this technology in large-aperture, high-power laser devices. Therefore, a self-interferometric spatiotemporally focused femtosecond laser pulse front tilt measurement device is proposed. This device utilizes a reference beam obtained after beam splitting the target beam, which has the opposite wavefront tilt direction to the target beam, to perform interferometric measurements. It eliminates the need for a compressor, simplifies assembly and adjustment, and is cost-effective. This invention provides a spatiotemporal focusing femtosecond laser pulse leading edge tilt measurement device based on self-interference. After splitting the spatiotemporal focusing pulse to be measured, a reference pulse with the same magnitude of leading edge tilt and opposite direction is obtained through optical path transformation. The two beams of light self-interfere at the focal plane of the focusing lens. By analyzing the linear relationship between the displacement of the center of the interference fringe and the inter-beam delay difference, the leading edge tilt of the pulse to be measured can be deduced.

[0006] The technical solution of the present invention is as follows: A spatiotemporal focusing femtosecond laser pulse leading-edge tilt measurement device based on self-interference comprises: a laser source system, a pair of parallel gratings, a 5:5 beam splitter, a 5:5 beam splitter, a 5:5 beam splitter, an electrically displaced mirror, a first mirror, a second mirror, a focusing lens, an imaging objective lens, and a CCD camera. The characteristics, functions, and connections of each component are detailed below: The laser source system consists of an oscillator, stretcher, and amplifier, used to generate femtosecond laser pulses with positive chirp. Typical output parameters are: spectral width 10-110 nm, center wavelength 750 nm–850 nm, repetition rate 1 kHz–10 kHz, single pulse energy 0.2 mJ–10 mJ, and beam waist radius 5 mm–15 mm. The laser beam emitted from the system propagates parallel through air to a pair of collimators, with the initial intensity defined as... I .

[0007] The parallel grating pair consists of two parallel gratings with a line density of 830 lines / mm–1600 lines / mm and a blaze angle of 38°–56°, with a grating spacing of 100 mm–800 mm. This grating pair is placed in the output beam path of the laser source system, introducing a quantitative angular chirp α1 into the femtosecond laser pulse, while simultaneously compensating for beam dispersion, causing the pulse to approach the Fourier transform limit at the focal position, and outputting a collimated parallel beam that is transmitted to a 5:5 beam splitter.

[0008] The first 5:5 beam splitter is a parallel planar substrate with a thickness of 0.5 mm–5 mm and an operating incident angle of 30°–60°. Both the front and rear surfaces are coated with a beam splitting film with a reflectivity and transmittance of 50%±2%. A parallel beam with an angular chirp α1 is split into two paths after incident: one path is reflected by the front surface to form the test beam TF1 (angular chirp α1, light intensity I / 2), which propagates vertically upward to the 5:5 beam splitter; the other path is transmitted and reflected by the rear surface to form the reference beam TF2 (angular chirp α1, light intensity I / 2), which is transmitted horizontally along the original direction to the electrically displaced mirror.

[0009] The second 5:5 beam splitter is a parallel planar substrate with a thickness of 0.5mm–5mm and an operating incident angle of 30°–60°. Both the front and rear surfaces are coated with a beam-splitting film with a reflectivity and transmittance of 50%±2%. The beam to be measured, TF1, is reflected horizontally by the front surface of the 5:5 beam splitter and then transmitted to the second mirror. The chirp rate is α1, the light intensity is I / 4, and the beam wavelength is distributed with shorter wavelengths at the top and longer wavelengths at the bottom.

[0010] The second reflecting mirror is a silver or gold film plane mirror with a diameter of 10 mm–70 mm, a reflectivity of not less than 98%, and a working incident angle of 30°–60°. It reflects the horizontally transmitted beam TF1 to be tested again, causing the propagation direction to be deflected downward by 90° and incident on the rear surface of the 5:5 beam splitter. During the transmission process, the angular chirp rate is maintained at α1, and the light intensity is maintained at I / 4.

[0011] The electrically driven displacement mirror has a reflectivity of no less than 98%, and the matching electrically driven displacement stage can precisely move within a range of 0–200 mm, with a minimum single movement of 0.1–0.5 μm, used to adjust the time difference between the reference light and the light under test. t d The reference beam TF2 is perpendicularly incident on the electrically displaced mirror and returns along the same path. After being reflected vertically downwards by the surface of the 5:5 beam splitter, it is transmitted to the first mirror, with the chirp rate maintained at α1 and the light intensity maintained at I / 4.

[0012] The first reflecting mirror is a silver or gold film plane mirror with a diameter of 10 mm–70 mm, a reflectivity of not less than 98%, and an operating incident angle of 30°–60°. It reflects the vertically downward-propagating reference beam TF2 into a horizontally propagating beam, which is then incident on the front surface of the 5:5 beam splitter, at which point the angular chirp is [value missing]. α1, the light intensity remains at I / 4, and the wavelength of the light beam is distributed with the shorter wavelength at the bottom and the longer wavelength at the top.

[0013] The third 5:5 beam splitter is a parallel planar substrate with a thickness of 0.5 mm–5 mm and an operating incident angle of 30°–60°. Both the front and rear surfaces are coated with a beam splitting film with a reflectivity and transmittance of 50%±2% each. It is used to spatially coaxially combine the test beam TF1 and the reference beam TF2 to form a collinear horizontally propagating beam.

[0014] The beam combining process is as follows: the reference beam TF2 is transmitted through the front surface of the 5:5 beam splitter and propagates along its original horizontal direction, with an angular chirp rate of... α1, light intensity I / 8; the beam to be tested TF1 is reflected from the rear surface of the 5:5 beam splitter, and its propagation direction turns to horizontal, with an angular chirp rate of α1 and a light intensity of I / 8. The two beams propagate coaxially horizontally to form a composite beam. The short wavelength of the reference beam coincides with the long wavelength of the beam to be tested, and the long wavelength of the reference beam coincides with the short wavelength of the beam to be tested. In front of the focusing lens, two beams with equal angular chirps and opposite directions are formed: the beam to be tested TF1 and the reference beam TF2.

[0015] The focusing lens has an effective aperture of 50 mm–150 mm and a focal length of 300 mm–1000 mm. It is placed in the beam combining optical path to focus two spatially chirped beams onto the same focal plane and form interference fringes at the focal point for measuring the tilt of the pulse leading edge. The fringe displacement is linearly related to the amount of pulse leading edge tilt.

[0016] The imaging objectives are microscope objectives with magnifications of 10×, 20×, and 30× and numerical apertures of 0.25, 0.4, and 0.45, respectively. They are placed on the image-side optical path of the focusing lens to magnify the interference fringe spots at the focal plane, making it easier for the CCD camera to acquire them.

[0017] The CCD camera has a resolution of 500×600 to 1024×1024 and a pixel pitch of 6.4–9.5 μm, and is optically aligned with the imaging objective (10). A motorized displacement mirror is moved to adjust the time difference between the pulse to be measured and the reference pulse. t d Images are captured and recorded by a CCD camera at different time delays. t d Record the displacement of the interference fringes x ,use x ∝ t d The pulse leading edge tilt amount is obtained by fitting the linear relationship.

[0018] This invention also provides a method for measuring the tilt of the leading edge of a spatiotemporally focused femtosecond laser pulse using the above-described apparatus, comprising the following steps: Beam preparation: Start the laser source system and output positively chirped femtosecond pulses. After passing through a parallel grating pair, a collimated parallel beam with an angular chirp rate α1 is obtained.

[0019] Beam splitting and transformation: The parallel beam is split into the test beam TF1 and the reference beam TF2 by the first 5:5 beam splitter.

[0020] After the reference beam TF2 returns via the electrically displaced mirror, it is reflected sequentially by the rear surface of the first 5:5 beam splitter and the first mirror, resulting in an angular chirp rate of [value missing]. α1, and is horizontally incident on the front surface of the third 5:5 beam splitter.

[0021] The beam to be tested, TF1, is reflected sequentially by the second 5:5 beam splitter and the second mirror, maintaining the chirp rate α1, and then incident downwards onto the rear surface of the third 5:5 beam splitter.

[0022] Beam combining and focusing: At the third 5:5 beam splitter, the reference beam TF2 is transmitted and the beam to be tested TF1 is reflected. The two beams are coaxially combined along the same horizontal direction to form a composite beam. This composite beam is focused to the same focal plane by a focusing lens, producing interference fringes at the focal point.

[0023] Data acquisition: Multiple different time delays t are set using an electrically driven displacement mirror. d (like 48fs (24fs, 24fs, 48fs), the center position x of the interference fringes on the focal plane under each time delay is recorded by a CCD camera.

[0024] Calculate the frontal inclination: for the measured t d By performing a linear fit with x, the slope Δt is obtained. d / Δx, and according to the self-interference principle formula P = (Δt) d The pulse leading edge tilt P of the beam under test is calculated as / Δx) / 2. This formula originates from the fact that, under self-interference conditions, the relative pulse leading edge tilt of the two beams is 2P, therefore the interference fringe displacement x corresponds to the delay difference t. d The response sensitivity is half that of the traditional method, and P can be obtained by dividing the slope by 2.

[0025] Compared with the prior art, the present invention achieves the following significant technical effects: 1. Significantly simplified system structure and greatly reduced cost: The compressor structure in traditional interferometry is completely eliminated, reducing the number of optical components. This lowers equipment costs, simplifies assembly and adjustment, and streamlines the assembly and adjustment process. The device of this invention is suitable for use in large-aperture, high-power laser devices, providing a high-performance, low-cost diagnostic tool for pulse spatiotemporal characteristics, specifically for measuring the tilt of the pulse leading edge at the focal point.

[0026] 2. Accurate and reliable measurement results: In a spatiotemporal focusing system, the group delay at the image-side focal plane can be expressed as... ,middle ω 0 is the center frequency. α For angular chirp rate, c At the speed of light, f Let be the focal length of the lens. Calculate the time delay for this group. x The first-order partial derivative yields the analytical expression for pulse leading edge tilt (PFT). This is used to measure the pulse leading edge tilt of the light pulse TF1 under test.P Traditional methods use a pulse with no leading edge tilt as the reference beam (CF). In this case, the relative tilt between the test beam and the reference beam is... P Displacement of the central interference fringe x The time difference between the two beams of light t d Satisfying P= t d / x However, if a self-interference method is used, that is, a reference light TF2 is selected with the same magnitude of tilt at the leading edge of the pulse of the light under test TF1 but in the opposite direction, the tilt of its leading edge is... P Then the spatial variation trends of the group delay of the two beams are opposite, and the relative tilt becomes 2. P Since interference only occurs in the region where the two beams of light completely overlap in space, under the same time delay... t d Under the condition, the displacement of the central fringe of the interference x This will reduce the size to half that of the traditional method. P = t d / 2 x Therefore, by making the focal planes of the spatiotemporally focused pulse and the reference pulse completely coincide in space, they interfere in the overlapping region of the time and spatial domains, forming banded interference fringes. Then, the inter-beam delay difference is adjusted... t d Observe the movement of the central interference fringes in the region where the focal planes coincide, and use the position of the central fringe... x and t d By establishing a linear relationship between the two, the PFT of the light pulse TF1 under test can be accurately obtained.

[0027] The effectiveness of this method was verified through numerical simulation and experiments. A beam with a center wavelength of 800 nm, a spectral width of 27 nm, a repetition frequency of 1 kHz, and a beam diameter of approximately 10 mm was used. The beam was then focused by a plano-convex lens with a focal length of 750 mm after passing through parallel gratings G1-G2 (grating line density 1500 lines / mm, incident angle 43°, grating spacing 195 mm). The angular inclination α introduced by the parallel gratings was approximately 1.15 × 10⁻⁶. -16 s·m / rad. Set a series of time delays. t dThe interference fringe displacements (-48 fs, -24 fs, 24 fs, 48 ​​fs) measured by the self-interferometry method in numerical simulations were 19.9 μm, 10 μm, -10 μm, and -19.9 μm, respectively. Linear fitting yielded a slope of -2.40 fs / μm, which translates to a PFT of -1.20 fs / μm, consistent with the theoretical value of -1.2 fs / μm. Experimental verification showed that the corresponding interference fringe displacements measured by the self-interferometry method were 19 μm, 9.3 μm, -10.7 μm, and -19.4 μm, with a linear fitting slope of -2.48 fs / μm, resulting in a PFT of -1.24 fs / μm. The experimental measurement error was only 3.3% relative to the theoretical value, and after repeated verification, the relative error remained within 4%. This result fully demonstrates that the improved interferometry method presented in this paper can accurately and effectively measure the leading edge tilt of a spatiotemporally focused pulse. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. The drawings are used to provide a further understanding of the present invention and constitute a part of the specification. They are used together with the embodiments of the present invention to explain the present invention and do not constitute a limitation of the present invention. Figure 1 This is a schematic diagram of the structure and principle of the spatiotemporal focusing femtosecond laser pulse leading edge tilt measurement device based on self-interference provided in an embodiment of the present invention; wherein, a is a diagram of the optical path structure of the device, and b is a schematic diagram of the self-interference measurement principle; Figure 2 This is a schematic diagram of a spatiotemporally focused femtosecond laser pulse leading edge tilt measurement device and its principle using traditional interferometry; where a represents the optical path structure of the traditional method, and b represents its measurement principle. Figure 3 The simulation provided in this embodiment of the invention has different delay differences. t d The center position of the corresponding interference fringes x Where a, b, c, and d are respectively t d Choose -48fs, -24fs, 24fs, and 48fs; Figure 4 This is a linear fitting graph obtained by data processing based on measurement points in the simulation provided by the embodiments of the present invention; where a, b, c, and d are respectively t d Choose -48fs, -24fs, 24fs, and 48fs; Figure 5 The different delay differences provided in the experiments of the embodiments of the present invention t d The center position of the corresponding interference fringesx .

[0029] Figure 6 This is a linear fitting graph obtained by data processing based on measurement points in the experiment provided in this embodiment of the invention.

[0030] The numbers in the diagram are: 1-Laser source system, 2-Parallel grating pair, 3-First 5:5 beam splitter, 4-Electrically shifted mirror, 5-First mirror, 6-Second 5:5 beam splitter, 7-Second mirror, 8-Third 5:5 beam splitter, 9-Focusing lens, 10-Imaging objective lens, 11-CCD camera; solid lines and dashed lines represent the spatial distribution of long wavelengths and short wavelengths in the beam, respectively. Detailed Implementation

[0031] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. In addition, the present invention may repeat reference numerals and / or reference text in different embodiments. Such repetition is for the purpose of simplification and clarity, and does not in itself indicate the relationship between the various implementation methods and / or settings discussed.

[0032] This embodiment proposes a self-interference-based spatiotemporal focusing femtosecond laser pulse leading-edge tilt measurement device, the optical path of which is as follows: Figure 1 As shown in Figure a, the device includes: a laser light source system 1, a pair of parallel gratings 2, a first 5:5 beam splitter 3, an electrically displaced mirror 4, a first mirror 5, a second 5:5 beam splitter 6, a second mirror 7, a third 5:5 beam splitter 8, a focusing lens 9, an imaging objective lens 10, and a CCD camera 11. Figure 1 Figure b illustrates the principle of measuring pulse leading edge tilt using the self-interference method. The reference beam TF2 and the beam under test TF1 have equal magnitudes but opposite directions of pulse leading edge tilt at the image-side focal plane of the lens. The time delay difference between the two beams is introduced by adjusting the electric displacement mirror 4. t d Different time delays correspond to different interference images. After passing through imaging objective 10, the interference fringes are imaged onto CCD camera 11; the center position of the interference fringes is fitted. x With delay t d The linear change of the pulse tip can be used to accurately measure the pulse leading edge tilt.

[0033] To compare the differences between traditional interferometry and self-interferometry, Figure 2The measurement apparatus and principle of the traditional interferometry method are presented. This method uses a pulse with a tilted leading edge (CF) as the reference light, which interferes with the spatiotemporally focusing pulse TF1 at the image focal plane. The time difference between the two pulses is adjusted. t d The interference fringes will shift within the region where the focal planes coincide; based on the center position of the interference fringes... x and t d The linear relationship allows for the measurement of the pulse front tilt P (see reference "Interferometric characterization of pulse front tilt of spatiotemporally focused femtosecond laser pulses"). From... Figure 2 As shown in b, the traditional method uses a pulse with no leading edge tilt as the reference light CF. In this case, the relative tilt between the test light and the reference light is: P Displacement of the central interference fringe x The time difference between the two beams of light t d Satisfying P= t d / x If a self-interference method is used, from Figure 1 As can be seen from b, a reference light TF2 is selected, which has the same magnitude of pulse leading edge tilt as the test light TF1 but opposite direction. Its pulse leading edge tilt is... If P, then the spatial variation trends of the group delay of the two beams are opposite, and the relative tilt becomes 2P. Interference only occurs in the region where the two beams completely overlap in space, under the same delay difference. t d Under the condition, the displacement of the central fringe of the interference x This will be reduced to half the size of the traditional method, at which point P = t d / 2 x .when t d When the time difference is ≠0, the self-interference method replaces the untilted reference light CF with a reference light TF2 that is tilted in the opposite direction to the light under test TF1, so that the same time delay difference is achieved. t d Below, the center position of the interference fringes in the overlapping region of the interference method. x The value is only half that of traditional interferometry. The slope measured by traditional interferometry directly corresponds to the PFT value, i.e., P = t d / xThe slope measured by the self-interferometry is twice that of the traditional method, corresponding to 2PFT. This shows that the self-interferometry proposed in this paper can achieve high-precision measurement of pulse leading edge tilt (PFT) without the need for a compressor to provide optical synchronization and a reference light without pulse leading edge tilt, as is the case with the traditional method.

[0034] To verify the correctness of the proposed method, the following analysis combines numerical simulation and experiments. The specific description of the self-interferometric measurement optical path and experimental parameters is as follows: Laser source system 1 consists of an oscillator, stretcher, and amplifier, emitting a femtosecond laser pulse with a center wavelength of 800 nm, a spectral width of 27 nm, a repetition frequency of 1 kHz, a beam diameter of approximately 10 mm, and a single pulse energy of 1 mJ, accompanied by positive chirp. After passing through parallel grating pair G1-G2 with a grating line density of 1500 lines / mm and an incident angle of 43°, the distance between the two gratings is adjusted to approximately 190 mm to fully compensate for the positive chirp of the amplifier output pulse. After passing through the grating pair, x The spot size in the direction is approximately 20 mm (1 / e) 2 The spot size in the y-direction is approximately 10 mm (1 / e). 2 The output is a parallel beam that propagates through the air to the 5:5 beam splitter 3. At this point, the angular chirp α1 introduced by the parallel grating to the pulse is approximately 1.15 × 10⁻⁶. - ¹ 6 The provided group delay dispersion is approximately -6.29 × 10⁻⁶ s·m / rad. 5 fs 2， The emitted light intensity is defined as I at this point.

[0035] The first 5:5 beam splitter 3 has a parallel planar structure with a substrate thickness of 3 mm and an operating incident angle of 45°. Its front and rear surfaces are coated with beam-splitting films with 50% reflectivity and 50% transmittance. The parallel beam emitted from the parallel grating G1-G2 is split into two paths: one path is reflected by the front surface to form the test beam TF1 (angular chirp α1, light intensity I / 2), which propagates vertically upward to the 5:5 beam splitter 6; the other path is transmitted and reflected by the rear surface to form the reference beam TF2 (angular chirp α1, light intensity I / 2), which is transmitted horizontally along the original direction to the electrically displaced mirror 4.

[0036] The second 5:5 beam splitter 6 is a parallel planar structure with a thickness of 3mm and an operating incident angle of 45°. Its front and rear surfaces are coated with a beam-splitting film with 50% reflectivity and 50% transmittance. The beam to be measured, TF1, is reflected by the front surface of the 5:5 beam splitter 6 and propagates horizontally in the air to the second reflecting mirror 7. At this point, the chirp rate is α1, the light intensity is I / 4, and the beam wavelength is shorter than the upper wavelength (…). Figure 1 (a is represented by dashed lines), with longer wavelengths distributed below ( Figure 1 (A is represented by a solid line).

[0037] The second reflecting mirror 7 is a silver-coated plane mirror with a diameter of 50 mm, a reflectivity of 99%, and a working incident angle of 45°. It reflects the horizontally transmitted beam TF1 again, causing the propagation direction to be deflected downward by 90° and incident on the rear surface of the 5:5 beam splitter 8. During the transmission process, the angular chirp rate remains α1, and the light intensity remains I / 4.

[0038] The motorized displacement mirror 4 has a reflectivity of 99% and a working incident angle of 90°. The matching motorized displacement stage allows for precise displacement within a range of 0–100 mm, with a minimum single movement of 0.16 μm. This is used to accurately adjust the time difference between the reference light and the light being measured. t d The reference beam TF2 is perpendicularly incident on the electrically displaced reflector 4 and returns along the same path. After being reflected vertically downwards by the 5:5 beam splitter 3, it is transmitted to the first reflector 5, with the chirp rate maintained at α1 and the light intensity maintained at 1 / 4.

[0039] The first reflecting mirror 5 is a silver-coated plane mirror with a diameter of 50 mm, a reflectivity of 99%, and an operating incident angle of 45°. It deflects the reference beam TF2, which is propagating vertically downwards in air, by 90° to achieve horizontal transmission, and directs it onto the front surface of the 5:5 beam splitter 8. The chirp rate is... α1, light intensity remains at I / 4, shorter wavelengths in the beam are in the lower half ( Figure 1 (a is represented by a dashed line), and the longer wavelength is in the upper half ( Figure 1 (A is represented by a solid line).

[0040] The third 5:5 beam splitter 8 is a parallel planar structure with a thickness of 3 mm and an operating incident angle of 45°. Both the front and rear surfaces are coated with a beam splitting film with a reflectivity and transmittance of 50% each. It is used to spatially coaxially combine the test beam TF1 and the reference beam TF2 to form a collinear horizontally propagating beam.

[0041] The beam combining process is as follows: Reference light TF2 is transmitted through the front surface of the 5:5 beam splitter 8 and propagates along its original horizontal direction, with an angular chirp rate of... α1, light intensity I / 8; the beam to be tested TF1 is reflected from the rear surface of the 5:5 beam splitter 8, and its propagation direction turns horizontal, with an angular chirp rate of α1 and a light intensity of I / 8. At the rear surface of the 5:5 beam splitter, the short-wavelength portion of the reference beam TF2 coincides with the long-wavelength portion of the beam to be tested TF1, and the long-wavelength portion of the reference beam TF2 coincides with the short-wavelength portion of the beam to be tested TF1. The two beams propagate coaxially horizontally to form a composite beam. Two beams, the beam to be tested TF1 and the reference beam TF2, with equal angular chirps and opposite directions, are formed in front of the focusing lens 9.

[0042] The focusing lens 9 is a plano-convex lens with a diameter of 100 mm and a focal length of 750 mm. It is positioned in the optical path of the combining beam to focus two beams with opposite angular chirps onto the same focal plane. The imaging objective 10 is a microscope objective with a magnification of 20× and a numerical aperture of 0.4. It is positioned in the image-side optical path of the focusing lens 9 to magnify the interference fringe spots at the focal plane, facilitating acquisition by the CCD camera. The CCD camera 11 has a resolution of 580×780 and is optically aligned with the imaging objective 10. The inter-beam delay is adjusted via an electrically driven displacement mirror 4. t d A corresponding linear displacement of the interference fringes on the focal plane can be observed. This displacement is recorded by... x Inter-beam delay time difference t d The linear proportional relationship between them, after dividing the slope by 2, can be used to accurately quantify the degree of tilt P of the pulse leading edge.

[0043] Traditional methods utilize a compressor to provide a reference beam with no pulse front tilt (α=0) and interfere with the beam under test (α=α1). By analyzing the linear relationship between the center position of the interference fringes and the inter-beam delay difference, the pulse front tilt (PFT) is obtained. An optical field transmission model is then established based on the reference "Interferometric characterization of pulse front tilt of spatiotemporally focused femtosecond laser pulses". In the self-interferometry method, based on the proportional relationship between the pulse front tilt and the spatial chirp of the incident pulse in spatiotemporally focused conditions, the traditional method, which requires a compressor to provide a chirp-free reference beam (α=0), replaces it with a reference beam (α=-α1) with a tilt direction opposite to that of the pulse under test (α=α1) to achieve interference.

[0044] Simulation results show that the delay difference t d The center shift of the interference fringes when set to -48 fs, -24 fs, 24 fs, and 48 fs respectively. x These correspond to 19.9 μm, 10 μm, -10 μm, and -19.9 μm, respectively. Linear fitting yielded a slope of -2.40 fs / μm, which translates to PFT = -1.20 fs / μm. Substituting the above parameters into the group delay formula at the image-side focal plane , ω 0 is the center frequency. α For angular chirp rate, c At the speed of light, f Let be the focal length of the lens. Calculate the group delay. xThe first derivative yields the analytical expression for the pulse leading edge tilt (PFT). The theoretical PFT is -1.2 fs / μm. It is evident that the PFT obtained from numerical simulation is consistent with the theoretically calculated PFT. Figure 4 The simulation provided in this embodiment of the invention has different delay differences. t d The center position of the corresponding interference fringes x . Figure 5 This is a linear fitting graph obtained by data processing based on measurement points in the simulation provided by the embodiments of the present invention.

[0045] In experimental verification, the corresponding interference fringe displacements measured by the self-interferometry were 19 μm, 9.7 μm, -10.3 μm, and -19.4 μm, respectively. Linear fitting yielded a slope of -2.48 fs / μm, which translates to PFT = -1.24 fs / μm. The experimental measurement error was only 3.3% relative to the theoretical value, and after repeated verification, the relative error remained within 4%. This result fully demonstrates that the improved interferometry proposed in this paper can accurately and effectively measure the leading edge tilt of a spatiotemporally focused pulse. Figure 6 The different delay differences provided in the experiments of the embodiments of the present invention The corresponding center position x of the interference fringes.

[0046] This invention effectively solves the problems of complex system assembly and high economic costs associated with traditional interferometric methods that rely on CPA system compressors to generate reference light. The measurement scheme based on self-interference significantly reduces the technical complexity of pulse front diagnosis. This technology provides a novel and efficient measurement tool for diagnosing the spatiotemporal characteristics of large-aperture, high-power laser devices, combining high performance with low cost, demonstrating significant engineering application value and development prospects.

[0047] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A device for measuring the tilt of the leading edge of a spatiotemporally focused femtosecond laser pulse based on self-interference, characterized in that, include: A laser source system (1) is used to generate femtosecond laser pulses with positive chirp; Parallel grating pair (2) is set in the output optical path of the laser source system (1) to introduce angular chirp rate α1 into the femtosecond laser pulse and output collimated parallel beam; The first 5:5 beam splitter (3) is inclinedly set on the outgoing light path of the parallel grating pair (2). Its front surface reflects the incident beam to form the test beam TF1 that propagates in the vertical upward direction, and its rear surface reflects the transmitted beam to form the reference beam TF2 that propagates in the original horizontal direction. An electrically driven displacement mirror (4) is positioned perpendicular to the propagation direction of the reference beam TF2. It is used to receive the perpendicularly incident reference beam TF2 and return it along the same path. At the same time, it precisely adjusts the time difference t between the reference beam and the beam under test by displacement. d ; The first reflector (5) is used to reflect the reference beam TF2, which has been returned by the electrically displaced reflector (4) and reflected by the rear surface of the first 5:5 beam splitter (3), into a horizontally transmitted beam, and to change the angular chirp rate of the reference beam TF2 to... α1; The second 5:5 beam splitter (6) is set in the optical path of the beam TF1 to be measured, and is used to reflect it and transmit it horizontally. The second reflector (7) is used to reflect the beam TF1 to be tested, which has been reflected by the second 5:5 beam splitter (6), so that its propagation direction is deflected by 90°. The third 5:5 beam splitter (8) is used to spatially coaxially combine the test beam TF1 from the second reflector (7) and the reference beam TF2 from the first reflector (5) to form a composite beam. After the beam is combined, the angular chirp rates of the two beams are equal in magnitude and opposite in direction. A focusing lens (9) is placed in the optical path of the synthesized beam to focus two beams with opposite angular chirps onto the same focal plane and generate interference fringes at the focal point. An imaging objective (10) is placed on the image-side optical path of the focusing lens (9) to magnify the interference fringe spot at the focal plane; A CCD camera (11) is optically aligned with an imaging objective (10) for recording different time delays t. d Interference fringe image below.

2. The spatiotemporal focusing femtosecond laser pulse leading edge tilt measurement device based on self-interference as described in claim 1, characterized in that, The laser source system (1) consists of an oscillator, a stretcher and an amplifier. Its output parameters are: spectral width 10nm~110nm, center wavelength 750nm~850nm, repetition frequency 1kHz~10kHz, single pulse energy 0.2mJ~10mJ, and beam waist radius 5mm~15mm.

3. The spatiotemporal focusing femtosecond laser pulse leading edge tilt measurement device based on self-interference as described in claim 1, characterized in that, The parallel grating pair (2) consists of two gratings with a line density of 830 lines / mm to 1600 lines / mm and a blaze angle of 38° to 56° placed in parallel, with a grating spacing of 100mm to 800mm.

4. The spatiotemporal focusing femtosecond laser pulse leading edge tilt measurement device based on self-interference as described in claim 1, characterized in that, The first 5:5 beam splitter (3), the second 5:5 beam splitter (6) and the third 5:5 beam splitter (8) are all parallel planar substrates with a thickness of 0.5mm to 5mm and an operating incident angle of 30° to 60°. The front and rear surfaces are coated with a beam splitting film with a reflectivity and transmittance of 50% ± 2%.

5. The spatiotemporal focusing femtosecond laser pulse leading edge tilt measurement device based on self-interference as described in claim 1, characterized in that, The reflectivity of the electric displacement mirror (4) is not less than 98%, and the matching electric displacement stage can precisely move within the range of 0 to 200 mm, with a minimum single movement of 0.1 μm to 0.5 μm.

6. The spatiotemporal focusing femtosecond laser pulse leading edge tilt measurement device based on self-interference as described in claim 1, characterized in that, The first reflector (5) and the second reflector (7) are silver or gold film plane mirrors with a diameter of 10mm to 70mm, a reflectivity of not less than 98%, and a working incident angle of 30° to 60°.

7. The spatiotemporal focusing femtosecond laser pulse leading edge tilt measurement device based on self-interference as described in claim 1, characterized in that, The effective aperture of the focusing lens (9) is 50mm to 150mm, and the focal length is 300mm to 1000mm.

8. The spatiotemporal focusing femtosecond laser pulse leading edge tilt measurement device based on self-interference as described in claim 1, characterized in that, The imaging objective (10) is a microscope objective with a magnification of 10×, 20× or 30× and a numerical aperture of 0.25 to 0.45; the CCD camera (11) has a resolution of 500×600 to 1024×1024 and a pixel pitch of 6.4μm to 9.5μm.

9. A method for measuring the leading edge tilt of a spatiotemporally focused femtosecond laser pulse using the apparatus according to any one of claims 1 to 8, characterized in that, Includes the following steps: Start the laser source system (1) and output positively chirped femtosecond pulses. After passing through the parallel grating pair (2), a collimated parallel beam with an angular chirp rate α1 is obtained. The parallel beam is divided into a test beam TF1 and a reference beam TF2 by the first 5:5 beam splitter (3), wherein the test beam TF1 propagates in the vertical upward direction and the reference beam TF2 propagates in the original horizontal direction to the electric displacement mirror (4). Adjust the displacement of the electric displacement mirror (4) and set the initial time delay difference t between the reference beam TF2 and the beam to be measured TF1. d ; After the reference beam TF2 returns via the electrically displaced mirror (4), it is reflected sequentially by the rear surface of the first 5:5 beam splitter (3) and the first mirror (5), resulting in an angular chirp rate of [value missing]. α1, and horizontally incident on the front surface of the third 5:5 beam splitter (8); The beam TF1 to be measured is reflected sequentially by the second 5:5 beam splitter (6) and the second mirror (7), with the chirp rate maintained at α1, and then incident downwards onto the rear surface of the third 5:5 beam splitter (8). At the third 5:5 beam splitter (8), the reference beam TF2 is transmitted and the beam to be tested TF1 is reflected. The two beams are coaxially combined along the same horizontal direction to form a composite beam. The composite beam is focused onto the same focal plane by the focusing lens (9), producing interference fringes at the focal point; The interference fringes are magnified by the imaging objective (10), and the current time delay t is recorded by the CCD camera (11). d The center position of the interference fringes is x; By changing the displacement of the electrically driven displacement mirror (4), multiple different time delays t are set. d Repeatedly record the corresponding interference fringe center position x; For the measured t d By performing a linear fit with x, the slope Δt is obtained. d / Δx, and according to the formula P = (Δt) d Calculate the pulse leading edge tilt P of the beam under test using ( / Δx) / 2.

10. The measurement method according to claim 9, characterized in that: The delay difference t d The range of values ​​is The center position x of the interference fringes is extracted using an image processing algorithm between 48fs and 48fs.

Images