A nonlinear compression system and method based on spatiotemporal beam splitting

Abstract

This invention discloses a nonlinear compression system and method based on spatiotemporal beam splitting, belonging to the field of ultrashort pulse laser technology. The method utilizes a spatiotemporal beam splitting module to separate an initial ultrashort pulse laser into multiple sub-pulses in both spatial and temporal dimensions. These sub-pulses are then injected into a multi-pass cavity structure, causing spectral broadening in each sub-pulse as it passes through the nonlinear medium multiple times. The spectrally broadened sub-pulses are then superimposed through spatial and temporal beam combining, followed by dispersion compensation to obtain the compressed ultrashort pulse output. By controlling the spatiotemporal distribution of the pulse in the nonlinear medium, the peak intensity of a single sub-pulse during the nonlinear process is reduced, which is beneficial for achieving stable nonlinear compression under high-energy conditions.

Description

Technical Field

[0001] This invention relates to the field of ultrashort pulse laser technology, and in particular to a nonlinear compression system and method based on spatiotemporal beam splitting. Background Technology

[0002] Modern ultrafast science requires reliable high-energy, short-period optical pulse sources. Currently, two methods exist for generating such pulses: post-compression of short laser pulses and optical parametric chirped pulse amplification. Post-compression techniques include hollow fiber technology, sheet compression technology, and multi-pass cell technology. Hollow fiber compression energy is limited to the hundreds of millijoules, while sheet compression can achieve joule-level pulse compression, but suffers from non-uniform spectral broadening and poor beam quality. Multi-pass cells (MPCs), due to their ability to achieve multiple nonlinear interactions in free space, maintain good beam quality and are widely studied and applied in high-energy, ultrashort pulse laser systems.

[0003] In existing technologies, typical multi-cavity systems typically employ a single laser beam that propagates multiple times through a gas medium, achieving spectral broadening through the Kerr nonlinear effect. However, with the continuous increase in input pulse energy, the peak intensity and nonlinear load experienced by a single beam propagating within a multi-cavity system increase significantly. Problems such as self-focusing effects, ionization, and damage to optical components gradually become the main factors limiting system performance improvement.

[0004] To reduce nonlinear effects per unit length, existing research typically disperses nonlinear phase accumulation by increasing the geometric length of the multi-pass cavity. Table 1 lists the parameters of representative multi-pass cavity systems reported in recent years.

[0005] Table 1. Comparison of parameters of typical multi-cavity nonlinear compression systems in existing technologies

[0006] As shown in Table 1, as the input pulse energy increases from the millijoule level to the hundreds of millijoules level, the length of the multi-channel cavity needs to increase from several meters to tens of meters, exhibiting a clear upward trend. For ultrashort pulse systems with joule-level or even higher energies, if a single-channel multi-channel cavity structure is still used, the required cavity length will further increase, significantly increasing the system size and alignment difficulty, and posing serious challenges to mechanical stability, thermal stability, and industrialization.

[0007] Existing technologies also explore the use of non-standard multipass cell geometries to further increase energy, such as plano-concave multipass cavities, which can reduce the cavity length by half under the same input conditions. However, the energy increase cannot reach the Joule level.

[0008] Time-division pulse technology effectively reduces the peak intensity of a single pulse by decomposing it into multiple time-delayed sub-pulses, thereby alleviating nonlinearity and damage limitations. In recent years, experiments have combined time-division pulse technology with nonlinear post-compression to achieve stable pulse compression under higher total energy conditions. However, with only time-division, increasing the number of sub-pulses necessitates complex optical path matching and timing control structures, increasing system complexity and reducing stability. Furthermore, each sub-pulse retains its original spatial beam size, resulting in a still relatively high peak intensity, thus limiting the system's ability to withstand a significant increase in total input energy. Currently, with only time-division, dividing the pulse into four sub-pulses doubles the overall output energy of the MPC compared to single-pulse operation, increasing the supported pulse energy from 1.8 mJ to 3.4 mJ.

[0009] Therefore, how to achieve nonlinear broadening and high-quality output of higher-energy ultrashort pulses without significantly increasing the geometric length of multi-channel cavities has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0010] One of the objectives of this invention is to provide a nonlinear compression method based on spatiotemporal beam splitting to solve the problems of nonlinear load concentration and limited single-pulse energy enhancement under single-beam propagation conditions in existing multi-cavity ultrashort pulse laser systems. This allows for the nonlinear broadening and high-quality output of higher-energy ultrashort pulses without significantly increasing the geometric length of the multi-cavity system.

[0011] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A nonlinear compression method based on spatiotemporal beam splitting includes the following steps: (1) An initial ultrashort pulse laser is split into at least two separate sub-pulses in the time and space domains by a time-space beam splitting module, such that the peak power of any sub-pulse is lower than the peak power of the initial ultrashort pulse laser under unsplit conditions. (2) Inject the at least two sub-pulses into the multi-cavity structure, so that each sub-pulse passes through the nonlinear medium at least twice in the multi-cavity to produce spectral broadening; (3) Perform temporal and spatial beam combining on at least two sub-pulses after spectral broadening to obtain the broadened pulse after beam combining; (4) Dispersion compensation is performed on the bundled and broadened pulse to obtain compressed ultrashort pulse output.

[0012] To address the aforementioned problems, this invention provides a method and system for multi-cavity laser energy expansion and beam combining based on time and space beam splitting. The purpose is to effectively disperse the nonlinear load of a single beam by performing joint time and space beam splitting on the laser pulse before it enters the multi-cavity, enabling multiple sub-pulses to propagate in parallel and undergo nonlinear evolution in the multi-cavity, thereby achieving high-quality laser beam combining at the output end.

[0013] As a preferred technical solution, in step (1), the time-domain beam splitting is achieved by a birefringent crystal, a structure based on a polarization beam splitter and a spatial delay line, or a pulse splitting scheme based on phase modulation.

[0014] As a preferred technical solution, in step (1), the spatial domain beam splitting is achieved by a polarization beam splitter, a polarization-independent partial mirror, or a diffractive optical element, wherein the diffractive optical element includes a two-dimensional phase grating.

[0015] As a preferred technical solution, in step (3), the time beam combining is achieved by an optical delay compensation structure corresponding to the time beam splitting process, so that at least two sub-pulses re-overlap in time.

[0016] As a preferred technical solution, in step (3), the spatial beam combining is achieved by optical elements that are optically conjugate with the spatial beam splitting module, so as to restore the spatial beam shape before beam splitting.

[0017] The second objective of this invention is to provide a nonlinear compression system based on spatial beam splitting. The technical solution adopted includes an ultrashort pulse laser source module, a two-dimensional phase grating module, a multi-cavity nonlinear stretching module, a two-dimensional phase grating compensation module, a far-field beam splitting module, and a dispersion compensation module arranged sequentially along the optical path.

[0018] A third objective of this invention is to provide another nonlinear compression system based on spatiotemporal beam splitting, comprising an ultrashort pulse laser source module, a birefringent crystal time-splitting module, a two-dimensional phase grating module, a multi-cavity nonlinear stretching module, a two-dimensional phase grating compensation module, a birefringent crystal time-combining module, and a compression module arranged sequentially along the optical path.

[0019] Specifically, the steps and principles of the method of the present invention are as follows: 1) Time-beam splitting processing First, the input ultrashort pulse laser is subjected to time-splitting. The initial pulse is precisely divided into N sub-pulses, which do not overlap in the time dimension. The segmented sub-pulses retain the same characteristics as the original pulse, but with reduced energy and different polarization directions. When using a sequence of birefringent crystals to achieve pulse segmentation, each crystal divides the incident pulse into two pulses of equal intensity: an ordinary wave (o-wave) and an extraordinary wave (e-wave). The arrangement of the crystal optical axes follows a specific pattern: the optical axes of the crystals at odd-numbered positions form a 45° angle with the pulse polarization direction, while the optical axes of the crystals at even-numbered positions are aligned with the pulse polarization direction. By precisely adjusting the length of the crystals, effective control over the temporal interval of the segmented pulses can be achieved. To minimize the impact of dispersion on system performance, birefringent materials with low group velocity dispersion (GVD) are preferred, such as calcite, α-BBO (α-BaB₂O₄), or YVO₄ birefringent crystals. Alternatively, crystals of different materials can be cleverly combined; for example, combining crystals with positive group velocity dispersion (such as α-BBO) with materials with relatively low or negative group velocity dispersion (such as calcite or quartz), and by rationally designing the crystal thickness and optical axis direction, the overall dispersion of the system can be made close to zero. Since the group velocities of o-waves and e-waves differ in the crystal (v₀ and v₀ respectively),... o and v e This causes them to gradually separate over time. The formula for calculating the time interval Δt is: Δt = L / (1 / v) e – 1 / v o ), where L represents the length of the crystal. When selecting the shortest crystal length L1, it is necessary to ensure that Δt is greater than the pulse width to ensure that the segmented pulses do not overlap in time.

[0020] 2) Spatial beam splitting processing After time-splitting, the laser pulse undergoes further spatial splitting. Using two-dimensional diffractive optical elements or equivalent spatial phase modulation elements, two-dimensional periodic or aperiodic spatial frequency modulation is applied to the transverse space of the laser. This operation causes the input laser to split into multiple sub-pulses that are spatially separated during propagation. Subsequently, these spatially split sub-pulses are injected together into a multi-channel cavity structure. A multipass cavity consists of at least one pair of reflective optical elements, and sub-pulses propagate back and forth multiple times within the cavity. A nonlinear medium is carefully placed within the multipass cavity. Through the equivalent focusing effect of the multipass cavity, multiple spatially separated sub-pulses form multiple spatially separated focused spots at the nonlinear medium. This arrangement allows each sub-pulse to interact nonlinearly independently in space, thereby achieving spectral broadening.

[0021] 3) Bundle assembly After the nonlinear evolution is completed in the multi-pass cavity, the multiple nonlinearly broadened sub-pulses are spatially and temporally combined at the output end of the multi-pass cavity, so that the sub-pulses that were originally separated in space and time are superimposed again, thereby obtaining an output laser with significantly improved energy and good beam quality. Spatial beam combining is achieved using two-dimensional diffractive optical elements conjugate to spatial beam splitting or equivalent spatial phase modulation elements. For temporal beam combining, a sequence of birefringent crystals, similar to that used in temporal beam splitting, is employed to recombine the pulses. Specifically, the optical axis of the birefringent crystals is rotated by 90 degrees. This operation rotates the polarization direction of the time-splittered pulses by 90 degrees, ensuring that e-waves transform into o-waves and vice versa, thus guaranteeing that all pulses experience the same total delay during recombination. The recombined pulses not only possess the same spectral and temporal profile as the multi-cavity output pulses but also exhibit significantly increased intensity. Finally, dispersion compensation is performed on the combined laser beam to obtain compressed ultrashort pulse output, which meets the needs of high-precision application scenarios.

[0022] 4) Flexible adaptability In another important aspect of this invention, the number of time-splitting and spatial-splitting beams is not fixed, but can be flexibly adjusted according to the actual needs of the system design, and is applicable to all existing multi-cavity devices. The number of beams can typically be set in the range of approximately 8 to 36 sub-pulses, and can be specifically determined based on factors such as the target output energy.

[0023] Under the condition that the MPC geometric parameters (such as cavity length and mirror curvature radius) and gas parameters (such as nonlinear exponent, i.e., the peak intensity of a single beam) remain basically unchanged, the total input energy allowed by the system is approximately proportional to the number of beams. That is, the total input energy that the system can withstand is approximately proportional to the product of the number of time beams and the number of spatial beams.

[0024] Etotal≈Nt×Ns×Esingle Where Nt is the number of time-based beams, Ns is the number of spatial-based beams, and Esingle is the input energy that a single beam can withstand.

[0025] Therefore, the spatiotemporal joint beam splitting structure proposed in this invention can be flexibly extended according to different laser system parameters, thus making it suitable for ultrashort pulse nonlinear compression systems ranging from sub-millijoule to joule levels.

[0026] This invention, building upon existing time-based beam splitting techniques, further introduces a spatial beam splitting structure to decompose the input beam into multiple spatial sub-beams that propagate within a multi-pass cavity. The simultaneous introduction of spatial beam splitting into the time-based beam splitting system is not a simple superposition; the research process encountered technical challenges such as the stability of multi-beam propagation, sub-beam phase consistency, and multi-pass cavity mode matching. Through extensive experimental research, the inventors have solved these problems.

[0027] Specifically, this invention achieves stable spatial beam splitting by introducing a two-dimensional grating, decomposing the input beam into multiple sub-beams with defined angular relationships and spatial intervals. Because the diffracted beams generated by the two-dimensional grating exhibit good directional consistency and phase stability, the stability of the multiple beams during propagation is guaranteed, and overlap or interference between different beam trajectories is avoided.

[0028] By rationally designing the diffraction efficiency of the two-dimensional grating, the energy distribution of each spatial sub-beam is made basically uniform, ensuring that each sub-beam undergoes approximately the same nonlinear propagation process in the multi-pass cavity, thereby obtaining similar B-integral phase shifts. This guarantees that each sub-beam maintains good phase consistency at the output end, achieving high-efficiency spatial beam combining and obtaining high-quality compressed output pulses.

[0029] In the mode-matching design of multi-pass cavities, the propagation of a single beam within the cavity must satisfy the periodic invariance condition of the q-parameter to ensure a stable spatial mode distribution after multiple round trips. Since time-splitting only introduces a time delay without altering the spatial distribution of the beam, mode matching after time-splitting is easily achieved. However, spatial beam splitting requires a well-designed two-dimensional grating that primarily alters the propagation direction of each sub-beam while maintaining a relatively constant transverse amplitude distribution. This ensures that each sub-pulse satisfies the same q-parameter matching condition within the cavity. Furthermore, because spatial beam splitting results in a multi-beam, laterally separated array, its overall transverse dimension is significantly larger than in the single-beam case. Therefore, the effective aperture of the reflector needs to be appropriately increased to avoid beam truncation.

[0030] Comparative results show that, under the same multi-cavity length (approximately 3.2 m), the maximum allowable pulse energy of the system without a beam splitting structure is approximately 0.4 mJ; with only a time-based beam splitting structure, the total energy can be increased to approximately 1.6 mJ; and with the time-space combined beam splitting structure proposed in this invention, the total allowable input energy of the system is further increased to 14.4 mJ. These results demonstrate that the unique time-space combined beam splitting of this invention not only achieves the superposition of two beam splitting methods but also exhibits a synergistic gain effect of "1+1>2" due to the significant reduction in single-beam peak intensity and nonlinear phase shift accumulation.

[0031] Compared with the prior art, the advantages of the present invention are as follows: Because this invention employs a structure that combines time-based and spatial-based beam splitting and performs nonlinear evolution in parallel within a multi-channel cavity, the following beneficial effects can be achieved: (1) Effectively reduces the peak intensity and nonlinear phase accumulation of a single subpulse in the multi-channel cavity, suppresses self-focusing effect, ionization generation, mirror damage and spatial mode distortion, and improves system stability; (2) To extend the laser energy that the multi-cavity system can withstand without significantly increasing the size of the multi-cavity system and the number of round trips; (3) By combining multiple sub-pulses, the output laser can be optimized in terms of energy, beam quality and coherence, which is suitable for high-energy, high-repetition-frequency laser systems. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the system structure of Embodiment 1 of the present invention; Figure 2 This is a schematic diagram of the system structure of Embodiment 2 of the present invention; Figure 3 Theoretical simulation of the beam splitting at the center of a multi-cavity grating in a two-dimensional grating; Figure 4 Theoretical simulation of the spot pattern for far-field beams after compensating for the two-dimensional grating; In the figure, 1 is the ultrashort pulse laser source module; 2 is the two-dimensional phase grating module; 3 is the multi-cavity nonlinear broadening module; 4 is the two-dimensional phase grating compensation module; 5 is the far-field beam combining module; 6 is the dispersion compensation module; 7 is the birefringent crystal time beam splitting module; and 8 is the birefringent crystal time beam combining module. Detailed Implementation

[0033] To explain the technical content, objectives, and effects of the present invention in detail, the following specific embodiments are provided to further illustrate the content of the present invention. However, the content of the present invention is far more than the following examples.

[0034] It should be noted that, unless otherwise specified, all components and modules designed in the following embodiments are well known in the art and can be obtained commercially. For example, the birefringent crystal can be an α-BBO crystal or a calcite crystal, the concave mirror can be a curved lens with a high-reflectivity coating, the two-dimensional grating can be realized using conventional DOE manufacturing methods in the art, and the multi-cavity structure is an existing structure. The required optical components can be provided by common optical component suppliers in the art, such as standard products from manufacturers like Thorlabs, Edmund Optics, Layertec, and EKSMA Optics.

[0035] Example 1 A nonlinear compression system based on spatiotemporal beam splitting is provided. Specifically, this embodiment provides an ultrashort pulse beam splitting-nonlinear broadening-combining system based on a two-dimensional phase grating. See also Figure 1 The system mainly includes the following modules: Ultrashort pulse laser source module 1: Used to generate femtosecond laser pulses with a wide spectral bandwidth in the near-infrared band. The input pulse energy and repetition frequency can be adjusted, providing the input light source for the entire system. Two-dimensional phase grating module 2: It performs two-dimensional spatial beam splitting on the input laser in the transverse space, generating multiple spatially separated sub-pulses; the design of its phase modulation function is crucial for achieving effective beam splitting; Multi-cavity nonlinear broadening module 3: It contains at least one pair of optical reflective elements, which can be selected from concave mirrors, convex mirrors, cylindrical mirrors or plane mirrors according to the cavity design to achieve the required beam transmission and multiple round-trip propagation; at the same time, the nonlinear medium (such as gas, bulk medium or liquid) set in the focal region of the cavity achieves spectral broadening through nonlinear effects such as self-phase modulation. Two-dimensional phase grating compensation module 4: Its phase distribution is set as conjugate with that of two-dimensional phase grating module 2, and it is used to spatially combine multiple sub-pulses to compensate for the spatial phase modulation introduced during the beam splitting stage. Far-field beam combining module 5: includes a lens or lens group for realizing Fourier transform, so that the combined beam forms a focal point at the far-field focal plane; Dispersion compensation module 6: Employs dispersion compensation elements such as grating pairs, prism pairs, or chirped mirrors to compensate for the dispersion accumulated during the propagation of the combined beam by adjusting the optical path difference, thereby achieving pulse compression and outputting ultrashort pulses with high peak power and a wide spectrum.

[0036] Its workflow is as follows: the laser generated by the ultrashort pulse laser source module 1 passes through the two-dimensional phase grating module 2, the multi-cavity nonlinear broadening module 3, the two-dimensional phase grating compensation unit 4, the far-field beam combining module 5, and the dispersion compensation module 6 in sequence to complete the entire beam splitting-nonlinear broadening-beam combining process.

[0037] The specific implementation steps of this embodiment are as follows: (1) Establishment of input ultrashort pulse optical field The laser generated by the ultrashort pulse laser source module 1 serves as the input laser. This laser is an ultrashort pulse laser with a center wavelength in the near-infrared band. In this embodiment, the center wavelength is approximately 800 nm, corresponding to a pulse width of approximately 70 fs. The above parameters are common and representative operating conditions in ultrafast laser systems, and can be adjusted according to specific needs in practical applications.

[0038] In terms of spatial distribution, the transverse distribution of the input laser can be approximated as a super-Gaussian beam with an order of 2. The beam aperture is approximately 0.1 mm to 1 mm. The input light field is discretized in the two-dimensional transverse coordinates (x, y) to construct a complex amplitude distribution. Simultaneously, the spectrum is discretized, and only wavelength components within the system passband are selected for subsequent propagation calculations.

[0039] 2) Spatial beam splitting realization of two-dimensional phase grating A two-dimensional phase grating module 2 is introduced into the input light field to perform two-dimensional spatial beam splitting of the input laser. The two-dimensional phase grating module 2 exhibits periodic phase modulation in both orthogonal transverse directions. This functional form is chosen because it can achieve beam splitting simultaneously in both directions. Its phase modulation function is exemplified as follows: , in and The spatial period parameter can be taken as the same value, such as 100μm, so that the two-dimensional phase grating module 2 has a square symmetrical structure. The corresponding phase modulation depth is designed according to the input laser wavelength. When the phase modulation depth is appropriate, effective two-dimensional spatial beam splitting can be achieved.

[0040] After the input light field passes through the two-dimensional phase grating module 2, its complex amplitude is modulated as follows: , This introduces two-dimensional discrete diffraction components into the spatial frequency domain.

[0041] 3) Formation of spatial beam splitting focus The light field modulated by the two-dimensional phase grating module 2 is further nonlinearly broadened by entering the multi-pass cavity nonlinear broadening module 3. In this example, two concave mirrors with the same radius of curvature R = 3.2 m are selected, and their equivalent effect on the propagating beam can be approximated as a focusing system with an equivalent focal length φ = 1.6 m. The cavity length is 3.2 m, and the cavity is filled with argon gas at a pressure of 1 bar. The equivalent focusing effect of the mirrors introduces a second phase term into the light field, so that different diffraction orders after beam splitting by the two-dimensional grating can form multiple spatially separated focal points near the focal plane, i.e., the cavity center, and propagate synchronously and interact nonlinearly within the multi-pass cavity.

[0042] Calculations using a nonlinear propagation model in a multi-cavity system show that multiple discrete focal spots are formed at the focal plane. These focal spots are separated from each other in the lateral space, and their number, spatial spacing, and energy distribution ratio are determined by the spatial frequency and phase modulation depth of the two-dimensional phase grating.

[0043] As shown in Figure 3, the two-dimensional phase grating module 2 divides the input beam into multiple spatially separated sub-spots at the center of the multi-pass cavity, thereby effectively reducing the peak light intensity of a single spot within the cavity. Since the sub-pulses are spatially separated and have different focal positions in the multi-pass cavity, problems such as self-focusing, ionization, and damage to optical components under strong nonlinear conditions are effectively suppressed.

[0044] In this embodiment, the geometric cavity length of the multi-cavity nonlinear stretching module 3 is approximately 3.2 m. Without employing a two-dimensional phase grating beam splitting structure, considering the nonlinear phase shift (B integral) of the gas medium within the multi-cavity and the limitation of the gas ionization threshold, the upper limit of the stable transmission single-pulse energy is approximately 0.4 mJ.

[0045] After introducing a two-dimensional grating beam splitting structure, the input beam is decomposed into multiple spatial sub-beams, each of which propagates independently within the multi-pass cavity and exhibits nonlinear broadening. Due to the significant reduction in single-beam peak power, nonlinear phase shift and gas ionization effects are effectively suppressed. Without significantly increasing the geometric length of the multi-pass cavity (approximately 3.2 m), if the light is split into 9 beams using a two-dimensional phase grating, the total input pulse energy that the system can withstand increases by approximately 9 times, corresponding to a laser output energy of approximately 3.6 mJ.

[0046] 4) Realization of spatial bundle convergence After the light field passes through the spatial beam splitting focal point, it continues to propagate a distance of 𝑓 (for example, the equivalent cavity length of a multi-pass cavity can be designed to be about 𝑓) and is re-collimated by a collimating optical element that is conjugate to the aforementioned focusing optical element.

[0047] Subsequently, a two-dimensional phase grating compensation module 4, conjugate with the aforementioned two-dimensional phase grating module 2, is introduced. Its phase modulation function has the opposite sign to that of the two-dimensional phase grating module 2 used in the beam splitting stage, thereby compensating for the previously introduced spatial phase modulation. By precisely adjusting the position and angle of the two-dimensional grating compensation module 4, accurate spatial beam combining of each sub-pulse in the far-field plane is ensured.

[0048] After being compensated by the two-dimensional phase grating, the spatially separated sub-pulses are re-superimposed in the spatial frequency domain.

[0049] 5) Formation and verification of the far-field convergence focal point After completing two-dimensional phase compensation, the light field passes through the focusing optical element again to form a beam-combining focus at the far-field focal plane.

[0050] The intensity distribution of the combined focal plane was calculated, as shown in Figure 4. It can be observed that the peak intensity of the combined focal plane is significantly improved compared with the spatially split focal point, and the focal spot morphology is restored to a single main peak structure.

[0051] By comparing the peak intensities of the spatial beam splitting focus and the beam combining focus, the reversibility of the two-dimensional phase grating in the spatial beam splitting and combining process can be verified, thus proving that the spatial beam splitting-combining scheme described in this invention can achieve peak intensity management in a multi-channel cavity without introducing significant energy loss.

[0052] After dispersion compensation, the combined beam can be compressed into an ultrashort pulse of approximately 18 fs (FWHM). At a pulse energy of 3.6 mJ, the peak power can reach ~171 GW, and the focal peak intensity is approximately 10... 9 W / m². Example

[0053] Implementation of a multi-pulse beam splitting system combining birefringent crystals and two-dimensional gratings This embodiment adds a birefringent crystal time-splitting module to the existing embodiment 1, forming a higher-order multi-pulse beam splitting system, as shown in the example below. Figure 2 As shown, it mainly includes the following modules: Ultrashort pulse laser source module 1: Outputs a single high-energy ultrashort pulse (femtosecond or picosecond) to provide the initial pulse source for the system.

[0054] Birefringent crystal time-splitting module 7: Divides the incident single pulse into multiple sub-pulses with controllable time intervals, providing a basis for subsequent time-splitting and spatial beam splitting.

[0055] Two-dimensional phase grating module 2: Spatially splits each time-divided sub-pulse to form multiple spatially separated sub-pulse beams, further dispersing the light intensity.

[0056] Multi-cavity nonlinear broadening module 3: Each sub-pulse propagates back and forth multiple times along different spatial paths in the multi-cavity, achieving spectral broadening in the nonlinear medium.

[0057] Two-dimensional phase grating compensation module 4: compensates for the phase of the two-dimensional grating, realizes the superposition and beaming of each sub-pulse in the far field space, and restores the spatial characteristics of the pulse.

[0058] Birefringent crystal time combining module 8: Combines sub-pulses from multiple time intervals into a single beam to restore the time characteristics of the pulse.

[0059] Dispersion compensation module 6: Employs dispersion compensation elements such as grating pairs, prism pairs, or chirped mirrors to compensate for the dispersion accumulated during the propagation of the combined beam by adjusting the optical path difference, thereby achieving pulse compression and outputting ultrashort pulses with high peak power and a wide spectrum.

[0060] Its workflow is as follows: The single high-energy ultrashort pulse output by the ultrashort pulse laser source module 1 passes sequentially through the birefringent crystal time beam splitting module 7, the two-dimensional phase grating module 2, the multi-cavity nonlinear broadening module 3, the two-dimensional phase grating compensation module 4, the birefringent crystal time beam combining module 8, and the compression module 6, and finally outputs an ultrashort pulse with high peak power and wide spectrum.

[0061] The specific implementation steps are as follows: 1) Time-splitting of birefringent crystal The incident pulse passes through one or two calcite blocks coated with an anti-reflection coating suitable for the 800 nm wavelength band, thereby reducing interface reflection loss and improving beam transmission efficiency. The calcite crystal splits the pulse into two to four low-energy pulses. For example, in this embodiment, the calcite blocks are 6 mm and 12 mm thick, used to space the split pulses at 3.8 ps intervals. The sub-pulse interval Δt can be adjusted by the crystal thickness and the birefringent crystal material, i.e., the refractive index difference. The time-splitting process is reversible, providing phase and polarization conditions for subsequent time-combining.

[0062] 2) Two-dimensional grating spatial beam splitting After each time-splitting sub-pulse, it is incident on the two-dimensional phase grating module 2 for spatial beam splitting. The same two-dimensional grating parameters as in Example 1 can be used. Spatial beam splitting can generate sub-pulse arrays with different arrangements, such as rectangles and circles, to distribute energy as evenly as possible. The spatially split sub-pulses propagate along different optical paths within the multi-pass cavity, further reducing the load on a single channel.

[0063] 3) Nonlinear widening of multi-cavity tunnels Each sub-pulse propagates back and forth multiple times in the multi-pass cavity, undergoing nonlinear effects such as self-phase modulation and spectral broadening in the nonlinear medium (gas or bulk). Time-splitting divides a single pulse into multiple sub-pulses, and spatial-splitting separates these sub-pulses in space, thereby dispersing the light intensity originally concentrated in a single channel to multiple channels. This effectively reduces the light intensity at the focal point of the multi-pass cavity, mitigating laser self-focusing, ionization, and damage to optical components.

[0064] 4) Two-dimensional grating compensation and spatial beam combining module The output sub-pulse passes through the two-dimensional phase grating compensation module 4, which introduces spatial phase modulation opposite to that of the beam splitter grating. By precisely adjusting the position and angle of the two-dimensional grating compensation module, it is ensured that the sub-pulses achieve spatial beam combining in the far-field plane, forming a single output beam.

[0065] 5) Time Bundle Module After the time-splittered subpulses are spatially combined, they pass through the same birefringent crystal as the time-splittered subpulses, with the optical axis rotated 90 degrees relative to the optical axis of the splitting crystal. When the optical axis of the birefringent crystal rotates 90 degrees, the propagation characteristics of the subpulses in the crystal change, and the time delay originally caused by the birefringence effect is eliminated, thus achieving time alignment. The output pulse energy after temporal beam combining is equal to the total energy of the original single pulse, retaining the broad spectral characteristics after nonlinear broadening; The output ultrashort pulse can be further compressed to achieve a final output with high peak power and a wide spectrum.

[0066] Simulation results show that when only a time-splitting structure is used, the maximum allowable input energy increase of the system is limited. For example, when using two birefringent crystals to achieve four-way time-splitting, the incident pulse is split into four time sub-pulses, each propagating independently in the multi-pass cavity. Since the peak power of a single sub-pulse is reduced by about four times, the maximum allowable input energy of the multi-pass cavity is correspondingly increased to about 1.6 mJ.

[0067] In the time-space joint beam splitting structure proposed in this embodiment, based on the above-mentioned 4-way time beam splitting, 9-way spatial beam splitting is further achieved through a two-dimensional phase grating, bringing the total number of beams in the system to 36. Since each sub-pulse propagates separately in both time and space dimensions, the peak power of a single beam and the nonlinear load are significantly reduced, further increasing the maximum allowable input energy of the multi-channel cavity to approximately 14.4 mJ.

[0068] Compared to using only a time-splitting structure (1.6 mJ) or only a spatial-splitting structure (3.6 mJ), the time-spatial joint splitting structure proposed in this invention can achieve higher energy expansion capability and exhibits a significant synergistic effect.

[0069] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A nonlinear compression method based on spatiotemporal beam splitting, characterized in that, Includes the following steps: (1) A supershort pulse laser is split into at least two separate sub-pulses in the time and space domains by a time-space beam splitting module, such that the peak power of any sub-pulse is lower than the peak power of the initial supershort pulse laser under the unsplit condition. (2) Inject the at least two sub-pulses into the multi-cavity structure, so that each sub-pulse passes through the nonlinear medium at least twice in the multi-cavity to produce spectral broadening; (3) Perform temporal and spatial beam combining on at least two sub-pulses after spectral broadening to obtain the broadened pulse after beam combining; (4) Dispersion compensation is performed on the bundled and broadened pulse to obtain compressed ultrashort pulse output.

2. The method according to claim 1, characterized in that, In step (1), the time-domain beam splitting is achieved through a birefringent crystal, a structure based on a polarization beam splitter and a spatial delay line, or a pulse splitting scheme based on phase modulation.

3. The method according to claim 1, characterized in that, In step (1), the spatial domain beam splitting is achieved by a polarization beam splitter, a polarization-independent partial mirror, or a diffractive optical element, wherein the diffractive optical element includes a two-dimensional phase grating.

4. The method according to claim 1, characterized in that, In step (3), the time beam combining is achieved by an optical delay compensation structure corresponding to the time beam splitting process, so that at least two sub-pulses re-overlap in time.

5. The method according to claim 1, characterized in that, In step (3), the spatial beam combining is achieved by optical elements that are optically conjugate with the spatial beam splitting module, so as to restore the spatial beam shape before beam splitting.

6. A nonlinear compression system based on spatial beam splitting, characterized in that, It includes an ultrashort pulse laser source module, a two-dimensional phase grating module, a multi-cavity nonlinear stretching module, a two-dimensional phase grating compensation module, a far-field beamforming module, and a dispersion compensation module arranged sequentially along the optical path.

7. A nonlinear compression system based on spatiotemporal beam splitting, characterized in that, It includes an ultrashort pulse laser source module, a birefringent crystal time-splitting module, a two-dimensional phase grating module, a multi-cavity nonlinear stretching module, a two-dimensional phase grating compensation module, a birefringent crystal time-combining module, and a compression module arranged sequentially along the optical path.

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