A method and system for femtosecond laser pulse compression

Abstract

The application relates to the field of femtosecond laser pulse compression technology and discloses a femtosecond laser pulse compression method and system, the method of which comprises generating a femtosecond laser pulse to be compressed; interacting the femtosecond laser pulse with plasma to make the femtosecond laser pulse generate linear negative frequency chirp while expanding the spectrum of the femtosecond laser pulse; and performing positive dispersion compensation on the linear negative frequency chirp, so as to obtain a compressed femtosecond laser pulse. The application can solve the problems existing in the prior art, such as the laser pulse energy being limited by the damage threshold, poor spatial uniformity of the compressed light beam, high cost of compression elements, difficulty in realizing nanosecond pulse width and the like.

Description

Technical Field

[0001] This invention belongs to the field of femtosecond laser pulse compression technology, specifically relating to a femtosecond laser pulse compression method and system. Background Technology

[0002] In the field of ultrafast laser science and technology, generating femtosecond laser pulses with extremely short pulse widths and high peak power is one of the core technologies driving the development of cutting-edge scientific research and high-end applications. With the rapid development of fields such as strong-field physics, attosecond science, ultrafast spectroscopy, and precision micro / nano fabrication, increasingly stringent requirements are being placed on the temporal resolution and peak power density of laser sources. Femtosecond laser pulse compression technology, as a key means to achieve ultrashort and ultra-intense laser output, is based on the principle of broadening the spectral bandwidth of the laser pulse through nonlinear effects, followed by precise dispersion compensation, thereby approaching or even reaching its Fourier transform-limited pulse width. Theoretically, pulse width is inversely proportional to spectral bandwidth; therefore, obtaining a broad and smooth spectrum is a prerequisite for realizing sub-10-femtosecond or even single-cycle (<5 fs) ultrashort pulses.

[0003] Currently, the mainstream femtosecond laser pulse compression technologies mainly include: thin-film compression based on solid nonlinear media, multi-cavity compression, gas-filled hollow fiber compression, and plasma-assisted compression.

[0004] 1. Solid-state thin-film compression technology utilizes highly transparent bulk nonlinear media (such as fused silica or sapphire) as the spectral broadening medium. A femtosecond laser sequentially passes through multiple parallel solid thin films, undergoing self-phase modulation (SPM) in each film to gradually broaden the spectrum. To avoid self-focusing and filamentation due to excessively high peak power, multiple thin films are typically combined to distribute the nonlinear effect across multiple low-nonlinear-length units. The broadened pulse needs to be corrected for group delay dispersion and third-order dispersion using chirped mirrors or prisms and equivalent dispersion compensation elements to ultimately achieve pulse width compression. This technology has a simple structure and high stability, and has been widely used in few-period pulse generation systems. However, the spectral uniformity after broadening is poor, often exhibiting oscillations or irregular structures, affecting the temporal quality of the compressed pulse. Furthermore, key parameters such as thin-film spacing, thickness, and material selection lack systematic theoretical modeling and optimization guidance, and experimental debugging heavily relies on experience, resulting in poor system repeatability and insufficient process stability, hindering standardization and mass production applications.

[0005] 2. Multi-cavity compression technology: The multi-cavity consists of two highly reflective concave mirrors, with the cavity filled with a nonlinear gas or placed in air. The input laser undergoes multiple round-trip reflections within the cavity, extending the interaction distance with the medium, thus achieving effective spectral broadening at lower peak power. The inner surface of the cavity mirrors is typically coated with an anti-reflection film, while the outer surface is coated with a high-reflectivity film to ensure stable laser circulation within the cavity. The amount of nonlinear phase accumulation can be flexibly controlled by adjusting parameters such as cavity length, incident angle, and gas pressure. The compressed pulse still requires dispersion compensation using chirped mirrors. This technology has advantages such as large aperture, high beam quality tolerance, and energy conversion efficiency exceeding 90%. However, limitations in the reflectivity of the cavity mirrors (typically <99.9%), group delay dispersion accumulation, and the laser damage threshold of optical components make it difficult to achieve compressed pulse output below 20 fs in a single-stage system. Furthermore, the spatial distortion and mode mismatch introduced by multiple reflections also affect the output beam quality.

[0006] 3. Gas-filled hollow fiber compression technology: This technology uses hollow glass fiber as the nonlinear medium, filled with an inert gas (such as argon, krypton, or xenon). When the laser propagates in the fiber, the intensity is significantly enhanced due to the narrow mode field diameter, thus efficiently exciting the Kerr effect and ionization effect, achieving broadband spectral broadening. Due to the low nonlinear coefficient and high damage threshold of the gas, extremely broad spectra can be obtained under relatively low nonlinear conditions. The dispersion after compression is usually compensated by prism pairs or chirped mirror groups. This technology has successfully achieved near-single-cycle pulse output. However, due to the mode field limitation and mechanical strength of the fiber itself, the input pulse energy is usually limited to the nJ range, making it difficult to meet the requirements of high-energy applications; in addition, the plasma effect caused by gas ionization may lead to mode instability, spectral distortion, or even fiber damage; and the hollow fiber is difficult to align, resulting in high system integration complexity.

[0007] 4. Plasma compression technology: This type of technology utilizes the nonlinear process in the interaction between laser and plasma to achieve pulse compression. It mainly includes: Plasma back-propagating Raman amplification: A long pump pulse and a short, back-propagating seed pulse are used to excite electron-plasma waves in the plasma, achieving energy transfer and pulse compression through three-wave coupling; Strongly coupled Brillouin amplification: Relying on the coupling of ion acoustic waves and light waves, pulse time compression is achieved; Rapidly extending plasma grating: A laser beam is first used to ionize gas to form a dynamic plasma grating, and then this moving grating reflects the back-pumping light, utilizing the Doppler frequency shift mechanism to achieve pulse compression.

[0008] Theoretically, this type of technology can withstand extremely high light intensities and is suitable for post-stage compression in petawatt-level ultrafast laser systems. Although it has extremely high damage threshold potential, it generally suffers from problems such as low gain efficiency (typically below 20%), stringent phase matching conditions, and poor system stability; moreover, most schemes require the coordinated action of multiple laser beams (such as pump light, seed light, and ionizing light), resulting in complex equipment, difficult control, and limited practicality.

[0009] All of the above techniques primarily introduce normal dispersion (positive group velocity dispersion) during spectral broadening, thus requiring external negative dispersion components (such as grating pairs, prism pairs, and chirped mirrors) for compensation. However, these components themselves have significant limitations: gratings and prism pairs are susceptible to material damage threshold limitations, requiring larger beam sizes to reduce power density, resulting in bulky systems and high manufacturing costs for large-size gratings; while chirped mirrors are limited by coating processes, making it difficult to cover ultra-wideband spectra (>500nm), and the design and fabrication of large-aperture chirped mirrors are complex and extremely costly. Summary of the Invention

[0010] To address the shortcomings of existing technologies, a femtosecond laser pulse compression method and system are proposed to solve problems such as laser pulse energy being limited by the damage threshold, poor spatial uniformity of the compressed beam, high cost of compression components, and difficulty in achieving femtosecond pulse widths during the pulse width compression process.

[0011] To achieve the above objectives, the present invention provides the following technical solution: In a first aspect, the present invention provides a femtosecond laser pulse compression method, comprising the following steps: S100: Generates the femtosecond laser pulse to be compressed; S200: The femtosecond laser pulse interacts with the plasma to broaden its spectrum while generating a linear negative frequency chirp in the femtosecond laser pulse; S300: Positive dispersion compensation is performed on the linear negative frequency chirp to obtain a compressed femtosecond laser pulse.

[0012] The technical solution is further configured such that, in step S200, the femtosecond laser pulse is focused so that it interacts with the plasma at the focal point.

[0013] The technical solution is further configured such that, in step S300, the femtosecond laser pulse after being acted upon by the plasma is transmitted through the dispersion compensation unit, and the positive dispersion provided by the dispersion compensation unit is used to perform dispersion compensation on the linear negative frequency chirp. The dispersion compensation unit is made of a dispersion compensation optical medium that has normal dispersion characteristics within the femtosecond laser pulse spectrum range.

[0014] The technical solution is further configured such that the thickness of the dispersion compensation optical medium is determined based on the group delay dispersion generated by the femtosecond laser pulse after being acted upon by plasma and the group velocity dispersion coefficient of the dispersion compensation optical medium. The group delay dispersion is calculated based on the pulse width and spectral width of the femtosecond laser pulse after plasma interaction.

[0015] This technical solution is further configured such that the dispersion compensation optical medium is composed of a single piece of optical glass; Alternatively, the dispersion-compensating optical medium may be composed of a combination of multiple optical glass sheets.

[0016] This technical solution is further configured such that, between step S200 and step S300, it also includes: The divergent laser beam emitted from the plasma is collimated to make it parallel before being incident on the dispersion-compensating optical medium.

[0017] The technical solution is further configured such that, in step S300, the divergent laser beam emitted from the plasma is first transmitted through the dispersion compensation optical medium for dispersion compensation, and then the laser beam is collimated.

[0018] In a second aspect, the present invention provides a femtosecond laser pulse compression system for implementing the femtosecond laser pulse compression method, comprising: The laser generation unit is used to generate the femtosecond laser pulses to be compressed; A plasma interaction unit is used to interact with the femtosecond laser pulse to broaden its spectrum and generate a linear negative frequency chirp; And a dispersion compensation unit, which is disposed in the laser pulse transmission optical path after being acted upon by the plasma action unit, is used to perform positive dispersion compensation on the laser pulse.

[0019] This technical solution is further configured to include a laser focusing unit disposed between the laser generating unit and the plasma interaction unit, for focusing the femtosecond laser pulse to the location of the plasma interaction unit.

[0020] This technical solution is further configured to include a beam collimation unit, which is disposed in the optical path between the plasma action unit and the dispersion compensation unit, or in the optical path after the dispersion compensation unit. The beneficial effects of this invention are: 1. Using plasma as a nonlinear medium for spectral broadening eliminates optical damage threshold limitations, enabling the compression of high-energy femtosecond laser pulses at the Joule level or even higher. Simultaneously, plasma possesses an extremely wide support bandwidth, allowing for sufficient spectral broadening and providing the possibility of generating extremely short pulses of several femtoseconds or even near-single-cycle duration. Furthermore, the plasma wake field excited by the laser pulse acts on the femtosecond laser, broadening the spectrum while simultaneously generating a linear negative frequency chirp in the output femtosecond laser pulse, providing the prerequisite for subsequent dispersion compensation using ordinary optical glass.

[0021] 2. By utilizing the normal dispersion characteristics of the dispersion compensation unit, positive dispersion compensation is provided for femtosecond laser pulses with negative chirp. This eliminates the expensive and complex chirped mirrors, grating pairs, or prism pairs used in traditional compression techniques. Instead, ordinary optical glass is innovatively used as the core dispersion management element, which greatly reduces system cost and complexity. Attached Figure Description

[0022] Figure 1 This is a flowchart of the femtosecond laser pulse compression method in an embodiment of the present invention; Figure 2 This is a schematic diagram of the electric field intensity and time of the femtosecond laser pulse to be compressed in an embodiment of the present invention; Figure 3 This is a schematic diagram of the waveform and time of the femtosecond laser pulse to be compressed in an embodiment of the present invention; Figure 4 This is a schematic diagram showing the electric field intensity and time of a femtosecond laser pulse after plasma interaction in an embodiment of the present invention; Figure 5 This is a schematic diagram of the femtosecond laser pulse waveform and time after plasma interaction in an embodiment of the present invention; Figure 6 This is a schematic diagram of the electric field intensity and time of the compressed femtosecond laser pulse in an embodiment of the present invention; Figure 7 This is a schematic diagram of the compressed femtosecond laser pulse waveform and time in an embodiment of the present invention; Figure 8 This is a schematic diagram of the dispersion curves of fused silica glass to laser pulses of different frequencies in an embodiment of the present invention; Figure 9 This is a schematic diagram of the laser pulse spectrum before and after plasma interaction in an embodiment of the present invention; Figure 10 This is a schematic diagram of the linear chirp distribution of the laser pulse after plasma interaction in an embodiment of the present invention; Figure 11 This is a schematic diagram of the lateral distribution of laser pulses after plasma interaction in an embodiment of the present invention; Figure 12This is a schematic diagram of a femtosecond laser pulse compression system in an embodiment of the present invention. Detailed Implementation

[0023] To enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. Based on the embodiments in this application, other similar embodiments obtained by those skilled in the art without creative effort should all fall within the scope of protection of this application. Furthermore, directional terms mentioned in the following embodiments, such as "up," "down," "left," and "right," are only for reference to the directions in the accompanying drawings; therefore, the directional terms used are for illustrative purposes and not for limiting the invention.

[0024] Example 1: According to an embodiment of the present invention, a femtosecond laser pulse compression method is provided. Please refer to [link to relevant documentation]. Figure 1 This includes the following steps: S100: Generates the femtosecond laser pulse to be compressed.

[0025] S200: The femtosecond laser pulse interacts with the plasma to broaden its spectrum while generating a linear negative frequency chirp in the femtosecond laser pulse.

[0026] Understandably, using plasma as a nonlinear medium for spectral broadening, without optical damage threshold limitations, can support the compression of high-energy femtosecond laser pulses at the Joule level or even higher. At the same time, plasma has an extremely wide support bandwidth, which can fully broaden the spectrum and make it possible to generate extremely short pulses of several femtoseconds or even near single cycles.

[0027] Compared to traditional plasma compression techniques, this invention does not rely on energy transfer. Instead, it utilizes the plasma wake field excited by a laser pulse to act on a femtosecond laser. This broadens the spectrum while simultaneously generating a linear negative frequency chirp in the output femtosecond laser pulse, meaning the laser pulse has a high leading-edge frequency (short wavelength) and a low trailing-edge frequency (long wavelength). In other words, by utilizing the self-modulation effect to broaden the spectrum and simultaneously introducing controllable chirp, the system is simplified (requiring only a single laser beam), and subsequent dispersion compensation using ordinary optical glass becomes possible.

[0028] Furthermore, focusing the femtosecond laser pulse so that it interacts with the plasma at the focal point means that the plasma needs to be precisely positioned at the focal point or focal region of the focused laser pulse.

[0029] Optionally, plasma can be obtained by ionization of gas jets from a gas nozzle or by ionization of a gas chamber.

[0030] The core mechanism of this invention lies in utilizing the interaction between femtosecond laser pulses and plasma to achieve spectral broadening. The strength of this interaction, i.e., the strength of the nonlinear effect, directly depends on the power density of the laser pulse. By focusing, the laser energy can be concentrated onto an extremely small focal spot area, typically tens of micrometers in size, thereby increasing the power density of the laser pulse in the focal region and achieving a high power density state. The spectral broadening process occurs in a highly focused, tiny plasma region, ensuring good spatial uniformity of the broadened beam. Simultaneously, because plasma has no damage threshold limitation, it can withstand extremely high power densities without being damaged. This is precisely the key to this invention's ability to handle high-energy laser pulses—it concentrates energy into plasma, a non-destructive medium, thereby circumventing the damage threshold limitations of traditional solid media.

[0031] S300: Positive dispersion compensation is performed on the linear negative frequency chirp to obtain a compressed femtosecond laser pulse.

[0032] Furthermore, the femtosecond laser pulse, after being processed by the plasma, is transmitted through a dispersion compensation unit, and the positive dispersion provided by the dispersion compensation unit is used to compensate for the linear negative frequency chirp. The dispersion compensation unit is made of a dispersion compensation optical medium that has normal dispersion characteristics within the spectral range of the femtosecond laser pulse.

[0033] Understandably, by utilizing the normal dispersion characteristics of the dispersion compensation unit, positive dispersion compensation is provided for femtosecond laser pulses with negative chirp. This eliminates the expensive and complex chirped mirrors, grating pairs, or prism pairs used in traditional compression techniques, and innovatively uses ordinary optical glass as the core dispersion management element, greatly reducing system cost and complexity.

[0034] Furthermore, the thickness of the dispersion compensation optical medium is determined based on the group delay dispersion generated by the femtosecond laser pulse after being subjected to plasma and the group velocity dispersion coefficient of the dispersion compensation optical medium; the group delay dispersion is calculated based on the pulse width and spectral width of the femtosecond laser pulse after being subjected to plasma.

[0035] The specific calculation process is as follows: 1. Calculation of laser dispersion after plasma treatment: The group delay dispersion of a laser pulse satisfies the following formula: Simplified calculations yield: Where GDD is the group delay dispersion of the laser pulse. The pulse width of the laser pulse after dispersion. The center wavelength of the laser pulse. The spectral width of the laser pulse. This represents the pulse width of the laser pulse in the dispersive state.

[0036] 2. Select a suitable dispersion compensation optical medium: its transmittance curve should cover the laser pulse spectrum after plasma action, and it should have normal dispersion within the laser pulse spectrum range. The higher the transmittance, the better.

[0037] 3. Calculate the group velocity dispersion coefficient (GVD) of the dispersion-compensating optical medium: Calculate the material's refractive index by referring to tables or using the Sellmeier equation. Then, it is obtained through the following relationship: , The speed of light in a vacuum. For the refractive index of the material, The wavelength of the laser is n, and the refractive index of the material is n = λ / 2. function ,Pick Then the dispersion-compensating optical medium can be used to calculate the center wavelength. GVD of laser pulses.

[0038] 4. Calculate the thickness of the required dispersion compensation optical medium: .

[0039] Furthermore, the dispersion compensation optical medium is composed of a monolithic optical glass, the thickness of which is equal to the thickness of the dispersion compensation optical medium. A monolithic optical glass of a specific thickness can provide the simplest structural layout.

[0040] Furthermore, the dispersion compensation optical medium can also achieve the same dispersion compensation effect by combining multiple optical glass sheets. The multi-sheet layout eliminates the need for customized thickness, offering operational flexibility. While stacking multiple sheets to achieve the same thickness can be achieved, some energy loss occurs due to multiple incident events.

[0041] Furthermore, the material of the dispersion compensation optical medium can be selected according to the required wavelength band. The primary principle is that its wavelength-transmittance curve has a high and flat transmittance within the spectral range of the femtosecond laser pulse to be compressed.

[0042] It is understandable that dispersion-compensating optical media have different refractive indices for different frequency components of a laser pulse, resulting in different group velocities for these different frequency components. Taking fused silica glass as an example, its refractive index for different frequency components, such as... Figure 8As shown in the attached figure, the typical wavelength range of ultrashort, ultra-intense lasers, from 750nm to 1200nm, is captured. This range falls within the normal dispersion range of fused silica glass, specifically exhibiting a decrease in refractive index as the laser pulse wavelength increases. Correspondingly, during laser pulse propagation through fused silica glass, the longer the wavelength, the faster the group velocity, and the shorter the wavelength, the slower the group velocity, with the group velocity curve exhibiting good approximate linearity. After plasma treatment, the high-frequency (short-wavelength) components of the laser pulse precede the low-frequency (long-wavelength) components, and the frequency exhibits a linear relationship with the preceding and following positions. By selecting a suitable thickness of fused silica glass, the lagging long-wavelength components in the laser pulse can catch up with the preceding short-wavelength components, thereby achieving a chirp-free, near-Fourier transform limit short pulse width.

[0043] In the femtosecond laser pulse compression method of this embodiment, please refer to... Figure 1 Between step S200 and step S300, the following is also included: The divergent laser beam emitted from the plasma is collimated to make it parallel before being incident on the dispersion-compensating optical medium.

[0044] It is understandable that the wavefront of collimated parallel light is a plane. When this parallel light beam passes perpendicularly through the dispersion-compensating optical medium, the physical path lengths of the light rays at all points within the beam's cross-section are strictly equal. This means that for the same frequency component, regardless of its location in the beam (center or edge), the amount of dispersion experienced is completely consistent, without introducing spatial phase distortion. The resulting compressed pulse not only has the narrowest time pulse width but also best preserves the spatial wavefront quality, which is beneficial for subsequent applications requiring high beam quality. Simultaneously, the collimated beam expands the spot area, avoiding damage to the dispersion-compensating optical medium. This method is suitable for cutting-edge research fields with stringent requirements for laser pulse quality.

[0045] In some other embodiments, the divergent laser beam emitted from the plasma is first transmitted through the dispersion-compensating optical medium for dispersion compensation, and then the laser beam is collimated.

[0046] Understandably, for a diverging laser beam, the central ray has the shortest path through the dispersion-compensating optical medium, while the peripheral rays, due to their oblique incidence, have longer actual paths. This results in slight differences in the physical paths experienced by the frequency components of the diverging laser beam at different spatial locations, leading to varying degrees of dispersion and introducing spatial phase distortion. This method is suitable for industrial applications or high-energy laser facilities.

[0047] In summary, the workflow of this invention embodies a unique integration of inventive points: femtosecond laser pulse to be compressed → focusing → interaction with plasma (generating spectral broadening and linear negative chirp) → beam collimation → passing through a dispersion-compensating optical medium (performing positive dispersion compensation and eliminating chirp) → outputting a compressed pulse close to the Fourier transform limit. This process seamlessly integrates the highly nonlinear effects of plasma with simple linear dispersion compensation, aiming to overcome the bottlenecks of existing technologies (such as solid-state thin films, multi-cavity structures, gas-filled hollow optical fibers, and plasma compression) in terms of energy tolerance limits, spatial uniformity, system cost, and compressed pulse width.

[0048] Specifically, the laser pulse to be compressed is often already at the Fourier transform-limited pulse width, exhibiting a chirp-free state in its frequency structure, meaning the frequency is consistent across different positions before and after the laser pulse, such as... Figure 2 , Figure 3 As shown. To further compress the femtosecond laser pulse to be compressed, the frequency of the laser pulse needs to be broadened. This invention utilizes the interaction between plasma and the femtosecond laser pulse to be compressed to achieve spectral broadening. Furthermore, to ensure that the laser pulse after spectral broadening by plasma can be effectively compressed, during the plasma interaction process, the laser pulse needs to simultaneously generate linear negative frequency chirp, i.e., a high leading-edge frequency (short wavelength) and a low trailing-edge frequency (long wavelength), such as... Figure 4 , Figure 5 As shown. Therefore, after being subjected to plasma, the spectrum of the femtosecond laser pulse to be compressed is broadened, while possessing a linear negative frequency chirp. Due to the presence of the frequency chirp, the laser pulse after plasma treatment is not at the Fourier transform limit state, and positive dispersion needs to be introduced through a dispersion compensation optical medium. Specifically, the long-wavelength components of the laser propagate faster in the dispersion compensation optical medium, while the short-wavelength components propagate slower. By matching a dispersion compensation optical medium of appropriate thickness, the frequency chirp of the laser pulse can be compensated. Ultimately, the frequency components distributed at different positions before and after can be tuned to the same time point, thereby obtaining the compressed laser pulse, as shown. Figure 6 , Figure 7 As shown.

[0049] Example 2: According to an embodiment of the present invention, a femtosecond laser pulse compression system for implementing the aforementioned femtosecond laser pulse compression method is provided. Please refer to [link to relevant documentation]. Figure 12 ,include: The laser generation unit is used to generate the femtosecond laser pulses to be compressed; A plasma interaction unit is used to interact with the femtosecond laser pulse to broaden its spectrum and generate a linear negative frequency chirp; And a dispersion compensation unit, which is disposed in the laser pulse transmission optical path after being acted upon by the plasma action unit, is used to perform positive dispersion compensation on the laser pulse.

[0050] Optionally, the laser generating unit may be a femtosecond laser.

[0051] In the femtosecond laser pulse compression system of this embodiment, please refer to Figure 12 It also includes a laser focusing unit disposed between the laser generating unit and the plasma interaction unit, used to focus the femtosecond laser pulse to the location of the plasma interaction unit.

[0052] Optionally, the laser focusing unit can be a convex lens, an off-axis parabolic mirror, or a concave mirror.

[0053] In the femtosecond laser pulse compression system of this embodiment, please refer to Figure 12 It also includes a beam collimation unit, which is disposed in the optical path between the plasma action unit and the dispersion compensation unit, or in the optical path after the dispersion compensation unit.

[0054] Optionally, the beam collimation unit can be a convex lens, an off-axis parabolic mirror, or a concave mirror.

[0055] In one specific embodiment, the initial femtosecond laser pulse to be compressed has an energy of 0.35 J, a pulse width of 25 fs, a center wavelength of 800 nm, and a spectral width of 60 nm (770-830 nm). The femtosecond laser to be compressed is focused onto the plasma interaction unit by a laser focusing unit, with a laser focal spot radius of 40 μm and a laser focusing normalized intensity a0 = 0.6. The plasma interaction unit generates plasma through a supersonic gas nozzle, which is ionized by the pre-pulse of the femtosecond laser pulse to be compressed, forming plasma with a plasma density of... The plasma length is 1 cm.

[0056] After the focused femtosecond laser pulse to be compressed interacts with the plasma, the spectrum broadens to 120 nm (810-930 nm), the center wavelength is 850 nm, the pulse width is 26.5 fs, and the corresponding GDD is 85 fs. 2 The Fourier-limited pulse width corresponding to this spectral range is 8.8 fs. The comparison of spectral broadening before and after plasma interaction is as follows: Figure 9 As shown. The linear spectral chirp distribution of the femtosecond laser pulse after plasma interaction is as follows. Figure 10 The figure shows that the generated frequency chirp has good linear characteristics. The transverse intensity distribution of the femtosecond laser pulse after plasma interaction is as follows: Figure 11As shown, the interaction process is spatially uniform, without splitting, twisting, or irregular hot spots, indicating that the beam quality is maintained and the nonlinear effect occurs uniformly across the cross-section. Regarding energy conversion efficiency, the laser pulse energy is 0.36 J before plasma interaction and 0.30 J after plasma interaction, with an energy efficiency of 83.3% during the plasma dispersion broadening stage.

[0057] The dispersion compensation unit used to compensate for dispersion employs BK7 glass material, with a thickness of 2.1 mm to compensate for GDD introduced by the plasma. This ultimately achieves near-Fourier transform limit with a laser pulse length of 8.8 fs.

[0058] In another embodiment, the dispersion compensation unit used to compensate for dispersion employs fused silica (FS) material, with a thickness of 2.6 mm to compensate for GDD introduced by the plasma. This ultimately achieves a compressed pulse width close to the Fourier transform limit.

[0059] Understandably, the successful compression using two different glass materials (BK7 and FS) demonstrates that this invention is insensitive to the specific material of the dispersion compensation unit, exhibiting material universality. Furthermore, both BK7 and FS are common and inexpensive materials, a stark contrast to the high-cost chirped mirrors or grating pairs traditionally required to achieve similar performance. Moreover, the calculation and application of different precise thicknesses for different materials strongly validates the accuracy and reliability of the thickness calculation method presented in this invention. It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of this application described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0060] Optionally, specific examples in this embodiment can refer to the examples described in the above embodiments, and will not be repeated here.

[0061] The sequence numbers of the embodiments in this application are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0062] In the above embodiments of this application, the descriptions of each embodiment have different focuses. For parts not described in detail in a certain embodiment, please refer to the relevant descriptions of other embodiments.

[0063] The above description is only a preferred embodiment of this application. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of this application, and these improvements and modifications should also be considered within the scope of protection of this application.

Claims

1. A femtosecond laser pulse compression method, characterized in that, Includes the following steps: S100: Generates the femtosecond laser pulse to be compressed; S200: The femtosecond laser pulse interacts with the plasma to broaden its spectrum while generating a linear negative frequency chirp in the femtosecond laser pulse; S300: The femtosecond laser pulse after being processed by the plasma is transmitted through the dispersion compensation unit, and the positive dispersion provided by the dispersion compensation unit is used to compensate for the dispersion of the linear negative frequency chirp, thereby obtaining a compressed femtosecond laser pulse. The dispersion compensation unit is made of a dispersion compensation optical medium that has normal dispersion characteristics within the femtosecond laser pulse spectrum range.

2. The femtosecond laser pulse compression method according to claim 1, characterized in that, In step S200, the femtosecond laser pulse is focused so that it interacts with the plasma at the focal point.

3. The femtosecond laser pulse compression method according to claim 1, characterized in that, The thickness of the dispersion compensation optical medium is determined based on the group delay dispersion generated by the femtosecond laser pulse after being acted upon by plasma and the group velocity dispersion coefficient of the dispersion compensation optical medium. The group delay dispersion is calculated based on the pulse width and spectral width of the femtosecond laser pulse after plasma interaction.

4. The femtosecond laser pulse compression method according to claim 3, characterized in that, The dispersion-compensating optical medium is composed of a single piece of optical glass. Alternatively, the dispersion-compensating optical medium may be composed of a combination of multiple optical glass sheets.

5. The femtosecond laser pulse compression method according to claim 1, characterized in that, Between step S200 and step S300, the following is also included: The divergent laser beam emitted from the plasma is collimated to make it parallel before being incident on the dispersion-compensating optical medium.

6. The femtosecond laser pulse compression method according to claim 1, characterized in that, In step S300, the divergent laser beam emitted from the plasma is first transmitted through the dispersion compensation optical medium for dispersion compensation, and then the laser beam is collimated.

7. A femtosecond laser pulse compression system for implementing the femtosecond laser pulse compression method according to any one of claims 1-6, comprising: The laser generation unit is used to generate the femtosecond laser pulses to be compressed; A plasma interaction unit is used to interact with the femtosecond laser pulse to broaden its spectrum and generate a linear negative frequency chirp; And a dispersion compensation unit, which is disposed in the laser pulse transmission optical path after being acted upon by the plasma action unit, is used to perform positive dispersion compensation on the laser pulse.

8. A femtosecond laser pulse compression system according to claim 7, characterized in that, It also includes a laser focusing unit disposed between the laser generating unit and the plasma interaction unit, used to focus the femtosecond laser pulse to the location of the plasma interaction unit.

9. A femtosecond laser pulse compression system according to claim 7, characterized in that, It also includes a beam collimation unit, which is disposed in the optical path between the plasma action unit and the dispersion compensation unit, or in the optical path after the dispersion compensation unit.

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