A method and system for measuring the self-focusing critical power of a femtosecond vortex beam

Abstract

The application discloses a kind of femtosecond vortex light beam self-focusing critical power determination method and system, belong to self-focusing critical power determination technical field.The present application includes femtosecond laser output energy adjusting device, vortex light beam generating device and fluorescence collection device, femtosecond laser output energy adjusting device is used to adjust the energy of incident femtosecond laser pulse, vortex light beam generating device is used to convert the femtosecond laser pulse after adjusting energy into femtosecond vortex light beam and generate plasma filament, fluorescence collection device is used to collect and monitor the lateral fluorescence signal of femtosecond vortex light beam.The present application can simply and sensitively quantitatively determine the self-focusing critical power of femtosecond vortex light beam, improve the accuracy and stability of determination result, is conducive to the further application of self-focusing critical power of femtosecond vortex light beam;Solve the problem that "self-focusing critical power of vortex light beam cannot be quantitatively determined" in prior art.

Description

Technical Field

[0001] This application relates to the field of vortex beam technology, and in particular to a method and system for measuring the self-focusing critical power of a femtosecond vortex beam. Background Technology

[0002] The statements in this section merely refer to the background art relevant to this application and do not necessarily constitute prior art.

[0003] In 1995, Professor G. Mourou's team (winner of the 2018 Nobel Prize in Physics) discovered that high-power femtosecond lasers could form plasma filaments up to 20 meters long and about 100 micrometers in diameter in the air, thus pioneering the field of femtosecond laser filamentation.

[0004] Femtosecond laser filamentation refers to the formation of plasma filaments by high-power ultrafast laser pulses propagating in an optical medium. These filaments, through the combined effects of Kerr self-focusing and plasma defocusing, travel a distance far exceeding the Rayleigh distance while maintaining a relatively constant diameter and laser intensity. Femtosecond laser filamentation exhibits very unique physical properties; for example, when a high-power femtosecond laser propagates through air, the laser intensity of the filament remains at 10⁻⁶. 13 -14W / cm 2 Electron density is 10 16-18 cm -3 With filament lengths reaching hundreds or even thousands of meters, the resulting supercontinuum radiation spectrum can cover the range from ultraviolet to mid-infrared. Therefore, femtosecond laser filamentation has attracted the attention of many researchers internationally since its discovery and has quickly become a cutting-edge field in ultrafast optics.

[0005] Vortex light, as a special type of optical field, has brought about many novel physical phenomena, created new opportunities for numerous application fields, and spurred entirely new scientific applications. It has already been widely used in optical tweezers or microparticle optical manipulation, optical communication, laser filamentation, laser micromachining, laser spectroscopy, astronomical observation, terahertz, and extreme ultraviolet radiation, greatly promoting the development of these fields. In recent years, the field of optical vortices has developed rapidly, with breakthroughs emerging continuously. Vortex femtosecond laser filamentation combines the unique properties of optical vortices with the ultra-intense and ultra-fast nature of femtosecond pulses, predictably producing many extraordinary physical phenomena and unprecedented applications. With the diversification and gradual maturation of high-power vortex femtosecond laser generation methods, many research institutions and companies both domestically and internationally have joined this field, conducting a series of studies on the filamentation and transmission characteristics, supercontinuum radiation, third harmonic generation, and micromachining of vortex femtosecond lasers. It has shown promising application prospects in waveguide laser micromachining, laser spectroscopy, ultrashort pulse generation, and even fast ignition laser nuclear fusion.

[0006] The self-focusing critical power of vortex femtosecond lasers in optical media is considered a key parameter for whether a laser can form a filament. It is also a key physical quantity for predicting whether multiple filaments can be generated and estimating the number of multiple filaments. Determining its value has important guiding significance for both basic research and its applications.

[0007] However, for the past two decades since the research on vortex femtosecond filamentation began, researchers have still been using the self-focusing critical power expression for continuous vortex light obtained by V. Kruglov et al. [J. Mod. Opt. 39, 2277 (1992)]. Although researchers have continuously conducted further explorations based on this expression [Phys. Rev. A 77, 045803 (2008); Phys. Rev. A100, 013836 (2019)], there are still significant deviations from experimental results. This is mainly because the expression is derived under the premise of continuous beams, without considering beam splitting caused by modulation instability, and without considering the influence of laser pulse width. This indicates that the current theoretical expression needs to be modified or reconstructed before it can be used for vortex femtosecond lasers, and such modification or reconstruction requires experimental data support. However, to date, there has been no research internationally on the quantitative determination of the self-focusing critical power of vortex femtosecond lasers. Currently, only one study related to self-focusing critical power exists: the experimental study of vortex femtosecond laser filamentation in air by P. Polynkin et al. at the University of Arizona [Phys. Rev. Lett. 111(2), 023901(2013)]. However, as the researchers stated in their report, the measurement method they used is unsuitable for quantitative measurement of self-focusing critical power. One main reason is the lack of a reference standard for selecting the threshold of the experimental signal, requiring subjective setting. The second main reason is that, as stated in their report, they did not find a suitable method for quantitatively measuring self-focusing critical power. Then, through fitting the experimental data, they obtained the relationship between self-focusing critical power and topological charge number, providing the only experimental data support for self-focusing critical power in the research and application of vortex beam filamentation.

[0008] As can be seen from the above analysis, femtosecond vortex lasers are not suitable for directly using the self-focusing critical power expression for continuous vortex beams. Existing methods cannot quantitatively measure the self-focusing critical power of vortex beams, which affects the accuracy of self-focusing critical power measurement and thus affects the research and various applications of femtosecond vortex lasers. Summary of the Invention

[0009] To address the shortcomings of existing technologies, this application provides a method and system for determining the self-focusing critical power of a femtosecond vortex beam. The method uses a photomultiplier tube (PMT) to measure the ionized fluorescence radiation in air to determine the self-focusing critical power of the femtosecond vortex beam in air. This method can obtain the self-focusing critical power of the femtosecond vortex beam in air and can also be used to determine the self-focusing critical power of other structured beams, such as Airy beams, ring Airy beams, Bessel beams, etc.

[0010] In one aspect, this application provides a femtosecond vortex beam self-focusing critical power measurement system;

[0011] A femtosecond vortex beam self-focusing critical power measurement system includes:

[0012] A femtosecond laser output energy adjustment device, wherein the femtosecond laser output energy adjustment device is used to adjust the energy of the incident femtosecond laser pulse;

[0013] A vortex beam generating device is used to convert a femtosecond laser pulse with adjusted energy into a femtosecond vortex beam and generate a plasma filament.

[0014] A fluorescence collection device for collecting and monitoring the lateral fluorescence signal of a femtosecond vortex beam.

[0015] Furthermore, the femtosecond laser output energy adjustment device includes a half-wave plate and a polarizer arranged sequentially on the same optical axis. The femtosecond laser pulse passes through the half-wave plate and the polarizer in sequence. By rotating the angle of the half-wave plate, the angle between the half-wave plate and the polarizer is changed, thereby realizing continuous adjustment of the energy of the input femtosecond laser pulse.

[0016] Furthermore, the vortex beam generating device includes a first quarter-wave plate, a vortex wave plate, and a focusing lens arranged sequentially on the same optical axis;

[0017] The first quarter-wave plate is used to convert the polarization state of the femtosecond laser pulse from linear polarization to circular polarization;

[0018] The vortex waveplate is used to convert circularly polarized femtosecond laser pulses into circularly polarized femtosecond vortex beams.

[0019] The focusing lens is used to focus and ionize air as the femtosecond vortex beam passes through, generating plasma filaments.

[0020] Furthermore, the vortex beam generating device also includes a second quarter-wave plate disposed between the vortex wave plate and the focusing lens;

[0021] The second quarter-wave plate is used to modulate a circularly polarized femtosecond vortex beam into a linearly polarized femtosecond vortex beam.

[0022] Furthermore, the fluorescence collection device includes a collection lens, a filter, a photomultiplier tube, and an oscilloscope;

[0023] The collecting lens is used to collect the lateral fluorescence signal of the femtosecond vortex beam. The lateral fluorescence signal enters the photomultiplier tube through a filter, and the oscilloscope is used to monitor and record the intensity of the lateral fluorescence signal.

[0024] Furthermore, the filter includes a bandpass filter and an ultraviolet lowpass filter sequentially arranged on the same optical axis;

[0025] The bandpass filter is used to filter the fluorescence spectrum in the gaseous medium, and the ultraviolet low-pass filter is used to filter stray light in the environment.

[0026] Secondly, this application provides a method for determining the critical power of a femtosecond vortex beam self-focusing;

[0027] A method for determining the self-focusing critical power of a femtosecond vortex beam, which employs the aforementioned femtosecond vortex beam self-focusing critical power determination system, includes the following steps:

[0028] Step 1: Turn on the femtosecond laser to generate femtosecond laser pulses. The femtosecond laser pulses enter the femtosecond laser output energy adjustment device to obtain energy-adjusted femtosecond laser pulses.

[0029] Step 2: The energy-adjusted femtosecond laser pulse enters the vortex beam generator to produce a femtosecond vortex beam and a plasma filament;

[0030] Step 3: The fluorescence collection device collects and monitors the lateral fluorescence signal of the femtosecond vortex beam;

[0031] Step 4: Record the signal intensity of femtosecond laser pulses with different energies and the corresponding lateral fluorescence signals. Calculate the self-focusing critical power of the femtosecond vortex beam based on the trend of signal intensity variation.

[0032] Furthermore, the energy of the femtosecond laser pulse can be adjusted by regulating the angle of the half-wave plate.

[0033] Furthermore, the energy-adjusted femtosecond laser pulses sequentially pass through the first quarter-wave plate, the vortex wave plate, and the second quarter-wave plate to generate a femtosecond vortex beam; the femtosecond vortex beam is focused in the air by a focusing lens to ionize the air and generate a plasma filament.

[0034] Furthermore, the collecting lens collects the lateral fluorescence signal of the femtosecond vortex beam. The lateral fluorescence signal passes sequentially through a bandpass filter and a UV low-pass filter into the photomultiplier tube. The photomultiplier tube converts the lateral fluorescence signal into an electrical signal and transmits it to an oscilloscope for monitoring and recording.

[0035] Furthermore, the femtosecond laser pulses are generated by a femtosecond laser amplifier with a center wavelength of 800 nm, a pulse duration of 65 fs, a repetition frequency of 1 kHz, and a maximum pulse capability of 6 mJ.

[0036] Compared with the prior art, the beneficial effects of this application are:

[0037] 1. The technical solution provided in this application measures the self-focusing critical power of a femtosecond vortex beam in air by measuring the ionized fluorescence radiation in the air using a photomultiplier tube, thereby achieving quantitative analysis of the self-focusing critical power. The method is simple and does not require expensive imaging equipment such as CCD or ICCD, thus reducing the measurement cost.

[0038] 2. The technical solution provided in this application enables the photomultiplier tube to collect and amplify lateral fluorescence signals, thereby enabling more sensitive measurement of weak signals. Attached Figure Description

[0039] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments of this application and their descriptions are used to explain this application and do not constitute an undue limitation of this application.

[0040] Figure 1 This is a schematic diagram of the structure of the femtosecond vortex beam self-focusing critical power measurement system provided in the embodiments of this application;

[0041] Figure 2 A schematic flowchart illustrating the method for determining the critical power of a femtosecond vortex beam self-focusing provided in this application embodiment;

[0042] Figure 3 Examples of typical results for determining the self-focusing critical power of a femtosecond vortex beam provided in the embodiments of this application are shown in (a) and (b) for the typical results of the self-focusing critical power of a femtosecond vortex beam with a topological charge of 1.

[0043] Among them, 1. half-wave plate; 2. polarizer; 3. first quarter-wave plate; 4. vortex wave plate; 5. second quarter-wave plate; 6. focusing lens; 7. collecting lens; 8. filter; 9. photomultiplier tube; 10. oscilloscope. Detailed Implementation

[0044] It should be noted that the following detailed descriptions are exemplary and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0045] Where there is no conflict, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0046] Terminology Explanation:

[0047] Self-focusing critical power: When a high-power femtosecond laser pulse propagates in an optical medium, it will experience Kerr self-focusing effect, diffraction, dispersion effect, etc. Only when the power of the incident laser exceeds a certain value can it overcome the diffraction and dispersion effects and produce self-focusing phenomenon. This specific value is called the self-focusing critical power of the laser in the medium.

[0048] Plasma filament: When the power of a femtosecond laser exceeds the self-focusing critical power, the optical pulse will undergo a self-focusing effect, which will cause the light intensity to gradually increase, causing the medium to ionize and generate plasma. Then, under the combined effect of self-focusing and plasma defocusing, a plasma filament with a transmission length much greater than its Rayleigh distance is formed.

[0049] Photomultiplier tube (PMT): A vacuum electronic device that converts weak light signals into electrical signals.

[0050] Femtosecond vortex beams: Combining ultrafast and ultra-intense speeds with a unique wavefront structure, they possess both high energy density and orbital angular momentum. They exhibit many unique phenomena in their transmission and interaction with matter, and provide new degrees of freedom for controlling laser-matter interactions.

[0051] Example 1

[0052] Existing power measurement techniques rely on expensive equipment and cannot quantitatively analyze the self-focusing critical power of femtosecond vortex beams. This results in unstable and inaccurate measurements of the self-focusing critical power, thus affecting its application. Therefore, this embodiment provides a femtosecond vortex beam self-focusing critical power measurement system.

[0053] Next, combined Figure 1 This embodiment provides a detailed description of a femtosecond vortex beam self-focusing critical power measurement system.

[0054] The femtosecond vortex beam self-focusing critical power measurement system includes a femtosecond laser output energy adjustment device, a vortex beam generation device, and a fluorescence collection device. The femtosecond laser output energy adjustment device is used to adjust the energy of the incident femtosecond laser pulse. The vortex beam generation device is used to convert the energy-adjusted femtosecond laser pulse into a femtosecond vortex beam and generate a plasma filament. The fluorescence collection device is used to collect and monitor the lateral fluorescence signal of the femtosecond vortex beam.

[0055] like Figure 1As shown, the femtosecond laser output energy adjustment device includes a half-wave plate 1 and a polarizer 2, and the vortex beam generating device includes a first quarter-wave plate 3, a vortex wave plate 4, a second quarter-wave plate 5, and a focusing lens 6. The half-wave plate 1, polarizer 2, first quarter-wave plate 3, vortex wave plate 4, second quarter-wave plate 5, and focusing lens 6 are arranged sequentially along the same optical axis in the aforementioned order. The femtosecond laser pulse sequentially passes through the half-wave plate 1 and the polarizer 2 to adjust its energy. After energy adjustment, the femtosecond laser pulse passes through the first quarter-wave plate 3, which converts the polarization state of the femtosecond laser pulse from linear polarization to circular polarization. Then, it passes through the vortex wave plate 4, which converts the circularly polarized femtosecond laser pulse into a circularly polarized femtosecond vortex beam. Finally, it passes through the second quarter-wave plate 5, which modulates the circularly polarized femtosecond vortex beam into a linearly polarized femtosecond vortex beam.

[0056] The fluorescence collection device includes a collection lens 7, a filter 8, a photomultiplier tube 9, and an oscilloscope 10. The filter 8 includes a bandpass filter and a low-pass ultraviolet filter arranged on the same optical axis. The collection lens 7 is used to collect the lateral fluorescence signal of the femtosecond vortex beam. The lateral fluorescence signal enters the photomultiplier tube 9 through the bandpass filter and the low-pass ultraviolet filter. The oscilloscope 10 is used to monitor and record the intensity of the lateral fluorescence signal.

[0057] The center wavelength of the bandpass filter is determined based on the gas medium. In this embodiment, the femtosecond vortex beam self-focusing critical power measurement system is used in air. The center wavelength of the bandpass filter is 337nm (the fluorescence spectral line at 337nm comes from nitrogen in the air). The focal length of the focusing lens 6 is 50cm, the focal length of the collecting lens 7 is 75mm, and the femtosecond laser pulse is generated by a titanium-doped sapphire chirped pulse amplification laser system with a center wavelength of 800nm, a repetition frequency of 1kHz, and a pulse width of 65fs.

[0058] The working principle of this embodiment is as follows:

[0059] A titanium-doped sapphire chirped pulse amplification laser system with a center wavelength of 800 nm, a repetition frequency of 1 kHz, and a pulse width of 65 fs generates femtosecond laser pulses. The femtosecond laser pulses are incident on half-wave plate 1, which changes the polarization direction of the incident femtosecond laser pulses. By rotating half-wave plate 1, the angle between the polarization direction of the incident femtosecond laser pulses on polarizer 2 and the polarization direction that polarizer 2 can transmit through is adjusted, thereby adjusting the energy of the femtosecond laser pulses. This is common knowledge in the field and will not be elaborated further here.

[0060] The energy-modulated femtosecond laser pulse is incident on the first quarter-wave plate 3, which converts the linearly polarized femtosecond laser pulse into a circularly polarized femtosecond laser pulse. The circularly polarized femtosecond laser pulse is then incident on the vortex wave plate 4, which modulates the circularly polarized femtosecond laser pulse into a circularly polarized femtosecond vortex beam. The circularly polarized femtosecond vortex beam is then incident on the second quarter-wave plate 5, which modulates the circularly polarized femtosecond vortex beam into a linearly polarized femtosecond vortex beam. The linearly polarized femtosecond vortex beam passes through the focusing lens 6, which focuses and ionizes the air as the femtosecond vortex beam passes, generating a plasma filament.

[0061] The collecting lens 7 collects the lateral fluorescence signal of the femtosecond vortex beam. The lateral fluorescence signal is incident on a bandpass filter, which filters the fluorescence spectrum of nitrogen in the air. The lateral fluorescence signal filtered by the bandpass filter is incident on a transmittance ultraviolet low-pass filter, which further filters stray light such as the fundamental frequency in the environment. The filtered lateral fluorescence signal enters a photomultiplier tube 9, which converts the lateral fluorescence signal into an electrical signal. The signal of the photomultiplier tube 9 is monitored and recorded using an oscilloscope 10.

[0062] The femtosecond laser pulse energy is continuously adjusted from small to large by the cooperation of half-wave plate 1 and polarizer 2. The corresponding electrical signal is recorded. Based on the trend of signal intensity change, linear fitting is performed to find the abrupt change position of signal intensity change. The self-focusing critical power of the femtosecond vortex beam is obtained from the intersection of the fitted lines.

[0063] In some embodiments, a circularly polarized femtosecond vortex beam can be directly measured. In this case, the vortex beam generating device includes a first quarter-wave plate 3, a vortex wave plate 4, and a focusing lens 6.

[0064] The energy-modulated femtosecond laser pulse is incident on the first quarter-wave plate 3, which converts the linearly polarized femtosecond laser pulse into a circularly polarized femtosecond laser pulse. The circularly polarized femtosecond laser pulse is incident on the vortex wave plate 4, which modulates the circularly polarized femtosecond laser pulse into a circularly polarized femtosecond vortex beam. The circularly polarized femtosecond vortex beam passes through the focusing lens 6, which focuses and ionizes the air as the femtosecond vortex beam passes through, generating a plasma filament.

[0065] Example 2

[0066] Combination Figure 2-3 This embodiment discloses a method for determining the self-focusing critical power of a femtosecond vortex beam, comprising the following steps:

[0067] Step 1: Perform a self-test on each device in the femtosecond vortex beam self-focusing critical power measurement system. If the self-test is normal, proceed to Step 2.

[0068] Step 2: Turn on the femtosecond laser to generate femtosecond laser pulses. The femtosecond laser pulses enter the femtosecond laser output energy adjustment device to obtain energy-adjusted femtosecond laser pulses. The center wavelength of the femtosecond laser pulse is 800nm, the pulse duration is 65fs, the repetition frequency is 1kHz, and the maximum pulse energy is 6mJ.

[0069] Specifically, by adjusting the angle of half-wave plate 1, the angle between half-wave plate 1 and polarizer 2 is changed. With the cooperation of half-wave plate 1 and polarizer 2, the femtosecond laser pulse energy can be continuously adjusted from small to large. During the continuous adjustment process, the adjusted femtosecond laser pulse is incident on the vortex beam generating device, and the following steps are executed until the last one.

[0070] Step 3: The energy-adjusted femtosecond laser pulse enters the vortex beam generating device to generate a femtosecond vortex beam and a plasma filament; specifically, the energy-adjusted femtosecond laser pulse passes sequentially through the first quarter-wave plate 3, the vortex wave plate 4 and the second quarter-wave plate 5 to generate a femtosecond vortex beam; the femtosecond vortex beam is focused in the air by a focusing lens 6 with a focal length of 50cm to ionize the air and generate a plasma filament.

[0071] Step 4: The collecting lens 7 with a focal length of 75mm collects the lateral fluorescence signal of the femtosecond vortex beam. The lateral fluorescence signal passes through the bandpass filter and the ultraviolet low-pass filter in sequence and enters the photomultiplier tube 9. The photomultiplier tube 9 converts the lateral fluorescence signal into an electrical signal and transmits it to the oscilloscope 10 for monitoring and recording.

[0072] Step 5: Record the signal intensity of femtosecond laser pulses with different energies and the corresponding lateral fluorescence signals. Based on the trend of signal intensity change, perform linear fitting to find the abrupt change position of signal intensity change, and obtain the self-focusing critical power of the femtosecond vortex beam from the intersection of the fitted lines.

[0073] Specifically, taking a femtosecond vortex beam with topological charges of 1, 2, and 3 as an example, based on the signal intensity of the lateral fluorescence signal recorded by oscilloscope 10 and the energy of the corresponding femtosecond laser pulse, the following plot is drawn: Figure 3 The graph shows that when the femtosecond laser pulse energy is very low, the signal intensity of the lateral fluorescence signal remains almost constant. With increasing energy, the fluorescence intensity increases rapidly at higher laser powers, exhibiting significant intensity changes at approximately 200 μJ, 340 μJ, and 380 μJ. This indicates that more laser energy is needed to excite the nonlinear increase in fluorescence intensity. The strong intensity change suggests that the self-focusing critical power should be within this range. A linear fitting method is used to find the self-focusing critical power of the vortex beam. Figure 3As shown, the intersection points of the linear fitting lines are located at 230.71 μJ, 410.66 μJ, and 512.71 μJ, which correspond to 3.55 GW, 6.32 GW, and 7.89 GW for vortex beams with topological charges m = 1, 2, and 3, respectively. Furthermore, it can be seen that the self-focusing critical power of the vortex beam increases with increasing topological charge.

[0074] The descriptions of each embodiment in the above embodiments have different focuses. For parts not described in detail in a certain embodiment, please refer to the relevant descriptions in other embodiments.

[0075] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A femtosecond vortex beam self-focusing critical power measurement system, characterized in that, include: A femtosecond laser output energy adjustment device, wherein the femtosecond laser output energy adjustment device is used to adjust the energy of the incident femtosecond laser pulse; A vortex beam generating device, wherein the vortex beam generating device is used to convert a femtosecond laser pulse with adjusted energy into a femtosecond vortex beam and generate a plasma filament; and... A fluorescence collection device is provided for collecting and monitoring the lateral fluorescence signal of a femtosecond vortex beam. The fluorescence collection device includes a filter, which includes a bandpass filter and a low-pass ultraviolet filter arranged sequentially on the same optical axis. The bandpass filter is used to filter the fluorescence spectrum in the gas medium, and the low-pass ultraviolet filter is used to filter stray light in the environment. The signal intensities of femtosecond laser pulses with different energies and their corresponding lateral fluorescence signals are recorded. Based on the trend of signal intensity changes, linear fitting is performed to find the abrupt change locations of signal intensity changes. The self-focusing critical power of the femtosecond vortex beam is obtained from the intersection of the fitted lines.

2. The femtosecond vortex beam self-focusing critical power measurement system as described in claim 1, characterized in that, The femtosecond laser output energy adjustment device includes a half-wave plate and a polarizer arranged sequentially on the same optical axis. The femtosecond laser pulse passes through the half-wave plate and the polarizer in sequence. By rotating the angle of the half-wave plate, the angle between the half-wave plate and the polarizer is changed, thereby continuously adjusting the energy of the input femtosecond laser pulse.

3. The femtosecond vortex beam self-focusing critical power measurement system as described in claim 1, characterized in that, The vortex beam generating device includes a first quarter-wave plate, a vortex wave plate, and a focusing lens arranged sequentially on the same optical axis. The first quarter-wave plate is used to convert the polarization state of the femtosecond laser pulse from linear polarization to circular polarization; The vortex waveplate is used to convert circularly polarized femtosecond laser pulses into circularly polarized femtosecond vortex beams. The focusing lens is used to focus and ionize air in the air as the femtosecond vortex beam passes by, generating plasma filaments; Furthermore, the vortex beam generating device also includes a second quarter-wave plate disposed between the vortex wave plate and the focusing lens; The second quarter-wave plate is used to modulate a circularly polarized femtosecond vortex beam into a linearly polarized femtosecond vortex beam.

4. The femtosecond vortex beam self-focusing critical power measurement system as described in claim 1, characterized in that, The fluorescence collection device also includes a collection lens, a photomultiplier tube, and an oscilloscope; The collecting lens is used to collect the lateral fluorescence signal of the femtosecond vortex beam. The lateral fluorescence signal enters the photomultiplier tube through a filter, and the oscilloscope is used to monitor and record the intensity of the lateral fluorescence signal.

5. A method for determining the self-focusing critical power of a femtosecond vortex beam, which can employ the femtosecond vortex beam self-focusing critical power determination system as described in any one of claims 1-4, characterized in that, Includes the following steps: Step 1: Turn on the femtosecond laser to generate femtosecond laser pulses. The femtosecond laser pulses enter the femtosecond laser output energy adjustment device to obtain energy-adjusted femtosecond laser pulses. Step 2: The energy-adjusted femtosecond laser pulse enters the vortex beam generator to produce a femtosecond vortex beam and a plasma filament; Step 3: The fluorescence collection device collects and monitors the lateral fluorescence signal of the femtosecond vortex beam; Step 4: Record the signal intensity of femtosecond laser pulses with different energies and the corresponding lateral fluorescence signals. Calculate the self-focusing critical power of the femtosecond vortex beam based on the trend of signal intensity variation.

6. The method for determining the critical power of a femtosecond vortex beam self-focusing as described in claim 5, characterized in that, By adjusting the angle of the half-wave plate, the angle between the half-wave plate and the polarizer is changed, thus enabling continuous adjustment of the energy of the input femtosecond laser pulse.

7. The method for determining the critical power of a femtosecond vortex beam self-focusing as described in claim 5, characterized in that, The energy-adjusted femtosecond laser pulses pass sequentially through the first quarter-wave plate, the vortex plate, and the second quarter-wave plate to generate a femtosecond vortex beam. The femtosecond vortex beam is focused in the air by a focusing lens to ionize the air and generate a plasma filament.

8. The method for determining the critical power of a femtosecond vortex beam self-focusing as described in claim 5, characterized in that, The collecting lens collects the lateral fluorescence signal of the femtosecond vortex beam. The lateral fluorescence signal passes through a bandpass filter and a UV low-pass filter in sequence before entering the photomultiplier tube. The photomultiplier tube converts the lateral fluorescence signal into an electrical signal and transmits it to an oscilloscope for monitoring and recording.

9. The method for determining the critical power of a femtosecond vortex beam self-focusing as described in claim 5, characterized in that, The femtosecond laser pulse is generated by a femtosecond laser amplifier with a center wavelength of 800 nm, a pulse duration of 65 fs, a repetition frequency of 1 kHz, and a maximum pulse capability of 6 mJ.

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