Measurement System and Measurement Method for Plasma Density Distribution of Femtosecond Laser Filaments

Abstract

The present invention discloses a measurement system and a measurement method for the plasma density distribution of a femtosecond laser filament. The system includes: a plasma charge quantity measurement system and an optical path for measuring the filament volume. The optical path for measuring the filament volume is used to measure the filament volume. The plasma charge quantity measurement system includes a pulsed laser light source, a first guiding optical path, a charge migration device, and a trigger optical path. The first guiding optical path is arranged between the pulsed laser light source and the charge migration device. The trigger optical path is used to adjust the time when the laser generated by the pulsed laser light source is incident on the charge migration device. The charge migration device includes a DC voltage generator, a high-voltage plate electrode, a shielding plate electrode, a sampling resistor, and at least a discrete plate electrode group. The high-voltage plate electrode is connected to the DC voltage generator. The discrete plate electrode group includes a plurality of discrete plate electrodes. Each discrete plate electrode is arranged at intervals along the laser direction and covers the entire filament. The discrete plate electrode and the shielding plate electrode are respectively grounded through the sampling resistor.

Description

Technical Field

[0001] The present invention relates to the new technical field of high-voltage applications, and particularly to a measurement system and a measurement method for the plasma density distribution of femtosecond laser filaments. Background Art

[0002] Femtosecond laser forms filaments in air through the Kerr self-focusing effect. After self-focusing, the light intensity can reach 1018 - 1022 W / cm2, and its intensity exceeds the Coulomb field inside the atom. The focused femtosecond laser pulse can strip electrons from the bondage of atoms, thus forming plasma. There are many complex non-linear processes in the femtosecond laser filament formation process, such as self-focusing, photoionization, self-phase modulation, self-steepening, diffraction, and defocusing, etc. Its filament formation characteristics have broad application prospects in the fields of high-voltage engineering and atmospheric detection.

[0003] The electrical characteristics of femtosecond laser mainly manifest as the plasma density characteristics of the filament channel. For various discharges caused or induced by femtosecond laser, the current characteristics of the discharge channel are closely related to the plasma density characteristics of the femtosecond laser filament. It is necessary to adopt reasonable research methods to study the plasma density characteristics of the filament channel.

[0004] Due to the characteristics of femtosecond laser filaments such as long distance, high power (TW), short duration (ns), and small channel radius (100μm), and the lack of repeatability of the characteristics of the laser ionization channel, common optical measurement methods such as laser interferometry and electrical measurement arrangements are difficult to obtain the plasma density distribution characteristics of the complete filament, and there is a lack of a good experimental system for measuring the plasma density distribution of the filament. Therefore, a measurement system for the plasma density distribution of femtosecond laser filaments is needed, which has the ability to measure the plasma density distribution of the filament through a single experiment, and promotes the development of fields such as laser-induced high-voltage discharge and long-distance detection.

[0005] In the related art, the Chinese invention patent application document with the application publication number of CN109100262A discloses a femtosecond laser filamentation plasma density measurement device and a measurement method. In this device, the femtosecond laser is divided into three beams of light by two beam splitters, which are respectively used as the pump light, the first probe light P1, and the second probe light P2. The pump light forms a plasma channel by filamentation in the gas cavity. The first probe light P1 passes through the plasma channel as a carrier of the change amount introduced by the plasma, and then the second probe light P2 observes the value of the change amount through cross-correlation measurement and thus inversely calculates the plasma density. However, this scheme uses an optical method for measurement. The detection range of the optical method depends on the spot diameter of the probe light. If the femtosecond laser filamentation distance is short, the optical method can be used for detection. However, if the filamentation distance is long and exceeds the spot diameter, the optical method is no longer applicable. Summary of the Invention

[0006] The technical problem to be solved by the present invention is how to measure the plasma density distribution of the entire filament through a single experiment.

[0007] The present invention solves the above technical problem through the following technical means:

[0008] The present invention provides a measurement system for the plasma density distribution of a femtosecond laser filament. The system includes: a plasma charge measurement system and a filament volume measurement optical path. The filament volume measurement optical path is used to measure the volume of the filament. The plasma charge measurement system includes a pulsed laser light source, a first guiding optical path, a charge migration device, and a trigger optical path. The first guiding optical path is arranged between the pulsed laser light source and the charge migration device. The trigger optical path is used to adjust the time when the laser generated by the pulsed laser light source is incident on the charge migration device;

[0009] The charge migration device includes a DC voltage generator, a high-voltage plate electrode, a shielding plate electrode, a sampling resistor, and at least a discrete plate electrode group. The high-voltage plate electrode is connected to the DC voltage generator. The discrete plate electrode group includes a plurality of discrete plate electrodes. Each discrete plate electrode is arranged at intervals along the laser direction and covers the entire filament. The discrete plate electrode and the shielding plate electrode are respectively grounded through the sampling resistor.

[0010] Further, the shielding plate electrode includes a first shielding plate electrode and a second shielding plate electrode. The first shielding plate electrode and the second shielding plate electrode are respectively arranged on both sides of the discrete plate electrode group.

[0011] Further, both the first shielding plate electrode and the second shielding plate electrode are arranged along the laser direction and the length is greater than or equal to 3 cm. The distance between the first shielding plate electrode and the second shielding plate electrode and the adjacent discrete plate electrode is less than 0.5 mm.

[0012] Further, the pulsed laser light source is a high-power ultrashort pulsed laser light source.

[0013] Further, the first guiding optical path includes a plurality of reflectors and a convex lens. The geometric focal length of the convex lens is 5 m.

[0014] Further, the filament volume measurement optical path includes a second guiding optical path, a quartz glass, a camera, and a filter. The included angle between the laser exiting from the first guiding optical path and the light incident surface of the quartz glass is greater than 120°. The light incident surface of the camera is parallel to the light incident surface of the quartz glass. The filter is arranged on the scattered light path of the quartz glass;

[0015] The camera is used to receive the light exiting from the filter and obtain the optical image area S of the camera photo.

[0016] In addition, the present invention also provides a method for measuring the plasma density distribution of a femtosecond laser filament, which is used to measure the plasma density distribution of the filament by using the measurement system for the plasma density distribution of a femtosecond laser filament as described above. The method includes the following steps:

[0017] Turn on the pulsed laser light source to emit laser light;

[0018] The laser light emitted by the pulsed laser light source is incident on the charge transfer device after passing through the first guiding optical path, and voltage measurement signals at both ends of each sampling resistor are read through a voltage probe;

[0019] Measure the segmented filament volume of the laser by using the filament volume measurement optical path;

[0020] Based on the voltage measurement signals at both ends of each sampling resistor and the segmented filament volume, measure the plasma density distribution of the entire filament.

[0021] Further, the step of measuring the segmented filament volume of the laser by using the filament volume measurement optical path includes:

[0022] The laser light emerging from the second guiding optical path is incident on the quartz glass, and the scattered light of the quartz glass is incident on a filter;

[0023] Use the camera to receive the light emerging from the filter to obtain the optical image area S of the camera photo;

[0024] Based on the optical image area S and the lengths of the discrete plate electrodes, calculate the segmented filament volume of the laser.

[0025] Further, the step of measuring the plasma density distribution of the entire filament based on the voltage measurement signals at both ends of each sampling resistor and the segmented filament volume, the calculation formula is:

[0026]

[0027] Where: u(t) represents the voltage measurement signal at both ends of the sampling resistor connected to each discrete plate electrode, R represents the resistance value of the sampling resistor, S represents the optical image area of the filament of the laser, L represents the length of the discrete plate electrode, and t represents time.

[0028] The advantages of the present invention are:

[0029] (1) The present invention provides a plasma charge quantity measurement system and an optical path for measuring the volume of an optical filament. Specifically, a charge migration device in the plasma charge quantity measurement system is provided with a discrete plate electrode group. Based on the voltage measurement information across the sampling resistors connected to each discrete plate electrode in the discrete plate electrode group, the charge quantity of the laser-segmented optical filament is determined. Additionally, the optical image area of the laser optical filament is measured through the optical path for measuring the volume of the optical filament, thereby determining the volume of the laser-segmented optical filament. According to the charge quantity and volume of the laser-segmented optical filament, the plasma density distribution of the entire laser optical filament is measured. Moreover, by changing the length L and number n of the discrete electrodes, the plasma density distribution of optical filaments with different lengths can be measured, which is of great significance for the development of technologies such as laser-guided long-distance high-voltage discharge, laser-induced lightning, and laser remote detection.

[0030] Additional aspects and advantages of the present invention will be partially presented in the following description, partially become apparent from the following description, or be understood through the practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 is a schematic structural diagram of the plasma charge quantity measurement system in the present invention;

[0032] Figure 2 is a waveform diagram of the electrical signal obtained through measurement in the present invention;

[0033] Figure 3 is a schematic structural diagram of the optical path for measuring the volume of the optical filament in the present invention;

[0034] Figure 4 The optical filament image obtained through measurement in the present invention;

[0035] Figure 5 is a schematic flow diagram of the method for measuring the plasma density distribution of a femtosecond laser optical filament proposed in the present invention.

[0036] In the figure:

[0037] 10 - pulsed laser light source; 20 - first guiding optical path; 30 - charge migration device;

[0038] 21 - mirror; 22 - convex lens;

[0039] 31 - DC voltage generator; 32 - high-voltage plate electrode; 33 - shielding plate electrode; 34 - sampling resistor; 35 - discrete plate electrode group; 41 - second guiding optical path; 42 - quartz glass; 43 - camera; 44 - filter. DETAILED DESCRIPTION OF THE INVENTION

[0040] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Apparently, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0041] As Figures 1 to 3 shown, a measurement system for the plasma density distribution of a femtosecond laser filament in the first embodiment of the present invention is proposed. The system includes: a plasma charge measurement system and a filament volume measurement optical path. The filament volume measurement optical path is used to measure the volume of the filament. The plasma charge measurement system includes a pulsed laser light source 10, a first guiding optical path 20, a charge migration device, and a trigger optical path. The first guiding optical path 20 is arranged between the pulsed laser light source 10 and the charge migration device. The trigger optical path is used to adjust the time when the laser generated by the pulsed laser light source 10 is incident on the charge migration device.

[0042] The charge migration device includes a DC voltage generator 31, a high-voltage plate electrode 32, a shielding plate electrode 33, a sampling resistor 34, and at least a discrete plate electrode group 35. The high-voltage plate electrode 32 is connected to the DC voltage generator 31. The discrete plate electrode group 35 includes a plurality of discrete plate electrodes. Each of the discrete plate electrodes is arranged at intervals along the laser direction and covers the entire filament. The discrete plate electrodes and the shielding plate electrode 33 are respectively grounded through the sampling resistor 34.

[0043] It should be noted that the electrical method uses the current formed by the migration of the plasma under an externally applied electric field to achieve the detection purpose. The detection range depends on the electrode length and the moving distance of the device. When the filamentation distance is long and exceeds the spot diameter, the electrical method can be used to obtain the plasma density distribution of the entire filament. In this embodiment, a plasma charge measurement system and a filament volume measurement optical path are set. Among them, the charge migration device in the plasma charge measurement system is provided with a discrete plate electrode group 35. Based on the voltage measurement information at both ends of the sampling resistor 34 connected to each discrete plate electrode in the discrete plate electrode group 35, the charge amount of the laser segmented filament is determined. And the optical image area of the laser filament is measured through the filament volume measurement optical path, so as to determine the volume of the laser segmented filament. According to the charge amount and volume of the laser segmented filament, the plasma density distribution of the entire laser filament is measured. And by changing the length L and the number n of the discrete electrodes, the plasma density distributions of filaments with different lengths can be measured, which is of great significance for the development of technologies such as laser-guided long-distance high-voltage discharge, laser-induced lightning, and laser remote detection.

[0044] It should be noted that the working principle of the charge migration device is as follows:

[0045] (1) Polarization: Before the optical filament plasma appears between the capacitor plates, the capacitor is charged by applying an external DC voltage, and no current flows through the sampling resistor 34. When the laser is incident on the capacitor gap to ionize and form an optical filament plasma, the positive / negative charges of the plasma move in opposite directions under the action of the external electric field, forming polarization.

[0046] (2) Charge redistribution and capacitor charge compensation: After the channel is polarized, on the one hand, the free charges inside the capacitor plates will redistribute, making the net electric field component near the capacitor plates perpendicular to the plates. The time scale of the charge redistribution process is about the plate spacing divided by the speed of light, which is in the ps range. This process will not be reflected in the waveform because it exceeds the detection bandwidth; on the other hand, the charge imbalance state inside the plates is compensated by the charges provided by the external circuit, and the time of this process is determined by the time constant of the circuit, which is in the ns - μs range;

[0047] (3) Charge migration: Under the action of the external electric field, the charges that do not participate in recombination will migrate to the plate electrodes. Considering that the electron mobility is 1 - 10 m 2 / V·s and the ion mobility is 10 -4 m 2 / V·s under standard atmospheric pressure, and the applied electric field is 100 - 400 V / cm, this process will last for the order of ms.

[0048] In one embodiment, each of the discrete plate electrodes is arranged along the laser direction and has a length of 3 cm, and the spacing between each of the discrete plate electrodes is less than 0.5 mm.

[0049] It should be noted that the length of the discrete plate electrodes and the spacing between each of the discrete plate electrodes are set based on the results of electric field simulation calculations. This design avoids the tip discharge at the electrode tips due to the shielding effect between the electrodes, making the source of charge generation only obtained by laser ionization; on the other hand, the gap electric field is uniform, and the electric field direction is perpendicular to the plate electrodes, enabling the charges to directly migrate to the nearest plate electrode, reducing the loop inductance.

[0050] It should be noted that the number of discrete plate electrodes provided in the discrete plate electrode group 35 is at least 3. The design of the size and spacing of the discrete plate electrodes is the basis for realizing the synchronous measurement of the density of multi - segment optical filament plasmas, enabling electrons to move in the direction perpendicular to the plate electrodes under the external electric field to achieve the purpose of maximizing the collection of charges.

[0051] In one embodiment, the shielding plate electrode 33 includes a first shielding plate electrode 33 and a second shielding plate electrode 33, and the first shielding plate electrode 33 and the second shielding plate electrode 33 are respectively arranged on both sides of the discrete plate electrode group 35.

[0052] It should be noted that the function of setting the shielding plate electrode 33 is to avoid the tip discharge of the electrode and form a new charge source.

[0053] In one embodiment, both the first shielding plate electrode 33 and the second shielding plate electrode 33 are arranged along the laser direction and have a length greater than or equal to 3 cm, and the distance between the first shielding plate electrode 33 and the second shielding plate electrode 33 and the adjacent discrete plate electrode is less than 0.5 mm.

[0054] It should be noted that the length of the shielding plate electrode 33 is the spatial resolution. Setting its length to 3 cm represents the amount of charge collected within a 3-cm range.

[0055] In one embodiment, the pulsed laser light source 10 is a high-power ultrashort pulsed laser light source 10.

[0056] Specifically, the high-power ultrashort pulsed laser light source 10 in this embodiment generates an ultrashort pulsed laser with a repetition frequency of 10 Hz, a wavelength of 800 nm, an energy of the mJ level, and a pulse width of the fs or ps level based on a titanium-doped sapphire (Ti:Sapphire) gain medium and chirped pulse amplification (CPA, Chirped Pulse Amplification) technology.

[0057] In this embodiment, by setting the high-power ultrashort pulsed laser light source 10, a 2-m-long plasma channel can be ionized and formed in the air after passing through the first guiding optical path 20, which is much longer than the plasma channel length that can be formed by a nanosecond pulsed laser in the air.

[0058] By setting the high-power ultrashort pulsed laser light source 10, a plasma channel of the meter level can be ionized and formed in the air.

[0059] In one embodiment, the first guiding optical path 20 includes a first group of reflectors 21 composed of several reflectors 21 and a convex lens 22. The convex lens 22 is arranged on the reflected light path of the reflector group 21, and the geometric focal length of the convex lens 22 is 5 m.

[0060] Due to the different positions of the measuring device, the laser is guided to different directions by using the reflector 21.

[0061] It should be noted that the focal length of the convex lens 22 is related to the length of the formed plasma channel. When it is 5 m, the plasma channel length is 2 m. Of course, those skilled in the art can also take other values according to the actual situation, and then the plasma channel length will change.

[0062] In one embodiment, the resistance value of the sampling resistor 34 is R = 10 MΩ. The voltage u(t) across the sampling resistor 34 is read by a voltage probe, and the measured electrical signal waveform diagram is as Figure 2As shown, since the value of the loop current is extremely small, a large resistor is required to obtain a better waveform on the oscilloscope.

[0063] In one embodiment, as Figure 3 shown, the filament volume measurement optical path includes a second guiding optical path 41, a quartz glass 42, a camera 43, and a filter 44. The angle between the laser beam exiting from the first guiding optical path 20 and the light incident surface of the quartz glass 42 is greater than 120°. The light incident surface of the camera 43 is parallel to the light incident surface of the quartz glass 42. The filter 44 is arranged on the scattered light path of the quartz glass 42;

[0064] The camera 43 is used to receive the light exiting from the filter 44 and obtain the optical image area S of the camera photo.

[0065] Since the camera 43 selects a CMOS camera, the CMOS camera is light-sensitive and will collect visible light while collecting the light spot. In this embodiment, by arranging the filter 44 on the scattered light path of the quartz glass 42, the visible light outside 700 - 900 nm of the light spot is filtered out, the noise of the image is reduced, and it is ensured that the optical image collected by the camera 43 is a light spot image.

[0066] It should be noted that the reason for setting the angle greater than 120° is to prevent the reflected laser from entering the camera 43 and damaging the photosensitive element of the camera 43.

[0067] The second guiding optical path 41 includes a second group of reflectors 21 composed of several reflectors 21, and the laser is guided to different directions by using the reflectors 21.

[0068] Specifically, the filament image obtained by the camera 43 in this embodiment is as Figure 4 shown.

[0069] It should be noted that by measuring the segmented filament charge quantity and the segmented filament volume through a single experiment, the measurement of the plasma density distribution of the full-segment filament can be realized through the formula where: u(t) represents the voltage measurement signal at both ends of the sampling resistor 34 connected to each discrete plate electrode, R represents the resistance value of the sampling resistor 34, S represents the optical image area of the laser filament, L represents the length of the discrete plate electrode, and t represents time.

[0070] Using the femtosecond laser filament plasma density distribution measurement system proposed in this embodiment, the spatial resolution reaches 3 cm, that is, the plasma density of a 2 m filament with a spatial accuracy of 3 cm can be calculated by this formula.

[0071] In addition, as Figure 5As shown in the figure, the second embodiment of the present invention proposes a method for measuring the plasma density distribution of a femtosecond laser filament, which is used to measure the plasma density distribution of the filament by using the measurement system for the plasma density distribution of the femtosecond laser filament proposed in the first embodiment above. The method includes the following steps:

[0072] S10. Turn on the pulsed laser light source and emit laser light.

[0073] S20. The laser light emitted by the pulsed laser light source is incident on the charge migration device after passing through the first guiding optical path, and voltage measurement signals at both ends of each sampling resistor are read through a voltage probe.

[0074] S30. Measure the segmented filament volume of the laser by using the filament volume measurement optical path.

[0075] S40. Measure the plasma density distribution of the entire filament based on the voltage measurement signals at both ends of each sampling resistor and the segmented filament volume.

[0076] In this embodiment, by setting a plasma charge measurement system and a filament volume measurement optical path, wherein a discrete plate electrode group is provided in the charge migration device of the plasma charge measurement system, and based on the voltage measurement information at both ends of the sampling resistors connected to each discrete plate electrode in the discrete plate electrode group, the charge of the laser segmented filament is determined, and the optical image area of the laser filament is measured through the filament volume measurement optical path, so as to determine the segmented filament volume of the laser. According to the charge and segmented filament volume of the laser segmented filament, the plasma density distribution of the entire laser filament is measured; and by changing the length L and the number n of the discrete electrodes, the plasma density distribution of filaments with different lengths can be measured, which is of great significance for the development of technologies such as laser-guided long-distance high-voltage discharge, laser-induced lightning, and laser remote detection.

[0077] In one embodiment, the step of measuring the segmented filament volume of the laser by using the filament volume measurement optical path includes the following steps:

[0078] The laser light emerging from the second guiding optical path is incident on the quartz glass, and the scattered light of the quartz glass is incident on the filter.

[0079] Use the camera to receive the light emerging from the filter to obtain the optical image area S of the camera photo.

[0080] Calculate the segmented filament volume of the laser based on the optical image area S and the lengths of the discrete plate electrodes.

[0081] In one embodiment, the step of measuring the plasma density distribution of the entire filament based on the voltage measurement signals at both ends of each sampling resistor and the segmented filament volume, the calculation formula is:

[0082]

[0083] Where: u(t) represents the voltage measurement signal across the sampling resistor connected to each of the discrete plate electrodes, R represents the resistance value of the sampling resistor, S represents the optical image area of the light filament of the laser, L represents the length of the discrete plate electrode, and t represents time.

[0084] The measurement method for the plasma density distribution of femtosecond laser filaments proposed in this embodiment can obtain the segmented filament charge quantity and the segmented filament volume through a single experiment measurement, and realize the measurement of the plasma density distribution of the entire filament.

[0085] In the description of this specification, the descriptions referring to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples", etc. mean that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples.

[0086] In addition, the terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include at least one of the features. In the description of the present invention, the meaning of "a plurality" is at least two, such as two, three, etc., unless otherwise specifically and clearly defined.

[0087] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and cannot be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention.

Claims

1. A measurement system for the plasma density distribution of a femtosecond laser filament, characterized in that, The system includes: a plasma charge quantity measurement system and an optical path for measuring the filament volume. The optical path for measuring the filament volume is used to measure the filament volume. The plasma charge quantity measurement system includes a pulsed laser light source, a first guiding optical path, a charge migration device, and a trigger optical path. The first guiding optical path is arranged between the pulsed laser light source and the charge migration device. The trigger optical path is used to adjust the time when the laser generated by the pulsed laser light source is incident on the charge migration device; The charge migration device includes a DC voltage generator, a high-voltage plate electrode, a shielding plate electrode, a sampling resistor, and at least a discrete plate electrode group. The high-voltage plate electrode is connected to the DC voltage generator. The discrete plate electrode group includes several discrete plate electrodes. Each of the discrete plate electrodes is arranged at intervals along the laser direction and covers the entire filament. The discrete plate electrodes and the shielding plate electrode are respectively grounded through the sampling resistor.

2. The measurement system for the femtosecond laser filament plasma density distribution according to claim 1, wherein Each of the discrete plate electrodes is arranged along the laser direction, and each of the discrete plate electrodes is arranged at equal intervals.

3. The measurement system for the femtosecond laser filament plasma density distribution according to claim 1, wherein The shielding plate electrode includes a first shielding plate electrode and a second shielding plate electrode. The first shielding plate electrode and the second shielding plate electrode are respectively arranged on both sides of the discrete plate electrode group.

4. The measurement system for the femtosecond laser filament plasma density distribution according to claim 3, wherein, Both the first shielding plate electrode and the second shielding plate electrode are arranged along the laser direction, and there is a distance between the first shielding plate electrode and the second shielding plate electrode and the adjacent discrete plate electrodes.

5. The measurement system for the femtosecond laser filament plasma density distribution according to claim 1, characterized in that, The pulsed laser light source is a high-power ultrashort pulsed laser light source.

6. The measurement system for the femtosecond laser filament plasma density distribution according to claim 1, wherein, The first guiding optical path includes a first mirror group composed of several mirrors and a convex lens. The convex lens is arranged on the reflected light optical path of the first mirror group.

7. The measurement system for the femtosecond laser filament plasma density distribution according to claim 1, wherein The optical path for measuring the filament volume includes a second guiding optical path, a quartz glass, a camera, and a filter. The angle between the laser exiting from the first guiding optical path and the light incident surface of the quartz glass is greater than 120°. The light incident surface of the camera is parallel to the light incident surface of the quartz glass. The filter is arranged on the scattered light path of the quartz glass; The camera is used to receive the light exiting from the filter and obtain the optical image area S of the camera photo.

8. A method for measuring the plasma density distribution of a femtosecond laser filament, characterized in that, For measuring the filament plasma density distribution using the measurement system for the femtosecond laser filament plasma density distribution according to any one of claims 1 to 7, the method includes: Turn on the pulsed laser light source and emit laser; The laser emitted by the pulsed laser light source is incident on the charge migration device after passing through the first guiding optical path, and voltage measurement signals at both ends of each sampling resistor are read through a voltage probe; Use the optical path for measuring the filament volume to measure the segmented filament volume of the laser; Based on the voltage measurement signals at both ends of each sampling resistor and the segmented filament volume, measure the plasma density distribution of the entire filament.

9. The method for measuring the femtosecond laser filament plasma density distribution according to claim 8, characterized in that, The step of using the optical path for measuring the filament volume to measure the segmented filament volume of the laser includes: The laser exiting from the second guiding optical path is incident on the quartz glass, and the scattered light of the quartz glass is incident on the filter; Use the camera to receive the light exiting from the filter and obtain the optical image area S of the camera photo; Based on the optical image area S of the laser and the lengths of the respective discrete plate electrodes, calculate the volume of the segmented optical filaments of the laser.

10. The measurement method of the femtosecond laser filament plasma density distribution according to claim 8, characterized in that, Based on the voltage measurement signals across the respective sampling resistors and the volume of the segmented optical filaments, measure the plasma density distribution of the entire optical filament. The calculation formula is as follows: In the formula: u(t) represents the voltage measurement signal across the sampling resistor connected to each discrete plate electrode, R represents the resistance value of the sampling resistor, S represents the optical image area of the optical filament of the laser, L represents the length of the discrete plate electrode, and t represents time.

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