Plasma electron density measuring device and method based on optical fiber dispersion interference

Abstract

The invention belongs to the field of plasma electron density measurement, and discloses a plasma electron density measurement device and method based on optical fiber dispersion interference. A part of an incident laser beam is subjected to frequency multiplication by the first optical fiber dispersion interference assembly, detection light becomes superposed light of a fundamental wave and an original second harmonic wave and penetrates through plasma, and the two waves both meet a characteristic phase shift; in order to obtain an interference light beam, the transmitted fundamental wave is multiplied by the second optical fiber dispersion interference assembly again, and the original second harmonic is kept unchanged; optical signals of two mutually interfered second harmonics are converted into electric signals through a balanced photoelectric detector, differential amplification is carried out on the electric signals, and finally the plasma electron density is obtained according to the electric signal differential amplification result; the influence of disturbance on the two partial waves is the same, so that the device has good stability in measuring the low-density plasma, and mutual compensation can be realized; and the measurement efficiency and effect are ensured.

Description

Technical Field

[0001] The present invention belongs to the field of plasma parameter diagnosis, and specifically relates to the field of plasma electron density measurement, and in particular to a plasma electron density measurement device and method based on optical fiber dispersion interferometry. Background Art

[0002] Plasma electron density measurement technology is mostly used to solve important scientific research and physical problems, and is widely used in many fields such as military, astronomy, resource exploration, and scientific research.

[0003] In the field of plasma electron density measurement technology, Z pinch refers to the process in which the plasma generated by pulsed current reaches a high temperature and high density state under the action of a magnetic field and produces strong X-radiation. Z pinch is the most effective means of producing strong pulsed soft X-rays in the laboratory, and plays an important role in radiation environment simulation, material science research, inertial confinement nuclear fusion and other fields. In the Z pinch device, the spatiotemporal distribution of plasma electron density is one of the core parameters, and is also the basis for further understanding the physical process of Z pinch and the development of Z pinch devices towards larger currents and faster pulses.

[0004] However, in terms of plasma electron density measurement, the Z-pinch device has many plasma types, complex generation mechanisms, large density coverage, short duration (usually less than 100ns), fast change speed, and is in an extreme environment of high voltage (MV), high current (MA) and strong radiation, which makes the measurement more challenging. Common plasma electron density measurements are performed using laser interferometry techniques, including Michelson and Mach-Zehnder interferometry, but currently, for some plasmas with line-integrated electron density below 10 -17 cm -2 The diagnostic capability of low-density plasma is limited, the measurement sensitivity is low and the measurement range is narrow, and the relevant physical models and numerical simulation programs are still imperfect, resulting in an unclear physical image of the low-density plasma generation mechanism.

[0005] It can be seen that the existing plasma electron density measurement measures have low measurement sensitivity and narrow measurement range for low-density plasma, resulting in low measurement efficiency and poor measurement results. Summary of the invention

[0006] The present invention provides a plasma electron density measurement device and method based on optical fiber dispersion interferometry, so as to solve the technical problems that the existing plasma electron density measurement measures have low measurement sensitivity and narrow measurement range for low-density plasma, resulting in low measurement efficiency and poor measurement effect.

[0007] In order to achieve the above object, the present invention adopts the following technical contents: A plasma electron density measuring device based on optical fiber dispersion interferometry, comprising: A pulse laser for emitting a pulsed laser beam; The first fiber dispersion interference component is used to double the frequency of the incident pulsed laser beam and emit superimposed light formed by superposition of the fundamental wave and the original second harmonic; A second fiber dispersion interference component is used to double the frequency of the superimposed light passing through the plasma, and emit a new second harmonic and an original second harmonic that interfere with each other; A balanced photodetector is used to convert the optical signal of the new second harmonic and the original second harmonic into an electrical signal, and to differentially amplify the electrical signal at the same time; so as to obtain the plasma electron density according to the differential amplification result of the electrical signal.

[0008] Furthermore, the first optical fiber dispersion interference component includes a first polarizer, a first adjustable aperture, a first lens, a first frequency doubling crystal, a second lens, and a second adjustable aperture, which are coaxially arranged in sequence from the receiving direction to the emitting direction; the second optical fiber dispersion interference component includes a third adjustable aperture, a second polarizer, a third lens, a second frequency doubling crystal, a fourth lens, a fourth adjustable aperture, and a third polarizer, which are coaxially arranged in sequence from the receiving direction to the emitting direction.

[0009] Furthermore, the first frequency doubling crystal and the second frequency doubling crystal are both nonlinear frequency doubling crystals with a frequency doubling efficiency greater than 50%.

[0010] Furthermore, the first lens, the second lens, the third lens and the fourth lens are plano-convex lenses with the same focal length.

[0011] Furthermore, a beam splitter, a first fiber coupler, a fiber collimator and a first reflector arranged in sequence are provided between the pulse laser and the first fiber dispersion interference assembly; the first fiber coupler and the fiber collimator are connected via an optical fiber; A fourth reflector, a filter and a polarization beam splitter are arranged between the second optical fiber dispersion interference component and the balanced photodetector. The first emitting end of the polarization beam splitter forms a first optical path with the balanced photodetector through a second optical fiber coupler and a first optical fiber array, and the second emitting end forms a second optical path with the balanced photodetector through a third optical fiber coupler and a second optical fiber array.

[0012] Furthermore, the first fiber array and the second fiber array both use a one-to-four fiber jumper; wherein the lengths of the four branch optical fibers of the one-to-four fiber jumper are designed in an arithmetical manner to control the time it takes for the laser to reach the balanced photodetector; the optical fiber, the first fiber array, and the second fiber array all use single-mode polarization-maintaining fiber jumpers; the first fiber coupler, the second fiber coupler, and the third fiber coupler all use fixed aspheric fiber collimators.

[0013] Furthermore, the plasma is arranged inside the vacuum chamber.

[0014] A method for measuring plasma electron density based on optical fiber dispersion interferometry, based on the above-mentioned plasma electron density measuring device based on optical fiber dispersion interferometry, comprising: The pulse laser emits a pulsed laser beam; The first fiber dispersion interference component doubles the frequency of the incident pulsed laser beam and emits superimposed light formed by the superposition of the fundamental wave and the original second harmonic; The second fiber dispersion interference component doubles the frequency of the superimposed light passing through the plasma, emitting a new second harmonic and the original second harmonic that interfere with each other; The balanced photodetector receives the optical signal of the interference between the new second harmonic and the original second harmonic, converts the optical signal into an electrical signal, and simultaneously differentially amplifies the electrical signal; the plasma electron density is obtained according to the differential amplification result of the electrical signal.

[0015] Further, wherein the fundamental wavelength is λ F The pulsed laser beam passes through the first fiber coupler, couples the free-space laser into the optical fiber, and then re-enters the free space through the fiber collimator; The pulsed laser beam is in a vertical state after passing through the first polarizer; after passing through the first frequency doubling crystal, the first wavelength generated is The original second harmonic of Two wavelengths of laser beams coaxially enter the vacuum chamber and pass through the plasma to produce a phase difference ; Then after a wavelength of The second polarizer rotates the fundamental frequency light by 90° and makes it horizontal, and rotates the original second harmonic by 180° and keeps the vertical polarization state, and then enters the second frequency doubling crystal; After passing through the second frequency doubling crystal and then the filter, the coaxial laser beam contains two second harmonics: the original second harmonic and the new second harmonic. The third polarizer deflects both second harmonics by 45°, and the two second harmonics remain perpendicular to each other to achieve interference. The total phase difference for ; The wavelength is The phase difference produced by the laser after passing through the plasma; The wavelength is The phase difference produced by the laser after passing through the plasma; The vertical component and horizontal component of the two mutually perpendicular second harmonics that interfere are then separated by a polarization beam splitter; The vertical component and the horizontal component of the two mutually perpendicular second harmonics separated by the polarization beam splitter are respectively coupled to the corresponding optical fiber arrays through the optical fiber coupler; The balanced photodetector converts the vertical components of the two second harmonic optical signals into and the horizontal component of the optical signal Convert it into an electrical signal, and achieve differential amplification of the electrical signal through TIA components and OPAMP differential amplifiers.

[0016] Furthermore, the specific formula for obtaining the plasma electron density according to the differential amplification result of the electrical signal is as follows:

[0017] In the formula, represents the total phase difference; represents the refractive index of electrons, is the effective electron number density; Represents the integrated density of plasma electron lines.

[0018] Compared with the prior art, the present invention has the following beneficial effects: The present invention provides a plasma electron density measuring device based on fiber dispersion interference. The device uses a first fiber dispersion interference component and a second fiber dispersion interference component surrounding the plasma, so that a part of the incident laser beam is frequency-doubled by the first fiber dispersion interference component, so that the detection light becomes a superposition of the fundamental wave and the original second harmonic, and passes through the plasma, wherein both waves encounter a characteristic phase shift; in order to obtain an interference beam, the transmitted fundamental wave is frequency-doubled again by the second fiber dispersion interference component, while the original second harmonic remains unchanged; the optical signals of the two mutually interfering second harmonics are converted into electrical signals by a balanced photodetector and differentially amplified, and finally the plasma electron density is obtained according to the differential amplification result of the electrical signal; since the influence of the disturbance on the two sub-waves is the same, the device has good stability in measuring low-density plasma, and can achieve mutual compensation; the efficiency and effect of the measurement are guaranteed. The device is finally used to achieve a plasma electron density measurement sensitivity that is 3-4 orders of magnitude higher than that of ordinary interference, thereby achieving efficient, reliable, and highly sensitive measurement of plasma electron density.

[0019] Preferably, in the present invention, the first optical fiber dispersion interference component and the second optical fiber dispersion interference component are respectively composed of a polarizer, an adjustable aperture, a lens and a frequency doubling crystal, which ensures the precise control of the optical path and the stable transmission of the optical signal; and the components of the first optical fiber dispersion interference component and the second optical fiber dispersion interference component are arranged in a coaxial system, which helps to improve the accuracy and stability of the interference, thereby further improving the measurement accuracy of the plasma electron density.

[0020] Further preferably, in the present invention, the first frequency doubling crystal and the second frequency doubling crystal both use nonlinear frequency doubling crystals with a frequency doubling efficiency greater than 50%, which improves the conversion efficiency of the optical signal, enhances the intensity of the second harmonic, and thereby improves the sensitivity and signal quality of the entire measurement system.

[0021] Further preferably, in the present invention, all lenses use plano-convex lenses with the same focal length, which ensures consistent focusing and collimation effects of the light beam in the optical path, reduces optical path errors caused by differences in lens focal lengths, and improves the accuracy of measurement results.

[0022] Preferably, in the present invention, precise optical path adjustment elements and optical fiber couplers are provided between the pulsed laser and the first optical fiber dispersion interferometer component, and between the second optical fiber dispersion interferometer component and the balanced photodetector, to ensure stable transmission and efficient coupling of optical signals; at the same time, through the design of optical fiber arrays and reflectors, flexible adjustment of the optical path and branching processing of signals are achieved, thereby improving the flexibility of the system and measurement efficiency.

[0023] Preferably, in the present invention, the fiber array adopts a one-to-four fiber jumper, and controls the time for the laser to reach the balanced photodetector by designing the fiber length arithmetic difference, which helps to eliminate the influence of time delay on the measurement results; the use of single-mode polarization-maintaining fiber jumpers and fixed aspheric fiber collimators further ensures the stability and transmission quality of the optical signal.

[0024] Preferably, in the present invention, placing the plasma inside a vacuum chamber can effectively control the environmental parameters of the plasma, reduce the impact of external interference on the measurement results, and improve the accuracy and reliability of the measurement.

[0025] The present invention also provides a method for measuring plasma electron density based on fiber optic dispersion interferometry. Based on the above-mentioned plasma electron density measuring device based on fiber optic dispersion interferometry, the method realizes high-sensitivity measurement of low-density plasma through the steps of emitting pulsed laser, frequency doubling and interference of optical signals, and differential amplification of electrical signals, thereby ensuring the accuracy and feasibility of the measurement results.

[0026] Preferably, in the present invention, the plasma electron density measurement method specifically includes key links such as the propagation of light signals in a vacuum cavity, adjustment of polarization states, generation and interference of second harmonics, and the method achieves accurate measurement of plasma electron density by precisely controlling the above links. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 A schematic diagram of the structure of a plasma electron density measurement device based on optical fiber dispersion interferometry provided in an embodiment of the present invention; Figure 2 A schematic structural diagram of another plasma electron density measuring device based on optical fiber dispersion interferometry provided in an embodiment of the present invention; Figure 3 A flow chart of a plasma electron density measurement method based on optical fiber dispersion interferometry provided by the present invention; Figure 4 A flowchart of a plasma electron density measurement device based on optical fiber dispersion interferometry provided in an embodiment of the present invention.

[0028] Reference numerals: Pulse laser-1, pulse laser beam-2, beam splitter 3, first fiber coupler-4, optical fiber-5, fiber collimator-6, first reflector-7, first polarizer-8, first adjustable aperture-9, first lens-10, first frequency doubling crystal-11, second lens-12, second adjustable aperture-13; second reflector-14, plasma-15, third reflector-16, third adjustable aperture-17, second polarizer-18, third lens-19, second frequency doubling crystal-20, fourth lens-21, fourth adjustable aperture-22, third polarizer-23, fourth reflector-24, filter-25, polarization beam splitter-26, second fiber coupler-27, third fiber coupler-28, first fiber array-29, balanced photodetector-30, second fiber array-31, vacuum chamber 32. DETAILED DESCRIPTION

[0029] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention more clearly understood, the present invention is further described in detail in the following specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0030] In order to make the purpose, 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 drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings here can be arranged and designed in various different configurations.

[0031] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0032] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, further definition and explanation thereof is not required in subsequent drawings.

[0033] In the description of the embodiments of the present invention, it should be noted that if the terms "upper", "lower", "horizontal", "inner", etc. indicate an orientation or positional relationship based on the orientation or positional relationship shown in the drawings, or the orientation or positional relationship in which the product of the invention is usually placed when in use, it is only for the convenience of describing the present invention and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention. In addition, the terms "first", "second", etc. are only used to distinguish the description, and cannot be understood as indicating or implying relative importance.

[0034] In addition, if the term "horizontal" appears, it does not mean that the component must be absolutely horizontal, but can be slightly tilted. For example, "horizontal" only means that its direction is more horizontal than "vertical", which does not mean that the structure must be completely horizontal, but can be slightly tilted.

[0035] In the description of the embodiments of the present invention, it is also necessary to explain that, unless otherwise clearly specified and limited, the terms "set", "install", "connect", and "connect" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal connection of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0036] The present invention is further described in detail below in conjunction with the accompanying drawings: Example 1 As mentioned in the background technology above, for some line-integrated electron densities below 10 -17 cm -2 The diagnostic capability of low-density plasma is limited due to low measurement sensitivity and narrow measurement range, and the relevant physical models and numerical simulation programs are still imperfect, resulting in unclear physical images of the low-density plasma generation mechanism. It can be seen that the measurement sensitivity of low-density plasma is low and the measurement range is narrow, resulting in low measurement efficiency and poor measurement effect.

[0037] In order to solve the above problems, this embodiment provides a plasma electron density measurement device based on fiber dispersion interferometry. The use of this device can solve the problems of low sensitivity and narrow measurement range of plasma electron density measurement using interference.

[0038] like Figure 1 As shown, this embodiment provides a plasma measurement device based on fiber-coupled dispersion interferometry, which is used to measure plasma electron density, including: a pulse laser 1, a beam splitter 3, a first fiber coupler 4, an optical fiber 5, a fiber collimator 6, a first reflector 7, a first adjustable aperture 8, a first polarizer 9, a first lens 10, a first frequency doubling crystal 11, a second lens 12, and a second adjustable aperture 13; a second reflector 14, a plasma 15, a third reflector 16, a third adjustable aperture 17, a second polarizer 18, a third lens 19, a second frequency doubling crystal 20, a fourth lens 21, a fourth adjustable aperture 22, a third polarizer 23, a fourth reflector 24, a filter 25, a polarization beam splitter 26, a second fiber coupler 27, a third fiber coupler 28, a first fiber array 29, a balanced photodetector 30, a second fiber array 31, and a vacuum cavity 32.

[0039] A pulse laser 1 is used as a light source to emit a pulse laser beam 2. The pulse laser beam 2 emitted by the pulse laser 1 is incident on a beam splitter 3; then incident on a first fiber coupler 4 from the beam splitter 3; then incident on a first fiber coupler 4 through an optical fiber 5 to a fiber collimator 6; then the fiber collimator 6 re-enters the free space and is incident on a first reflector 7; then the reflected light of the first reflector 7 passes through a first adjustable aperture 8, a first polarizer 9, and a first lens 10 in sequence and then enters a first frequency doubling crystal 11; then the first frequency doubling crystal 11 is emitted and passes through a second lens 12, a second adjustable aperture 13, and a second reflector 4 and then passes through a plasma 15 to a third reflector 16; then the reflected light of the first reflector 7 passes through a first adjustable aperture 8, a first polarizer 9, and a first lens 10 and then enters a first frequency doubling crystal 11; then the reflected light of the first frequency doubling crystal 1 ... The mirror 16 is incident on the second frequency doubling crystal 20 through the second adjustable aperture 17, the second polarizer 18, and the third lens 19; it is emitted from the second frequency doubling crystal 20 and is incident on the fourth reflector 24 through the fourth lens 21, the fourth adjustable aperture 22, and the third polarizer 23; it is reflected by the fourth reflector 24, split by the filter 25 and the polarization beam splitter 26, and is incident on the second fiber coupler 27 and the third fiber coupler 28; it is incident coupled by the second fiber coupler 27 to the first fiber array 29 to the positive input port of the balanced photodetector 30; it is incident coupled by the third fiber coupler 28 to the second fiber array 31 to the negative input port of the balanced photodetector 30.

[0040] like Figure 2 As shown, in this embodiment, the plasma 15 is disposed inside the vacuum chamber 32. The vacuum chamber 32 is sealed with a steel plate and evacuated to a vacuum using a vacuum pump; the plasma 15 is generated by applying a pulse current to a metal wire array load, the peak value of the pulse current is 450 kA, and the rising edge is 400 ns; the pulse width is in the order of ns or less to ensure the time resolution of the measurement.

[0041] The first optical fiber array 29 and the second optical fiber array 31 use a one-to-four optical fiber jumper, and the lengths of the four branch optical fibers of the one-to-four optical fiber are designed to be equidistant to control the time when the laser reaches the balanced photodetector; the optical fiber 5, the first optical fiber array 29 and the second optical fiber array 31 are all single-mode polarization-maintaining optical fiber jumpers with high return loss and low insertion loss.

[0042] The first polarizer 8, the first adjustable aperture 9, the first lens 10, the first frequency doubling crystal 11, the second lens 12 to the second adjustable aperture 13 are all connected by a 30mm coaxial system, and the third adjustable aperture 17, the second polarizer 18, the third lens 19, the second frequency doubling crystal 20, the fourth lens 21, the fourth adjustable aperture 22, and the third polarizer 23 optical elements are all connected by a 30mm coaxial system.

[0043] The first fiber coupler 4, the second fiber coupler 27 and the third fiber coupler 28 are fixed aspheric fiber collimators.

[0044] The first frequency doubling crystal 11 and the second frequency doubling crystal 20 are frequency doubling crystals with a frequency doubling efficiency greater than 50%.

[0045] The first lens 10 , the second lens 12 , the third lens 19 , and the fourth lens 21 are all plano-convex lenses with the same focal length.

[0046] This embodiment provides a plasma electron density measurement device based on fiber dispersion interferometry, and the specific working principle is as follows: The fiber interferometer (i.e., the first fiber dispersion interferometer and the second fiber dispersion interferometer) is composed of two nonlinear frequency-doubling crystals surrounding the plasma to be measured. A part of the incident pulsed laser beam 2 is frequency-doubled by the first nonlinear crystal, so that the detection light becomes a superposition of the fundamental wave F and its second harmonic SH, passing through the plasma, where both waves encounter a characteristic phase shift; in order to obtain an interference beam, the transmitted fundamental wave F is frequency-doubled again by the second nonlinear crystal, while the second harmonic SH remains unchanged; the optical signals of the two mutually interfering second harmonics are converted into electrical signals through a balanced photodetector and differentially amplified. The differentially amplified electrical signal is used to realize the calculation and measurement of the plasma electron density.

[0047] Example 2 like Figure 3 As shown, this embodiment provides a plasma electron density measurement method based on optical fiber dispersion interferometry, based on the plasma electron density measurement device based on optical fiber dispersion interferometry provided in Example 1, specifically comprising: Step 1: Pulse laser 1 emits pulse laser beam 2; Step 2: The first fiber dispersion interferometer doubles the frequency of the incident pulsed laser beam and emits a superimposed light formed by the superposition of the fundamental wave and the original second harmonic; Step 3: The second fiber dispersion interference component doubles the frequency of the superimposed light passing through the plasma 15, and emits a new second harmonic and the original second harmonic that interfere with each other; Step 4: The balanced photodetector 30 receives the optical signal of the interference between the new second harmonic and the original second harmonic, converts the optical signal into an electrical signal, and differentially amplifies the electrical signal at the same time; the plasma electron density is obtained according to the differential amplification result of the electrical signal.

[0048] The specific implementation process of the above method steps is as follows Figure 4 As shown, specifically including: Firstly, a plasma electron density measurement system based on fiber dispersion interferometry was built based on a pulsed laser beam to measure the interference phase difference. The balanced photodetector 30 receives the light signal and converts it into an electrical signal parameter according to the light intensity; The electrical signal parameters pass through the differential amplifier module in the balanced photodetector 30 to amplify the interference phase difference; The distribution of electron line integral density (electron density) is obtained by calculating the result of differential amplification based on the interference phase difference.

[0049] In this embodiment, more specific steps are as follows: The fundamental wavelength of the pulsed laser beam is λ F The free-space laser is coupled into the optical fiber through the first optical fiber coupler and then re-enters the free space through the optical fiber collimator; the wavelength is The laser beam is in a vertical state after passing through the first polarizer; after passing through the first frequency doubling crystal, the second harmonic is generated for the first time, with a wavelength of ; Two wavelengths of laser beams coaxially enter the vacuum cavity and pass through the plasma to produce a phase difference ; The wavelength is The second polarizer rotates the fundamental frequency light by 90° to a horizontal state, rotates the second harmonic by 180° to retain a vertical polarization state, and then enters the second frequency doubling crystal; after passing through the second frequency doubling crystal and the filter, the coaxial laser beam contains the second harmonic generated for the first time and the second time; after passing through the wavelength The second polarizer deflects the second harmonic by 45°, and the two second harmonics still remain in a vertical state to achieve interference, with a phase difference of , and then the polarization beam splitter separates the vertical and horizontal components of the two interfering, mutually perpendicular second harmonics.

[0050] The vertical component and horizontal component of the two mutually perpendicular second harmonics separated by the polarization beam splitter are coupled to the fiber array through the fiber coupler; the first fiber array and the second fiber array are respectively connected to the first four-in-one fiber jumper and the second four-in-one fiber jumper, and the lengths of the four-branch fiber branches are designed to be equidistant to control the time interval for the laser to reach the balanced photodetector. The first four-in-one optical fiber jumper and the second four-in-one optical fiber jumper are respectively connected to the positive input terminal and the negative input terminal of the balanced photodetector to receive the separated vertical component and horizontal component optical signal. and , and convert it into an electrical signal according to the light intensity; The balanced photodetector converts the vertical and horizontal components of the optical signal into and Convert it into an electrical signal, and achieve differential amplification of the electrical signal through TIA components and OPAMP differential amplifiers.

[0051] When the density of the illuminated medium changes, the refractive index of the medium changes accordingly, which is intuitively reflected as the bending or movement of the interference fringes. Neutral particles and ions change the plasma dispersion relation, producing an effect with the opposite sign to that of electrons. The refractive index can be expressed as: (1) In the formula, The wavelength is The refractive index of the laser is represents the refractive index of electrons, represents the refractive index of neutral particles and ions, represents the effective electron number density, represents the number density of neutral particles and ions, where It can be expressed as , is the charge of the electron, is the dielectric constant of vacuum, is the mass of the electron, is the speed of light; But in a fully ionized plasma ,and , so the refractive index can be expressed as: (2) After passing through the interference device, the phase change formula can be obtained based on the optical path difference: (3) In the formula, is the phase change; Simplifying a bit, we can say that the wavelength is The phase change is based on the wavelength The total phase difference is thus: (4) For example, when the fundamental wavelength is 1064 nm, the relationship between the phase difference and the plasma line integral density can be expressed as: (5) In the formula, is the plasma electron line integrated density; The phase change of the laser light after passing through the low-density plasma is small, so the interferometer must be very sensitive. Assuming parallel plane waves, the intensity modulation in the interference pattern becomes: (6) In the formula, , is the intensity of the second harmonic laser beam, and the operating point of the sensitive measurement interferometer is fixed to ,in is an integer greater than or equal to 0; For maximum sensitivity, the system is further extended with a polarizing beam splitter to produce two complementary interference patterns: (7) In the formula include and , represents the two complementary interference intensities after polarization splitting The differential signal can be expressed as: (8) Balanced photodetector spacing Output a signal at different spatial locations, where: (9) In the formula, To balance the photodetector sampling frequency.

[0052] The plasma electron density can be inferred from the differential signal result and formula (5).

[0053] In summary, the present invention provides a plasma electron density measurement device and method based on fiber dispersion interferometry, which has the following advantages over existing measurement measures: First, high-precision measurement: Through fiber-optic interferometry technology, the pulsed laser beam is frequency-doubled to generate new second harmonics and original second harmonics that interfere with each other. A balanced photodetector is used to convert the optical signal into an electrical signal and perform differential amplification, thereby accurately measuring the plasma electron density and improving the accuracy and reliability of the measurement.

[0054] Second, compact structure and good stability: The device uses coaxially arranged optical elements and fiber optic components to reduce interference and errors in the optical path. At the same time, the use of optical fiber enhances the stability and anti-electromagnetic interference capability of the system, making the entire measuring device compact and easy to integrate.

[0055] Third, the high efficiency of nonlinear frequency doubling crystals: Using nonlinear frequency doubling crystals with a frequency doubling efficiency greater than 50% improves the light-to-frequency conversion efficiency and enhances the light signal intensity, thereby improving the signal-to-noise ratio of the measurement signal.

[0056] Fourth, flexible optical path design: By adjusting optical elements such as adjustable apertures, lenses and reflectors, the optical path can be flexibly adjusted to achieve accurate measurements under different plasma environments and conditions.

[0057] Fifth, multi-channel differential amplification: The interference light signal is separated into vertical and horizontal components using a polarization beam splitter, and differential amplification is performed through two independent fiber arrays and balanced photodetectors, further improving the sensitivity and accuracy of the measurement.

[0058] Sixth, strong adaptability: the device can adapt to plasma measurements in different density ranges. By adjusting optical elements and measurement parameters, accurate monitoring and analysis of different plasma states can be achieved.

[0059] Seventh, easy to operate and maintain: The entire device adopts a modular design, and the connections between the components are simple, which is easy to operate and maintain, while reducing maintenance costs and usage complexity.

[0060] The above embodiment is only one of the implementation methods that can realize the technical solution of the present invention. The scope of protection claimed by the present invention is not limited only to this embodiment, but also includes changes, replacements and other implementation methods that can be easily thought of by any technician familiar with the technical field within the technical scope disclosed by the present invention.

Claims

1. A plasma electron density measurement device based on fiber dispersion interferometry, characterized in that: include: A pulse laser (1) for emitting a pulse laser beam (2); A first optical fiber dispersion interference component is used to double the frequency of an incident pulsed laser beam (2) and emit superimposed light formed by superposition of a fundamental wave and an original second harmonic; A second fiber dispersion interference component is used to double the frequency of the superimposed light passing through the plasma (15) to emit a new second harmonic and the original second harmonic that interfere with each other; The balanced photodetector (30) is used to convert the optical signal of the new second harmonic and the original second harmonic into an electrical signal, and to differentially amplify the electrical signal at the same time, so as to obtain the plasma electron density according to the differential amplification result of the electrical signal.

2. The plasma electron density measuring device based on optical fiber dispersion interferometry according to claim 1, characterized in that: The first optical fiber dispersion interference component comprises a first polarizer (8), a first adjustable iris (9), a first lens (10), a first frequency doubling crystal (11), a second lens (12) and a second adjustable iris (13) which are coaxially arranged in sequence from a receiving direction to a transmitting direction; and the second optical fiber dispersion interference component comprises a third adjustable iris (17), a second polarizer (18), a third lens (19), a second frequency doubling crystal (20), a fourth lens (21), a fourth adjustable iris (22) and a third polarizer (23) which are coaxially arranged in sequence from a receiving direction to a transmitting direction.

3. The plasma electron density measuring device based on optical fiber dispersion interferometry according to claim 2, characterized in that: The first frequency doubling crystal (11) and the second frequency doubling crystal (20) are both nonlinear frequency doubling crystals with a frequency doubling efficiency greater than 50%.

4. The plasma electron density measuring device based on optical fiber dispersion interferometry according to claim 2, characterized in that: The first lens (10), the second lens (12), the third lens (19) and the fourth lens (21) are plano-convex lenses having the same focal length.

5. The plasma electron density measuring device based on optical fiber dispersion interferometry according to claim 1, characterized in that: A beam splitter (3), a first optical fiber coupler (4), an optical fiber collimator (6) and a first reflector (7) arranged in sequence are provided between the pulse laser (1) and the first optical fiber dispersion interference component; the first optical fiber coupler (4) and the optical fiber collimator (6) are connected via an optical fiber (5); A fourth reflector (24), a filter (25) and a polarization beam splitter (26) are arranged between the second optical fiber dispersion interference component and the balanced photodetector (30); a first emission end of the polarization beam splitter (26) forms a first optical path with the balanced photodetector (30) through a second optical fiber coupler (27) and a first optical fiber array (29); and a second emission end forms a second optical path with the balanced photodetector (30) through a third optical fiber coupler (28) and a second optical fiber array (31).

6. The plasma electron density measuring device based on optical fiber dispersion interferometry according to claim 5, characterized in that: The first optical fiber array (29) and the second optical fiber array (31) both use a one-to-four optical fiber jumper; wherein the lengths of the four branch optical fibers of the one-to-four optical fiber jumper are designed to be arithmetic difference, so as to control the time when the laser reaches the balanced photodetector (30); the optical fiber (5), the first optical fiber array (29) and the second optical fiber array (31) all use single-mode polarization-maintaining optical fiber jumpers; and the first optical fiber coupler (4), the second optical fiber coupler (27) and the third optical fiber coupler (28) all use fixed aspheric optical fiber collimators.

7. The plasma electron density measuring device based on optical fiber dispersion interferometry according to claim 1, characterized in that: The plasma (15) is arranged inside the vacuum chamber (32).

8. A method for measuring plasma electron density based on fiber dispersion interferometry, characterized in that: The plasma electron density measuring device based on optical fiber dispersion interferometry according to any one of claims 1 to 7 comprises: The pulse laser (1) emits a pulse laser beam (2); The first fiber dispersion interference component doubles the frequency of the incident pulsed laser beam (2) and emits superimposed light formed by the superposition of the fundamental wave and the original second harmonic; The second fiber dispersion interference component doubles the frequency of the superimposed light passing through the plasma (15) to emit a new second harmonic and the original second harmonic that interfere with each other; The balanced photodetector (30) receives the optical signal of the new second harmonic and the original second harmonic interfering with each other, converts the optical signal into an electrical signal, and simultaneously differentially amplifies the electrical signal; so as to obtain the plasma electron density according to the differential amplification result of the electrical signal.

9. The plasma electron density measurement method based on optical fiber dispersion interferometry according to claim 8, characterized in that: in, The fundamental wavelength is λ F The pulsed laser beam (2) passes through a first optical fiber coupler (4), couples the free-space laser into an optical fiber (5), and then re-enters the free space through an optical fiber collimator (6); The pulsed laser beam (2) is in a vertical state after passing through the first polarizer (8); after passing through the first frequency doubling crystal (11), a wavelength of The original second harmonic of The two wavelength laser beams coaxially enter the vacuum chamber (32) and pass through the plasma (15) to generate a phase difference. ; Then after a wavelength of The second polarizer (18) rotates the fundamental frequency light by 90° to a horizontal state, rotates the original second harmonic by 180° to retain a vertical polarization state, and then enters the second frequency doubling crystal (20); After passing through the second frequency doubling crystal (20) and then the filter, the coaxial laser beam contains two second harmonics, the original second harmonic and the new second harmonic; The third polarizer (23) deflects both second harmonics by 45°, and the two second harmonics still maintain a vertical state to achieve interference; The total phase difference for ; The wavelength is The phase difference produced by the laser after passing through the plasma; The wavelength is The phase difference produced by the laser after passing through the plasma; Then, the vertical component and the horizontal component of the two mutually perpendicular interfering second harmonics are separated by a polarization beam splitter (26); The vertical component and the horizontal component of the two mutually perpendicular second harmonics separated by the polarization beam splitter are respectively coupled to the corresponding optical fiber arrays through the optical fiber coupler; The balanced photodetector (30) converts the vertical components of the two second harmonic optical signals into and the horizontal component of the optical signal Convert it into an electrical signal, and achieve differential amplification of the electrical signal through TIA components and OPAMP differential amplifiers.

10. The plasma electron density measurement method based on optical fiber dispersion interferometry according to claim 9, characterized in that: The specific formula for obtaining the plasma electron density based on the electrical signal differential amplification result is as follows: In the formula, represents the total phase difference; represents the refractive index of electrons, is the effective electron number density; Represents the integrated density of plasma electron lines.

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