A system for measuring the spatial distribution of the polarization state of a laser beam in real time

Abstract

A system for measuring the spatial distribution of the polarization state of a laser beam in real time mainly comprises a collimating system, a liquid crystal polarizer, a birefringent element, a CCD camera and a control unit. The incident light beam is incident on the liquid crystal polarizer through the collimating system, and then propagates to the rotating birefringent element. The rotation axis of the birefringent element has a displacement from the center, and the displacement is equal to the diameter of the incident light beam. The P light and S light split by the birefringent element rotate with the birefringent element and are incident on the CCD camera. The control unit analyzes the image of the CCD camera and calculates the polarization direction of the incident light beam through a predictive learning algorithm, and then adjusts the liquid crystal polarizer. The polarization orientation of the liquid crystal polarizer is as close as possible to the polarization spatial distribution of the incident light beam, which improves the measurement sensitivity of the polarization state spatial distribution and provides a high-precision polarization state distribution measurement technology with time resolution.

Description

Technical Field

[0001] This invention relates to a laser beam polarization state measurement technique, and more particularly to a system for real-time measurement of the spatial distribution of laser beam polarization state. Background Technology

[0002] In existing technologies, the measurement of the polarization state of a laser beam typically consists of a polarizer, an analyzer, and a power meter. This approach has been in use for many years. Starting with the initial use of birefringent crystals as polarizers and analyzers, numerous improvements and enhancements have been made to the device structures of polarizers and analyzers, such as instrument digitization and multi-parameter measurement schemes. However, the basic approach still relies on the fundamental combination of a polarizer, analyzer, and power meter. In recent years, the emergence of advanced laser beams with complex polarizations, such as radially polarized light, vortex light, higher-order Laguerre Gaussian beams, and photon spin states, has spurred market demand for the spatial distribution and real-time measurement of polarization states. For measuring the spatial distribution of polarization states, a CCD camera is generally used instead of a power meter to record the spatial distribution of beam power in P-polarized and S-polarized states, thus obtaining the spatial distribution of polarization states. However, the acquisition speed of CCD cameras is difficult to improve; research-grade CCD cameras currently only achieve about 100 frames per second, and such cameras are very expensive, making them unacceptable to the industrial sector.

[0003] In lidar applications, a continuously rotating polarizer is used to detect the polarization state of the received laser beam, which can effectively improve the detection sensitivity of lidar. However, this method is only suitable for simple linearly polarized light and circularly polarized light, and cannot measure the spatial distribution details of complex spatially polarized beams. In the direct polarization detection satellite launched by China, nine polarizers with different orientations are installed on a large rotating disk. When these nine polarizers sweep across the detection beam, the polarization state of reflected light from the same area can be detected time-divisionally, thus achieving multi-angle polarization detection. However, this solution is not only large in size, but also more expensive and more difficult to maintain than most users can afford.

[0004] In summary, both industry and academia need to measure the spatial distribution of polarization states in real time, but there is currently no reliable, user-friendly, and cost-effective technical solution. Summary of the Invention

[0005] To address the aforementioned problems, this invention proposes a system for real-time measurement of the spatial distribution of laser beam polarization states. The system mainly comprises a collimation system, a liquid crystal polarizer, a birefringent element, a CCD camera, and a control unit. The incident beam passes through the collimation system onto the liquid crystal polarizer. After passing through the liquid crystal polarizer, the beam propagates to the birefringent element, which rotates around its rotation axis. The rotation axis has a displacement equal to the diameter of the incident beam from the center of the birefringent element. The beam split by the birefringent element consists of P-beams and S-beams, which rotate with the birefringent element and are then incident on the CCD camera. The control unit analyzes and calculates the polarization direction of the incident beam based on the image from the CCD camera. The liquid crystal polarizer is then adjusted to ensure that the polarization orientation of each polarization unit of the liquid crystal polarizer is as close as possible to the spatial distribution of the incident beam's polarization.

[0006] In this system, the centers of the liquid crystal polarizer and the birefringent element are on the same axis, the center of the CCD camera is on the same straight line as the optical axis containing the rotation axis of the birefringent element, the CCD camera is a camera that uses a charge-coupled device as a photosensitive element, P-light represents parallel polarized light, and S-light represents vertical polarized light.

[0007] Furthermore, a system for real-time measurement of the spatial distribution of laser beam polarization states also includes an adjustable neutral attenuator, which is positioned in front of a liquid crystal polarizer.

[0008] Furthermore, a system for real-time measurement of the spatial distribution of laser beam polarization states also includes a focusing lens positioned in front of the CCD camera.

[0009] Furthermore, in a system for real-time measurement of the spatial distribution of the polarization state of a laser beam, the birefringent element is a Voss-Raton prism.

[0010] Furthermore, in a system for real-time measurement of the spatial distribution of the polarization state of a laser beam, the birefringent element is a thin-film polarizer.

[0011] Furthermore, in a system for real-time measurement of the spatial distribution of the polarization state of a laser beam, the birefringent element is a ring-shaped Dammann grating.

[0012] Furthermore, in a system for real-time measurement of the spatial distribution of the polarization state of a laser beam, the focusing lens is an angle corrector.

[0013] Furthermore, the control unit predicts the polarization orientation of each polarization unit of the liquid crystal polarizer using a predictive learning algorithm.

[0014] Furthermore, the predictive learning algorithm predicts the polarization orientation of each polarization unit of the liquid crystal polarizer by including the following steps:

[0015] S1: Process the image incident on the CCD camera;

[0016] S2: Polarization analysis, to determine the spatial polarization state distribution of the incident beam;

[0017] S3: Predict the spatial polarization distribution of the incident beam;

[0018] S4: Set the orientation of each polarization unit of the liquid crystal polarizer.

[0019] The system for real-time measurement of the spatial distribution of laser beam polarization states provided by this invention mainly includes a collimation system, a liquid crystal polarizer, a birefringent element, a CCD camera, and a control unit. The incident beam is collimated by the collimation system and then propagates onto the liquid crystal polarizer. The beam then propagates to the rotating birefringent element, whose rotation axis is displaced from its center by a displacement equal to the diameter of the incident beam. The P-beam and S-beam, split by the birefringent element, rotate with the element and are then incident on the CCD camera. The control unit analyzes the image from the CCD camera and calculates the polarization direction of the incident beam using a predictive learning algorithm. The control unit then adjusts the liquid crystal polarizer to make the polarization orientation of each polarization unit as close as possible to the spatial distribution of the incident beam's polarization. This invention can detect polarization state distributions with millisecond-level temporal resolution and micrometer-level spatial resolution, providing a high-precision measurement technology for polarization state distribution with time resolution. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of a system for real-time measurement of the spatial distribution of laser beam polarization state according to the present invention;

[0021] Figure 2 This is a schematic diagram of a first embodiment of a system for real-time measurement of the spatial distribution of laser beam polarization states according to the present invention;

[0022] Figure 3 This is a schematic diagram of a second embodiment of a system for real-time measurement of the spatial distribution of laser beam polarization states according to the present invention;

[0023] Figure 4 This is a schematic diagram of a third embodiment of a system for real-time measurement of the spatial distribution of laser beam polarization state according to the present invention.

[0024] Explanation of reference numerals in the attached figures

[0025] 101 Incident beam; 102 Collimation system; 103 Adjustable neutral density filter

[0026] 104 Liquid crystal polarizer; 105 Rotation axis of birefringent element; 106 Birefringent element

[0027] 107 Focusing lens; 108 CCD camera; 109 Control unit

[0028] 400 Beam splitter, 401 First reflecting mirror, 402 Beam splitter prism

[0029] 403 ITO film; 404 Second reflecting mirror; 405 Composite prism

[0030] 406 First Worthraton Prism; 407 Angle Corrector; 408 Third Reflector

[0031] 501 Emitting light source; 502 Object under test; 503 Glan prism; 504 Plaza intensity analyzer

[0032] 505 Second Wollardon Prism; 506 Circular Pattern; 507 Relationship between Light Intensity and Time

[0033] 601 Active fiber of fiber laser; 602 Polarizing element; 603 Output cavity mirror

[0034] 604 Circular Damman Grating; 605 Circular Image. Detailed Implementation

[0035] To provide a further understanding of the purpose, structure, features, and functions of the present invention, detailed descriptions are provided below with reference to specific embodiments.

[0036] To address the above problems, this invention proposes a system for real-time measurement of the spatial distribution of laser beam polarization states, see [link to relevant documentation]. Figure 1 , Figure 1This is a schematic diagram of a system for real-time measurement of the spatial distribution of laser beam polarization states according to the present invention. It mainly includes a collimation system 102, a liquid crystal polarizer 104, a birefringent element 106, a CCD camera 108, and a control unit 109. An incident beam 101 passes through the collimation system 102 and is incident on the liquid crystal polarizer 104. The beam propagates through the liquid crystal polarizer 104 to the birefringent element 106. The birefringent element 106 rotates around its rotation axis, and its rotation axis has a displacement equal to the diameter of the incident beam 101. The beam passing through the birefringent element 106 is split into P-beams and S-beams. P-beams represent parallel polarized light, and S-beams represent perpendicular polarized light. The split P-beams and S-beams rotate with the birefringent element 106 and are incident on the CCD camera 108. Because the center of the birefringent element 106 has a displacement equal to the diameter of the incident beam 101 relative to its rotation axis, the split beams are thus... The P-beam and S-beam do not overlap in the image formed by the CCD camera. The time it takes for the P-beam and S-beam, which are split by the birefringent element 106, to complete one rotation is equal to the time it takes for the CCD camera 108 to update one frame. At this time, the CCD camera 108 records the integrated light intensity distribution of the P-beam and S-beam after one rotation. The control unit 109 analyzes and calculates the polarization direction of the incident beam 101, i.e., the polarization spatial distribution, based on the integrated light intensity distribution image formed by the CCD camera 108. Then, the liquid crystal polarizer 104 is adjusted so that the polarization orientation of each polarization unit of the liquid crystal polarizer 104 is as close as possible to the polarization spatial distribution of the incident beam 101. The centers of the liquid crystal polarizer 104 and the birefringent element 106 are on the same axis, and the center of the CCD camera 108 is on the same straight line as the optical axis containing the rotation axis of the birefringent element 106. The CCD camera 106 is a camera that uses a charge-coupled device as a photosensitive element.

[0037] When the polarization orientation of each polarization unit in the liquid crystal polarizer 104 is completely perpendicular to the corresponding local polarization direction of the incident beam 101, the incident beam 101 will be completely unable to pass through the liquid crystal polarizer 104. At this time, the image recorded by the CCD camera 108 is pure black. When the polarization orientation of any polarization unit in the liquid crystal polarizer 104 is not perpendicular to the corresponding local polarization direction of the incident beam 101, part of the incident beam 101 will pass through the liquid crystal polarizer 104 and then be transmitted to the birefringent element 106, where it will be split into P-beams and S-beams, and illuminate different parts of the photosensitive element of the CCD camera 108. By analyzing the brightness of the P-beams and S-beams, the angle between the polarization angle of the specific polarization unit in the liquid crystal polarizer 104 and the corresponding local polarization direction of the incident beam 101 can be determined, thereby determining the polarization direction of the incident beam 101, that is, the spatial polarization state distribution. Specifically, taking the 01x02 polarization unit in the liquid crystal polarizer 104 as an example, this polarization unit represents the polarization unit with unit number 01 in the X-axis and unit number 02 in the Y-axis. Assuming that the relative light intensity of the P light at the corresponding position of the CCD camera 108 at a certain moment is 0.7 and the relative light intensity of the S light is 0.2, then the angle between the polarization orientation of the incident beam 101 and the 01x02 polarization unit in the liquid crystal polarizer 104 is θ1 = argtan(0.2 / 0.7). If the angle between the polarization orientation of the 01x02 polarization unit and the X-axis is θ2, then the polarization angle of the incident beam 101 is θ1 + θ2.

[0038] Preferably, a system for real-time measurement of the spatial distribution of the polarization state of a laser beam further includes an adjustable neutral attenuator 103, which is disposed in front of the liquid crystal polarizer 104. Generally, an adjustable neutral attenuator 103 is required for strong light detection.

[0039] Preferably, the system for real-time measurement of the spatial distribution of the polarization state of a laser beam provided by the present invention further includes a focusing lens 107, which is set in front of the CCD camera 108. The focusing lens 107 is added when the full aperture beam needs to be acquired, and the focusing lens 107 is not needed if only a part of the aperture beam is acquired.

[0040] Preferably, the birefringent element 106 is a Wollardon prism, but it can also be a thin-film polarizer or other similar components. The present invention does not impose any limitations on this.

[0041] Preferably, the focusing lens 107 is an angle corrector or other similar components, but the present invention does not impose any restrictions.

[0042] To further improve the temporal resolution of the CCD camera 108 image, it is necessary to minimize the number and intensity of bright spots in the CCD camera 108 image. Bright spots are formed when the incident light beam 101 directly shines on the CCD camera 108. The control unit 109 predicts the polarization orientation of each polarization unit of the liquid crystal polarizer 104 through a predictive learning algorithm, so that the polarization orientation of each polarization unit of the liquid crystal polarizer 104 is as close as possible to the polarization spatial distribution of the incident light beam 101, thereby reducing the number and intensity of bright spots in the CCD camera 108 image.

[0043] Preferably, the predictive learning algorithm predicts the polarization orientation of each polarization unit of the liquid crystal polarizer 104 mainly by calculating the temporal coherence of the incident beam 101 based on the spatial coherence of its polarization state, and then predicting the change of the spatial polarization state of the incident beam 101 at the next time point based on the temporal coherence, thereby using this as the basis for adjusting the polarization orientation of each polarization unit of the liquid crystal polarizer 104. Specifically, this includes the following steps:

[0044] S1: Process the image incident on the CCD camera 108;

[0045] It mainly includes image denoising, image segmentation, and time track reading operations.

[0046] S2: Polarization analysis, to determine the spatial polarization state distribution of the incident beam 101;

[0047] When the polarization orientation of any polarization unit in the liquid crystal polarizer 104 is not perpendicular to the local polarization direction corresponding to the incident beam 101, part of the incident beam 101 will pass through the liquid crystal polarizer 104 and then be transmitted to the birefringent element 106, where it will be split into P-beams and S-beams, and illuminate different parts of the photosensitive element of the CCD camera 108. By analyzing the brightness of the P-beams and S-beams, the angle between the polarization angle of the specific polarization unit in the liquid crystal polarizer 104 and the corresponding local polarization direction of the incident beam 101 can be determined, thereby determining the polarization direction of the incident beam 101, that is, the spatial polarization state distribution of the incident beam 101.

[0048] S3: Predict the spatial polarization distribution of the incident beam 101;

[0049] Based on the relationship between temporal coherence and spatial coherence, i.e. L s =c*L t L s Let L be the spatial coherence length, c be the speed of light in air, and L be the velocity of light in air. tLet L be the temporal coherence length. The spatial coherence of the incident beam's optical field is determined from the spatial image distributions of the P-beam and S-beam, and further, the temporal coherence of the incident beam's optical field is derived. The optical field propagation function is Γ(t)=∫∫S(t)T(x,y)exp{-i(ωt+βz)}dxdy, where S(t) is the temporal coherence coefficient, T(x,y) is the optical field distribution of the incident beam, ω is the angular frequency of the incident beam, and βz is the wave vector in the z-direction. The spatial coherence length L is... s This refers to the second-order matrix <Γ(t)Γ(t) of the propagation function, the optical field propagation function. * The difference between (x, y, z) when >= 0, the time coherence length L t This refers to the second-order matrix <Γ(t)Γ(t) of the propagation function, the optical field propagation function. * The difference between t and t when >= 0 can be quickly solved using the Fast Fourier Transform (FFT) of the light field propagation function. Furthermore, due to the relationship between spatial and temporal coherence, we...

[0050] S4: Set the orientation of each polarization unit of the liquid crystal polarizer 104;

[0051] Based on the predicted spatial polarization distribution of the incident beam 101 at the next moment t, the polarization direction of the liquid crystal polarizer 104 is adjusted in advance to be perpendicular to the spatial polarization distribution of the incident beam 101. This can effectively reduce the number and intensity of bright spots in the image of the CCD camera 108 and improve the sensitivity of polarization state spatial distribution measurement.

[0052] This invention provides a system for real-time measurement of the spatial distribution of laser beam polarization states, which can be used for polarization measurement in space laser coherent communication. (See also...) Figure 2 , Figure 2This is a schematic diagram of a first embodiment of a system for real-time measurement of the spatial distribution of laser beam polarization state according to the present invention. The incident beam is first split into beam A and beam B by a beam splitter 400 with an intensity ratio of 50:50. Beam A is reflected by a first reflecting mirror 401 and a second reflecting mirror 404, and after passing through a collimation system 102 and an adjustable center attenuator 103, it illuminates a liquid crystal polarizer 104. The liquid crystal polarizer 104 initially selects an arbitrary linear polarization direction, such as the S-polarization direction. Due to disturbances from various media during laser transmission, the polarization state is no longer linearly polarized light. Regardless of the polarization direction selected by the polarization unit of the liquid crystal polarizer 104, a laser beam will pass through the liquid crystal polarizer 104. The laser beam passing through the liquid crystal polarizer 104 is then incident on a rotating first Wollaston prism 406. The rotation axis of the first Wollaston prism 406 is offset from its center by an amount equal to the diameter of the beam incident on the first Wollaston prism 406. The laser beam is split into two mutually orthogonally polarized beams by the first Woslaton prism 406. These two beams pass through the angle corrector 407 and finally reach the CCD camera 108. Assume that the optical axis of one beam coincides with the rotation axis of the first Woslaton prism 406, and the direction of this rotation axis also coincides with the optical axis of this beam after passing through the angle corrector 407. The center of the CCD camera 108 is on the same line as the optical axis of this beam, that is, on the same straight line as the rotation axis of the first Woslaton prism 406 in the direction of light propagation. In this embodiment, the angle corrector 407 acts as a focusing lens. The image formed on the CCD camera 108 is calculated by the control unit 109 based on a predictive learning algorithm to determine the polarization state distribution of beam A. The calculation result is then used to adjust the polarization orientation of the polarization units of the liquid crystal polarizer 104 so that the image of the CCD camera 108 is as close to zero as possible. At this point, the polarization orientation of the polarization units of the liquid crystal polarizer 104 is the polarization state distribution of beam A.

[0053] The split beam B is incident on a polarization correction system consisting of a beam splitter prism 402, an ITO film 403, a third reflecting mirror 408, and a combining prism 405. The ITO film 403 is a transparent conductive film of indium tin oxide. Beam B is split into P-beams and S-beams by the beam splitter prism 402. Since the refractive index of the ITO film 403 can be changed by applying a voltage to its surface, the delay of the P-beam relative to the S-beam in the correction system is altered. The voltage on the surface of the ITO film 403 is adjusted by the control unit 109 using the measured polarization distribution information, as the refractive index of the ITO film is linearly related to its surface voltage. Adjusting the surface voltage of the ITO film 403 is equivalent to adjusting the optical path length of the beam through the ITO film 403, thus adjusting the time delay of the P-beam and S-beam. When the time delays of the P-beam and S-beam are equal or differ by half an oscillation period, the final synthesized light is linearly polarized, thus correcting the polarization state of the incident laser to linear polarization and improving the detection signal-to-noise ratio of laser coherent communication.

[0054] This invention provides a system for real-time measurement of the spatial distribution of laser beam polarization states, which can be used in polarization measurement systems for polarized light detection. See [link to relevant documentation]. Figure 3 , Figure 3 This is a schematic diagram of a second embodiment of a system for real-time measurement of the spatial distribution of laser beam polarization state according to the present invention.

[0055] The purpose of the polarization measurement system for polarization detection is to study the properties of the object under test by measuring the change in polarization state of the laser beam after it passes through the object under test. When the polarized light emitted by the emitting light source 501 is reflected by the object under test 502, it enters the system provided by this invention for real-time measurement of the spatial distribution of the polarization state of the laser beam. The beam reflected by the object under test 502 first passes through the collimation system 102 and a Glan prism 503. The rotation axis of the Glan prism 503 is offset from its center by a displacement equal to the incident beam entering the Glan prism 503. Regarding the diameter, it should be noted that in this embodiment, a convex lens is also provided after the collimation system 102. This is because the observation distance of the polarization measurement system for polarization detection is relatively far, and an additional convex lens can adjust the focal length for more precise alignment with the object under test 502. The Glan prism 503 splits the beam into P-beams and S-beams. The P-beams, whose polarization direction is consistent with that of the emitting light source 501, are not altered by the object under test 502. The P-beams are incident on the light field intensity analyzer 504 to measure their light field intensity value. Since the light emitted from the Glan prism 503 consists of multiple locally polarized S-beams, when these locally polarized S-beams pass through the adjustable neutral density filter 103 and illuminate the liquid crystal polarizer 104, if the polarization orientation of the local polarization unit of the liquid crystal polarizer 104 is S, and the S-beams passing through this local area are not zero, then the S-beams will pass through the liquid crystal polarizer 104. If the polarization orientation of the local polarization unit of the liquid crystal polarizer 104 is P, then no light will pass through the liquid crystal polarizer 104 regardless of whether the S-polarized light in that region is zero. The polarization orientation of the local polarization unit of the liquid crystal polarizer 104 switches between a P-polarization orientation and an S-polarization orientation at a frequency of 1000 Hz. The light passing through the liquid crystal polarizer 104 forms a circular pattern 506 with alternating bright and dark colors on the CCD camera 108 after passing through the rotating Wollaston prism 505. The center of the CCD camera 108 is on the same axis as the rotation axis of the Wollaston prism 505, and the time interval between the bright and dark colors of the circular pattern 506 is equal to the rotation frequency of the polarization orientation of the polarization unit on the liquid crystal polarizer 104.

[0056] The sum of the optical field intensity value of the P-beam measured by the optical field intensity analyzer 504 and the integral of the optical field intensity of the S-beam on the pattern on the CCD camera 108 is the total optical field intensity distribution of the beam reflected by the object under test 502. A circular pattern 506 with alternating bright and dark areas is formed on the CCD camera 108. Time analysis is performed along the circumference of the circular pattern 506, with different angular positions corresponding to different times, as shown in the light intensity versus time relationship 507. Through intensity and time analysis of the alternating bright and dark pattern 506, in the light intensity versus time relationship 507, the ratio of peaks to troughs represents the intensity of the S-beam of the incident light within a certain time period. Simultaneously, the relative integral optical field intensity of the P-beam at the corresponding position of the optical field intensity analyzer 504 during this time period (relative integral optical field intensity refers to the integral of the optical field intensity on a ring with the same radius as the center of the beam and the circular pattern 506) is read, which is the intensity of the P-beam. The polarization orientation, i.e., the polarization angle, of this local polarization unit can be obtained from the ratio of the P-beam intensity to the S-beam intensity. Since the intensity of S-rays is time-dependent, the polarization angle of the local polarization unit can also be considered as a function of time variation. It can be used as a numerical value of the time resolution of the polarization state of the incident light. The time and spatial distribution of the polarization state of the beam reflected by the object under test 502 can be obtained. Since the polarization distribution of the beam reflected by the object under test 502 is related to the characteristics of the object under test, the properties of the object under test can be obtained by measuring the polarization state distribution of the beam after passing through the object under test 502 using the liquid crystal polarizer 104.

[0057] This invention provides a system for real-time measurement of the spatial distribution of laser beam polarization states, applicable to polarization monitoring systems of fiber lasers that output vortex light. See also: Figure 4 , Figure 4This is a schematic diagram of a third embodiment of a system for real-time measurement of the spatial distribution of laser beam polarization states according to the present invention. The active fiber 601 of the fiber laser outputs vortex light, the output characteristics of which are achieved by the polarization element 602 within the fiber laser and output externally through the output cavity mirror 603. The output polarized light enters the system for real-time measurement of the spatial distribution of laser beam polarization states provided by the present invention, which consists of a collimation system 102, an adjustable neutral density attenuator 103, a liquid crystal polarizer 104, a birefringent element (here, a ring-shaped Dammann grating 604), a CCD camera 108, and a control unit 109 that analyzes the ring image 605. Because the light to be measured is directly emitted from the laser, no focusing lens is placed in front of the CCD camera 108. The control unit 109 analyzes the ring image 605 formed on the CCD camera 108 and controls the polarization element 602 in the fiber laser to improve the output characteristics and stability of the fiber laser. Since the output laser beam from the active fiber 601 of the fiber laser is vortex light, the rotating birefringent element uses a ring-shaped Damman grating 604 with the same scale as the vortex light. When the output laser beam from the active fiber 601 of the fiber laser is pure vortex light without linear polarization components, the liquid crystal polarizer 104 only needs to select a linear polarization direction. At this time, the image on the CCD camera 108 is a regular ring image 605, which simplifies the complexity and computation time of the prediction learning algorithm. Since the rotation axis of the ring-shaped Damman grating 604 has a small offset from the center of the ring-shaped Damman grating 604, the offset is the diameter of the beam incident on the ring-shaped Damman grating 604. When the ring-shaped Damman grating 604 does not rotate, the image formed on the CCD camera 108 is an ellipse with alternating bright and dark areas. The center of the ring-shaped Damman grating 604 and the point where the rotation axis of the ring-shaped Damman grating 604 is located constitute the two foci of the ellipse. When the annular Dammann grating 604 rotates, the alternating bright and dark ellipses formed on the CCD camera 108 also rotate around the rotation axis of the annular Dammann grating 604, ultimately forming an alternating bright and dark annular image 605. If the polarization state of the vortex light emitted by the active fiber 601 of the fiber laser is ideal, the annular image 605 is a standard ring. When the polarization state of the vortex light emitted by the active fiber 601 of the fiber laser is impure, the annular image 605 will be distorted. The location of the distortion is the polarization state distribution of the vortex light emitted by the active fiber 601 of the fiber laser at that moment. Based on this distribution, the control unit 109 adjusts the polarization orientation of the polarization unit at the corresponding position of the liquid crystal polarizer 104 through a predictive learning algorithm, so that the distortion of the annular image 605 disappears.

[0058] The system for real-time measurement of the spatial distribution of laser beam polarization states provided by this invention mainly includes a collimation system, a liquid crystal polarizer, a birefringent element, a CCD camera, and a control unit. The incident beam is collimated by the collimation system and then propagates onto the liquid crystal polarizer. The beam then propagates to the rotating birefringent element, whose rotation axis is displaced from its center by a displacement equal to the diameter of the incident beam. The P-beam and S-beam, split by the birefringent element, rotate with it and are then incident on the CCD camera. The control unit analyzes the image from the CCD camera and calculates the polarization direction of the incident beam using a predictive learning algorithm. The control unit then adjusts the liquid crystal polarizer to make the polarization orientation of each polarization unit as close as possible to the spatial distribution of the incident beam's polarization. This invention can detect polarization state distributions with millisecond-level temporal resolution and micrometer-level spatial resolution, providing a high-precision measurement technology for polarization state distribution with time resolution and improving the sensitivity of polarization state spatial distribution measurement.

[0059] The present invention has been described in the above-described embodiments; however, these embodiments are merely examples for implementing the present invention. It must be noted that the disclosed embodiments do not limit the scope of the present invention. Conversely, any modifications and refinements made without departing from the spirit and scope of the present invention are within the scope of patent protection of the present invention.

Claims

1. A system for real-time measurement of the spatial distribution of the polarization state of a laser beam, characterized in that, The system includes a collimation system, a liquid crystal polarizer, a birefringent element, a CCD camera, and a control unit. The incident light beam is collimated by the collimation system and then incident on the liquid crystal polarizer. The light beam after passing through the liquid crystal polarizer propagates to the birefringent element, which rotates around its rotation axis. The rotation axis has a displacement from the center of the birefringent element, which is equal to the diameter of the incident light beam. The light beam split by the birefringent element is divided into P-beams and S-beams. The P-beams and S-beams rotate with the birefringent element and are incident on the CCD camera. The control unit analyzes and calculates the polarization direction of the incident light beam based on the image from the CCD camera, and then adjusts the liquid crystal polarizer so that the polarization orientation of each polarization unit of the liquid crystal polarizer is as close as possible to the polarization spatial distribution of the incident light beam. The centers of the liquid crystal polarizer and the birefringent element are on the same axis, the center of the CCD camera is on the same straight line as the optical axis containing the rotation axis of the birefringent element, the CCD camera is a camera that uses a charge-coupled element as a photosensitive element, the P-light represents parallel polarized light, and the S-light represents vertical polarized light; the system also includes an adjustable neutral attenuator, which is disposed in front of the liquid crystal polarizer; The system also includes a focusing lens positioned in front of the CCD camera.

2. The system for real-time measurement of the spatial distribution of laser beam polarization state according to claim 1, characterized in that, The birefringent element is a Worsleyton prism.

3. The system for real-time measurement of the spatial distribution of laser beam polarization state according to claim 1, characterized in that, This birefringent element is a thin-film polarizer.

4. The system for real-time measurement of the spatial distribution of laser beam polarization state according to claim 1, characterized in that, The birefringent element is a ring-shaped Dammann grating.

5. The system for real-time measurement of the spatial distribution of laser beam polarization state according to claim 1, characterized in that, This focusing lens is an angle corrector.

6. The system for real-time measurement of the spatial distribution of laser beam polarization state according to claim 1, characterized in that, The control unit predicts the polarization orientation of each polarization unit of the liquid crystal polarizer using a predictive learning algorithm.

7. The system for real-time measurement of the spatial distribution of laser beam polarization state according to claim 6, characterized in that, The predictive learning algorithm for predicting the polarization orientation of each polarization unit in a liquid crystal polarizer includes the following steps: S1: Process the image incident on the CCD camera; S2: Polarization analysis, to determine the spatial polarization state distribution of the incident beam; S3: Predict the spatial polarization distribution of the incident beam; S4: Set the orientation of each polarization unit of the liquid crystal polarizer.

Images