2-micron high-order Poincare sphere vector vortex ultrafast laser polarization detection method

Abstract

The invention provides a 2-micron high-order Poincare sphere vector vortex ultrafast laser polarization detection method, which comprises the following steps of: performing out-cavity and out-cavity beam splitting on 2-micron high-order Poincare sphere vector vortex ultrafast laser through a beam splitting module, directly detecting light intensity spatial distribution by one beam, and dynamically regulating and controlling the out-cavity of a polarization state by the other beam after the other beam enters a polarization regulation and control module; the computer communicates with the ESP32 development board to control the hollow rotary displacement table to change the forward included angle between the fast axis and the x axis of the vortex quarter-wave plate so as to change the polarization state of incident light; through the ultrafast laser polarization beam splitter, the polarization state of passing light is limited, and horizontal line polarized light perpendicular to a beam splitting surface passes through the ultrafast laser polarization beam splitter; dynamically detecting real-time data of light intensity which is dynamically adjusted outside the cavity through the polarization regulation and control module each time through a light intensity detection module; and the polarization analysis module receives regulation and control parameters of the polarization regulation and control module and light intensity parameters of the power detection module, analyzes the polarization state of a light beam input into the system, and completes 2-micron high-order Poincare sphere vector vortex ultrafast laser polarization detection.

Description

Technical Field

[0001] The present invention relates to the field of laser technology, and particularly to a method for detecting the polarization of a 2-micron high-order Poincaré sphere vector vortex ultrafast laser. Background Art

[0002] Due to their unique phase and polarization characteristics, high-order Poincaré sphere vector vortex beams have become a research hotspot in the field of modern optics. Currently, the detection methods for high-order Poincaré sphere beams mainly include the point-by-point detection method, digital holography method, and machine learning method. The point-by-point detection method involves independent measurement of each point on the beam cross-section. Although this method can provide detailed spatial resolution, the process is complex and time-consuming, and it cannot meet the need for rapid detection. The digital holography method extracts polarization information by recording and reconstructing the interference pattern of the beam. Although it can measure dynamic beams, when facing high-power lasers, high-precision equipment is required to ensure high accuracy, and it is not suitable for mass production. Machine learning methods, such as convolutional neural networks and principal component analysis, have been used to identify and classify complex polarization patterns. Although they provide high accuracy and flexibility, their internal mechanisms are often difficult to explain.

[0003] It is not difficult to find by comparing the above methods that these methods usually start from the perspective of the traditional Poincaré sphere to detect non-uniformly polarized vector beams. However, the polarization state on the cross-section of the vector beam varies with position, which also causes the positions or coordinates of the polarization states of each point on the high-order Poincaré sphere to be different, making the detection process relatively troublesome. Summary of the Invention

[0004] Aiming at the above technical deficiencies, the purpose of the present invention is to provide a method for detecting the polarization of a 2-micron high-order Poincaré sphere vector vortex ultrafast laser. By utilizing the advantages of the high-order Poincaré sphere and combining advanced optical designs and mathematical models and algorithms, efficient, accurate, and rapid detection of the polarization of the high-order Poincaré sphere vector vortex ultrafast laser is achieved. The sensitivity and resolution of laser polarization / detection are significantly improved, providing strong technical support for research and applications in related fields.

[0005] To achieve the above purpose, the present invention adopts the following technical solutions:

[0006] A method for detecting the polarization of a 2-micron high-order Poincaré sphere vector vortex ultrafast laser, comprising the following steps:

[0007] S1. Split the 2-micron high-order Poincaré sphere vector vortex ultrafast laser outside the cavity through a beam splitting module. One beam is directly detected by an optical intensity detection module to detect the spatial distribution of the optical intensity, and the other beam enters a polarization control module to dynamically control the polarization state outside the cavity;

[0008] S2. Communicate with the ESP32 development board through a computer to control the polarization modulation module to change the polarization state of the incident light; restrict the polarization state of the transmitted light so that only the horizontally polarized light perpendicular to the beam splitting plane can pass through.

[0009] S3. Dynamically detect the real-time data of the light intensity after each dynamic adjustment outside the cavity of the polarization modulation module through the light intensity detection module.

[0010] S4. Receive the modulation parameters of the polarization modulation module and the light intensity parameters of the power detection module through the polarization analysis module, analyze the polarization state of the light beam input to the system, and complete the polarization detection of the 2-micron high-order Poincaré sphere vector vortex ultrafast laser.

[0011] Preferably, the beam splitting module includes an ultrafast laser collimating beam splitter, which collimates the incident 2-micron cylindrical vector ultrafast laser into parallel light by using a gradient refractive index, and divides the 2-micron vector vortex ultrafast laser into two beams of uniformly incident light in time and space through the birefringence principle of the crystal.

[0012] Preferably, the polarization modulation module includes:

[0013] A vortex quarter-wave plate, which is used to change the polarization state of light, and generates a spiral phase structure through its non-uniform thickness distribution, combining the polarization information and spatial information of light.

[0014] An electric rotary displacement stage, which is used to control the rotation angle of the vortex quarter-wave plate, thereby adjusting the spiral phase and polarization state of the output beam.

[0015] An ultrafast laser polarization beam splitter, which is used to restrict the polarization state of the transmitted light so that only the component with a polarization state perpendicular to the beam splitting plane can pass through.

[0016] Preferably, the light intensity detection module includes a first CCD camera and a second CCD camera, which are used to detect and record the intensity distribution of the light beam passing through the vortex quarter-wave plate and the ultrafast laser polarization beam splitter.

[0017] Preferably, the polarization analysis module includes a computer, which connects to a high-precision control of the electric rotary displacement stage through a TTL serial port or Bluetooth, electrically adjusts the rotation angle of the vortex quarter-wave plate, finely adjusts the spiral phase and polarization state of the output beam, and processes the data transmitted back by the electric rotary displacement stage and the first CCD camera and the second CCD camera by using the adaptive weight fuzzy downsampling method. Then, combined with the Fourier analysis high-order Poincaré sphere model, it calculates the spatial distribution of the polarization state of the 2-micron high-order Poincaré sphere vector vortex ultrafast laser and the high-order Poincaré sphere parameters.

[0018] The computer includes:

[0019] An intelligent control unit, which is used to control the movement of the electric rotary displacement stage.

[0020] A data transmission unit, configured to receive and process the light intensity data of the first CCD camera and the second CCD camera, and the angle data transmitted back by the motorized rotary displacement stage during each rotation.

[0021] A data processing unit, configured to receive and process the light intensity data of the first CCD camera and the second CCD camera, and the angle data transmitted back by the motorized rotary displacement stage during each rotation, perform dimensionality reduction processing on the measurement data, and use Fourier analysis and singular value decomposition method to obtain the accurate mapping of the polarization state spatial distribution and the polarization state on the high-order Poincaré sphere.

[0022] Preferably, it further includes preliminary preparation work: placing an ultrafast laser collimating beam splitter, a vortex quarter-wave plate, a motorized rotary displacement stage, a first CCD camera, a second CCD camera, and an ultrafast laser polarization beam splitter in the x - o - y plane.

[0023] Among them, the beam splitting surface of the ultrafast laser collimating beam splitter forms a 45° angle with the input beam, and is collinear with the optical axes of the ultrafast laser collimating beam splitter and the vortex quarter-wave plate. The motorized rotary displacement stage is controlled by a computer through serial communication of wired USB to RS232 or wireless Bluetooth transmission. The angle of the ultrafast laser polarization beam splitter in the x - o - y plane is such that the main axis is parallel to the z-axis and the beam splitting surface forms a 45° angle with the x - o - y plane.

[0024] The processing method after obtaining the polarized light beam includes: the computer acquires the rotation angle of the motorized rotary displacement stage through serial communication of wired USB to RS232 or wireless Bluetooth transmission, and uses singular value decomposition to inversely deduce the accurate mapping of the polarization state spatial distribution and the position of the polarization state on the high-order Poincaré sphere formed by the vortex quarter-wave plate and the ultrafast laser polarization beam splitter through the Mueller matrix.

[0025] Preferably, the Mueller matrix of the vortex quarter-wave plate is:

[0026]

[0027] where θ is the angle between the fast axis of the vortex quarter-wave plate and the positive x - direction.

[0028] The angle with the positive x - axis is The Mueller matrix of the ultrafast laser polarization beam splitter is:

[0029]

[0030] where is the angle between the ultrafast laser polarization beam splitter and the positive x - direction.

[0031] Preferably, the Stokes matrix when the intensity of the light received by the second CCD after passing through the vortex quarter-wave plate and the ultrafast laser polarization beam splitter is at its maximum is:

[0032]

[0033] Preferably, using singular value decomposition, the Stokes matrix of the polarizer composed of the vortex quarter-wave plate and the ultrafast laser polarization beam splitter can be deduced as:

[0034]

[0035] where, S p is the Stokes matrix of the polarizer, and S0, S1, S2, S3 are the Stokes parameters of the polarizer. is the transpose of the product of the Mueller matrix of the vortex quarter-wave plate and the Mueller matrix of the ultrafast laser polarization beam splitter, and S out_max = [1, cos(2θ p ), sin(2θ p ), 0] T is the Stokes matrix of the output beam after passing through the vortex quarter-wave plate and the ultrafast laser polarization beam splitter.

[0036] Preferably, the coordinates of the polarizer composed of the vortex quarter-wave plate and the ultrafast laser polarization beam splitter on the high-order Poincaré sphere are:

[0037]

[0038] where, is the order of the high-order Poincaré sphere, and r, θ, are the position coordinates of the polarizer composed of the vortex quarter-wave plate and the ultrafast laser polarization beam splitter on the high-order Poincaré sphere, and S0, S1, S2, S3 are the Stokes parameters of the polarizer.

[0039] The computer collects the initial light intensity distribution of the laser and the light intensity after the vortex quarter-wave plate and the ultrafast laser polarization beam splitter are rotated by three different angles through wired USB to RS232 serial communication transmission or wireless Bluetooth transmission. Using the generalized Malus' theorem and the high-order Poincaré sphere model, combined with the adaptive weighted fuzzy downsampling method, it calculates the Stokes parameters of the input laser and outputs and stores the calculated data, and displays the polarization state of the input laser.

[0040] Preferably, the light intensity data of the first CCD and the second CCD transmitted back through wired USB to RS232 serial communication transmission or wireless Bluetooth are, where n is the resolution of the first CCD and the second CCD:

[0041]

[0042] Among them, I ij is the light intensity of a point in the spot plane, and n is the resolution of the spot plane.

[0043] Preferably, divide the light intensity region:

[0044] I(x, y) → I(k, l)

[0045]

[0046] Among them, x and y are the horizontal and vertical coordinates of the position of a point in the spot respectively, and r is the distance from the position of the spot to the origin.

[0047] Preferably, the light intensity after Gaussian blur is:

[0048]

[0049] Among them, x and y are the horizontal and vertical coordinates of the position of a point in the spot respectively, r is the distance from the position of the spot to the origin, and σ is the standard deviation of the light intensity in the spot plane.

[0050] Preferably, re - distribute the light intensity weights:

[0051]

[0052] Among them, x and y are the horizontal and vertical coordinates of the position of a point in the spot respectively, c x and c y are the horizontal and vertical coordinates of the position of the reference light intensity on the spot plane.

[0053] Preferably, obtain the compressed light intensity distribution:

[0054]

[0055] Among them, ω(x, y) is the re - distributed light intensity weight, and I' k,l (k, l) is the light intensity after Gaussian blur.

[0056] Preferably, processing the data transmitted back by the first CCD camera and the second CCD camera after compression by the adaptive weight fuzzy down - sampling method can obtain the light intensity factor λ α , and n is the dimension of the light intensity distribution after compression by the adaptive weight fuzzy down - sampling method:

[0057]

[0058] Among them, I new_α is the light intensity after passing through the analyzer composed of a vortex quarter - wave plate and an ultrafast laser polarization beam splitter, and I new_0is the light intensity before the analyzer composed of a vortex quarter-wave plate and an ultrafast laser polarization beam splitter, λ ij is the light intensity factor at each point in the spot plane, and n is the spatial resolution of the spot.

[0059] Preferably, according to the generalized Malus' theorem and the high-order Poincaré sphere model, the polarizer composed of a vortex quarter-wave plate and an ultrafast laser polarization beam splitter is projected onto the high-order Poincaré sphere, and the geometric figure formed is a spherical circle. The center of the spherical circle and the radius of the spherical circle are (S α , λ α ), (S β , λ β ), and (S γ , λ γ ), respectively.

[0060] Preferably, according to Fourier analysis, the Stokes parameters representing the spatial polarization state distribution of the incident light are:

[0061]

[0062] where S ij is the Stokes matrix at a point on the spot plane, and n is the spatial resolution of the spot;

[0063] Finally, according to the light intensity weight, the parameters of the 2-μm high-order Poincaré sphere vector vortex ultrafast laser are obtained:

[0064] [l, 2θ, 2φ];

[0065] where l is the order of the high-order Poincaré sphere, 2θ is the polar angle of the high-order Poincaré sphere, and 2φ is the azimuth angle of the high-order Poincaré sphere.

[0066] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0067] 1. Effectiveness: Combining software and hardware technologies, efficient and precise regulation and data processing of the polarization state of incident light in the 2-μm band are achieved.

[0068] The rotation of the electric rotary displacement stage is controlled by using a cosine acceleration / deceleration curve, which shortens the time for realizing the polarization regulation of incident light by using a vortex quarter-wave plate, and realizes the angle control of the vortex quarter-wave plate at the level of 0.1 arcseconds; the adaptive weight fuzzy downsampling method is used to reduce the dimension of the light intensity data transmitted back by the first CCD camera and the second CCD camera and extract key features, realizing a high data processing rate and reducing the data storage burden; the singular value decomposition method (SVD) is used to process the position information of the electric rotary displacement stage, realizing the determination of the position information of the analyzer composed of the vortex quarter-wave plate and the ultrafast laser polarization beam splitter on the high-order Poincaré sphere; the polarization state of the incident light is accurately deduced by using Fourier analysis and the high-order Poincaré sphere theory, effectively avoiding the multi-value situation. The complex beam processing has high precision and high automation. Users can control the electric rotary displacement stage through a computer, collect, process and store the data of the CCD camera, and display the spatial polarization state distribution and the high-order Poincaré sphere, and the operation is simple and intuitive.

[0069] 2. Low cost: The device is simple and there is no additional polarization modulation device, which is suitable for mid-infrared high-power applications.

[0070] Using a vortex quarter-wave plate and an air-gap ultrafast laser polarization beam splitter as polarization response devices, compared with the existing spatial light modulators and polarization response metasurface materials, first, it has a higher damage threshold, and the components can be quickly replaced according to the wavelength of the incident ultrafast laser, which is more convenient for the polarization state detection of high-power lasers; second, the detection optical components adopt multi-layer dielectric coating technology and glue-free design, reducing the use of materials such as glue and reducing the additional costs brought by complex manufacturing processes (such as: nano-etching); third, the optical components have high thermal conductivity and heat dissipation capacity, reducing the need for additional cooling systems (such as: water cooling) or complex structural designs, withstanding high power and having a wide range of applications.

[0071] 3. Universality: The device is simple and there is no additional polarization modulation device, which is a closed-type laser.

[0072] The device has a small volume, supports two modes of free-space coupling and fiber coupling, and has two ways of wired and wireless control command and data transmission, which is suitable for open-type and sealed lasers; there is no need to place convex lenses, spatial light modulators and polarization response metasurface materials in the device. The beam can be collimated into parallel light through the ultrafast laser collimating beam splitter, and the polarization state of the incident light can be detected by rotating the vortex quarter-wave plate and the ultrafast laser polarization beam splitter, greatly reducing the complexity of the system, having a reusable degree and being easier to mass-produce. Description of the Drawings

[0073] Figure 1 It is a schematic connection diagram of the overall structure of the present invention;

[0074] Figure 2This is the overall algorithm flowchart of the present invention;

[0075] Figure 3 It is the polarization state distribution of the high-order Poincaré sphere vector vortex beam;

[0076] Among them, (a) is the polarization state of the high-order Poincaré sphere vector vortex beam at and (b) is the polarization state of the high-order Poincaré sphere vector vortex beam at

[0077] Figure 4 This is the schematic diagram of the light intensity change before and after using the 2-micron radial light adaptive weight fuzzy downsampling method of the present invention;

[0078] Among them, (a) is the original light intensity before the processing of the adaptive weight fuzzy downsampling method and the light intensity distribution after passing through the vortex quarter-wave plates at three different angles and the ultrafast laser polarization beam splitter, and (b) is the original light intensity after the processing of the adaptive weight fuzzy downsampling method and the light intensity distribution after passing through the vortex quarter-wave plates at three different angles and the ultrafast laser polarization beam splitter;

[0079] Figure 5 This is the polarization characteristic diagram of the 2-micron radial light measured by the present invention;

[0080] Among them, (a1) is the simulation diagram of the polarization state spatial distribution of the 2-micron radial light, (b1) is the simulation diagram of the polarization high-order Poincaré sphere of the vortex quarter-wave plate, the ultrafast laser polarization beam splitter and the 2-micron radial light, (a2) is the experimental diagram of the polarization state spatial distribution of the 2-micron radial light, (b2) is the experimental diagram of the polarization high-order Poincaré sphere of the vortex quarter-wave plate, the ultrafast laser polarization beam splitter and the 2-micron radial light. The triangles, quadrilaterals and hexagrams in the figure are the characterizations of the Stokes parameters of the three vortex quarter-wave plates and the ultrafast laser polarization beam splitter on the high-order Poincaré sphere;

[0081] Figure 6 This is the schematic diagram of the predicted detection result of the radial light polarization characteristic measured in Example 1 of the present invention;

[0082] Among them, (a1), (b1), (c1), (d1), (e1) are simulation diagrams, and (a2), (b2), (c2), (d2), (e2) are experimental diagrams. The arrows in the figure are the polarization directions of the PBS;

[0083] Figure 7 This is the schematic diagram of the light intensity change before and after using the 2-micron angular light adaptive weight fuzzy downsampling method of the present invention;

[0084] Figure 8 This is the polarization characteristic diagram of the 2-micron angular light measured by the present invention;

[0085] Among them, (a1) is the simulation diagram of the spatial distribution of the polarization state of the 2-μm angular light, (b1) is the simulation diagram of the high-order Poincaré sphere of the polarization of the vortex quarter-wave plate, the ultrafast laser polarization beam splitter, and the 2-μm angular light, (a2) is the experimental diagram of the spatial distribution of the polarization state of the 2-μm radial light, (b2) is the experimental diagram of the high-order Poincaré sphere of the polarization of the vortex quarter-wave plate, the ultrafast laser polarization beam splitter, and the 2-μm radial light. Triangles, quadrilaterals, and hexagrams in the figure represent the characterization of the Stokes parameters of the cubic vortex quarter-wave plate and the ultrafast laser polarization beam splitter on the high-order Poincaré sphere;

[0086] Figure 9 Schematic diagram of the predicted detection results of the polarization characteristics of the angular light measured in Example 2 of the present invention;

[0087] Among them, (a1), (b1), (c1), (d1), (e1) are simulation diagrams, and (a2), (b2), (c2), (d2), (e2) are experimental diagrams. Arrows in the figure represent the polarization directions of the PBS;

[0088] Figure 10 Schematic diagram of the device and method for detecting the polarization characteristics of the radial light and angular light measured in the embodiment of the present invention;

[0089] Among them, PBS is a rotatable ultrafast laser polarization beam splitter, and L1 is a focusing lens with a focal length of 100.

[0090] Among them:

[0091] 1. Ultrafast laser collimating beam splitter; 2. First CCD camera; 3. Vortex quarter-wave plate; 4. Electric rotating displacement stage; 5. Ultrafast laser polarization beam splitter; 6. Second CCD camera; 7. Computer. Detailed implementation mode

[0092] The present invention will be further described below with reference to the accompanying drawings.

[0093] As Figure 1 shown, the specific implementation of the method of the present invention is as follows:

[0094] Step 1: Set the incident light. The incident beam is a 2-μm ultrafast laser with an arbitrary polarization direction. The beam enters the device through free space coupling or multimode fiber coupling. In the experiment, 2-μm thulium (Tm) system radial light and angular light are selected for measurement and verification;

[0095] Step 2: Adjust the ultrafast laser collimating beam splitter 1. The optical axis is collinear with the propagation direction of the incident light, and the beam splitting surface forms a 45° angle with the y-o-z plane. The incident ultrafast laser is collimated into parallel light and evenly split into two beams in terms of time and space, and the time and space polarization states of the incident light are kept unchanged during the beam splitting process;

[0096] Step 3: Adjust the electric rotary displacement stage 4 and the vortex quarter-wave plate 3. The vortex quarter-wave plate 3 is fixed on the electric rotary displacement stage 4 and rotates around its optical axis ( Figure 1 the z-axis in it). The light beam passes vertically through the vortex quarter-wave plate 3, and the center of the light beam coincides with the center of the vortex quarter-wave plate 3. The initial angle and the rotation angle of the fast axis of the vortex quarter-wave plate 3 can be programmed and adjusted by the computer 7. The vortex quarter-wave plate 3 of the experimental system is applicable to the 2-micron band, and the initial position of the fast axis of the vortex quarter-wave plate 3 coincides with the x-axis;

[0097] Step 4: Adjust the ultrafast laser polarization beam splitter. The polarization beam splitter is a cubic air-gap coated PBS. The splitting surface of it makes an angle of 45° with the x-o-y axis, and the distance between its optical axis and the z-axis does not exceed 2 mm. It is used to divide the light passing through it into two beams of light with perpendicular polarizations (P-polarized light and S-polarized light), and make the light beam with the polarization direction along the direction parallel to the y-axis continue to propagate along the z-axis direction. The PBS selected in the experiment is for the 2-micron band, which is consistent with the applicable band of the vortex quarter-wave plate 3.

[0098] Step 4: Control the first CCD camera 2 and the second CCD camera 6 to record the beam power. The probe frame rate of the CCD camera is 90 frames per second (fps), which is applicable to the detection of the laser power in the 2-16 micron band. In this system, the probe of the CCD camera coincides with the z-axis, and the light beam is perpendicularly incident on the probe absorption array; the data of the CCD camera is transmitted back to the computer 7 for processing through USB to RS232 or Bluetooth.

[0099] Step 5: Adjust the computer 7 to control the electric rotary displacement stage 4, process and visualize the data, and deduce the spatial distribution of the polarization state of the final cylindrical vector ultrafast laser and its characterization on the high-order Poincaré sphere.

[0100] In summary, the present invention enables efficient, accurate, and rapid measurement of the polarization state of the 2-micron ultrafast cylindrical vector laser. The system structure is simple and easy to operate and control. By replacing the vortex quarter-wave plate 3 and the ultrafast laser polarization beam splitter 5, multi-wavelength measurement can be realized, and the applicable range is wide. The use of the electric rotary displacement stage 4 and the computer 7 for control can achieve automated measurement, reduce human operation errors, and improve the measurement efficiency. The computer 7 can accurately calculate the high-order Poincaré sphere parameters and draw the high-order Poincaré sphere by combining the generalized Malus theorem and the high-order Poincaré sphere model with the adaptive weight fuzzy downsampling method, the singular value decomposition method, and the Fourier analysis method, which is convenient for users to analyze and understand the laser polarization state in real time.

[0101] The following further illustrates the present invention with reference to the embodiments:

[0102] Embodiment 1

[0103] Set the parameters of a 2-micron high-order Poincaré sphere vector vortex ultrafast laser polarization detection device and method as follows:

[0104] For the incident light laser, select an a-cut yttrium aluminum garnet (CYA) disordered crystal solid laser, whose main function is to generate mid-infrared 2-micron band annular radial light;

[0105] The ultrafast laser collimating beam splitter 11 is made of N-BK7 with a size of 10*10*10 mm, n0 of 1.46, and α of 10 - 4 mm -2 , with an optical thin film with high transmittance for the 1900nm - 2100nm band coated on the incident surface, the transmittance is greater than 99.8%, the optical axis is collinear with the incident light, and the beam splitting surface forms a 45° angle with the y-o-z plane in Figure 1 the middle, which can split the 2-micron incident cylindrical vector ultrafast laser into two mutually perpendicular light beams according to the light intensity of 1:1;

[0106] The vortex quarter-wave plate 33 can fixedly generate a 90° phase delay, the vortex order l = 1, and it is made of liquid crystal polymer (LCP), with a size of 1 inch, and an anti-reflection film with a high damage threshold and a fused silica protective optical thin film for the 1900 - 2100 band are coated on the surface. It is fixed on the electric rotary displacement stage 44 by a threaded mounting ring to ensure that its optical axis is collinear with the rotation axis of the beam splitter and the rotary stage, and to avoid excessive mechanical stress causing lens deformation, modulate the vector field of the light beam to generate a spiral phase structure, and at the same time maintain its cylindrical vector characteristics, forming a part of the analyzer;

[0107] The electric rotary displacement stage 44 is driven by a DC servo motor with an input voltage of 12V, the rotation range is from 0° to 360 degrees, the repeat positioning accuracy is 0.01°, the angle encoder accuracy is ±3 arcsec, and the cosine acceleration / deceleration curve is used to control the takeoff and landing, which provides the possibility for high-precision control of the angle of the vortex quarter-wave plate 3 mounted on it;

[0108] The ultrafast laser polarization beam splitter 55 is located on the right side of the vortex quarter-wave plate 33, made of UV-grade fused silica, with a size of 15*15*15 mm, the optical axis is collinear with the vortex quarter-wave plate 33, and the laser damage threshold is ≥100 J / cm 2 , the transmittance of p-polarized light is 98%, and an anti-reflection film with a high damage threshold for the 1900 - 2100 band is coated on the surface. The thickness of the intermediate air gap is 0.3 mm, which reduces the interference effect and the thermal lens effect, and ensures that the spot size of the incident light does not change;

[0109] The first CCD camera 22 and the second CCD camera 66 have a frame rate of 30 Hz, enabling faster response capabilities in high-speed dynamic scenarios and being applicable to the detection of laser intensity in the 2 - 16 micron wavelength band. In addition, the signal-to-noise ratio (SNR) of the camera is 30 decibels (dB), and it can still provide high-quality image quality under low light intensity conditions.

[0110] A computer 77 with a Windows 11 system, using an NVIDIA GeForce RTX 3060 independent graphics card, and built-in python platform, is used to send control instructions and process angular data and light intensity data.

[0111] Data transmission method: In this embodiment, a USB to RS232 is used to transmit control instructions, angular data, and light intensity data. The baud rate is 9600 bps, and the data transmission signals are TXD (transmission), RXD (reception), and the control signals are RTS (request to send), CTS (clear to send), DTR (data terminal ready), DSR (data set ready), DCD (data carrier detect), RI (ring indicator), to achieve serial communication with the motorized rotary displacement stage 44, the first CCD camera 22, and the second CCD camera 66.

[0112] A commercially available high-power fiber-coupled laser diode with a wavelength of 793 nm is used as the pump source to pump the thulium-doped anisotropic a-cut CYA disordered crystal, thereby generating a mid-infrared 2-micron band cylindrical vector ultrafast laser as the incident light.

[0113] As Figure 1 shown, the radial light with a central wavelength of 1960 nm output by the laser is first collimated into parallel light by the ultrafast laser collimating beam splitter 11, with a spot diameter of 20 μm, and then divided into two beams with the same light intensity and polarization state after passing through the beam splitting surface of the ultrafast laser collimating beam splitter 1, respectively along the z-axis and parallel to the x-axis. The light parallel to the x-axis reaches the first CCD camera 22 after passing through free space d1 and is captured.

[0114] The optical distance Δ that the beam along the z-axis passes through is Δ = d2 + n1h1 + d3 + n2h2 + d4, where h1 and h2 are the thicknesses of the vortex quarter-wave plate 33 and the ultrafast laser polarization beam splitter 55, respectively, and n1 and n2 are the refractive indices of the vortex quarter-wave plate 33 and the ultrafast laser polarization beam splitter 55, respectively. To ensure the coherence of the two beams of light passing through the ultrafast laser collimating beam splitter 11, the optical distance between each component is adjusted such that |d1 - Δ| ≤ 20 μm.

[0115] Then the computer 77 sends pulses to the motorized rotary displacement stage 44 in a wired manner, so that the angle between the fast axis of the vortex quarter-wave plate 3 fixed on the motorized rotary displacement stage 44 and the positive x-direction is α = 0°. At this time, the Mueller matrix of the vortex quarter-wave plate 3 is

[0116]

[0117] Among them, M VQWP_α is the Mueller matrix of the vortex quarter-wave plate 3.

[0118] The beam-splitting surface of the ultrafast laser polarization beam splitter 55 makes an angle of 45° with the y-o-z plane, and its Mueller matrix is:

[0119]

[0120] Among them, M LP is the Mueller matrix of the ultrafast laser polarization beam splitter 55.

[0121] Then, the computer 77 combines the angular data of the vortex quarter-wave plate 33 and can calculate the Stokes parameters S of the polarizer composed of the vortex quarter-wave plate 33 and the ultrafast laser polarization beam splitter 55 at this angle through the high-order Poincaré sphere model combined with the singular value decomposition algorithm α is

[0122]

[0123] Among them, S p is the Stokes matrix of the polarizer, and S0, S1, S2, S3 are the Stokes parameters of the polarizer. is the transpose of the product of the Mueller matrix of the vortex quarter-wave plate and the Mueller matrix of the ultrafast laser polarization beam splitter.

[0124] Then, the computer 77 sends instructions to the first CCD 2 and the second CCD 6 in a wired manner, and transmits back the light intensity data of the two beams of light detected currently. After visualization, as shown in Figure 3 (a), combined with the light intensity data after the adaptive weight fuzzy downsampling method, after visualization, as shown in Figure 3 (b), calculates the light intensity factor λ of the vortex quarter-wave plate 33 and the ultrafast laser polarization beam splitter 55 at this angle α is

[0125]

[0126] Among them, I ij is the light intensity of a point in the spot plane, and n is the resolution of the spot plane.

[0127] Repeat the above method 2 times. The computer 77 controls the vortex quarter-wave plate 33 fixed on the motorized rotary displacement stage 44 to rotate by a fixed angle of β = 22.5°, γ = 45°, and inversely deduces the Stokes parameters S of the polarizer composed of the vortex quarter-wave plate 33 and the ultrafast laser polarization beam splitter 55 β and S γ, read the light intensity data of the first CCD2 and the second CCD6 and calculate the light intensity factor λ β and λ γ ;

[0128] Finally, the computer 77 calculates the representation of the incident light on the high-order Poincaré sphere according to the generalized Malus' theorem and the high-order Poincaré sphere model as

[0129]

[0130] where 1 is the order of the high-order Poincaré sphere vector vortex beam represented this time, is the azimuth angle of the high-order Poincaré sphere vector vortex beam represented this time, and 0 is the polar angle of the high-order Poincaré sphere vector vortex beam represented this time.

[0131] The spatial distribution of the polarization state of the incident light is as Figure 4 (a) shown, and the positions of the three polarizers and the incident light on the high-order Poincaré sphere are as Figure 4 (b) shown;

[0132] Therefore, by using the vortex quarter-wave plate 33 and the ultrafast laser polarization beam splitter 55, the spatial distribution of the polarization state of the radial light of the 2-μm thulium (Tm)-doped solid laser is directly measured.

[0133] To verify whether the incident light is radial light, a rotatable polarization beam splitter (PBS) with a rotation accuracy of 1° is placed outside the cavity by using a rotatable device. The incident light passes through the rotatable PBS, a focusing lens (focal length f = 100 mm), and a CCD detection device and method, as Figure 9 shown. As Figure 5 shown, the incident light shows the characteristics of two lobes after passing through the PBS. According to the symmetry of the two lobes on both sides of the polarization direction of the PBS, it can be known that the incident light is radial light.

[0134] Embodiment 2

[0135] In this embodiment, the 2-μm angular ultrafast laser generated by a 2-μm thulium-doped fiber laser is used as the incident light, and wireless Bluetooth is used to transmit commands and data, and the spatial polarization distribution characteristics of the 2-μm angular light and its representation on the high-order Poincaré sphere are obtained.

[0136] On the basis of Embodiment 1, the solid laser of a thulium-doped anisotropic a-cut CYA disordered crystal pumped by a 793-nm high-power fiber-coupled laser diode is replaced with a 2-μm thulium-doped fiber laser with a 2-m long thulium-doped multimode fiber pumped by a 793-nm LD, which has higher nonlinearity and dispersion and is more flexible in generating high-order modes.

[0137] First, fuse the OC end of the 2-μm thulium-doped fiber laser with a multimode fiber, and adjust the optical axes of the ultrafast laser collimating beam splitter 1, the vortex quarter-wave plate 3, and the ultrafast laser polarization beam splitter 5 to be collinear with the laser output from the multimode fiber. To ensure the coherence of the two beams passing through the ultrafast laser collimating beam splitter 11, adjust the optical distance |d1 - Δ| ≤ 20 μm between each component, and set the angle between the ultrafast laser polarization beam splitter 56 and the x-axis to 45°.

[0138] Next, the computer 77 sends instructions to the electric rotary displacement stage 44 according to the SPP (Bluetooth Serial Port Profile) protocol, making the angle between the fixed wave plate optical axis and the x-axis become 22.5°, and transmitting back the current angle information. Use singular value decomposition (SVD) to deduce the state of the current analyzer on the high-order Poincaré sphere. Then send instructions to the first CCD camera 22 and the second CCD camera 66 to transmit back the current light intensity, and then calculate the light intensity factor at this time according to the adaptive weighted fuzzy downsampling method, as Figure 6 shown.

[0139] Then, change the rotation angle of the electric rotary displacement stage 44 and repeat the above steps twice. The angles are 45° and 67.5° respectively, and combine the generalized Malus' law and the high-order Poincaré sphere model to calculate the spatial distribution of the angular light polarization state and its representation on the high-order Poincaré sphere at this time.

[0140]

[0141] Among them, -1 is the order of the high-order Poincaré sphere vector vortex beam characterized this time, is the azimuth angle of the high-order Poincaré sphere vector vortex beam characterized this time, and π is the polar angle of the high-order Poincaré sphere vector vortex beam characterized this time.

[0142] Finally, visualize the spatial distribution of the polarization state and its representation on the high-order Poincaré sphere as Figure 7 shown, and use fiber coupling to sequentially pass the incident light through a rotatable PBS, a focusing lens (focal length f = 100 mm), and a CCD detection device and method. Figure 8 shown, the incident light presents the characteristics of two lobes after passing through the PBS. According to the symmetry of the two lobes on both sides of the perpendicular bisector of the polarization direction, it can be known that the incident light is angular light.

[0143] The parts not elaborated in detail in the present invention belong to the well-known technologies of those skilled in the art.

[0144] Although the specific implementation methods of the present invention have been described above, those skilled in the art should understand that these are only examples. Without departing from the principles and implementation of the present invention, various changes or modifications can be made to these embodiments (for example: replacing PBS with a polarizer or other optical elements that can polarize, replacing the CCD camera with other types of cameras, etc.). Therefore, the protection scope of the present invention is defined by the appended claims.

Claims

1. A method for detecting the polarization of a 2-micron high-order Poincaré sphere vector vortex ultrafast laser, characterized in that, It includes the following steps: S1. The 2-μm high-order Poincaré sphere vector vortex ultrafast laser is externally split by a beam splitting module outside the cavity. One beam is directly detected by an optical intensity detection module for the spatial distribution of the optical intensity, and the other beam enters a polarization regulation module for dynamic external cavity regulation of the polarization state; S2. Communicate with the ESP32 development board through a computer (7) to control the polarization regulation module to change the polarization state of the incident light; limit the polarization state of the transmitted light, and only allow the horizontally polarized light perpendicular to the beam splitting plane to pass through; S3. Dynamically detect the real-time data of the optical intensity after each dynamic external cavity adjustment by the polarization regulation module through the optical intensity detection module; S4. Receive the regulation parameters of the polarization regulation module and the optical intensity parameters of the power detection module through the polarization analysis module, analyze the polarization state of the beam input to the system, and complete the polarization detection of the 2-μm high-order Poincaré sphere vector vortex ultrafast laser.

2. The 2-μm high-order Poincaré sphere vector vortex ultrafast laser polarization detection method according to claim 1, characterized in that, The beam splitting module includes an ultrafast laser collimating beam splitter (1), which collimates the incident 2-μm cylindrical vector ultrafast laser into parallel light by using a gradient refractive index, and divides the 2-μm vector vortex ultrafast laser into two beams of incident light uniformly in time and space by the birefringence principle of the crystal.

3. The 2-μm high-order Poincaré sphere vector vortex ultrafast laser polarization detection method according to claim 1, wherein The polarization regulation module includes: A vortex quarter-wave plate (3), which is used to change the polarization state of light, and generates a spiral phase structure through a non-uniform thickness distribution to combine the polarization information and spatial information of light; An electric rotating displacement stage (4), which is used to control the rotation angle of the vortex quarter-wave plate (3), so as to adjust the spiral phase and polarization state of the outgoing beam; An ultrafast laser polarization beam splitter (5), which is used to limit the polarization state of the transmitted light, and only allows the component with a polarization state perpendicular to the beam splitting plane to pass through.

4. The 2-μm high-order Poincaré sphere vector vortex ultrafast laser polarization detection method according to claim 3, wherein The optical intensity detection module includes a first CCD camera (2) and a second CCD camera (6), which are used to detect and record the intensity distribution of the beam passing through the vortex quarter-wave plate (3) and the ultrafast laser polarization beam splitter (5).

5. The 2-micron high-order Poincaré sphere vector vortex ultrafast laser polarization detection method according to claim 4, characterized in that, The polarization analysis module includes a computer (7), which controls the electric rotating displacement stage (4) with high precision through a TTL serial port or Bluetooth, electrically adjusts the rotation angle of the vortex quarter-wave plate (3), finely adjusts the spiral phase and polarization state of the outgoing beam, and processes the data transmitted back by the electric rotating displacement stage (4), the first CCD camera (2), and the second CCD camera (6) by using the adaptive weight fuzzy downsampling method. Then, combined with the Fourier analysis high-order Poincaré sphere model, it calculates the spatial distribution of the polarization state of the 2-μm high-order Poincaré sphere vector vortex ultrafast laser and the high-order Poincaré sphere parameters; The computer (7) includes: An intelligent control unit, which is used to control the movement of the electric rotating displacement stage (4); A data transmission unit, which is used to receive and process the optical intensity data of the first CCD camera (2) and the second CCD camera (6) and the angle data transmitted back by the electric rotating displacement stage (4) during each rotation; A data processing unit, which is used to receive and process the light intensity data of the first CCD camera (2) and the second CCD camera (6) and the angle data transmitted back by the motorized rotary displacement stage (4) during each rotation, perform dimensionality reduction processing on the measurement data, and use Fourier analysis and singular value decomposition method to obtain the accurate mapping of the polarization state spatial distribution and the polarization state on the high-order Poincaré sphere.

6. The 2-μm high-order Poincaré sphere vector vortex ultrafast laser polarization detection method according to claim 5, characterized in that It also includes preliminary preparation work: placing an ultrafast laser collimating beam splitter (1), a vortex quarter-wave plate (3), a motorized rotary displacement stage (4), a first CCD camera (2), a second CCD camera (6), and an ultrafast laser polarization beam splitter (5) in the x-o-y plane; Among them, the beam splitting surface of the ultrafast laser collimating beam splitter (1) forms a 45° angle with the input beam and is collinear with the optical axes of the ultrafast laser collimating beam splitter (1) and the vortex quarter-wave plate (3). The computer (7) is used to control the motorized rotary displacement stage (4) through wired USB to RS232 serial communication transmission or wireless Bluetooth transmission, and the angle of the ultrafast laser polarization beam splitter (5) in the x-o-y plane, so that the main axis is parallel to the z-axis and the beam splitting surface forms a 45° angle with the x-o-y plane; The processing method after obtaining the polarized light beam includes: the computer (7) collects the rotation angle of the motorized rotary displacement stage (4) through wired USB to RS232 serial communication transmission or wireless Bluetooth transmission, and uses singular value decomposition to inversely deduce the accurate mapping of the polarization state spatial distribution and the polarization state on the high-order Poincaré sphere composed of the vortex quarter-wave plate (3) and the ultrafast laser polarization beam splitter (5) and the position on the high-order Poincaré sphere.

7. The 2-micron high-order Poincaré sphere vector vortex ultrafast laser polarization detection method according to claim 6, characterized in that, Using singular value decomposition to calculate the Stokes matrix of the polarizer composed of the vortex quarter-wave plate (3) and the ultrafast laser polarization beam splitter (5) is: Among them, S p is the Stokes matrix of the analyzer, and S0, S1, S2, S3 are the Stokes parameters of the analyzer. M VQWP is the Mueller matrix of the vortex quarter-wave plate, and M LP is the Mueller matrix of the ultrafast laser polarization beam splitter. S out = [1, cos(2θ p ), sin(2θ p ), 0] T is the Stokes matrix of the output light beam after passing through the vortex quarter-wave plate (3) and the ultrafast laser polarization beam splitter (5).

8. The 2-μm high-order Poincaré sphere vector vortex ultrafast laser polarization detection method according to claim 6, wherein The coordinates of the polarizer composed of the vortex quarter-wave plate (3) and the ultrafast laser polarization beam splitter (5) on the high-order Poincaré sphere are: where l is the order of the high-order Poincaré sphere, and r, θ, are the position coordinates of the analyzer composed of the vortex quarter-wave plate (3) and the ultrafast laser polarization beam splitter (5) on the high-order Poincaré sphere, and S0, S1, S2, S3 are the Stokes parameters of the analyzer; The computer (7) collects the initial light intensity distribution of the laser and the light intensity after the vortex quarter-wave plate (3) and the ultrafast laser polarization beam splitter (5) are rotated at three different angles through wired USB to RS232 serial communication transmission or wireless Bluetooth transmission, uses the generalized Malus theorem and the high-order Poincaré sphere model, combines the adaptive weight fuzzy downsampling method to calculate the Stokes parameters of the input laser, outputs and stores the calculation data, and displays the polarization state of the input laser.

9. The 2-μm high-order Poincaré sphere vector vortex ultrafast laser polarization detection method according to claim 6, characterized in that Processing the data transmitted back by the first CCD camera (2) and the second CCD camera (6) after being compressed by the adaptive weight fuzzy downsampling method can obtain the light intensity factor λ, and n is the dimension of the light intensity distribution after being compressed by the adaptive weight fuzzy downsampling method: Among them, I new_α is the light intensity after passing through the analyzer composed of the vortex quarter-wave plate (3) and the ultrafast laser polarization beam splitter (5), and I new_0 is the light intensity before passing through the analyzer composed of the vortex quarter-wave plate (3) and the ultrafast laser polarization beam splitter (5), λ ij is the light intensity factor at each point in the spot plane, and n is the spatial resolution of the spot.

10. The 2-μm high-order Poincaré sphere vector vortex ultrafast laser polarization detection method according to claim 6, wherein According to Fourier analysis, the Stokes parameters representing the spatial polarization state distribution of the incident light are: Among them, S ij is the Stokes matrix of a point on the light spot plane, and n is the spatial resolution of the light spot; Finally, according to the light intensity weight, the parameters of the 2-micron high-order Poincaré sphere vector vortex ultrafast laser are obtained: [l, 2θ, 2φ]; Among them, l is the order of the high-order Poincaré sphere, 2θ is the polar angle of the high-order Poincaré sphere, and 2φ is the azimuth angle of the high-order Poincaré sphere.

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