Multi-color Vortex Light Generation System and Method Based on Superfluorescent Light Source and Coherent Beam Combining

Abstract

The present invention relates to a system and method for realizing a multi-color vortex beam based on a superluminescent light source and coherent beam combining, so as to solve the technical problems of complex structure, limited output power and types existing in the generation of existing multi-wavelength or multi-color vortex beams. The system includes a superluminescent light source, a multi-channel laser generation unit containing an optical fiber splitter and a coherent optical fiber array, and a spatial phase regulation unit containing an optical transmission component, a computer, an optical fiber phase control module, and M optical fiber phase modulators all connected to the optical fiber phase control module. The method is to set the optical paths of each input optical fiber sub-module to be consistent and the output wavelengths to be consistent, and the polarization states of the laser beams of each path at the output end of the optical fiber collimating output head to be consistent. The spatial phase regulation unit is adopted to adjust the voltage applied to the optical fiber phase modulator through the SPGD algorithm to obtain a locked spatial phase, and then the required multi-color vortex beam is obtained.

Description

Technical Field

[0001] The present invention relates to a device and method for generating a complex-color vortex beam, and particularly to a system and method for a complex-color vortex beam based on a super-fluorescent light source and coherent beam combining. Background Art

[0002] A vortex beam is a kind of spatial structured light field whose wavefront phase varies spirally around the center of the light field. The spiral phase structure of the vortex beam makes its wave vector point spirally around the optical axis, similar to an electron orbiting around the atomic nucleus, so it has orbital angular momentum. And the orbital angular momentum L carried by a single vortex photon is proportional to its topological charge l, that is ( where h is the Planck constant). Characteristics such as the spiral phase structure, quantized orbital angular momentum, and hollow intensity distribution have enabled the vortex beam to be widely applied in fields such as optical micro-manipulation, quantum computing, material topography processing, and coronagraphs.

[0003] The rapidly growing application requirements have put forward higher demands on the vortex light field and its related devices, especially the vortex light source. In response to this problem, in recent years, people have successively proposed various methods, such as the passive method of diffraction or transmission elements and the active method in a laser cavity. Currently, the more widely used optical elements include spatial light modulators, twisted nematic liquid crystals, spiral phase plates, metasurfaces, etc. Such optical elements do not involve the optical path structure of a laser resonator, avoiding the problem of optical path adjustment and can conveniently and flexibly realize the generation of various vortex light fields. However, the power-carrying capacity and diffraction efficiency of such optical elements are relatively low, and the output power and energy utilization rate are not high. In addition, factors such as complex preparation processes and high preparation costs severely restrict large-scale applications. In addition, the above-mentioned optical elements can only realize the generation of a single-wavelength vortex beam, while in some applications based on vortex beams, such as vortex optical communication systems and sensing systems, the light source is usually required to have multi-wavelength characteristics. Therefore, the research on multi-wavelength or complex-color vortex light sources is very necessary.

[0004] In 2006, in the paper "Generation of achromatic bessel beams using a compensated spatial light modulator" published by J. Leach et al. in Volume 14, Issue 12 of Optics Express, a complex-color vortex light was generated by using a spatial phase modulation device and a white light source, but an additional prism was required in the optical path for chromatic aberration compensation. In 2018, In the paper "Multispectral Management of the Photon Orbital Angular Momentum" published in PHYSICAL REVIEW LETTERS, Volume 121, Issue 21, a programmable liquid crystal spatial light modulator with a special design and "topological pixels" was used to generate multi-wavelength vortex beams. Although the above studies have all achieved the generation of multi-wavelength or polychromatic vortex beams, there are certain deficiencies, such as the need for specially designed optical elements, limited output power, and so on. In addition, the multi-wavelength or polychromatic vortex beams generated in the above studies are only limited to the type where the spatial phase (or topological charge distribution) is independent of the wavelength, and there are currently no reports on multi-wavelength or polychromatic vortex beams with a spatial phase distribution related to the wavelength. Summary of the Invention

[0005] The object of the present invention is to solve the technical problems of the existing generation of multi-wavelength or polychromatic vortex beams, such as complex structure, limited output power and types, and to propose a system and method for generating polychromatic vortex beams based on a superluminescent light source and coherent beam combining.

[0006] The technical solution provided by the present invention is as follows:

[0007] A system for generating polychromatic vortex beams based on a superluminescent light source and coherent beam combining, which is characterized in that it includes a superluminescent light source, a multi-channel laser generation unit and a spatial phase regulation unit;

[0008] The multi-channel laser generation unit includes an optical fiber splitter and a coherent optical fiber array;

[0009] The optical fiber splitter receives the fluorescent laser beam emitted by the superluminescent light source and divides the superluminescent laser beam into M beams of laser, M≥3;

[0010] The coherent optical fiber array includes M optical fiber sub-modules and M optical fiber collimating output heads corresponding to the optical fiber sub-modules. The M optical fiber collimating output heads are at least evenly arranged in a circle along the circumferential direction, and each circle is arranged at an equal radius along the radial direction; the M optical fiber sub-modules respectively receive the M beams of laser output by the optical fiber splitter;

[0011] The optical fiber sub-module includes an optical fiber tunable filter and a polarization-maintaining single-mode optical fiber fused between the optical fiber tunable filter and the optical fiber collimating output head; the input ends of the M optical fiber tunable filters are respectively connected to the M laser output ends of the optical fiber splitter; the M optical fiber collimating output heads are used to output M collimated optical signals;

[0012] The spatial phase modulation unit includes an optical transmission component, a computer, a fiber optic phase control module connected in sequence, and M fiber optic phase modulators respectively connected to the M output ends of the fiber optic phase control module; the fiber optic phase modulators are arranged between the corresponding polarization maintaining single-mode fibers and the fiber optic collimated output heads;

[0013] The optical transmission component includes a beam splitter, a reflector, a 4f system, a half-wave plate, a spatial light modulator, a third lens, and a charge coupled device arranged in sequence along the optical signal;

[0014] The complex conjugate phase of the required complex color vortex beam is loaded on the spatial light modulator;

[0015] The output end of the charge coupled device is connected to the computer, and the computer is used to set modulation parameters and adjust the voltage loaded on the fiber optic phase modulators through the fiber optic phase control module.

[0016] Further, the superluminescent light source is a superluminescent fiber light source.

[0017] Further, the 4f system includes a first lens, a spatial filter, and a second lens arranged in sequence on the transmission optical path of the beam splitter;

[0018] The aperture size of the spatial filter is adjustable.

[0019] The present invention also provides a method for realizing a complex color vortex beam based on a superluminescent light source and coherent beam combining, which is characterized in that the system for realizing a complex color vortex beam based on a superluminescent light source and coherent beam combining as described above is adopted, and includes the following steps:

[0020] S1. The laser emitted by the superluminescent light source is injected into M fiber optic sub-modules through a fiber optic splitter, M≥3, and the laser output from each fiber optic sub-module is output to the beam splitter through M fiber optic collimated output heads;

[0021] The optical paths of each fiber optic sub-module are set to be consistent, the output wavelengths of each fiber optic sub-module are the same, and the polarization states of the pulsed laser beams at the output ends of the fiber optic collimated output heads are the same;

[0022] S2. The computer is used to set the initial voltage U (k) as the current voltage, k is the current voltage adjustment times, k≥1 and is an integer, and is loaded on each fiber optic phase modulator through the fiber optic phase control module to perform phase modulation on the laser in each fiber optic sub-module;

[0023] S3. The laser after phase modulation is output from each fiber optic collimated output head to the beam splitter, the transmitted optical signal of the beam splitter is reflected by the reflector to form a combined beam field and input into the 4f system, and the 4f system processes the combined beam field and outputs it to the half-wave plate;

[0024] S4. Rotate the half-wave plate to adjust the polarization state of the combined light field so that it meets the polarization requirements of the incident laser on the spatial light modulator. Load the complex conjugate phase information of the required polychromatic vortex light field on the spatial light modulator. After passing through the spatial light modulator, the combined light field forms a combined light field carrying the complex conjugate phase information;

[0025] The combined light field carrying the complex conjugate phase information is collected by a charge-coupled device at the focal plane of the third lens;

[0026] S5. The charge-coupled device transmits the collected information to a computer to obtain the performance evaluation function J, and randomly generates a perturbation vector. According to the perturbation vector, the fiber optic phase control module generates the random perturbation voltage δU corresponding to the M-way lasers (k) ;

[0027] S6. Determine whether the performance evaluation function J has evolved to an extreme value. If so, obtain the locked spatial phase of the required polychromatic vortex beam and execute step S9; otherwise, execute step S7;

[0028] S7. After adding the random perturbation voltage and the initial voltage, load them onto the corresponding fiber optic phase modulator to apply a positive perturbation to the lasers of each fiber optic sub-module to obtain the positive performance evaluation function J (k) + ;

[0029] After subtracting the random perturbation voltage from the initial voltage, load them onto the corresponding fiber optic phase modulator to apply a negative perturbation to the lasers of each fiber optic sub-module to obtain the negative performance evaluation function J (k) - ;

[0030] According to the change amount of the performance evaluation function δJ (k) =(J (k) + -J (k) - ), adjust the voltage U loaded on the fiber optic phase modulator through the SPGD algorithm (k+1) =U (k) +γδJ (k) δU (k) , where γ is the step gain;

[0031] S8. Use the voltage loaded on the fiber optic phase modulator in step S7 as the current voltage and return to step S2;

[0032] S9. Based on the locked spatial phase in step S8, adjust the output wavelengths of the fiber optic tunable filters in each fiber optic sub-module to obtain the required polychromatic vortex beam with a spatial phase distribution related to the wavelength.

[0033] Further, in step S1, the optical paths of the input optical fiber sub-modules are set to be consistent, the output wavelengths of each optical fiber sub-module are consistent, and the polarization states of the pulsed laser beams at the output ends of the optical fiber collimating output heads are consistent. Specifically:

[0034] Based on the pulse ranging method, measure the time delay Δt of the pulsed laser beams at the output ends of the optical fiber collimating output heads of the optical fiber array. By splicing polarization-maintaining single-mode optical fibers with a length of ΔL = cΔt / N in the corresponding optical fiber sub-modules, the optical paths of each optical fiber sub-module are made consistent; where c is the speed of light and N is the refractive index of the laser wavelength in the optical fiber.

[0035] Set an optical fiber tunable filter to make the output wavelengths of each optical fiber sub-module consistent;

[0036] By setting the polarization directions of the optical fiber collimating output heads arranged radially and equally in diameter in the optical fiber array, the polarization states of the pulsed laser beams at the output ends of the optical fiber collimating output heads are made consistent.

[0037] Further, in step S3, the laser beams after phase modulation are output through the optical fiber collimating output heads to a beam splitter. Among them, the optical field distribution E m (x, y) of the m-th laser beam is:

[0038]

[0039] where ω 0 are the beam waist radii of each laser sub-module respectively, r 0 is the radius of the optical fiber array, i is the imaginary unit;

[0040] Then the optical field distribution of the laser beams output through the optical fiber collimating output heads is:

[0041]

[0042] Further, in step S3, the transmitted optical signal of the beam splitter is reflected by a mirror to form a combined beam field and is transmitted into a 4f system. Specifically: The optical field distribution E 1 (ξ, ζ) at the spatial filter is:

[0043]

[0044] where f 1 is the focal length of the first lens, λ is the laser wavelength, F represents the Fourier transform, R 0 is the aperture radius of the spatial filter, (ξ, ζ) are the Fourier plane coordinates of the first lens, and the spatial filtering characteristics are described by the circ function;

[0045] After passing through the second lens, the optical field distribution of the combined beam field is:

[0046]

[0047] where (μ, v) are the Fourier plane coordinates of the second lens, and f 2 is the focal length of the second lens.

[0048] Further, in step S4, the optical field distribution of the combined beam carrying the complex conjugate phase information is:

[0049] E 3 (μ, v) = E 2 (μ, v) exp(-inv)

[0050] where n is the topological charge of the target complex-color vortex beam.

[0051] Further, in step S4, the combined beam carrying the complex conjugate phase information is collected by a charge-coupled device at the focal plane of the third lens to obtain a performance evaluation function J, specifically:

[0052] S4.1. Set that when the fiber phase modulator is adjusted and the coherent combination of the fiber array generates an optical field that is exactly a vortex optical field, the standard optical field intensity I ideal (ε, η) is formed on the charge-coupled device;

[0053] S4.2. During the modulation process, the optical field intensity formed on the charge-coupled device is:

[0054]

[0055] where f 3 is the focal length of the third lens, and (ε, η) are the Fourier plane coordinates of the third lens;

[0056] S4.3. Calculate the performance evaluation function J, and the formula is as follows:

[0057]

[0058] where R PIB is the radius defined by the power in the bucket, and R PIB = 1.22λL / D, where λ is the laser wavelength, L is the laser transmission distance, and D is the radius of the combined optical field carrying the complex conjugate phase information before the laser is incident on the third lens.

[0059] Further, in step S9, adjust the output wavelengths of the fiber tunable filters in each fiber sub-module to obtain a complex-color vortex beam with a spatial phase distribution related to the wavelength, specifically:

[0060] Adjust the fiber tunable filters in each fiber sub-module so that their output wavelength is λm , the obtained light field distribution E of the composite-color vortex beam is as follows:

[0061]

[0062] where λ m =(λ max -λ min )m / M, λ max and λ min are respectively the maximum wavelength and the minimum wavelength output by the super-fluorescent light source, and (x 0 , y 0 ) is the central coordinate corresponding to the circle formed by the circumferential connection of M fiber collimator output heads.

[0063] Advantages of the present invention:

[0064] 1. The system for realizing the composite-color vortex beam based on the super-fluorescent light source and coherent beam combination of the present invention uses the super-fluorescent light source and the coherent fiber array in cooperation, and locks the spatial phase through the spatial phase control unit, thereby obtaining the composite-color vortex beam, providing a new idea for realizing a complex composite-color structured light field, and the device process is simple and the cost is low.

[0065] 2. In the present invention, the fiber collimator output heads can be flexibly set in the radial direction as needed. At the same time, fiber tunable filters, polarization-maintaining single-mode fibers and fiber phase modulators are arranged on each fiber sub-module, so that the phases of each fiber sub-module can be independently regulated, and further various complex composite-color structured light fields can be obtained.

[0066] 3. The present invention adopts a coherent fiber array composed of multiple fiber sub-modules and multiple fiber collimator output heads, which can greatly improve the output power without increasing the cost.

[0067] 4. The method for realizing the composite-color vortex beam based on the super-fluorescent light source and coherent beam combination of the present invention makes the optical paths input to each fiber sub-module consistent, the output wavelengths of each fiber sub-module consistent, and the polarization states of the pulsed lasers at the output ends of each fiber collimator output head consistent. By using the spatial phase control unit and adjusting the voltage applied to the fiber phase modulator through the SPGD algorithm, the locked spatial phase of the required composite-color vortex beam is obtained, and then the required composite-color vortex beam is obtained. The operation is simple, and various complex composite-color vortex beams can be obtained by adjusting the spatial distribution of the fiber collimator output heads and the independent regulation of the phases of the fiber sub-modules. Description of the Drawings

[0068] Figure 1 is a schematic diagram of an embodiment of the system structure for realizing the composite-color vortex beam based on the super-fluorescent light source and coherent beam combination of the present invention;

[0069] Figure 2Schematic diagram of the multi-channel laser generation unit in the embodiment of the present invention;

[0070] Figure 3 Flowchart of the method for realizing a polychromatic vortex beam based on a superluminescent light source and coherent beam combining in the present invention;

[0071] Figure 4 Schematic diagram of the simulation analysis result of the vortex beam in the embodiment of the present invention.

[0072] The reference signs are as follows:

[0073] 1 - Superluminescent light source, 2 - Fiber optic splitter, 3 - Fiber optic tunable filter, 4 - Polarization-maintaining single-mode fiber, 5 - Fiber optic phase modulator, 6 - Fiber optic collimated output head, 7 - Beam splitter, 8 - Power meter, 9 - Reflector, 10 - First lens, 11 - Spatial filter, 12 - Second lens, 13 - Half-wave plate, 14 - Spatial light modulator, 15 - Third lens, 16 - Charge-coupled device, 17 - Computer, 18 - Fiber optic phase control module. Detailed implementation manners

[0074] Refer to Figures 1 - 2 , this embodiment provides a system for realizing a polychromatic vortex beam based on a superluminescent light source and coherent beam combining. The system includes a superluminescent light source 1, a multi-channel laser generation unit, and a spatial phase regulation unit. The superluminescent light source 1 provides broadband laser. The output of the superluminescent light source 1 is amplified spontaneous emission, and its state is between fluorescence and laser. The superluminescent light source 1 adopted in this embodiment is a superluminescent fiber light source. Because it does not have a resonator, its output spectrum is relatively wide, up to dozens of nanometers. In addition, the output spectrum range of the superluminescent light source 1 can be adjusted by means such as temperature regulation. There are three advantages in adopting a superluminescent fiber light source: 1. The superluminescent fiber light source is a superluminescent light source 1 with doped fiber as the gain medium, having low temporal coherence and good spatial coherence; 2. The superluminescent fiber light source has the advantages of no relaxation oscillation, no mode competition, no self-pulsing, and good temporal stability; 3. Since fiber optic devices have the advantages of miniaturization and light weight, the superluminescent fiber light source system is compact and light in weight.

[0075] The multi-channel laser generating unit includes an optical fiber splitter 2 and a coherent optical fiber array; the optical fiber splitter 2 receives the fluorescent laser beam emitted by the superluminescent light source 1 and divides the fluorescent laser beam into M laser beams, where M≥3; the coherent optical fiber array includes M optical fiber sub-modules and M optical fiber collimating output heads 6 correspondingly connected to the optical fiber sub-modules. The M optical fiber collimating output heads 6 are arranged at least evenly in a circle along the circumferential direction, and each circle is arranged with equal radii along the radial direction. In this embodiment, it is a single-circle distribution in the radial direction, and multiple circles can also be arranged according to needs; the M optical fiber sub-modules correspondingly receive the M laser beams output by the optical fiber splitter 2; the optical fiber sub-module includes an optical fiber tunable filter 3 and a polarization-maintaining single-mode optical fiber 4 fused between the optical fiber tunable filter 3 and the optical fiber collimating output head 6; the input ends of the M optical fiber tunable filters 3 are respectively connected to the M laser output ends of the optical fiber splitter 2; the M optical fiber collimating output heads 6 are used to output M collimated optical signals; by using the polarization-maintaining single-mode optical fiber 4, on the one hand, it ensures the single-mode property of the laser output by each optical fiber sub-module of the optical fiber array, and on the other hand, it ensures that the laser output by each optical fiber sub-module is linearly polarized light, guaranteeing the polarization consistency of the laser output by the optical fiber array. When in use, the superluminescent light source 1 provides a broadband laser output, and multi-channel lasers (each channel corresponds to a wavelength) are obtained by using a beam splitting device and a filtering device. The coherent optical fiber array is arranged in a single-layer radial pattern, and the phase of each optical fiber sub-module of the optical fiber array can be phase-regulated by a phase modulation device, so that the phase distribution of the radially arranged optical fiber array shows a discrete angular gradient change, that is, the phase distribution spirals around the center, forming a vortex phase distribution. Then, the laser of each channel is injected into the corresponding optical fiber collimating output head 6 of the optical fiber array, thereby realizing the generation of a complex color or multi-wavelength vortex beam with a spatial phase distribution related to the wavelength.

[0076] The spatial phase control unit includes an optical transmission component, a computer 17, an optical fiber phase control module 18 connected in sequence, and M optical fiber phase modulators 5 correspondingly connected to the M output ends of the optical fiber phase control module 18; the optical fiber phase modulators 5 are arranged between the corresponding polarization-maintaining single-mode optical fibers 4 and the optical fiber collimating output heads 6; the optical transmission component includes a beam splitter 7 arranged in sequence along the optical signal. The transmitted light of the beam splitter 7 is sequentially provided with a reflecting mirror 9, a 4f system, a half-wave plate 13, a spatial light modulator 14, a third lens 15, and a charge-coupled device 16; the reflected light of the beam splitter 7 enters the power meter 8. The 4f system includes a first lens 10, a spatial filter 11, and a second lens 12; among them, the aperture size of the spatial filter 11 is adjustable, and by changing the aperture radius size of the spatial filter, the sidelobes of the optical field generated by the coherent beam combination of the optical fiber array can be truncated, improving the purity of the beam generated by the coherent beam combination.

[0077] The complex conjugate phase of the required polychromatic vortex beam is loaded on the spatial light modulator 14; the output end of the charge-coupled device 16 is connected to the computer 17, which is used to set the modulation parameters and adjust the voltage loaded on the fiber optic phase modulator 5 through the fiber optic phase control module 18.

[0078] The optical transmission process of the above system for realizing a polychromatic vortex beam based on a superluminescent light source and coherent beam combining is as follows:

[0079] The broadband laser emitted from the superluminescent fiber light source 1 passes through the 1×M fiber optic splitter 2 and is then injected into the fiber optic array composed of M fiber optic sub-modules; in each fiber optic sub-module, the injected laser can be flexibly adjusted in output laser wavelength through the fiber optic tunable filter 3, and then its phase is adjusted by the fiber optic phase modulator 5, and finally it is output by the fiber optic collimating output head 6. The M fiber optic collimating output heads 6 are radially arranged in space to form a fiber optic collimating output array.

[0080] The polychromatic vortex beam is obtained by using the above system for realizing a polychromatic vortex beam based on a superluminescent light source and coherent beam combining by splicing a polarization-maintaining single-mode fiber 4 with a length of ΔL in the corresponding fiber optic sub-module to ensure the consistency of the optical path of each fiber optic sub-module in the fiber optic array; in order to obtain a multi-wavelength or polychromatic vortex beam with a spatial phase distribution related to the wavelength, the premise is to achieve vortex phase locking at the same wavelength. First, the fiber optic tunable filter 3 is set so that the output wavelengths of each fiber optic sub-module are kept consistent. Most of the laser output from the fiber optic array is reflected by the beam splitter 7 into the power meter 8 for power recording and then used, and a small part is used for sampling. It passes through the mirror 9 and the 4f optical system with the spatial filter 11 in turn to truncate the sidelobes of the combined light field and improve the purity of the coherent combined light field. The polarization state of the combined light field is adjusted by using the half-wave plate 13 to meet the polarization requirements of the incident laser for the spatial light modulator 14. The complex conjugate phase information of the required vortex light field is loaded on the spatial light modulator 14. The spot pattern of the combined light field carrying the complex conjugate phase information after being focused by the third lens is collected by the charge-coupled device (CCD) near the focal plane to obtain the performance evaluation function J. According to the change amount δJ of the performance evaluation function (k) , the control voltage loaded on the fiber optic phase modulator 5 is adjusted through the SPGD algorithm and iterated continuously until the performance evaluation function J gradually evolves to an extreme value, thus realizing the spatial phase locking of the required vortex beam. Finally, the output wavelength of the fiber optic tunable filter 3 in each fiber optic sub-module is changed, thereby obtaining a multi-wavelength (or polychromatic) vortex beam with a spatial phase distribution related to the wavelength.

[0081] See Figure 3 , and the specific steps are as follows:

[0082] S1. The superluminescent fiber light source is fused with the fiber optical splitter 2. The superluminescent light source 1 emits broadband laser, which is divided into M paths of laser by the fiber optical splitter 2 and respectively injected into M fiber sub-modules, where M≥3. The laser power of each path is about 1 / M of the input laser, but its spectral range is consistent with that of the input laser. After each path of laser entering the fiber array passes through the fiber tunable filter 3, a narrowband laser with adjustable central wavelength can be obtained by means of electric control or manual operation. After each path of narrowband laser passes through the fiber phase modulator 5, phase regulation is carried out, and the laser output from each fiber sub-module is output to the beam splitter 7 through M fiber collimating output heads 6.

[0083] Set the optical path of each path input into the fiber sub-module to be consistent, the output wavelength of each fiber sub-module to be consistent, and the polarization states of the pulsed laser of each path at the output end of the fiber collimating output head 6 to be consistent. Specifically, in order to ensure the consistency of the optical path of each fiber sub-module in the fiber array, passive fiber is used to regulate the optical path in this embodiment. Its basic principle is the pulse ranging method. The picosecond pulsed laser is input into the fiber optical splitter 2. After splitting, the picosecond pulsed laser is transmitted in each fiber sub-module. A high-speed photodetector is used at the output end of the fiber array to measure the time delay Δt of the pulsed laser of each path at the output end of the fiber collimating output head 6 of the fiber array. By splicing polarization-maintaining single-mode fiber 4 with a length of ΔL = cΔt / N in the corresponding fiber sub-module, the optical paths of each fiber sub-module are made consistent, where c is the speed of light and N is the refractive index of the laser wavelength in the fiber. Set the fiber tunable filter 3 to make the output wavelength of each fiber sub-module consistent. By rotating the fiber collimating output head 6, set the polarization direction of the fiber collimating output heads 6 arranged radially and equidiametrically in the fiber array, so that the polarization states of the pulsed laser of each path at the output end of the fiber collimating output head 6 are consistent.

[0084] S2. Use the computer 17 to set the initial voltage U (k) , and take the initial voltage U (k) as the current voltage, where k is the current voltage adjustment times, k≥1 and k is an integer; and load it onto each fiber phase modulator 5 through the fiber phase control module 18 to perform phase modulation on the laser in each fiber sub-module.

[0085] S3. The laser after phase modulation is output to the beam splitter 7 through the fiber collimating output head 6 for each path of laser. Among them, the optical field distribution E m (x, y) of the m-th path of laser output from the fiber collimating output head 6 is:

[0086]

[0087] Among them, ω 0 are the waist radii of each laser sub-module respectively. r 0 is the radius of the fiber array, i is the imaginary unit;

[0088] Then the optical field distributions of the laser beams output from the fiber collimating output head 6 are as follows:

[0089]

[0090] After the laser array of the same wavelength output from the fiber collimating output head 6 passes through the beam splitter 7, most of it is reflected into the power meter 8. The transmitted part is reflected by the mirror 9, and the laser beams with different phases interact with each other during spatial transmission to form a combined beam optical field. It successively passes through the 4f system composed of the first lens 10, the spatial filter 11, and the second lens 12. The reflected light signal of the beam splitter 7 enters the power meter 8, and the transmitted light signal of the beam splitter 7 is reflected by the mirror 9 to form a combined beam optical field and input into the 4f system. Specifically: The optical field distribution E 1 at the spatial filter 11 of the combined beam optical field is:

[0091]

[0092] where f 1 is the focal length of the first lens 10, λ is the laser wavelength, F represents the Fourier transform, R 0 is the aperture radius of the spatial filter 11, (ξ, ζ) are the Fourier plane coordinates of the first lens 10, and the spatial filtering characteristics are described by the circ function; Since there are sidelobes in the optical field generated by the coherent beam combination of the fiber array, changing the aperture radius R 0 of the spatial filter 11 can truncate the sidelobes of the optical field generated by the coherent beam combination of the fiber array and improve the purity of the beam generated by the coherent beam combination.

[0093] After passing through the second lens 12, the optical field distribution of the combined beam optical field is:

[0094]

[0095] where (μ, v) are the Fourier plane coordinates of the second lens 12, and f 2 is the focal length of the second lens 12.

[0096] The 4f system processes the combined beam optical field and outputs it to the half-wave plate 13.

[0097] S4. Rotate the half-wave plate 13 to adjust the polarization state of the combined beam optical field to meet the polarization requirements of the spatial light modulator 14 for the incident laser. The complex conjugate phase information of the required complex-color vortex optical field is loaded on the spatial light modulator 14. After passing through the spatial light modulator 14, the combined beam optical field forms a combined beam optical field carrying the complex conjugate phase information; The optical field distribution of the combined beam optical field carrying the complex conjugate phase information is:

[0098] E 3(μ, v) = E 2 (μ, v) exp(-inv)

[0099] Where n is the topological charge of the target complex-color vortex beam.

[0100] The combined beam field carrying the complex conjugate phase information is collected by the charge-coupled device 16 at the focal plane of the third lens 15 to obtain the performance evaluation function J, specifically:

[0101] S4.1. Set that when the fiber optic phase modulator 5 is adjusted so that the light field generated by the coherent combination of the fiber optic array is exactly a vortex light field, the standard light field intensity formed on the charge-coupled device 16 is I ideal (ε, η);

[0102] S4.2. During the modulation process, the light field intensity formed on the charge-coupled device 16 is:

[0103]

[0104] Where f 3 is the focal length of the third lens 15, and (ε, η) are the Fourier plane coordinates of the third lens 15;

[0105] S4.3. Calculate the performance evaluation function J, and the formula is as follows:

[0106]

[0107] Where R PIB is the radius defined by the power in the bucket, and R PIB = 1.22λL / D, where λ is the laser wavelength, L is the laser transmission distance, and D is the radius of the combined light field carrying the complex conjugate phase information before the laser is incident on the third lens 15.

[0108] S5. The charge-coupled device 16 transmits the collected information to the computer 17 and randomly generates a perturbation vector. According to the randomly generated perturbation vector, the fiber optic phase control module 18 generates the random perturbation voltage δU corresponding to the M-channel laser (k) ;

[0109] S6. Determine whether the performance evaluation function J has evolved to an extreme value. If so, obtain the locked spatial phase of the required complex-color vortex beam and execute step S9; otherwise, execute step S7;

[0110] S7. After adding the random perturbation voltage and the initial voltage, load it onto the corresponding fiber optic phase modulator 5 to apply a positive perturbation to the lasers of each fiber optic sub-module to obtain the positive performance evaluation function J (k) + ;

[0111] After subtracting the random perturbation voltage from the initial voltage, it is loaded onto the corresponding fiber optic phase modulator 5 to apply a negative perturbation to the laser of each fiber optic sub-module, and the forward performance evaluation function J is obtained. (k) - ;

[0112] According to the change amount δJ of the performance evaluation function (k) =(J (k) + -J (k) - ), the voltage loaded on the fiber optic phase modulator 5 is adjusted through the SPGD algorithm: U (k+1) =U (k) +γδJ (k) δU (k) , where γ is the step gain;

[0113] S8. Use the voltage loaded on the fiber optic phase modulator 5 in step S7 as the current voltage, and return to step S2;

[0114] S9. Based on the locked spatial phase in step S8, adjust the output wavelength of the fiber optic tunable filter 3 in each fiber optic sub-module to obtain a complex vortex beam with a spatial phase distribution related to the wavelength, specifically: adjust the fiber optic tunable filter 3 in each fiber optic sub-module so that its output wavelength is λ m , then the optical field distribution E of the obtained complex vortex beam is:

[0115]

[0116] where λ m =(λ max -λ min )m / M, λ max and λ min are respectively the maximum wavelength and the minimum wavelength output by the superluminescent light source 1, and (x 0 , y 0 ) is the central coordinate corresponding to the circle formed by the circumferential connection of M fiber collimated output heads.

[0117] In order to verify the effectiveness of the method for realizing a complex vortex beam based on the superluminescent light source 1 and coherent beam combination proposed in this embodiment, a simulation analysis was carried out through a numerical simulation method. See Figure 4 , in the analysis process, it is assumed that in the radially arranged fiber array, the laser output of each fiber array is single-frequency single-mode Gaussian laser, and the loaded phase of the m-th fiber optic sub-module is (M is the number of fiber optic sub-modules, M = 8). If the output laser wavelengths of all fiber optic sub-modules are the same (the spot pattern at the output end of the fiber array is shown in Figure 4-1), then a vortex beam is obtained in the far field, and its intensity and phase distributions are as shown in Figure 4 - 2As shown in FIGS. 4-3; if the wavelength is set (the wavelength range is 1058 nm - 1072 nm, and the wavelength interval is 2 nm) and also shows a spatial gradient change (the spot pattern at the output end of the fiber array is as Figure 4 - 4 shown), then a polychromatic vortex beam with a spatial phase distribution related to the wavelength is obtained in the far field, and its intensity and phase distributions are as Figure 4 shown in FIGS. 4-5 and 4-6. Multi-wavelength lasers with a constant phase difference can be superimposed and interfered within a certain range, but the contrast of the interference fringes will decrease. Comparing Figure 4 - 2 with FIGS. 4-4, the maximum intensity value of the polychromatic vortex beam with a spatial phase distribution related to the wavelength is lower than that of the single-wavelength vortex optical field, which is in good agreement with the theoretical analysis results. In addition, in the real environment, the laser has a certain line width, and at the same time, it is affected by environmental and temperature perturbations, and there are also certain perturbations in the vortex phase of the fiber array. Therefore, the contrast of the intensity of the polychromatic vortex beam with a spatial phase distribution related to the wavelength will be further reduced.

[0118] The method for generating a polychromatic vortex beam proposed in this embodiment uses a coherent fiber array and can achieve high-power laser output; in addition, the flexible spatial arrangement and the independent phase control of the fiber sub-modules provide a new way to realize various complex polychromatic structured light fields.

Claims

1. A system for realizing a composite vortex beam based on a superluminescent light source and coherent beam combining, characterized in that: it includes a superluminescent light source (1), a multi-channel laser generating unit and a spatial phase modulation unit; the multi-channel laser generating unit includes an optical fiber splitter (2) and a coherent optical fiber array; the optical fiber splitter (2) receives the fluorescent laser beam emitted by the superluminescent light source (1) and divides the superluminescent laser beam into M beams of laser, where M≥3; the coherent optical fiber array includes M optical fiber sub-modules and M optical fiber collimating output heads (6) correspondingly connected to the optical fiber sub-modules. The M optical fiber collimating output heads (6) are arranged at least evenly in a circle along the circumferential direction, and are arranged at equal radii along the radial direction in each circle; the M optical fiber sub-modules correspondingly receive the M beams of laser output by the optical fiber splitter (2); the optical fiber sub-module includes an optical fiber tunable filter (3) and a polarization-maintaining single-mode optical fiber (4) fused between the optical fiber tunable filter (3) and the optical fiber collimating output head (6); the input ends of the M optical fiber tunable filters (3) are respectively connected to the M laser output ends of the optical fiber splitter (2); the M optical fiber collimating output heads (6) are used for outputting M collimated optical signals; the spatial phase modulation unit includes an optical transmission component, a computer (17), an optical fiber phase control module (18) connected in sequence, and M optical fiber phase modulators (5) correspondingly connected to the M output ends of the optical fiber phase control module (18); the optical fiber phase modulator (5) is arranged between the corresponding polarization-maintaining single-mode optical fiber (4) and the optical fiber collimating output head (6); the optical transmission component includes a beam splitter (7), a reflecting mirror (9), a 4f system, a half-wave plate (13), a spatial light modulator (14), a third lens (15) and a charge-coupled device (16) arranged in sequence along the optical signal; the beam splitter (7) is used for receiving the M collimated optical signals output from the M optical fiber collimating heads (6); the complex conjugate phase of the required composite vortex beam is loaded on the spatial light modulator (14); the output end of the charge-coupled device (16) is connected to the computer (17), and the computer (17) is used for setting modulation parameters and realizing the adjustment of the voltage loaded on the optical fiber phase modulator (5) through the optical fiber phase control module (18).

2. The system for realizing a composite vortex beam based on a superluminescent light source and coherent beam combining according to claim 1, characterized in that: the superluminescent light source (1) is a superluminescent fiber light source.

3. The system for realizing a composite vortex beam based on a superluminescent light source and coherent beam combining according to claim 1 or 2, characterized in that: the 4f system includes a first lens (10), a spatial filter (11) and a second lens (12) arranged in sequence on the transmission optical path of the beam splitter (7); the aperture size of the spatial filter (11) is adjustable.

4. A method for realizing a composite vortex beam based on a superluminescent light source and coherent beam combining, characterized in that, using the system for realizing a composite vortex beam based on a superluminescent light source and coherent beam combining according to claim 1, includes the following steps: S1. The laser emitted by the superluminescent light source (1) is injected into M fiber sub-modules through an optical fiber splitter (2), where M≥3. The laser output from each fiber sub-module is then output to a beam splitter (7) through M fiber collimating output heads (6). The optical path of each fiber sub-module is set to be the same, the output wavelength of each fiber sub-module is the same, and the polarization states of the pulsed laser beams at the output ends of the fiber collimating output heads (6) are the same. S2. Use a computer (17) to set the initial voltage U (k) as the current voltage, k is the current voltage adjustment times, k ≥ 1 and is an integer, and load it onto each fiber optic phase modulator (5) through the fiber optic phase control module (18) to perform phase modulation on the laser in each fiber optic sub-module; S3. The laser after phase modulation is output as laser beams of each path through the fiber collimating output heads (6) to the beam splitter (7). The transmitted optical signal of the beam splitter (7) is reflected by a mirror (9) to form a combined beam field, which is input into a 4f system. The 4f system processes the combined beam field and outputs it to a half-wave plate (13). S4. Rotate the half-wave plate (13) to adjust the polarization state of the combined beam field so that it meets the polarization requirements of the incident laser on the spatial light modulator (14). The complex conjugate phase information of the required complex-color vortex beam field is loaded on the spatial light modulator (14). After passing through the spatial light modulator (14), the combined beam field forms a combined beam field carrying the complex conjugate phase information. The combined beam field carrying the complex conjugate phase information is collected by a charge-coupled device (16) at the focal plane of the third lens (15). S5. The charge-coupled device (16) transmits the collected information to the computer (17) to obtain the performance evaluation function J, and randomly generates a perturbation vector. According to the perturbation vector, the optical fiber phase control module (18) generates the random perturbation voltage δU corresponding to M paths of lasers (k) ; S6. Determine whether the performance evaluation function J evolves to an extreme value. If so, obtain the locked spatial phase of the required complex-color vortex beam and execute step S9; otherwise, execute step S7. S7. After adding the random perturbation voltage and the initial voltage, load them onto the corresponding fiber optic phase modulator (5) to apply a positive perturbation to the laser of each fiber optic sub-module, and obtain the positive performance evaluation function J (k) + ; After subtracting the random perturbation voltage from the initial voltage, it is loaded onto the corresponding fiber optic phase modulator (5) to apply a negative perturbation to the laser of each fiber optic sub-module, and a negative performance evaluation function J is obtained. (k) - ; According to the change amount δJ of the performance evaluation function (k) =(J (k) + -J (k) - ), the voltage U applied to the fiber optic phase modulator (5) is adjusted by the SPGD algorithm (k+1) =U (k) +γδJ (k) δU (k) , where γ is the step gain; S8. Use the voltage loaded on the fiber optic phase modulator (5) in step S7 as the current voltage and return to step S2. S9. Based on the locked spatial phase in step S8, adjust the output wavelengths of the fiber optic tunable filters (3) in each fiber sub-module to obtain the required complex-color vortex beam with a spatial phase distribution related to the wavelength.

5. The method for realizing a complex-color vortex beam based on a superluminescent light source and coherent beam combination according to claim 4, characterized in that: In step S1, setting the optical path of each fiber sub-module to be the same, the output wavelength of each fiber sub-module to be the same, and the polarization states of the pulsed laser beams at the output ends of the fiber collimating output heads (6) to be the same specifically includes: Based on the pulse ranging method, measure the time delay Δt of the pulsed laser beams at the output ends of the fiber collimating output heads (6) of the fiber array. By splicing polarization-maintaining single-mode fibers (4) with a length of ΔL = cΔt / N in the corresponding fiber sub-modules, the optical paths of each fiber sub-module are made the same; where c is the speed of light and N is the refractive index of the laser wavelength in the optical fiber. Set the fiber optic tunable filters (3) to make the output wavelengths of each fiber sub-module the same. By setting the polarization directions of the fiber collimating output heads (6) arranged radially and equally in diameter in the fiber array, the polarization states of the pulsed laser beams at the output ends of the fiber collimating output heads (6) are made the same.

6. The method for realizing a complex-color vortex beam based on a superluminescent light source and coherent beam combination according to claim 5, characterized in that: In step S3, the phase-modulated laser is output through the fiber collimation output head (6) to output multiple paths of laser to the beam splitter (7). Among them, the optical field distribution E m (x, y) of the m-th path of laser is as follows: where ω 0 is the beam waist radius of each laser sub-module, r 0 is the radius of the fiber array, and i is the imaginary unit; Then the optical field distributions of the laser beams output from each path through the fiber collimating output heads (6) are:

7. The method for realizing a complex-color vortex beam based on a superluminescent light source and coherent beam combination according to claim 6, characterized in that: In step S3, the transmitted light signal of the beam splitter (7) is reflected by the mirror (9) to form a combined beam field and is introduced into the 4f system. Specifically, the light field distribution of the combined beam field at the spatial filter (11) is as follows: where f 1 is the focal length of the first lens (10), λ is the laser wavelength, F represents the Fourier transform, R 0 is the aperture radius of the spatial filter (11), (ξ, ζ) are the Fourier plane coordinates of the first lens (10), and the spatial filtering characteristic is described by the circ function; After passing through the second lens (12), the light field distribution of the combined beam field is as follows: where (μ, v) are the Fourier plane coordinates of the second lens (12), and f 2 is the focal length of the second lens (12).

8. The method for realizing a polychromatic vortex beam based on a superluminescent light source and coherent beam combining according to claim 7, characterized in that: In step S4, the light field distribution of the combined beam field carrying the complex conjugate phase information is as follows: E 3 (μ, v) = E 2 (μ, v) exp(-inv) where n is the topological charge of the target polychromatic vortex beam.

9. The method for realizing a polychromatic vortex beam based on a superluminescent light source and coherent beam combining according to claim 8, characterized in that: In step S4, the combined beam field carrying the complex conjugate phase information is collected by the charge-coupled device (16) at the focal plane of the third lens (15) to obtain the performance evaluation function J. Specifically: S4.

1. Set that when the optical fiber phase modulator (5) is adjusted and the coherent beam combination of the optical fiber array generates an optical field that is exactly a vortex optical field, a standard optical field intensity I is formed on the charge coupled device (16). ideal (ε,η); S4.

2. During the modulation process, the light field intensity formed on the charge-coupled device (16) is: where f 3 is the focal length of the third lens (15), and (ε, η) are the Fourier plane coordinates of the third lens (15); S4.

3. Calculate the performance evaluation function J, and the formula is as follows: Among them, R PIB is the radius defined by the power in the barrel, R PIB = 1.22λL / D, where λ is the laser wavelength, L is the laser transmission distance, and D is the radius of the synthetic optical field carrying the complex conjugate phase information before the laser is incident on the third lens (15).

10. The method for realizing a polychromatic vortex beam based on a superluminescent light source and coherent beam combining according to claim 9, characterized in that: In step S9, adjust the output wavelengths of the fiber tunable filters (3) in each fiber sub-module to obtain a polychromatic vortex beam with a spatial phase distribution related to the wavelength. Specifically: Adjust the fiber tunable filter (3) in each fiber sub-module so that its output wavelength is λ m , then the optical field distribution E of the obtained polychromatic vortex beam is: where λ m =(λ max -λ min )m / M, λ max and λ min are the maximum wavelength and the minimum wavelength output by the superluminescent light source (1) respectively, and (x 0 , y 0 ) are the central coordinates corresponding to the circle formed by the circumferential connection of M fiber collimated output heads.

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