CN112198668B - Optical field reconstruction system and method for generating vortex light beam by coherent synthesis of fiber laser - Google Patents
Optical field reconstruction system and method for generating vortex light beam by coherent synthesis of fiber laser Download PDFInfo
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Abstract
The invention provides a light field reconstruction system and a light field reconstruction method for generating vortex light beams through optical fiber laser coherent synthesis. The optical field reconstruction method for generating the vortex light beam based on the optical fiber laser coherent synthesis is compatible with the vortex light beam generation system, and the vortex light beam generation and the optical field reconstruction for generating the vortex light beam based on the coherent synthesis can be realized through one set of system. The invention overcomes the difficulty of representing the vortex light beam light field generated by the coherent synthesis of the fiber laser in the prior technical scheme.
Description
Technical Field
The invention relates to the technical field of optical fiber laser coherent synthesis, in particular to a light field reconstruction method for generating vortex light beams based on optical fiber laser coherent synthesis.
Background
With the rapid development of laser and photonics technologies, the generation and regulation of structured light fields with special spatial distribution of amplitude, phase, polarization and coherence gain wide attention of researchers at home and abroad. As a typical structure optical field, the intensity is distributed annularly, the vortex optical field with a spiral phase structure and carrying orbital angular momentum is a hotspot of recent academic research, discloses a plurality of novel physical phenomena and is widely applied to the fields of free space optical communication, super-resolution optical imaging, laser processing and manufacturing, optical tweezers and the like. For special application scenes of long-distance free space optical communication, laser ablation and processing and nonlinear frequency conversion, the vortex light beam is required to be utilized, and higher requirements are also put forward on the power of the vortex light beam.
Most current methods of generating a vortex beam have severe power limitations. The fiber laser coherent synthesis has the potential of improving the output power, can keep good beam quality, applies the fiber laser coherent synthesis technology to generate vortex beams, has the advantages of large power improvement potential and high mode switching speed, and is paid attention by researchers in recent years.
At present, deep research has been carried out on the aspects of the arrangement mode, the phase control method and the light beam transmission characteristic of the optical fiber laser array on the emitting surface of the vortex light beam generated based on the coherent synthesis of the optical fiber laser. However, the technical route for generating the vortex beam is still insufficient in the aspect of optical field characterization, so that the research on the optical field distribution characteristics of the vortex beam generated based on the coherent synthesis of the fiber laser is less, and the further development of the technical route is not facilitated. In order to solve the problem, a method for detecting and reconstructing the optical field which generates the vortex light beam based on the coherent synthesis of the fiber laser needs to be provided.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides a light field reconstruction system and a light field reconstruction method for generating vortex light beams by coherent synthesis of fiber lasers. By adopting the method and the device, the light intensity distribution of the vortex light beam generated by coherent synthesis can be obtained according to the detection signal, and the phase distribution of the vortex light beam generated by coherent synthesis is reconstructed according to the detection signal, so that the light field characterization of the vortex light beam generated by the coherent synthesis based on the fiber laser is realized. The invention realizes the generation of vortex beams and the light field reconstruction of the vortex beams based on coherent synthesis through a set of system.
In order to achieve the technical purpose, the invention adopts the following specific technical scheme:
the optical field reconstruction system for generating vortex beams by fiber laser coherent synthesis comprises a vortex beam generation subsystem and an optical field reconstruction subsystem; the vortex light beam generating subsystem generates vortex light beams based on optical fiber laser coherent synthesis, a small part of array light beams output by the vortex light beam generating subsystem are collected and input into the light field reconstruction subsystem, and the light field reconstruction subsystem comprises a 4-F system, a 1# space light phase modulator, a 3# focusing lens, a 1# objective lens, a 2# space light phase modulator, a 4# focusing lens, a 2# objective lens and a light spot collecting module which are sequentially arranged. The array light beam is transmitted to a 1# space optical phase modulator for modulation after passing through a 4-F system, and the 1# space optical phase modulator controls the amplitude and the phase of a central unit light beam of the array light beam; the array light beam modulated by the 1# space optical phase modulator is transmitted to the 2# space optical phase modulator through a 3# focusing lens and a 1# objective lens, and the 1# objective lens enables the array light beam to be matched with the 2# space optical phase modulator in size; the 2# space optical phase modulator is loaded with phase distribution used for resolving light field information and modulates the array light beam; the light beam modulated by the 2# space light phase modulator passes through the 4# focusing lens and the 2# objective lens and then is collected and processed by the light spot collection module, and the 2# objective lens enables the light beam transmitted to the light spot collection module to be matched with the light spot collection module in size. The light spot acquisition module captures and acquires light spots in real time and analyzes and processes light intensity information, so that light field reconstruction of vortex light beams generated by coherent synthesis is realized.
Preferably, the 4-F system is composed of a 1# focusing lens and a 2# focusing lens.
Preferably, the vortex beam generation subsystem comprises a seed source, a fiber splitter, a phase modulator array, a fiber amplification module, a collimator array, a high-reflection mirror, a spectroscope, an array beam processing module, a photoelectric detection module and a control system.
The optical fiber beam splitter is provided with N output ends, the seed laser output by the seed source is divided into N beams of unit light beams by the optical fiber beam splitter, and the output ends of the optical fiber beam splitters are respectively and sequentially connected with the phase modulator, the optical fiber amplification module and the collimator; n phase modulators form a phase modulator array; the N collimators are arranged in a circular array to form a collimator array. The multi-path unit light beams output by the beam splitter are transmitted to a collimator array positioned on the emitting surface after being subjected to phase control of a phase modulator and power expansion of an optical fiber amplification module, and the collimator array splices the multi-path unit light beams into array light beams distributed in a circular array and collimates and outputs the array light beams.
The array light beam of the emitting surface output by the collimator array is divided into two parts by the high reflecting mirror, wherein most of the array light beam with power reflected by the high reflecting mirror is transmitted to a far field in free space, and forms a vortex light beam in the far field. In addition, the array beam with small power transmitted from the high reflecting mirror is used for closed-loop phase control of the vortex beam generation subsystem and light field information acquisition of the light field reconstruction subsystem.
The array light beam with small power transmitted from the high reflector is divided into two parts again by the spectroscope, wherein the light beam transmitted from the spectroscope is transmitted to the light field reconstruction subsystem, the light beam reflected from the spectroscope is received by the photoelectric detection module after being processed by the array light beam processing module, and the photoelectric detection module converts the optical signal of the received light field into an electric signal and transmits the electric signal to the control system. The control system is loaded with a phase optimization control algorithm, processes the electric signals transmitted by the photoelectric detection module by operating the phase optimization control algorithm, acquires phase control signals of each path of unit light beam, transmits the phase control signals to the corresponding phase modulator, and realizes compensation of the phase noise of the vortex light beam generation subsystem and piston phase regulation of the array light beam.
Wherein: the array beam processing module consists of a spatial light phase modulator and a focusing lens, or consists of a spatial light phase modulator and a lens group, or consists of liquid crystal and a focusing lens, or consists of liquid crystal and a lens group; the array light beam processing module and the photoelectric detection module are used for processing light field information of the emitted area array light beam and collecting light spots containing phase control information after processing.
Preferably, the seed laser output by the seed source is uniformly divided into N unit beams by the optical fiber beam splitter after being preliminarily amplified by the preamplifier.
Preferably, the fiber amplification module is composed of a single fiber amplifier, or is composed of cascaded fiber amplifier chains, or is realized by a common aperture coherent synthesis technology.
Preferably, the phase optimization control algorithm is a random parallel gradient descent algorithm, a simulated annealing algorithm, a particle swarm optimization algorithm, or the like.
Based on the light field reconstruction system, the invention provides a light field reconstruction method for generating vortex light beams by coherent synthesis of fiber lasers, which comprises the following steps:
the emission area array light beam output by the vortex light beam generation subsystem in the optical field reconstruction system for generating the vortex light beam by the fiber laser coherent synthesis comprises a central unit light beam positioned at the central position of the array light beam and NcircEach circular sub-array takes the center of the central unit light beam as the center of a circle, the unit light beams on each circular sub-array are uniformly distributed along the angular direction, and the beam waist radius of each path of unit light beam and the central unit light beam on each circular sub-array is w0Wavelength of λ, beam diameter of d, and amplitude of A0And the center of the unit beam and the central unit beam on each circular subarrayHas a pitch of rj，j＝1、2、3…Ncirc。
The optical field distribution of the emission area array light beam output by the vortex light beam generation subsystem is as follows:
wherein, (x, y) is the coordinates of the emitting surface, NjFor the number of unit beams contained in the jth circular sub-array, (x)j,h,yj,h) And phij,hThe center coordinate and the piston phase of the h unit beam on the j circular sub-array are shown. U shapec(x, y) is the complex amplitude of the central unit beam, when the vortex beam generation subsystem works in the state of generating vortex beams, the central unit beam does not emit light, and U is the complex amplitude of the central unit beamc(x,y)＝0。
The central coordinate parameter of the h unit beam on the jth circular sub-array meets the following conditions:
wherein r isjThe distance between the beam center of the jth circular sub-array unit and the emitting area array center is shown.
For generating vortex light beams with topological charge number l, the piston phase parameters of the light beams of the emission surface unit satisfy the following conditions:
in the light field reconstruction subsystem, the light field distribution of the array light beam transmitted to the spatial light phase modulator through the 4-F system is consistent with the light field distribution of the array light beam of the emitting area, and the light field distribution of the array light beam transmitted to the spatial light phase modulator through the 3# focusing lens and the 1# objective lens after passing through the 1# spatial light phase modulator is the far field light field distribution of the array light beam
Wherein
Is the plane polar coordinate of the 2# space optical phase modulator; far field light field distribution of array beam
Can be decomposed into a plurality of angular modes as follows:
wherein, cl(ρ)＝αl(ρ)exp[iΔθl(ρ)]Being coefficients of angular modes, alphal(ρ) is the amplitude of the azimuthal mode coefficient, Δ θl(ρ) is the phase of the azimuthal mode coefficient;
the transmission function of the 2# spatial light phase modulator is
Wherein, the delta R is the bandwidth of the light-transmitting ring of the 2# space optical phase modulator, and the R is the radius of the light-transmitting ring of the 2# space optical phase modulator;
after a light beam reflected by the 2# spatial light phase modulator passes through the 4# focusing lens, the complex amplitude at the center position of the Fourier plane meets the following requirements:
where f is the focal length and k is the wavenumber.
The light spot acquisition module detects the light intensity I of the central position of the Fourier planel(R), calculating the amplitude of the angular mode coefficients:
when measuring the phase of the angular mode coefficient, the central unit light beam emits light, and the light field distribution of the central unit light beam is as follows:
the transmission function of the 2# spatial light phase modulator is as follows:
wherein R is0The minimum light-passing annular radius of the 2# space optical phase modulator is obtained, the 2# space optical phase modulator loads the phase to enable the phase gamma of the central unit light beam to evolve from 0 to 2 pi, and the light spot acquisition module detects the light intensity I at the central position of the Fourier planel(R) and recording the phase [ gamma ] of the beam added to the center cell corresponding to the maximum value of the intensity of light and the phase [ delta ] theta ] of the angular mode coefficientlAnd (rho ═ R) ═ gamma, and reconstructing the optical field distribution of the coherent synthesis generated vortex light beam according to the angular mode coefficients.
The invention has the following beneficial effects:
the method can obtain the light intensity distribution of the vortex light beams generated by coherent synthesis according to the detected light intensity signals, and simultaneously calculate the phase distribution of the vortex light beams generated by coherent synthesis, thereby realizing the light field characterization of the vortex light beams generated by coherent synthesis.
The optical field reconstruction method for generating the vortex light beam based on the optical fiber laser coherent synthesis is compatible with the vortex light beam generation system, and the vortex light beam generation and the optical field reconstruction for generating the vortex light beam based on the coherent synthesis can be realized through one set of system.
The method overcomes the difficulty of representing the vortex light beam light field generated by the coherent synthesis of the optical fiber laser in the prior technical scheme, and provides a technical basis for the mode analysis and the mode control of the vortex light beam generated by the coherent synthesis of the optical fiber laser.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the structures shown in the drawings without creative efforts.
FIG. 1 is a schematic structural diagram of the present invention.
Fig. 2 is a schematic arrangement diagram of the emission area array light beams.
Fig. 3 is a schematic diagram of the structure of the light-passing ring band of the spatial optical phase modulator.
FIG. 4 is a graph of the generation of vortex beams with topological charge number-1 and the light field reconstruction result.
FIG. 5 is a graph of the generation of vortex beams with a topological charge number of 2 and the reconstruction result of the optical field.
Detailed Description
In order to make the technical scheme and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
Example 1:
referring to fig. 1, a light field reconstruction system for generating vortex beams by fiber laser coherent synthesis is provided, which includes a vortex beam generation subsystem and a light field reconstruction subsystem.
The vortex light beam generation subsystem comprises a seed source 1, a preamplifier 2, an optical fiber beam splitter 3, a phase modulator array 4, an optical fiber amplification module 5, a collimator array 6, a high reflection mirror 7, a spectroscope 8, an array light beam processing module 9, a photoelectric detection module 10 and a control system 11.
The seed laser output by the seed source 1 is primarily amplified by the preamplifier 2 and then transmitted to the fiber beam splitter 3. The optical fiber beam splitter 3 is provided with N output ends, the seed laser output by the seed source 1 is divided into N beams of unit light beams through the optical fiber beam splitter 3, and the output end of each optical fiber beam splitter 3 is respectively connected with the phase modulator, the optical fiber amplification module 5 and the collimator in sequence; the N phase modulators form a phase modulator array 4. The optical fiber amplification module 5 may be composed of a single optical fiber amplifier, or a cascade optical fiber amplifier link, or implemented by a common aperture coherent synthesis technology, and functions to increase the output power of each laser. The N collimators are arranged in a circular array to form a collimator array 6. The multipath unit beams output by the optical fiber beam splitter 3 are transmitted to a collimator array 6 positioned on an emitting surface after being subjected to phase control of a phase modulator and power expansion of an optical fiber amplification module, and the collimator array 6 splices the multipath unit beams into array beams arranged in a circular array and collimates and outputs the array beams.
The array beam whose emission surface is outputted by the collimator array is divided into two parts by the high reflection mirror 7, wherein most of the power of the array beam reflected from the high reflection mirror 7 is transmitted to a far field in a free space, and a vortex beam is formed in the far field. In addition, the array beam with small power transmitted from the high reflecting mirror 7 is used for closed-loop phase control of the vortex beam generation subsystem and light field information acquisition of the light field reconstruction subsystem.
The array light beam with small power transmitted from the high reflecting mirror 7 is divided into two parts again by the beam splitter 8, wherein the light beam transmitted from the beam splitter 8 is transmitted to the light field reconstruction subsystem, the light beam reflected from the beam splitter 8 is processed by the array light beam processing module 9 and then received by the photoelectric detection module 10, and the photoelectric detection module 10 converts the optical signal of the received light field into an electrical signal and transmits the electrical signal to the control system 11. The control system 11 is loaded with a phase optimization control algorithm, and the control system 11 processes the electric signals transmitted by the photoelectric detection module 10 by operating the phase optimization control algorithm to obtain phase control signals of each path of unit light beam, and transmits the phase control signals to corresponding phase modulators, so that compensation of phase noise of the vortex light beam generation subsystem and piston phase regulation of the array light beam are realized. Wherein: the array beam processing module 9 is composed of a spatial light phase modulator and a focusing lens, or composed of a spatial light phase modulator and a lens group, or composed of a liquid crystal and a focusing lens, or composed of a liquid crystal and a lens group; the array light beam processing module and the photoelectric detection module are used for processing light field information of the emitted area array light beam and collecting light spots containing phase control information after processing.
The light field reconstruction subsystem comprises a 4-F system 12, a 1# space optical phase modulator 13, a 3# focusing lens 14, a 1# objective lens 15, a 2# space optical phase modulator 16, a 4# focusing lens 17, a 2# objective lens 18 and a light spot collection module 19 which are arranged in sequence. The 4-F system 12 is composed of a # 1 focusing lens 12a and a # 2 focusing lens 12 b.
The array beam transmitted from the beam splitter 8 is modulated by the 1# spatial optical phase modulator 13 after passing through the 4-F system, and the 1# spatial optical phase modulator 13 is used for controlling the amplitude and phase of the central unit beam of the array beam. The array beam modulated by the 1# spatial optical phase modulator 13 passes through the 3# focusing lens 14 and the 1# objective lens 15 and is transmitted to the 2# spatial optical phase modulator 16, and the 1# objective lens 15 is used for realizing the amplification of the size of an optical field so as to match the size of the array beam with the size of the 2# spatial optical phase modulator 16. The 2# spatial optical phase modulator 16 loads a complex phase distribution for resolving optical field information and modulates the array light beam. The light beam modulated by the 2# spatial light phase modulator 16 is transmitted to the 4# focusing lens 17 and the 2# objective lens 18, and then collected and processed by the spot collection module 19. The 2# objective lens 18 is used for realizing the amplification of the size of the light field, and the purpose is to match the light field with the size of the light spot collecting module. The light spot acquisition module 19 can be composed of a CCD camera and a computer, captures and acquires light spots in real time, and analyzes and processes light intensity information, so that light field reconstruction of vortex light beams generated by coherent synthesis is realized.
In the vortex beam generation subsystem, the emission surface array beams are arranged as shown in fig. 2, and the emission surface array beams include a central unit beam located at the central position of the array beam, and also include NcircEach circular sub-array takes the center of the central unit light beam as the center of a circle, the unit light beams on each circular sub-array are uniformly distributed along the angular direction, and the beam waist radius of each path of unit light beam and the central unit light beam on each circular sub-array is w0Wavelength of λ, beam diameter of d, and amplitude of A0And the distance between the center of the unit beam and the central unit beam on each circular subarray is rj，j＝1、2、3…Ncirc。
Spatial optical phase modulators are common optical devices in the art. In the present embodiment, both the 1# spatial optical phase modulator and the 2# spatial optical phase modulator use a pure phase type spatial optical modulator to realize amplitude modulation. The phase structure design of the 1# space optical phase modulator and the 2# space optical phase modulator can adopt a checkerboard phase lattice method, the checkerboard phase lattice refers to a phase structure formed by loading liquid crystal lattices with phases of 0 and pi alternately, and zero amplitude modulation is realized through the checkerboard phase lattice. For the 1# space optical phase modulator, when the optical field reconstruction subsystem works, the central unit light beam realizes the switching of loading/unloading checkerboard lattices in the circular area covered by the diffraction light spot on the 1# space optical phase modulator, and the checkerboard lattices are not loaded in the other areas. For the 2# space optical phase modulator, when the optical field reconstruction subsystem works, checkerboard lattices are loaded in the region except the light-passing annular band. The number and the width of light-passing annular zones of the 1# space light phase modulator and the 2# space light phase modulator are flexibly adjusted according to the size of a light spot generating a vortex light beam, the bandwidth of the light-passing annular zones determines the precision of a reconstructed light field, and the narrower the light-passing annular zones, the higher the precision of the reconstructed light field.
Based on the light field reconstruction system for generating the vortex light beam by the fiber laser coherent synthesis, the present embodiment provides a light field reconstruction method for generating the vortex light beam by the fiber laser coherent synthesis, and the specific method has been described in detail in the summary of the invention, which is not described herein again.
Example 2:
the optical field reconstruction system and method for generating vortex beams by coherent synthesis of fiber lasers provided in embodiment 1 are applied, and this embodiment provides a specific application example. First, consider the case of generating and detecting a vortex beam with a topological charge number of-1. Referring to fig. 4, the emitting surface array beam in this embodiment is composed of a circular subarray composed of six circularly arranged unit beams and a central unit beam, when the central unit beam does not emit light, the optical field reconstruction system for generating a vortex beam by fiber laser coherent synthesis works in a state of generating the vortex beam, the light intensity distribution of the emitting surface array beam is shown in fig. 4(a), and the phase distribution is shown in fig. 4(b), where: laser beam waist radius w00.89mm, 2mm beam diameter d, and the distance r between the center of each sub-beam and the origin of the first circular sub-array12.2mm and 1064nm as the laser operating wavelength. Center unit beam waist radius w00.89mm, 2mm beam diameter d and 1064nm laser operating wavelength. The focal lengths of the focusing lens 3 and the focusing lens 4 are both 200mm, the magnification of the objective lens 1 and the magnification of the objective lens 2 are both 10 times, the light field distribution of the array light beam before the array light beam enters the No. 2 blank front light phase modulator, namely the light field distribution of the vortex light beam generated in the far field is obtained through numerical simulation by utilizing an angular spectrum transmission method, the light intensity distribution is shown in a figure 4(c), and the phase distribution is shown in a figure 4 (d). The 1# space optical phase modulator 13 and the 2# space optical phase modulator 16 load phase distribution to detect the optical field generating vortex light beams, and the radius R of the minimum light-passing annular zone00.125mm, 25 μm for the clear ring bandwidth Δ R, 28 clear ring bands, and angular modes l of-7, -1, and 5. According to the light intensity of the central position of the Fourier plane detected by the light spot acquisition module, the light intensity distribution of the generated vortex light beam can be reconstructed, as shown in the attached drawing 4(e), the phase distribution of the generated vortex light beam is reconstructed, as shown in the attached drawing 4(f), the result shows that the reconstructed light field distribution and the generated vortex light beam light field distribution have higher similarity, the system has the function of reconstructing the generated vortex light beam light field distribution, and the precision is higher.
Consider further the case of generating and detecting a vortex beam with a topological charge number of 2. The emitting surface array light beam is composed of two circular ring sub-arrays and a central unit light beam, wherein the inner circular ring sub-array (inner ring) comprises twelve paths of sub-light beams, the outer circular ring sub-array (outer ring) comprises eighteen paths of sub-light beams, when the light beam at the central position does not emit light, the system works in a state of generating vortex light beams, the light intensity distribution of the emitting surface array light beam is shown in a figure 5(a), the phase distribution is shown in a figure 5(b), and the emitting surface array light beam is characterized in that: laser beam waist radius w00.89mm, 2mm beam diameter d, and distance r between center of each unit beam and origin of the first circular sub-array12.2mm, the distance r between the center of each unit beam and the origin of the second circular ring sub-array24.4mm, and the laser working wavelength lambda is 1064 nm. Central unit beam laserBeam waist radius w00.89mm, 2mm beam diameter d and 1064nm laser operating wavelength. By using an angular spectrum transmission method, the light field distribution of the array light beam before entering the 2# space front light phase modulator 16, namely the light field distribution of the vortex light beam generated by the far field is obtained through numerical simulation, the light intensity distribution is shown in fig. 5(c), and the phase distribution is shown in fig. 5 (d). The 1# space optical phase modulator 13 and the 2# space optical phase modulator 16 load phase distribution to detect the optical field generating vortex light beams, and the radius R of the minimum light-passing annular zone00.05mm, clear ring bandwidth Δ R25 μm, clear ring number 30, angular mode l-16, -10, 2, 14 and 20. According to the light intensity of the central position of the Fourier plane detected by the light spot acquisition module, the light intensity distribution of the generated vortex light beam can be reconstructed, as shown in the attached figure 5(e), and the phase distribution of the generated vortex light beam is reconstructed, as shown in the attached figure 5 (f). The result shows that for the condition that the emission surface array light beam is composed of more unit light beams and generates a vortex light beam with higher-order topological charge number, the reconstructed light field distribution still has higher precision, and the complexity of the light field reconstruction subsystem cannot be influenced by the increase of the emission surface unit light beam and the increase of the topological charge number of the generated vortex light beam.
In summary, although the present invention has been described with reference to the preferred embodiments, it should be understood that various changes and modifications can be made by those skilled in the art without departing from the spirit and scope of the invention.
Claims (9)
1. The optical field reconstruction system for generating vortex beams by coherent synthesis of fiber lasers is characterized in that: the system comprises a vortex light beam generation subsystem and a light field reconstruction subsystem; the vortex light beam generating subsystem comprises a seed source, an optical fiber beam splitter, a phase modulator array, an optical fiber amplifying module and a collimator array;
the optical fiber beam splitter is provided with N output ends, the seed laser output by the seed source is divided into N beams of unit light beams by the optical fiber beam splitter, and the output ends of the optical fiber beam splitters are respectively and sequentially connected with the phase modulator, the optical fiber amplification module and the collimator; n phase modulators form a phase modulator array; the N collimators are arranged in a circular array to form a collimator array; the multi-path unit light beams output by the beam splitter are transmitted to a collimator array positioned on an emitting surface after being subjected to phase control of a phase modulator and power expansion of an optical fiber amplification module, and the collimator array splices the multi-path unit light beams into array light beams distributed in a circular array and collimates and outputs the array light beams;
the vortex light beam generating subsystem generates vortex light beams based on optical fiber laser coherent synthesis, collects a small part of array light beams output by the vortex light beam generating subsystem and inputs the small part of array light beams into the light field reconstruction subsystem, and the light field reconstruction subsystem comprises a 4-F system, a 1# space light phase modulator, a 3# focusing lens, a 1# objective lens, a 2# space light phase modulator, a 4# focusing lens, a 2# objective lens and a light spot collecting module which are sequentially arranged; the array light beam is transmitted to a 1# space optical phase modulator for modulation after passing through a 4-F system, and the 1# space optical phase modulator controls the amplitude and the phase of a central unit light beam of the array light beam; the array light beam modulated by the 1# space optical phase modulator is transmitted to the 2# space optical phase modulator through a 3# focusing lens and a 1# objective lens, and the 1# objective lens enables the array light beam to be matched with the 2# space optical phase modulator in size; the 2# space optical phase modulator is loaded with phase distribution used for resolving light field information and modulates the array light beam; the light beam modulated by the 2# space light phase modulator passes through the 4# focusing lens and the 2# objective lens and then is collected and processed by the light spot collection module, and the 2# objective lens enables the light beam transmitted to the light spot collection module to be matched with the light spot collection module in size; the light spot acquisition module captures and acquires light spots in real time and analyzes and processes light intensity information, so that light field reconstruction of vortex light beams generated by coherent synthesis is realized.
2. The optical field reconstruction system for generating vortex beams by coherent combination of fiber lasers according to claim 1, characterized in that: the emission area array light beam output by the vortex light beam generation subsystem comprises a central unit light beam positioned at the central position of the array light beam and NcircCircular ring sub-arrays with the center of the central unit beam as the center of circle, and unit beam angle on each circular ring sub-arrayUniformly arranged, the beam waist radius of each unit beam and the central unit beam on each circular sub-array is w0Wavelength of λ, beam diameter of d, and amplitude of A0And the distance between the center of the unit beam and the central unit beam on each circular subarray is rj，j＝1、2、3…Ncirc。
3. The optical field reconstruction system for generating vortex beams by coherent combination of fiber lasers according to claim 2, characterized in that: the 4-F system consists of a 1# focusing lens and a 2# focusing lens.
4. The optical field reconstruction system for generating vortex beams by coherent synthesis of fiber lasers according to any one of claims 1 to 3, characterized in that: the vortex light beam generation subsystem further comprises a high reflecting mirror, a spectroscope, an array light beam processing module, a photoelectric detection module and a control system;
the array light beam of the emission surface, which is output by the collimator array, is divided into two parts by the high reflecting mirror, wherein most of the array light beam with power reflected by the high reflecting mirror is transmitted to a far field in a free space, and a vortex light beam is formed in the far field; in addition, the array beam with small power transmitted from the high reflector is used for closed-loop phase control of the vortex beam generation subsystem and light field information acquisition of the light field reconstruction subsystem;
the array light beam with small power transmitted from the high reflector is divided into two parts again by the spectroscope, wherein the light beam transmitted from the spectroscope is transmitted to the light field reconstruction subsystem, the light beam reflected from the spectroscope is received by the photoelectric detection module after being processed by the array light beam processing module, and the photoelectric detection module converts the optical signal of the received light field into an electric signal and transmits the electric signal to the control system; the control system is loaded with a phase optimization control algorithm, processes the electric signals transmitted by the photoelectric detection module by operating the phase optimization control algorithm, acquires phase control signals of each path of unit light beam, transmits the phase control signals to the corresponding phase modulator, and realizes compensation of the phase noise of the vortex light beam generation subsystem and piston phase regulation of the array light beam.
5. The optical field reconstruction system for generating vortex beams by coherent combination of fiber lasers according to claim 4, wherein: the array beam processing module consists of a spatial light phase modulator and a focusing lens, or consists of a spatial light phase modulator and a lens group, or consists of liquid crystal and a focusing lens, or consists of liquid crystal and a lens group; the array light beam processing module and the photoelectric detection module are used for processing light field information of the emitted area array light beam and collecting light spots containing phase control information after processing.
6. The optical field reconstruction system for generating vortex beams by coherent combination of fiber lasers according to claim 4, wherein: the seed laser output by the seed source is uniformly divided into N paths of unit beams through the optical fiber beam splitter after being preliminarily amplified by the preamplifier.
7. The optical field reconstruction system for generating vortex beams by coherent combination of fiber lasers according to claim 4, wherein: the optical fiber amplification module is composed of a single optical fiber amplifier, or is composed of a cascade optical fiber amplifier link, or is realized by a common aperture coherent synthesis technology.
8. The optical field reconstruction system for generating vortex beams by coherent synthesis of fiber lasers according to claim 5, 6 or 7, characterized in that: the phase optimization control algorithm is a random parallel gradient descent algorithm, a simulated annealing algorithm or a particle swarm optimization algorithm.
9. The optical field reconstruction method for generating vortex beams by fiber laser coherent synthesis is characterized in that the optical field reconstruction system for generating the vortex beams by the fiber laser coherent synthesis according to claim 1 is adopted, wherein the emission surface array beams output by the vortex beam generation subsystem comprise a central unit beam positioned at the central position of the array beams and NcircA circular ring subarray with the center of the central unit beam as the center of circle, each subarrayThe unit beams on the circular ring sub-arrays are uniformly distributed along the angular direction, and the beam waist radius of each unit beam and the central unit beam on each circular ring sub-array is w0Wavelength of λ, beam diameter of d, and amplitude of A0And the distance between the center of the unit beam and the central unit beam on each circular subarray is rj，j＝1、2、3…Ncirc；
The optical field distribution of the emission area array light beam output by the vortex light beam generation subsystem is as follows:
wherein, (x, y) is the coordinates of the emitting surface, NjFor the number of unit beams contained in the jth circular sub-array, (x)j,h,yj,h) And phij,hThe central coordinate and the piston phase of the h unit beam on the j circular sub-array are obtained; u shapec(x, y) is the complex amplitude of the central unit beam, when the vortex beam generation subsystem works in the state of generating vortex beams, the central unit beam does not emit light, and U is the complex amplitude of the central unit beamc(x,y)＝0；
The central coordinate parameter of the h unit beam on the jth circular sub-array meets the following conditions:
wherein r isjThe distance between the beam center of the jth circular sub-array unit and the emitting area array center is set;
for generating vortex light beams with topological charge number l, the piston phase parameters of the light beams of the emission surface unit satisfy the following conditions:
in the light field reconstruction subsystem, the array light beam is transmitted to the light field of the spatial light phase modulator through the 4-F systemThe distribution is consistent with the light field distribution of the light beam of the emitting area array, after passing through the 1# space light phase modulator, the light field distribution before being transmitted to the space light phase modulator through the 3# focusing lens and the 1# objective lens is the far field light field distribution of the array light beam
Wherein
Is the plane polar coordinate of the 2# space optical phase modulator; far field light field distribution of array beam
Decomposition into multiple azimuthal modes is as follows:
wherein, cl(ρ)＝αl(ρ)exp[iΔθl(ρ)]Being coefficients of angular modes, alphal(ρ) is the amplitude of the azimuthal mode coefficient, Δ θl(ρ) is the phase of the azimuthal mode coefficient;
the transmission function of the 2# spatial light phase modulator is
Wherein, the delta R is the bandwidth of a light-transmitting ring of the 2# space optical phase modulator, and the R is the radius of the light-transmitting ring of the 2# space optical phase modulator;
after a light beam reflected by the 2# spatial light phase modulator passes through the 4# focusing lens, the complex amplitude at the center position of the Fourier plane meets the following requirements:
wherein f is the focal length and k is the wavenumber;
the light spot acquisition module detects the light intensity I of the central position of the Fourier planel(R), calculating the amplitude of the angular mode coefficients:
when measuring the phase of the angular mode coefficient, the central unit light beam emits light, and the light field distribution of the central unit light beam is as follows:
the transmission function of the 2# spatial light phase modulator is as follows:
wherein R is0The minimum light-passing annular radius of the 2# space optical phase modulator is obtained, the 2# space optical phase modulator loads the phase to enable the phase gamma of the central unit light beam to evolve from 0 to 2 pi, and the light spot acquisition module detects the light intensity I at the central position of the Fourier planel(R) and recording the phase [ gamma ] of the beam added to the center cell corresponding to the maximum value of the intensity of light and the phase [ delta ] theta ] of the angular mode coefficientlAnd (rho ═ R) ═ gamma, and reconstructing the optical field distribution of the coherent synthesis generated vortex light beam according to the angular mode coefficients.
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