CN114485967B - Method and device for measuring vortex beam topological charge under extremely low coherence condition - Google Patents
Method and device for measuring vortex beam topological charge under extremely low coherence condition Download PDFInfo
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Abstract
The invention provides a method and a device for measuring vortex beam topological charge under the condition of extremely low coherence, wherein the device comprises the following steps: a Gaussian Shell mode beam generating unit for generating a Gaussian Shell mode beam; the spatial light modulator is used for loading independent controllable cross phases on Gaussian Schhell mode light beams to generate a to-be-detected partially coherent vortex light beam; the charge coupling device is used for collecting a plurality of groups of light intensity patterns of the to-be-detected partially coherent vortex light beams which are coupled through the cross phases; the processing unit is used for reading the light intensity graph, obtaining a coherence degree function mode distribution graph through calculation, and obtaining the size and the sign of the topological load according to the number of the dark rings and the arrangement direction of the dark rings which are separated from the coherence degree function mode distribution graph. The invention overcomes the defect that the topology charge cannot be measured under the low coherence condition in the existing measuring method, and can realize the synchronous detection of the size and the symbol of the topology charge; the limitation that the existing measuring method can only measure the topological charge at the near focal plane is broken, and the flexibility of the topological charge measurement is improved.
Description
Technical Field
The invention belongs to the technical field of optical communication, and particularly relates to a method and a device for measuring vortex beam topological charge under the condition of extremely low coherence.
Background
The statements in this section merely provide background information related to the present disclosure and may not necessarily constitute prior art.
With the advent of the information age, people have been increasingly demanding in terms of information processing, mainly in terms of both information processing speed and communication capacity. Compared with the traditional communication means (beacon signaling, pigeon book transmission, radio waves and the like), the optical communication is an important means of modern communication, has the characteristics of large communication capacity, high transmission speed, low transmission loss, strong anti-interference capability and the like, and is therefore receiving great attention.
Vortex is a type of swirl around the periphery of a singular point, which is widely found in nature. For example: eddies in water eddies, tornados in atmospheric eddies, and spiral stars in the lunar system, etc. Also, it exists in the optical field and has evolved into an important branch-singularity optics of optics. Vortex beams and applications thereof are one of the hot spots of research at home and abroad in recent years. A common vortex beam is a ragel gaussian beam. The vortex beam carries physical characteristics such as phase singular points, orbital angular momentum and the like because of a unique spiral wave front structure, so that the vortex beam has important application in the aspects of free space optical communication, optical micromanipulation, super-resolution imaging and the like. Since most of the above applications are based on known topology charges, measurement of topology charges is particularly important.
However, conventional methods of measuring topological charge, such as: interferometry, diffraction, fourier transform, etc. are only applicable to perfectly coherent beams, and fail when coherence is low. In practical application, the laser loses energy due to phenomena such as scattering, refraction and absorption, and is influenced by atmospheric turbulence disturbance effects such as beam expansion, drift and flickering, so that the coherence of the laser is seriously reduced and the laser becomes partially coherent light. For partial coherent light, the main flow topology load measurement method is a correlation function method, but the method can only effectively measure the topology load when the coherence is relatively high and can also fail when the coherence is smaller than the beam waist radius for the current experimental detection. There are two main limitations of this approach: only the size of the topological charge can be measured, and the sign of the topological charge cannot be measured; only at the near focal plane, which also limits the flexibility of topological charge measurement.
Disclosure of Invention
In order to overcome the defects in the prior art, the invention provides a method and a device for measuring vortex beam topological charge under the condition of extremely low coherence, and has important application in the aspect of free space optical communication.
To achieve the above object, one or more embodiments of the present invention provide the following technical solutions:
in a first aspect, an apparatus for measuring vortex beam topology charges under very low coherence conditions is disclosed, comprising:
a Gaussian Shell mode beam generating unit for generating a Gaussian Shell mode beam;
the spatial light modulator is used for loading Laguerre amplitude, vortex phase and cross phase on the Gaussian Shell mode light beam to generate a to-be-detected partially coherent vortex light beam coupled with the cross phase;
the charge coupling device is used for collecting a plurality of groups of light intensity patterns of the to-be-detected partially coherent vortex light beams which are coupled through the cross phases;
the processing unit is used for reading the light intensity graph, obtaining a coherence degree function mode distribution graph through calculation, and obtaining the size and the sign of the topological load according to the number of the dark rings and the arrangement direction of the dark rings which are separated from the coherence degree function mode distribution graph.
According to a further technical scheme, the Gaussian Schhell mode light beam generating unit comprises a helium-neon laser, a linear polaroid, a beam expander, a first lens, rotary ground glass, a second lens and a Gaussian filter which are sequentially arranged;
the helium-neon laser generates Gaussian beam, selects light with transverse polarization through a linear polarizer, expands the light through a beam expander, focuses the light through a first lens, strikes the rotating frosted glass to generate incoherent light, collimates the incoherent light through a second lens, and generates Gaussian-Shell mode beam through a Gaussian filter.
According to a further technical scheme, the spatial light modulator loads Laguerre amplitude, vortex phase and cross phase on Gaussian-Shell mode light beams.
Further technical scheme still includes: 4f system and diaphragm;
the Gaussian Schlemm mode beam is loaded with Laguerre amplitude, vortex phase and cross phase by a spatial light modulator, then passes through a 4f system consisting of a third lens and a fourth lens, and then a first-order diffraction light spot is selected by a diaphragm.
Further technical scheme still includes: and the fifth lens is used for focusing and transmitting the first-order diffraction light spots, and the light spots are shot by a charge coupled device at the receiving surface.
According to a further technical scheme, the CCD shoots light spots on the receiving surface, and shooting speed and the number of shot pictures are controlled through a computer.
According to a further technical scheme, incoherent light generated in the Gaussian Schlemm light beam generation unit is changed into partially coherent light through a certain transmission distance, and the degree of coherence is adjusted by controlling the distance between the first lens and the rotating frosted glass, the roughness degree of the rotating frosted glass and the focal length of the second lens.
In a second aspect, a method of measuring vortex beam topology charge under low coherence conditions is disclosed, comprising:
generating a Gaussian Shell mode beam;
loading Laguerre amplitude, vortex phase and cross phase on Gaussian Shell mode light beam to generate partially coherent vortex light beam to be detected coupled with the cross phase;
collecting a plurality of groups of light intensity patterns of the partially coherent vortex light beams to be detected, which are coupled by the cross phases;
and reading the light intensity graph, obtaining a coherence degree function mode distribution graph through calculation, and obtaining the size and the sign of the topological load according to the number of the dark rings and the arrangement direction of the dark rings which are separated from the coherence degree function mode distribution graph.
According to a further technical scheme, after the light intensity graph is read, the light intensity graph is subjected to superposition processing, and a mode distribution diagram of the coherence function is obtained according to a relation between the partial coherent light beam fourth-order correlation function and the coherence function.
Further technical proposal, the Gaussian Shell mode beam cross spectral density expression at the light source surface is:
wherein W is 1 (r 1 ,r 2 ) Representation of Gaussian Shell mode beam cross spectral density expression at light source face, r i Is a position vector at the light source face of the gaussian scher-mode beam, i=1, 2; ω and δ are beam waist width and coherence width, respectively.
The one or more of the above technical solutions have the following beneficial effects:
the invention couples the cross phase with the partial coherent Laguerre Gaussian beam, and discovers that the cross phase can separate concentric dark rings of the partial coherent Laguerre Gaussian beam, and can more clearly identify the topological charge through the separated dark rings, wherein the sign (positive and negative) of the topological charge can be judged according to the arrangement direction of the dark rings, and the number of the dark rings is equal to the size of the topological charge. And the effective measurement of the size and the sign of the topological load carried by the light beam to be measured in the transmission process can be realized by regulating and controlling the cross phase, so that the flexibility of the topological load measurement is greatly improved. The invention has important application in free space optical communications.
The method aims at solving the technical problem that the topology cannot be effectively measured because the coherence of the light beam is reduced due to environmental influence in the field of optical communication at present. The invention overcomes the defect that the topology charge cannot be measured under the low coherence condition in the existing measuring method, and can realize the synchronous detection of the size and the symbol of the topology charge. The invention breaks through the limitation that the existing measuring method can only measure the topological charge at the near focal plane, and improves the flexibility of the topological charge measurement.
Additional aspects of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
Drawings
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the invention.
FIG. 1 is a diagram of a method and apparatus for measuring vortex beam topology charge under very low coherence conditions in accordance with the present invention;
fig. 2 (a), a theoretical result diagram of the mode distribution of the coherence function of the target beam to be measured (i= ±3) at the focal plane without cross phase coupling;
fig. 2 (b), a theoretical result diagram of the coherence function mode distribution of the target beam (l= ±3) to be measured at the focal plane after the cross phase coupling;
fig. 2 (c), which is a diagram of experimental results of coherence function mode distribution of the target beam (l= ±3) to be measured at the focal plane after the cross phase coupling;
fig. 3 (a), a theoretical result diagram of the mode distribution of the coherence function of the target beam to be measured (i= ±3) without cross phase coupling at different transmission distances;
fig. 3 (b), a theoretical result diagram of the mode distribution of the coherence function of the target beam (i= ±3) to be measured at different transmission distances after the cross phase coupling;
fig. 3 (c) is a diagram of experimental results of mode distribution of coherence functions of the object beam (l= ±3) to be measured at different transmission distances after cross phase coupling.
The reference numerals in the figures illustrate: 1. 2 parts of helium-neon laser, 2 parts of linear polaroid, 3 parts of beam expander, 4 parts of first lens, 5 parts of rotary ground glass, 6 parts of second lens, 7 parts of Gao Silv wave plate, 8 parts of spatial light modulator, 9 parts of first computer, 10 parts of third lens, 11 parts of diaphragm, 12 parts of fourth lens, 13 parts of fifth lens, 14 parts of charge coupled device, 15 parts of second computer.
Detailed Description
It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the invention. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.
It is noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of exemplary embodiments according to the present invention.
Embodiments of the invention and features of the embodiments may be combined with each other without conflict.
Example 1
The embodiment discloses a device for measuring vortex beam topological charge under the condition of low coherence, which comprises a helium-neon laser 1, a linear polaroid 2, a beam expander 3, a first lens 4, a second lens 6, a third lens 10, a fourth lens 12, a fifth lens 13, a rotary ground glass 5, a Gaussian filter 7, a spatial light modulator 8, a diaphragm 11, a charge coupled device 14, a first computer 9 and a second computer 15.
The helium-neon laser 1 generates Gaussian beams, selects light with transverse polarization through the linear polaroid 2, expands the light through the beam expander 3, focuses the light through the first lens 4, and strikes the light on the rotary frosted glass 5 to generate incoherent light; the light is collimated by a second lens 6 and passed through a gaussian filter 7 to produce a conventional gaussian schel-mode beam (partially coherent light).
Loading Laguerre amplitude, vortex phase and cross phase through a spatial light modulator 8, and selecting a first-order diffraction light spot through a diaphragm 11 after passing through a 4f system consisting of a third lens 10 and a fourth lens 12 to obtain a required light beam model; the spatial light modulator 8 is controlled by a first computer 9.
The focal transmission is carried out through the fifth lens 13, the focal spot is shot by a charge coupler 14 at the receiving surface, and the shooting process is controlled by the second computer 15.
The light intensity map of the CCD is led into Matlab, the coherence degree function mode distribution map is obtained through calculation, and the size and the sign of the topological charge can be obtained according to the number and the direction of dark rings separated from the mode distribution map.
A beam waist radius equal to 3 times is generally default to be considered as a completely coherent beam (the light emitted by the laser is the completely coherent beam); the beam waist radius with the coherence width smaller than 1 time is regarded as a low coherence beam, and the beam waist radius with the coherence width equal to 0.3 time in the experiment can be theoretically 0.1 time, so that the beam waist radius is under the condition of extremely low coherence. Extremely low in this application means that the coherence width can be as low as 0.3 times the beam waist radius.
Example two
Referring to fig. 1, it is an object of the present embodiment to provide a method of measuring a vortex beam topology charge under very low coherence conditions, the method comprising: loading an independent controllable cross phase into the to-be-detected partially-coherent vortex beam by means of a spatial light modulator, and specifically controlling the to-be-detected partially-coherent vortex beam by a first computer to generate the to-be-detected partially-coherent vortex beam coupled with the cross phase; then shooting a plurality of groups of light intensity patterns of the target light beams to be detected which are coupled by the cross phases by using the charge-coupled devices, and controlling the charge-coupled devices by using a second computer so as to further control the shooting process; and finally, reading a light intensity graph by Matlab, and calculating the coherence function mode distribution of the to-be-detected partially coherent vortex light beams coupled with the cross phases according to the Gaussian momentum theorem by the recorded light intensity graph, wherein the number of dark rings separated from the coherence function mode distribution corresponds to the size of the topological charges, and the sign of the topological charges can be identified according to the arrangement direction of the dark rings.
The method is mainly used for measuring the topological charge of the vortex beam under the condition of low coherence and in the transmission process, and has the advantages of overcoming the defect that the traditional correlation function method cannot identify the topological charge due to the fact that the coherence of the beam is reduced due to environmental influence in practical application, breaking the limitation that the traditional measuring method can only measure the topological charge at the near-focal plane, and improving the flexibility of the topological charge measurement.
The method comprises the following steps:
step 1: generating Gaussian Shell mode beams
The helium-neon laser generates laser, light with transverse polarization is selected through a linear polarizer, the light is expanded through a beam expander, the purpose of the beam expansion is to enlarge light spots, the light is focused on the rotary frosted glass through a first lens and scattered, the light coming out of the surface of the rotary frosted glass can be regarded as completely incoherent light, the incoherent light is transmitted for a certain distance to become partially coherent light according to Van Citert-Zernike theorem, the partially coherent light is collimated through a second lens, and then the light beams are obtained through a Gaussian filter, and the expression is that:
wherein W is 1 (r 1 ,r 2 ) Representation of Gaussian Shell mode beam cross spectral density expression at light source face, r i (i=1, 2) is a position vector at the gaussian schorl mode beam source face; ω and δ are beam waist width and coherence width, respectively.
A fully coherent light beam (e.g., light from a laser), characterized by an amplitude; whereas a partially coherent beam is characterized by a cross spectral density, which is only a representation of the partially coherent beam, "cross spectral density" is a proper term. The Gaussian-mode beam is an intermediate process for obtaining a beam model, and the Laguerre amplitude, the vortex phase and the cross phase are added on the basis of the Gaussian-mode beam to obtain the required beam model.
In this step, the first lens focuses the light onto the rotating frosted glass, breaks up the completely coherent light beam generated by the laser, and emits an incoherent light beam, which becomes partially coherent light through a certain transmission distance according to Van Citert-Zernike theorem, and the degree of coherence can be adjusted by controlling the distance between the first lens and the rotating frosted glass, the roughness of the rotating frosted glass, and the focal length of the second lens.
Distance between the first lens and the rotary frosted glass: the size of the light spot which is hit on the frosted glass can be changed by adjusting the distance between the lens and the frosted glass, and the larger the light spot is, the more scattered the light spot is, and the lower the coherence is.
Step 2: loading amplitude and phase terms by a spatial light modulator
The resulting gaussian-mode beam is directed onto the spatial light modulator that loads the hologram, by complex amplitude modulation, with the lager amplitude, vortex phase, and cross phase being loaded on the hologram, with different magnitudes of cross phase factors depending on the level of coherence, this example taking: ω=1 mm, l= ±3, δ=0.3ω, δ=0.5ω, δ=1ω, δ=3ω, when the corresponding cross-phase factor takes u=140 mm -2 ,u＝50mm -2 ,u＝12mm -2 ,u＝4mm -2 . According to the holographic principle, the light reflected from the spatial light modulator is a partially coherent Laguerre Gaussian beam coupled across the phase. Then, a 4f system is used for screening out first-order diffraction light spots by using a diaphragm to obtain a target light beam, wherein the expression is as follows:
wherein W is 2 (r 1 ,r 2 ) Representation of cross spectral density at the source face of a partially coherent Laguerre Gaussian beam coupled with cross phase, r i (i=1, 2) andradial coordinates and angular coordinates at the light source face of the partially coherent lager gaussian beam; />Represents a Laguerre polynomial; l is the topology charge; exp (iux) i y i ) (i=1, 2) is a cross-phase structure, u is a cross-phase factor; the 4f system consists of a third lens and a fourth lens.
The beam is modulated (amplitude, phase) by a hologram to obtain the desired beam. Holograms are computer generated and can be flexibly controlled as desired. Also, since the beam coming out of the laser is gaussian, the present invention requires loading information (amplitude, phase) on the hologram to obtain the desired beam.
The Gaussian-mode beam of the invention, a partially coherent Gaussian beam, does not contain Laguerre amplitude, vortex phase and cross phase. The final beam model is a partially coherent Laguerre beam coupled in cross phase (Laguerre beam itself contains Laguerre amplitude, vortex phase).
The beam pattern is the desired light source plane beam pattern, which is typically a partially coherent vortex beam coupled across the phase, and is subsequently transmitted.
Step 3: focusing and transmitting light beam through lens
Referring to fig. 1, focusing transmission is performed through a fifth lens, and the expression is:
wherein W (ρ) 1 ,ρ 2 ) Representing the coupling-over cross-phaseCross spectral density expression, ρ, of partially coherent Laguerre Gaussian beam after focused transmission i (i=1, 2) is the position vector of the observation plane, a=1-z/f, b=z, c= -1/f, d=1, is the matrix element of the transmission system, where the matrix represents the transmission after lens to z, k=2pi/λ is the wave number, λ=633 mm is the wavelength, and focal length f=400 mm.
Step 4: CCD shooting light intensity map
Referring to fig. 1, the light beam is transmitted through a lens system, and then the light spot on the observation surface is shot by a charge coupled device, and the shooting speed and the number of shot pictures are controlled by a computer.
Step 5: reading the light intensity pattern
The light intensity map of the CCD is led into Matlab, and the relation between the fourth-order correlation function and the coherence function of the partial coherent light beam is as follows:
wherein,
wherein: i (ρ) represents the instantaneous light intensity at ρ points; < g > represents an ensemble averaging operation.
The light intensity map of the CCD is led into Matlab, the coherence degree function mode distribution map is obtained by calculation, the size and the sign of the topological charge can be obtained according to the number and the arrangement direction of dark rings separated from the mode distribution map, the method comprises the following steps:
firstly, importing a shot light intensity graph into Matlab, and performing superposition processing on the light intensity graph; then, through Gaussian momentum theorem, the mode distribution |u (rho) of the coherence function is obtained according to the relation between the partial coherent light beam fourth-order correlation function and the coherence function 1 ,ρ 2 ) The I, ρ represents the position vector of the observation plane, where ρ 1 ＝-ρ 2 Representing the relationship of the two position vectors; size of final topology payloadAnd the number of the dark rings separated from the module distribution of the coherence degree function is equal, and the sign of the topological load is judged according to the arrangement direction of the dark rings. The size of the topology payload is equal to the number of dark rings, for example: 3 dark rings, the topology load size is equal to 3; the dark rings are longitudinally arranged, and the topological charge symbol is positive; the dark rings are arranged transversely, and the topological charge sign is negative.
Effect verification see the following detailed analytical description:
referring to fig. 2 (a), with the conventional correlation function method (i.e., the case without cross phase), the size of the topological charge (|l|=3) can only be identified by the number of concentric dark rings (3) presented in the coherence degree function mode distribution, and when under other coherence conditions (such as δ=0.3ω and δ=3ω), the identifiable number of concentric dark rings does not correspond to the number of topological charges, or even the number of concentric dark rings cannot be identified, i.e., the method fails under the coherence, and the method cannot identify the sign of the topological charge (i.e., the positive and negative of l) regardless of the coherence.
Referring to fig. 2 (b), after the cross phases are coupled to the target beam to be detected, it is found that the original concentric rings are separated no matter the coherence is high or low, the number of the separated dark rings is clearly distinguishable (3), and is just equal to the number of topological charges (|l|=3), so that the partially coherent vortex beam to be detected can effectively detect the topological charges by means of the mode distribution of the coherence function of the partially coherent vortex beam to be detected after the cross phases are coupled, the defect that the traditional correlation function method cannot measure the topological charges under the condition of low coherence is overcome, and the method can identify the sign of the topological charges according to the arrangement direction of the split dark rings (namely, the vertical direction is positive and the horizontal direction is negative).
Referring to fig. 2 (c), which is an experimental result graph of the mode distribution of the coherence function of the target beam (l= ±3) to be detected at the focal plane after the cross phase coupling effect, we find that the number of the separated dark rings is 3 no matter the coherence is high or low, and the number of the separated dark rings corresponds to the size of the topological load, and the positive and negative of the topological load can be distinguished through the arrangement direction of the dark rings in the mode distribution, in a word, the experimental result is identical with the theoretical result of fig. 2 (b), and the feasibility and the accuracy of the method are also proved;
referring to fig. 3 (a), the coherence δ=1ω, the cross phase is not coupled, we find that only the number (3) of concentric dark rings exhibited by the mode distribution of the coherence function of the target beam to be measured at the near focal plane can be effectively identified, the further the detection plane is from the focal plane, the lower the identifiable degree of the number of dark rings, especially at the far focal plane (such as z=0.5f), and the sign of the topological load cannot be identified in the whole transmission process;
referring to fig. 3 (b), coherence δ=1ω, and a theoretical result diagram of coherence function mode distribution of a target light beam (l= ±3) to be measured after cross phase coupling at different transmission distances, compared with a case without cross phase coupling (fig. 3 (a)), we find that due to coupling effect of cross phase and vortex phase, we can effectively identify topology charges at different transmission distances, i.e. the magnitude of topology charges is detected by separating the number of dark rings, and the sign of topology charges is detected by the arrangement direction of the dark rings, which greatly improves flexibility of topology charge measurement, so that the detection distance is not limited to near focal plane any more;
referring to fig. 3 (c), the coherence δ=1ω, the experimental result graph of the coherence function mode distribution of the object beam (l= ±3) to be measured at different transmission distances after the cross phase coupling effect can be easily identified that the number of the separated dark rings is 3 by adjusting and controlling different cross phases at different transmission distances, and the positive and negative of the topology load can be resolved by the arrangement direction of the dark rings in the mode distribution, so that the experimental result is identical with the theoretical result.
While the foregoing description of the embodiments of the present invention has been presented in conjunction with the drawings, it should be understood that it is not intended to limit the scope of the invention, but rather, it is intended to cover all modifications or variations within the scope of the invention as defined by the claims of the present invention.
Claims (9)
1. An apparatus for measuring the topological charge of a vortex beam under extremely low coherence conditions, comprising:
a Gaussian Shell mode beam generating unit for generating a Gaussian Shell mode beam;
the spatial light modulator is used for loading Laguerre amplitude, vortex phase and cross phase on the Gaussian Shell mode light beam to generate a to-be-detected partially coherent vortex light beam coupled with the cross phase;
the charge coupling device is used for collecting a plurality of groups of light intensity patterns of the to-be-detected partially coherent vortex light beams which are coupled through the cross phases;
the processing unit is used for reading the light intensity graph, obtaining a coherence degree function mode distribution graph through calculation, and obtaining the size and the sign of the topological load according to the number of the dark rings and the arrangement direction of the dark rings separated from the coherence degree function mode distribution graph;
the extremely low coherence width can be 0.3 times of the beam waist radius at the lowest;
generating a gaussian schorl mode beam of very low coherence by controlling the distance between the first lens and the rotating frosted glass, the roughness of the rotating frosted glass, and the focal length of the second lens;
the generated Gaussian Shell mode beam irradiates a spatial light modulator loaded with a hologram, and Laguerre amplitude, vortex phase and cross phase are loaded on the hologram through complex amplitude modulation, and cross phase factors with different sizes are loaded according to the coherence.
2. The apparatus for measuring vortex beam topology charges under very low coherence conditions of claim 1, wherein said gaussian schhelter mode beam generating unit comprises a helium neon laser, a linear polarizer, a beam expander, a first lens, a rotating ground glass, a second lens, and a gaussian filter in sequence;
the helium-neon laser generates Gaussian beam, selects light with transverse polarization through a linear polarizer, expands the light through a beam expander, focuses the light through a first lens, strikes the rotating frosted glass to generate incoherent light, collimates the incoherent light through a second lens, and generates Gaussian-Shell mode beam through a Gaussian filter.
3. The apparatus for measuring vortex beam topology charges under very low coherence conditions of claim 1, further comprising: 4f system and diaphragm;
the Gaussian Schlemm mode beam is loaded with Laguerre amplitude, vortex phase and cross phase by a spatial light modulator, then passes through a 4f system consisting of a third lens and a fourth lens, and then a first-order diffraction light spot is selected by a diaphragm.
4. The apparatus for measuring the topological charge of a vortex beam under very low coherence conditions of claim 3, further comprising: and the fifth lens is used for focusing and transmitting the first-order diffraction light spots, and the light spots are shot by a charge coupled device at the receiving surface.
5. The apparatus for measuring the topological charge of a vortex beam under the condition of extremely low coherence according to claim 4, wherein the charge coupled device shoots a light spot on a receiving surface, and the shooting speed and the shooting picture number are controlled by a computer.
6. The apparatus for measuring a vortex beam topology charge under an extremely low coherence condition as recited in claim 2, wherein incoherent light generated in the gaussian schorl mode beam generating unit is changed into partially coherent light through a certain transmission distance, and the degree of coherence is adjusted by controlling a distance between the first lens and the rotating frosted glass, a roughness degree of the rotating frosted glass, and a focal length of the second lens.
7. A method of measuring a vortex beam topology charge under very low coherence conditions comprising:
generating a Gaussian Shell mode beam;
loading Laguerre amplitude, vortex phase and cross phase on Gaussian Shell mode light beam to generate partially coherent vortex light beam to be detected coupled with the cross phase;
collecting a plurality of groups of light intensity patterns of the partially coherent vortex light beams to be detected, which are coupled by the cross phases;
reading a light intensity graph, obtaining a coherence degree function mode distribution graph through calculation, and obtaining the size and the sign of topological charges according to the number of dark rings and the arrangement direction of the dark rings separated from the coherence degree function mode distribution graph;
the extremely low coherence width can be 0.3 times of the beam waist radius at the lowest;
the distance between the first lens and the rotary frosted glass, the roughness of the rotary frosted glass and the focal length of the second lens are controlled to generate Gaussian Shell mode light beams with extremely low coherence.
8. The method for measuring the topological charge of a vortex beam under the condition of extremely low coherence according to claim 7, wherein after the light intensity graph is read, the light intensity graph is subjected to superposition processing, and a mode distribution diagram of a coherence function is obtained according to a relation between a partial coherent beam fourth-order correlation function and the coherence function.
9. The method of measuring a vortex beam topology charge under very low coherence conditions of claim 7, wherein a gaussian schel-mode beam cross spectral density expression at a source face is:
wherein W is 1 (r 1 ,r 2 ) Representation of Gaussian Shell mode beam cross spectral density expression at light source face, r i Is a position vector at the light source face of the gaussian scher-mode beam, i=1, 2; ω and δ are beam waist width and coherence width, respectively.
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