Circularly polarized light generating device based on chiral fiber grating separation three-wavelength coupling and implementation method of circularly polarized light generating device

Abstract

The invention discloses a circularly polarized light generating device based on chiral fiber grating separation three-wavelength coupling and an implementation method thereof. The circularly polarized light generating device utilizes the characteristic that the effective refractive index difference of transverse electricity, transverse magnetism and a mixed mode which are degenerated to be in the same linear polarization mode in an optical fiber is increased along with the increase of the radial order; and the waveguide dispersion coefficient enhancement effect corresponding to a specific grating period coupling wavelength interval is combined with the unique angular momentum matching condition of the chiral fiber grating, so that different circularly polarized light beams are coupled and generated at three separated and controllable wavelength positions. The circularly polarized light generating device is an all-fiber device and is easy to be compatible with other fiber devices, the manufacturing process is simple and easy to realize, and two different circularly polarized light beams can be respectively generated at three separated and controllable wavelengths.

Description

Technical Field

[0001] The invention relates to the field of optical communication, and in particular to a circularly polarized light generating device based on chiral fiber grating separation and three-wavelength coupling. Background Art

[0002] Circular polarization is a free dimension of light that can be used for orthogonal multiplexing optical communications, as well as for measurement and sensing. When left- and right-circularly polarized light interact with matter, they may exhibit optical activity (circular birefringence) and circular dichroism. Optical activity (circular birefringence) refers to the different refractive indices of left- and right-circularly polarized light propagating through matter, and circular dichroism refers to the different transmittance (absorption) rates of left- and right-circularly polarized light in a specific wavelength region when propagating through matter. Using this property, measurement or sensing of the chirality of materials based on circularly polarized light can be achieved. When realizing various applications based on circularly polarized light, it is first necessary to effectively separate different circularly polarized lights. However, most existing polarization separation devices can only separate different circularly polarized light beams in different spaces, but cannot achieve separation at different wavelengths.

[0003] Common spatial polarization separation devices include Rochon, Wollaston and Nomarski prisms, which are generally made of birefringent materials and can split an incident light beam of arbitrary polarization into two orthogonal linearly polarized light beams emitted in different spatial directions. Combined with a quarter-wave plate, the spatial separation of circularly polarized light beams can be achieved. Common spatial polarization separation devices also include polarization gratings made of liquid crystal polymers, which can directly separate different circularly polarized light beams in space. In addition, Chinese patent CN202310588773.5 is a nanoimprint-based wire grid circular polarizer, which integrates two nanowire grids on the same substrate to realize a miniaturized "polarizer-quarter wave plate" structure, thereby realizing the spatial separation of circularly polarized light beams; Chinese patent CN202310620671.7 is an ultra-narrow-band dichroic mirror based on Faraday anomalous dispersion effect and its implementation method, which realizes the spatial separation of ultra-narrow-band circularly polarized light beams based on Faraday anomalous dispersion effect; Chinese patent CN2020800 20940.0 A polarization separation device and a differential interferometer and a differential contrast optical microscope including the device, which realize the spatial separation of circularly polarized light beams based on geometric phase; Chinese patent CN202110996178.6 A perfect circular polarization separator based on magneto-electric coupling, which realizes the spatial separation of circularly polarized light beams based on magneto-electric coupling; Chinese patent CN202410236718.4 A coaxial separation device and method for orthogonal linear polarized light beams, which uses a polarization splitting lens to realize the spatial separation of circularly polarized light beams.

[0004] In the paper "Cylindrical vector modes based Mach-Zehnder interferometer with vortex fiber for sensing applications, Appl. Phys. Lett. 115, 051103 (2019)", TE in ring core fiber was applied. 0v ,TM 0v and HE 2v The three modes have the characteristics of large effective refractive index difference. Fiber Bragg gratings are made on the ring-core fiber to achieve TE 0v ,TM 0v and HE 2v The three coupling modes are separated at different wavelengths, but what is generated at the three wavelength positions is a vector mode with a changing polarization distribution at different light field positions, and no circular polarization mode (light beam) can be generated. Summary of the invention

[0005] Purpose of the invention: The purpose of the present invention is to provide a circularly polarized light generating device based on chiral fiber grating separation three-wavelength coupling, which can be easily compatible with other optical fiber devices, has a simple manufacturing process and is easy to realize an all-fiber device, and can generate two different circularly polarized light beams at three separated and controllable wavelength positions.

[0006] Technical solution: A method for realizing circularly polarized light based on separation of three-wavelength coupling by chiral fiber grating, which uses a chiral fiber grating with a specific design period to generate different circularly polarized beams at three separated and controllable wavelength positions;

[0007] The period Λ of the chiral fiber grating is:

[0008] Λ=λ 2v / (n e,11 -n e,2v )

[0009] Among them, λ 2v It is the basic model HE 11 Mode and circular polarization HE 2v The coupling wavelength of the mode is HE 2v The wavelength of the mode; n e,11 、n e,2v HE 11 Mode and circular polarization HE 2v The effective refractive index of the mode, v is the radial order of the coupled mode.

[0010] Furthermore, the expression of the wavelength interval difference of different circular polarization modes is as follows:

[0011] Δλ AB =Λ×γAB ×Δn e,AB

[0012] In the formula, A and B are TE 0v ,TM 0v and HE 2v Three modes; Δλ AB =λ A -λ B is the wavelength interval difference between the A and B modes, where λ A , B are the wavelengths of the A and B modes respectively; Δn e,AB =Δn e,A -Δn e,B is the effective refractive index difference between modes A and B, Δn e,A , Δn e,B are the effective refractive indices of the two modes A and B respectively;

[0013] γ AB =Δn e,AB / Δn g,AB is the waveguide dispersion coefficient, Δn g,AB =Δn e,AB -λ×(dΔn g,AB / dλ) is the group refractive index difference between modes A and B, λ is an arbitrary wavelength, dΔn g,AB / dλ represents the group refractive index difference Δn g,AB Differentiation with respect to wavelength.

[0014] Furthermore, the expression of the effective refractive index of each mode is:

[0015] n e =f(nco,ncl,nsur,rco,rcl,v,λ)

[0016] Among them, n e Represents the effective refractive index; nco is the core refractive index of the optical fiber, ncl is the cladding refractive index of the optical fiber, nsur is the external environment refractive index of the optical fiber, rco is the core radius of the optical fiber, and rcl is the cladding radius of the optical fiber.

[0017] Furthermore, when a left-handed fiber Bragg grating is selected, three separated wavelength positions from short wavelength to long wavelength are coupled to generate left-handed, right-handed and left-handed circularly polarized beams respectively;

[0018] When a right-handed fiber Bragg grating is selected, three separated wavelength positions from short wavelength to long wavelength are coupled to generate right-handed, left-handed and right-handed circularly polarized beams respectively.

[0019] A circularly polarized light generating device based on chiral fiber Bragg grating separated three-wavelength coupling utilizes the property that the effective refractive index difference of transverse electric, transverse magnetic and mixed modes degenerate to the same linear polarization mode in the optical fiber increases with the increase of radial order, and the waveguide dispersion coefficient enhancement effect corresponding to the coupling wavelength interval of a specific grating period, combined with the angular momentum matching condition of the chiral fiber Bragg grating, adopts any of the above-mentioned circularly polarized light realization methods, and couples different circularly polarized light beams at three separated and controllable wavelength positions.

[0020] Furthermore, when a left-handed fiber Bragg grating is used, the left-handed circular polarization fundamental mode, the right-handed circular polarization fundamental mode and the left-handed circular polarization fundamental mode are coupled with the left-handed circular polarization mode, the right-handed circular polarization mode and the left-handed circular polarization mode respectively from three wavelength positions from short wavelength to long wavelength, and left-handed circular polarization light beams with orbital angular momentum of +1 are generated at the three wavelength positions respectively;

[0021] A right-handed fiber Bragg grating is selected, and the right-hand circular polarization fundamental mode, the left-hand circular polarization fundamental mode and the right-hand circular polarization fundamental mode are coupled with the right-hand circular polarization mode, the left-hand circular polarization mode and the right-hand circular polarization mode at three wavelength positions from short wavelength to long wavelength, respectively, to generate right-hand, left-hand and right-hand circularly polarized light beams with orbital angular momentum of -1 at these three wavelength positions, respectively.

[0022] Compared with the prior art, the present invention has the following significant effects:

[0023] 1. The circularly polarized light realization method of the present invention adopts a chiral fiber grating with a specific design period to generate different circularly polarized light beams at three separate and controllable wavelength positions, thereby realizing the separation of two circularly polarized light beams of different wavelengths;

[0024] 2. The circularly polarized light generating device of the present invention is an all-fiber device, which is easily compatible with other fiber optic devices and has a simple and easy-to-implement manufacturing process; it can be applied to the fields of chiral sensors, polarization multiplexing communications, and chiral measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a structural schematic diagram of the present invention;

[0026] Figure 2 for Figure 1 Schematic diagram of the structure's characteristics;

[0027] Figure 3 Schematic diagram of the equipment for preparing chiral fiber Bragg gratings;

[0028] Figure 4a Schematic diagram of the transmission spectrum of left-handed fiber Bragg grating;

[0029] Figure 4b The distribution diagram of the circular polarization mode generated by the left-handed fiber Bragg grating;

[0030] Figure 5a Schematic diagram of the transmission spectrum of right-handed fiber Bragg grating;

[0031] Figure 5b Distribution diagram of circular polarization mode generated by right-handed fiber Bragg grating;

[0032] In the figure, L-CFG: left-handed fiber Bragg grating; R-CFG: right-handed fiber Bragg grating; LCP: left-handed circularly polarized light; RCP: right-handed circularly polarized light; TE ⅠⅤ : Transverse electric mode; TM ⅠⅤ : Transverse magnetic mode; HE ⅠⅤ : mixed mode; subscripts Ⅰ and Ⅴ in the mode are the angular and radial orders respectively; ∧: grating period. DETAILED DESCRIPTION

[0033] The present invention is further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0034] The present invention provides a circularly polarized light generating device based on three-wavelength coupling separated by chiral fiber grating. The device utilizes the characteristics that the effective refractive index difference of the transverse electric (TE), transverse magnetic (TM) and hybrid (HE) modes degenerate into the same linear polarization (LP) mode in the optical fiber increases with the increase of radial order, and the waveguide dispersion coefficient enhancement effect corresponding to the specific grating period coupling wavelength interval, combined with the unique angular momentum matching condition of the chiral fiber grating, to realize a device that couples and generates different circularly polarized light beams at three separated and controllable wavelength positions. When the left-handed fiber grating is selected, the three separated wavelength positions from short wavelength to long wavelength are coupled to generate left-handed, right-handed and left-handed circularly polarized light beams respectively; and when the right-handed fiber grating is selected, the three separated wavelength positions from short wavelength to long wavelength are coupled to generate right-handed, left-handed and right-handed circularly polarized light beams respectively.

[0035] like Figure 1 The figure shows the specific structure of the circularly polarized light generating device. Figure 2 for Figure 1 Schematic diagram of the structure characteristics; left-handed fiber Bragg grating (L-CFG) is selected, and the left-handed circularly polarized fundamental mode HE 11 , right-handed circularly polarized fundamental mode HE 11 and left-handed circularly polarized fundamental mode HE 11 The three wavelength positions from short wavelength to long wavelength are related to left-handed circular polarization TM 0v Mode, right-hand circular polarization HE 2v and left-handed circularly polarized TE 0v Mode coupling generates left-handed, right-handed and left-handed circularly polarized beams with orbital angular momentum of +1 at these three wavelength positions; and right-handed fiber Bragg grating (R-CFG) is used, right-handed circularly polarized fundamental mode HE 11 , left-handed circularly polarized fundamental mode HE 11and right-handed circularly polarized fundamental mode HE 11 The three wavelength positions from short wavelength to long wavelength and right circular polarization TM 0v Mode, left-hand circularly polarized HE 2v and right-handed circularly polarized TE 0v Mode coupling generates right-handed, left-handed and right-handed circularly polarized light beams with orbital angular momentum of -1 at these three wavelength positions respectively.

[0036] The chiral fiber grating with periodic refractive index change is produced, which can be caused by a spiral refractive index change or a quarter period difference between horizontal and vertical refractive index changes.

[0037] First, according to the fiber parameters, calculate the fundamental mode HE 11 Mode and its coupled mode TE 0v ,TM 0v and HE 2v The effective refractive index n of the three modes at any wavelength λ e The calculation expression of is as follows (this formula is the general calculation expression of effective refractive index):

[0038] n e =f(nco,ncl,nsur,rco,rcl,v, λ) (1)

[0039] Among them, nco is the core refractive index of the optical fiber, ncl is the cladding refractive index of the optical fiber, nsur is the external environment refractive index of the optical fiber, rco is the core radius of the optical fiber, rcl is the cladding radius of the optical fiber, and v is the radial order of the coupled mode.

[0040] According to the set basic model HE 11 Mode and HE 2v Mode coupling wavelength λ 2v , determine the specific period Λ of the spiral fiber grating as:

[0041] Λ=λ 2v / (n e,11 -n e,2v ) (2)

[0042] Among them, λ 2v It is the basic model HE 11 Mode and HE 2v The coupling wavelength of the mode, that is, the circularly polarized HE 2v The wavelength of the generated mode, n e,11 and n e,2v HE 11 Mode and HE 2v The effective refractive index (phase refractive index) of the mode, v is the radial order of the mode.

[0043] The wavelength interval difference Δλ between different modes (circularly polarized beams) is AB for:

[0044] Δλ AB =Λ×γ AB ×Δn e,AB (3)

[0045] In the formula, A and B can be TE 0v ,TM 0v and HE 2v Three modes; Δλ AB =λ A -λ B is the wavelength interval difference between the A and B modes, where λ A , B are the wavelengths of the A and B modes respectively; Δn e,AB =Δn e,A -Δn e,B is the refractive index difference between modes A and B, Δn e,A , Δn e,B are the effective refractive indices of the A and B modes respectively; γ AB =Δn e,AB / Δn g,AB , where Δn g,AB is the group refractive index difference between the A and B modes, expressed as Δn g,AB =Δn e,AB -λ×(dΔn g,AB / dλ),λ is any wavelength, dΔn g,AB / dλ represents the group refractive index difference Δn g,AB Differentiation with respect to wavelength.

[0046] By optimizing the parameters nco, ncl, nsur, rco, rcl, v and λ, the fundamental mode HE can be obtained. 11 Mode and its coupled mode TE 0v ,TM 0v and HE 2v The three modes have specific effective refractive index n at any wavelength λ e , and combined with the specific period selection Λ, three wavelength-separated circularly polarized beams that meet the required wavelength interval difference Δλ can be generated at any wavelength λ position.

[0047] The following two cases are used for experiments. Both cases use the same single-mode optical fiber (Yangtze Optical Fiber's step-index single-mode optical fiber). The specific parameters are: the radii of the optical fiber core and cladding are rco=4.1μm and rcl=62.5μm, respectively, and the refractive indices of the core and cladding are nco=1.4580 and ncl=1.4536, respectively.

[0048] Both cases adopted Figure 3 The experimental platform shown in the figure includes a translation stage 1, a translation stage 2, a translation stage 3, a fixture, a rotator and a reflector. The fixture and the rotator are fixed on the translation stage 1 and the translation stage 2 respectively, and the translation stage 1 and the translation stage 2 are placed on the translation stage 3; it also includes a light source and a spectrum analyzer for observing the transmission spectrum characteristics of the chiral fiber grating being produced. The experimental platform can synchronously control the heating, movement and rotation of the optical fiber, and synchronously move and rotate the optical fiber heated to a molten state to form a chiral fiber grating, wherein clockwise rotation forms a left-handed fiber grating, and counterclockwise rotation forms a right-handed fiber grating. The period Λ' of the formed chiral fiber grating is determined by the optical fiber movement speed s (mm / s-millimeter / second) controlled by the translation stage 3 and the rotation speed w (turn / s-turn / second) controlled by the rotator, and is expressed as:

[0049] Λ'=s / w (4)

[0050] The unit of period is mm / turn.

[0051] In addition, the translation stage 3 may not move, but the translation stages 1 and 2 may move instead. In this way, a chiral fine-diameter fiber Bragg grating may be generated by controlling the speed difference between the translation stages 1 and 2. The diameter of the generated fine-diameter grating is expressed as:

[0052]

[0053] The period at this time is expressed as: Λ = v2 / w (mm / turn-millimeter / turn)

[0054] Where D t and D o is the fiber diameter before and after thinning, v1 and v2 are the translation speeds of translation stage 1 and translation stage 2 respectively (v1 <v2)。

[0055] The specific production steps are:

[0056] Step A1, fix the fixer and the rotator on the translation stage 1 and the translation stage 2 respectively, and place the translation stage 1 and the translation stage 2 on the translation stage 3;

[0057] Step A2, obtain an optical fiber to be manufactured, such as a single-mode optical fiber, fix one end of it in a fixture to keep the original optical fiber being processed straight (when manufacturing chiral fine-diameter fiber gratings, the straightening can be maintained by making v1 slightly lower than v2), and fix the other end into a rotator.

[0058] Step A3, prepare a grating with a period of Λ: In order to ensure the uniformity of the heating zone, a sapphire tube is used to form the heating zone, and the optical fiber is heated to a molten state by a carbon dioxide laser. If a normal chiral fiber grating is produced, a grating with a period of Λ is obtained by controlling the moving speed s of the translation stage 3 and the rotation rate w of the rotator, wherein the moving speed s and the rotation rate w satisfy s / w=Λ, and clockwise and counterclockwise rotations form left-handed fiber gratings and right-handed fiber gratings, respectively. If a chiral fine-diameter fiber grating needs to be produced (the waveguide dispersion coefficient can be controlled by reducing the optical fiber diameter), the speed difference between the translation stage 1 and the translation stage 2 is controlled, and the diameter of the fine-diameter grating produced is expressed as The period at this time is expressed as Λ = s2 / w (mm / turn-millimeter / turn), where D t , D o are the fiber diameters before and after thinning, s1 and s2 are the translation speeds of translation stage 1 and translation stage 2, respectively.

[0059] Example 1: Circular polarization wavelength three-separation device based on left-handed fiber grating

[0060] First, according to the above fiber parameters, calculate the fundamental mode HE 11 Mode and its coupled mode TE 0v ,TM 0v and HE 2v The effective refractive index n of the three modes at any wavelength λ e左 , which can be expressed as:

[0061] n e左 =f(nco,ncl,nsur,rco,rcl,v,λ)

[0062] Among them, nco=1.4580 is the core refractive index of the optical fiber, ncl=1.4536 is the cladding refractive index of the optical fiber, nsur=1 is the external environment refractive index of the optical fiber, rco=4.1μm is the core radius of the optical fiber, rcl=62.5μm is the cladding radius of the optical fiber, and v=13 is the radial order of the coupled mode.

[0063] Next, a left-handed fiber Bragg grating is selected to generate left-handed, right-handed and left-handed circularly polarized beams respectively at three separated wavelength positions from short wavelength to long wavelength.

[0064] Finally, according to the set basic model HE 11 Mode and HE 2v Mode coupling wavelength λ 2v =1558nm, determine the specific period Λ of the helical fiber grating 左 for:

[0065] Λ 左 =λ 2v / (n e,11 -n e,2v )=144μm

[0066] Among them, λ 2v It is the basic model HE 11 Mode and HE 2v The coupling wavelength of the mode, that is, HE 2v The wavelength of the mode; n e,11 、n e,2v HE 11 Mode and HE 2v The effective refractive index (phase refractive index) of the mode.

[0067] The obtained TE 0v and HE 2v The mode coupling mode spacing is Δλ TM，HE =4nm; TM obtained 0v and HE 2v The mode coupling mode spacing is Δλ TE，HE =7nm.

[0068] Its transmission spectrum is Figure 4a As shown, left-hand circular polarization + 1st-order orbital angular momentum mode, right-hand circular polarization + 1st-order orbital angular momentum mode, and left-hand circular polarization + 1st-order orbital angular momentum mode are generated at 1554nm, 1558nm, and 1565nm, respectively. Figure 4b From the interference pattern, we can see that all the generated orbital angular momentum modes are +1 order.

[0069] Example 2: Circular polarization wavelength three-separation device based on right-handed thin-diameter fiber Bragg grating

[0070] First, according to the above fiber parameters, calculate the fundamental mode HE 11 Mode and its coupled mode TE 0v ,TM 0v and HE 2v The effective refractive index n of the three modes at any wavelength λ e右 , which can be expressed as:

[0071] n e右 =f(nco,ncl,nsur,rco,rcl,v,λ)

[0072] Among them, nco=1.4580 is the core refractive index of the thin-diameter optical fiber, ncl=1.4536 is the cladding refractive index of the thin-diameter optical fiber, nsur=1 is the external environment refractive index of the thin-diameter optical fiber, rco=2.87μm is the core radius of the thin-diameter optical fiber, rcl=43.75μm is the cladding radius of the thin-diameter optical fiber, and v=12 is the radial order of the coupled mode. In this embodiment, in order to obtain a better waveguide dispersion coefficient, a chiral thin-diameter fiber Bragg grating is used, and the waveguide dispersion coefficient is adjusted by pulling the fiber diameter thinner.

[0073] Next, a right-handed fiber Bragg grating is selected to generate right-handed, left-handed and right-handed circularly polarized beams respectively at three separated wavelength positions from short wavelength to long wavelength.

[0074] Finally, according to the set basic model HE 11 Mode and HE 2v Mode coupling wavelength λ 2v =1519nm, determine the specific period Λ of the helical fiber grating 右 for:

[0075] Λ 右 =λ 2v / (n e,11 -n e,2v )=94μm

[0076] Among them, λ 2v It is the basic model HE 11 Mode and HE 2v The coupling wavelength of the mode, that is, HE 2v The wavelength of the generated mode, n e,11 and n e,2v HE 11 Mode and HE 2v The effective refractive index (phase refractive index) of the mode. Then the obtained TE 0v and HE 2v The mode coupling mode spacing is Δλ TM，HE =5nm; TM obtained 0v and HE 2v The mode coupling mode spacing is Δλ TE，HE =3nm.

[0077] Its transmission spectrum is Figure 5a As shown, right-hand circular polarization-1st-order orbital angular momentum mode, left-hand circular polarization-1st-order orbital angular momentum mode, and right-hand circular polarization-1st-order orbital angular momentum mode are generated at 1514nm, 1519nm, and 1522nm, respectively. Figure 5b From the interference pattern, we can see that all the generated orbital angular momentum modes are -1 order.

[0078] The above disclosure is only a preferred embodiment of the present invention, which cannot be used to limit the scope of the present invention. Therefore, equivalent changes made according to the claims of the present invention are still within the scope of the present invention.

Claims

1. A method for realizing circularly polarized light separation of three-wavelength coupling based on chiral fiber grating, characterized in that: A chiral fiber Bragg grating with a specific design period is used to generate different circularly polarized beams at three separate and controllable wavelength positions; The period Λ of the chiral fiber grating is: L=l 2v / (n e,11 -n e,2v ) Among them, λ 2v It is the basic model HE 11 Mode and circular polarization HE 2v The coupling wavelength of the mode is HE 2v The wavelength of the mode; n e,11 、n e,2v HE 11 Mode and circular polarization HE 2v The effective refractive index of the mode, v is the radial order of the coupled mode.

2. The method for realizing circularly polarized light separation of three-wavelength coupling based on chiral fiber grating according to claim 1, characterized in that: The expression for the wavelength spacing difference of different circular polarization modes is as follows: Dl AB =Λ×γ AB ×Δn e,AB In the formula, A and B are TE 0v ,TM 0v and HE 2v Three modes; Δλ AB =λ A -λ B is the wavelength difference between the A and B modes, λ A , B are the wavelengths of the two modes A and B respectively; Δn e,AB =Δn e,A -Δn e,B is the effective refractive index difference between modes A and B, Δn e,A , Δn e,B are the effective refractive indices of the A and B modes respectively; γ AB =Δn e,AB / Δn g,AB is the waveguide dispersion coefficient; Δn g,AB =Δn e,AB -λ×(dΔn g,AB / dλ) is the group refractive index difference between modes A and B, λ is an arbitrary wavelength, dΔn g,AB / dλ represents the group refractive index difference Δn g,AB Differentiation with respect to wavelength.

3. The method for realizing circularly polarized light separation of three-wavelength coupling based on chiral fiber grating according to claim 1, characterized in that: The expression of the effective refractive index of each mode is: n e =f(nco,ncl,nsur,rco,rcl,v,λ) Among them, n e Represents the effective refractive index; nco is the core refractive index of the optical fiber, ncl is the cladding refractive index of the optical fiber, nsur is the external environment refractive index of the optical fiber, rco is the core radius of the optical fiber, and rcl is the cladding radius of the optical fiber.

4. The method for realizing circularly polarized light separation of three-wavelength coupling based on chiral fiber grating according to any one of claims 1 to 3, characterized in that: When a left-handed fiber Bragg grating is selected, three separated wavelength positions from short wavelength to long wavelength are coupled to produce left-handed, right-handed and left-handed circularly polarized beams respectively; When a right-handed fiber Bragg grating is selected, three separated wavelength positions from short wavelength to long wavelength are coupled to generate right-handed, left-handed and right-handed circularly polarized beams respectively.

5. A circularly polarized light generating device based on chiral fiber grating separation of three-wavelength coupling, characterized in that: Utilizing the property that the effective refractive index difference of the transverse electric, transverse magnetic and mixed modes degenerate into the same linear polarization mode in the optical fiber increases with the increase of the radial order, and the waveguide dispersion coefficient enhancement effect corresponding to the coupling wavelength interval of a specific grating period, combined with the angular momentum matching condition of the chiral fiber Bragg grating, the circularly polarized light realization method as described in any one of claims 1 to 3 is adopted to couple different circularly polarized light beams at three separate and controllable wavelength positions.

6. The circularly polarized light generating device based on chiral fiber grating separation three-wavelength coupling according to claim 1, characterized in that: When a left-handed fiber Bragg grating is selected, the left-handed circular polarization fundamental mode, the right-handed circular polarization fundamental mode and the left-handed circular polarization fundamental mode are coupled with the left-handed circular polarization mode, the right-handed circular polarization mode and the left-handed circular polarization mode from three wavelength positions from short wavelength to long wavelength, respectively, to generate left-handed, right-handed and left-handed circularly polarized beams with orbital angular momentum of +1 at these three wavelength positions; A right-handed fiber Bragg grating is selected, and the right-hand circular polarization fundamental mode, the left-hand circular polarization fundamental mode and the right-hand circular polarization fundamental mode are coupled with the right-hand circular polarization mode, the left-hand circular polarization mode and the right-hand circular polarization mode at three wavelength positions from short wavelength to long wavelength, respectively, to generate right-hand, left-hand and right-hand circularly polarized light beams with orbital angular momentum of -1 at these three wavelength positions, respectively.

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