CN111323903B - Optical fiber orbital angular momentum mode separation method based on spiral structure - Google Patents
Optical fiber orbital angular momentum mode separation method based on spiral structure Download PDFInfo
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Abstract
The invention discloses a method for separating an optical fiber orbital angular momentum mode based on a helical structure, which comprises the following steps: calculating the angular effective refractive index difference between the newly formed modes in the spiral optical fiber, which meets the requirement of the effective separation of the orbital angular momentum mode, according to the requirement of the total effective refractive index difference between the modes required by the effective separation of the orbital angular momentum mode and the longitudinal effective refractive index value of the orbital angular momentum mode in the original optical fiber; calculating the specific designed spiral period length meeting the requirement of the angular effective refractive index difference calculated in the previous step by utilizing the characteristic that the angular effective refractive index newly formed in the spiral optical fiber is related to the spiral period length; heating the original optical fiber, and controlling the movement and rotation speed of the original optical fiber to make it twist to form the spiral optical fiber. The invention ensures the reliable, stable and multiplexed long-distance transmission of the orbital angular momentum mode in the designed spiral optical fiber and realizes the common multiplexing of the circular polarization direction and the orbital angular momentum order.
Description
Technical Field
The invention relates to the technical field of optical communication and optical sensing, in particular to a method for separating an optical fiber orbital angular momentum mode based on a helical structure.
Background
The optical orbital angular momentum is widely applied in the fields of optical communication, optical sensing, particle control and the like, and the optical fiber supporting stable multiplexing transmission of various orbital angular momentum modes is an important optical device for realizing the application of the optical orbital angular momentum. In a conventional optical fiber, the refractive index difference between the core and the cladding is increased, or the refractive index difference between the HE mode and the EH mode (or TE/TM mode) belonging to the same degenerate mode is increased by a method of designing an optical fiber with a special structure, so that mode separation of an orbital angular momentum mode (orbital angular momentum formed based on the HE mode) in which spin and orbital angular momentum are the same and an orbital angular momentum mode (orbital angular momentum formed based on the EH or TE/TM mode) in which spin and orbital angular momentum are the same is realized. Common methods include few-mode, multi-mode, hollow-core, ring-core, multi-core, and photonic crystal fiber schemes. However, the above methods cannot separate orbital angular momentum modes having the same order of orbital angular momentum but different spin directions, and require complicated optical fiber design techniques and processing processes. Meanwhile, such specially designed optical fibers are difficult to realize low-loss connection with conventional optical fibers in optical fiber communication and sensing systems.
A great number of patents and papers at home and abroad discuss how to design optical fibers capable of stably transmitting a plurality of orbital angular momentum modes, but all the optical fibers are designed from the angle of increasing the effective refractive index of the light propagation direction (longitudinal direction), and a method for increasing the angular effective refractive index by adopting optical fibers with a spiral structure to realize reliable, stable and multiplexed long-distance transmission of different orbital angular momentum modes in the optical fibers is not adopted. In addition, the method of increasing the refractive index in the propagation direction can only increase the refractive index difference between the orbital angular momentum modes having the same orbital angular momentum order but different spin directions (HE mode angular momentum) and the opposite direction (EH mode angular momentum), and cannot increase the refractive index difference between the orbital angular momentum modes having the same orbital angular momentum order, thereby realizing effective separation and multiplexing transmission of such modes.
There are also a lot of papers at home and abroad discussing the manufacture of spiral long-period fiber gratings for various orbital angular momentum generators, or circular polarization converters, and sensors for torsion, etc. However, most of such orbital angular momentum generators generate a high-order mode through mode coupling, and then a phase difference of pi/2 is generated between an odd mode and an even mode belonging to the same vector mode HE mode (EH mode or TE/TM mode) to form an orbital angular momentum mode, and a method for realizing mode separation of different orbital angular momenta by increasing an angular effective refractive index through a helical structure in an optical fiber is not provided. In addition, the spiral fiber grating has mode conversion characteristics near the coupling wavelength, so that the modes participating in coupling cannot be stably and reliably transmitted.
Disclosure of Invention
The technical problem to be solved by the present invention is to provide a method for separating an optical fiber orbital angular momentum mode based on a spiral structure, which can ensure reliable, stable and multiplexed long-distance transmission in a designed spiral optical fiber, and realize common multiplexing of a circular polarization direction and an orbital angular momentum order.
In order to solve the technical problem, the invention provides a method for separating an optical fiber orbital angular momentum mode based on a helical structure, which comprises the following steps:
(1) calculating the angular effective refractive index difference between the modes newly formed in the spiral optical fiber and meeting the requirement of the effective separation of the orbital angular momentum mode according to the requirement of the total effective refractive index difference between the modes required by the effective separation of the orbital angular momentum mode and the longitudinal effective refractive index value of the orbital angular momentum mode in the original optical fiber, wherein the total effective refractive index of the orbital angular momentum mode in the spiral optical fiber consists of the longitudinal effective refractive index in the original optical fiber and the angular effective refractive index newly formed in the spiral optical fiber;
(2) calculating the specially designed spiral period length required by the angular effective refractive index difference calculated in the step (1) by utilizing the characteristics of the newly formed angular effective refractive index in the spiral fiber, the circular polarization state of the orbital angular momentum mode in the spiral fiber, the orbital angular momentum order, the working wavelength and the spiral period length of the spiral fiber;
(3) heating the original optical fiber, and controlling the movement and rotation speed of the original optical fiber to make the original optical fiber be twisted to form the spiral optical fiber with the specific designed spiral period length calculated in the step (2).
Preferably, in step (1), the original fiber is any fiber with a circularly symmetric structure, and the fiber types include single-mode fiber, few-mode fiber, multi-mode fiber, hollow-core fiber, ring-core fiber, multi-core fiber and photonic crystal fiber.
Preferably, in step (2), the specifically designed helical period length Λ should satisfy the formula:
Λ＝min{Λp,q}
wherein subscripts p and q denote two orbital angular momentum modes p and q in the original fiber; min { } represents taking a minimum value operation sign for obtaining the minimum spiral period length required by any two mode separations; lambdap,qThe length of the spiral period of a particular design required to separate orbital angular momentum modes p and q is expressed specifically as:
wherein the subscripts p and q represent two orbital angular momentum modes p and q in the helical fiber; the | | is an absolute value calculation symbol; spAnd sqThe spin quantum numbers of the two orbital angular momentum modes are respectively, and the values of +1 or-1 respectively represent the left-handed and right-handed circular polarization states; lpAnd lqThe orders of the orbital angular momentum of the two orbital angular momentum modes are respectively integers; lambda [ alpha ]minA minimum wavelength within a range of operating wavelengths for the orbital angular momentum mode; Δ n is the effective refractive index difference between the orbital angular momentum modes desired to be obtained, and is typically taken to be Δ n > 1 × 10-4；
Respectively the longitudinal effective refractive indexes of the two orbital angular momentum modes in the original optical fiber before torsion, and meet the condition
Preferably, in the step (3), the heating and twisting method is to perform coaxial twisting around the center of the heated original optical fiber, and finally form the spiral optical fiber with a coaxial spiral structure with a specially designed spiral period length; the heating mode of the optical fiber is selected from an oxyhydrogen flame or a carbon dioxide laser.
The invention has the beneficial effects that: the manufacturing method is simple, can be realized on the traditional optical fiber processing equipment, and solves the problems that the traditional orbital angular momentum optical fiber (optical fibers with few modes, hollow cores, ring cores, multiple cores, photonic crystals and the like) needs special optical fiber design and processing technology and is difficult to produce in large scale and at low cost; the surface of the optical fiber processed according to the proposed design and manufacturing method basically has no deformation and damage, can keep the original transmission characteristic and can be connected with the original optical fiber before torsion in a low-loss manner; the invention can also solve the problem that the prior orbital angular momentum optical fiber can not separate orbital angular momentum modes with the same orbital angular momentum order and different circular polarization directions, namely, the spiral structure designed and manufactured by the invention can realize multiplexing transmission of orbital angular momentum and circular polarization; the application range of the invention not only comprises the spiral optical fiber, but also comprises devices such as a spiral optical fiber-based orbital angular momentum mode coupler, a converter, a generator, an amplifier and the like which are designed by using the method.
Drawings
FIG. 1 is a schematic diagram of the structure of the spiral optical fiber according to the present invention.
FIG. 2 is a schematic diagram of the experimental principle of fabricating a spiral optical fiber according to the present invention.
FIG. 3 is a schematic view of a single mode fiber based spiral fiber actually fabricated by the microscope of the present invention.
FIG. 4 is a diagram showing the simulation result of the mode effective refractive index of the original single mode fiber before the spiral structure is formed according to the present invention.
FIG. 5 is a schematic diagram showing the mode effective refractive index of the helical single-mode optical fiber after the helical structure is formed according to the present invention.
FIG. 6(a) is a schematic diagram of interference fringes of the +1 st order orbital angular momentum mode and Gaussian spherical wave separated by the present invention.
FIG. 6(b) is a schematic diagram of interference fringes of-1 order orbital angular momentum mode and Gaussian spherical wave separated by the present invention.
FIG. 6(c) is a schematic diagram of a separated light spot of the +1 orbital angular momentum mode according to the present invention.
FIG. 6(d) is a schematic diagram of a light spot of the separated-1 orbital angular momentum mode according to the present invention.
FIG. 7 is a graph showing the simulation results of the mode effective refractive index of the original four-mode optical fiber before the spiral structure is formed according to the present invention.
FIG. 8 is a diagram illustrating the simulation result of the mode effective refractive index of the spiral four-mode optical fiber after the spiral structure is formed.
FIG. 9(a) is a schematic diagram of interference fringes of the +2 order orbital angular momentum mode and Gaussian spherical wave separated by the present invention.
FIG. 9(b) is a schematic diagram of interference fringes of-2 order orbital angular momentum mode and Gaussian spherical wave separated by the present invention.
FIG. 9(c) is a schematic diagram of a separated spot of +2 orbital angular momentum mode according to the present invention.
FIG. 9(d) is a schematic diagram of a separated-2 orbital angular momentum mode of the invention.
FIG. 10 is a schematic view of the observation principle of observing and confirming mode separation according to the present invention.
Detailed Description
The technical scheme of the invention is realized by synchronously controlling the heating, moving and rotating of the original optical fiber. The original fiber heated to the molten state is moved and rotated synchronously to form a spiral fiber having a spiral periodic length structure of a specific design. The period length Λ of the spiral of the particular design formed is determined by the speed v (mm/s-mm/sec) of travel and the speed w (turn/s-rev/sec) of rotation of the original fiber, denoted as Λ -v/w (mm/turn-mm/rev). Due to the torsion tangential acting force when being heated, a spiral periodic length structure with specific design is formed in the cooled spiral optical fiber, and the spiral optical fiber has uniform spiral refractive index distribution characteristics. Under the influence of the spiral refractive index distribution, the orbital angular momentum mode in the spiral optical fiber not only has the longitudinal propagation characteristic of a common optical fiber mode, but also has the angular phase change characteristic related to the propagation direction. The characteristic of this variation of the angular phase along the propagation direction can be expressed in terms of the angular refractive index. The azimuthal and longitudinal effective indices act together to exhibit a phase change along the propagation direction (equivalently the total propagation constant or total effective index change). The schematic diagram of the structure of the spiral optical fiber is shown in fig. 1, and the spiral structure in the diagram is only for explaining the structural principle, and reflects the refractive index change characteristics of the spiral optical fiber, and the physical structure cannot be observed in the actually manufactured spiral optical fiber. As can be seen, the effective propagation direction of light in the optical fiber forming the helical structure is a helical direction, and the propagation of the helical direction can be orthogonally decomposed into propagation along the longitudinal direction and propagation along the angular direction of the cross-sectional direction, so that the total effective refractive index of the propagation thereof includes the longitudinal effective refractive index and the angular effective refractive index.
Assuming that the effective longitudinal refractive index of the orbital angular momentum mode (l, m) in the original fiber before twisting is
Where l and m denote the angular and radial order, respectively, in the cross-section. The effective refractive index of the orbital angular momentum mode in the twisted helical optical fiber forming the helical structure
Can be expressed as:
in the above formula
And is partly the manifestation of the change in angular phase along the direction of propagation of orbital angular momentum modes (l, m) in an optical fiber having a helical structure, i.e., the angular effective refractive index. Where s is the spin quantum number, s-1 and-1 represent the left-and right-hand circular polarization states, respectively; σ is the twist direction of the helical fiber, σ ═ 1, or-1, representing the left-handed or right-handed helical structure, respectively; lambda is the working period of the orbital angular momentum mode; Λ is the spiral period length of a particular design.
To effectively separate the different orbital angular momentum modes, it is generally necessary that the effective refractive index difference between the two modes satisfies Δ n > 1 × 10-4The conditions of (1). Since the angular effective refractive index is equivalent to the longitudinal effective refractive index, the longitudinal effective refractive index difference can be realized only by specially designed optical fiber structures or by increasing the refractive index difference between the fiber core and the cladding. Therefore, in the invention, the requirement of the effective refractive index difference is innovatively provided to be realized through the change of the angular effective refractive index, and in order to realize the effective refractive index difference required by the separation of any two orbital angular momentum modes p and q, the spiral period in the manufactured spiral optical fiber at least meets the following requirements:
wherein subscripts p and q represent any two orbital angular momentum modes p and q in the helical fiber; the | | is an absolute value calculation symbol; spAnd sqThe spin quantum numbers of the two modes are respectively, and when the value is 1 or-1, the spin quantum numbers respectively represent the left-handed and right-handed circular polarization states; lpAnd lqThe orders of orbital angular momentum of the two modes are respectively, and the values are integers; lambda [ alpha ]minIs the minimum wavelength within the operating wavelength range of the mode; Δ n is the effective refractive index difference between the desired modes, and is typically taken to be Δ n > 1 × 10-4；
The longitudinal effective refractive indices of the two modes in the original fiber before twisting,
when there are multiple orbital angular momentum modes in a helical fiber, the helix period length Λ for a particular design needs to satisfy the formula:
Λ＝min{Λp,q}(3)
wherein subscripts p and q denote two orbital angular momentum modes p and q in the original fiber; min { } represents the minimum operator sign, i.e., takes the minimum value in the spiral period required to separate any two modes.
When the above conditions are satisfied, the difference in effective refractive index between two orbital angular momentum modes having the same angular momentum order and different spin directions, that is, an HE mode in which spin and orbital angular momentums belong to the same degenerate mode and an EH mode in which spin and orbital angular momentums are in the same direction can be up to 2 × 10-4In the above, effective mode separation can be realized. Meanwhile, the effective refractive index difference between two orbital angular momentum modes in the same spin direction and different angular momentum directions can reach 1 multiplied by 10-4In the above, the effective mode separation can be realized. For any other different orbital angular momentum modes, due to the different spin directions or orbital angular momentum orders, the effective refractive index difference between any two orbital angular momentum modes can reach at least 1 × 10-4By the method, the effective separation of the optical fiber in the designed spiral optical fiber can be realized, and the requirements of long-distance stable, reliable and multiplexing transmission of different modes in the optical fiber are met.
When the period Λ satisfies the resonance condition between certain modes in the spiral fiber grating in a specific wavelength interval, the coupled energy exchange between the modes at a specific wavelength will be generated, i.e. the orbital angular momentum mode conversion is realized. Such periods should be avoided if reliable and efficient long distance transmission of these modes at specific wavelengths is to be ensured.
The invention has the following implementation steps: the optical fiber to be processed is fixed between a holder and a rotator, which are respectively fixed on two translation stages. When the optical fiber is heated to a molten state, the moving speed and the rotating speed of the spiral optical fiber are synchronously controlled by synchronously controlling the movement of the translation stage and the rotation of the rotator. The period Λ for forming the helical structure is determined by the moving speed v (mm/s-mm/sec) and the rotating speed w (turn/s-rev/sec), and is expressed as Λ ═ v/w (mm/turn-mm/rev). In manufacturing, it is necessary to control the speed of movement and rotation at the same time so that the period of the formed spiral structure satisfies the above formula (3).
Example 1: taking a single-mode fiber as an example, the separation and transmission of 1 st order orbital angular momentum in the cladding are realized.
The structure of figure 2 was used to align a pristine single mode fiber (Fujikura Inc,
) Heating and twisting are carried out. In the figure, in order to ensure the uniformity of the heating zone, a Sapphire tube (Sapphire tube) is used to form the heating zone, and a carbon dioxide laser (CO) is controlled by a photoswitch (Shutter)2laser) on/off. The processed original Optical fiber (Optical fiber) is fixed (Clamp) at one end, and the straightening of the processed original Optical fiber is kept by a Weight (Weight), and the other end is fixedly arranged in a Rotator (Rotator). A broad optical source (ASE) and an Optical Spectrum Analyzer (OSA) are used to observe characteristics of the spiral fiber during fabrication, such as loss characteristics. When the original optical fiber is heated to a certain temperature, a Stage (Stage) and a Rotator (Rotator) work synchronously under the control of control software in a Computer (Computer) to realize simultaneous movement and rotation. The period Λ for forming the helical structure is determined by the moving speed v (mm/s-mm/sec) and the rotating speed w (turn/s-rev/sec), and is expressed as Λ ═ v/w (mm/turn-mm/rev). Figure 3 is a microscopic observation of a single mode fiber based (Fujikura Inc,
) Actually fabricated spiral optical fiber. As can be seen from the figure, the diameter deformation of the optical fiber after heating and twisting is about 1-3um, the transmission characteristic of the original single-mode optical fiber is hardly changed, and the formed spiral optical fiber can realize low-loss connection with the original optical fiber. Single mode fibers before and after forming the helix-structure (Fujikura Inc,
) The different mode effective refractive indices in (a) are shown in fig. 4 and 5. Figure 4 is a graph of the original single mode fiber before the helix-structure was formed (Fujikura Inc,
) The mode effective refractive index simulation result of (1). In the figure, the radius of the fiber core and the radius of the cladding are respectively 4.1um and 62.5um, the refractive index is respectively 1.4580 and 1.4536, and the effective refractive index result in the figure is obtained by simulation calculation by a finite element method. As can be seen, HE23、TE03And TM03The effective refractive index difference of the modes (the mode subscripts sequentially indicate the azimuthal and radial orders of the modes, respectively) is extremely small, and degenerates in the original fiber, making it difficult to separate independent propagation. Figure 5 shows a spiral single mode fiber after forming the spiral structure (Fujikura Inc,
) The radius of the fiber core and the cladding in the graph are respectively 4.1um and 62.5um, the refractive index is respectively 1.4580 and 1.4536, the spiral radius is 1cm, and the result in the graph is obtained by finite element method simulation calculation. It can be seen that different orbital angular momentum modes HE in the wavelength range of 1450nm to 1650nm23 +、HE23 -And TE/TM03 +/-(the signs in the superscript indicate different circular polarizations, + indicates a left-handed circular polarization state, -indicates a right-handed circular polarization state), respectively), the effective refractive index difference between the different orbital angular momentum modes is about at least 3 x 10-4And the requirement of effective separation of orbital angular momentum modes is completely met.
With the structure of fig. 10, the transmission conditions of the optical fiber in the helical structure formed by different orbital angular momentum modes are observed. The Tunable Laser (Tunable Laser) outputs light with a corresponding observation wavelength, the light is coupled by a Coupler (Coupler), one path of the light is input into a Mode converter (Mode converter) to obtain a corresponding orbital angular momentum Mode, the light is transmitted by a fiber forming a helical structure and then interferes with a Gaussian spherical wave reference signal output by the other path of the light, interference fringes are observed by a CCD, and a Polaroid (PC) and an Attenuator (attentuator) are used for adjusting the definition of the interference fringes. When no Gaussian spherical wave reference signal is added, the circular light spot corresponding to the orbital angular momentum mode is displayed, and the observed results are shown in FIGS. 6(c) and (d); when the gaussian spherical wave reference signal is added, it shows helical interference fringes, and the observed results are shown in fig. 6(a), (b). Wherein FIGS. 6(a), (c) show the post-interference fringes and pre-interference spots of the +1 order orbital angular momentum mode, respectively; FIGS. 6(b) and (d) show interference fringes of the-1 st order orbital angular momentum mode and a spot before interference, respectively.
Example 2: taking a four-mode optical fiber as an example, the separation transmission of 2-order orbital angular momentum in the fiber core is realized.
The structure of fig. 2 is used for heating and twisting an original four-mode optical fiber (long flying company, step-type four-mode optical fiber), and the heating and twisting mode is consistent with that of a single-mode optical fiber. The effective refractive indices of the different modes in the four-mode fiber (long-fiber, step-type four-mode fiber) before and after the formation of the spiral structure are shown in fig. 7 and 8. Fig. 7 shows the mode effective index simulation results of the original four-mode fiber (long femtolier, step four-mode fiber) before the spiral structure is formed. In the figure, the radius of the fiber core and the radius of the cladding are respectively 9.5um and 62.5um, the refractive index is respectively 1.4499 and 1.444, and the effective refractive index result in the figure is obtained by simulation calculation by a finite element method. As can be seen, HE31And EH11(the mode indices sequentially indicate the azimuthal and radial order of the mode, respectively.) the mode's effective index difference is extremely small, creating degenerate modes in the fiber that are difficult to separate for independent propagation. Fig. 8 shows a simulation result of the mode effective refractive index of a spiral four-mode fiber (long fiber, step-type four-mode fiber) after the spiral structure is formed. In the figure, the radius of the fiber core and the radius of the cladding are respectively 9.5um and 62.5um, the refractive index is 1.4499 and 1.444, the radius of the helix is 1cm, and the effective refractive index result in the figure is obtained by finite element method simulation calculation. It can be seen that different orbital angular momentum modes HE are available in the wavelength range of 1450nm to 1650nm31 +、HE31 -、EH11 +And EH11 -(the signs in the superscript denote different circular polarizations, + denotes the HE-mode left-hand circular polarization state and the EH-mode right-hand circular polarization state, -denotes the HE-mode right-hand circular polarization state and the EH-mode left-hand circular polarization state), respectively), and the difference in refractive index between the different orbital angular momentum modes is at least about 3 x 10-4And the requirement of effective separation of orbital angular momentum modes is completely met.With the structure of fig. 10, the transmission conditions of the optical fiber in the helical structure formed by different orbital angular momentum modes are observed. When no Gaussian spherical wave reference signal is added, the circular light spot corresponding to the orbital angular momentum mode is displayed, and the observed results are shown in FIGS. 9(c) and (d); when the gaussian spherical wave reference signal is added, it shows helical interference fringes, and the observed results are shown in fig. 9(a), (b). Wherein FIGS. 9(a), (c) show the post-interference fringes and pre-interference spots of the +2 order orbital angular momentum mode, respectively; FIGS. 9(b) and (d) show interference fringes of the-2 nd order orbital angular momentum mode and a spot before interference, respectively.
Claims (3)
1. A method for separating an optical fiber orbital angular momentum mode based on a helical structure is characterized by comprising the following steps:
(1) calculating the angular effective refractive index difference between the modes newly formed in the spiral optical fiber and meeting the requirement of the effective separation of the orbital angular momentum mode according to the requirement of the total effective refractive index difference between the modes required by the effective separation of the orbital angular momentum mode and the longitudinal effective refractive index value of the orbital angular momentum mode in the original optical fiber, wherein the total effective refractive index of the orbital angular momentum mode in the spiral optical fiber consists of the longitudinal effective refractive index in the original optical fiber and the angular effective refractive index newly formed in the spiral optical fiber;
(2) calculating the specially designed spiral period length required by the angular effective refractive index difference calculated in the step (1) by utilizing the characteristics of the newly formed angular effective refractive index in the spiral fiber, the circular polarization state of the orbital angular momentum mode in the spiral fiber, the orbital angular momentum order, the working wavelength and the spiral period length of the spiral fiber; the spiral period length Λ for a particular design needs to satisfy the formula:
Λ＝min{Λp,q}
wherein subscripts p and q denote two orbital angular momentum modes p and q in the original fiber; min { } represents taking a minimum value operation sign for obtaining the minimum spiral period length required by any two mode separations; lambdap,qThe period of the helix of the particular design required to represent the separation of orbital angular momentum modes p and q is longThe degree, specifically expressed as:
wherein the subscripts p and q represent two orbital angular momentum modes p and q in the helical fiber; the | | is an absolute value calculation symbol; spAnd sqThe spin quantum numbers of the two orbital angular momentum modes are respectively, and the values of +1 or-1 respectively represent the left-handed and right-handed circular polarization states; lpAnd lqThe orders of the orbital angular momentum of the two orbital angular momentum modes are respectively integers; lambda [ alpha ]minA minimum wavelength within a range of operating wavelengths for the orbital angular momentum mode; Δ n is the effective refractive index difference between the orbital angular momentum modes desired to be obtained, and Δ n > 1 × 10-4；
Respectively the longitudinal effective refractive indexes of the two orbital angular momentum modes in the original optical fiber before torsion, and meet the condition
(3) Heating the original optical fiber, and controlling the movement and rotation speed of the original optical fiber to make the original optical fiber be twisted to form the spiral optical fiber with the specific designed spiral period length calculated in the step (2).
2. The method for separating orbital angular momentum modes of an optical fiber based on a helical structure according to claim 1, wherein in the step (1), the original optical fiber is any optical fiber having a circularly symmetric structure.
3. The method for separating orbital angular momentum mode of optical fiber based on helical structure as claimed in claim 1, wherein in the step (3), the heating and twisting method is to perform coaxial twisting around the center of the heated original optical fiber, and finally form the helical optical fiber of coaxial helical structure with specially designed helical period length; the heating mode of the optical fiber is selected from an oxyhydrogen flame or a carbon dioxide laser.
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| CN112698440A (en) * | 2021-01-28 | 2021-04-23 | 深圳大学 | Spiral fiber grating, preparation method and all-fiber orbital angular momentum beam generator |
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