CN111323903A - Optical fiber orbital angular momentum mode separation method based on spiral structure - Google Patents

Abstract

The invention discloses an optical fiber orbital angular momentum mode separation method based on a helical structure. Take the value of the longitudinal effective refractive index in the original fiber, and calculate the angular effective refractive index difference between the newly formed modes in the helical fiber to meet the requirements for effective separation of the orbital angular momentum modes; use the newly formed angular direction in the helical fiber. The characteristic of the effective refractive index in relation to the helical period length, calculate the helical period length of the specific design required to satisfy the angular effective refractive index difference calculated in the previous step; heat the original fiber while controlling the movement and rotation speed of the original fiber , twist it to form a helical fiber. The invention ensures the reliable, stable and multiplexed long-distance transmission of the orbital angular momentum mode in the designed helical fiber, and realizes the common multiplexing of the circular polarization direction and the order of the orbital angular momentum.

Description

A Method for Separating Optical Fiber Orbital Angular Momentum Mode Based on Helical Structure

technical field

The invention relates to the technical fields of optical communication and optical sensing, in particular to a method for separating optical fiber orbital angular momentum modes based on a helical structure.

Background technique

Optical orbital angular momentum is widely used in optical communication, optical sensing and particle manipulation. Optical fibers that support stable multiplexing of multiple orbital angular momentum modes are important optical devices for optical orbital angular momentum applications. The traditional fiber is to increase the refractive index difference between the core and the cladding, or design a special structure of the fiber to increase the refractive index difference between the HE mode and the EH mode (or TE/TM mode) belonging to the same degenerate mode. , so that it can realize the orbital angular momentum mode with the same spin and orbital angular momentum (based on the HE mode) and the orbital angular momentum mode with the same spin and orbital angular momentum (based on the EH or TE/TM mode) of orbital angular momentum). 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 with the same order of orbital angular momentum but different spin directions, and require complex fiber design technology and processing technology. At the same time, it is difficult for such specially designed optical fibers to achieve low-loss connections with traditional optical fibers in optical fiber communication and sensing systems.

A large number of patents and papers at home and abroad discuss how to design optical fibers that can stably transmit various orbital angular momentum modes, but they are all designed from the perspective of increasing the effective refractive index in the direction of light propagation (longitudinal), and no helical structure is used. Fiber to increase the angular effective refractive index to achieve reliable, stable and multiplexed long-distance transmission of different orbital angular momentum modes in fiber. In addition, the method of increasing the refractive index in the propagation direction can only increase the refractive index difference with spin and orbital angular momentum in the same direction (HE mode angular momentum) and opposite (EH mode angular momentum), but cannot increase the refractive index difference with the same orbital The angular momentum order, but the refractive index difference between the orbital angular momentum modes of different spin directions, enables efficient separation and multiplexing of such modes.

There are also a large number of papers at home and abroad discussing the fabrication of helical long-period fiber gratings, which are used in various orbital angular momentum generators, or circular polarization converters, and torsion sensors and other applications. However, most of these orbital angular momentum generators generate higher-order modes through mode coupling, and then generate a phase difference of π/2 between the odd and even modes belonging to the same vector mode HE mode (EH mode or TE/TM mode) to To form orbital angular momentum modes, no method has been proposed to increase the angular effective refractive index through the helical structure in the fiber to achieve mode separation of different orbital angular momentums. In addition, the helical fiber grating has mode conversion characteristics near the coupling wavelength, which leads to the unstable and reliable transmission of the modes participating in the coupling.

SUMMARY OF THE INVENTION

The technical problem to be solved by the present invention is to provide an optical fiber orbital angular momentum mode separation method based on a helical structure, which can ensure its reliable, stable and multiplexed long-distance transmission in the designed helical fiber, and realize the circular polarization direction and Common multiplexing of orbital angular momentum orders.

In order to solve the above-mentioned technical problems, the present invention provides a method for separating optical fiber orbital angular momentum modes based on a helical structure, comprising the following steps:

(1) According to the requirement of the total effective refractive index difference between the modes required for the effective separation of the orbital angular momentum mode, and the value of the longitudinal effective refractive index of the orbital angular momentum mode in the original fiber, the calculation meets the effective separation of the orbital angular momentum mode. The required angular effective refractive index difference between newly formed modes in the helical fiber, the total effective refractive index of the orbital angular momentum modes in the helical fiber is determined by the longitudinal effective refractive index in the original fiber and the newly formed in the helical fiber The angular effective refractive index composition of ;

(2) Using the newly formed angular effective refractive index in the helical fiber, which is related to the circular polarization state of the orbital angular momentum mode in the helical fiber, the order of the orbital angular momentum, the working wavelength and the helical period length of the helical fiber, Calculate the helical period length of the specific design that meets the needs of the angular effective refractive index difference calculated in step (1);

(3) Heating the original fiber while controlling the movement and rotation speed of the original fiber to twist the original fiber to form a helical fiber with a specifically designed helical period length calculated in step (2).

Preferably, in step (1), the original optical fiber is any optical fiber with a circularly symmetric structure, and the types of optical fibers include single-mode optical fibers, few-mode optical fibers, multi-mode optical fibers, hollow-core optical fibers, ring-core optical fibers, multi-core optical fibers, and photonic crystal fibers. .

Preferably, in step (2), the specially designed helical period length Λ needs to satisfy the formula:

Λ=min{Λ _p,q }

Among them, the subscripts p and q represent the two orbital angular momentum modes p and q in the original fiber; min{} represents the minimum value operation symbol, which is used to obtain the minimum helical period length required for the separation of any two modes; Λ _{p ,q} denotes the specially designed helical period length required to separate the orbital angular momentum modes p and q, and is expressed as:

分别是两个轨道角动量模式在扭转前原始光纤中的纵向有效折射率，满足条件

Among them, the subscripts p and q represent the two orbital angular momentum modes p and q in the helical fiber; || is the absolute value operation symbol; s _p and s _q are the spin quantum numbers of the two orbital angular momentum modes, respectively, The value of +1 or -1 represents the left-handed and right-handed circular polarization states, respectively; l _p and l _q are the order of the orbital angular momentum of the two orbital angular momentum modes, and the value is an integer; λ _min is the orbital angular momentum mode of the The minimum wavelength in the working wavelength range; Δn is the effective refractive index difference between the desired orbital angular momentum modes, generally Δn>1×10 ^-4 ;

are the longitudinal effective refractive indices of the two orbital angular momentum modes in the original fiber before twisting, satisfying the condition

Preferably, in step (3), the heating and twisting method is to perform coaxial twisting around the center of the heated original optical fiber, and finally form a helical optical fiber with a coaxial helical structure of a specific designed helical period length; the method of heating the optical fiber is to use oxyhydrogen The heating method of flame or carbon dioxide laser heating.

The beneficial effects of the present invention are as follows: the manufacturing method of the present invention is simple, can be realized on traditional optical fiber processing equipment, and overcomes the need for special special Optical fiber design and processing technology are difficult to produce on a large scale and at low cost; the surface of the optical fiber processed according to the proposed design and production method has basically no deformation and damage, and the original transmission characteristics can be maintained, and the energy is lower than that of the original optical fiber before twisting. Loss connection; through the present invention, the problem that the conventional orbital angular momentum optical fibers cannot be separated with the same orbital angular momentum order and different circular polarization directions can also be solved, that is, the design and manufacture of the present invention can be realized by the spiral structure. Multiplexed transmission of orbital angular momentum and circular polarization; the application scope of the present invention includes not only helical fibers, but also devices such as orbital angular momentum mode couplers, converters, generators and amplifiers based on helical fibers designed using this method.

Description of drawings

FIG. 1 is a schematic diagram of the structural principle of forming a helical optical fiber according to the present invention.

FIG. 2 is a schematic diagram of the experimental principle of making a helical optical fiber according to the present invention.

FIG. 3 is a schematic diagram of a helical optical fiber actually produced based on a single-mode optical fiber observed by the microscope of the present invention.

FIG. 4 is a schematic diagram of the simulation result of the mode effective refractive index of the original single-mode optical fiber before the helical structure is formed in the present invention.

FIG. 5 is a schematic diagram of 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 the interference fringes of the +1-order orbital angular momentum mode and Gaussian spherical wave separated by the present invention.

Fig. 6(b) is a schematic diagram of the interference fringes of the -1 order orbital angular momentum mode and Gaussian spherical wave separated by the present invention.

FIG. 6( c ) is a schematic diagram of the light spot of the +1 orbital angular momentum mode separated by the present invention.

FIG. 6(d) is a schematic diagram of the light spot of the -1 orbital angular momentum mode separated by the present invention.

FIG. 7 is a schematic diagram showing the simulation result of the mode effective refractive index of the original four-mode optical fiber before the helical structure is formed in the present invention.

FIG. 8 is a schematic diagram showing the simulation result of the mode effective refractive index of the helical four-mode optical fiber after the helical structure is formed according to the present invention.

FIG. 9( a ) is a schematic diagram of the interference fringes of the +2-order orbital angular momentum mode and the Gaussian spherical wave separated by the present invention.

FIG. 9(b) is a schematic diagram of the interference fringes of the -2-order orbital angular momentum mode and Gaussian spherical wave separated by the present invention.

FIG. 9( c ) is a schematic diagram of the light spot of the +2 orbital angular momentum mode separated by the present invention.

FIG. 9(d) is a schematic diagram of the light spot of the -2 orbital angular momentum mode separated by the present invention.

FIG. 10 is a schematic diagram of the observation principle of the present invention for observing and confirming mode separation.

Detailed ways

The technical solution of the present invention is realized by synchronously controlling the heating, movement and rotation of the original optical fiber. The raw fiber heated to the molten state is moved and rotated synchronously to form a helical fiber with a specially designed helical period length structure. The helical period length Λ of the particular design formed is determined by the moving speed v (mm/s-mm/sec) and the rotation speed w (turn/s-turn/sec) of the original fiber, expressed as Λ=v/w (mm/ turn-mm/turn). Due to the torsional tangential force when heated, a specially designed helical period length structure is formed in the cooled helical fiber, which has a uniform helical refractive index distribution. Under the influence of the helical refractive index distribution, the orbital angular momentum mode in the helical fiber not only has the longitudinal propagation characteristics of the ordinary fiber mode, but also has the angular phase change characteristics related to the propagation direction. This angular phase variation along the propagation direction can be represented by the angular refractive index. The angular refractive index and the longitudinal effective refractive index work together and appear as a phase change along the direction of propagation (equivalent to the total propagation constant or total effective refractive index change). The schematic diagram of the structure of the helical fiber is shown in Figure 1. The helical structure in the figure is only used to illustrate the structural principle and reflects the refractive index change characteristics of the helical fiber. Such a physical structure cannot be observed in the actual helical fiber. It can be seen from the figure that the effective propagation direction of the light in the optical fiber forming the helical structure is the helical direction, and the propagation in the helical direction can be orthogonally decomposed into the propagation along the longitudinal direction and the angular propagation along the cross-sectional direction. The total effective refractive index includes the effective refractive index in the longitudinal direction and the effective refractive index in the angular direction.

其中l和m分别表示横截面中角向和径向的阶数。则该轨道角动量模式在扭转后的形成螺旋结构的螺旋光纤中的有效折射率

可表示为：Let the longitudinal effective refractive index of the orbital angular momentum mode (l,m) in the original fiber before twisting be

where l and m represent the angular and radial orders in the cross section, respectively. Then the effective refractive index of the orbital angular momentum mode in the twisted helical fiber forming the helical structure

can be expressed as:

部分是轨道角动量模式(l,m)在具有螺旋结构的光纤中角向相位沿着传播方向变化的体现，即角向有效折射率。其中s为自旋量子数，s＝1和-1分别表示左旋和右旋圆偏振状态；σ为螺旋光纤的扭转方向，σ＝1，或者-1分别表示左手或者右手螺旋结构；λ为轨道角动量模式工作周期；Λ为特定设计的螺旋周期长度。in the above formula

The part is the embodiment of the orbital angular momentum mode (l,m) in the fiber with the helical structure of the angular phase change along the direction of propagation, that is, the angular effective refractive index. where s is the spin quantum number, s=1 and -1 represent the left-handed and right-handed circular polarization states, respectively; σ is the twist direction of the helical fiber, σ=1, or -1 represents the left-handed or right-handed helical structure, respectively; λ is the orbital Angular momentum mode duty cycle; Λ is the helical cycle length for a specific design.

In order to effectively separate different orbital angular momentum modes, the effective refractive index difference between the two modes generally needs to satisfy the condition of Δn>1×10 ⁻⁴ . Since the angular effective refractive index can be equivalent to the longitudinal effective refractive index, the longitudinal effective refractive index difference can generally only be achieved by a specially designed fiber structure or by increasing the refractive index difference between the core and the cladding. Therefore, in the present invention, it is innovatively proposed to realize the effective refractive index difference requirement through the change of the angular effective refractive index. The helical period in the helical fiber should at least satisfy:

分别是两个模式在扭转前的原始光纤中的纵向有效折射率，

Among them, the subscripts p and q represent any two orbital angular momentum modes p and q in the helical fiber; || is the absolute value operation symbol; s _p and s _q are the spin quantum numbers of the two modes, respectively, and the value 1 or -1 represents the left-handed and right-handed circular polarization states, respectively; l _p and l _q are the orbital angular momentum orders of the two modes, respectively, which are integers; λ _min is the minimum wavelength within the operating wavelength range of the mode; Δn is the effective refractive index difference between the desired modes, generally Δn>1×10 ^-4 ;

are the longitudinal effective refractive indices of the two modes in the pristine fiber before twisting, respectively,

When there are multiple orbital angular momentum modes in the helical fiber, the helical period length Λ of a specific design needs to satisfy the formula:

Λ=min{Λp _,q }(3)

Among them, the subscripts p and q represent the two orbital angular momentum modes p and q in the original fiber; min{} represents the minimum value operation symbol, that is, the minimum value is taken in the helical period required to separate any two modes.

When the above conditions are satisfied, there are two orbital angular momentum modes with the same angular momentum order and different spin directions, that is, the HE mode and the spin and orbital angles belonging to the same degenerate mode with the same spin and orbital angular momentum in the same direction The effective refractive index difference between EH modes with reversed momentum can reach more than 2×10 ^-4 , which can achieve effective mode separation. At the same time, the effective refractive index difference between the two orbital angular momentum modes in the same spin direction and different angular momentum directions can reach more than 1×10 ^-4 , which can also achieve effective mode separation. For any other different orbital angular momentum modes, due to the existence of 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 ^-4 or more, It can realize its effective separation in the designed helical fiber, and meet the requirements of long-distance stable, reliable and multiplexing transmission of different modes in the fiber.

When the period Λ satisfies the resonance condition between some modes in the helical fiber grating in a specific wavelength range, the coupling energy exchange between these modes at a specific wavelength will occur, that is, the orbital angular momentum mode conversion will be realized. Such periods should be avoided to ensure reliable and efficient long-distance transmission of these modes at specific wavelengths.

The implementation steps of the present invention are as follows: the optical fiber to be processed is fixed between the holder and the rotator, and the holder and the rotator are respectively fixed on two translation stages. During production, the fiber is first heated by a carbon dioxide laser or a hydrogen-oxygen flame. When the fiber is heated to a molten state, the moving speed and rotation speed of the helical fiber are synchronously controlled by synchronously controlling the movement of the translation stage and the rotation of the rotator. The period Λ of the helical structure is determined by the moving speed v (mm/s-mm/sec) and the rotation speed w (turn/s-revolution/sec), expressed as Λ=v/w (mm/turn-mm/revolution) . During production, it is necessary to control the speed of movement and rotation at the same time, so that the period of the formed helical structure satisfies the above formula (3).

Example 1: Taking a single-mode fiber as an example, the separated transmission of the first-order orbital angular momentum in the cladding is realized.

)进行加热扭转。图中为保证加热区的均匀性，采用蓝宝石管(Sapphire tube)形成加热区，用光开关(Shutter)控制二氧化碳激光器(CO₂ laser)的通断。被加工原始光纤(Optical fiber)一头固定(Clamp)，并用重物(Weight)保持加工中的原始光纤的拉直，另一头固定装入旋转器(Rotator)。宽光源(ASE)和光谱分析仪(OSA)用于观测制作中的螺旋光纤特性，如损耗特性等。当原始光纤加热到一定温度时，移动台(Stage)和旋转台(Rotator)在计算机(Computer)中控制软件控制下，同步工作，实现同时移动和旋转。形成螺旋结构的周期Λ由移动速度v(mm/s-毫米/秒)和旋转速度w(turn/s-转/秒)决定，表示为Λ＝v/w(mm/turn-毫米/转)。图3为显微镜观测的基于单模光纤(Fujikura Inc，

)实际制作的螺旋光纤。由图可见，加热扭转后的光纤直径形变约为1-3um，几乎不会改变原来单模光纤的传输特性，形成的螺旋光纤可以和原光纤实现低损耗的连接。形成螺旋结构前后的单模光纤(Fujikura Inc，

)中的不同模式有效折射率如图4和图5所示。图4为形成螺旋结构之前的原始单模光纤(Fujikura Inc，

)的模式有效折射率仿真结果。图中光纤纤芯和包层的半径分别为4.1um和62.5um，折射率分别为1.4580和1.4536，图中的有效折射率结果采用有限元方法仿真计算得到。由图可见，HE₂₃、TE₀₃和TM₀₃模式(模式下标分别顺序表示模式的角向和径向的阶数)的有效折射率差极小，会在原始光纤里简并，难以分离独立传播。图5为形成螺旋结构之后的螺旋单模光纤(Fujikura Inc，

)的模式有效折射率，图中光纤纤芯和包层的半径分别为4.1um和62.5um，折射率分别为1.4580和1.4536，螺旋半径为1cm，图中结果采用有限元方法仿真计算得到。由图可见在1450nm到1650nm波长范围内的不同的轨道角动量模式HE₂₃ ⁺、HE₂₃ ^-和TE/TM₀₃ ^+/-(上标中的正负符号分别表示不同的圆偏振，+表示左旋圆偏振状态，-表示右旋圆偏振状态)，不同轨道角动量模式之间的有效折射率差约为至少3×10^-4，完全满足轨道角动量模式有效分离的要求。The original single-mode fiber (Fujikura Inc,

) for heating and twisting. In the figure, in order to ensure the uniformity of the heating area, a sapphire tube is used to form the heating area, and an optical switch (Shutter) is used to control the on-off of the carbon dioxide laser (CO ₂ laser). One end of the processed original optical fiber (Optical fiber) is fixed (Clamp), and a weight (Weight) is used to keep the processed original optical fiber straight, and the other end is fixed into a rotator (Rotator). A broad light source (ASE) and an optical spectrum analyzer (OSA) were used to observe the properties of the helical fiber in production, such as loss characteristics. When the original optical fiber is heated to a certain temperature, the mobile stage (Stage) and the rotary stage (Rotator) work synchronously under the control of the control software in the computer (Computer) to achieve simultaneous movement and rotation. The period Λ of the helical structure is determined by the moving speed v (mm/s-mm/sec) and the rotation speed w (turn/s-revolution/sec), expressed as Λ=v/w (mm/turn-mm/revolution) . Figure 3 is a microscope observation based on a single-mode fiber (Fujikura Inc,

) actually fabricated helical fibers. It can be seen from the figure that the diameter deformation of the fiber after heating and twisting is about 1-3um, which hardly changes the transmission characteristics of the original single-mode fiber, and the formed helical fiber can achieve low-loss connection with the original fiber. Single-mode fiber before and after formation of the helical structure (Fujikura Inc,

) in different modes of effective refractive index are shown in Fig. 4 and Fig. 5. Figure 4 shows the original single-mode fiber (Fujikura Inc,

) mode effective refractive index simulation results. In the figure, the radii of the fiber core and cladding are 4.1um and 62.5um, respectively, and the refractive indices are 1.4580 and 1.4536, respectively. The effective refractive index results in the figure are calculated by the finite element method. It can be seen from the figure that the effective refractive index difference of the HE ₂₃ , TE ₀₃ and TM ₀₃ modes (the mode subscripts represent the angular and radial orders of the modes respectively) is extremely small, which will be degenerate in the original fiber and difficult to separate and independent. spread. Figure 5 is a helical single-mode optical fiber (Fujikura Inc,

), the radii of the fiber core and cladding in the figure are 4.1um and 62.5um, the refractive indices are 1.4580 and 1.4536, respectively, and the helix radius is 1cm. The results in the figure are calculated by the finite element method. It can be seen from the figure that the different orbital angular momentum modes HE ₂₃ ⁺ , HE ₂₃ ^- and TE/TM ₀₃ ^+/- in the wavelength range from 1450nm to 1650nm (the positive and negative symbols in the superscript represent different circular polarizations, + represents left-handed Circular polarization state, - denotes right-handed circular polarization state), the effective refractive index difference between different orbital angular momentum modes is about at least 3×10 ^-4 , which fully meets the requirement of effective separation of orbital angular momentum modes.

Through the structure of FIG. 10 , the transmission conditions of the optical fibers forming the helical structure with different orbital angular momentum modes are observed. The Tunable Laser outputs the light corresponding to the observed wavelength, and after being coupled by the Coupler, it is input to the Mode Converter all the way to obtain the corresponding orbital angular momentum mode. The Gaussian spherical wave output from the other channel interferes with the reference signal, and the interference fringes are observed by CCD, and the polarizer (PC) and the attenuator (Attenuator) are used to adjust the clarity of the interference fringes. When no Gaussian spherical wave reference signal is added, it is displayed as a ring spot corresponding to the orbital angular momentum mode, and the observed results are shown in Figure 6(c), (d); when the Gaussian spherical wave reference signal is added, it is displayed as a spiral shape interference fringes, and the observed results are shown in Fig. 6(a), (b). Figures 6(a) and (c) show the interference fringes and pre-interference spots of the +1-order orbital angular momentum mode, respectively; Figures 6(b) and (d) show the interference fringes and the -1-order orbital angular momentum mode, respectively. Spot before interference.

Example 2: Taking a four-mode fiber as an example, the separated transmission of the second-order orbital angular momentum in the fiber core is realized.

The original four-mode optical fiber (YOFC, step-type four-mode optical fiber) is heated and twisted using the structure shown in Figure 2, and the heating and twisting method is the same as that of the single-mode optical fiber. The effective refractive indices of different modes in the four-mode fiber (YOFC, step-type four-mode fiber) before and after forming the helical structure are shown in Figures 7 and 8. Fig. 7 shows the simulation results of the mode effective refractive index of the original four-mode fiber (YOFC, step-type four-mode fiber) before the helical structure is formed. In the figure, the radii of the fiber core and cladding are 9.5um and 62.5um, respectively, and the refractive indices are 1.4499 and 1.444, respectively. The effective refractive index results in the figure are calculated by the finite element method. It can be seen from the figure that the effective refractive index difference of the modes HE ₃₁ and EH ₁₁ (the mode subscripts indicate the angular and radial orders of the modes respectively) is extremely small, which will form a degenerate mode in the fiber, which is difficult to separate and propagate independently. FIG. 8 is the simulation result of the mode effective refractive index of the helical four-mode fiber (YOFC, step-type four-mode fiber) after the helical structure is formed. In the figure, the radii of the fiber core and cladding are 9.5um and 62.5um respectively, the refractive indices are 1.4499 and 1.444, respectively, and the helix radius is 1cm. The effective refractive index results in the figure are calculated by the finite element method. It can be seen from the figure that the different orbital angular momentum modes HE ₃₁ ⁺ , HE ₃₁ ^- , EH ₁₁ ⁺ and EH ₁₁ ^- in the wavelength range from 1450nm to 1650nm (the positive and negative symbols in the superscript represent different circular polarizations, respectively, + represents the HE mode Left-handed circular polarization state and EH mode right-handed circular polarization state, - means HE-mode right-handed circular polarization state and EH mode left-handed circular polarization state), and the refractive index difference between different orbital angular momentum modes is about at least 3×10 ^-4 , which fully meets the requirements for effective separation of orbital angular momentum modes. Through the structure of FIG. 10 , the transmission conditions of the optical fibers forming the helical structure with different orbital angular momentum modes are observed. When no Gaussian spherical wave reference signal is added, it is displayed as a ring spot corresponding to the orbital angular momentum mode, and the observed results are shown in Figure 9(c), (d); when the Gaussian spherical wave reference signal is added, it is displayed as a spiral shape interference fringes, and the observed results are shown in Fig. 9(a), (b). Figures 9(a) and (c) show the interference fringes and pre-interference spots of the +2-order orbital angular momentum mode, respectively; Figures 9(b) and (d) show the interference fringes and the -2-order orbital angular momentum mode, respectively. Spot before interference.

Claims (4)

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;

(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 optical fibers based on a helical structure according to claim 1, wherein in the step (1), the original optical fiber is any optical fiber with a circularly symmetric structure, and the optical fiber types include single mode optical fiber, few mode optical fiber, multi-mode optical fiber, hollow core optical fiber, ring core optical fiber, multi-core optical fiber and photonic crystal optical fiber.

3. The method for separating orbital angular momentum modes of an optical fiber based on a helical structure as claimed in claim 1, wherein in the step (2), the period length Λ of the specially designed helix satisfies the following formula:

Λ＝min{Λ_p,q}

wherein subscripts p and q represent two orbital angular momentum modes p and q in the original fiber, min { } represents the minimum operation sign for obtaining the minimum spiral period length required by any two mode separations, Λ_p,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; s_pAnd s_qThe 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; l_pAnd l_qThe orders of the orbital angular momentum of the two orbital angular momentum modes are respectively integers; lambda [ alpha ]_minIs the minimum wavelength in the working wavelength range of the orbital angular momentum mode, and is the effective refractive index difference between the orbital angular momentum modes expected to be obtained, and is taken as delta 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

4. 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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