LP01-LP11-OAM mode conversion method for a single single-mode optical fiber

Abstract

The present disclosure provides a method for converting the LP01-LP11-OAM modes of a single single-mode optical fiber in the field of all-optical fiber technology. The method can be used to convert a single single-mode optical fiber from the LP01 mode to the LP11 mode and the OAM mode at low cost and quickly. The method is carried out according to the following steps: twisting an ordinary single-mode optical fiber with an LP01 mode at an odd multiple of 45°, using a heating and pulling method to stretch and break the end of the optical fiber and then cut it to obtain the LP11 mode, and using electrode discharge technology to prepare a hemispherical optical fiber lens at the cut to obtain the OAM mode. When the method proposed in the present disclosure performs optical fiber mode conversion, no other additional optical elements are inserted, and low-cost and safe conversion based on the LP01-LP11-OAM mode of a single single-mode optical fiber can be achieved.

Description

Technical Field

[0001] The present disclosure relates to the field of all-optical fibers, and in particular to a method for performing LP01-LP11-OAM optical mode conversion on ordinary single-mode optical fibers. Background Art

[0002] In the field of optical fiber waveguides, linear polarization (LP) mode and orbital angular momentum (OAM) mode are regarded as the basic entities in optical waveguides. At present, the methods for optical mode conversion can be roughly divided into two categories, one is to obtain optical mode conversion by using optical device modulation technology, and the other is to achieve the conversion of LP mode or OAM mode by optical fiber modulation method.

[0003] In the existing optical device modulation technology, H. Guo et al. use spatial light modulators (SLM) and phase plates to achieve spatial mode modulation, but this method has high requirements on the precision of spatial structure, requires careful integration of various components, and the cost of optical components is relatively high. K. Li et al. use silicon-based waveguide models to modulate optical devices, which can confine mode manipulation to the inside of the chip and realize the mutual conversion of LP mode and OAM mode through algorithmic control of the chip. However, this method relies heavily on the effectiveness of the algorithm and the computing power of the platform.

[0004] In the existing fiber modulation methods, the conversion from LP01 mode to LP11 mode is generally achieved by fusing two single-mode fibers (SMF) or one single-mode fiber and one multimode fiber (MMF). For example, Chen Zilun et al. proposed a method for preparing a fiber end-face pump coupler based on a twisting method (Chinese patent document: CN 112397983 A), and Xiao Limin et al. proposed a low-loss and high-polarization-maintaining fiber mode field adapter and its preparation method (Chinese patent document: CN

[0005] 116184564A). In these technical solutions, the method of fiber taper is adopted, but the operation form of coupling two optical fibers is selected in the preparation process. Therefore, it will be subject to certain limitations of fiber coupling operation. In addition, although Zhang Cong et al. proposed an optical fiber with a mode field diameter that changes slowly along the taper direction (patent: CN 115857088 A), which also obtains the change of the fiber mode field through the form of fiber taper, it uses a device bridge fiber with a gradually distributed core refractive index. Compared with ordinary single-mode optical fibers, the cost is higher and the conversion from LP11 to OAM mode cannot be realized. In addition, Z.Bai et al. proposed the use of LPFG (Long Period Fiber Grating) to realize the conversion from LP mode to OAM mode. Although this method combines stress with optical device modulation, it puts forward certain requirements on the spatial structure of optical devices and optical fibers.

[0006] In summary, the prior art lacks a relatively simple and low-cost method for optical fiber mode conversion. Summary of the invention

[0007] In order to solve the technical problems existing in the background technology, the present disclosure proposes a LP01-LP11-OAM mode conversion method for a single single-mode optical fiber. By using this method, only one ordinary single-mode optical fiber (SMF) is used. Through twisting, tapering, and flattening processing methods, the single-mode optical fiber can excite the LP11 mode with a twisted orthogonal angle characteristic from the LP01 mode. Subsequently, a hemispherical lens is manufactured at the end of the optical fiber cut by an electrode discharge method, and finally the orbital angular momentum (OAM) mode is realized, which can realize the LP01-LP11-OAM mode conversion without performing other complex operations such as multi-fiber coupling or optical device modulation.

[0008] The LP01-LP11-OAM mode conversion method of a single single-mode optical fiber disclosed in the present invention is unique in that it is carried out according to the following steps: one end of the single single-mode optical fiber is not processed, and the other end of the single single-mode optical fiber is twisted, tapered, cut and electrode-discharged in sequence, so that after the processing, the other end of the single single-mode optical fiber has a tapered transition area and a hemispherical optical fiber lens area.

[0009] Further: using an XZ-axis optical fiber electric taper machine 2 and a laser 4;

[0010] Fix the unprocessed end of the single single-mode optical fiber to the optical fiber holder of the X-axis slide rail of the electric taper drawing machine as the optical fiber input end, and fix the other end to the optical fiber holder at the center axis of the Z-axis rotating table of the electric taper drawing machine; adjust the angle of the Z-axis rotating table to an odd multiple of 45°, so that the optical fiber fixed on the axis of the rotating table is also twisted around the central axis by a corresponding angle;

[0011] Adjust the distance between the Z-axis rotary table and the X-axis electric slide rail to within the range of (10cm, 13cm) to ensure that there is enough space for the subsequent motor to pull the optical fiber;

[0012] The optical fiber input end is connected to the output end of the laser 4 .

[0013] Further, an alcohol lamp 5 and a host computer PC terminal 3 are used;

[0014] An alcohol lamp 5 is placed in the middle position between the optical fiber holder of the X-axis slide rail of the electric taper pulling machine and the optical fiber holder at the center axis of the Z-axis rotating table of the electric taper pulling machine. After the optical fiber is heated to the melting point, the electric taper pulling machine is started using the host computer PC terminal 3. The optical fiber is stretched to form a tapered transition zone and then broken to obtain a tapered optical fiber tip. After breaking, the single-mode optical fiber with the LP01 mode is converted into a tapered optical fiber with multiple mode aliasing.

[0015] Further, a fiber optic cutter 6 is applied;

[0016] The tip of the tapered optical fiber is flattened until the fiber core diameter is in the range of 2.52 μm to 3.95 μm. After the processing, the tapered optical fiber with multiple mode aliasing is converted into a flat-end optical fiber with LP11 mode. The processed tapered optical fiber can obtain two LP11 light spots with corresponding torsional orthogonal angles when a laser with a wavelength of 650 nm is passed through it.

[0017] Further, a fiber splicing machine 7 is used;

[0018] The flat-end optical fiber end is placed in a fiber splicing machine 7, and a hemispherical optical fiber lens with a diameter of 15.5 μm is prepared by electrode discharge. The flat-end optical fiber in LP11 mode is converted into a hemispherical optical fiber in OAM mode.

[0019] Furthermore, the single single-mode optical fiber adopts G.652D Corning SMF-28e, the cladding diameter is 125 μm, and the optical fiber core diameter is 8.2 μm;

[0020] Use a fiber stripper to strip off the outermost protective sheath and coating of the optical fiber.

[0021] At least one of the above technical solutions adopted in one or more embodiments of this specification can achieve the following beneficial effects:

[0022] Firstly, the method provided by the present disclosure can obtain corresponding mode conversion by operating only a single common single-mode optical fiber, and has the characteristics of low cost and good applicability.

[0023] Secondly, after twisting, tapering and flattening the optical fiber according to the method provided by the present disclosure, since an SMF optical fiber with LP01 mode is subjected to When the stress of twisting angle is stretched, the circular symmetry of the optical fiber will be destroyed. According to the birefringence effect of the stressed optical fiber, the optical fiber will be decomposed into two linear polarization components along the fast axis F and the slow axis S. The fast axis component F leads the slow axis component S. Therefore, when the specified area is cut flat, the polarization LP11 mode with the same twist angle can be excited in the ordinary single-mode fiber LP01. On this basis, a hemispherical fiber lens is prepared by using the electrode discharge function of the fiber fusion machine, which is equivalent to introducing A circular polarizer is used, and the hemispherical lens converges at the tip of the optical fiber output to superimpose the light spots with phase differences to form an OAM mode. This method can obtain the high-order polarization mode and OAM mode of LP11 by combining the fiber twisting taper and the hemispherical lens prepared at the tip. There are four steps in total, which are simple to operate and can be repeated many times. In addition, the entire process does not require any fiber coupling or optical component modulation, and the operation cost is low and the success rate is high.

[0024] In addition, the mode conversion method provided by the present disclosure can convert single-mode optical fibers into different modes, and adjusting the torsion angle can produce different light intensities and light distributions. The OAM mode has better light transmission characteristics. Through experimental data analysis, it can be seen that the 270° LP11 mode and the 225° OAM mode have better coupling efficiency. This mode conversion method facilitates the conversion of a single SMF into various modes. The light distribution of both modes has one or more high-low-high fluctuations, and its unique trend makes it suitable for use in a variety of fields such as optical signal transmission and optical tweezers capture technology. This optimized optical mode conversion is expected to improve the multi-dimensional capture efficiency of optical tweezers for microsphere particles, and provide a simpler method for the operation and analysis of microscale particles.

[0025] In summary, the method provided by the present invention can achieve the mode conversion of LP01-LP11-OAM by combining four steps of fiber twisting, tapering, flattening and electrode discharge, without the need for other complex operations such as multi-fiber coupling or optical device modulation. It is a relatively simple and low-cost fiber mode conversion method.

[0026] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the disclosure.

[0027] Further features and aspects of the present disclosure will become apparent from the following detailed description of exemplary embodiments with reference to the attached drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The drawings herein are incorporated into the specification and constitute a part of the specification. These drawings illustrate embodiments consistent with the present disclosure and are used to illustrate the technical solutions of the present disclosure together with the specification.

[0029] Figure 1 This is a model diagram of a tapered output port of an optical fiber after tapering in a specific embodiment provided by the present disclosure;

[0030] Figure 2 It is a single-mode optical fiber LP01-LP11-OAM mode conversion model diagram under a specific embodiment provided by the present disclosure;

[0031] Figure 3is a schematic diagram of the angle between the light polarization direction and the optical fiber fast axis in a specific embodiment provided by the present disclosure;

[0032] Figure 4 is a three-dimensional model diagram of LP light field distribution under a specific embodiment provided by the present disclosure;

[0033] Figure 5 is a three-dimensional model diagram of OAM light field distribution under a specific embodiment provided by the present disclosure;

[0034] Figure 6 It is a diagram of LP01-LP11-OAM mode conversion results at different angles under a specific embodiment provided by the present disclosure;

[0035] Figure 7 is a diagram of an experimental device under a specific embodiment provided by the present disclosure;

[0036] Figure 8 is a light field distribution diagram under the LP11 mode in a specific embodiment provided by the present disclosure;

[0037] Fig. 9 is a light field distribution diagram under the OAM mode of a specific embodiment provided by the present disclosure;

[0038] Fig.10 It is a diagram of the taper tip, LP mode and OAM mode output according to a specific embodiment provided by the present disclosure.

[0039] Fig.11 It is a coupling efficiency diagram of LP mode and OAM mode output under a specific embodiment provided by the present disclosure.

[0040] In the figure, 1-ordinary single-mode optical fiber G.652D, 2-electric taper machine, 3-host computer PC, 4-laser, 5-alcohol lamp, 6-optical fiber cutter, 7-optical fiber melting machine DETAILED DESCRIPTION

[0041] Various exemplary embodiments, features and aspects of the present disclosure will be described in detail below with reference to the accompanying drawings. The same reference numerals in the accompanying drawings represent elements with the same or similar functions. Although various aspects of the embodiments are shown in the accompanying drawings, the drawings are not necessarily drawn to scale unless otherwise specified.

[0042] In addition, in order to better illustrate the present disclosure, numerous specific details are given in the following specific embodiments. It should be understood by those skilled in the art that the present disclosure can also be implemented without certain specific details. In some examples, methods and means well known to those skilled in the art are not described in detail in order to highlight the main purpose of the present disclosure.

[0043] The present disclosure provides a method for converting LP01-LP11-OAM modes of a single single-mode optical fiber. The obtained LP11 mode and OAM mode can be applied to the field of fiber optic tweezers particle capture.

[0044] The following specific embodiment uses an SMF optical fiber as the operating object, utilizes the twisting and taper method to excite the LP11 mode with the corresponding twist orthogonal angle from the LP01 mode of the SMF, and utilizes the electrode discharge method to prepare a hemispherical lens at the tip of the optical fiber to obtain the OAM mode.

[0045] The single-mode optical fiber is G.652D Corning (SMF-28e, cladding diameter is 125μm, fiber core diameter is 8.2μm), and a fiber stripper is needed to strip the outermost protective sheath and coating of the optical fiber. One end is unprocessed, and the other end is twisted, tapered, cut, and electrode-discharged to have a tapered transition zone and a hemispherical optical fiber lens zone.

[0046] The single-mode optical fiber LP01-LP11 mode conversion is obtained by cutting the waist region of the optical fiber after twisting and taper processing of the optical fiber.

[0047] Furthermore, the single-mode optical fiber LP11-OAM mode conversion is obtained by discharging the electrodes at the ends of the cut optical fibers to form a hemispherical optical fiber lens.

[0048] The mode conversion method of single-mode optical fiber LP01-LP11-OAM includes the following specific steps:

[0049] Step 1: After stripping off the outermost protective cover and coating of the common single-mode optical fiber, place one end of the optical fiber on the XZ-axis electric fiber taper machine. One end of the optical fiber is fixed on the optical fiber holder of the X-axis slide rail of the taper machine, and the other end is fixed on the optical fiber holder at the center axis of the Z-axis rotating table of the taper machine. Twist the Z-axis rotating table to an odd multiple of 45°, and the end of the optical fiber will also be twisted around the center axis by a corresponding angle. Adjust the distance between the Z-axis rotating table and the X-axis electric slide rail to within the range of 10cm-13cm to ensure that there is enough space for the subsequent motor to pull the optical fiber. The input end of the optical fiber is connected to the output end of the laser 4.

[0050] The twist angle of the Z-axis rotating table on the XZ-axis optical fiber electric taper machine 2 is selected to be an odd multiple of 45°, which has the following characteristics: when a single-mode optical fiber of LP01 mode is subjected to stress stretching with an odd multiple of θ=45° twist angle, the circular symmetry of the optical fiber will be destroyed, thereby causing the birefringence effect of the optical fiber. The optical fiber will be decomposed into two linear polarization components along the fast axis F and the slow axis S. Due to the twist with θ, the fast axis component F of the optical fiber leads the slow axis component S. Phase difference. When the fast axis F direction of SUS forms an angle θ with the x polarization direction, the transmission of the input polarization state in the fiber can be regarded as a superposition process. Therefore, when the fiber end is twisted by an angle θ and then tapered, the relationship between the input and output polarization states is:

[0051]

[0052] Therefore, when the SMF fiber of LP01 mode is twisted at an angle of θ at the end and then tapered, and then flattened in a specified area, a polarized LP11 mode with the same twist angle can be excited, such as Figure 3 shown.

[0053] The XZ-axis optical fiber electric taper drawing machine 2 is configured as follows: the electric taper drawing machine is connected to a 220V power supply, and one port is connected to a host computer PC terminal 3 via a USB data cable.

[0054] The laser 4 is configured as follows: the output of the laser is a wavelength, and the output power is Pout=30 mW.

[0055] Step 2: Place the alcohol lamp 5 between the optical fiber holder of the X-axis slide rail of the electric taper puller 1 and the optical fiber holder at the center axis of the Z-axis rotating table of the electric taper puller 1, heat the optical fiber to the melting point, start the taper puller 2 using the host computer PC terminal 3, stretch the optical fiber 1 to form a tapered transition zone and then break it, so that the single-mode optical fiber with the LP01 mode is converted into a tapered optical fiber with multiple mode aliasing.

[0056] The host computer PC terminal 3 is configured as follows: the host computer PC terminal is used to control the motion state of the X-axis electric slide rail of the XZ-axis optical fiber electric taper drawing machine 2. About 5 seconds after the alcohol lamp 5 is ignited and the optical fiber melts, the X-axis electric slide rail motor of the XZ-axis optical fiber electric taper drawing machine is started through the host computer PC terminal 3. The initial acceleration of the X-axis electric slide rail of the XZ-axis optical fiber electric taper drawing machine in the front stage is set to 0.03mm / s 2 , the duration is 3s, making the fiber thinner. Set the acceleration of the latter stage to 0.05mm / s 2 The optical fiber is quickly broken, forming a cone-like crack at the output end, so that the output light mode is transformed from the LP01 spot to a hollow mixed-mode aperture with multiple mode aliasing.

[0057] Step 3: Use the optical fiber cutter 6 to cut the tip of the tapered optical fiber flat, so that the optical fiber core diameter ranges from 2.52 μm to 3.95 μm, thereby converting the tapered optical fiber with multiple mode aliasing into a flat-end optical fiber with LP11 mode.

[0058] The selection of the optical fiber core diameter range is characterized in that the number of modes propagating in the optical fiber depends on the normalized frequency parameter V of the transmitted light wave in the optical fiber. The normalized parameter V is related to the wavelength of the laser, the optical fiber core radius, the core refractive index and the cladding refractive index, and thus:

[0059]

[0060] When V<2.405, the LP01 mode is excited in the optical fiber; when 2.405≤V≤3.832, the LP11 mode is excited in the optical fiber; and when V>3.832, higher-order modes will be excited in the optical fiber. Therefore, after cutting within the range of 2.52μm to 3.95μm in diameter of the optical fiber core, 2.405≤V≤3.832 can be achieved, thereby realizing the conversion of single-mode optical fiber mode LP01-LP11.

[0061] Step 4: Place the flat-end optical fiber end in a fiber splicing machine (7), and use electrode discharge to prepare a hemispherical optical fiber lens with a diameter of 15.5 μm, so that the LP11 mode is converted into the OAM mode.

[0062] The hemispherical optical fiber lens prepared at the end of the optical fiber is characterized in that: when the twist angle θ of the optical fiber is an odd multiple of 45°, a hemispherical optical fiber lens is prepared on this basis by using the electrode discharge function of the fiber fusion machine, which is equivalent to introducing A circular polarizer. Using the two-dimensional Jones matrix as the basic model, the LP11 mode generated in step 3 is used to generate the LP11 mode in the four directions (LP 11ax , L.P. 11bx , L.P. 11ay ,LP 11by ) as the Jones vector basis, it can be expressed as:

[0063]

[0064] Among them, F lm (r) is the radiation field distribution function, is the azimuth coordinate. and is the polarization direction along the HV coordinate system, a and b are the mode petal orientation angles along the HV coordinate system. When the LP11 mode is twisted by an angle θ at the tip of the optical fiber, then:

[0065]

[0066] When θ is an odd multiple of 45°, the hemispherical fiber lens has a convergence and superposition effect on the light spot, making LP11 a and LP11b conduct The phases are superimposed, and finally the OAM mode is obtained. Where the superscript ±l represents the topological charge number, then:

[0067]

[0068] Hereinafter, another specific embodiment of the present disclosure is given:

[0069] Figure 1 The figure shows the tapered output port model of the optical fiber after tapering. Figure 1 As shown in the figure, the tip of the tapered fiber can be regarded as a non-uniform waveguide structure, and the waveguide parameters will change due to the tapered fiber. The relationship between the propagation constants of the fundamental mode and the second-order mode is:

[0070]

[0071] Among them, ρ = ρ (z) represents the core radius of the tapered area at z, and Ω = Ω (z) represents the cone angle of the tapered area at z. If the speed of the optical fiber taper is faster, the tapered area at the tip of the optical fiber will be shorter, and the rate of change of the tapered area will be greater. When the local cone angle is greater than Ω, the fundamental mode can be coupled to the higher-order mode, thus forming multimode interference. The number of modes propagating in the optical fiber depends on the normalized frequency parameter V of the transmitted light wave in the optical fiber. The normalized parameter V is related to the wavelength of the laser, the radius of the optical fiber core, the core refractive index and the cladding refractive index, and then:

[0072]

[0073] When V<2.405, the LP01 mode is excited in the fiber; when 2.405≤V≤3.832, the LP11 mode is excited in the fiber; and when V>3.832, higher-order modes will be excited in the fiber. Therefore, after cutting the fiber at a diameter of about 15um, V=2.2184 can be made, and the LP11 mode can be excited, such as Figure 2 shown.

[0074] When a single-mode fiber with LP01 mode is subjected to stress stretching with a θ twist angle, the circular symmetry of the fiber will be destroyed. According to the birefringence effect of the stressed fiber, the fiber will be decomposed into two linear polarization components along the fast axis F and the slow axis S. The fast axis component F leads the slow axis component S. Phase difference. When the fast axis F direction of SUS is an odd multiple of θ = 45° with the x polarization direction, the transmission of the input polarization state in the fiber can be regarded as a superposition process. Therefore, when the fiber end is twisted by an angle of θ and then tapered, the relationship between the input and output polarization states is:

[0075]

[0076] Therefore, when the SMF fiber of LP01 mode is twisted at an angle of θ at the end and then tapered, and then flattened in a specified area, a polarized LP11 mode with the same twist angle can be excited, such as Figure 3 shown.

[0077] When θ is an odd multiple of 45°, a hemispherical optical fiber lens is prepared based on θ using the electrode discharge function of the fiber fusion machine, which is equivalent to introducing A circular polarizer. Using the two-dimensional Jones matrix as the basic model, LP11 is used in the four directions (LP 11ax , L.P. 11bx , L.P. 11ay ,LP 11by ) as the Jones vector basis, it can be expressed as:

[0078]

[0079] Among them, F lm (r) is the radiation field distribution function, is the azimuth coordinate. and is the polarization direction along the HV coordinate system, a and b are the mode petal orientation angles along the HV coordinate system. When the LP11 mode is twisted by θ at the tip of the optical fiber, then:

[0080]

[0081] When θ is an odd multiple of 45°, the hemispherical fiber lens has a convergence and superposition effect on the light spot, making LP11 a and LP11 b conduct Phase superposition, where the superscript ±l represents the number of topological charges, then:

[0082]

[0083] Figure 4 and Figure 5 It is divided into three-dimensional module output of LP11 mode and OAM mode under θ twist. Figure 6 The LP01-LP11-OAM mode conversion results at different angles according to an embodiment of the present disclosure are shown.

[0084] Figure 7The diagram of the experimental device according to the embodiment of the present disclosure is shown. The LP01-LP11-OAM mode conversion device of the embodiment of the present disclosure includes two parts: an FRT module and an FHDL module. The FRT module is responsible for twisting and tapering the single-mode optical fiber and cutting the optical fiber to obtain an LP11 mode light spot with a corresponding twist angle. The module device includes: 1. Ordinary single-mode optical fiber G.652D, 2. Electric taper machine, 3. Host computer PC end, 4. Laser 5. Alcohol lamp 6. Optical fiber cutter. The FHDL module is responsible for preparing a hemispherical optical fiber lens at the tip of the flat optical fiber. The module device includes: 7. Single-mode optical fiber processed by the optical fiber cutter, 8. Optical fiber melting machine.

[0085] The common single-mode optical fiber is configured as follows: the outermost protective sleeve and coating layer of the common single-mode optical fiber G.652D (Corning SMF-28e) are stripped using a fiber stripper. After stripping, the fiber cladding diameter is 125 μm, and the fiber core diameter is 8.2 μm.

[0086] The electric taper machine is configured as follows: the electric taper machine is connected to a power source, and one port is connected to the host PC via a USB data cable. The distance between the Z-axis rotating table and the X-axis electric slide is adjusted to 10 cm. The optical fiber is fixed to two optical fiber holders of the Z-axis rotating table (the end of the optical fiber) and the X-axis electric slide (the middle of the optical fiber), and the optical fiber input end is connected to the output end of the laser. The Z-axis rotating table is twisted to an odd multiple of 45°, and the end of the optical fiber will twist around the central axis by a corresponding angle.

[0087] The host computer PC is configured as follows: the host computer PC is used to control the motion state of the X-axis electric slide of the electric taper machine. About 5 seconds after the alcohol lamp is ignited and the optical fiber melts, the X-axis electric slide motor is started through the PC. The initial acceleration of the X-axis electric slide of the electric taper machine is set to 0.03 mm / s 2 , the duration is 3s, making the fiber thinner. Set the acceleration of the latter stage to 0.05mm / s 2 , the optical fiber is quickly broken, forming a cone-shaped crack at the output end, so that the output light mode is transformed from the LP01 spot to a hollow mixed-mode aperture.

[0088] The laser is configured as follows: the laser output has a wavelength of λ=650nm and an output power of P out =30mW.

[0089] The alcohol lamp is configured as follows: the alcohol lamp is placed in the middle position between the optical fiber holder of the X-axis slide rail of the electric taper machine and the optical fiber holder at the center axis of the Z-axis rotating table of the electric taper machine. The alcohol lamp is lit to heat the optical fiber to the melting point, and then the electric taper machine is started and continued to heat until the optical fiber taper is melted.

[0090] In the second step, the tapered end of the optical fiber is fixed in a fiber cutter and cut to obtain a flat-end optical fiber, so that the single-mode optical fiber is excited to produce the LP11 mode from the LP01.

[0091] The number of modes propagating in the optical fiber depends on the normalized frequency parameter V of the transmitted light wave in the optical fiber. The normalized parameter V is related to the wavelength of the laser, the radius of the optical fiber core, the core refractive index and the cladding refractive index. When the optical fiber is cut at a diameter of 15um, V = 2.2184 can be achieved, which can excite the LP11 mode.

[0092] At the same time, the ordinary single-mode optical fiber can excite a high-order hybrid mode from the original LP01 mode under the heating taper of the alcohol lamp. Therefore, after the wavelength laser is introduced, the tapered output end of the optical fiber can generate a hybrid mode interference spot. At the same time, since the optical fiber is twisted by an odd multiple of 45° with the help of the Z-axis rotating table of the electric taper machine during the heating taper, a ring-shaped hollow hybrid interference aperture is finally formed after taper. When the optical fiber diameter is selected to be 15μm (core radius is about 2.78μm) and a flat-end optical fiber is obtained after cutting, the LP11 mode with the corresponding orthogonal angle of the spot lobe can be obtained.

[0093] The third step is to configure the fiber splicing machine of the FHDL module.

[0094] The fiber fusion machine is configured as: Sumitomo Type-82C optical fiber fusion machine. In the discharge test mode, the discharge intensity is set to standard +100bit, the discharge time is 0.25s, and the optical fiber model is SMF60mm.

[0095] The fourth step is to place the flat-end optical fiber in the fiber fusion machine of the FHDL module, and use the discharge program to complete the electrode discharge heating, so that a hemispherical optical fiber lens with a spherical diameter of 15.5 μm is formed at the tip of the optical fiber.

[0096] The hemispherical optical fiber lens formed at the flat optical fiber tip can play a role similar to circular polarization. When the optical fiber is twisted to an odd multiple of 45°, Phase difference, due to the convergence and superposition effect of the optical fiber lens on the light spot, such as Figure 7 -FHDL ​​module, which enables LP11a and LP11b to perform phase superposition and finally obtain the OAM mode.

[0097] The single-mode optical fiber LP01-LP11-OAM mode conversion method provided by the embodiments of the present disclosure can be applied to the multi-dimensional capture technology of optical fiber tweezers. Figure 8 and Fig. 9The light field distribution diagrams under the LP11 and OAM modes according to the embodiments of the present disclosure are shown respectively. In the light field distribution of the LP11 mode and the OAM mode, the light intensity at the four twist angles of 45°, 135°, 225° and 270° has one or more high-low-high large light intensity fluctuations along the x-axis. The structural arrangement of the light intensity in this configuration produces a local light gradient, thereby producing a spatial gradient of the optical force. In an area with multiple high-low-high light traps, microsphere particles with a refractive index lower than the refractive index of the light field may be affected by the gradient force or scattering force and change the original state of motion. The modulation of the light intensity distribution can be changed by adjusting the twist angle of the optical fiber, thereby generating the corresponding LP11 mode. Similarly, a black hole is generated in the middle of the OAM spot (y=0). The particles are bound to move in the central area. Therefore, optical forces can be formed in the light intensity fluctuation area in the LP and OAM optical modes, and microsphere particles with different refractive indices can show different states of motion, thereby realizing multi-dimensional micro-nanoparticle capture and manipulation.

[0098] The specific data and experimental results of the implementation are given below:

[0099] The distance between the Z-axis rotating table and the X-axis electric slide of the electric optical fiber taper machine is set to 10 cm. The initial acceleration of the X-axis electric slide of the electric taper machine in the first stage is set to 0.03 mm / s 2 , duration is 3s, the second stage acceleration is 0.05mm / s 2 The optical fiber is twisted at an odd multiple of 45° on the Z-axis rotating stage. The rotation angles used in the experiment are 45°, 135°, 225°, and 270°. The wavelength of the laser is 650nm, and the output power is set to 30mW.

[0100] Sumitomo Type-82C fiber fusion machine, in discharge test mode, set the discharge intensity to standard +100bit, the discharge time to 0.25s, and the fiber model to SMF60mm.

[0101] Fig.10 The outputs of the tapered tip, LP mode, and OAM mode at four twist angles of 45°, 135°, 225°, and 270° according to an embodiment of the present disclosure are shown. Fig.10 (a) is the mixed interference mode output result after taper, both of which are hollow annular interference halos. Fig.10 (b) is the LP11 mode output after cutting when the fiber diameter is about 15μm. The two lobes of LP11 light form an orthogonal angle corresponding to the twist angle. Fig.10 (c) is the OAM mode light spot output by the end hemispherical fiber lens. This is consistent with the above principle inference, proving the feasibility of this method.

[0102] Fig.11 The coupling efficiency of the LP mode and OAM mode output according to the embodiment of the present disclosure is shown. Among the four angles, the LP11 mode at 270° and the OAM mode at 225° have better coupling efficiency. Comparing the two modes, the change trend of the OAM mode curve is generally higher than that of the LP11 mode, which verifies that OAM has better optical transmission characteristics.

[0103] In summary, the technical solution of the disclosed embodiment can easily convert the optical mode of a single single-mode optical fiber, which is easy to operate and has low cost. Since the optical fiber can have a unique light field distribution during the conversion process, it is suitable for a variety of fields such as optical signal transmission and optical tweezers capture technology. This optimized optical mode conversion is expected to improve the multi-dimensional capture efficiency of microsphere particles by optical tweezers, and provide a simpler method for the operation and analysis of microscale particles.

[0104] The embodiments of the present disclosure have been described above, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and changes will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The selection of terms used herein is intended to best explain the principles of the embodiments, practical applications, or technical improvements in the market, or to enable other persons of ordinary skill in the art to understand the embodiments disclosed herein.

Claims

1. A method for converting LP01-LP11-OAM mode of a single single-mode optical fiber, characterized in that Follow these steps: One end of a single single-mode optical fiber is not processed, and the other end of the single single-mode optical fiber is subjected to twisting, tapering, cutting and electrode discharge treatment in sequence. After the treatment, the other end of the single single-mode optical fiber has a tapered transition zone and a hemispherical optical fiber lens zone. The specific path is as follows: A single-mode fiber with LP01 mode is twisted at an odd multiple of 45° and then tapered. The fiber is stretched to form a tapered transition zone and then broken, so that the single-mode fiber with LP01 mode is converted into a tapered fiber with multiple mode aliasing. Then, the tip of the tapered optical fiber is cut flat to convert the tapered optical fiber with multiple mode aliasing into a flat-end optical fiber with LP11 mode; A hemispherical fiber lens is prepared at the end of the flat fiber using the electrode discharge function of the fiber splicer, which is equivalent to introducing a 1 / 4 circular polarizer to obtain the OAM mode. The mode conversion of single-mode optical fiber LP01-LP11 is obtained by cutting the waist area of ​​the optical fiber after twisting and taper processing; The single-mode fiber LP11-OAM mode conversion is achieved by discharging the electrode at the end of the cut fiber to form a hemispherical fiber lens.

2. The LP01-LP11-OAM mode conversion method of a single single-mode light according to claim 1, characterized in that: Using an XZ-axis optical fiber electric taper machine (2) and a laser (4); Fix the unprocessed end of the single single-mode optical fiber to the optical fiber holder of the X-axis slide rail of the electric taper drawing machine as the optical fiber input end, and fix the other end to the optical fiber holder at the center axis of the Z-axis rotating table of the electric taper drawing machine; adjust the angle of the Z-axis rotating table to an odd multiple of 45°, so that the optical fiber fixed on the axis of the rotating table is also twisted around the central axis by a corresponding angle; Adjust the distance between the Z-axis rotary table and the X-axis electric slide to within the range of (10cm, 13cm) to ensure that there is enough space for the subsequent motor to pull the optical fiber; The optical fiber input end is connected to the output end of the laser (4).

3. The LP01-LP11-OAM mode conversion method of a single single-mode optical fiber according to claim 2, characterized in that: Use alcohol lamp (5) and host PC (3); An alcohol lamp (5) is placed between the optical fiber holder of the X-axis slide rail of the electric taper machine and the optical fiber holder at the center axis of the Z-axis rotating table of the electric taper machine. After the optical fiber is heated to the melting point, the electric taper machine is started using the host computer PC terminal (3). The optical fiber is stretched to form a tapered transition zone and then broken to obtain a tapered optical fiber tip. After breaking, the single-mode optical fiber with the LP01 mode is converted into a tapered optical fiber with multiple mode aliasing.

4. The LP01-LP11-OAM mode conversion method of a single single-mode optical fiber according to claim 3, characterized in that: Applying a fiber optic cutter (6); The tip of the tapered optical fiber is flattened until the fiber core diameter is in the range of 2.52 μm to 3.95 μm. After the processing, the tapered optical fiber with multiple mode aliasing is converted into a flat-end optical fiber with LP11 mode. The processed tapered optical fiber can obtain two LP11 light spots with corresponding torsional orthogonal angles when a laser with a wavelength of 650 nm is passed through it.

5. The LP01-LP11-OAM mode conversion method of a single single-mode optical fiber according to claim 4, characterized in that: Application of fiber splicing machine (7); The flat-end optical fiber end is placed in a fiber fusion machine (7), and a hemispherical optical fiber lens with a diameter of 15.5 μm is prepared by electrode discharge, and the flat-end optical fiber in LP11 mode is converted into a hemispherical optical fiber in OAM mode.

6. The LP01-LP11-OAM mode conversion method of a single single-mode optical fiber according to claim 5, characterized in that: The single single-mode optical fiber adopts G.652D Corning SMF-28e, with a cladding diameter of 125 μm and an optical fiber core diameter of 8.2 μm; Use a fiber stripper to strip off the outermost protective sheath and coating of the optical fiber.

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