WO2020107254A1 - 基于手征光纤光栅的偏振无关型轨道角动量调制器及其制备方法和轨道角动量光束发生器 - Google Patents

基于手征光纤光栅的偏振无关型轨道角动量调制器及其制备方法和轨道角动量光束发生器 Download PDF

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WO2020107254A1
WO2020107254A1 PCT/CN2018/117890 CN2018117890W WO2020107254A1 WO 2020107254 A1 WO2020107254 A1 WO 2020107254A1 CN 2018117890 W CN2018117890 W CN 2018117890W WO 2020107254 A1 WO2020107254 A1 WO 2020107254A1
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angular momentum
polarization
orbital angular
optical fiber
spiral
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PCT/CN2018/117890
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English (en)
French (fr)
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白志勇
张岩
王义平
付彩玲
刘申
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深圳大学
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Priority to PCT/CN2018/117890 priority Critical patent/WO2020107254A1/zh
Priority to US17/297,647 priority patent/US11536895B2/en
Publication of WO2020107254A1 publication Critical patent/WO2020107254A1/zh

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    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
    • G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
    • G02B6/02123—Refractive index modulation gratings, e.g. Bragg gratings characterised by the method of manufacture of the grating
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02033—Core or cladding made from organic material, e.g. polymeric material
    • G02B6/02038—Core or cladding made from organic material, e.g. polymeric material with core or cladding having graded refractive index
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
    • G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
    • G02B6/0208—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response
    • G02B6/02085—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response characterised by the grating profile, e.g. chirped, apodised, tilted, helical
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/13306—Circuit arrangements or driving methods for the control of single liquid crystal cells
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/1333—Constructional arrangements; Manufacturing methods
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
    • G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
    • G02B6/0208—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response
    • G02B6/02085—Refractive index modulation gratings, e.g. Bragg gratings characterised by their structure, wavelength response characterised by the grating profile, e.g. chirped, apodised, tilted, helical
    • G02B2006/0209—Helical, chiral gratings
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02057—Optical fibres with cladding with or without a coating comprising gratings
    • G02B6/02076—Refractive index modulation gratings, e.g. Bragg gratings
    • G02B6/02123—Refractive index modulation gratings, e.g. Bragg gratings characterised by the method of manufacture of the grating
    • G02B6/02152—Refractive index modulation gratings, e.g. Bragg gratings characterised by the method of manufacture of the grating involving moving the fibre or a manufacturing element, stretching of the fibre

Definitions

  • the present application relates to the technical field of application of orbital angular momentum, and in particular to a polarization-independent orbital angular momentum modulator based on chiral fiber gratings, a preparation method thereof, and an orbital angular momentum beam generator.
  • the vortex light carrying orbital angular momentum is very different from the conventional plane wave, spherical wave and other polarized light in wavefront structure.
  • the vortex light presents a unique spiral wavefront around the center of the beam
  • phase factor can be expressed as exp(il ⁇ ) by mathematical formula, where ⁇ is the rotation phase angle, and l is the topological charge number of the optical vortex.
  • Spiral Phase Plates can improve conversion efficiency without changing the mode.
  • the spiral phase plate is an optical modulation device with uneven thickness, one side of which is a plane, and the other side is a spiral curved medium disc whose thickness changes with azimuth.
  • the topological charge can be 1 if one rotation makes a phase difference of 2l ⁇ .
  • This method can intuitively generate the spiral phase without the need for other auxiliary equipment.
  • using one spiral phase plate cannot simultaneously generate multiple orbital angular momentum beams of different orders, and can only generate orbital angular momentum beams at specific operating wavelengths.
  • the helical phase plate has strict requirements on the pitch in the angular direction, requiring very precise manufacturing techniques.
  • the computational hologram generated by the spatial modulator can be used to realize the function of Diffractive Optical Elements (DOEs).
  • DOEs Diffractive Optical Elements
  • Different voltages can be loaded on the LCD screen of the spatial light modulator to change the shape of the crystal, so that different holograms can be loaded on the LCD screen.
  • Different holograms can be used to flexibly control the topological charge of the generated orbital angular momentum beam.
  • spatial light modulators have the disadvantages of being expensive, having polarization dependence, low coupling efficiency, and only producing low-power orbital angular momentum beams.
  • long-period fiber grating realizes the coupling of the mode from the fundamental mode to the higher-order mode, while other accessories can produce a phase difference of ⁇ /2 between the coupled higher-order modes.
  • this method has the advantages of high coupling efficiency, low insertion loss, light weight, low cost, anti-electromagnetic interference and full compatibility with optical fiber communication systems.
  • the excitation of the spiral phase of long-period fiber grating requires additional accessories to achieve phase modulation, which has polarization dependence.
  • the related art has the problems of wavelength dependence, polarization dependence, complicated structure and difficult preparation to realize spiral phase excitation.
  • a polarization-independent orbital angular momentum modulator including:
  • the optical fiber main body has a spiral optical fiber structure with a long period fiber grating effect
  • the axial direction of the optical fiber body has periodic spiral refractive index modulation, the spiral refractive index modulation period is in the order of hundreds of micrometers, the spiral refractive index modulation is distributed in the axial, radial and angular directions of the optical fiber body, It is used to excite the spiral phase and generate the orbital angular momentum beam.
  • a preparation method of polarization-independent orbital angular momentum modulator including:
  • the preparation method using the above polarization-independent orbital angular momentum modulator has high stability, and the chiral optical fiber obtained by processing is very uniform, can be close to the theoretical limit value, and has high coupling efficiency.
  • An orbital angular momentum beam generator including:
  • the input end is connected to the light source
  • a polarization-independent orbital angular momentum modulator connected to the output of the polarization controller
  • a first optical fiber jumper connected to an end of the polarization-independent orbital angular momentum modulator away from the polarization controller;
  • the polarization-independent orbital angular momentum modulator includes:
  • the optical fiber main body has a spiral optical fiber structure with a long period fiber grating effect
  • the axial direction of the optical fiber body has periodic spiral refractive index modulation, the spiral refractive index modulation period is in the order of hundreds of micrometers, the spiral refractive index modulation is distributed in the axial, radial and angular directions of the optical fiber body, It is used to excite the spiral phase and generate the orbital angular momentum beam.
  • the above-mentioned polarization-independent orbital angular momentum modulator uses a fiber body as a base, the fiber body has a periodically distributed spiral fiber structure, and can excite the spiral phase within a certain bandwidth.
  • the spiral optical fiber structure formed by the optical fiber body has spiral symmetry, does not form a specific direction sensitive to polarization, and has polarization independence.
  • the polarization-independent orbital angular momentum modulator is an all-fiber structure, which is beneficial to integration in communication, has good compatibility, does not require the assistance of other devices, and has the advantages of simple structure and easy preparation. Further, the polarization-independent orbital angular momentum modulator has high coupling efficiency, and the generated OAM can carry any polarization state and has high purity.
  • the preparation method of the polarization-independent orbital angular momentum modulator adopts a high-temperature heat source to heat the optical fiber body, and at the same time, the optical fiber body is kept in a stretched state to translate and twist, which can complete the polarization-independent orbital angular momentum Preparation of the modulator.
  • the polarization-independent orbital angular momentum modulator of a specific period can be prepared by setting the speed of twisting and translation, which has the advantages of simple method and easy operation.
  • the orbital angular momentum beam generator can generate a beam with a spiral phase within a certain bandwidth, the beam does not form a specific direction sensitive to polarization, and has polarization independence.
  • FIG. 1 is a schematic structural diagram of a polarization-independent orbital angular momentum modulator provided by an embodiment of the present application
  • FIG. 2 is a schematic diagram of a spiral refractive index modulation surface of a polarization-independent orbital angular momentum modulator provided by an embodiment of the present application;
  • FIG. 3 is a flowchart of a method for preparing a polarization-independent orbital angular momentum modulator provided by an embodiment of the present application
  • FIG. 4 is a flowchart of another method for preparing a polarization-independent orbital angular momentum modulator provided by an embodiment of the present application
  • FIG. 6 is a schematic diagram of a polarization-independent orbital angular momentum modulator testing device provided by an embodiment of the present application
  • FIG. 7 is a graph of test spectrum results of a polarization-independent orbital angular momentum modulator provided by an embodiment of the present application.
  • FIG. 8 is a test result diagram of a polarization-independent orbital angular momentum modulator with different polarization states provided by an embodiment of the present application.
  • Polarization independent orbital angular momentum modulator 100 fiber main body 110, orbital angular momentum beam generator 400, light source 410, polarization controller 420, first fiber jumper 430, second fiber jumper 440, single mode fiber 450, less Mode fiber 460.
  • an element when referred to as being “fixed” to another element, it can be directly on the other element or there can also be a centered element. When an element is considered to be “connected” to another element, it may be directly connected to another element or there may be a center element at the same time.
  • the polarization-independent orbital angular momentum modulator 100 includes a fiber body 110.
  • the optical fiber body 110 has a spiral fiber structure, and the spiral fiber structure has a long-period fiber grating effect.
  • the optical fiber body 110 has periodic helical refractive index modulation in the axial direction, please refer to FIG. 2 as well.
  • the spiral refractive index modulation period is on the order of one hundred micrometers.
  • the spiral refractive index modulation is distributed in the axial, radial, and angular directions of the optical fiber body 110 and is used to excite the spiral phase and generate the orbital angular momentum beam.
  • the optical fiber body 110 has periodic helical refractive index modulation along the fiber axis.
  • the period of grating modulation is in the range of 200 ⁇ m-2000 ⁇ m. In the period of this range, the resonance peak of the grating is generated in the C+L band, which is conducive to integration with the communication system.
  • the refractive index modulation period is on the order of one hundred microns, so that the spiral fiber structure has a long period fiber grating effect.
  • the spiral optical fiber structure is a spiral core structure, which can generate spiral refractive index modulation on incident light.
  • the refractive index modulation of different traditional long-period fiber gratings has distribution along the axial, radial and angular directions of the fiber.
  • the light incident into the fiber grating will not only be reflected by the grating to generate higher-order modes, but also be affected by the spiral refractive index distribution to produce an additional spiral phase.
  • the high-order mode and the spiral phase are enhanced by resonance, thereby forming a high-order mode with a spiral phase, that is, an orbital angular momentum beam. Since the orbital angular momentum beam formed is only affected by the spiral refractive index modulation and has nothing to do with the direction of the electric field vibration of the incident light, the orbital angular momentum modulator 100 can be used to excite polarization-independent orbital angular momentum mode.
  • the polarization-independent orbital angular momentum modulator 100 uses a chiral fiber grating as a substrate, which can directly realize the coupling of orbital angular momentum without the need for other auxiliary equipment such as a polarization controller and a stress modulation plate.
  • the polarization-independent orbital angular momentum modulator 100 has a uniform spiral refractive index modulation structure, and can periodically scatter the fundamental mode light. Due to the existence of spiral perturbation in the polarization-independent orbital angular momentum modulator 100, the plane phase of the fundamental mode can be changed periodically.
  • the orbital angular momentum modulator 100 has two functions, one is the generation of the orbital angular momentum beam, and the other is the conversion of the orbital angular momentum beam.
  • the incidence of the spiral phase can be achieved by using any polarization input light incident, thereby generating orbital angular momentum beams.
  • the energy of the fundamental mode light can be coupled to the higher-order mode through resonance, and its plane phase is converted to Spiral shape, so as to realize the excitation of spiral phase, that is, the generation of orbital angular momentum.
  • the fiber body 110 is a dual-mode fiber or a quad-mode fiber.
  • the optical fiber body may also be one of other few-mode optical fibers.
  • the optical fiber body 110 may include an optical fiber core and a cladding.
  • the fiber core diameter may be 19 microns, and the cladding diameter may be 125 microns.
  • the polarization-independent orbital angular momentum modulator 100 is an all-fiber device, and has a spiral fiber structure that can directly excite the spiral phase without the assistance of other accessories. It is good for integration in communication and has good compatibility.
  • the optical fiber body 110 has the spiral refractive index modulation with uniform period and depth in the axial direction, and the spiral refractive index modulation amount ranges from 2x10 -4 to 2x10 -3 .
  • the spiral refractive index modulation has a spiral refractive index modulation with a uniform period and depth along the fiber axis.
  • the spiral refractive index modulation of any cross section of the optical fiber body 110 has an asymmetry, and along the radial direction of the optical fiber body 110, the spiral refractive index modulation decreases as the radius increases.
  • the spiral refractive index modulation depth has a helicity distribution and is used to excite the spiral phase of the light beam.
  • the asymmetric spiral refractive index modulation is conducive to the coupling excitation of higher-order asymmetric modes.
  • the corresponding refractive index modulation is uneven and asymmetric, and a phase delay amount of an integer multiple of 2 ⁇ can be achieved within one grating period.
  • the refractive index modulation of the chiral fiber grating is in the range of 2x10 -4 -2x10 -3 .
  • the length of the optical fiber body 110 ranges from 1 cm to 6 cm.
  • the chiral fiber grating can realize the excitation of high-order core mode according to the change of the period.
  • the spiral refractive index modulation of the polarization-independent orbital angular momentum modulator 100 is an inherent form of the grating. Therefore, the spiral refractive index modulation function does not change with external changes, and has the advantage of not being affected by external conditions such as temperature, vibration, distortion, and bending. It can be understood that the helical phase generated by the polarization-independent orbital angular momentum modulator 100 is only related to the spiral modulation of the chiral fiber grating itself constituting the polarization-independent orbital angular momentum modulator 100, and does not depend on the input. The polarization state of light changes. Further, when there is a spiral phase node 1 but the rotatability is different, the two do not couple with each other.
  • the spiral fiber structure has polarization independence.
  • the spiral phase is modulated by the spiral refractive index and is independent of the polarization state of the incident light.
  • the rotation of the spiral phase is determined by the rotation of the spiral refractive index of the grating itself, regardless of the polarization state of the incident light.
  • the mode purity represented by the spiral phase is determined by the period and intensity of the spiral refractive index, and is independent of the polarization state of the incident light.
  • the spiral fiber structure has spiral symmetry so that it does not form a specific direction sensitive to polarization. Therefore, the polarization-independent orbital angular momentum modulator 100 has polarization-independent characteristics.
  • the higher-order modes coupled by the polarization-independent orbital angular momentum modulator 100 all have a spiral phase, so The orbital angular momentum modulator 100 is polarization-independent, and the OAM generated by excitation can carry any polarization state.
  • the spiral phase can be excited after passing through the polarization-independent orbital angular momentum modulator 100. And the polarization characteristics of the excited high-order mode are consistent with the input fundamental mode light.
  • the polarization-independent orbital angular momentum modulator 100 has a high coupling efficiency, the purity of the generated spiral phase is independent of the polarization state, and the purity of the generated OAM is high.
  • the polarization-independent orbital angular momentum modulator 110 can generate OAM within a certain bandwidth. The bandwidth may be 3dB.
  • the spiral fiber structure has the same rotation direction, and the rotation direction is clockwise or counterclockwise. According to the difference of the rotation directions, it can be divided into the polarization-independent orbital angular momentum modulator 100 composed of a right-handed chiral fiber grating and the polarization-independent orbital angular momentum modulator 100 composed of a left-handed chiral fiber.
  • the polarization-independent orbital angular momentum modulator 100 further includes a protective layer.
  • the protective layer is located outside the optical fiber body 110 to form a mechanical protection for the optical fiber body 110 and prolong the service life of the polarization-independent orbital angular momentum modulator 100.
  • the polarization-independent orbital angular momentum modulator 100 is an all-optical device, which is convenient for connection and coupling with other optical fiber devices and has high compatibility. It can be understood that the polarization-independent orbital angular momentum modulator 100 has the characteristics of light weight and resistance to electromagnetic interference.
  • This application provides a method for preparing a polarization-independent orbital angular momentum modulator.
  • the preparation method of the polarization-independent orbital angular momentum modulator includes: S10, heating the optical fiber body 110 to a molten state. S20, twisting the optical fiber body 110 in a melted state. S30, cooling the optical fiber body 110.
  • step S10 the two ends of the optical fiber body 110 are fixed, one end is fixed to the fiber rotator, and the other end is fixed to the fiber clamp.
  • the optical fiber clamp and the optical fiber rotator are respectively fixed on a displacement platform.
  • step S10 under high temperature conditions, the optical fiber body 110 melts uniformly.
  • the high temperature condition is above 1800°C.
  • the optical fiber body 110 is a low-mode optical fiber, and the low-mode optical fiber is a channel for mode high-quality transmission.
  • the few-mode optical fiber may be a dual-mode, quad-mode, etc. optical fiber.
  • the few-mode optical fiber has the advantages of light weight and resistance to electromagnetic interference.
  • step S20 the melted optical fiber body 110 is twisted uniformly at high speed.
  • step S30 when the optical fiber body 110 is removed from the heating zone, the optical fiber body 110 is rapidly cooled. During the cooling process of the optical fiber body 110, part of the residual stress is frozen, thereby forming a spiral refractive index modulation, obtaining a chiral fiber grating, and completing the preparation of the polarization-independent orbital angular momentum modulator 100.
  • the optical fiber body 110 is translated and twisted, and a chiral fiber grating with a spiral refractive index distribution can be prepared.
  • the chiral fiber grating can be used as the polarization-independent orbital angular momentum modulator 100.
  • the polarization-independent orbital angular momentum modulator 100 has a high coupling efficiency.
  • the coupling efficiency is the proportion in the higher-order mode of optically coupling the input fundamental mode to the optical fiber.
  • the coupling efficiency is related to the spiral grating structure and processing method.
  • the preparation method using the polarization-independent orbital angular momentum modulator has high stability, and the chiral optical fiber obtained by processing is very uniform, can be close to the theoretical limit value, and has high coupling efficiency.
  • any heat source such as CO2 laser, continuous arc discharge, high temperature furnace or coil is used to heat the optical fiber body 100 to the melted state.
  • the optical fiber body 110 in the melting state is twisted in an instantaneous or counterclockwise direction.
  • the optical fiber body 110 in the step of cooling the optical fiber body 110 is removed from the heating zone, and the residual stress generated during the twisting process is frozen in the optical fiber body 110.
  • the present application provides an orbital angular momentum beam generator 400.
  • the orbital angular momentum generator 400 includes a light source 410, a polarization controller 420, a polarization-independent orbital angular momentum modulator 100, and a first fiber jumper 430.
  • the input terminal of the polarization controller 420 is connected to the light source 410.
  • the polarization-independent orbital angular momentum modulator 100 is connected to the output of the polarization controller 420.
  • the first fiber jumper 430 is connected to an end of the polarization-independent orbital angular momentum modulator 100 away from the polarization controller 420.
  • the polarization-independent orbital angular momentum modulator 100 includes an optical fiber body 110.
  • the optical fiber body 110 has a spiral fiber structure, and the spiral fiber structure has a long-period fiber grating effect.
  • the optical fiber body 110 has the same diameter.
  • the optical fiber body 110 has a periodic spiral refractive index modulation in the axial direction, the spiral refractive index modulation period is in the order of hundreds of micrometers, and the spiral refractive index modulation is distributed in the axial direction, radial direction and angle of the optical fiber body 110 The direction is used to excite the spiral phase and generate the orbital angular momentum beam.
  • the light source 410 may be a tunable laser.
  • the tunable laser can be adjusted in a desired wavelength band.
  • the orbital angular momentum beam generator 400 can generate the required OAM beam in a wide bandwidth. In one embodiment, the bandwidth is 3dB.
  • the first optical fiber jumper 430 is used to connect with other instruments to facilitate the application of the orbital angular momentum beam generator 400 in communication. It can be understood that various types of optical fibers are connected by corresponding adapters, that is, flanges.
  • the orbital angular momentum beam generator 400 further includes a second fiber jumper 440.
  • the input terminal of the polarization controller 420 is connected to the light source 410 through the second fiber jumper 440.
  • the second optical fiber jumper 440 can ensure stable transmission of the light beam.
  • the second optical fiber jumper 440 may be a single mode.
  • the orbital angular momentum beam generator 400 further includes a single-mode optical fiber 450.
  • the polarization-independent orbital angular momentum modulator 100 is connected to the output end of the polarization controller 420 through the single-mode optical fiber 450.
  • the single-mode optical fiber 450 can realize low-loss transmission of the light beam between the polarization-independent orbital angular momentum modulator 100 and the polarization controller 420.
  • the orbital angular momentum beam generator 400 further includes a few mode fiber 460.
  • the first optical fiber jumper 430 is connected to the polarization-independent orbital angular momentum modulator 100 through the few-mode optical fiber 460.
  • the few-mode optical fiber 460 can ensure the stable transmission of the light beam with orbital angular momentum.
  • FIG. 6 Please refer to FIG. 6 together to test the polarization dependence of the polarization-independent orbital angular momentum modulator 100 by using a testing device.
  • a system based on spatial interference is constructed to detect the orbital angular momentum beam generated by the polarization-independent orbital angular momentum modulator 100.
  • OAM beams interfere with spherical waves or plane waves to detect the phase of OAM.
  • the method is to interfere the Gaussian beam and the OAM beam with topological charge l generated by the polarization-independent orbital angular momentum modulator 100 based on a long-period chiral grating in space, if the two beams are coaxial in space,
  • the interference fringes for spherical waves appear as a spiral phase pattern with topological charge number l.
  • the counterclockwise or clockwise rotation of the interference fringes depends on the sign of the topological charge number l of the OAM beam.
  • the light output by the tunable single-wavelength laser is output through an optical fiber jumper and collimated by a 10x objective lens, and then enters an adjustable splitting ratio polarization beam splitter (Polarization Beam Splitter, PBS) into two optical paths, namely, a reference optical path and a test optical path.
  • PBS Polarization Beam Splitter
  • the input light of the reference optical path enters the beam combiner after passing through the 1/2 wave plate.
  • the input light of the test optical path passes through a polarizer, a 1/4 wave plate, a 10x objective lens, the polarization-independent orbital angular momentum modulator 100 sample, a 40x objective lens and an analyzer, and then enters a beam combiner.
  • the two beams of light meet each other on the beam combiner to cause interference.
  • infrared CCD Charge Coupled Device
  • the light emitted by the tunable laser is first collimated and amplified by the 10x objective lens.
  • the model VA5-1550, Thorlabs' PBS is used to align the directly amplified beam for beam splitting.
  • the split ratio of the PBS is adjustable and the two split beams are p-polarized light and s-polarized light, respectively.
  • the input light includes linear polarization (LP), left-handed circular polarization (CP-) and right-handed circular polarization (CP+).
  • the input light of the test optical path then passes through the 10x objective lens, the polarization-independent orbital angular momentum modulator 100, and the 40x objective lens, and then enters the analyzer.
  • the analyzer analyzes the polarization state of the generated OAM beam.
  • the input light of the reference optical path adjusts the vibration direction of the linearly polarized input light to the same as the vibration direction of the test light path light exiting from the analyzer through a 1/2 wave plate to ensure that the two beams of reference light and test light occur put one's oar in.
  • a beam combiner Beam Combiner, BS
  • the BS model is VC5-1550, Thorlabs.
  • the reference light is not added first, but the distribution of the mode field is measured using the test optical path.
  • a precision cutting device is used to cut both ends of the sample of the polarization-independent orbital angular momentum modulator 100, and the flatness of the end surface of the sample is ensured.
  • the end face of the sample fiber will be fixed on the platform placed between the objective lenses by using the fiber holder 220.
  • the fiber holder 220 may be a double-arm fiber holder.
  • the polarizer, the 1/4 wave plate and the analyzer After completing the preparation work, first, remove the polarizer, the 1/4 wave plate and the analyzer from the test optical path, and set the wavelength of the tunable single-wavelength laser at the At the resonance wavelength of the polarization-independent orbital angular momentum modulator 100.
  • the three-dimensional stage at both ends of the polarization-independent orbital angular momentum modulator 100 is precisely adjusted.
  • the two objective lenses before and after the polarization-independent orbital angular momentum modulator 100 are respectively focused on the left and right end surfaces of the polarization-independent orbital angular momentum modulator 100 sample.
  • the infrared CCD detects the spot intensity in real time until the spot brightness observed from the screen of the infrared CCD is at the maximum position, from the objective lens to the polarization-independent orbital angular momentum modulator at this time 100 has the highest coupling efficiency.
  • the polarizer, the quarter wave plate and the analyzer in the test optical path.
  • the input light of the polarization-independent orbital angular momentum modulator 100 sample is linearly polarized light is tested.
  • the direction parallel to the paper surface is specified as 0 degrees. Since the output light of the tunable laser is linearly polarized light, the direction of the light axis of the polarizer and the fast axis of the 1/4 wave plate are both placed at 0 degrees to obtain linearly polarized light. Input light.
  • the CCD is used to record separate mode field information when the analyzer transmission axis is located at four different angles: 0 degrees, 90 degrees, 180 degrees, and 270 degrees.
  • the CCD is used to record the interference information when the analyzer is located at four angles: 0 degrees, 90 degrees, 180 degrees and 270 degrees, and the phase information of the orbital angular momentum can be obtained by interference.
  • the polarizer, the wave plate, etc. are added to realize the input light of any polarization state, and the polarization test is combined with the spiral phase to test the polarization-independent type Characteristics of the orbital angular momentum modulator 100.
  • Adjusting the angle between the light transmission axis of the analyzer and the fast axis of the quarter-wave plate can generate light of any polarization state such as linear polarization and circular polarization, and thus light of different polarization states can be input.
  • the analyzer at the rear end can be used to analyze the polarization state of the output light. In this way, the effect of different polarization states on the mode excitation can be tested.
  • FIGS. 7(a) and 7(b) the right-handed and left-handed spectra of the polarization-independent orbital angular momentum modulator 100 are shown in FIGS. 7(a) and 7(b).
  • the sample period of the two polarization-independent orbital angular momentum modulators 100 is 1192 um.
  • the resonance peak of the right-handed polarization-independent orbital angular momentum modulator 100 is located at 1554.5 nm and the coupling depth is -24.6 dB.
  • the position of the resonance peak of the left-handed polarization-independent orbital angular momentum modulator 100 is 1550.9 nm, and the coupling depth is -22.5 dB.
  • the small difference in the resonance peaks of the two samples of the polarization-independent orbital angular momentum modulator 100 is that the structure of the grating is partially uneven due to external jitter during the manufacturing process of the polarization-independent orbital angular momentum modulator 100. It can be understood that the position period of the resonance peak is determined.
  • the coupling depth is related to the number of grating periods, that is, the number of spirals, and the uniformity of the processing process.
  • the polarization-independent orbital angular momentum modulator 100 sample described in the right hand is used to test the excitation of the orbital angular momentum under different polarization states. The results are shown in FIG. 8(a).
  • the input light is linearly polarized light
  • the mode field without the reference light path is shown in Fig. 8(a11).
  • the fundamental mode is excited to a higher-order orbital angular momentum. From the figure, you can see the hollow ring, and when the analyzer rotates once, there will be changes in light and dark, indicating that the generated light beam has the characteristics of linear polarization.
  • the reference optical path is added and interferes with the test optical path, and the phase information is determined by the interference method.
  • the period is in the order of hundreds of micrometers, the spiral modulation is uniform, the spectral quality is high, and the mode coupling is clear.
  • the polarization-independent orbital angular momentum modulator 100 is based on a few-mode fiber and is an all-fiber type modulator. It has the advantages of small size, low cost, high coupling efficiency and easy integration. There are important applications in areas such as particle manipulation.

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Abstract

本申请涉及一种基于手征光纤光栅的偏振无关型轨道角动量调制器及其制备方法和轨道角动量光束发生器。轨道角动量调制器包括光纤主体，所述光纤主体具有螺旋光纤结构，所述螺旋光纤结构具有长周期光纤光栅效应。所述光纤主体轴向具有周期性的螺旋折射率调制，所述螺旋折射率调制周期位于百微米量级，所述螺旋折射率调制分布于所述光纤主体的轴向、径向和角向，用于激发螺旋相位，产生轨道角动量光束。基于手征光纤光栅的偏振无关型轨道角动量调制器为全光纤结构，具有偏振无关性且耦合效率高。

Description

基于手征光纤光栅的偏振无关型轨道角动量调制器及其制备方法和轨道角动量光束发生器 技术领域
本申请涉及轨道角动量应用技术领域，特别是涉及一种基于手征光纤光栅的偏振无关型轨道角动量调制器及其制备方法和轨道角动量光束发生器。
背景技术
携带有轨道角动量(orbital angular momentum，OAM)的涡旋光与传统的平面波、球面波及其他偏振光在波前结构方面存在着很大的差别，涡旋光围绕光束中心呈现出特有的螺旋型波前结构，其相位因子中存在用数学公式可表示为exp(ilθ)的一项，其中θ为旋转相位角，l为光学涡旋的拓扑荷数。
相关技术中，螺旋相位片(Spiral phase plates，SPP)可以在不改变模式的前提下提高转换效率。螺旋相位片是一块厚度不均匀的光学调制器件，其中一面为平面，另外一面是厚度随方位角变化的螺旋曲面介质圆盘。其螺旋相位片平板厚度随方位角按h＝lλθ/2πr(n-1)关系变化，其中n为材料折射率，l为产生的轨道角动量光束拓扑荷值，θ为方位角。当一束高斯光经过该透射型介质平板时，不同半径处所经过的厚度与折射角均不相同，得到不同的光程差，进而引起不同的相位差。对于特定波长，若旋转一周形成2lπ的相位差即可产生的拓扑荷数为l。该方法可以直观的产生螺旋相位，不需要其他辅助设备。但是，使用一个螺旋相位片不能同时产生多个不同阶数的轨道角动量光束，且只能在特定工作波长处产生轨道角动量光束。此外，螺旋相位片沿角向的螺距要求严格，需要非常精密的制备技术。
可以利用空间调制器产生的计算全息图实现衍射光学元件(Diffractive Optical Elements，DOEs)的功能。通过电脑程序控制可以在空间光调制器的液晶屏上加载不同的电压，改变晶体的形态，从而在液晶屏上加载不同的全 息图。利用不同全息图即可灵活的控制产生的轨道角动量光束的拓扑荷数。但是，空间光调制器存在价格昂贵、具有偏振相关性、耦合效率低以及只能产生低功率轨道角动量光束等缺点。
长周期光纤光栅与压力板、旋转器或偏振控制器相结合可以激发出螺旋相位。长周期光纤光栅实现模式由基模到高阶模式的耦合，而其他附件可以在耦合的高阶模式间产生±π/2的相位差。虽然该方法具有耦合效率高、插损低、质量轻、成本低、抗电磁干扰以及与光纤通信系统完全兼容等优势。但是，长周期光纤光栅激发螺旋相位需要外加附件以实现相位调制，具有偏振相关性。
综上所述，相关技术实现螺旋相位激发具有波长依赖性、偏振相关性、结构复杂以及制备困难等问题。
发明内容
基于此，有必要针对激发螺旋相位的相关技术存在波长依赖性和偏振相关性、结构复杂以及制备困难的问题，提供一种基于手征光纤光栅的偏振无关型轨道角动量调制器及其制备方法和轨道角动量光束发生器。
一种偏振无关型轨道角动量调制器，包括：
光纤主体，具有螺旋光纤结构，所述螺旋光纤结构具有长周期光纤光栅效应；
所述光纤主体轴向具有周期性的螺旋折射率调制，所述螺旋折射率调制周期位于百微米量级，所述螺旋折射率调制分布于所述光纤主体的轴向、径向和角向，用于激发螺旋相位，产生轨道角动量光束。
一种偏振无关型轨道角动量调制器制备方法，包括：
加热光纤主体至融熔状态；
扭曲融熔状态的所述光纤主体；
冷却所述光纤主体。
采用上述偏振无关型轨道角动量调制器制备方法具有较高的稳定性，加 工得到的手征光纤非常均匀，能够较好的接近理论的极限值，具有较高的耦合效率。
一种轨道角动量光束发生器，包括：
光源；
偏振控制器，输入端与所述光源连接；
偏振无关型轨道角动量调制器，与所述偏振控制器输出端连接；以及
第一光纤跳线，与所述偏振无关型轨道角动量调制器远离所述偏振控制器的一端连接；
其中，所述偏振无关型轨道角动量调制器包括：
光纤主体，具有螺旋光纤结构，所述螺旋光纤结构具有长周期光纤光栅效应；
所述光纤主体轴向具有周期性的螺旋折射率调制，所述螺旋折射率调制周期位于百微米量级，所述螺旋折射率调制分布于所述光纤主体的轴向、径向和角向，用于激发螺旋相位，产生轨道角动量光束。
上述偏振无关型轨道角动量调制器，通过采用光纤主体作为基底，所述光纤主体具有周期性分布的螺旋光纤结构，可以在一定带宽内激发螺旋相位。所述光纤主体形成的螺旋光纤结构具有螺旋对称性，不会形成对偏振敏感的特定方向，具有偏振无关性。所述偏振无关型轨道角动量调制器为全光纤结构，通信中有利于集成，具有良好的兼容性，无需其他器件辅助，具有结构简单、容易制备等优势。进一步的，所述偏振无关型轨道角动量调制器耦合效率高，产生的OAM可以携带任意偏振态，且具有高纯度。所述偏振无关型轨道角动量调制器制备方法，采用高温热源对所述光纤主体进行加热的同时，将所述光纤主体保持拉伸状态进行平移和扭曲，可以完成所述偏振无关型轨道角动量调制器的制备。且可以通过设置扭曲和平移的速度制备特定周期的所述偏振无关型轨道角动量调制器，具有方法简单、容易操作等优势。所述轨道角动量光束发生器，可以在一定带宽内产生带有螺旋相位的光束，所述光束不会形成对偏振敏感的特定方向，具有偏振无关性。
附图说明
为了更清楚地说明本申请实施例或现有技术中的技术方案，下面将对实施例或现有技术描述中所需要使用的附图作简单地介绍，显而易见地，下面描述中的附图仅仅是本申请的一些实施例，对于本领域普通技术人员来讲，在不付出创造性劳动的前提下，还可以根据这些附图获得其它的附图。
图1为本申请实施例提供的一种偏振无关型轨道角动量调制器结构示意图；
图2为本申请实施例提供的一种偏振无关型轨道角动量调制器螺旋折射率调制面示意图；
图3为本申请实施例提供的一种偏振无关型轨道角动量调制器制备方法流程图；
图4为本申请实施例提供的另一种偏振无关型轨道角动量调制器制备方法流程图；
图5为本申请实施例提供的一种轨道角动量光束发生器；
图6为本申请实施例提供的一种偏振无关型轨道角动量调制器测试装置示意图；
图7为本申请实施例提供的一种偏振无关型轨道角动量调制器测试光谱结果图；
图8为本申请实施例提供的一种不同偏振状态偏振无关型轨道角动量调制器测试结果图。
附图标号说明
偏振无关型轨道角动量调制器100、光纤主体110、轨道角动量光束发生器400、光源410、偏振控制器420、第一光纤跳线430、第二光纤跳线440、单模光纤450、少模光纤460。
具体实施方式
为使本申请的上述目的、特征和优点能够更加明显易懂，下面结合附图对本申请的具体实施方式做详细的说明。在下面的描述中阐述了很多具体细节以便于充分理解本申请。但是本申请能够以很多不同于在此描述的其它方式来实施，本领域技术人员可以在不违背本申请内涵的情况下做类似改进，因此本申请不受下面公开的具体实施的限制。
需要说明的是，当元件被称为“固定于”另一个元件，它可以直接在另一个元件上或者也可以存在居中的元件。当一个元件被认为是“连接”另一个元件，它可以是直接连接到另一个元件或者可能同时存在居中元件。
除非另有定义，本文所使用的所有的技术和科学术语与属于本申请的技术领域的技术人员通常理解的含义相同。本文中在本申请的说明书中所使用的术语只是为了描述具体的实施例的目的，不是旨在于限制本申请。本文所使用的术语“及/或”包括一个或多个相关的所列项目的任意的和所有的组合。
请参见图1，本申请提供一种偏振无关型轨道角动量调制器100。所述偏振无关型轨道角动量调制器100包括光纤主体110。所述光纤主体110具有螺旋光纤结构，所述螺旋光纤结构具有长周期光纤光栅效应。所述光纤主体110轴向具有周期性的螺旋折射率调制，请一并参见图2。所述螺旋折射率调制周期位于百微米量级，所述螺旋折射率调制分布于所述光纤主体110的轴向、径向和角向，用于激发螺旋相位，产生轨道角动量光束。
所述光纤主体110沿光纤轴向具有周期性的螺旋折射率调制。沿光纤轴向上，光栅调制的周期在200μm-2000μm范围内。在该范围的周期上，光栅谐振峰产生在C+L波段，有利于与通信系统集成使用。所述折射率调制周期在百微米量级，使得该螺旋光纤结构具有长周期光纤光栅效应。所述螺旋光纤结构为呈螺旋状的纤芯结构，可以对入射光产生螺旋状折射率调制。不同传统长周期光纤光栅的折射率调制，所述折射率调制沿光纤轴向、径向和角向均具有分布。因而，在一个光栅周期内，入射到光纤光栅中的光除受到光栅反射产生高阶模式外，还将受到螺旋折射率分布的影响而产生一个附加的 螺旋相位。在多个光栅调制周期的作用下，高阶模式和螺旋相位被谐振增强，从而形成具有螺旋相位的高阶模式，即轨道角动量光束。由于所形成的轨道角动量光束仅受螺旋折射率调制的影响，与入射光的电场振动方向无关，故而所述轨道角动量调制器100可以用来激发偏振无关的轨道角动量模式。
所述偏振无关型轨道角动量调制器100采用手征光纤光栅作为基底，可以直接实现轨道角动量的耦合，无需如偏振控制器和应力调制板等其他附属设备。所述偏振无关型轨道角动量调制器100具有均匀的螺旋折射率调制结构，可以对基模光进行周期性的散射。由于所述偏振无关型轨道角动量调制器100中螺旋微扰的存在，可以使基模的平面相位发生周期的变化。所述轨道角动量调制器100具有两种作用，一是轨道角动量光束的产生，另一种是轨道角动量光束的转换。对于轨道角动量光束的产生，因为螺旋的折射率调制，采用任一偏振态的输入光入射均可以实现螺旋相位的产生，从而产生轨道角动量光束。在满足与所述偏振无关型轨道角动量调制器100中光纤光栅相位匹配的条件下，基模光的能量可以通过谐振耦合到高阶模式上，其平面相位在周期性谐振的作用下转换为螺旋形态，从而实现螺旋相位的激发，即实现轨道角动量的产生。
在一个实施例中，所述光纤主体110为双模光纤或四模光纤。所述光纤主体也可以为其他少模光纤中的一种。所述光纤主体110可以包括光纤纤芯和包层。所述光纤纤芯直径可以为19微米，所述包层直径可以为125微米。所述偏振无关型轨道角动量调制器100为全光纤器件，具有的螺旋光纤结构可以直接激发螺旋相位，不需要其他附件辅助。在通信中有利于集成，具有良好的兼容性。
所述光纤主体110轴向具有周期和深度均匀的所述螺旋折射率调制，所述螺旋折射率调制量的范围为2ⅹ10 -4至2ⅹ10 -3。所述螺旋折射率调制沿光纤轴向具有周期和深度均匀的螺旋折射率调制。在一个实施例中，所述光纤主体110任一截面的所述螺旋折射率调制具有非对称性，沿所述光纤主体110径向，所述螺旋折射率调制随半径的增大而减小。所述螺旋折射率调制深度 具有螺旋性分布，用于激发光束的螺旋相位。非对称的螺旋折射率调制，有利于高阶非对称模式的耦合激发。在所述光纤主体110轴向任一位置的截面内，沿角向旋转一周，对应的折射率调制不均匀且不对称，可以在一个光栅周期内实现2π的整数倍的相位延迟量。为实现高耦合效率、低插损的模式耦合，在任一周期内，手征光纤光栅的折射率调制量2ⅹ10 -4-2ⅹ10 -3范围内。在一个实施例中，所述光纤主体110的长度范围为1厘米至6厘米。在模式耦合上，该手征光纤光栅根据周期的变化，可以实现高阶纤芯模式的激发。
所述偏振无关型轨道角动量调制器100的螺旋折射率调制为光栅固有形态，因此螺旋折射率调制功能不随外界变化而变化，具有不受温度、震动、扭曲和弯曲等外界条件影响的优点。且可以理解，所述偏振无关型轨道角动量调制器100产生的螺旋相位的旋性只与构成所述偏振无关型轨道角动量调制器100的手征光纤光栅本身的螺旋调制有关，而不随输入光的偏振态改变而改变。进一步的，当具有螺旋相位节点l但旋性不同时，两者不发生相互耦合。
在一个实施例中，所述螺旋光纤结构具有偏振无关性。螺旋相位由所述螺旋折射率调制而成，与入射光偏振态无关。螺旋相位的旋向由光栅本身的螺旋折射率旋向决定，与入射光偏振态无关。螺旋相位的所代表的模式纯度由螺旋折射率的周期和强度决定，与入射光偏振态无关。
所述螺旋光纤结构具有螺旋对称性，从而不会形成对偏振敏感的特定方向。因此，所述偏振无关型轨道角动量调制器100具有偏振无关特性。当对所述偏振无关型轨道角动量调制器100输入任意偏振态的基模光时，所述偏振无关型轨道角动量调制器100耦合的高阶模式均具有螺旋相位，因此所述偏振无关型轨道角动量调制器100具有偏振无关性，激发产生的OAM可以携带任意偏振态。即所述偏振无关型轨道角动量调制器100的输入光的偏振态任意变化时，经过所述偏振无关型轨道角动量调制器100均可激发螺旋相位。且激发的高阶模式的偏振特性与输入的基模光一致。在一个实施例中，所述偏振无关型轨道角动量调制器100具有较高的耦合效率，产生的螺旋相 位的纯度与偏振态无关，同时产生的OAM纯度高。所述偏振无关型轨道角动量调制器110在一定带宽内可以产生OAM。所述带宽可以为3dB。
在一个实施例中，所述螺旋光纤结构的旋向相同，所述旋向为顺时针或逆时针。依据所述旋向的不同可以分为右手手征光纤光栅构成的所述偏振无关型轨道角动量调制器100和左手手征光栅光纤构成的所述偏振无关型轨道角动量调制器100。所述偏振无关型轨道角动量调制器100可以产生拓扑电荷为l＝+1或者l＝-1的轨道角动量光束。
在一个实施例中，所述偏振无关型轨道角动量调制器100还包括保护层。所述保护层位于所述光纤主体110外部，对所述光纤主体110形成机械保护，延长所述偏振无关型轨道角动量调制器100的使用寿命。所述偏振无关型轨道角动量调制器100为全光型器件，方便与其他光纤器件连接耦合，且具有很高的兼容。可以理解，所述偏振无关型轨道角动量调制器100具有质量轻、抗电磁干扰的特性。
请一并参见图3，本申请提供一种偏振无关型轨道角动量调制器制备方法。所述偏振无关型轨道角动量调制器制备方法包括：S10，加热光纤主体110至融熔状态。S20，扭曲融熔状态的所述光纤主体110。S30，冷却所述光纤主体110。
在步骤S10之前，将所述光纤主体110两端固定，一端固定于光纤旋转器上，另一端固定于光纤夹上。光纤夹具与光纤旋转器分别固定在一个位移平台上。在步骤S10中，在高温条件下，所述光纤主体110发生均匀的融熔。所述高温条件为1800℃以上。所述光纤主体110为少模光纤，所述少模光纤为模式高质量传输的通道。所述少模光纤可以为双模、四模等光纤。所述少模光纤具有质量轻、抗电磁干扰等优点。在步骤S20中，均匀地高速扭曲融熔的所述光纤主体110。在步骤S30中，待所述光纤主体110移除加热区时，迅速冷却所述光纤主体110。在所述光纤主体110的冷却过程中，冻结了部分残余应力，从而形成螺旋折射率调制，得到手征光纤光栅，完成偏振无关型轨道角动量调制器100的制备。
利用高温热源对所述光纤主体100局部加热至熔融状态的同时，对所述光纤主体110进行平移和扭曲，可以制备出具有螺旋型折射率分布的手征光纤光栅。所述手征光纤光栅可以作为所述偏振无关型轨道角动量调制器100。所述偏振无关型轨道角动量调制器100具有较高的耦合效率。所述耦合效率为将输入的基模光耦合到光纤的高阶模式中的比例。所述耦合效率与螺旋光栅结构和加工的方式有关。采用上述偏振无关型轨道角动量调制器制备方法具有较高的稳定性，加工得到的手征光纤非常均匀，能够较好的接近理论的极限值，具有较高的耦合效率。
请一并参见图4。在一个实施例中，所述加热光纤主体110至融熔状态步骤中，采用CO2激光、电弧持续放电、高温炉或线圈任一种热源加热所述光纤主体100至融熔状态。
在一个实施例中，在所述扭曲融熔状态的所述光纤主体110步骤中，沿瞬时针或逆时针方向扭曲融熔状态的所述光纤主体110。在一个实施例中，在所述冷却所述光纤主体110步骤中，将所述光纤主体110移除加热区，将扭曲过程中产生的残余应力冻结在所述光纤主体110中。
请一并参见图5，本申请提供一种轨道角动量光束发生器400。所述轨道角动量发生器400包括光源410、偏振控制器420、偏振无关型轨道角动量调制器100以及第一光纤跳线430。所述偏振控制器420的输入端与所述光源410连接的。所述偏振无关型轨道角动量调制器100与所述偏振控制器420的输出端连接。所述第一光纤跳线430与所述偏振无关型轨道角动量调制器100远离所述偏振控制器420的一端连接。其中，所述偏振无关型轨道角动量调制器100包括光纤主体110。所述光纤主体110具有螺旋光纤结构，所述螺旋光纤结构具有长周期光纤光栅效应。所述光纤主体110直径相同。所述光纤主体110轴向具有周期性的螺旋折射率调制，所述螺旋折射率调制周期位于百微米量级，所述螺旋折射率调制分布于所述光纤主体110的轴向、径向和角向，用于激发螺旋相位，产生轨道角动量光束。
所述光源410可以为可调激光器。所述可调激光器可以调节在所需波段。 轨道角动量光束发生器400可以在一个较宽的带宽内产生所需的OAM光束。在一个实施例中，所述带宽为3dB。通过调节所述偏振控制器420可以得到所需偏振态的输入光。所述第一光纤跳线430用于与其他仪器连接，方便所述轨道角动量光束发生器400在通信中进行应用。可以理解，各类光纤之间通过对应的适配器，即法兰进行连接。
在其中一个实施例中，所述轨道角动量光束发生器400还包括第二光纤跳线440。所述偏振控制器420输入端通过所述第二光纤跳线440与所述光源410连接。通过所述第二光纤跳线440可以保证光束的稳定传输。所述第二光纤跳线440可以为单模。
在其中一个实施例中，所述轨道角动量光束发生器400还包括单模光纤450。所述偏振无关型轨道角动量调制器100通过所述单模光纤450与所述偏振控制器420输出端连接。所述单模光纤450可以实现所述偏振无关型轨道角动量调制器100与所述偏振控制器420之间光束的低损耗传输。
在其中一个实施例中，所述轨道角动量光束发生器400还包括少模光纤460。所述第一光纤跳线430通过所述少模光纤460与所述偏振无关型轨道角动量调制器100连接。所述少模光纤460可以保证具有轨道角动量的光束的稳定传输。
请一并参见图6，采用测试装置测试所述偏振无关型轨道角动量调制器100的偏振相关性。搭建一套基于空间干涉的系统来检测所述偏振无关型轨道角动量调制器100产生的轨道角动量光束。通过OAM光束与球面波或平面波进行干涉，从而检测OAM的相位。该方法是将高斯光束和基于长周期手征光栅的所述偏振无关型轨道角动量调制器100产生的带有拓扑荷l的OAM光束在空间中干涉，若两束光在空间中同轴，对于球面波干涉条纹表现为拓扑荷数为l的螺旋相位图案。干涉条纹逆时针或顺时针的旋向取决于OAM光束的拓扑荷数l的符号。
可调谐单波长激光器输出的光经光纤跳线输出再经10倍物镜准直后进入可调分光比偏振分束器(Polarization Beam Splitter，PBS)分成两个光路， 即参考光路和测试光路。所述参考光路的输入光经过1/2波片后，进入合束器。所述测试光路的输入光顺序经过起偏器、1/4波片、10倍物镜、所述偏振无关型轨道角动量调制器100样品、40倍物镜和检偏器后进入合束器。两路光在所述合束器上相遇进而产生干涉，最后利用红外CCD(Charge Coupled Device)实时记录干涉条纹。
进一步的，所述可调激光器发出的光首先经过所述10倍物镜进行准直放大。在一个实施例中，采用型号为VA5-1550，Thorlabs的PBS对准直放大后的光束进行分束。所述PBS的分光比可调且分出的两束光分别是p偏振光和s偏振光。所述测试光路的输入光经反射镜改变光路方向后，依次经所述起偏器和所述1/4波片后产生不同偏振态的输入光。所述输入光包括线偏振(linear polarization，LP)，左旋圆偏振(left-handed circular polarization，CP-)和右旋圆偏振(left-handed circular polarization，CP+)。所述测试光路的输入光随后经过10倍物镜、所述偏振无关型轨道角动量调制器100以及40倍物镜后，进入所述检偏器。所述检偏器对产生的OAM光束的偏振态进行分析。所述参考光路的输入光经1/2波片将线偏振输入光的振动方向调整到与从所述检偏器出来的测试光路光的振动方向相同，确保参考光和测试光两束光发生干涉。最后采用合束器(Beam Combiner，BS)收集两路光进行干涉。在一个实施例中，所述BS型号为VC5-1550，Thorlabs。
完成光路搭建后，首先不加入参考光，只是用所述测试光路测量模场的分布。同时使用精密切割装置对所述偏振无关型轨道角动量调制器100样品的两端进行切割，并保证样品的端面平整度。然后将利用光纤夹持器220将其样品光纤端面固定在置于物镜之间的平台上。在一个实施例中，所述光纤夹持器220可以为双臂光纤夹具。
完成准备工作后，首先，从所述测试光路移除所述起偏器、所述1/4波片和所述检偏器，并将所述可调谐单波长激光器的波长设定在所述偏振无关型轨道角动量调制器100的谐振波长处。精密调节所述偏振无关型轨道角动量调制器100两端的三维位移台。使得所述偏振无关型轨道角动量调制器100 前后两个物镜分别聚集到所述偏振无关型轨道角动量调制器100样品的左右两个端面。在调试的过程中，通过所述红外CCD实时的检测光斑强度，直到从所述红外CCD的屏幕上观察到的光斑亮度最大位置，此时的从物镜到所述偏振无关型轨道角动量调制器100的耦合效率最高。
其次，在所述测试光路放入所述起偏器、所述1/4波片和所述检偏器。首先测试所述偏振无关型轨道角动量调制器100样品的输入光为线偏振光的情况。为了便于描述，规定平行于纸面的方向为0度。由于所述可调激光器的输出光为线偏振光，将所述起偏器的通光轴方向和所述1/4波片的快轴方向都置于0度位置，即可得到线偏振的输入光。所述线偏振光经过所述偏振无关型轨道角动量调制器100样品后，再经过所述检偏器。利用所述CCD分别记录当所述检偏器透光轴位于四个不同角度：0度，90度，180度和270度时单独的模场信息。
再次，加入所述参考光路，使所述参考光路与所述测试光路中的光进行干涉。利用所述CCD记录检偏器位于所述：0度，90度，180度和270度四个角度时的干涉信息，通过干涉的方式可以得到轨道角动量的相位信息。
最后，通过调节起偏器的透光轴与1/4波片的快轴夹角，可以得到左旋圆偏振和右旋圆偏振。每次输入不同的偏振态的输入光时，都重复上述操作，即可完成对所述偏振无关型轨道角动量调制器100的测试过程。
在同轴干涉测试螺旋相位测试装置的光路中，增加所述起偏器和所述波片等，实现任意偏振态的输入光，将偏振测试与螺旋相位相结合，可以测试所述偏振无关型轨道角动量调制器100的特性。调节所述检偏器透光轴和所述1/4波片快轴的夹角，可以产生线偏振和圆偏光等任意偏振态的光，由此可以输入不同偏振态的光。同时后端的所述检偏器可以用来分析输出光所具有的偏振态。通过这种方式就可以测试出在不同偏振态对于模式激发影响。
请一并参见图7，所述偏振无关型轨道角动量调制器100的右旋和左旋光谱如图7(a)和图7(b)所示。两个所述偏振无关型轨道角动量调制器100样品周期都是1192um，右旋偏振无关型轨道角动量调制器100的谐振峰的位 置在1554.5nm，耦合深度-24.6dB。左旋偏振无关型轨道角动量调制器100的谐振峰的位置在1550.9nm，耦合深度-22.5dB。结果表明所述偏振无关型轨道角动量调制器100的耦合效率高达86％。两根所述偏振无关型轨道角动量调制器100样品的谐振峰的细小差异在于所述偏振无关型轨道角动量调制器100制作过程中的外界抖动导致光栅的结构局部不均匀所致。可以理解，所述谐振峰的位置周期决定。所述耦合深度与光栅周期数，即螺旋个数，和加工过程的均匀性相关。
请一并参见图8，首先用右手所述偏振无关型轨道角动量调制器100样品测试在不同偏振态下轨道角动量的激发情况，其结果如图8(a)所示。当输入光为线偏振光时，不加参考光路的模场如图8(a11)所示。当光经过所述偏振无关型轨道角动量调制器100后，基模被激发到高阶的轨道角动量。从图上可以看到空心的圆环，并且当所述检偏器旋转一周，会出现明暗的变化，说明产生的光束具有线偏振的特点。为了观察螺旋相位信息，加入所述参考光路，并与所述测试光路进行干涉，通过干涉方法确定其相位信息。从图8(a12)可以发现逆时针旋转的螺旋光场分布，即拓扑电荷l＝+1。通过上述实验可以得知，当输入光为线偏振光时可以激发出带有线偏振的轨道角动量。同样的当输入光为右旋圆偏振(CP+)时，从图8(a21)可以看到空心的圆环，并且当所述检偏器旋转一周时，光斑的强度没有发生改变，这说明该光束的偏振态为圆偏振。此外可以观察到顺势旋转的干涉条纹，说明产生了拓扑电荷为l＝+1的轨道角动量。当输入光的偏振态为左旋圆偏振(CP-)时也有和CP+一样的现象。通过上述实验表明所述偏振无关型轨道角动量调制器100在不同偏振态时都可以激发l＝+1的轨道角动量，具有偏振不相关的特性。此外我们利用左手螺旋的所述偏振无关型轨道角动量调制器100，在不同的偏振态下也可以激发出l＝-1的轨道角动量。实验表明，通过高温热源局部加热并平移扭曲的方法制备的所述偏振无关型轨道角动量调制器100具有偏振无关的特点，该器件有望在通信领域发挥巨大作用。
本申请所述偏振无关型轨道角动量调制器100基于手征光纤光栅的偏振 不相关特性，可以在任意偏振态的条件下，激发l＝±1阶的轨道角动量光束，并且激发的l＝-1或者l＝+1仅取决于所述偏振无关型轨道角动量调制器100螺旋光纤光栅的旋性。其周期在百微米量级，螺旋调制均匀，光谱质量高，模式耦合清晰。同时所述偏振无关型轨道角动量调制器100基于少模光纤，是一种全光纤型的调制器，具有体积小，成本低，耦合效率高，易集成等优势，在光纤通信，光镊，微粒操控等领域有重要应用。
以上所述实施例的各技术特征可以进行任意的组合，为使描述简洁，未对上述实施例中的各个技术特征所有可能的组合都进行描述，然而，只要这些技术特征的组合不存在矛盾，都应当认为是本说明书记载的范围。
以上所述实施例仅表达了本申请的几种实施方式，其描述较为具体和详细，但并不能因此而理解为对申请专利范围的限制。应当指出的是，对于本领域的普通技术人员来说，在不脱离本申请构思的前提下，还可以做出若干变形和改进，这些都属于本申请的保护范围。因此，本申请专利的保护范围应以所附权利要求为准。

Claims (16)

  1. 一种偏振无关型轨道角动量调制器(100)，其特征在于，包括：
    光纤主体(110)，具有螺旋光纤结构，所述螺旋光纤结构具有长周期光纤光栅效应；
    所述光纤主体(110)轴向具有周期性的螺旋折射率调制，所述螺旋折射率调制周期位于百微米量级，所述螺旋折射率调制分布于所述光纤主体(110)的轴向、径向和角向，用于激发螺旋相位，产生轨道角动量光束。
  2. 根据权利要求1所述的偏振无关型轨道角动量调制器(100)，其特征在于，所述光纤主体(110)为双模光纤或四模光纤。
  3. 根据权利要求1所述的偏振无关型轨道角动量调制器(100)，其特征在于，所述光纤主体(110)轴向具有周期和深度均匀的所述螺旋折射率调制，所述螺旋折射率调制量的范围为2ⅹ10 -4至2ⅹ10 -3。
  4. 根据权利要求1所述的偏振无关型轨道角动量调制器(100)，其特征在于，所述光纤主体(110)包括光纤纤芯和包层。
  5. 根据权利要求4所述的偏振无关型轨道角动量调制器(100)，其特征在于，所述光纤纤芯直径为19微米，所述包层直径为125微米。
  6. 根据权利要求1所述的偏振无关型轨道角动量调制器(100)，其特征在于，所述光纤主体(110)任一截面的所述螺旋折射率调制具有非对称性，沿所述光纤主体(110)径向，所述螺旋折射率调制随半径的增大而减小。
  7. 根据权利要求1所述的偏振无关型轨道角动量调制器(100)，其特征在于，所述光纤主体(110)的长度范围为1厘米至6厘米。
  8. 根据权利要求1所述的偏振无关型轨道角动量调制器(100)，其特征在于，还包括保护层。
  9. 一种偏振无关型轨道角动量调制器制备方法，其特征在于，包括：
    加热光纤主体(110)至融熔状态；
    扭曲融熔状态的所述光纤主体(110)；
    冷却所述光纤主体(110)。
  10. 根据权利要求9所述的偏振无关型轨道角动量调制器制备方法，其特征在于，在所述加热光纤主体(110)至融熔状态步骤中，采用CO2激光、电弧持续放电、高温炉或线圈任一种热源加热所述光纤主体(100)至融熔状态。
  11. 根据权利要求9所述的偏振无关型轨道角动量调制器制备方法，其特征在于，在所述扭曲融熔状态的所述光纤主体(110)步骤中，沿瞬时针或逆时针方向扭曲融熔状态的所述光纤主体(110)。
  12. 根据权利要求9所述的偏振无关型轨道角动量调制器制备方法，其特征在于，在所述冷却所述光纤主体(110)步骤中，将所述光纤主体(110)移除加热区，将扭曲过程中产生的残余应力冻结在所述光纤主体(110)中。
  13. 一种轨道角动量光束发生器(400)，其特征在于，包括：
    光源(410)；
    偏振控制器(420)，输入端与所述光源(410)连接；
    偏振无关型轨道角动量调制器(100)，与所述偏振控制器(420)输出端连接；以及
    第一光纤跳线(430)，与所述偏振无关型轨道角动量调制器(100)远离所述偏振控制器(420)的一端连接；
    其中，所述偏振无关型轨道角动量调制器(100)包括：
    光纤主体(110)，具有螺旋光纤结构，所述螺旋光纤结构具有长周期光纤光栅效应；
    所述光纤主体(110)轴向具有周期性的螺旋折射率调制，所述螺旋折射率调制周期位于百微米量级，所述螺旋折射率调制分布于所述光纤主体(110)的轴向、径向和角向，用于激发螺旋相位，产生轨道角动量光束。
  14. 根据权利要求13所述的轨道角动量光束发生器(400)，其特征在于，还包括：
    第二光纤跳线(440)，所述偏振控制器(420)输入端通过所述第二光纤跳线(440)与所述光源(410)连接。
  15. 根据权利要求13所述的轨道角动量光束发生器(400)，其特征在于，还包括：
    单模光纤(450)，所述偏振无关型轨道角动量调制器(100)通过所述单模光纤(450)与所述偏振控制器(420)输出端连接。
  16. 根据权利要求13所述的轨道角动量光束发生器(400)，其特征在于，还包括：
    少模光纤(460)，所述第一光纤跳线(430)通过所述少模光纤(460)与偏振无关型轨道角动量调制器(100)连接。
PCT/CN2018/117890 2018-11-28 2018-11-28 基于手征光纤光栅的偏振无关型轨道角动量调制器及其制备方法和轨道角动量光束发生器 WO2020107254A1 (zh)

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