CN112698440A - Spiral fiber grating, preparation method and all-fiber orbital angular momentum beam generator - Google Patents
Spiral fiber grating, preparation method and all-fiber orbital angular momentum beam generator Download PDFInfo
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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
- G02B6/02147—Point by point fabrication, i.e. grating elements induced one step at a time along the fibre, e.g. by scanning a laser beam, arc discharge scanning
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
- G02B27/09—Beam shaping, e.g. changing the cross-sectional area, not otherwise provided for
- G02B27/0927—Systems for changing the beam intensity distribution, e.g. Gaussian to top-hat
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
- G02B27/09—Beam shaping, e.g. changing the cross-sectional area, not otherwise provided for
- G02B27/0938—Using specific optical elements
- G02B27/0994—Fibers, light pipes
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- 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
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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
- 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
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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
- G02B6/0208—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
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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
- G02B2006/02157—Grating written during drawing of the fibre
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Abstract
The application discloses a helical fiber grating, which comprises an optical fiber and a modulation fiber grating formed on the cladding of the optical fiber and having a helical line with a variable refractive index. The application also discloses a preparation method of the spiral fiber grating. The application also discloses an all-fiber orbital angular momentum beam generator, which comprises a few-mode fiber and a helical long-period fiber grating formed on the cladding of the few-mode fiber, wherein the generator is of an all-fiber structure.
Description
Technical Field
The invention relates to the technical field of fiber gratings, in particular to a spiral fiber grating, a preparation method and an all-fiber orbital angular momentum beam generator.
Background
Once proposed, the spiral fiber grating has received a lot of attention, and the existing preparation method is to first locally melt the optical fiber by the thermal effect and then physically twist the optical fiber geometry to form a spiral structure. The heating mode can be a high-temperature furnace, oxyhydrogen flame or laser heating.
For example, a method for preparing a spiral fiber grating by oxyhydrogen flame comprises the following steps: the thermal effect of oxyhydrogen flame is utilized to locally melt the optical fiber, then the geometric structure of the optical fiber is deformed in a spiral shape, and the optical fiber is periodically heated to form periodic refractive index change. The method has the advantages that the method is suitable for any optical fiber, other processing is not needed to be carried out on the optical fiber, the grating period can be flexibly changed according to the preparation requirement, the oxyhydrogen flame temperature is stable, the heating efficiency is high, a mask plate is not needed, photosensitivity is not needed, the grating can be formed by the common communication optical fiber, and the grating forming efficiency and the grating quality are greatly improved. However, the method has low efficiency, low repetition rate of grating preparation, uncontrollable grating resonant wavelength and resonant peak depth, and unstable insertion loss and insufficiently deep resonant peak of the manufactured grating. In addition, the method is mainly used for the long-period fiber grating because the method is mainly twisted through the physical geometrical structure, and is not suitable for manufacturing the short-period fiber grating.
Disclosure of Invention
In order to overcome the problems in the prior art, the invention provides a preparation method of a spiral fiber grating, which comprises the following steps:
step S1: preparing an optical fiber, and stripping a coating layer of the optical fiber;
step S2: fixing the optical fiber on a clamp of a mobile platform;
step S3: introducing CO2Focusing a laser beam on a cladding of the optical fiber; the optical fiber is made to rotate around the axis of the optical fiber and translate along the horizontal direction at the same time, and CO is utilized2The thermal effect of the laser beam is used for carrying out spiral refractive index modulation on the optical fiber cladding and forming the spiral refractive index change modulation type optical fiber grating.
As an improvement of the preparation method of the spiral fiber grating provided by the invention, in step S3, CO is used2Laser is radiated from the surface to the inner side on the surface of the cladding, so that the spiral fiber grating forms continuous spiral refractive index modulation on the fiber cladding.
As an improvement of the method for manufacturing the spiral fiber grating provided by the present invention, in step S3, control parameters of the relative motion of the optical fiber, which is rotated around its own axis and translated in the horizontal direction, are set according to a phase matching formula; the phase matching formulas include formula (1), formula (2), and formula (3):
nN＝nF-m.lambda/T equation (1)
JN＝JF+ m.sigma equation (2)
Formula (3) of T2 pi · ν/ω
Wherein n isFAnd nNRespectively representing the effective refractive indices of the fundamental and coupled modes, λ is the resonance wavelength for the helical fiber grating, T is the corresponding grating period, and JFAnd JNIs the total angular momentum of the two corresponding modes, which is the sum of the orbital angular momentum and the spin angular momentum of the corresponding modes, m is in the order of harmonics, σ represents the spin direction of the helical fiber grating, v is the horizontal motion velocity of the moving platform, and ω is the fiber rotation angular velocity.
As an improvement of the preparation method of the spiral fiber grating provided by the invention, before the fiber cladding is modulated by the spiral refractive index, the numerical operation method is used for simulating the periodic parameters which are corresponding to the fiber and generate the high-order mode, so as to obtain partial control parameters.
As an improvement of the preparation method of the spiral fiber grating provided by the invention, the writing of the grating with different periods is realized by controlling the horizontal moving speed and the rotating speed of the optical fiber, and the tuning of the writing wavelength of the spiral fiber grating is realized.
As an improvement of the preparation method of the spiral fiber grating provided by the invention, one end of the optical fiber is connected with the light source, and the other end of the optical fiber is connected with the spectrometer;
observing the spectrum pattern of the fiber grating by a spectrometer, and turning off CO focused on the fiber after obtaining the required spectrum pattern2The laser beam stops the fiber from moving horizontally and rotationally.
As an improvement of the preparation method of the spiral fiber grating provided by the invention, the clamp comprises two rotary clamps which are respectively arranged at two ends of the optical fiber; the two rotary fixtures are driven by the two rotary motors, and the two rotary motors have the same rotating speed and the same rotating direction.
The application also provides a helical fiber grating, which comprises an optical fiber and a modulation fiber grating formed on the optical fiber cladding and with the change of the helical line refractive index.
As an improvement of the preparation method of the spiral fiber grating provided by the invention, the spiral fiber grating is prepared by the preparation method.
The application also provides an all-fiber orbital angular momentum beam generator, which comprises a few-mode fiber and a spiral long-period fiber grating formed on the cladding of the few-mode fiber, wherein the generator is of an all-fiber structure; the light incident into the fiber grating is influenced by the grating action to generate a high-order mode on one hand and the spiral refractive index distribution to generate an additional spiral phase so as to generate a high-order orbital angular momentum beam on the other hand.
As an improvement of the full-fiber orbital angular momentum beam generator provided by the invention, the spiral long-period fiber grating is prepared by the preparation method.
As an improvement of the full-fiber orbital angular momentum beam generator provided by the invention, the few-mode fiber is a graded or jump-index fiber supporting high-order mode independent stable propagation.
The application has the following beneficial effects:
the invention adopts low-energy CO2A weak coupling method for modulating the spiral refractive index of the optical fiber cladding by laser beam includes modulating the spiral refractive index of the optical fiber cladding and forming a spiral refractive index variation modulation type optical fiber grating on the cladding. Compared with the method of exciting a high-order vector mode on an optical fiber through large-energy strong modulation, the preparation method has a good protection effect on the grating, the obtained grating is small in damage, the insertion loss of the manufactured grating is low, the depth of a resonance peak is deep, the mode conversion efficiency can reach 99%, and the grating with the same characteristic can be repeatedly prepared for many times by the same parameter.
In addition, the existing long-period fiber grating which is written by the point-by-point exposure of the few-mode fiber can only generate a linear polarization mode, and cannot directly generate orbital angular momentum beams. In the application, by modulating the helical refractive index of the cladding of the few-mode optical fiber, light entering the grating is influenced by the helical refractive index distribution to generate an additional helical phase besides a high-order mode generated by the grating action, so that the generated high-order mode has the helical phase, and the high-order mode and the helical phase are resonantly enhanced under the action of a plurality of grating modulation periods to form the high-order mode of the helical phase, so that the high-order orbital angular momentum light beam can be directly generated under the condition that a polarization controller, a pressure plate or an optical fiber twister is not suitable. Most importantly, the generated high-order orbital angular momentum light beam can be directly transmitted in the few-mode optical fiber.
Drawings
FIG. 1 is a schematic diagram of an apparatus for manufacturing a spiral fiber grating according to an embodiment of the present invention;
fig. 2 is a schematic diagram of an all-fiber orbital angular momentum beam generator according to an embodiment of the invention.
Reference numerals:
CO2the device comprises a laser 1, a reflector 2, an attenuator 3 for controlling laser energy, a beam expander 4, a laser switch 5 for controlling on-off of a light path, a focusing lens 6, a rotary clamp 7, a moving platform 8, a spectrometer 9, a motion controller 10, a light source 11, a control device 12, a fiber core 10, a cladding 20 and a spiral fiber grating 30.
The specific implementation mode is as follows:
in order to make the technical solutions of the present invention better understood, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
In the description of the present invention, it is to be understood that the terms "central," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," "circumferential," and the like are used in the orientations and positional relationships indicated in the drawings for convenience in describing the invention and to simplify the description, and are not intended to indicate or imply that the referenced devices or elements must have a particular orientation, be constructed and operated in a particular orientation, and are therefore not to be considered limiting of the invention.
Furthermore, the terms "first", "second" and "first" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one such feature. In the description of the present invention, "a plurality" means at least two, e.g., two, three, etc., unless specifically limited otherwise.
In the present invention, unless otherwise expressly stated or limited, the terms "mounted," "connected," "secured," and the like are to be construed broadly and can, for example, be fixedly connected, detachably connected, or integrally formed; may be mechanically coupled, may be electrically coupled or may be in communication with each other; they may be directly connected or indirectly connected through intervening media, or they may be connected internally or in any other suitable relationship, unless expressly stated otherwise. The specific meanings of the above terms in the present invention can be understood by those skilled in the art according to specific situations.
The technical solution of the present invention is further described in detail by the accompanying drawings and embodiments.
The method for manufacturing the spiral fiber grating 30 according to the embodiment of the present application includes the following steps:
step S1: preparing an optical fiber, and stripping a coating layer of the optical fiber;
step S2: fixing the optical fiber on a clamp of a mobile platform;
step S3: introducing CO2Package with laser beam focused on optical fiberA layer 20; the optical fiber is made to rotate around the axis of the optical fiber and simultaneously translate along the horizontal direction, CO2The laser beam is modulated in the helical refractive index in the fiber cladding 20 and forms a helical refractive index change modulation type long period fiber grating in the cladding 20.
In step S3, the light path is adjusted to reduce the energy of CO2The laser beam is focused on the fiber cladding 20. Then, the optical fiber is enabled to rotate and simultaneously horizontally translate, so that the residual stress is eliminated by performing laser heating along the spiral path irradiated by the laser, and spiral refractive index modulation is formed on the optical fiber cladding 20, thereby realizing CO2The laser writes a high precision spiral path in the fiber cladding 20.
In the above step S3, the control parameters of the relative movement of the optical fiber, which is rotated around its own axis and translated in the horizontal direction, may be set according to the phase matching formula; the phase matching formulas include formula (1), formula (2), and formula (3):
nN＝nF-m.lambda/T equation (1)
JN＝JF+ m.sigma equation (2)
Formula (3) of T2 pi · ν/ω
Wherein n isFAnd nNRespectively, the effective refractive indices of the fundamental and coupled modes, λ is the resonance wavelength corresponding to the helical fiber grating, T is the grating period, and J isFAnd JNIs the total angular momentum of two corresponding modes, which is the sum of the orbital angular momentum and the spin angular momentum of the respective modes, m is of harmonic magnitude, σ represents the spin direction of the helical fiber grating, and σ -1 are denoted as left-hand and right-hand, respectively. v is the horizontal motion rate of the moving platform and ω is the angular velocity of rotation of the fiber.
Moving objects have momentum and objects of different motion types have different momentum. For the light beam each photon has
And has two kinds of angular momentum, namely spin angular momentum and orbitAngular momentum. If the electric field of a light beam rotates along the optical axis, i.e. circularly polarized, the light beam has a spin angular momentum and the spin orbital angular momentum of each photon is
Wherein "±" represents levorotation and dextrorotation; a light beam has orbital angular momentum if its wavevector rotates along the optical axis to form a helical phase plane. The expression of the helical phase plane is exp (il Φ), where Φ is the angular coordinate and l is the topological charge of the orbital angular momentum. The value of l is an integer and can be positive, negative and zero, corresponding to clockwise, counterclockwise helical and no-helical phases, respectively. The spiral direction of the spiral phase wave surface of the light beam and the number of the spiral phase wave surfaces depend on the positive and negative sum of l. In 1992, Allen et al demonstrated that all vortex beams with helical phase factor exp (il Φ) were in band under paraxial propagation conditions
And all the spiral phase beams can be represented by linear superposition. The conclusion is drawn to the heat of the study of orbital angular momentum by scholars, and various new phenomena related to the orbital angular momentum are generated from various generation technologies, detection methods and applications of the orbital angular momentum.
The phase matching of the helical fiber grating can be satisfied by the above-described formula (1), formula (2), and formula (3).
Before the optical fiber cladding 20 is modulated by the helical refractive index, a numerical operation method is used for simulating the corresponding periodic parameters of the optical fiber, which generate high-order modes, so as to obtain part of control parameters in the formula (1), the formula (2) and the formula (3).
The clamp comprises two rotary clamps which are respectively arranged at two ends of the optical fiber. The two rotary clamps are driven by the two rotary motors, and the two rotary motors have the same rotating speed and the same rotating direction. Two rotating fixtures are arranged on the moving platform. The optical fiber rotating fixture adopts two rotating fixtures, and if one rotating fixture is adopted, the optical fiber can be off-axis due to the action of centrifugal force in the rotating process because the rotating fixture can not be accurately coaxial. Compared with the single rotary clamp, the coaxial rotary clamp has the advantages that the coaxial rotation technology that the two rotary motors rotate the optical fiber in the same direction is adopted, so that the good coaxiality of the optical fiber rotation can be realized, and the coaxial precision requirement of the optical fiber rotation is ensured; and the optical fiber is kept stable in the rotating process, and the writing quality and writing efficiency of the grating are improved. ω is the rotational angular velocity of the rotating electrical machine.
In the preparation process, the rotating motor drives the rotating clamp and the optical fiber to rotate. Meanwhile, the rotary clamp, the rotary motor and the optical fiber are arranged on the moving platform and can be driven by the moving platform to simultaneously move horizontally.
In the preparation process, the optical fiber is horizontally translated while being ensured to rotate, so that CO is realized2The laser writes a high precision spiral path in the fiber cladding 20. The laser spiral refractive index modulation technology requires precise synchronization between laser pulses and optical fiber movement, and if the laser pulses and the optical fiber movement cannot be kept synchronized at any time, the grid period and the refractive index modulation depth are not uniform, so that the writing efficiency is reduced, and the spectral quality is reduced.
The control parameters of the rotary motion and the relative motion of the translation along the horizontal direction can be written by an autonomous writing control program to carry out accurate control. The control device such as a computer can control the rotating motor, the mobile platform and the laser to be started simultaneously, and then grating writing can be carried out.
The adjustability of the resonance wavelength and the resonance peak value can be realized by controlling the speed and the distance of the moving platform and the rotating speed and the number of turns of the rotating clamp. For example, the self-programmed program is used for realizing the control of the synchronism and the consistency of the mobile platform and the rotary clamp, namely ensuring the simultaneous operation of the mobile platform and the rotary clamp and stopping the same.
The control parameters include, for example, the direction of rotation of the rotating motor, the angle of rotation, the speed and direction of movement of the mobile platform. By controlling the above control parameters, the scanning path and direction of the laser can be controlled, so as to obtain the desired spiral fiber grating 30.
The writing of the grating with different periods is realized by controlling the horizontal moving speed and the rotating speed of the optical fiber, and the tuning of the writing wavelength of the spiral optical fiber grating is realized.
Further, in the grating writing process of step S3, one end of the optical fiber is connected to the light source, and the other end is connected to the spectrometer. Observing the spectrum pattern of the fiber grating by a spectrometer, and turning off CO focused on the fiber after obtaining the required spectrum pattern2The laser beam stops the fiber from moving horizontally and rotationally. The spectrum of the grating is observed in the preparation process, the adjustability of the resonance wavelength and the resonance peak value can be realized, and the grating with the spectrum meeting the requirement is searched without continuously reducing the length of the grating after the grating with a certain length is prepared.
The spiral fiber grating 30 prepared by the method of the present application is a long-period fiber grating having a period greater than 30 μm.
The preparation method is applicable to all optical fiber types. For example, the class of optical fibers includes, but is not limited to, single mode fibers, few mode fibers.
An apparatus for manufacturing a spiral fiber grating 30 according to the present application is shown in fig. 1. The method comprises the following steps: CO22A laser processing optical path system, a clamping and moving device and a control device 12. The control device 12 is CO-operated with CO2The laser processing light path system is connected with the clamping moving device.
CO2The laser processing optical path system comprises CO2The laser device comprises a laser 1, a reflector 2, an attenuator 3 for controlling laser energy, a beam expander 4, a laser switch 5 for controlling the on-off of a light path and a focusing lens 6. The gripping and moving device comprises a moving platform 8, a motion controller 10 and a rotating gripper 7.
The preparation device also comprises a spectrometer 9 and a light source 11. The light source 11 and the spectrometer 9 are respectively connected to two ends of the optical fiber.
The moving platform 8 is, for example, a three-dimensional precision moving platform. The laser switch 5 may be an electronic shutter (electronic switch) controlled by the control device 12. The control device 12 may be an intelligent terminal, such as a computer.
The present application uses a CO2 laser as the light source, and a CO2 laser may be provided by a CO2 laser. The stability of the CO2 laser reaches +/-2%, the quality of a focusing light spot is good, the size and the energy density of the light spot can be ensured to be uniform and consistent when the optical fiber is periodically heated, the stability and the repeatability of repeated exposure at different periods can be ensured, and the heating area and a focal spot in the scanning process can be well overlapped. It can be understood that by selecting the appropriate laser power, it is ensured that the grating can be normally written without damaging the optical fiber.
CO2The laser emitted by the laser 1 passes through the reflector 2 and the attenuator 3 in sequence to obtain laser with proper energy and mode, the laser is expanded by the beam expander 4 to reach the focusing lens 6 for focusing, and finally, CO is absorbed by the laser2The laser beam is focused on the cladding 20 of the fiber. The rotating clamp 7 enables the optical fiber to rotate, the rotating clamp 7 and the optical fiber are arranged on the moving platform, and the moving platform translates to enable the optical fiber to rotate and also translate in the horizontal direction.
The rotary jig 7 includes two rotary jigs respectively provided at both ends of the optical fiber. The two rotary clamps are driven by the two rotary motors, and have the same rotating speed and the same rotating direction.
The laser switch 5, the rotary clamp 7 and the mobile platform 8 are connected with the motion controller 10 and then controlled by software on the control device 12, and the spectrometer 9 and the light source 11 are combined to observe the spectrum condition of the grating in real time. The autonomously written program may also control the CO2An electronic shutter for the laser.
The self-programmed program controls the mobile platform, the rotating motor and the electronic shutter, so that the horizontal movement of the three-dimensional mobile platform, the rotation of the rotating clamp and the opening and closing of the electronic shutter are combined, and the optical fiber is positioned in the CO2Uniform helical refractive index modulation is achieved under laser light. The writing of the grating with different periods is realized by controlling the horizontal moving speed of the optical fiber and the rotating speed of the rotating clamp, and the tuning of the writing wavelength of the spiral optical fiber grating is realized.
When the moving distance and the number of rotation turns of the moving platform meet the writing length of the fiber bragg grating and the resonance wavelength and the resonance depth meet the requirements, the electronic shutter is controlled to enable CO to be in contact with the fiber bragg grating2The laser is in a shielding state, namely any requirement can be metSpectrum of (a).
The spiral fiber grating 30 of the present application does not require the use of high temperatures or large amounts of energy to melt the fiber and then spin it to physically twist the fiber geometry. The invention adopts low-energy CO2In the weak coupling method for modulating the spiral refractive index of the fiber cladding 20 with the laser beam, the spiral refractive index is modulated in the fiber cladding 20 to form a spiral refractive index change modulation fiber grating in the cladding 20. Compared with a large-energy-emphasis mode, the preparation method has a good protection effect on the grating, the obtained grating is very small in damage, the manufactured grating is low in insertion loss, deep in resonant peak depth, high in temperature stability and capable of achieving 99% of mode conversion efficiency, the grating with the same characteristic can be repeatedly prepared for many times by the same parameter, and tuning of the spectral parameter of the fiber grating can be flexibly achieved.
In the present application, the spiral structure of the spiral fiber grating is a continuous spiral line extending spirally along the axial direction in the cladding 20, and the continuous spiral line passes through CO2The thermal effect of the laser is formed in the cladding. By CO2Laser is radiated from the surface to the inner side on the surface of the cladding, so that the spiral fiber grating forms continuous spiral refractive index modulation on the fiber cladding. Using lower energy CO by a weakly coupled process2The laser carries out spiral refractive index modulation on the cladding 20 of the few-mode fiber, and a full-fiber high-order orbital angular momentum beam generator which directly generates a high-order orbital angular momentum beam can be obtained.
This application is writing system screw-tupe fiber grating at the cladding, compares with writing system screw-tupe fiber grating at the fibre core, writes system on the cladding and is minimum to the whole damage of optic fibre, hardly influences the fiber structure, and the insertion loss of production is also very low, can directly be used for large capacity communication system.
The existing methods for generating orbital angular momentum beams include a spiral phase plate method, an SLM conversion method, a lens conversion method, a Q-disk conversion method and an integrated device conversion method, and the methods are orbital angular momentum beam methods based on own space and integrated devices. However, the actual communication system uses the optical fiber as the main transmission medium, and the orbital angular momentum beam generated by the above method needs to be coupled into the optical fiber to be actually used in the actual communication system, which brings extra workload and technical difficulty. For example, when the vortex optical wavelength division multiplexing technology based on integrated devices such as free space or silicon-based chips is used for optical fiber transmission, the multiplexed light must be coupled into the optical fiber at the signal transmitting end and coupled out of the optical fiber at the receiving end. Loss and crosstalk are often introduced during coupling, and therefore, the vortex optical wavelength division multiplexing applied to the optical fiber is greatly influenced.
Example two
The present application also provides a spiral fiber grating 30. Which is a modulation type fiber grating formed in the fiber cladding 20 and having a change in the refractive index of the spiral line
The application also provides a spiral fiber grating 30 which is prepared by the preparation method.
Preferably, by the preparation method of the first embodiment, a modulation type long-period fiber grating with a spiral refractive index variation can be formed on the few-mode fiber cladding 20, and the grating is a spiral type long-period fiber grating with a few-mode fiber as a substrate.
Applications use graded or step index few-mode fibers that support stable propagation of high-order modes alone. By the method of modulating the spiral refractive index in the few-mode fiber cladding 20, under the condition of meeting the phase matching condition, the fundamental mode is coupled to a specific high-order mode in the few-mode spiral fiber grating, is excited into the high-order mode and carries the spiral phase, so that the fundamental mode is directly converted into a high-order orbital angular momentum beam, and the fundamental mode passing through the few-mode spiral fiber grating is converted into the high-order orbital angular momentum beam.
EXAMPLE III
The present application further provides an all-fiber orbital angular momentum beam generator, and fig. 2 is a schematic structural diagram of an orbital angular momentum beam generator according to an embodiment of the present invention.
In the specific embodiment of the application, the orbital angular momentum beam generator is made using a few-mode fiber, which includes a core 10 and a cladding 20. A helical fiber grating 30 is formed on the few-mode fiber cladding 20. The spiral fiber grating is a spiral long-period fiber grating, which is a spiral long-period fiber grating with few-mode fiber as a substrate.
The spiral long period fiber grating is prepared by the preparation method of the first embodiment.
Few-mode fibers are graded or step-index fibers that support stable propagation of high-order modes alone.
The orbital angular momentum beam generator is a high-order orbital angular momentum beam generator, a spiral long-period fiber grating is obtained by modulating a spiral refractive index in a few-mode fiber cladding 20, a basic mode is coupled to a specific high-order mode in the few-mode spiral fiber grating under the condition of meeting phase matching, light incident into the fiber grating is subjected to the grating action to generate the high-order mode on one hand, and is also subjected to the influence of spiral refractive index distribution to generate an additional spiral phase on the other hand, so that the basic mode is directly converted into the high-order orbital angular momentum beam. The spiral fiber grating 30 on the few-mode fiber is formed by a plurality of continuously distributed spiral structures with the same rotation direction, and the direction of the spiral structure of the grating main body is the same as that of the generated high-order orbital angular momentum light beam. Multiple helical structures can excite helical phases within a certain bandwidth.
The high-order orbital angular momentum light beam generator is of an all-fiber structure, has a spiral structure, and can directly convert a basic mode into a light beam with high-order orbital angular momentum.
The utility model provides an all-fiber orbit angular momentum beam generator can support the orbit angular momentum beam transmission of first order, second order, third order, fourth order simultaneously. The all-fiber orbital angular momentum beam generator can be directly applied to an optical fiber transmission system, and can realize the communication transmission of the all-fiber orbital angular momentum beams without any space debugging.
According to the method, the cladding of the few-mode optical fiber is modulated by the spiral refractive index, light entering the grating is influenced by the spiral refractive index distribution to generate an additional spiral phase besides a high-order mode generated by the grating action, so that the light has the spiral phase, and the high-order mode and the spiral phase are resonantly enhanced under the action of a plurality of grating modulation periods to form the high-order mode with the spiral phase, so that the stable high-order orbital angular momentum light beam can be directly obtained under the condition of not using a polarization controller, a pressure plate or an optical fiber twister. Most importantly, the high-order orbital angular momentum light beam can be stably transmitted in the optical fiber directly.
The invention utilizes CO2The technology for preparing the helical refractive index change modulation type long-period fiber grating by laser is used for preparing the helical long-period fiber grating based on the few-mode fiber as a high-order orbital angular momentum beam generator, and the device is simple in structure, clear in principle, low in manufacturing cost and easy to realize; the period can be accurately controlled, and the stability is high.
The optical fiber orbital angular momentum beam generator can directionally generate a specific orbital angular momentum beam, has high efficiency and high purity, can support stable transmission, adopts an all-fiber structure, has a compact structure, and is easily compatible with an optical fiber communication network.
The high-order orbital angular momentum light beam generator provided by the application has unique physical characteristics, and has wide prospects and important application values in a plurality of fields such as optical tweezers, biomedical imaging particle control, microscopic imaging, quantum information processing and high-capacity optical communication.
It is to be understood that the above-described embodiments are merely illustrative of some, but not restrictive, of the broad invention, and that the appended drawings illustrate preferred embodiments of the invention and do not limit the scope of the invention. This application is capable of embodiments in many different forms and is provided for the purpose of enabling a thorough understanding of the disclosure of the application. Although the present application has been described in detail with reference to the foregoing embodiments, it will be apparent to one skilled in the art that the present application may be practiced without modification or with equivalents of some of the features described in the foregoing embodiments. All equivalent structures made by using the contents of the specification and the drawings of the present application are directly or indirectly applied to other related technical fields and are within the protection scope of the present application.
Claims (12)
1. A preparation method of a spiral fiber grating is characterized by comprising the following steps:
step S1: preparing an optical fiber, and stripping a coating layer of the optical fiber;
step S2: fixing the optical fiber on a clamp of a mobile platform;
step S3: introducing CO2Focusing a laser beam on a cladding of the optical fiber; the optical fiber is made to rotate around the axis of the optical fiber and translate along the horizontal direction at the same time, and CO is utilized2The thermal effect of the laser beam is used for carrying out spiral refractive index modulation on the optical fiber cladding and forming the spiral refractive index change modulation type optical fiber grating.
2. The method for preparing a spiral fiber grating as claimed in claim 1, wherein in step S3, the grating is prepared by CO2Laser is radiated from the surface to the inner side on the surface of the cladding, so that the spiral fiber grating forms continuous spiral refractive index modulation on the fiber cladding.
3. The method for manufacturing a helical fiber grating as claimed in claim 1, wherein in step S3, control parameters of the relative motion of the optical fiber, which is rotation around its axis and translation in the horizontal direction, are set according to a phase matching formula; the phase matching formulas include formula (1), formula (2), and formula (3):
nN＝nF-m.lambda/T equation (1)
JN＝JF+ m.sigma equation (2)
Formula (3) of T2 pi · ν/ω
Wherein n isFAnd nNRespectively representing the effective refractive indices of the fundamental and coupled modes, λ is the resonance wavelength for the helical fiber grating, T is the corresponding grating period, and JFAnd JNIs the total angular momentum of two corresponding modes, which is the sum of the orbital angular momentum and the spin angular momentum of the corresponding modes, m is in the order of harmonics, σ represents the spin direction of the spiral fiber grating, and v is that of the moving platformThe horizontal movement rate, ω, is the fiber rotation angular velocity.
4. The method of claim 1, wherein before the modulation of the helical refractive index of the fiber cladding, the periodic parameters of the fiber corresponding to the higher order modes are simulated by using a numerical operation method to obtain the partial control parameters.
5. The method according to claim 1, wherein the writing of the grating with different periods is realized by controlling the horizontal moving speed and the rotating speed of the optical fiber, so as to realize the tuning of the writing wavelength of the spiral fiber grating.
6. The method for preparing a spiral fiber grating according to claim 1, wherein one end of the optical fiber is connected with a light source, and the other end is connected with a spectrometer;
observing the spectrum pattern of the fiber grating by a spectrometer, and turning off CO focused on the fiber after obtaining the required spectrum pattern2The laser beam stops the fiber from moving horizontally and rotationally.
7. The method for preparing a spiral fiber grating according to claim 1, wherein the jig comprises two rotary jigs respectively disposed at two ends of the optical fiber; the two rotary fixtures are driven by the two rotary motors, and the two rotary motors have the same rotating speed and the same rotating direction.
8. A helical fiber grating includes an optical fiber and a modulation fiber grating formed in a cladding of the optical fiber and having a refractive index of a helix varied.
9. The spiral fiber grating according to claim 8, wherein the spiral fiber grating is prepared by the preparation method according to any one of claims 1 to 7.
10. The all-fiber orbital angular momentum beam generator is characterized by comprising a few-mode fiber and a spiral long-period fiber grating formed on the cladding of the few-mode fiber, wherein the generator is of an all-fiber structure; the light incident into the fiber grating is influenced by the grating action to generate a high-order mode on one hand and the spiral refractive index distribution to generate an additional spiral phase so as to generate a high-order orbital angular momentum beam on the other hand.
11. The all-fiber orbital angular momentum beam generator of claim 10, wherein the spiral long-period fiber grating is prepared by the preparation method of any one of claims 1 to 7.
12. The all-fiber orbital-angular-momentum beam generator of claim 10, wherein the few-mode fiber is a graded-index or a step-index fiber supporting stable propagation of high-order modes alone.
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| PCT/CN2021/083595 WO2022160453A1 (en) | 2021-01-28 | 2021-03-29 | Spiral fiber grating and preparation method therefor, and all-fiber orbital angular momentum beam generator |
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| CN113376736A (en) * | 2021-06-18 | 2021-09-10 | 天津工业大学 | Vortex integrated multiplexing/demultiplexing optical fiber suitable for 6G communication |
| CN114047576A (en) * | 2021-11-23 | 2022-02-15 | 深圳大学 | Preparation method of spiral refraction type fiber grating for all-fiber orbital angular momentum beam generator |
| CN114964328A (en) * | 2022-05-10 | 2022-08-30 | 南京师范大学 | All-fiber angular momentum interference sensing measurement system and method based on spiral grating |
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| CN115236798B (en) * | 2022-09-20 | 2022-11-22 | 中国航空工业集团公司北京长城计量测试技术研究所 | Fiber grating and preparation device and preparation method thereof |
| CN115639640B (en) * | 2022-10-20 | 2023-04-21 | 中国航天三江集团有限公司 | Optical fiber for actively and precisely controlling laser polarization state, preparation method and application thereof |
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| US20020069676A1 (en) * | 2000-12-12 | 2002-06-13 | Kopp Victor Il?Apos;Ich | Apparatus and method of manufacturing chiral fiber bragg gratings |
| DE602005006383T2 (en) * | 2005-03-04 | 2009-06-04 | Fujitsu Ltd., Kawasaki | Chiral optical fiber |
| CN106526741A (en) * | 2016-12-27 | 2017-03-22 | 南京理工大学 | Device for manufacturing spiral core long-period fiber grating |
| CN111323903B (en) * | 2020-02-17 | 2021-09-28 | 南京师范大学 | Optical fiber orbital angular momentum mode separation method based on spiral structure |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN113376736A (en) * | 2021-06-18 | 2021-09-10 | 天津工业大学 | Vortex integrated multiplexing/demultiplexing optical fiber suitable for 6G communication |
| CN114047576A (en) * | 2021-11-23 | 2022-02-15 | 深圳大学 | Preparation method of spiral refraction type fiber grating for all-fiber orbital angular momentum beam generator |
| CN114047576B (en) * | 2021-11-23 | 2022-12-13 | 深圳大学 | Preparation method of spiral refraction type fiber grating for all-fiber orbital angular momentum beam generator |
| WO2023092484A1 (en) * | 2021-11-23 | 2023-06-01 | 深圳大学 | Method for preparing helical refractive-index-change-type fiber grating for all-fiber orbital angular momentum beam generator |
| CN114964328A (en) * | 2022-05-10 | 2022-08-30 | 南京师范大学 | All-fiber angular momentum interference sensing measurement system and method based on spiral grating |
| CN114964328B (en) * | 2022-05-10 | 2023-12-05 | 南京师范大学 | All-fiber angular momentum interferometry sensing measurement system and method based on spiral grating |
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