WO2022160453A1

WO2022160453A1 - Spiral fiber grating and preparation method therefor, and all-fiber orbital angular momentum beam generator

Info

Publication number: WO2022160453A1
Application number: PCT/CN2021/083595
Authority: WO
Inventors: 白志勇; 黄政; 刘朝; 王义平
Original assignee: 深圳大学
Priority date: 2021-01-28
Filing date: 2021-03-29
Publication date: 2022-08-04
Also published as: CN112698440A

Abstract

A preparation method for a spiral fiber grating (30), a spiral fiber grating (30) prepared by the method, and an all-fiber orbital angular momentum beam generator. The spiral fiber grating (30) comprises a fiber, and a modulated fiber grating that is formed on an optical fiber cladding (20) and has a varying spiral refractive index. The all-fiber orbital angular momentum beam generator comprises a few-mode fiber and a spiral long-period fiber grating that is formed on the few-mode fiber cladding (20), and the generator is of an all-fiber structure.

Description

Helical fiber grating, preparation method and all-fiber orbital angular momentum beam generator

technical field

The invention relates to the technical field of fiber gratings, in particular to a helical fiber grating, a preparation method and an all-fiber orbital angular momentum beam generator.

Background technique

Helical fiber gratings have attracted widespread attention since they were proposed. The existing preparation method is to partially melt the optical fiber through thermal effect, and then physically twist the optical fiber geometry to form a helical structure. The heating method can be high temperature furnace, hydrogen-oxygen flame or laser heating. For example, a method for preparing a helical fiber grating with a hydrogen-oxygen flame: it uses the thermal effect of the hydrogen-oxygen flame to partially melt the optical fiber and deform the optical fiber geometry in a helical shape, and then periodically heat the optical fiber to form a periodic refractive index change .

technical problem

The advantage of the above method is that it is applicable to any optical fiber, no other processing is required for the optical fiber, and the grating period can be flexibly changed according to the preparation requirements. The grating can be formed by ordinary communication fibers, which greatly improves the grating efficiency and the grating quality. However, this method has low efficiency, low grating preparation repetition rate, uncontrollable grating resonant wavelength and resonant peak depth, and at the same time, the manufactured grating has unstable insertion loss and insufficient resonant peak depth. In addition, since this method mainly twists through the physical geometric structure, this method can only be used for long-period fiber gratings, and is not suitable for fabricating short-period fiber gratings.

technical solutions

In order to overcome the problems existing in the prior art, the present invention proposes a preparation method of a helical fiber grating, comprising the following steps:

Step S1: prepare the optical fiber, and strip the coating layer of the optical fiber;

Step S2: fixing the optical fiber on the fixture of the mobile platform;

Step S3: focus the CO ₂ laser beam on the cladding of the fiber; rotate the fiber around its own axis and also translate in the horizontal direction, and use the thermal effect of the CO ₂ laser beam to perform helical refractive index modulation on the fiber cladding and form a helical line Refractive index modulated fiber grating.

As an improvement to the preparation method of the helical fiber grating provided by the present invention, in step S3, the CO ₂ laser is radiated from the surface to the inside of the cladding, so that the helical fiber grating forms a continuous helical shape on the fiber cladding Refractive index modulation.

As an improvement of the preparation method of the helical fiber grating provided by the present invention, in step S3, the control parameters of the relative motion of the optical fiber rotating around its own axis and translating along the horizontal direction are set according to the phase matching formula; the phase matching formula includes the formula (1), formula (2) and formula (3):

n _N =n _F -m·λ/T Formula (1)

J _N =J _F +m·σ Formula (2)

T=2π·ν/ω Formula (3)

where n _F and n _N represent the effective refractive indices of the fundamental mode and the coupled mode, respectively, λ is the resonant wavelength corresponding to the helical fiber grating, T is the corresponding grating period, and J _F and J _N are the sum of the two corresponding modes Angular momentum, which is the sum of the orbital angular momentum and spin angular momentum of the corresponding mode, m is the harmonic order of magnitude, σ is the handedness of the helical fiber grating, v is the horizontal motion rate of the mobile platform, and ω is the fiber rotation angular velocity.

As an improvement to the preparation method of the helical fiber grating provided by the present invention, before the modulation of the helical refractive index of the fiber cladding is performed, a numerical operation method is used to simulate the period parameters corresponding to the optical fiber for generating high-order modes, so as to obtain some control parameters .

As an improvement of the preparation method of the helical fiber grating provided by the present invention, by controlling the horizontal movement rate and rotation speed of the optical fiber, writing of different periodic gratings is realized, and the tuning of the writing wavelength of the helical fiber grating is realized.

As an improvement of the preparation method of the helical fiber grating provided by the present invention, one end of the optical fiber is connected with the light source, and the other end is connected with the spectrometer;

Observe the spectral pattern of the fiber grating through a spectrometer. After obtaining the required spectral pattern, turn off the _CO2 laser beam focused on the fiber and stop the horizontal movement and rotation of the fiber.

As an improvement of the preparation method of the helical fiber grating provided by the present invention, the fixture includes two rotating fixtures, which are respectively arranged at both ends of the optical fiber; the two rotating fixtures are driven by two rotating motors, and the two rotating fixtures are The rotating motors have the same rotational speed and the same direction of rotation.

The present application also provides a helical fiber grating, which includes an optical fiber, and a helical refractive index-varying modulation fiber grating formed on the cladding of the optical fiber.

As an improvement of the preparation method of the helical fiber grating provided by the present invention, the helical fiber grating is prepared by the above preparation method.

The application also provides an all-fiber orbital angular momentum beam generator, comprising a few-mode fiber, and a helical long-period fiber grating formed on the few-mode fiber cladding, the generator is an all-fiber structure; incident on the fiber grating On the one hand, the light in the beam is affected by the grating to generate a high-order mode, and on the other hand, it is also affected by the helical refractive index distribution to generate an additional helical phase, thereby generating a high-order orbital angular momentum beam.

As an improvement of the all-fiber orbital angular momentum beam generator provided by the present invention, the helical long-period fiber grating is prepared by the above-mentioned preparation method.

As an improvement of the all-fiber orbital angular momentum beam generator provided by the present invention, the few-mode optical fiber is a graded or transition-index optical fiber that supports independent and stable propagation of high-order modes.

beneficial effect

The present invention adopts the weak coupling method of low-energy CO ₂ laser beam to modulate the helical refractive index of the fiber cladding layer, modulates the helical refractive index in the optical fiber cladding layer, and forms a helical refractive index change modulation fiber grating on the cladding layer. Compared with the excitation of high-order vector modes on the optical fiber through the modulation of large energy intensity, the preparation method has a good protection effect on the grating, the obtained grating has little damage, and the fabricated grating has low insertion loss and deep resonance peak depth. , the mode conversion efficiency can reach 99%, and the same parameter can realize the repeated preparation of the same characteristic grating for many times.

In addition, the existing long-period fiber gratings written by point-by-point exposure of few-mode fibers can only generate linear polarization mode modes, and cannot directly generate orbital angular momentum beams. In this application, by modulating the helical refractive index of the cladding of the few-mode fiber, the light incident into the grating will not only be affected by the grating to generate high-order modes, but also be affected by the helical refractive index distribution to generate an additional helical phase, so that the high-order mode has a helical phase. Under the action of multiple grating modulation periods, the high-order mode and the helical phase are resonantly enhanced, thereby forming a high-order mode of the helical phase. Therefore, when the polarization controller and pressure plate are not applicable Or under the condition of a fiber twister, high-order orbital angular momentum beams can be directly generated. Crucially, the resulting high-order orbital angular momentum beams can be directly transmitted in few-mode fibers.

Description of drawings

1 is a schematic diagram of a preparation device of a helical 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 present invention.

Reference number:

_CO2 laser 1, mirror 2, attenuator 3 for controlling laser energy, beam expander 4, laser switch 5 for controlling the on-off of the optical path, focusing lens 6, rotating fixture 7, mobile platform 8, spectrometer 9, The motion controller 10 , the light source 11 , the control device 12 , the fiber core 10 , the cladding 20 , and the helical fiber grating 30 .

Embodiments of the present invention

In order to make those skilled in the art better understand the solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only Embodiments are part of the present invention, but not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", " Rear, Left, Right, Vertical, Horizontal, Top, Bottom, Inner, Outer, Clockwise, Counterclockwise, Axial, The orientations or positional relationships indicated by "radial direction", "circumferential direction", etc. are based on the orientations or positional relationships shown in the accompanying drawings, which are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying the indicated devices or elements. It must have a specific orientation, be constructed and operate in a specific orientation, and therefore should not be construed as a limitation of the present invention.

In addition, the terms "first" and "second" are only used for descriptive purposes, and should not be construed as indicating or implying relative importance or implying the number of indicated technical features. Thus, a feature delimited with "first", "second" may expressly or implicitly include at least one of that feature. In the description of the present invention, "plurality" means at least two, such as two, three, etc., unless otherwise expressly and specifically defined.

In the present invention, unless otherwise expressly specified and limited, the terms "installed", "connected", "connected", "fixed" and other terms should be understood in a broad sense, for example, it may be a fixed connection or a detachable connection , or integrated; it can be a mechanical connection or an electrical connection or can communicate with each other; it can be directly connected or indirectly connected through an intermediate medium, it can be the internal connection of two components or the interaction relationship between the two components, unless otherwise expressly qualified. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations.

The technical solutions of the present invention will be further described in detail below through the accompanying drawings and embodiments.

A method for preparing a helical fiber grating 30 provided by a specific embodiment of the present application includes the following steps:

Step S2: fixing the optical fiber on the fixture of the mobile platform;

Step S3: focus the CO ₂ laser beam on the cladding 20 of the fiber; rotate the fiber around its own axis and also translate in the horizontal direction, the CO ₂ laser beam modulates the helical index of A helical refractive index change modulation type long-period fiber grating is formed.

In the above step S3, the low-energy CO ₂ laser beam is focused on the fiber cladding 20 by adjusting the optical path. Then, it is ensured that the fiber rotates and also translates in the horizontal direction. Therefore, in the present application, the residual stress is eliminated by laser heating along the helical path of laser irradiation, and the helical refractive index modulation is formed in the fiber cladding 20, so as to realize the CO ₂ laser in the fiber. The cladding 20 is written with a high precision spiral path.

In the above-mentioned step S3, the control parameters of the rotation of the optical fiber around its own axis and the related motion of the horizontal translation can be set according to the phase matching formula; the phase matching formula includes formula (1), formula (2) and formula (3):

n _N =n _F -m·λ/T Formula (1)

J _N =J _F +m·σ Formula (2)

T=2π·ν/ω Formula (3)

where n _F and n _N represent the effective refractive indices of the fundamental mode and the coupled mode, respectively, λ is the resonant wavelength corresponding to the helical fiber grating, T is the grating period, and J _F and J _N are the total angles of the two corresponding modes Momentum, they are the sum of the orbital angular momentum and spin angular momentum of the corresponding mode, m is the harmonic order, σ is the handedness of the helical fiber grating, σ=-1 and σ=1 are denoted as left-handed and right-handed, respectively. v is the horizontal motion rate of the moving platform, and ω is the rotational angular velocity of the fiber.

Objects in motion have momentum, and objects of different types of motion have different momentums. For a beam of light, each photon has

The linear momentum of , and has two kinds of angular momentum, namely spin angular momentum and orbital angular momentum. If the electric field of the beam rotates along the optical axis i.e. circularly polarized light, the beam has spin angular momentum and the spin-orbit angular momentum of each photon is

where "±" represents left-handed and right-handed; if the wave vector of the beam is rotated along the optical axis to form a helical phase plane, the beam has orbital angular momentum. The expression of the helical phase surface is exp(ilΦ), where Φ is the angular coordinate and l is the topological charge of the orbital angular momentum. The value of l is an integer, which can be positive, negative, and zero, corresponding to clockwise, counterclockwise, and non-spiral phases, respectively. The handedness of the helical phase wavefront of the beam and the number of the helical phase planes depend on the positive, negative and magnitude of l. In 1992, Allen et al. proved that under paraxial propagation conditions, all vortex beams with a helical phase factor exp(ilΦ) have

The orbital angular momentum and can be represented by linear superposition of all helical phase beams. The conclusion of this conclusion has triggered an upsurge in the study of orbital angular momentum by scholars. From various generation technologies, detection methods and applications of orbital angular momentum, various new phenomena of orbital angular momentum have emerged.

The phase matching of the helical fiber grating can be satisfied by the above formula (1), formula (2) and formula (3).

Before the fiber cladding 20 modulates the helical index of refraction, a numerical operation method is used to simulate the period parameters corresponding to the high-order mode of the fiber to obtain some control parameters in formula (1), formula (2), and formula (3). .

The above-mentioned clamp includes two rotating clamps, which are respectively arranged at both ends of the optical fiber. The two rotating fixtures are driven by two rotating motors, and the two rotating motors have the same rotation speed and the same rotation direction. Two rotating fixtures are set on the mobile platform. If the present invention adopts two rotating fixtures, if one rotating fixture is used, the optical fiber will be off-axis due to centrifugal force during the rotation process because the rotating fixture cannot be precisely coaxial. Compared with using a single rotating fixture, the present invention can realize the good coaxiality of the optical fiber rotation by using the coaxial rotation technology of two rotating motors to rotate the optical fiber in the same direction, and ensure the coaxial precision requirement of the optical fiber rotation; It is stable during the rotation process, which improves the writing quality and writing efficiency of the grating. ω is the rotational angular velocity of the rotating electrical machine.

During the preparation process, the rotating motor drives the rotating fixture and the optical fiber to rotate. At the same time, the rotating fixture, the rotating motor and the optical fiber are arranged on the moving platform, and can be translated in the horizontal direction at the same time under the driving of the moving platform.

In the preparation process, it is necessary to ensure that the fiber rotates and also performs horizontal translation, so as to realize the high-precision helical path writing of the CO ₂ laser in the fiber cladding 20 . The laser helical refractive index modulation technology requires precise synchronization between the laser pulse and the fiber movement. If the two cannot be synchronized at all times, the grid period and the refractive index modulation depth will be uneven, resulting in a decrease in the writing efficiency. , the spectral quality decreases.

The above-mentioned control parameters of the movement in the rotation direction and the related movement in the horizontal direction can be written in the self-written control program for accurate control. Using a control device such as a computer, the rotating motor, the moving platform and the laser can be controlled to turn on at the same time, and the raster writing can be performed.

Adjustability of resonance wavelength and resonance peak can be achieved by controlling the speed and distance of the moving platform and the rotation speed and number of turns of the rotating fixture. For example, using self-written programs to realize the synchronization and consistency control of the mobile platform and the rotating fixture can ensure that the mobile platform and the rotating fixture run and stop at the same time.

The control parameters include, for example, the rotation direction, rotation angle, and rotation speed of the rotary motor, and the movement speed and direction 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 helical fiber grating 30 .

By controlling the horizontal movement rate and rotation speed of the optical fiber, the writing of different periodic gratings is realized, and the tuning of the writing wavelength of the helical fiber grating is realized.

Furthermore, 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. Observe the spectral pattern of the fiber grating through a spectrometer. After obtaining the required spectral pattern, turn off the _CO2 laser beam focused on the fiber and stop the horizontal movement and rotation of the fiber. By observing the spectrum of the grating during the preparation process, the tunability of the resonance wavelength and the resonance peak can be realized, and it is not necessary to continuously cut the length of the grating after preparing the grating of a certain length to find a grating whose spectrum meets the requirements.

The helical fiber grating 30 using the preparation method of the present application is a long-period fiber grating with a period greater than 30 μm. This preparation method is applicable to all fiber types. For example, the types of optical fibers include but are not limited to single-mode optical fibers and few-mode optical fibers.

A preparation device of a helical fiber grating 30 of the present application is shown in FIG. 1 . Including: CO ₂ laser processing optical path system, clamping moving device, control device 12 . The control device 12 is simultaneously connected with the CO ₂ laser processing optical path system and the clamping moving device.

The CO ₂ laser processing optical path system includes a CO ₂ laser 1 , a mirror 2 , an attenuator 3 for controlling laser energy, a beam expander 4 , a laser switch 5 for controlling the on-off of the optical path, and a focusing lens 6 . The gripping moving device includes a moving platform 8 , a motion controller 10 and a rotating fixture 7 .

The preparation device further includes a spectrometer 9 and a light source 11 . The light source 11 and the spectrometer 9 are respectively connected at both ends of the optical fiber.

The moving platform 8 is, for example, a three-dimensional precision moving platform. The laser switch 5 can be an electronic shutter (electronic switch), and the electronic shutter is controlled by the control device 12. The control device 12 may be an intelligent terminal, such as a computer.

This application uses a CO2 laser as the light source, and the CO2 laser can be provided by a CO2 laser. The CO2 laser has a stability of up to ±2%, and its focusing spot quality is good. When the fiber is periodically heated, the spot size and energy density can be guaranteed to be uniform, which can ensure the stability and repeatability of repeated exposure at different periods. And the heating area and the focal spot in the scanning process can overlap well. It is understandable that by selecting a suitable laser power, it is ensured that the grating can be written normally without damaging the optical fiber.

The CO ₂ laser is emitted by the laser 1 and then passes through the mirror 2 and the attenuator 3 in turn to obtain a laser with suitable energy and mode, and then passes through the beam expander 4 for beam expansion, and reaches the focusing lens 6 for focusing. Finally, the CO ₂ laser beam is focused 20 in the cladding of the fiber. The rotating fixture 7 rotates the optical fiber, and the rotating fixture 7 and the optical fiber are arranged on the mobile platform, and the mobile platform translates, so that the optical fiber also performs horizontal translation while rotating.

The above-mentioned rotating fixture 7 includes two rotating fixtures, which are respectively arranged at both ends of the optical fiber. The two rotating fixtures are driven by two rotating motors, and the two rotating fixtures have the same rotation speed and the same rotation direction.

The laser switch 5, the rotating fixture 7, and the mobile platform 8 are all connected to the motion controller 10, and are controlled by the software on the control device 12. The spectrometer 9 and the light source 11 can be combined to observe the grating spectrum in real time. The self-written program can also control the electronic shutter of the _CO2 laser.

The moving platform, the rotating motor and the electronic shutter are controlled by the self-written program, so that the horizontal movement of the three-dimensional moving platform, the rotation of the rotating fixture and the opening and closing of the electronic shutter are combined, so that the fiber can achieve uniform helical refractive index modulation under the _CO laser. By controlling the horizontal movement rate of the optical fiber and the rotation speed of the rotating fixture, the writing of different periodic gratings is realized, and the tuning of the writing wavelength of the helical fiber grating is realized.

When the moving distance and number of rotations of the mobile platform satisfy the writing length of the fiber grating, and the resonant wavelength and resonant depth satisfy the requirements, the electronic shutter is controlled to keep the CO ₂ laser in a blocking state, that is, any required spectrum can be obtained.

The helical fiber grating 30 of the present application does not need to use high temperature or high energy to make the optical fiber in a molten state and then rotate the optical fiber to physically twist the geometric structure of the optical fiber. The present invention adopts the weak coupling method of the low-energy CO ₂ laser beam to modulate the helical refractive index of the fiber cladding layer 20 , the helical refractive index modulation is performed in the optical fiber cladding layer 20 and the helical refractive index change modulation fiber is formed in the cladding layer 20 grating. Compared with modulating the mode by a large energy intensity, the preparation method has a good protective effect on the grating, the obtained grating has little damage, and the manufactured grating has a low insertion loss, and the resonance peak depth is deep, and the mode conversion efficiency can reach 99%. %, high temperature stability, the same parameter can realize the repeated preparation of the same characteristic grating for many times, and can also flexibly realize the tuning of the spectral parameters of the fiber grating.

In the present application, the helical structure of the helical fiber grating is in the cladding layer 20 in the form of a continuous helical pattern spirally extending along the axial direction, which is formed on the cladding layer by the thermal effect of the CO ₂ laser. The CO ₂ laser is radiated from the surface of the cladding to the inside, so that the helical fiber grating forms a continuous helical refractive index modulation in the fiber cladding. By using a low-energy _CO laser to perform helical refractive index modulation on the cladding 20 of the few-mode fiber by the weak coupling method, an all-fiber high-order orbital angular momentum beam generator that directly generates high-order orbital angular momentum beams can be obtained.

The present application is to write helical fiber gratings on the cladding. Compared with the helical fiber gratings that are written on the core, writing on the cladding has very little damage to the fiber as a whole, hardly affects the fiber structure, and the resulting insertion loss is also low. Very low, can be directly used in large-capacity communication systems.

The existing methods for generating orbital angular momentum beams include the helical phase plate method, the SLM conversion method, the lens conversion method, the Q-disk conversion method, and the integrated device conversion method. These methods are based on the orbits of their own space and integrated devices. Angular Momentum Beam Method. However, the actual communication system uses optical fiber as the main transmission medium. The orbital angular momentum beam generated by the above method needs to be coupled into the optical fiber before it can be used in the actual communication system, which brings additional 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 fiber at the signal transmitting end, and the multiplexed light must be coupled from the fiber at the receiving end. come out. However, loss and crosstalk are often introduced during coupling, which has a great impact on vortex optical wavelength division multiplexing applied in optical fibers.

Embodiment 2

The present application also provides a helical fiber grating 30 . It is a modulated fiber grating with a change in the refractive index of the helical line formed in the fiber cladding 20

The present application also provides a helical fiber grating 30, which is prepared by the above-mentioned preparation method.

Preferably, through the preparation method of Embodiment 1, a modulated long-period fiber grating with a variable helical refractive index can be formed on the few-mode fiber cladding 20, and the grating is a helical long-period fiber grating based on a few-mode fiber.

Apply for the use of graded or jump-index few-mode fibers that support stable propagation of higher-order modes alone. By modulating the helical refractive index in the few-mode fiber cladding 20, the fundamental mode is coupled to a specific high-order mode in the few-mode helical fiber grating under the condition of phase matching, and is excited into a high-order mode and carries the helix phase, so that the fundamental mode is directly converted into a high-order orbital angular momentum beam, and the fundamental mode after passing through the few-mode helical fiber grating will be converted into a high-order orbital angular momentum beam.

Embodiment 3

The present application also provides an all-fiber orbital angular momentum beam generator. 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 of a few-mode optical fiber, and the few-mode optical fiber includes a core 10 and a cladding 20 . A helical fiber grating 30 is formed on the few-mode fiber cladding 20 . The helical fiber grating here is specifically a helical long-period fiber grating, which is a helical long-period fiber grating based on a few-mode fiber.

The helical long-period fiber grating is prepared by the preparation method of the above-mentioned first embodiment.

Few-mode fibers are graded or jump-index fibers that support the stable propagation of higher-order modes alone.

The orbital angular momentum beam generator of the present application is a high-order orbital angular momentum beam generator, and a helical long-period fiber grating is obtained by performing helical refractive index modulation in the few-mode fiber cladding 20, and under the condition of phase matching, The fundamental mode is coupled to a specific higher-order mode by the few-mode helical fiber grating. The light incident into the fiber grating is affected by the grating to generate high-order modes on the one hand, and it is also affected by the helical refractive index distribution to generate an additional one. Helical phase, which in turn enables the direct conversion of the fundamental mode into a high-order orbital angular momentum beam. The helical fiber grating 30 on the few-mode fiber is continuous with a plurality of helical structures distributed continuously with the same rotation direction, and the direction of the helical structure of the grating main body is the same as the direction of the generated high-order orbital angular momentum beam. Multiple helical structures can excite helical phases within a certain bandwidth.

The high-order orbital angular momentum beam generator provided by the present application is an all-fiber structure with a helical structure, and can directly convert the fundamental mode into a beam with high-order orbital angular momentum.

An all-fiber orbital angular momentum beam generator provided by the present application can simultaneously support the transmission of first-order, second-order, third-order, and fourth-order orbital angular momentum beams. The all-fiber orbital angular momentum beam generator can be directly applied to the optical fiber transmission system, and can realize the communication transmission of the all-fiber orbital angular momentum beam without any space debugging.

In the present application, by modulating the helical refractive index of the cladding of the few-mode fiber, the light incident into the grating will not only be affected by the grating to generate high-order modes, but also be affected by the distribution of the helical refractive index to generate an additional helical phase. , so that the light has a helical phase. Under the action of multiple grating modulation periods, the higher-order mode and the helical phase are enhanced by resonance, thereby forming a higher-order mode with a helical phase, so no polarization controller, pressure plate or fiber twist is used. Under the condition of the detector, a stable high-order orbital angular momentum beam can be directly obtained. Most importantly, the high-order orbital angular momentum beam can be stably transmitted directly in the fiber.

The invention utilizes CO ₂ laser to prepare helical refractive index change modulation type long-period fiber grating technology to prepare helical long-period fiber grating based on few-mode fiber as a high-order orbital angular momentum beam generator, and has simple structure, clear principle and relatively low cost. Low, easy to implement; the cycle can be precisely controlled and the stability is high.

The optical fiber orbital angular momentum beam generator can directionally generate a specific orbital angular momentum beam, with high efficiency and high purity, and can support its stable transmission.

A high-order orbital angular momentum beam generator provided by this application, the generated high-order orbital angular momentum beam has unique physical characteristics, and is used in optical tweezers, biomedical imaging particle manipulation, microscopic imaging, quantum information processing and large capacity Optical communication and many other fields have broad prospects and important application value.

Obviously, the above-described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. The accompanying drawings show the preferred embodiments of the present application, but do not limit the scope of the patent of the present application. This application may be embodied in many different forms, rather these embodiments are provided so that a thorough and complete understanding of the disclosure of this application is provided. Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing specific embodiments, or perform equivalent replacements for some of the technical features. . Any equivalent structure made by using the contents of the description and drawings of the present application, which is directly or indirectly used in other related technical fields, is also within the scope of protection of the patent of the present application.

Claims

A preparation method of helical fiber grating is characterized in that, comprises the following steps:

Step S1: prepare the optical fiber, and strip the coating layer of the optical fiber;

Step S2: fixing the optical fiber on the fixture of the mobile platform;

Step S3: focus the CO 2 laser beam on the cladding of the fiber; rotate the fiber around its own axis and also translate in the horizontal direction, and use the thermal effect of the CO 2 laser beam to perform helical refractive index modulation on the fiber cladding and form a helical line Refractive index modulated fiber grating.
The preparation method of the helical fiber grating according to claim 1, characterized in that, in step S3, the CO 2 laser is radiated from the surface to the inside on the surface of the cladding, so that the helical fiber grating forms a continuous pattern on the fiber cladding. Helical refractive index modulation.
The preparation method of the helical fiber grating according to claim 1 is characterized in that, in step S3, according to the phase matching formula, the control parameters of the relative motion of the optical fiber rotating around its own axis and translating along the horizontal direction are set; the phase matching formula Including formula (1), formula (2) and formula (3):

n N =n F -m·λ/T Formula (1)

J N =J F +m·σ Formula (2)

T=2π·ν/ω Formula (3)

where n F and n N represent the effective refractive indices of the fundamental mode and the coupled mode, respectively, λ is the resonant wavelength corresponding to the helical fiber grating, T is the corresponding grating period, and J F and J N are the sum of the two corresponding modes Angular momentum, which is the sum of the orbital angular momentum and spin angular momentum of the corresponding mode, m is the harmonic order of magnitude, σ is the handedness of the helical fiber grating, v is the horizontal motion rate of the mobile platform, and ω is the fiber rotation angular velocity.
The preparation method of the helical fiber grating according to claim 1, characterized in that, before the modulation of the helical refractive index of the fiber cladding is performed, a numerical operation method is used to simulate the period parameters corresponding to the high-order mode of the fiber to obtain the partial control parameter.
The preparation method of the helical fiber grating according to claim 1 is characterized in that, by controlling the horizontal movement rate and the rotation speed of the optical fiber, the writing of different periodic gratings is realized, and the tuning of the writing wavelength of the helical fiber grating is realized.
The preparation method of the helical fiber grating according to claim 1, wherein one end of the optical fiber is connected with the light source, and the other end is connected with the spectrometer;

Observe the spectral pattern of the fiber grating through a spectrometer. After obtaining the required spectral pattern, turn off the CO2 laser beam focused on the fiber and stop the horizontal movement and rotation of the fiber.
The preparation method of the helical fiber grating according to claim 1, wherein the fixture comprises two rotating fixtures, which are respectively arranged at both ends of the optical fiber; the two rotating fixtures are driven by two rotating motors, The two rotating electrical machines have the same rotational speed and the same direction of rotation.
A helical fiber grating is characterized by comprising an optical fiber and a modulation fiber grating with a change in the refractive index of the helical line formed on the cladding of the optical fiber.
The helical fiber grating according to claim 8, wherein the helical fiber grating is prepared by the preparation method of any one of claims 1-7.
An all-fiber orbital angular momentum beam generator, characterized in that it comprises a few-mode fiber and a helical long-period fiber grating formed on the few-mode fiber cladding, the generator is an all-fiber structure; incident on the fiber grating On the one hand, the light generated by the grating produces a high-order mode, and on the other hand, it is also affected by the helical refractive index distribution to generate an additional helical phase, thereby generating a high-order orbital angular momentum beam.
The all-fiber orbital angular momentum beam generator according to claim 10, wherein the helical long-period fiber grating is prepared by the preparation method of any one of claims 1-7.
The all-fiber orbital angular momentum beam generator according to claim 10, wherein the few-mode optical fiber is a graded or transition-index optical fiber that supports independent and stable propagation of high-order modes.