WO2023092484A1

WO2023092484A1 - Method for preparing helical refractive-index-change-type fiber grating for all-fiber orbital angular momentum beam generator

Info

Publication number: WO2023092484A1
Application number: PCT/CN2021/133649
Authority: WO
Inventors: 白志勇; 陈嘉燕; 廖常锐; 王义平
Original assignee: 深圳大学
Priority date: 2021-11-23
Filing date: 2021-11-26
Publication date: 2023-06-01
Also published as: CN114047576A; CN114047576B

Abstract

A method for preparing a helical refractive-index-change-type fiber grating for an all-fiber orbital angular momentum beam generator, comprising: step 200, determining a starting position of a long-period helical fiber grating in an optical fiber; and step 300, focusing a laser beam at the starting position of the long-period helical fiber grating, and then driving the optical fiber to perform three-dimensional helical movement, such that the focus of the laser beam is written and prepared in the optical fiber, so as to form the long-period helical fiber grating.

Description

Preparation method of helical deflection fiber grating for all-fiber orbital angular momentum beam generator

technical field

The invention relates to the field of optical fiber gratings, in particular to a method for preparing a helical deflection optical fiber grating for an all-fiber orbital angular momentum beam generator. Background technique

technical problem

Orbital angular momentum beam generators can be divided into two types: space type and fiber type according to the different generation media. The optical fiber-type methods for generating orbital angular momentum beams include holographic pattern generation technology, off-axis incident generation technology, and mode conversion generation technology. Among them, the mode conversion generation technology is divided into fiber coupler orbital angular momentum beam generation technology and fiber grating generation technology. The preparation method based on fiber grating is a hot research topic because of its advantages of high coupling efficiency, specific wavelength selection, low cost, low insertion loss, and easy preparation. Among them, the helical deflection fiber grating is the key component of the fiber-optic angular momentum beam generator. The orbital angular momentum beam can be directly generated by using the helical fiber grating without additional auxiliary devices such as polarization controllers and pressure plates. It is more stable and flexible, and the system is more compact, so it has wide application prospects.

In an invention patent with the publication number CN112698440A, a method for preparing a spiral fiber grating is disclosed, which focuses the CO ₂ laser beam on the cladding of the optical fiber, and at the same time makes the optical fiber rotate around its own axis and translate in the horizontal direction. The CO ₂ laser beam forms a helical refractive index writing in the fiber cladding 20, so as to realize the high-precision helical path writing of the CO ₂ laser beam in the fiber cladding 20, and finally obtain a helical refractive index change writing type long-period fiber grating. However, in this preparation method, the optical fiber is clamped on the rotating fixture when the grating is written, and the rotating fixture is driven by a rotating motor to realize the rotation of the optical fiber. At the same time, a moving platform is used to drive the rotating motor, rotating fixture and optical fiber to move horizontally. It needs to rotate around the same rotation axis with the rotating motor, and the requirements for coaxiality are relatively high. Therefore, the relative position between the optical fiber, the rotating fixture and the rotating motor must be adjusted before preparation to make the three coaxial, otherwise during the rotation process , the optical fiber will be offset relative to the CO ₂ laser beam, resulting in the CO ₂ laser beam not hitting the correct position, making the writing position of the spiral fiber grating offset, or even not writing on the optical fiber.

technical solution

In order to solve the above-mentioned deficiencies in the prior art, the present invention provides a preparation method for preparing a helical deflection fiber grating for an all-fiber orbital angular momentum beam generator.

The technical problem to be solved by the present invention is realized through the following technical solutions:

A method for preparing a helical deflection fiber grating for an all-fiber orbital angular momentum beam generator, comprising the steps of:

Step 200: determining the initial position of the long-period spiral fiber grating in the optical fiber;

Step 300: focusing the laser beam on the starting position of the long-period spiral fiber grating, and then driving the optical fiber to perform three-dimensional helical movement, so that the focal point of the laser beam is written in the optical fiber to form a long-period spiral fiber grating.

Further, before step 200, the following steps are also included:

Step 100: peel off the coating layer on the surface of the optical fiber to expose the cladding and core inside the optical fiber.

Further, in step 300, it also includes: launching a detection beam into one end of the optical fiber, receiving the outgoing detection beam from the other end of the optical fiber to obtain a real-time spectrum of the detection beam, when the detection When the real-time spectrum of the light beam is consistent with the design spectrum, the writing of the long-period spiral fiber grating is stopped.

Further, in step 300, the step of driving the optical fiber to perform three-dimensional helical movement is as follows:

Step 301: Decompose the three-dimensional path of the long-period spiral fiber grating into moving paths along the X-axis, Y-axis, and Z-axis respectively, and the X-axis, Y-axis, and Z-axis are perpendicular to each other;

Step 302: Drive the optical fiber to move synchronously along the X-axis, Y-axis and Z-axis according to the moving path of the long-period spiral fiber grating along the X-axis, Y-axis and Z-axis.

Further, define the axial direction of the optical fiber as the X axis, define the direction perpendicular to the axial direction of the optical fiber on the horizontal plane as the Y axis, and define the direction perpendicular to the axial direction of the optical fiber on the vertical plane The direction of is defined as the Z axis.

Further, the speed at which the optical fiber moves in one direction along the X-axis is Vx, the speed at which the optical fiber moves back and forth along the Y-axis in both directions is Vy, and the speed at which the optical fiber moves back and forth in both directions along the Z-axis is Vz, satisfying the following formula:

Where r is the design radius of the long-period spiral fiber grating, and T is the design pitch of the long-period spiral fiber grating.

Further, as the orbital angular momentum generator, the helical deflection fiber grating should satisfy the following phase matching conditions:

Among them, n _F and n _N represent the effective refractive index of the fundamental mode and the coupled mode respectively, λ is the resonant peak wavelength corresponding to the helical deflection fiber grating, J _F and J _N are the total angular momentum of the two corresponding modes , which are the sum of the orbital angular momentum and spin angular momentum of the corresponding mode, m is the diffraction order of the grating, and σ represents the handedness of the helical deflection fiber grating.

Further, before step 200, the following steps are also included:

Step 100: Set the design radius r of the long-period spiral fiber grating, the design pitch T, and the unidirectional movement speed Vx of the optical fiber along the X-axis.

Further, the design pitch T is a constant value or a variable value.

Further, the design radius r is a constant value or a variable value.

Beneficial effect

The present invention has following beneficial effect:

1. In this case, the three-dimensional moving device directly drives the optical fiber to perform three-dimensional spiral movement, so as to cooperate with the laser light source to write a long-period spiral fiber grating in the optical fiber. Compared with the prior art by driving the optical fiber Rotating to write the spiral fiber grating method, because there is no need to rotate the optical fiber, so the coaxiality requirement is low;

2. In this case, the femtosecond laser is used to prepare the long-period helical fiber grating. There is no need to rotate the optical fiber when the optical fiber is in a molten state at high temperature or high energy, and a fiber grating that can directly generate orbital angular momentum beam mode can be obtained. The all-fiber generator, the written long-period spiral fiber grating has good performance, effectively guarantees that the long-period spiral fiber grating has low insertion loss and high temperature stability, and can flexibly realize the long-period fiber The tuning of the spectral parameters of the grating, while making the mode conversion efficiency reach 98%;

3. In this case, not only can start to write the helix at each position of the optical fiber, but also can flexibly write complex and multiple helixes in combination with the three-dimensional displacement device, fully exploiting the potential of femtosecond laser beams to prepare long-period helical fiber gratings, making all-fiber tracks The angular momentum beam mode generator can generate a specific orbital angular momentum beam mode in a direction, with high efficiency and high purity, and can support its stable transmission. It adopts an all-fiber structure, compact structure, and is easy to be compatible with optical fiber communication networks;

4. The period of the long-period spiral grating fiber can be changed arbitrarily through the control device to realize the design of the resonance wavelength of the long-period spiral fiber grating. When the moving distance of the three-dimensional displacement device meets the requirements of the long-period spiral fiber After writing the grating length, resonant wavelength and formant depth, the control device can be used to control the three-dimensional moving device to stop moving. Continue to write the long-period spiral fiber grating at the end position until the requirements are met, so as to realize the adjustability of the resonance depth of the long-period spiral fiber grating, and then obtain any required modulation spectrum.

Description of drawings

Fig. 1 is the schematic diagram of the principle of the preparation system of the helical deflection fiber grating provided by the present invention;

2 is a schematic diagram of the steps of the preparation method of the helical deflection fiber grating provided by the present invention;

Fig. 3 is a schematic diagram of a helical deflection fiber grating in the all-fiber orbital angular momentum beam generator provided by the present invention.

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Embodiments of the present invention

The present invention will be described in detail below in conjunction with the accompanying drawings and embodiments.

Embodiment one

As shown in Figures 1 and 3, a preparation system for a helical deflection fiber grating for an all-fiber orbital angular momentum beam generator includes a laser light source 1, an optical processing device, a three-dimensional moving device 10 and a control device 12,

The laser light source 1 is used to emit a laser beam for writing a grating;

The light processing device is used to process the laser beam, and project the laser beam at the focal point;

The three-dimensional moving device 10 is used to drive the optical fiber to perform three-dimensional helical movement, so that the focus of the laser beam writes a long-period spiral fiber grating in the optical fiber;

The control unit is used to control the laser light source 1 and the three-dimensional moving device 10 , and is connected in communication with the laser light source 1 and the three-dimensional moving device 10 respectively.

In this embodiment, the laser light source 1 is a femtosecond laser light source 1. The femtosecond laser beam has an extremely short laser beam and super strong peak power. When the femtosecond laser beam irradiates the transparent dielectric material, due to its very The high electric field intensity induces strong nonlinear effects, including "nonlinear photoionization" and "avalanche ionization". The high nonlinear effect causes a localized high-concentration plasmoid to be generated at the laser focus, making the density increase instantaneously, the medium is densified and an expanding micro-explosion occurs. After the micro-explosion, a micro-cavity remains in the center, and the adjacent area Microcompression occurs, changing the material density in that region, thus creating a permanent change in the refractive index. Femtosecond laser beams based on the above principles to make long-period fiber gratings have the following characteristics: the thermal diffusion effect is very weak, and a relatively "cold" processing is realized: the energy exchange between femtosecond pulses and matter is based on photon ionization, energy The transfer is limited to a small area of the laser focus, rather than the entire focus spot, so femtosecond laser beam processing can break through the limitation of beam diffraction wavelength and achieve ultra-precision processing; the femtosecond laser beam focus scans and moves inside the transparent medium, which can realize Three-dimensional processing can even create unconstrained structures; the grating period written by femtosecond laser beams can be less than 100 μm.

Of course, with the development of light source technology, this case does not rule out the use of other types of laser light sources1.

The control device 12 can be a terminal such as a notebook computer, a desktop PC, a tablet computer, or a smart phone. Through software, the three-dimensional path of the long-period spiral fiber grating is converted into corresponding point cloud coordinate data, and one coordinate point corresponds to a writing point of the optical fiber, and then sequentially position each writing point of the optical fiber to the focus of the laser beam for writing.

The three-dimensional mobile device has a mobile controller 11 and is connected to the control device 12 through the mobile controller 11 .

The light processing module includes an attenuation unit, a reflection unit and a focusing objective lens 6,

The attenuation unit is used to attenuate the energy of the laser beam;

The reflection unit is used to control the propagation direction of the laser beam;

The focusing objective lens 6 is used for focusing the laser beam and projecting it to a focal point.

The attenuation unit includes an attenuator 2 or at least one attenuation sheet. The attenuation of the attenuator 2 is adjustable. If the attenuator 2 is used, the attenuator 2 is connected to the control device 12 by communication. The control device 12 is used to control; if attenuators are used, the number of the attenuators depends on the required laser energy, the smaller the required laser energy, the more the number of the attenuators 2, and the required laser energy The greater the energy, the fewer the number of attenuators 2 .

The reflective unit includes at least one reflective sheet 3, the number of the reflective sheets 3 depends on the required laser path, the more complex the required laser path, the more the number of the reflective sheet 3, the required laser path The simpler it is, the fewer the number of reflective sheets 3 is.

The preparation system also includes an imaging device 4 for imaging the optical fiber, connected to the control device 12 through communication, and controlled by the control device 12 .

The light processing module further includes a dichroic mirror 5 , the focusing objective lens 6 is disposed on the reflection side of the dichroic mirror 5 , and the imaging device 4 is disposed on the projection side of the dichroic mirror 5 .

The dichroic mirror 5 can reflect the short-wave laser and transmit the long-wave laser. Before the laser beam is grating-written on the optical fiber, the laser beam has higher energy and shorter wavelength, which belongs to the short-wave laser. The mirror 5 can be reflected by the dichroic mirror 5 into the focusing objective lens 6. After the laser beam is grating-written on the optical fiber, most of the energy is absorbed by the optical fiber, and the wavelength becomes longer, which belongs to long-wave laser After being reflected by the optical fiber, it passes through the focusing objective lens 6 and the dichroic mirror 5 in sequence, and when passing through the dichroic mirror 5, it directly passes through the dichroic mirror 5 to form an image on the imaging device 4 at the back.

The optical fiber has different refractive indices before and after the grating is written, so the writing of the long-period spiral fiber grating in the optical fiber can be judged according to the imaging of the laser beam on the imaging device 4 .

The imaging device 4 may be, but not limited to, a CCD camera.

In this embodiment, the attenuation unit includes an attenuator 2, the reflection unit includes a reflection sheet 3, the attenuation unit is arranged in front of the emitting port of the light source device, and the reflection sheet 3 is arranged at the attenuation Between the exit port of the device 2 and the incident side of the dichroic mirror 5 , the focusing objective lens 6 is arranged on the emitting side of the dichroic mirror 5 , and the imaging device 4 is arranged on the projecting side of the dichroic mirror 5 .

The preparation system also includes an optical fiber clamp 8, which is used to clamp the two ends of the optical fiber, and is arranged on the three-dimensional moving device 10, and is driven by the three-dimensional moving device 10 to perform three-dimensional helical movement.

The preparation system also includes a detection light source 7 and a spectrometer 9, both of the detection light source 7 and the spectrometer 9 are connected to the control device 12 through communication, and are controlled by the control device 12;

The detection light source 7 is used to emit a detection beam into one end of the optical fiber, and is connected to the control device 12 through communication;

The spectrometer 9 is configured to receive the outgoing detection light beam from the other end of the optical fiber to obtain a real-time spectrum of the detection light beam.

During grating writing, the control device 12 can judge whether the real-time spectrum of the detection beam obtained by the spectrometer 9 is consistent with the preset spectrum, and then judge whether the writing of the long-period spiral fiber grating is completed.

Embodiment two

As shown in Figures 2 and 3, a method for preparing a helical deflection fiber grating for an all-fiber orbital angular momentum beam generator is applied to the preparation system described in Embodiment 1, including the following steps:

Step 200: Determine the initial position of the long-period spiral fiber grating in the optical fiber.

In this step 200, the initial position of the long-period spiral fiber grating can be determined according to the length of the optical fiber and parameters such as the period and modulation spectrum of the long-period spiral fiber grating.

Wherein, the optical fiber used to modulate the helical deflection fiber grating can be any optical fiber, and the structure of the optical fiber includes a coating layer, a cladding layer and a fiber core from the outer layer to the inner layer, and the coating layer is a non-transparent material, It mainly plays the role of protecting the cladding and the fiber core and shielding interference; the cladding and the fiber core are both transparent materials, but the refractive index of the cladding and the fiber core is different, so the light beam can pass through the cladding Total reflection occurs at the interface between the fiber and the core, while propagating forward in the core.

The femtosecond laser beam used in this case can pass through the coating layer and directly act on the fiber core for refractive index modulation, so when writing the long-period spiral fiber grating, it is not necessary to peeling off of the coating layer. However, in order to facilitate the cutting of the optical fiber after the writing of the long-period spiral fiber grating is completed, before step 200, the following steps may also be included:

Step 100: stripping off the coating layer on the surface of the optical fiber to expose the cladding and core inside the optical fiber.

Step 300: focusing the laser beam on the starting position of the long-period spiral fiber grating, and then driving the optical fiber to perform three-dimensional helical movement, so that the focus of the laser beam writes the long-period spiral fiber in the optical fiber raster.

In this step 300, the optical fiber is first driven to move along its own axis by the three-dimensional moving device 10, so that the initial position of the long-period spiral fiber grating is reset to the focal point of the laser light source 1, and then the The laser light source 1 emits a laser beam to the optical fiber through the focusing objective lens 6, so that the laser beam is projected onto the initial position of the long-period spiral fiber grating after being focused by the focusing objective lens 6, and then kept The focus of the laser beam does not move, and at the same time, the three-dimensional moving device 10 drives the optical fiber to perform a three-dimensional helical movement, so that the focus of the laser beam forms a helical modulation path in the optical fiber, and finally obtains the Long-period helical fiber gratings.

In step 300, it also includes: transmitting a detection beam into one end of the optical fiber, receiving the outgoing detection beam from the other end of the optical fiber to obtain a real-time spectrum of the detection beam, when the real-time spectrum of the detection beam is When the spectrum is consistent with the design spectrum, the writing of the long-period spiral fiber grating is stopped.

Before performing step 300, one end of the optical fiber is connected to the detection light source 7, and the other end is connected to the spectrometer 9, and then the coating layer on the surface of the optical fiber is stripped, and then the optical fiber is Both ends of are fixed on the fiber holder 8 of the three-dimensional moving device 10 .

In step 300, the steps of driving the optical fiber to perform three-dimensional helical movement are as follows:

Step 301: Decompose the three-dimensional path of the long-period spiral fiber grating into moving paths along the X-axis, Y-axis, and Z-axis, and the X-axis, Y-axis, and Z-axis are perpendicular to each other.

In this step 301, in order to facilitate the calculation of the movement parameters of the optical fiber, the axial direction of the optical fiber is defined as the X axis, and the direction perpendicular to the axial direction of the optical fiber on the horizontal plane is defined as the Y axis, and the The direction perpendicular to the axial direction of the optical fiber on the vertical plane is defined as the Z axis.

After decomposition, the moving path of the long-period spiral fiber grating along the X-axis is unidirectional, and the moving paths along the Y-axis and Z-axis are both bidirectional reciprocating movements.

In this step 302 , the optical fiber is driven by the three-dimensional moving device 10 to move unidirectionally along the X axis, and to reciprocate bidirectionally along the Y axis and the Z axis.

Wherein, the speed of the unidirectional movement of the optical fiber along the X axis is Vx, the speed of the bidirectional reciprocating movement along the Y axis is Vy, and the speed of the bidirectional reciprocating movement along the Z axis is Vz, satisfying the following formula:

Where r is the design radius of the long-period helical fiber grating, T is the design pitch of the long-period helical fiber grating (that is, the design period of the long-period helical fiber grating); and Vx can be arbitrary according to actual modulation requirements value.

When the helical deflection fiber grating is used as an orbital angular momentum generator, it should meet the following phase matching conditions:

Before step 200, the preparation method also includes the following steps:

In step 100, the design pitch T can be a constant value or a variable value. When the design pitch T is a constant value, the long-period spiral fiber grating formed by modulation has the same pitch everywhere, so The long-period helical fiber grating is a uniform-period helical grating. When the design pitch T is a variable value, the pitch of the long-period helical fiber grating formed by modulation changes regularly, and the long-period helical fiber grating is a chirp Chirped spiral grating; the design radius r can be a constant value or a variable value. When the design radius r is a constant value, the long-period spiral fiber grating formed by modulation is uniformly distributed in the transverse modulation section everywhere, When the design radius r is a variable value, the transverse modulation sections of the long-period helical fiber grating formed by modulation are distributed regularly.

As for stripping the coating layer on the optical fiber first, or setting the values of r, T and Vx first, it depends on the habit of the operator and the actual situation, without any limitation.

The above-described embodiments only express the implementation manner of the present invention, and its description is more specific and detailed, but it should not be interpreted as limiting the scope of the patent of the present invention, as long as the technical solutions obtained in the form of equivalent replacement or equivalent transformation are adopted , should fall within the protection scope of the present invention.

Industrial Applicability

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Claims

A method for preparing a helical deflection fiber grating for an all-fiber orbital angular momentum beam generator, characterized in that it comprises the following steps:

Step 200: determine the initial position of the long-period spiral fiber grating in the optical fiber;

Step 300: focusing the laser beam on the starting position of the long-period spiral fiber grating, and then driving the optical fiber to perform three-dimensional helical movement, so that the focal point of the laser beam is written in the optical fiber to form a long-period spiral fiber grating.
The method for preparing a helical deflection fiber grating for an all-fiber orbital angular momentum beam generator according to claim 1, wherein, before step 200, further comprising the following steps:

Step 100: peel off the coating layer on the surface of the optical fiber to expose the cladding and core inside the optical fiber.
The method for preparing a helical deflection fiber grating for an all-fiber orbital angular momentum beam generator according to claim 1, characterized in that, in step 300, it also includes: launching a detection beam into one end of the optical fiber, from The other end of the optical fiber receives the outgoing detection beam to obtain the real-time spectrum of the detection beam. When the real-time spectrum of the detection beam is consistent with the design spectrum, the writing of the long-period spiral fiber grating is stopped.
The method for preparing a helical deflection fiber grating for an all-fiber orbital angular momentum beam generator according to claim 1, wherein in step 300, the step of driving the optical fiber to perform a three-dimensional helical movement is as follows:

Step 301: Decompose the three-dimensional path of the long-period spiral fiber grating into moving paths along the X-axis, Y-axis, and Z-axis respectively, and the X-axis, Y-axis, and Z-axis are perpendicular to each other;

Step 302: Drive the optical fiber to move synchronously along the X-axis, Y-axis and Z-axis according to the moving path of the long-period spiral fiber grating along the X-axis, Y-axis and Z-axis.
The preparation method of the helical deflection fiber grating for the all-fiber orbital angular momentum beam generator according to claim 4, characterized in that, the axial direction of the optical fiber is defined as the X axis, and the axis of the optical fiber on the horizontal plane is A direction perpendicular to the axis direction is defined as a Y axis, and a direction perpendicular to the axis direction of the optical fiber on a vertical plane is defined as a Z axis.
The preparation method of the helical deflection fiber grating for the all-fiber orbital angular momentum beam generator according to claim 5, wherein the speed of the optical fiber moving in one direction along the X axis is Vx, and the speed of the bidirectional reciprocating movement along the Y axis The speed is Vy and the speed of bidirectional reciprocating movement along the Z axis is Vz, which satisfies the following formula:

Where r is the design radius of the long-period spiral fiber grating, and T is the design pitch of the long-period spiral fiber grating.
The preparation method of the helical deflection fiber grating for the all-fiber orbital angular momentum beam generator according to claim 6, wherein the helical deflection fiber grating as the orbital angular momentum generator should satisfy the following phase matching conditions:

Among them, n F and n N represent the effective refractive index of the fundamental mode and the coupled mode respectively, λ is the resonant peak wavelength corresponding to the helical deflection fiber grating, J F and J N are the total angular momentum of the two corresponding modes , which are the sum of the orbital angular momentum and spin angular momentum of the corresponding mode, m is the diffraction order of the grating, and σ represents the handedness of the helical deflection fiber grating.
The method for preparing a helical deflection fiber grating for an all-fiber orbital angular momentum beam generator according to claim 6, wherein, before step 200, further comprising the following steps:

Step 100: Set the design radius r of the long-period spiral fiber grating, the design pitch T, and the unidirectional movement speed Vx of the optical fiber along the X-axis.
The method for preparing a helical deflection fiber grating for an all-fiber orbital angular momentum beam generator according to claim 8, wherein the design pitch T is a constant value or a variable value.
The method for manufacturing a helical deflection fiber grating for an all-fiber orbital angular momentum beam generator according to claim 8, wherein the design radius r is a constant value or a variable value.