Multi-core fiber grating and rapid preparation method thereof

Abstract

The invention discloses a multi-core fiber bragg grating, which comprises a multi-core fiber and a fiber bragg grating, the fiber bragg grating is formed in the multi-core fiber, the multi-core fiber comprises a cladding and a plurality of fiber cores, and the fiber cores are arranged in the cladding in parallel; the fiber bragg grating comprises a plurality of grating modulation surfaces which are periodically arranged along the axial direction of the fiber; each grating modulation surface consists of a plurality of refractive index modulation points which are periodically arranged in the radial plane of the fiber; each fiber core contains all or part of at least one refractive index modulation point in each grating modulation plane. According to the multi-core fiber grating, the frequency of fiber core positioning of the multi-core fiber can be reduced, so that the grating preparation speed is increased. The invention also discloses a rapid preparation method of the multi-core fiber grating.

Description

Technical Field

[0001] This invention relates to multi-core fiber fabrication technology, and more particularly to a multi-core fiber grating and its rapid fabrication method. Background Technology

[0002] Multi-core fiber is a new type of optical fiber structure that integrates multiple independent fiber cores in the same cladding area. Each fiber core can transmit optical signals independently. Its core advantage lies in breaking through the capacity bottleneck of traditional single-mode fiber while maintaining the physical size of the fiber. It is particularly suitable for long-distance, high-bandwidth scenarios.

[0003] Multi-core fiber gratings are formed by fabricating corresponding fiber gratings in each core of a multi-core optical fiber. They can continuously acquire physical quantity information such as temperature, strain, and vibration of the object under test in space, and play an important role in national and civil safety monitoring, oil and gas pipeline monitoring, wind turbine health monitoring, and deep-sea and seabed earthquake monitoring.

[0004] Existing methods for fabricating multi-core fiber gratings mainly include ultraviolet exposure mask method, femtosecond laser point-by-point writing method, and femtosecond laser writing method based on spatial light modulation. Each of these three methods has its own advantages and disadvantages. Among them, the femtosecond laser point-by-point writing method has extremely high flexibility, but in the fabrication of multi-core fiber gratings, in order to ensure that each refractive index modulation point can be accurately written in the corresponding fiber core, a lot of time needs to be spent on positioning each fiber core before writing, so that the spot focus of the femtosecond laser can be moved sequentially to each fiber core during writing. Summary of the Invention

[0005] To address the shortcomings of the prior art, this invention provides a multi-core fiber grating and a rapid fabrication method thereof, which can reduce the number of times the multi-core fiber is positioned, thereby improving the fabrication speed of the grating.

[0006] The technical problem to be solved by the present invention is achieved through the following technical solution:

[0007] A multi-core fiber grating includes a multi-core optical fiber and a fiber grating, wherein the fiber grating is formed within the multi-core optical fiber, the multi-core optical fiber includes a cladding and multiple fiber cores, each fiber core being arranged in parallel within the cladding; the fiber grating includes multiple grating modulation surfaces periodically arranged along the optical fiber axis, each grating modulation surface being composed of multiple refractive index modulation points periodically arranged in the radial plane of the optical fiber; each fiber core contains all or part of at least one refractive index modulation point in each grating modulation surface.

[0008] Furthermore, at least some of the refractive index modulation points are periodically arranged in the radial plane of the optical fiber along the first and second optical fiber radial directions that are perpendicular to each other.

[0009] Furthermore, within each grating modulation surface, the center-to-center distance between two adjacent refractive index modulation points along the radial direction of the first or second fiber is... R is the radius of the fiber core, and r is the radius of the refractive index modulation point.

[0010] Furthermore, within each grating modulation surface, the center-to-center distance d between two adjacent refractive index modulation points along the radial direction of the first or second optical fiber is greater than 2ω0, where ω0 is the effective mode field radius of the refractive index modulation point.

[0011] Furthermore, at least some of the refractive index modulation points are periodically arranged along the circumference of the fiber and from the inside out within the radial plane of the fiber.

[0012] Furthermore, within each grating modulation surface, the central angle between two adjacent refractive index modulation points on the i-th modulation point is... The difference in circumferential radius between the modulation point of the i-th cycle and the modulation point of the (i-1)-th cycle R is the radius of the fiber core, r is the radius of the refractive index modulation point, and Si is the circumference radius of the i-th modulation point.

[0013] Furthermore, within each grating modulation surface, the central angle between two adjacent refractive index modulation points on the i-th modulation point is... The difference in circumferential radius ΔS between the modulation point of the i-th cycle and the modulation point of the (i-1)-th cycle i,i-1 <2ω, where ω is the effective mode field radius of the refractive index modulation point, and Si is the circumference radius of the i-th modulation point.

[0014] Furthermore, the grating modulation surface is a rectangular modulation surface or a circular modulation surface, and the rectangular modulation surface or the circular modulation surface covers all the fiber cores in the multi-core optical fiber.

[0015] Furthermore, the grating modulation surface includes a first sub-modulation surface and a second sub-modulation surface, the first sub-modulation surface covering a portion of the fiber core within the multi-core optical fiber, and the second sub-modulation surface covering another portion of the fiber core within the multi-core optical fiber.

[0016] Furthermore, the multi-core optical fiber includes a central core and multiple peripheral cores, with each peripheral core surrounding the same peripheral circumference of the central core; the first sub-modulation surface is a rectangular or circular modulation surface to cover the central core, and the second sub-modulation surface is an annular modulation surface to cover all peripheral cores.

[0017] Furthermore, the refractive index modulation points are prepared using a femtosecond laser point-by-point writing method.

[0018] A rapid fabrication method for multi-core fiber gratings, used to fabricate the aforementioned multi-core fiber gratings; the rapid fabrication method includes the following steps:

[0019] Step 1: Provide a section of multi-core optical fiber;

[0020] Step 2: Determine the distribution position of each refractive index modulation point in the grating modulation surface according to the arrangement of each fiber core in the multi-core optical fiber;

[0021] Step 3: Locate the calibration point within the multi-core optical fiber and move the multi-core optical fiber to position the focal spot of the femtosecond laser at the calibration point;

[0022] Step 4: Based on the distribution position of each refractive index modulation point in the grating modulation surface, drive the multi-core optical fiber to move in the radial plane of the optical fiber, so that the femtosecond laser sequentially creates each refractive index modulation point in the radial plane of the optical fiber.

[0023] Step 5: Move the multi-core optical fiber along the optical fiber axis and repeat step 4 until multiple grating modulation surfaces constituting the fiber grating are formed.

[0024] The present invention has the following beneficial effects: The multi-core fiber grating of the present invention fabricates a dense and periodically arranged plurality of refractive index modulation points in the radial plane of the fiber without differentiation (without distinguishing between the cladding and the core), such that each refractive index modulation point in the same radial plane of the fiber constitutes a corresponding grating modulation surface, and each grating modulation surface periodically arranged along the fiber axis further constitutes the fiber grating; each refractive index modulation point does not need to distinguish between the cladding and the core during fabrication, as long as the density is large enough so that each core contains all or part of at least one refractive index modulation point in each grating modulation surface, so only the entire multi-core fiber needs to be positioned once to avoid the position of each refractive index modulation point exceeding the radial plane of the fiber, without the need for individual positioning of each core, thus reducing the number of core positioning operations for the multi-core fiber and improving the grating fabrication speed. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the axial structure of the multi-core fiber grating provided by the present invention.

[0026] Figure 2 This is a schematic diagram of the structure of the grating modulation surface in the multi-core fiber grating provided by the present invention.

[0027] Figure 3 This is a schematic diagram of another structure of the grating modulation surface in the multi-core fiber grating provided by the present invention.

[0028] Figure 4 This is a schematic diagram of another structure of the grating modulation surface in the multi-core fiber grating provided by the present invention.

[0029] Figure 5 Five schematic diagrams illustrating the relative positions between the fiber core and the refractive index modulation point in a multi-core fiber grating provided by this invention.

[0030] Figure 6 This is a schematic diagram of the structure of each refractive index modulation point periodically arranged along the radial direction of the first fiber and the second fiber in the multi-core fiber grating provided by the present invention.

[0031] Figure 7 A schematic diagram of the structure of each refractive index modulation point arranged periodically from the inside to the outside along the circumference of the optical fiber in a multi-core fiber grating provided by the present invention.

[0032] Figure 8 The distance domain spectrum of the multi-core fiber optic grating provided by this invention.

[0033] Figure 9 This is a flowchart illustrating the steps of the rapid fabrication method for multi-core fiber Bragg gratings provided by the present invention. Detailed Implementation

[0034] The present invention will now be described in detail with reference to the accompanying drawings and embodiments, examples of which are shown in the drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0035] In the description of this invention, it should be understood that the terms "length", "width", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0036] Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first," "second," or "third" may explicitly or implicitly include one or more of that feature. In the description of this invention, "multiple" means two or more, unless otherwise explicitly specified.

[0037] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," "fixing," and "setting," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0038] Example 1

[0039] like Figure 1-3 As shown, a multi-core fiber grating includes a multi-core fiber 1 and a fiber grating 2. The fiber grating 2 is formed within the multi-core fiber 1. The multi-core fiber 1 includes a cladding 11 and multiple fiber cores 12, with each fiber core 12 arranged in parallel within the cladding 11. The fiber grating 2 includes multiple grating modulation surfaces 21 arranged periodically along the Z-axis of the fiber. Each grating modulation surface 21 is composed of multiple refractive index modulation points 211 arranged periodically in the radial plane of the fiber. Each fiber core 12 contains all or part of at least one refractive index modulation point 211 within each grating modulation surface 21.

[0040] The multi-core fiber grating of the present invention fabricates a plurality of densely and periodically arranged refractive index modulation points 211 in the radial plane of the fiber without differentiation (without distinguishing between the cladding 11 and the fiber core 12). Each refractive index modulation point 211 in the same radial plane of the fiber constitutes a corresponding grating modulation surface 21, and each grating modulation surface 21 periodically arranged along the Z-axis of the fiber further constitutes the fiber grating 2. During fabrication, the position of each refractive index modulation point 211 within the multi-core fiber 1 does not need to be distinguished. As long as the density is large enough, each fiber core 12 contains all or part of at least one refractive index modulation point 211 in each grating modulation surface 21. Therefore, only one overall positioning of the multi-core fiber 1 is required to avoid the position of each refractive index modulation point 211 exceeding the radial plane of the fiber, without the need for individual positioning of each fiber core 12. This reduces the number of times the fiber core 12 needs to be positioned, thereby improving the grating fabrication speed.

[0041] In the first example, such as Figure 5 As shown in Figure a, a portion of the fiber core 12 contains only the entirety of a single refractive index modulation point 211 within each grating modulation surface 21; in the second example, as... Figure 5 As shown in Figure b, a portion of the fiber core 12 contains only one refractive index modulation point 211 within each grating modulation surface 21; in the third example, as... Figure 5As shown in Figure c, a portion of the fiber core 12 contains all of two or more refractive index modulation points 211 within each grating modulation surface 21; in the fourth example, as... Figure 5 As shown in d, a portion of the fiber core 12 contains two or more refractive index modulation points 211 within each grating modulation surface 21; in the fifth example, as... Figure 5 As shown in Figure e, each portion of the fiber core 12 contains all of at least one refractive index modulation point 211 and a portion of at least one refractive index modulation point 211 within each grating modulation surface 21.

[0042] It should be noted that the number or area of ​​refractive index modulation points 211 contained in each fiber core 12 within each grating modulation surface 21 only affects the reflectivity of the fiber grating 2, and does not affect the center wavelength of the fiber grating 2. The center wavelength of the fiber grating 2 is determined by the center distance (i.e., the grating period Λ) between two adjacent grating modulation surfaces 21.

[0043] The specific shape of the grating modulation surface 21 can be determined according to the arrangement of the fiber cores 12 within the multi-core optical fiber 1. In its simplest form, as shown below... Figure 2 and 3 As shown, the grating modulation surface 21 can be a rectangular modulation surface or a circular modulation surface, and the rectangular modulation surface or the circular modulation surface covers all the fiber cores 12 in the multi-core optical fiber 1.

[0044] Of course, in order to reduce the number of refractive index modulation points 211 within each grating modulation surface 21, such as Figure 4 As shown, the grating modulation surface 21 may include a first sub-modulation surface 21a and a second sub-modulation surface 21b. The first sub-modulation surface 21a covers a portion of the fiber cores 12 within the multi-core optical fiber 1, and the second sub-modulation surface 21b covers another portion of the fiber cores 12 within the multi-core optical fiber 1. For example, the multi-core optical fiber 1 includes a central fiber core 12a and multiple peripheral fiber cores 12b, with each peripheral fiber core 12b surrounding the same peripheral circumference of the central fiber core 12a. In this case, the first sub-modulation surface 21a can be a rectangular modulation surface or a circular modulation surface to cover the central fiber core 12a, and the second sub-modulation surface 21b can be an annular modulation surface to cover all the peripheral fiber cores 12b.

[0045] Of course, the grating modulation surface 21 may also include three or more sub-modulation surfaces, depending on the specific requirements.

[0046] For any shape of modulation surface, each refractive index modulation point 211 can be periodically arranged along the first fiber radial direction X and the second fiber radial direction Y, which are perpendicular to each other, in the radial plane of the optical fiber.

[0047] For each refractive index modulation point 211 periodically arranged along the radial direction X of the first fiber and the radial direction Y of the second fiber, in order to ensure that each fiber core 12 contains all or part of at least one refractive index modulation point 211 in each grating modulation surface 21, preferably, as follows: Figure 6 As shown, within each grating modulation surface 21, the center-to-center distance between two adjacent refractive index modulation points 211 along the radial direction X of the first optical fiber or the radial direction Y of the second optical fiber is... R is the radius of the fiber core 12, and r is the radius of the refractive index modulation point 211.

[0048] Meanwhile, in order to avoid optical coupling between two adjacent refractive index modulation points 211, which would cause the optical signal to be transmitted sequentially from one fiber core 12 to another fiber core 12 through the optically coupled refractive index modulation point 211, preferably, within each grating modulation surface 21, the center distance d between two adjacent refractive index modulation points 211 along the radial direction X of the first optical fiber or the radial direction Y of the second optical fiber is greater than 2ω, where ω is the effective mode field radius of the refractive index modulation point 211.

[0049] For special modulation surfaces such as circular or annular modulation surfaces, each refractive index modulation point 211 can be arranged periodically along the first fiber radial direction X and the second fiber radial direction Y in the radial plane of the optical fiber, and can also be arranged periodically from the inside to the outside along the circumferential direction O of the optical fiber.

[0050] For each refractive index modulation point 211 arranged periodically from the inside to the outside along the circumferential direction O of the optical fiber, in order to ensure that each fiber core 12 contains all or part of at least one refractive index modulation point 211 in each grating modulation surface 21, preferably, as follows: Figure 7 As shown, within each grating modulation surface 21, the central angle between two adjacent refractive index modulation points 211 on the i-th modulation point is... The difference in circumferential radius between the modulation point of the i-th cycle and the modulation point of the (i-1)-th cycle R is the radius of the fiber core 12, r is the radius of the refractive index modulation point 211, and Si is the circumference radius of the i-th modulation point.

[0051] Meanwhile, to avoid optical coupling between two adjacent refractive index modulation points 211, which would cause the optical signal to be transmitted sequentially from one fiber core 12 to another fiber core 12 through the optically coupled refractive index modulation points 211, preferably, within each grating modulation surface 21, the central angle between two adjacent refractive index modulation points 211 on the i-th ring of modulation points is... The difference in circumferential radius ΔS between the modulation point of the i-th cycle and the modulation point of the (i-1)-th cycle i,i-1<2ω, where ω is the effective mode field radius of the refractive index modulation point 211, and Si is the circumference radius of the i-th modulation point.

[0052] In this embodiment, the refractive index modulation point 211 is prepared by femtosecond laser point-by-point writing method.

[0053] The number of fiber gratings 2 can be only one, or two or more. When there are two or more fiber gratings 2, each fiber grating 2 is distributed sequentially at intervals along the Z-axis of the optical fiber. The interval between two adjacent fiber gratings 2 is determined according to the actual required sensing position.

[0054] Figure 8 The figure shows the distance-domain spectra obtained by testing the multi-core fiber grating of the present invention using an OFDR (Optical Frequency Domain Reflectometry) system. The left side of the figure shows the distance-domain spectrum of the central fiber core 12a, and the right side shows the distance-domain spectrum of one of the outer fiber cores 12b. As can be seen from the figure, both the central fiber core 12a and the outer fiber core 12b exhibit distinct peaks in their distance-domain spectra at the length positions corresponding to each fiber grating 2 within the multi-core fiber 1. This indicates that the multi-core fiber grating of the present invention has relatively ideal performance in the distance domain.

[0055] Example 2

[0056] like Figure 9 As shown, a rapid fabrication method for multi-core fiber gratings is provided for fabricating the multi-core fiber grating described in Example 1; the rapid fabrication method includes the following steps:

[0057] Step 1: Provide a section of multi-core optical fiber 1;

[0058] Step 2: Determine the distribution position of each refractive index modulation point 211 in the grating modulation surface 21 according to the arrangement of each fiber core 12 in the multi-core optical fiber 1.

[0059] Step 3: Locate the calibration point within the multi-core optical fiber 1 and move the multi-core optical fiber 1 to position the focal spot of the femtosecond laser at the calibration point;

[0060] Step 4: Based on the distribution position of each refractive index modulation point 211 in the grating modulation surface 21, drive the multi-core optical fiber 1 to move in the radial plane of the optical fiber so that the femtosecond laser sequentially creates each refractive index modulation point 211 in the radial plane of the optical fiber.

[0061] Step 5: Move the multi-core optical fiber 1 along the optical fiber axis Z, and repeat step 4 until multiple grating modulation surfaces 21 constituting the fiber grating 2 are fabricated.

[0062] In step 3, the calibration point can be any point within the multi-core optical fiber 1, as long as it is accessible for identification and positioning by the vision system. In this embodiment, the calibration point is the center point of the multi-core optical fiber 1.

[0063] In step 4, the calibration point is used as the origin to construct a rectangular coordinate system or a polar coordinate system for the radial plane of the optical fiber, thereby determining the coordinate values ​​of each refractive index modulation point 211 in the rectangular coordinate system or polar coordinate system. If the refractive index modulation points 211 are periodically arranged along the first fiber radial direction X and the second fiber radial direction Y in the radial plane of the optical fiber, then a rectangular coordinate system is constructed using the first fiber radial direction X and the second fiber radial direction Y as coordinate axes, and the multi-core optical fiber 1 is moved stepwise along the first fiber radial direction X and the second fiber radial direction Y respectively. The stepwise movement distance in both directions is d (the center-to-center distance between two adjacent refractive index modulation points 211). If the refractive index modulation points 211 are periodically arranged along the circumferential direction O of the optical fiber in the radial plane of the optical fiber and from the inside out, then a polar coordinate system is constructed using the incident direction of the femtosecond laser as the polar axis (the incident direction of the femtosecond laser is perpendicular to the fiber axis Z), and the multi-core optical fiber 1 is moved stepwise along the incident direction of the femtosecond laser and rotated stepwise around the fiber axis Z. The stepwise movement distance is ΔR. i,i-1 (The difference in circumferential radius between the modulation point of the i-th cycle and the modulation point of the (i-1)-th cycle), the angle of each step rotation is θ. i (The central angle between two adjacent refractive index modulation points 211 on the i-th modulation point).

[0064] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the embodiments of the present invention and not to limit them. Although the embodiments of the present invention have been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the embodiments of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A multi-core fiber grating, comprising a multi-core optical fiber and a fiber grating, wherein the fiber grating is formed within the multi-core optical fiber, the multi-core optical fiber comprising a cladding and multiple fiber cores, each fiber core being arranged in parallel within the cladding; the fiber grating comprising a plurality of grating modulation surfaces periodically arranged along the optical fiber axis, characterized in that, Each grating modulation surface consists of multiple refractive index modulation points arranged periodically in the radial plane of the fiber; each fiber core contains all or part of at least one refractive index modulation point in each grating modulation surface.

2. The multi-core fiber optic grating according to claim 1, characterized in that, At least some of the refractive index modulation points are periodically arranged in the radial plane of the optical fiber along the first and second optical fiber radial directions that are perpendicular to each other.

3. The multi-core fiber optic grating according to claim 2, characterized in that, Within each grating modulation surface, the center-to-center distance between two adjacent refractive index modulation points along the radial direction of the first or second fiber. R is the radius of the fiber core, and r is the radius of the refractive index modulation point.

4. The multi-core fiber optic grating according to claim 2 or 3, characterized in that, Within each grating modulation surface, the center-to-center distance d between two adjacent refractive index modulation points along the radial direction of the first or second optical fiber is greater than 2ω0, where ω0 is the effective mode field radius of the refractive index modulation point.

5. The multi-core fiber optic grating according to claim 1, characterized in that, At least some of the refractive index modulation points are periodically arranged in the radial plane of the optical fiber along the circumference of the fiber and from the inside to the outside.

6. The multi-core fiber optic grating according to claim 5, characterized in that, Within each grating modulation surface, the central angle θ between two adjacent refractive index modulation points on the i-th modulation point is... i < The difference in circumferential radius between the modulation point of the i-th cycle and the modulation point of the (i-1)-th cycle R is the radius of the fiber core, r is the radius of the refractive index modulation point, and Si is the circumference radius of the i-th modulation point.

7. The multi-core fiber optic grating according to claim 5 or 6, characterized in that, Within each grating modulation surface, the central angle θ between two adjacent refractive index modulation points on the i-th modulation point is... i >2arcsin The difference in circumferential radius ΔS between the modulation point of the i-th cycle and the modulation point of the (i-1)-th cycle i,i-1 <2ω, where ω is the effective mode field radius of the refractive index modulation point, and Si is the circumference radius of the i-th modulation point.

8. The multi-core fiber optic grating according to claim 1, characterized in that, The grating modulation surface is a rectangular modulation surface or a circular modulation surface, and the rectangular modulation surface or the circular modulation surface covers all the fiber cores in the multi-core optical fiber.

9. The multi-core fiber optic grating according to claim 1, characterized in that, The grating modulation surface includes a first sub-modulation surface and a second sub-modulation surface. The first sub-modulation surface covers a portion of the fiber core within the multi-core optical fiber, and the second sub-modulation surface covers another portion of the fiber core within the multi-core optical fiber.

10. The multi-core fiber optic grating according to claim 9, characterized in that, The multi-core optical fiber includes a central core and multiple peripheral cores, with each peripheral core surrounding the same outer circumference of the central core; the first sub-modulation surface is a rectangular or circular modulation surface to cover the central core, and the second sub-modulation surface is an annular modulation surface to cover all peripheral cores.

11. The multi-core fiber optic grating according to claim 1, characterized in that, The refractive index modulation points are formed using a femtosecond laser point-by-point writing method.

12. A rapid fabrication method for multi-core fiber Bragg gratings, characterized in that, The rapid fabrication method is used to prepare the multi-core fiber grating of claim 1, comprising the following steps: Step 1: Provide a section of multi-core optical fiber; Step 2: Determine the distribution position of each refractive index modulation point in the grating modulation surface according to the arrangement of each fiber core in the multi-core optical fiber; Step 3: Locate the calibration point within the multi-core optical fiber and move the multi-core optical fiber to position the focal spot of the femtosecond laser at the calibration point; Step 4: Based on the distribution position of each refractive index modulation point in the grating modulation surface, drive the multi-core optical fiber to move in the radial plane of the optical fiber, so that the femtosecond laser sequentially creates each refractive index modulation point in the radial plane of the optical fiber. Step 5: Move the multi-core optical fiber along the optical fiber axis and repeat step 4 until multiple grating modulation surfaces constituting the fiber grating are formed.

Images