Method for manufacturing large-area fiber gratings and fiber grating

Abstract

The application relates to the technical field of fiber gratings, and discloses a preparation method of a large-area fiber grating and the fiber grating. The method is to make a glass tube with a refractive index close to that of a fiber coating adhere to the fiber under the action of high temperature by using the collapsing glass tube method, so that the femtosecond laser damage threshold of the fiber boundary is improved, the writing area of the fiber grating is increased, and even the modulation area introduced by the femtosecond laser can cross the two areas of the fiber and the glass tube, so that the fiber grating covering the whole cross section of the fiber is prepared. Meanwhile, the refractive index of the glass tube is close to that of the coating, so that the mode deformation and loss are not caused.

Description

Technical Field

[0001] This invention mainly relates to the field of fiber laser technology, and in particular to a method for fabricating a large-area fiber grating and the fiber grating itself. Background Technology

[0002] Fiber Bragg gratings (FBGs) are passive filtering devices composed of a series of periodic refractive index modulation regions distributed along the axial direction within an optical fiber, and they have found important applications in many fields. Femtosecond laser direct writing technology uses an objective lens to directly focus a femtosecond laser beam within the fiber and perform periodic relative motion, generating periodic refractive index modulation to form a fiber grating. Compared to ultraviolet laser writing technology and phase mask writing technology, femtosecond laser direct writing technology has become an important technology for fiber grating fabrication due to its flexibility, controllability, lack of hydrogen loading requirement, and ability to process various materials.

[0003] However, in femtosecond laser direct writing technology, the femtosecond laser is directly focused into the fiber through the objective lens, resulting in a very small refractive index modulation area near the focal point, typically 2–4 μm. Currently, the core diameter of commonly used multimode fibers and single-crystal fibers is usually greater than 100 μm, making it difficult to fabricate high-reflectivity fiber gratings on these fibers, and resulting in severe scattering loss and heat generation. Increasing the area of ​​the fiber grating can significantly improve the reflectivity of fiber gratings fabricated on these fibers, while simultaneously reducing scattering loss and heat generation. However, traditional femtosecond laser direct writing technology struggles to write large-area fiber gratings on fibers with core diameters greater than 100 μm. Improvements to traditional femtosecond laser direct writing technology have been reported to increase the writing area of ​​fiber gratings through techniques such as beam shaping, introducing spherical aberration, optical field manipulation, splicing, and stacking. Furthermore, immersing the fiber in a refractive index matching solution and using a long working distance objective lens can increase the Rayleigh distance at the focal point, thereby introducing a longer refractive index modulation region. However, immersing the optical fiber in the refractive index matching liquid can cause other problems. Specifically, when the femtosecond laser focus approaches the fiber boundary, if the fiber coating is not stripped, it will cause damage to the coating. If the fiber coating is stripped, it will cause bubbles in the refractive index matching liquid, affecting the writing of the fiber grating.

[0004] Therefore, there is an urgent need in this field for a technology that can fabricate large-area fiber gratings in optical fibers with a core diameter of 100 μm or more. Summary of the Invention

[0005] In view of the technical problems existing in the prior art, the present invention proposes a method for fabricating large-area fiber gratings and a fiber grating.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] On one hand, the present invention provides a method for fabricating a large-area fiber grating, which enables the fabrication of a fiber grating with a refractive index modulation region covering the entire core or cladding on an optical fiber with a core diameter or cladding diameter greater than 100 μm. The method includes:

[0008] Prepare optical fibers, determine the position of the fiber grating to be inscribed on the optical fibers, and remove the coating layer at the location of the fiber grating to be inscribed.

[0009] A glass tube is prepared, the glass tube including an inner wall and an outer wall, the refractive index of the inner wall of the glass tube is lower than the refractive index of the cladding of the optical fiber or the same as the refractive index of the coating layer of the optical fiber, and the refractive index of the outer wall of the glass tube is the same as the refractive index of the refractive index matching liquid.

[0010] The optical fiber is passed through the glass tube, the outer diameter of the optical fiber matches the inner diameter of the glass tube, and the fiber grating to be written is located in the middle section of the glass tube.

[0011] The glass tube at the location where the fiber grating is to be written is collapsed to ensure that the inner wall of the glass tube at the location where the fiber grating is to be written is tightly attached to the fiber cladding, while the core and cladding dimensions of the fiber remain unchanged.

[0012] Immerse the glass tube at the location of the fiber grating to be inscribed in a refractive index matching solution;

[0013] The femtosecond laser is used to write fiber gratings with the required refractive index modulation region shape and size.

[0014] On the other hand, the present invention provides fiber gratings fabricated based on the above-described method for fabricating large-area fiber gratings. Using the method for fabricating large-area fiber gratings provided by the present invention, fiber gratings can be fabricated on silicate, tellurite, fluoride, sulfide optical fibers or single-crystal optical fibers with a core diameter or cladding diameter greater than 100 μm.

[0015] The key to improving reflectivity is increasing the overlap area between the fiber grating and the fiber mode field. However, in these large-diameter fibers, the mode field area is also very large, thus requiring advanced fabrication techniques for large-area fiber gratings. Furthermore, in conventional fiber grating writing methods, the writing boundary of the fiber grating is often constrained by air, coatings, or refractive index matching fluids, preventing the fiber grating from covering a larger fiber area. This is because when the femtosecond laser focus is too close to air, coatings, or refractive index matching fluids, the cylindrical mirror effect between air and the fiber surface, as well as the significant difference in femtosecond laser damage thresholds between the coating / refractive index matching fluid and the fiber material, can damage the fiber surface and coating, or induce air bubbles in the refractive index matching fluid that interfere with the focusing of the femtosecond laser. Therefore, existing technologies lack a process for writing large-area fiber gratings surface-by-surface, making the fabrication of large-area fiber gratings a significant challenge. Compared to existing technologies, the technical advantages of this invention are:

[0016] This invention increases the area for writing fiber gratings on large-core optical fibers by collapsing a glass tube onto the fiber cladding, thus achieving the fabrication of large-area fiber gratings. Simultaneously, the refractive index of the inner wall of the glass tube is lower than or similar to that of the fiber cladding, avoiding deformation and leakage of the transmission mode field in the fiber core and reducing deformation and leakage of the transmission mode field in the fiber cladding.

[0017] This invention utilizes a collapsed glass tube method, whereby a glass tube with a refractive index similar to that of the fiber optic coating is bonded to the fiber under high temperature. This increases the femtosecond laser damage threshold at the fiber boundary, thereby increasing the writing area of ​​the fiber grating. It can even be achieved by allowing the modulation region introduced by the femtosecond laser to span both the fiber and the glass tube, fabricating a fiber grating covering the entire cross-section of the fiber. Simultaneously, because the refractive index of the inner wall of the glass tube is lower than or similar to that of the fiber cladding, deformation and leakage of the transmission mode field in the fiber core can be avoided, and deformation and leakage of the transmission mode field in the fiber cladding can be reduced. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0019] Figure 1 This is a schematic diagram showing the relative positions of the optical fiber and glass tube before collapse, according to an embodiment of the present invention:

[0020] Figure 2 This is a schematic diagram showing the relative positions of the collapsed optical fiber and glass tube according to an embodiment of the present invention:

[0021] Figure 1 and Figure 2 Numbered items: 1. Glass tube; 2. Optical fiber with coating not removed; 3. Optical fiber stripping point; 4. Location of fiber grating to be written; 5. Collapsed glass tube;

[0022] Figure 3 This is a schematic diagram of the refractive index modulation region of the fiber optic grating to be inscribed in one embodiment.

[0023] Figure 3 Numbered: 5. Collapsed glass tube; 6. Refractive index modulation rectangle; 7. Fiber cladding; 8. Fiber core. Detailed Implementation

[0024] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0025] In one embodiment, a method for fabricating a large-area fiber grating is provided. This method enables the fabrication of fiber gratings with a refractive index modulation region covering the entire core or cladding on optical fibers with a core diameter or cladding diameter greater than 100 μm. Specifically, the method includes the following steps:

[0026] Prepare an optical fiber, determine the position of the fiber grating to be inscribed on the optical fiber, and remove the coating layer at the location of the fiber grating to be inscribed. Let the cladding diameter of the optical fiber be D1 and the coating diameter be D2.

[0027] A glass tube is prepared, the glass tube comprising an inner wall and an outer wall, the refractive index of the inner wall of the glass tube being lower than the refractive index of the fiber cladding or the same as the refractive index of the coating layer of the fiber, and the refractive index of the outer wall of the glass tube being the same as the refractive index of the refractive index matching liquid.

[0028] The optical fiber is passed through the glass tube, with the outer diameter of the optical fiber matching the inner diameter of the glass tube. The fiber grating to be written is located in the middle section of the glass tube. Further, the outer diameter of the glass tube is at least 100 μm larger than its inner diameter.

[0029] The glass tube at the location where the fiber grating is to be written is collapsed to ensure that the inner wall of the glass tube at the location where the fiber grating is to be written is tightly attached to the fiber cladding, while the core and cladding dimensions of the fiber remain unchanged.

[0030] Immerse the glass tube at the location of the fiber grating to be inscribed in a refractive index matching solution;

[0031] The femtosecond laser is used to write fiber gratings with the required refractive index modulation region shape and size.

[0032] In one embodiment, the glass tube at the location where the fiber Bragg grating is to be written is subjected to a collapse process, including the following steps:

[0033] Seal one end of the glass tube together with the optical fiber (the sealing method is not limited, such as using UV-curing adhesive for sealing).

[0034] Connect the air pump to the other end of the glass tube and use the air pump to make the air pressure inside the glass tube lower than the air pressure outside the glass tube.

[0035] The heat source is used to heat and soften the glass tube at the location where the fiber Bragg grating (FBG) is to be written. Under the influence of negative pressure inside the tube, it collapses, causing the inner wall of the glass tube at the FBG location to adhere tightly to the fiber cladding. Note that the heat source power depends on the thermal properties of the glass tube material; for materials that are difficult to soften, the heat source power needs to be increased, while for materials that are easy to soften, the power needs to be decreased. The heat source moves at a constant speed v1, ultimately ensuring that the entire glass tube at the FBG location is adhered to the fiber cladding, while maintaining the core and cladding dimensions of the fiber. The heat source speed v1 must ensure a tight fit between the glass tube and the fiber cladding without applying excessive heat to the fiber, which could cause deformation.

[0036] Reference Figure 1 and Figure 2 , Figure 1 This is a schematic diagram of the relative positions of the optical fiber and the glass tube before collapse, provided in one embodiment. The optical fiber at position 4 where the fiber grating is to be written has had its coating removed. The two ends of position 4 where the fiber grating is to be written are the fiber removal points 3. Position 4 where the fiber grating is to be written is located in the middle section of the glass tube 1. The optical fiber 2 whose coating has not been removed is located outside the glass tube 2. Figure 2 This is a schematic diagram of the relative positions of the collapsed optical fiber and the glass tube according to an embodiment of the present invention. The inner wall of the collapsed glass tube 2 is tightly attached to the cladding of the optical fiber.

[0037] After the collapse process, the optical fibers on both sides of the glass tube are clamped and straightened using optical fiber clamps to proceed to the next step, which is to immerse the glass tube at the location where the fiber grating is to be written in a refractive index matching liquid, wherein the refractive index of the refractive index matching liquid is the same as or close to the refractive index of the outer wall of the glass tube, and then cover the upper surface of the glass tube at the location where the fiber grating is to be written with a cover glass.

[0038] Alternatively, when immersing the glass tube at the location where the fiber grating is to be inscribed in the refractive index matching liquid, the glass tube can be placed on a flat glass plate or slide and the refractive index matching liquid can be dripped until it covers the glass tube.

[0039] Alternatively, when immersing the glass tube at the location of the fiber grating to be inscribed in the refractive index matching liquid, the glass tube at the location of the fiber grating to be inscribed can be placed in a transparent oil bath and the refractive index matching liquid can be added until the oil bath is filled.

[0040] The femtosecond laser is used to write fiber gratings with the required refractive index modulation region shape and size. The specific writing process depends on the refractive index modulation region shape and size and the grating period.

[0041] In one embodiment, the refractive index modulation region of the fiber grating to be etched is rectangular. The process of etching a fiber grating with the desired refractive index modulation region shape and size using the femtosecond laser includes the following steps:

[0042] (a) The refractive index modulation region of the fiber grating to be written is rectangular in shape, and the length of the rectangle along the z-axis is determined. l 1 and length along the y-axis l 2;

[0043] (b) Determine the position of the refractive index modulation region of the fiber optic grating to be inscribed inside the fiber, and determine the initial inscription position of the refractive index modulation region;

[0044] (c) Based on the length of the rectangle along the z-axis l 1. Determine the single-pulse energy and beam diameter of the femtosecond laser corresponding to the inscription of the rectangle;

[0045] (d) Turn on the femtosecond laser, control the femtosecond laser to emit a single pulse energy and beam diameter femtosecond laser that is incident from the z-axis direction and focused onto the initial marking position of the refractive index modulation region inside the fiber, and simultaneously control the fiber to move at a constant speed along the y-axis direction by a set distance. l 2. Complete the writing of a single-plane fiber grating with the shape and size of the refractive index modulation region.

[0046] Given a constant average power of a femtosecond laser, the length of the refractive index modulation region introduced by the femtosecond laser along the z-axis is... l 1. As the beam diameter d of the femtosecond laser decreases, the length of the refractive index modulation region introduced by the femtosecond laser along the z-axis can be controlled. l 1.

[0047] In a preferred embodiment, when the femtosecond laser wavelength is 515 nm, the repetition frequency is 1 kHz, the single pulse energy is 0.94 μJ, the beam diameter is 1.5 mm, the scanning speed is 0.6 mm / s, the coverslip thickness is 1.1 mm, the refractive index of the matching fluid is 1.464, and the outer diameter of the collapsed glass tube is 1.05 mm, lThe length is 50 μm. This parameter combination can produce longer lengths at once. l 1. Length.

[0048] In one embodiment, the refractive index modulation region of the fiber grating to be etched is shaped as a plurality of rectangles arranged parallel to each other along the z-axis and spaced apart from each other. The etching of the fiber grating with the desired refractive index modulation region shape and size using the femtosecond laser includes:

[0049] (a) The refractive index modulation region of the fiber grating to be written is a plurality of rectangles arranged parallel to each other along the z-axis and spaced apart from each other. The rectangles are numbered 1, 2...N from bottom to top along the z-axis, and the length of the i-th rectangle along the z-axis is... l 1i The length of the i-th rectangle along the y-axis is l 2i The distance between the i-th rectangle and the (i+1)-th rectangle on the z-axis is l 3i ;

[0050] (b) Determine the position of the refractive index modulation region of the fiber optic grating to be written inside the fiber, and determine the initial writing rectangle of the refractive index modulation region and its initial writing position. The initial writing rectangle is the first rectangle.

[0051] (c) Based on the length of each rectangle along the z-axis in the refractive index modulation region l 1i Determine the single-pulse energy P of the femtosecond laser corresponding to each rectangle being inscribed. i and beam diameter d i ;

[0052] (d) Turn on the femtosecond laser and control it to emit a single pulse energy P. i Beam diameter d i The femtosecond laser is incident from the z-axis and focused onto the initial writing position, while the optical fiber is controlled to move uniformly a set distance along the negative y-axis. l 2i Complete the inscription of the i-th rectangle;

[0053] (e) Turn off the femtosecond laser and control the fiber to move at a constant speed along the positive y-axis for a set distance. l 2i Then move at a constant speed a set distance in the positive z-axis direction. l 3i That is, to the initial writing position of the (i+1)th rectangle, i=i+1, the current writing rectangle is updated to the (i+1)th rectangle, the single pulse energy and beam diameter of the femtosecond laser are updated, and the cycle returns to step (d) until the writing of the fiber grating with the shape and size of the refractive index modulation region is completed.

[0054] In a preferred embodiment, when the femtosecond laser wavelength is 515 nm, the repetition rate is 1 kHz, the single pulse energy is 0.94 μJ, the beam diameter is 1.5 mm, the scanning speed is 0.6 mm / s, the coverslip thickness is 1.1 mm, the refractive index of the matching liquid is 1.464, and the outer diameter of the collapsed glass tube is 1.05 mm, l The length is 50 μm. This parameter combination can produce longer lengths at once. l 1. Length.

[0055] like Figure 3 The diagram shown illustrates the refractive index modulation region of a fiber optic grating to be inscribed in one embodiment, including a collapsed glass tube 5, a refractive index modulation rectangle 6, an optical fiber cladding 7, and an optical fiber core 8. The refractive index modulation region of the fiber optic grating to be inscribed is shaped as a plurality of rectangles arranged parallel to each other along the z-axis in the yz plane and spaced apart from each other. The length of the first rectangle along the z-axis is... l 11 The length of the first rectangle along the y-axis is l 21 The distance between the first rectangle and the second rectangle on the z-axis is... l 31 The length of the second rectangle along the z-axis is... l 12 The length of the first rectangle along the y-axis is l 22 The distance between the second and third rectangles on the z-axis is... l 32 The length of the third rectangle along the z-axis is... l 13 The length of the third rectangle along the y-axis is l 23 The distance between the 3rd and 4th rectangles on the z-axis is... l 33 The length of the 4th rectangle along the z-axis is l 14 The length of the fourth rectangle along the y-axis is l 24 The distance between the 4th and 5th rectangles on the z-axis is... l 34 The length of the 5th rectangle along the z-axis is... l 15 The length of the 5th rectangle along the y-axis is l 25 .

[0056] In practical applications, the number of rectangles in the refractive index modulation region of the fiber grating to be inscribed and the size of each rectangle are determined according to the requirements and are not required. The size of each rectangle can be the same, partially the same, or different from each other.

[0057] In one embodiment, the fiber grating to be written is a periodic fiber grating, and the refractive index modulation region of each grating period is a plurality of rectangles arranged parallel to each other along the z-axis and spaced apart from each other. The writing of the fiber grating with the required refractive index modulation region shape and size using the femtosecond laser includes:

[0058] (a) Determine the grating period of the fiber grating to be inscribed. The refractive index modulation region of each grating period consists of multiple rectangles arranged parallel to each other along the z-axis and spaced apart. The rectangles are numbered 1, 2...N from bottom to top along the z-axis. The length of the i-th rectangle along the z-axis is... l 1i The length of the i-th rectangle along the y-axis is l 2i The distance between the i-th rectangle and the (i+1)-th rectangle on the z-axis is l 3i ;

[0059] (b) Determine the position of the refractive index modulation region of the fiber optic grating to be written inside the fiber, and determine the initial writing rectangle and its initial writing position of the refractive index modulation region corresponding to each grating period. The initial writing rectangle of each grating period is the first rectangle of the corresponding period.

[0060] (c) The length of each rectangle along the z-axis in the refractive index modulation region of each grating period l 1i Determine the single-pulse energy P of the femtosecond laser corresponding to each rectangle being inscribed. i and beam diameter d i

[0061] (d) Turn on the femtosecond laser and control it to emit a single pulse energy P. i Beam diameter d i A femtosecond laser is incident from the z-axis and focused onto the initial writing position of the refractive index modulation region corresponding to the current grating period, while simultaneously controlling the fiber to move uniformly a set distance along the negative y-axis. l 2i Complete the inscription of the i-th rectangle;

[0062] (e) Turn off the femtosecond laser and control the fiber to move at a constant speed along the positive y-axis for a set distance. l 2i Then move at a constant speed a set distance in the positive z-axis direction. l 3iThat is, to the initial writing position of the (i+1)th rectangle of the refractive index modulation region corresponding to the current grating period, i=i+1, the writing rectangle of the current grating period is updated to the (i+1)th rectangle, the single pulse energy and beam diameter of the femtosecond laser are updated, and the process returns to step (d) and repeats until the writing of N rectangles of the refractive index modulation region corresponding to the current grating period is completed, that is, the writing of the fiber grating with the shape and size of the refractive index modulation region in the current grating period is completed.

[0063] (f) Control the fiber to move along the x direction by one grating period, update the current grating period, return to step (d), until the writing of the periodic fiber grating of the required period is completed.

[0064] The optical fiber is a silicate, tellurite, fluoride, sulfide, or single-crystal optical fiber with a core diameter or cladding diameter greater than 100 μm. Using the large-area fiber grating fabrication method provided by this invention, fiber gratings can be fabricated on silicate, tellurite, fluoride, sulfide, or single-crystal optical fibers with a core diameter or cladding diameter greater than 100 μm. The refractive index modulation region etched on these gratings can cover a large area of ​​the core or cladding region as needed, achieving a maximum of 100% coverage of the entire core or cladding region. That is, if the grating is etched on the core of an optical fiber with a core diameter greater than 100 μm, 100% coverage of the entire core region can be achieved; if the grating is etched on the cladding of an optical fiber with a cladding diameter greater than 100 μm, 100% coverage of the entire cladding region can be achieved.

[0065] As in one embodiment, a method for fabricating a large-area fiber Bragg grating is provided, comprising:

[0066] The optical fiber to be processed is a large-mode-field double-clad 20 / 400 passive fiber with a cladding diameter D1 of 400 μm and a coating diameter D2 of 550 μm. The location of the fiber grating to be inscribed on the fiber is determined, and the coating at the inscribed fiber grating location is stripped.

[0067] A semi-fluorine-doped glass tube is used, in which a certain amount of fluorine is added to the material of the inner wall of the glass tube, making the refractive index of the inner wall of the glass tube lower than that of the optical fiber cladding. The outer wall material of the glass tube is pure quartz.

[0068] The optical fiber is passed through the glass tube, with the outer diameter of the optical fiber matching the inner diameter of the glass tube. The fiber grating to be written is located in the middle section of the glass tube. In this embodiment, the inner diameter of the glass tube is 580 μm, and the outer diameter is 1070 μm.

[0069] The glass tube at the location where the fiber Bragg grating is to be written is collapsed: one end of the glass tube, along with the optical fiber, is sealed with UV-curing adhesive and a UV lamp. Then, a vacuum pump connected to the other end of the glass tube is used to evacuate air, lowering the air pressure inside the tube compared to the outside. The torch power is set to 190 W, which effectively softens the glass tube for collapse. The torch speed v1 is 0.1 mm / s, ensuring a tight fit between the glass tube and the fiber cladding without applying excessive heat to the fiber, which could cause deformation. After collapse, the inner wall of the glass tube at the location where the fiber Bragg grating is to be written is tightly fitted to the fiber cladding. The fiber cladding diameter remains unchanged, while the inner diameter of the glass tube becomes 400 μm and the outer diameter becomes 1050 μm.

[0070] The optical fibers on both sides of the glass tube are clamped and straightened using fiber optic clamps. The glass tube to which the fiber Bragg grating positions are to be inscribed is placed in a square oil bath and immersed in a refractive index matching solution. The side length of the oil bath is 1.5 mm, and the refractive index of the refractive index matching solution is 1.464. A cover glass with a thickness of 1.1 mm is placed on top of the oil bath.

[0071] Next, the femtosecond laser is used to write a fiber grating with the required refractive index modulation region shape and size.

[0072] In this embodiment, the refractive index modulation region shape of the fiber grating to be written in each fiber period is: nine rectangles arranged parallel to each other along the z-axis in the yz plane and spaced apart from each other. Each rectangle is 450 μm long and 50 μm wide, and the distance between adjacent rectangles on the z-axis is 50 μm. The combination of these nine rectangles can cover the entire inner cladding of the 20 / 400 fiber. From bottom to top along the z-axis, they are the 1st rectangle, the 2nd rectangle, ..., the 9th rectangle.

[0073] The single-pulse energy of the femtosecond laser corresponding to each rectangle was determined to be 0.94 μJ, and the beam diameter was determined to be 1.5 mm. The initial writing rectangle and its initial writing position of the refractive index modulation region corresponding to each grating period were determined: at the endpoint with the largest y-axis coordinate in the first rectangle.

[0074] Write periodic fiber Bragg gratings:

[0075] (1) Turn on the femtosecond laser and control the femtosecond laser to emit a single pulse energy of 0.94 μJ and a beam diameter of 1.5 mm. The femtosecond laser is incident from the z-axis direction and focused on the initial writing position of the refractive index modulation region corresponding to the current grating period. At the same time, control the fiber to move at a constant speed along the negative y-axis for a set distance of 450 μm to complete the writing of the i-th rectangle. When writing for the first time, i=1.

[0076] (2) Turn off the femtosecond laser, control the fiber to move at a constant speed along the positive y-axis for a set distance of 450 μm, and then move at a constant speed along the positive z-axis for a set distance of 50 μm, that is, to the initial writing position of the (i+1)th rectangle of the refractive index modulation region corresponding to the current grating period, i=i+1, the writing rectangle of the current grating period is updated to the (i+1)th rectangle, return to step (1) and repeat until the writing of 9 rectangles of the refractive index modulation region corresponding to the current grating period is completed, that is, the writing of the fiber grating with the shape and size of the refractive index modulation region in the current grating period is completed.

[0077] (f) Control the fiber to move along the x direction for one grating cycle and move to the initial writing position of the next cycle, update the current grating cycle and return to step (1) until the writing of the periodic fiber grating for the required cycle is completed.

[0078] Matters not covered in this invention are common knowledge.

[0079] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0080] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this application should be determined by the appended claims.

[0081] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for fabricating large-area fiber gratings, characterized in that, The method described above enables the fabrication of fiber gratings with refractive index modulation regions covering the entire core or cladding on optical fibers with core or cladding diameters greater than 100 μm. The method includes: Prepare optical fibers, determine the position of the fiber grating to be inscribed on the optical fibers, and remove the coating layer at the location of the fiber grating to be inscribed. A glass tube is prepared, the glass tube including an inner wall and an outer wall, the refractive index of the inner wall of the glass tube is lower than the refractive index of the cladding of the optical fiber or the same as the refractive index of the coating layer of the optical fiber, and the refractive index of the outer wall of the glass tube is the same as the refractive index of the refractive index matching liquid. The optical fiber is passed through the glass tube, the outer diameter of the optical fiber matches the inner diameter of the glass tube, and the fiber grating to be written is located in the middle section of the glass tube. The glass tube at the location where the fiber grating is to be written is collapsed to ensure that the inner wall of the glass tube at the location where the fiber grating is to be written is tightly attached to the fiber cladding, while the core and cladding dimensions of the fiber remain unchanged. Immerse the glass tube at the location of the fiber grating to be inscribed in a refractive index matching solution; Femtosecond lasers are used to write fiber gratings with the desired refractive index modulation region shape and size.

2. The method for fabricating a large-area fiber grating according to claim 1, characterized in that, The outer diameter of the glass tube is more than 100 μm larger than its inner diameter.

3. The method for fabricating a large-area fiber grating according to claim 1 or 2, characterized in that, The glass tube at the location where the fiber Bragg grating is to be written is collapsed, including: Seal one end of the glass tube together with the optical fiber. Connect the air pump to the other end of the glass tube and use the air pump to make the air pressure inside the glass tube lower than the air pressure outside the glass tube. The glass tube at the location where the fiber grating is to be written is heated and softened using a flame. Under the action of negative pressure inside the tube, the glass tube collapses and the inner wall of the glass tube at the location where the fiber grating is to be written adheres tightly to the fiber cladding.

4. The method for fabricating a large-area fiber grating according to claim 1, characterized in that, Using the femtosecond laser to write fiber gratings with the desired refractive index modulation region shape and size includes: (a) The refractive index modulation region of the fiber grating to be written is rectangular in shape, and the length of the rectangle along the z-axis is determined. l 1 and length along the y-axis l 2; (b) Determine the position of the refractive index modulation region of the fiber optic grating to be inscribed inside the fiber, and determine the initial inscription position of the refractive index modulation region; (c) Based on the length of the rectangle along the z-axis l 1. Determine the single-pulse energy and beam diameter of the femtosecond laser corresponding to the inscription of the rectangle; (d) Turn on the femtosecond laser, control the femtosecond laser to emit a single pulse energy and beam diameter femtosecond laser that is incident from the z-axis direction and focused onto the initial marking position of the refractive index modulation region inside the fiber, and simultaneously control the fiber to move at a constant speed along the y-axis direction by a set distance. l 2. Complete the writing of a single-plane fiber grating with the shape and size of the refractive index modulation region.

5. The method for fabricating a large-area fiber grating according to claim 1, characterized in that, Using the femtosecond laser to write fiber gratings with the desired refractive index modulation region shape and size includes: (a) The refractive index modulation region of the fiber grating to be written is a plurality of rectangles arranged parallel to each other along the z-axis and spaced apart from each other. The rectangles are numbered 1, 2...N from bottom to top along the z-axis, and the length of the i-th rectangle along the z-axis is... l 1i The length of the i-th rectangle along the y-axis is l 2i The distance between the i-th rectangle and the (i+1)-th rectangle on the z-axis is l 3i ; (b) Determine the position of the refractive index modulation region of the fiber optic grating to be written inside the fiber, and determine the initial writing rectangle of the refractive index modulation region and its initial writing position. The initial writing rectangle is the first rectangle. (c) Based on the length of each rectangle along the z-axis in the refractive index modulation region l 1i Determine the single-pulse energy P of the femtosecond laser corresponding to each rectangle being inscribed. i and beam diameter d i ; (d) Turn on the femtosecond laser and control it to emit a single pulse energy P. i Beam diameter d i The femtosecond laser is incident from the z-axis and focused onto the initial writing position, while the optical fiber is controlled to move uniformly a set distance along the negative y-axis. l 2i Complete the inscription of the i-th rectangle; (e) Turn off the femtosecond laser and control the fiber to move at a constant speed along the positive y-axis for a set distance. l 2i Then move at a constant speed a set distance in the positive z-axis direction. l 3i That is, to the initial writing position of the (i+1)th rectangle, i=i+1, the current writing rectangle is updated to the (i+1)th rectangle, the single pulse energy and beam diameter of the femtosecond laser are updated, and the cycle returns to step (d) until the writing of the fiber grating with the shape and size of the refractive index modulation region is completed.

6. The method for fabricating a large-area fiber grating according to claim 1, characterized in that, The femtosecond laser is used to fabricate a fiber grating with the desired refractive index modulation region shape and size. The fiber grating is a periodic fiber grating, comprising: (a) Determine the grating period of the fiber grating to be inscribed. The refractive index modulation region of each grating period consists of multiple rectangles arranged parallel to each other along the z-axis and spaced apart. The rectangles are numbered 1, 2...N from bottom to top along the z-axis. The length of the i-th rectangle along the z-axis is... l 1i The length of the i-th rectangle along the y-axis is l 2i The distance between the i-th rectangle and the (i+1)-th rectangle on the z-axis is l 3i ; (b) Determine the position of the refractive index modulation region of the fiber optic grating to be written inside the fiber, and determine the initial writing rectangle and its initial writing position of the refractive index modulation region corresponding to each grating period. The initial writing rectangle of each grating period is the first rectangle of the corresponding period. (c) The length of each rectangle along the z-axis in the refractive index modulation region of each grating period l 1i Determine the single-pulse energy P of the femtosecond laser corresponding to each rectangle being inscribed. i and beam diameter d i ; (d) Turn on the femtosecond laser and control it to emit a single pulse energy P. i Beam diameter d i A femtosecond laser is incident from the z-axis and focused onto the initial writing position of the refractive index modulation region corresponding to the current grating period, while simultaneously controlling the fiber to move uniformly a set distance along the negative y-axis. l 2i Complete the inscription of the i-th rectangle; (e) Turn off the femtosecond laser and control the fiber to move at a constant speed along the positive y-axis for a set distance. l 2i Then move at a constant speed a set distance in the positive z-axis direction. l 3i That is, to the initial writing position of the (i+1)th rectangle of the refractive index modulation region corresponding to the current grating period, i=i+1, the writing rectangle of the current grating period is updated to the (i+1)th rectangle, the single pulse energy and beam diameter of the femtosecond laser are updated, and the process returns to step (d) and repeats until the writing of N rectangles of the refractive index modulation region corresponding to the current grating period is completed, that is, the writing of the fiber grating with the shape and size of the refractive index modulation region in the current grating period is completed. (f) Control the fiber to move along the x direction by one grating period, update the current grating period, return to step (d), until the writing of the periodic fiber grating of the required period is completed.

7. The method for fabricating a large-area fiber grating according to claim 4, 5, or 6, characterized in that, The optical fiber is a silicate, tellurite, fluoride, sulfide optical fiber or a single-crystal optical fiber with a core diameter or cladding diameter greater than 100 μm.

8. A fiber grating prepared by the fabrication method of a large-area fiber grating as described in claim 1, 2, 4, 5, or 6.

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