Preparation method of large-area fiber bragg grating and fiber bragg grating

Abstract

The invention relates to the technical field of fiber bragg gratings, and provides a preparation method of a large-area fiber bragg grating and a fiber bragg grating. A method of collapsing a glass tube is adopted, so that the glass tube with the refractive index similar to that of an optical fiber coating layer is attached to an optical fiber under the action of high temperature, and the femtosecond laser damage threshold value of the optical fiber boundary is increased; therefore, the inscribing area of the fiber grating is increased, and even a modulation area introduced by femtosecond laser crosses two areas of the optical fiber and the glass tube, so that the fiber grating covering the whole cross section of the optical fiber is prepared. Meanwhile, the refractive index of the glass tube is close to that of the coating layer, so that deformation and loss of a mode cannot be caused.

Description

Technical Field

[0001] The invention mainly relates to the field of optical fiber laser technology, in particular to a preparation method of a large-area optical fiber grating and the optical fiber grating. Background Art

[0002] Fiber Bragg gratings are passive filter devices consisting of a series of periodic refractive index modulation zones distributed along the axial direction of an optical fiber, and have important applications in many fields. Femtosecond laser direct writing technology uses an objective lens to focus the femtosecond laser directly into the optical fiber and perform periodic relative motion, which can produce periodic refractive index modulation to form a fiber Bragg grating. Compared with ultraviolet laser writing technology and phase mask writing technology, femtosecond laser direct writing technology has become an important technology for fiber Bragg grating preparation due to its flexible and controllable nature, no need for hydrogen carrier, and the ability to process a variety of materials.

[0003] However, in femtosecond laser direct writing technology, the femtosecond laser is directly focused inside the optical fiber through the objective lens, so the refractive index modulation area near the focus is very small, usually 2~4 μm. The core diameter of the commonly used multimode optical fiber, single crystal optical fiber, etc. is usually greater than 100 μm, so it is difficult to prepare high-reflectivity fiber Bragg gratings on these optical fibers, and there are serious scattering losses and heat generation. By increasing the area of ​​the fiber Bragg grating, the reflectivity of the fiber Bragg grating prepared on these optical fibers can be greatly improved, and the scattering loss and heat generation can be reduced. However, it is difficult to write large-area fiber Bragg gratings on optical fibers with a core diameter greater than 100 μm with traditional femtosecond laser direct writing technology. Through the improvement of traditional femtosecond laser direct writing technology, it is currently reported that the writing area of ​​fiber Bragg gratings can be increased by beam shaping, introduction of spherical aberration, light field control, splicing and stacking. In addition, by immersing the optical fiber in a refractive index matching liquid and using a long working distance objective lens, the Rayleigh distance of the focus can be increased, thereby introducing a longer refractive index modulation area. However, immersing the optical fiber in the refractive index matching liquid will cause other problems. That is, when the focus of the femtosecond laser approaches the boundary of the optical fiber, if the coating of the optical fiber is not stripped, it will cause damage to the coating; if the coating of the optical fiber 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 the art for a technology that can prepare large-area fiber Bragg gratings with a core diameter of more than 100 μm. Summary of the invention

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

[0006] To achieve the above purpose, the technical solution adopted by the present invention is as follows: On the one hand, the present invention provides a method for preparing a large-area fiber Bragg grating, by which a fiber Bragg grating with a refractive index modulation region covering the entire core or cladding can be prepared on an optical fiber with a core diameter or a cladding diameter greater than 100 μm, and the method comprises: Prepare the optical fiber, determine the position of the optical fiber where the optical fiber Bragg grating is to be written, and remove the coating layer at the position where the optical fiber Bragg grating is to be written; A glass tube is provided, wherein the glass tube comprises 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 optical fiber cladding or is 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; Passing the optical fiber through the glass tube, the outer diameter of the optical fiber matches the inner diameter of the glass tube, and the position of the fiber grating to be written is located in the middle section of the glass tube; The glass tube at the position where the fiber grating is to be written is collapsed so that the inner wall of the glass tube at the position where the fiber grating is to be written is closely attached to the fiber cladding, and the core and cladding size of the fiber remain unchanged; Immerse the glass tube at the position where the fiber grating is to be written in the refractive index matching liquid; The femtosecond laser is used to write a fiber grating with a desired refractive index modulation area shape and size.

[0007] On the other hand, the present invention provides a fiber Bragg grating prepared based on the above-mentioned large-area fiber Bragg grating preparation method. Using the large-area fiber Bragg grating preparation method provided by the present invention, a fiber Bragg grating can be prepared on a silicate, tellurite, fluoride, sulfide optical fiber or single crystal optical fiber with a core diameter or cladding diameter greater than 100 μm.

[0008] The key to improving the reflectivity is to increase the overlapping area between the fiber grating and the fiber mode field, but in these large diameter fibers, the area of ​​the mode field is also large, so it is very necessary to have a process for preparing large-area fiber gratings. In addition, in general fiber grating writing methods, the writing boundary of the fiber grating is often restricted by air, coating or refractive index matching liquid, resulting in the fiber grating being unable to cover a larger fiber area. The reason is that when the focus of the femtosecond laser is too close to the air, coating or refractive index matching liquid, due to the cylindrical mirror effect of the air and the fiber surface and the femtosecond laser damage threshold that is very different from the coating and refractive index matching liquid and the fiber material, the femtosecond laser will damage the surface and coating of the fiber, or induce air bubbles in the refractive index matching liquid to interfere with the focusing of the femtosecond laser. Therefore, the prior art lacks a process for writing fiber gratings over a large area face by face, making the preparation of large-area fiber gratings a difficult problem. Compared with the prior art, the technical effects that the present invention can produce are: The present invention increases the area for writing fiber gratings on large-core optical fibers by collapsing the glass tube on the optical fiber cladding, thereby realizing the preparation of large-area fiber gratings. At the same time, the refractive index of the inner wall of the glass tube is lower than the refractive index of the optical fiber cladding or is close to the refractive index of the optical fiber coating layer, thereby avoiding the deformation and leakage of the transmission mode field in the optical fiber core and reducing the deformation and leakage of the transmission mode field in the optical fiber cladding.

[0009] The present invention uses a method of collapsing a glass tube to make a glass tube with a refractive index close to that of an optical fiber coating fit with an optical fiber under the action of high temperature, thereby increasing the femtosecond laser damage threshold of the optical fiber boundary, thereby increasing the writing area of ​​the optical fiber grating, and even making the modulation area introduced by the femtosecond laser span the optical fiber and the glass tube, so as to prepare an optical fiber grating covering the entire optical fiber cross section. At the same time, since the refractive index of the inner wall of the glass tube is lower than the refractive index of the optical fiber cladding or close to that of the optical fiber coating, the deformation and leakage of the transmission mode field in the optical fiber core can be avoided, and the deformation and leakage of the transmission mode field in the optical fiber cladding can be reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying creative work.

[0011] Figure 1 This is a schematic diagram of the relative positions of the optical fiber and the glass tube before collapse provided by an embodiment of the present invention: Figure 2 This is a schematic diagram of the relative positions of the collapsed optical fiber and the glass tube provided by an embodiment of the present invention: Figure 1 and Figure 2 Bid numbers: 1. Glass tube; 2. Optical fiber whose coating has not been stripped; 3. Optical fiber stripping point; 4. Position where the fiber grating is to be inscribed; 5. Collapsed glass tube; Figure 3 Schematic diagram of the refractive index modulation region of the fiber grating to be inscribed in one embodiment.

[0012] Figure 3 Winning bid numbers: 5. Collapsed glass tube; 6. Refractive index modulation rectangle; 7. Optical fiber cladding; 8. Optical fiber core. DETAILED DESCRIPTION

[0013] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0014] In one embodiment, a method for preparing a large-area fiber Bragg grating is provided, wherein the method can be used to prepare a fiber Bragg grating whose refractive index modulation region covers the entire core or cladding on an optical fiber whose core diameter or cladding diameter is greater than 100 μm. Specifically, the method comprises the following steps: Prepare an optical fiber, determine the position of the optical fiber to be inscribed with the optical fiber grating and remove the coating layer at the position where the optical fiber grating is to be inscribed, and assume that the cladding diameter of the optical fiber is D 1 The coating diameter is D 2 .

[0015] A glass tube is prepared, the glass tube comprises 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 optical fiber cladding or is 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.

[0016] 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 position of the fiber grating to be written is located in the middle section of the glass tube. Furthermore, the outer diameter of the glass tube is greater than the inner diameter by more than 100 μm.

[0017] The glass tube at the position where the fiber grating is to be written is collapsed so that the inner wall of the glass tube at the position where the fiber grating is to be written is closely attached to the fiber cladding, and the core and cladding size of the fiber remain unchanged; Immerse the glass tube at the position where the fiber grating is to be written in the refractive index matching liquid; The femtosecond laser is used to write a fiber grating with a desired refractive index modulation area shape and size.

[0018] In one embodiment, collapsing the glass tube at the position where the fiber grating is to be written comprises the following steps: Seal one end of the glass tube together with the optical fiber (the sealing method is not limited, such as using UV curing glue for sealing); Use a vacuum pump to connect the other end of the glass tube, and use the vacuum pump to make the air pressure inside the glass tube lower than the air pressure outside the glass tube; Use a torch to heat and soften the glass tube at the location where the fiber grating is to be engraved. The glass tube collapses under the negative pressure inside the tube, and the inner wall of the glass tube at the location where the fiber grating is to be engraved fits tightly with the fiber cladding. Note that the torch power depends on the thermal properties of the glass tube material. For materials that are difficult to soften, the torch power needs to be increased, and for materials that are easy to soften, the torch power needs to be reduced. The torch is heated at a speed v 1 The uniform speed movement eventually makes the glass tube at the position where the fiber grating is to be engraved fit with the fiber cladding, while ensuring that the core and cladding dimensions of the fiber remain unchanged. 1 It is necessary to ensure that the glass tube fits tightly to the optical fiber cladding while not applying excessive high temperature to the optical fiber, which may cause deformation of the optical fiber.

[0019] 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 by an embodiment, the optical fiber at the optical fiber grating position 4 to be written has its coating stripped, both ends of the optical fiber grating position 4 to be written are optical fiber stripping points 3, the optical fiber grating position 4 to be written is located in the middle section of the glass tube 1, and the optical fiber 2 whose coating has not been stripped is located outside the glass tube 2. Figure 2 It is a schematic diagram of the relative positions of the collapsed optical fiber and the glass tube provided by an embodiment of the present invention. The inner wall of the collapsed glass tube 2 is tightly fitted with the optical fiber cladding.

[0020] After the collapse treatment, use a fiber optic clamp to clamp the optical fibers on both sides of the glass tube and straighten the optical fibers to proceed to the next step, which is to immerse the glass tube at the position 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 use a cover glass to cover the upper surface of the glass tube at the position where the fiber grating is to be written.

[0021] Alternatively, when the glass tube at the position where the fiber grating is to be written is immersed in the refractive index matching liquid, the glass tube can be placed on a flat glass or a slide glass and the refractive index matching liquid can be dripped until the glass tube is covered.

[0022] Alternatively, when the glass tube at the position of the fiber grating to be written is immersed in the refractive index matching liquid, the glass tube at the position of the fiber grating to be written can also be placed in a transparent oil tank and the refractive index matching liquid is added dropwise until the oil tank is filled.

[0023] The femtosecond laser is used to write the fiber grating with the required refractive index modulation area shape and size. The specific writing process is determined according to the refractive index modulation area shape and size and the grating period.

[0024] In one embodiment, the shape of the refractive index modulation area of ​​the fiber grating to be written is a rectangle, and the femtosecond laser is used to complete the writing of the fiber grating with the required refractive index modulation area shape and size, including the following steps: (a) The refractive index modulation region of the fiber grating to be inscribed is in the shape of a rectangle, and the length of the rectangle along the z-axis is determined. l 1 And the length along the y-axis l 2 ; (b) determining the position of the refractive index modulation region of the fiber grating to be inscribed inside the optical fiber, and determining the initial writing position of the refractive index modulation region; (c) According to the length of the rectangle along the z-axis l 1 , determining the single pulse energy and beam diameter of the femtosecond laser corresponding to writing the rectangle; (d) turning on the femtosecond laser, controlling the femtosecond laser to emit the single pulse energy and beam diameter of the femtosecond laser from the z-axis direction and focusing it to the initial writing position of the refractive index modulation region inside the optical fiber, and controlling the optical fiber to move at a uniform speed along the y-axis direction for a set distance l 2 , completing the writing of a single-plane fiber grating having the shape and size of the refractive index modulation area.

[0025] When the average power of the femtosecond laser is constant, the length of the refractive index modulation area 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 area introduced by the femtosecond laser along the z-axis direction can be controlled by changing the beam diameter d of the femtosecond laser. l 1 .

[0026] In a preferred embodiment, when the wavelength of the femtosecond laser 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 cover glass thickness is 1.1 mm, the refractive index of the refractive index matching liquid is 1.464, and the outer diameter of the collapsed glass tube is 1.05 mm, l 1 The length is 50 μm. This parameter combination can produce longer l 1 length.

[0027] In one embodiment, the refractive index modulation area of ​​the fiber grating to be written is in the shape of a plurality of rectangles parallel to each other and spaced apart along the z-axis direction, and the femtosecond laser is used to complete the writing of the fiber grating having the desired refractive index modulation area shape and size, including: (a) The refractive index modulation region of the fiber grating to be inscribed is in the shape of a plurality of rectangles parallel to each other and spaced apart from each other along the z-axis, and the rectangles are the 1st, 2nd, ...Nth rectangles 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) determining the position of the refractive index modulation region of the fiber grating to be inscribed inside the optical fiber, determining the initial writing rectangle of the refractive index modulation region and its initial writing position, the initial writing rectangle being the first rectangle; (c) According to the length of each rectangle along the z-axis in the refractive index modulation area l 1i , determine the single pulse energy P of the femtosecond laser corresponding to each rectangle 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 direction and focused to the initial writing position, while the optical fiber is controlled to move uniformly along the negative direction of the y-axis for a set distance. l 2i , complete the inscription of the i-th rectangle; (e) Turn off the femtosecond laser and control the optical fiber to move at a constant speed along the positive direction of the y-axis for a set distance. l 2i , and then move the set distance at a constant speed in the positive direction of the z axis l 3i , that is, to the initial writing position of the i+1th rectangle, i=i+1, the current writing rectangle is updated to the i+1th 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 Bragg grating with the shape and size of the refractive index modulation area is completed.

[0028] In a preferred embodiment, when the wavelength of the femtosecond laser 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 cover glass thickness is 1.1 mm, the refractive index of the refractive index matching liquid is 1.464, and the outer diameter of the collapsed glass tube is 1.05 mm, l 1 The length is 50 μm. This parameter combination can produce longer l 1 length.

[0029] like Figure 3 As shown in FIG. 1 , a schematic diagram of the refractive index modulation region of the fiber grating to be inscribed in one embodiment includes a collapsed glass tube 5, a refractive index modulation rectangle 6, a fiber cladding 7, and a fiber core 8. The refractive index modulation region of the fiber grating to be inscribed is shaped as: a plurality of rectangles arranged parallel to and spaced from each other along the z-axis in the yz plane. 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 third and fourth rectangles on the z-axis is l 33 ; The length of the fourth 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 rectangle and the 5th rectangle on the z-axis is l 34 ; The length of the fifth rectangle along the z-axis is l 15 , the length of the fifth rectangle along the y-axis is l 25 .

[0030] 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 demand and are not required. The sizes of the rectangles can be the same, partially the same, or different from each other.

[0031] In one embodiment, the fiber grating to be written is a periodic fiber grating, and the refractive index modulation area of ​​each grating period is a plurality of rectangles parallel to each other and arranged at intervals along the z-axis direction. The femtosecond laser is used to complete the writing of the fiber grating having the desired refractive index modulation area shape and size, including: (a) Determine the grating period of the fiber Bragg grating to be inscribed. The refractive index modulation area of ​​each grating period is a plurality of rectangles parallel to each other and arranged at intervals along the z-axis. The rectangles are the 1st, 2nd, ...Nth rectangles 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) determining the position of the refractive index modulation region of the fiber grating to be inscribed inside the optical fiber, determining the initial writing rectangle and initial writing position of the refractive index modulation region corresponding to each grating period, and the initial writing rectangle of each grating period is the corresponding first rectangle; (c) The length of each rectangle along the z-axis in the refractive index modulation area of ​​each grating period l 1i , determine the single pulse energy P of the femtosecond laser corresponding to each rectangle 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 direction and focused to the initial writing position of the refractive index modulation area corresponding to the current grating period, and at the same time the optical fiber is controlled to move uniformly along the negative direction of the y-axis for a set distance. l 2i , complete the inscription of the i-th rectangle; (e) Turn off the femtosecond laser and control the optical fiber to move at a constant speed along the positive direction of the y-axis for a set distance. l 2i , and then move the set distance at a constant speed in the positive direction of the z axis l 3i , that is, to the initial writing position of the i+1th rectangle of the refractive index modulation area corresponding to the current grating period, i=i+1, the writing rectangle of the current grating period is updated to the i+1th rectangle, the single pulse energy and beam diameter of the femtosecond laser are updated, and the loop is returned to step (d) until the writing of N rectangles of the refractive index modulation area corresponding to the current grating period is completed, that is, the writing of the fiber Bragg grating with the shape and size of the refractive index modulation area of ​​the current grating period is completed; (f) Control the optical fiber to move one grating period along the x direction, update the current grating period, and return to step (d) until the writing of the periodic fiber grating of the required period is completed.

[0032] The optical fiber is a silicate, tellurite, fluoride, sulfide optical fiber or a single crystal optical fiber with a core diameter or a cladding diameter greater than 100 μm. Utilizing the method for preparing a large-area optical fiber grating provided by the present invention, an optical fiber grating can be prepared on a silicate, tellurite, fluoride, sulfide optical fiber or a single crystal optical fiber with a core diameter or a cladding diameter greater than 100 μm, and the refractive index modulation region written can achieve large-area coverage of the core region or the cladding region according to actual needs, and can achieve 100% coverage of the entire core region or the cladding region at most. That is, if the grating is written 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 written 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.

[0033] In one embodiment, a method for preparing a large-area fiber grating is provided, comprising: The optical fiber to be processed is a large mode field double-clad 20 / 400 passive optical fiber with a cladding diameter of D 1 The coating diameter D is 400 μm. 2 The position of the fiber grating to be inscribed is determined on the optical fiber and the coating layer is stripped off at the position of the fiber grating to be inscribed.

[0034] A semi-fluorine-doped glass tube is used. A certain amount of fluorine is doped into the inner wall of the glass tube, so that the refractive index of the inner wall of the glass tube is lower than the refractive index of the optical fiber cladding. The outer wall of the glass tube is made of pure quartz.

[0035] 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 position of 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.

[0036] The glass tube at the position where the fiber grating is to be written is collapsed: one end of the glass tube and the optical fiber are sealed with UV curing glue and UV lamp, and then the other end of the glass tube is connected with a vacuum pump to evacuate the air so that the air pressure inside the glass tube is lower than the air pressure outside the tube. The flame head power is set to 190 W, which can better soften the glass tube for collapse operation. The flame head speed v 1 The speed is 0.1 mm / s, which can ensure that the glass tube fits tightly with the fiber cladding while not applying excessive high temperature to the fiber to cause deformation of the fiber. After the collapse, the inner wall of the glass tube where the fiber grating is to be written fits tightly with the fiber cladding, the fiber cladding diameter remains unchanged, the inner diameter of the glass tube becomes 400 μm, and the outer diameter becomes 1050 μm.

[0037] The optical fibers on both sides of the glass tube are clamped and straightened using a fiber clamp, and the glass tube where the fiber grating position is to be engraved is placed in a square oil tank and immersed in a refractive index matching liquid. The side length of the oil tank is 1.5 mm, and the refractive index of the refractive index matching liquid is 1.464. The top of the oil tank is covered with a cover glass with a thickness of 1.1 mm.

[0038] Next, the femtosecond laser is used to write the fiber grating with the required refractive index modulation area shape and size.

[0039] In this embodiment, the refractive index modulation area of ​​the fiber grating to be written in each fiber period is shaped as follows: 9 rectangles arranged parallel to and spaced from each other along the z-axis in the yz plane. 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 the 9 rectangles can cover the entire inner cladding of the 20 / 400 optical fiber. From bottom to top along the z-axis, they are the first rectangle, the second rectangle, ..., and the ninth rectangle.

[0040] The single pulse energy of the femtosecond laser corresponding to the writing of each rectangle is determined to be 0.94 μJ, and the beam diameter is 1.5 mm. The first writing rectangle of the refractive index modulation area corresponding to each grating period and its initial writing position are determined: the endpoint position with the largest y-axis coordinate in the first rectangle.

[0041] Writing periodic fiber gratings: (1) Turn on the femtosecond laser and control it to emit a single pulse with an energy of 0.94 μJ and a beam diameter of 1.5 mm. The laser is incident from the z-axis direction and focused to the initial writing position of the refractive index modulation area corresponding to the current grating period. At the same time, the optical fiber is controlled to move at a constant speed along the negative direction of the y-axis for a set distance of 450 μm to complete the writing of the i-th rectangle. During the initial writing, i=1. (2) Turn off the femtosecond laser, control the optical fiber to move uniformly along the positive direction of the y-axis for a set distance of 450 μm, and then uniformly move along the positive direction of the z-axis for a set distance of 50 μm, that is, to the initial writing position of the i+1th rectangle of the refractive index modulation area corresponding to the current grating period, i=i+1, the writing rectangle of the current grating period is updated to the i+1th rectangle, return to step (1) and loop until the writing of the 9 rectangles of the refractive index modulation area corresponding to the current grating period is completed, that is, the writing of the fiber Bragg grating with the shape and size of the refractive index modulation area of ​​the current grating period is completed; (f) Control the optical fiber to move one grating period in the x direction and move to the initial writing position of the next period, update the current grating period and return to step (1) until the writing of the periodic fiber grating of the required period is completed.

[0042] Matters not covered by the present invention are known technologies.

[0043] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, 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.

[0044] The above-described embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention. It should be pointed out that, for a person of ordinary skill in the art, several modifications and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the attached claims.

[0045] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing a large-area fiber Bragg grating, characterized in that: The method can be used to prepare a fiber grating whose refractive index modulation region covers the entire core or cladding on an optical fiber with a core diameter or cladding diameter greater than 100 μm. The method comprises: Prepare the optical fiber, determine the position of the optical fiber where the optical fiber Bragg grating is to be written, and remove the coating layer at the position where the optical fiber Bragg grating is to be written; A glass tube is provided, wherein the glass tube comprises 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 optical fiber cladding or is 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; Passing the optical fiber through the glass tube, the outer diameter of the optical fiber matches the inner diameter of the glass tube, and the position of the fiber grating to be written is located in the middle section of the glass tube; The glass tube at the position where the fiber grating is to be written is collapsed so that the inner wall of the glass tube at the position where the fiber grating is to be written is closely attached to the fiber cladding, and the core and cladding size of the fiber remain unchanged; Immerse the glass tube at the position where the fiber grating is to be written in the refractive index matching liquid; The femtosecond laser is used to write a fiber grating with a desired refractive index modulation area shape and size.

2. The method for preparing a large-area fiber Bragg grating according to claim 1, characterized in that: The outer diameter of the glass tube is greater than the inner diameter by more than 100 μm.

3. The method for preparing a large-area fiber Bragg grating according to claim 1 or 2, characterized in that: The glass tube at the position where the fiber grating is to be written is collapsed, including: Seal one end of the glass tube together with the optical fiber; Use a vacuum pump to connect the other end of the glass tube, and use the vacuum pump to make the air pressure inside the glass tube lower than the air pressure outside the glass tube; A torch is used to heat and soften the glass tube at the position where the fiber grating is to be written. The glass tube collapses under the action of negative pressure inside the tube, and the inner wall of the glass tube at the position where the fiber grating is to be written fits tightly with the fiber cladding.

4. The method for preparing a large-area fiber Bragg grating according to claim 1, characterized in that: The femtosecond laser is used to write a fiber grating having a desired refractive index modulation region shape and size, including: (a) The refractive index modulation region of the fiber grating to be inscribed is in the shape of a rectangle, and the length of the rectangle along the z-axis is determined. l 1 and the length along the y-axis l 2; (b) determining the position of the refractive index modulation region of the fiber grating to be inscribed inside the optical fiber, and determining the initial writing position of the refractive index modulation region; (c) According to 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 writing the rectangle; (d) turning on the femtosecond laser, controlling the femtosecond laser to emit the single pulse energy and beam diameter of the femtosecond laser from the z-axis direction and focusing it to the initial writing position of the refractive index modulation region inside the optical fiber, and controlling the optical fiber to move at a uniform speed along the y-axis direction for a set distance l 2. Complete the writing of a single-plane fiber grating having the shape and size of the refractive index modulation area.

5. The method for preparing a large-area fiber Bragg grating according to claim 1, characterized in that: The femtosecond laser is used to write a fiber grating having a desired refractive index modulation region shape and size, including: (a) The refractive index modulation region of the fiber grating to be inscribed is in the shape of a plurality of rectangles parallel to each other and spaced apart from each other along the z-axis, and the rectangles are the 1st, 2nd, ...Nth rectangles 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) determining the position of the refractive index modulation region of the fiber grating to be inscribed inside the optical fiber, determining the initial writing rectangle of the refractive index modulation region and its initial writing position, the initial writing rectangle being the first rectangle; (c) According to the length of each rectangle along the z-axis in the refractive index modulation area l 1i , determine the single pulse energy P of the femtosecond laser corresponding to each rectangle 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 direction and focused to the initial writing position, while the optical fiber is controlled to move uniformly along the negative direction of the y-axis for a set distance. l 2i , complete the inscription of the i-th rectangle; (e) Turn off the femtosecond laser and control the optical fiber to move at a constant speed along the positive direction of the y-axis for a set distance. l 2i , and then move the set distance at a constant speed in the positive direction of the z axis l 3i , that is, to the initial writing position of the i+1th rectangle, i=i+1, the current writing rectangle is updated to the i+1th 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 Bragg grating with the shape and size of the refractive index modulation area is completed.

6. The method for preparing a large-area fiber Bragg grating according to claim 1, characterized in that: The femtosecond laser is used to write a fiber grating having a desired refractive index modulation region shape and size, wherein the fiber grating is a periodic fiber grating, comprising: (a) Determine the grating period of the fiber Bragg grating to be inscribed. The refractive index modulation area of ​​each grating period is a plurality of rectangles parallel to each other and arranged at intervals along the z-axis. The rectangles are the 1st, 2nd, ...Nth rectangles 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) determining the position of the refractive index modulation region of the fiber grating to be inscribed inside the optical fiber, determining the initial writing rectangle and initial writing position of the refractive index modulation region corresponding to each grating period, and the initial writing rectangle of each grating period is the corresponding first rectangle; (c) The length of each rectangle along the z-axis in the refractive index modulation area of ​​each grating period l 1i , determine the single pulse energy P of the femtosecond laser corresponding to each rectangle 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 direction and focused to the initial writing position of the refractive index modulation area corresponding to the current grating period, and at the same time the optical fiber is controlled to move uniformly along the negative direction of the y-axis for a set distance. l 2i , complete the inscription of the i-th rectangle; (e) Turn off the femtosecond laser and control the optical fiber to move at a constant speed along the positive direction of the y-axis for a set distance. l 2i , and then move the set distance at a constant speed in the positive direction of the z axis l 3i , that is, to the initial writing position of the i+1th rectangle of the refractive index modulation area corresponding to the current grating period, i=i+1, the writing rectangle of the current grating period is updated to the i+1th rectangle, the single pulse energy and beam diameter of the femtosecond laser are updated, and the loop is returned to step (d) until the writing of N rectangles of the refractive index modulation area corresponding to the current grating period is completed, that is, the writing of the fiber Bragg grating with the shape and size of the refractive index modulation area of ​​the current grating period is completed; (f) Control the optical fiber to move one grating period along the x direction, update the current grating period, and return to step (d) until the writing of the periodic fiber grating of the required period is completed.

7. The method for preparing a large-area fiber Bragg 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 a cladding diameter greater than 100 μm.

8. A fiber Bragg grating prepared based on the method for preparing a large-area fiber Bragg grating as claimed in claim 1, 2, 4, 5 or 6.

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