Fiber grating writing method and device and fiber grating

Abstract

The application provides a fiber grating writing method and device and a fiber grating, which comprises the following steps: determining the shape and size of a refractive index modulation region of a fiber grating to be written; determining the average power of femtosecond laser and the beam diameter of the femtosecond laser according to the size of the refractive index modulation region of the fiber grating to be written; determining the position of the refractive index modulation region in the fiber, and determining the initial writing position of the refractive index modulation region; the femtosecond laser is incident from the z-axis direction and focused on the initial writing position in the fiber of the fiber grating to be written, while the fiber is controlled to move at a constant speed along the y-axis direction by a set distance, and the writing of the single-plane fiber grating with the shape and size of the refractive index modulation region is completed. The application effectively improves the processing efficiency of the fiber grating written plane by plane.

Description

Technical Field

[0001] This invention relates to the field of grating writing technology, and in particular to a method, apparatus and fiber optic grating for writing fiber optic gratings. Background Technology

[0002] Fiber Bragg gratings (FBGs) are passive filtering devices composed of a series of periodic refractive index modulations along the fiber axis within an optical fiber. FBGs are widely used in fiber lasers, fiber sensing, and fiber communication. With the development of femtosecond laser sources, femtosecond laser direct writing technology has become the primary tool for fabricating FBGs. Compared to traditional phase masking techniques, direct writing technology allows for more flexible adjustment of grating structural parameters and makes it easier to write gratings through the fiber coating. Methods for fabricating FBGs based on femtosecond laser direct writing technology can be categorized into point-by-point processing, line-by-line processing, and surface-by-surface processing. Surface-by-surface processing involves introducing a series of periodic two-dimensional refractive index modulation planes along the fiber axis using a femtosecond laser to construct the FBG. Surface-by-surface processing significantly improves the fabrication efficiency of FBGs and effectively reduces insertion loss, scattering loss, and polarization-dependent loss. Currently, grating surfaces fabricated using femtosecond laser surface-by-surface writing are mostly composed of lines stitched together to form a surface, with the size of a single grating surface written in a single step being around 10 μm, and the refractive index uniformity of the grating surface needs further improvement. Therefore, there is an urgent need in this field for a face-by-face grating technique that can form a large-size grating surface with uniform refractive index modulation in one step. Summary of the Invention

[0003] In view of the technical problems existing in the prior art, the present invention proposes a method, apparatus and fiber grating for writing fiber gratings.

[0004] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0005] On one hand, the present invention provides a method for writing fiber Bragg gratings, comprising:

[0006] Determine the fiber to be inscribed with the fiber Bragg grating, wherein the fiber is placed horizontally along the x-direction;

[0007] The shape and size of the refractive index modulation region to be inscribed in the fiber grating are determined. The refractive index modulation region is rectangular, with a length of l1 along the z-axis and a length of l2 along the y-axis.

[0008] The average power P of the femtosecond laser is determined based on the length l1 of the refractive index modulation region along the z-axis of the fiber grating to be inscribed.

[0009] The beam diameter d of the femtosecond laser is determined based on the length l1 of the refractive index modulation region of the fiber grating to be inscribed along the z-axis.

[0010] Determine the location of the refractive index modulation region of the fiber optic grating to be inscribed within the fiber, and determine the initial inscription position of the refractive index modulation region.

[0011] A femtosecond laser with an average power P and a beam diameter d is incident from the z-axis and focused onto the initial writing position inside the fiber of the fiber grating to be written. At the same time, the fiber is controlled to move at a constant speed along the y-axis for a set distance l2, thus completing the writing of a single-plane fiber grating with the shape and size of the refractive index modulation region.

[0012] On one hand, the present invention provides a method for writing fiber Bragg gratings, comprising:

[0013] Determine the fiber to be inscribed with the fiber Bragg grating, wherein the fiber is placed horizontally along the x-direction;

[0014] The shape of the refractive index modulation region of the fiber grating to be inscribed is determined. The refractive index modulation region is rectangular, with a length of l1 along the z-axis and a length of l2 along the y-axis.

[0015] Determine the average power P of the femtosecond laser;

[0016] When the average power P of a femtosecond laser is constant, the length l1 of the refractive index modulation region introduced by the femtosecond laser along the z-axis increases as the beam diameter d of the femtosecond laser decreases. The length l1 of the refractive index modulation region introduced by the femtosecond laser along the z-axis can be controlled by changing the beam diameter d of the femtosecond laser.

[0017] Determine the location of the refractive index modulation region of the fiber optic grating to be inscribed within the fiber, and determine the initial inscription position of the refractive index modulation region.

[0018] A femtosecond laser with an average power P and a beam diameter d is incident from the z-axis and focused onto the initial writing position inside the fiber of the fiber grating to be written. At the same time, the fiber is controlled to move at a constant speed along the y-axis for a set distance l2, thus completing the writing of a single-plane fiber grating with the shape and size of the refractive index modulation region.

[0019] On one hand, the present invention provides a method for writing fiber Bragg gratings, comprising:

[0020] Determine the fiber to be inscribed with the fiber Bragg grating, wherein the fiber is placed horizontally along the x-direction;

[0021] The shape of the refractive index modulation region of the fiber grating to be inscribed is determined. The refractive index modulation region is rectangular, with a length of l1 along the z-axis and a length of l2 along the y-axis.

[0022] Determine the beam diameter d of the femtosecond laser;

[0023] When the beam diameter d of a femtosecond laser is constant, the length l1 of the refractive index modulation region introduced by the femtosecond laser along the z-axis is positively correlated with the average power P of the femtosecond laser. By changing the average power P of the femtosecond laser, the length l1 of the refractive index modulation region introduced by the femtosecond laser along the z-axis can be controlled.

[0024] Determine the location of the refractive index modulation region of the fiber optic grating to be inscribed within the fiber, and determine the initial inscription position of the refractive index modulation region.

[0025] A femtosecond laser with an average power P and a beam diameter d is incident from the z-axis and focused onto the initial writing position inside the fiber of the fiber grating to be written. At the same time, the fiber is controlled to move at a constant speed along the y-axis for a set distance l2, thus completing the writing of a single-plane fiber grating with the shape and size of the refractive index modulation region.

[0026] On one hand, the present invention provides a method for writing fiber Bragg gratings, comprising:

[0027] Determine the fiber to be inscribed with the fiber Bragg grating, wherein the fiber is placed horizontally along the x-direction;

[0028] The shape and size of the refractive index modulation region to be inscribed in the fiber grating are determined. The refractive index modulation region is rectangular, with a length of l1 along the z-axis and a length of l2 along the y-axis.

[0029] The average power P of the femtosecond laser is determined based on the length l1 of the refractive index modulation region along the z-axis of the fiber grating to be inscribed.

[0030] The beam diameter d of the femtosecond laser is determined based on the length l1 of the refractive index modulation region of the fiber grating to be inscribed along the z-axis.

[0031] Determine the grating period of the fiber grating to be written, determine the position of the refractive index modulation region of the fiber grating to be written inside the fiber, and determine the initial writing position of the refractive index modulation region.

[0032] The writing of periodic fiber Bragg gratings includes:

[0033] (1) Turn on the femtosecond laser. The femtosecond laser with average power P and beam diameter d emitted by the femtosecond laser is incident from the z-axis direction and focused on the initial writing position inside the fiber 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 l2 to complete the writing of the first single-plane fiber grating with the shape and size of the refractive index modulation region.

[0034] (2) Turn off the femtosecond laser, control the fiber to move at a constant speed along the positive y-axis for a set distance l2, that is, return to the initial position of the fiber, and then control the fiber to move along the x-direction for one grating cycle, return to step (1), until the writing of the required fiber grating is completed.

[0035] Furthermore, when the average power P of the femtosecond laser is constant, the length l1 of the refractive index modulation region introduced by the femtosecond laser along the z-axis increases as the beam diameter d of the femtosecond laser decreases. By changing the beam diameter d of the femtosecond laser, the length l1 of the refractive index modulation region introduced by the femtosecond laser along the z-axis can be controlled.

[0036] Furthermore, when the beam diameter d of the femtosecond laser is constant, the length l1 of the refractive index modulation region introduced by the femtosecond laser along the z-axis is positively correlated with the average power P of the femtosecond laser. By changing the average power P of the femtosecond laser, the length l1 of the refractive index modulation region introduced by the femtosecond laser along the z-axis can be controlled.

[0037] On the other hand, the present invention provides a fiber optic grating writing apparatus, characterized in that it comprises:

[0038] A femtosecond laser is used to output femtosecond laser light, the average power of which is P.

[0039] An adjustable aperture is used to control the beam size of the femtosecond laser;

[0040] The objective lens focuses a femtosecond laser with an average power P and a beam diameter d onto the initial writing position inside the fiber of the fiber grating to be written.

[0041] An electrically controlled displacement platform is used to set the optical fiber to be inscribed in the fiber grating, and to control the displacement of the optical fiber in the x and y directions during the inscription of the fiber grating, wherein the optical fiber is placed horizontally along the x direction.

[0042] Furthermore, it also includes an oil tank, which is set on an electrically controlled displacement platform. The oil tank contains refractive index matching oil. The optical fiber is suspended in the oil tank and completely immersed in the refractive index matching oil. The two ends of the optical fiber are fixed by clamping or supporting structures at both ends of the oil tank. A cover glass is set above the oil tank.

[0043] Furthermore, by changing the positional relationship between the objective lens and the optical fiber, the femtosecond laser after passing through the objective lens is focused onto the initial writing position inside the optical fiber of the fiber grating to be written.

[0044] Furthermore, it also includes a CCD camera, which is used to observe the focusing position of the femtosecond laser after passing through the objective lens.

[0045] On the other hand, the present invention provides a fiber grating obtained by any of the above-described fiber grating writing methods.

[0046] Since existing technologies lack efficient processes for large-area, surface-by-surface fiber grating writing, this invention provides a method for surface-by-surface grating writing within an optical fiber using femtosecond laser direct writing technology. By adjusting the size of the aperture, a large-area, uniform "surface" modulation region can be introduced into the optical fiber in one step. Furthermore, the fiber does not require hydrogen loading or removal of the fiber coating, effectively improving the processing efficiency of surface-by-surface fiber grating writing. Simultaneously, compared to existing fiber grating writing techniques that utilize line splicing to form a surface, the refractive index modulation region obtained by this invention is more uniform. Attached Figure Description

[0047] 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.

[0048] Figure 1 This is a schematic diagram of the structure of a fiber Bragg grating writing device provided in an embodiment of the present invention;

[0049] Figure 2 A schematic diagram showing the position of the optical fiber placed in the oil tank;

[0050] Figure 3 The relationship between the length l1 of the refractive index modulation region introduced for femtosecond laser along the z-axis and the beam diameter d of the femtosecond laser;

[0051] Figure 4 A graph showing the relationship between the size of the refractive index modulation region introduced for femtosecond lasers and the average laser power;

[0052] Figure 5 This is a schematic diagram of the inscription path in one embodiment;

[0053] Figure 6 The transmission spectrum of a high-reflectivity fiber grating prepared in one embodiment;

[0054] Figure 7 This is a cross-sectional view of the refractive index distribution of an optical fiber after the grating surface has been written, as measured in one embodiment.

[0055] Numbering on the map:

[0056] 1. Femtosecond laser; 2. Adjustable aperture; 3. Objective lens; 4. Electrically controlled displacement platform; 5. Optical fiber; 6. Oil tank; 7. Refractive index matching oil; 8. Bottom wall of oil tank; 9. Cover glass; 10. CCD camera. Detailed Implementation

[0057] 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.

[0058] In one embodiment, a method for writing fiber Bragg gratings is provided, comprising:

[0059] Determine the fiber to be inscribed with the fiber Bragg grating, wherein the fiber is placed horizontally along the x-direction;

[0060] The shape and size of the refractive index modulation region to be inscribed in the fiber grating are determined. The refractive index modulation region is rectangular, with a length of l1 along the z-axis and a length of l2 along the y-axis.

[0061] The average power P of the femtosecond laser is determined based on the length l1 of the refractive index modulation region along the z-axis of the fiber grating to be inscribed.

[0062] The beam diameter d of the femtosecond laser is determined based on the length l1 of the refractive index modulation region of the fiber grating to be inscribed along the z-axis.

[0063] Determine the location of the refractive index modulation region of the fiber optic grating to be inscribed within the fiber, and determine the initial inscription position of the refractive index modulation region.

[0064] A femtosecond laser with an average power P and a beam diameter d is incident from the z-axis and focused onto the initial writing position inside the fiber of the fiber grating to be written. At the same time, the fiber is controlled to move at a constant speed along the y-axis for a set distance l2, thus completing the writing of a single-plane fiber grating with the shape and size of the refractive index modulation region.

[0065] The above embodiments, by setting the average power P and beam diameter d of the femtosecond laser and combining it with displacement control technology during grating writing, can etch fiber gratings with the desired refractive index modulation region shape and size within the fiber. Moreover, regardless of whether the refractive index modulation region is located in the fiber core or cladding region, the refractive index of the refractive index modulation region introduced by the femtosecond laser is relatively uniform. Compared to existing fiber grating writing techniques that utilize line-splicing to form a surface, the refractive index modulation region obtained by this invention is more uniform. This is because the uniformity of the refractive index distribution on a line-splicing grating surface depends on the spacing between the writing lines; obtaining a uniform refractive index modulation plane requires shortening the spacing between lines, further increasing the time cost of writing a large-area processing plane.

[0066] In another embodiment, a method for writing fiber Bragg gratings is provided, comprising:

[0067] Determine the fiber to be inscribed with the fiber Bragg grating, wherein the fiber is placed horizontally along the x-direction;

[0068] The shape of the refractive index modulation region of the fiber grating to be inscribed is determined. The refractive index modulation region is rectangular, with a length of l1 along the z-axis and a length of l2 along the y-axis.

[0069] Determine the average power P of the femtosecond laser;

[0070] When the average power P of a femtosecond laser is constant, the length l1 of the refractive index modulation region introduced by the femtosecond laser along the z-axis increases as the beam diameter d of the femtosecond laser decreases. The length l1 of the refractive index modulation region introduced by the femtosecond laser along the z-axis can be controlled by changing the beam diameter d of the femtosecond laser.

[0071] Determine the location of the refractive index modulation region of the fiber optic grating to be inscribed within the fiber, and determine the initial inscription position of the refractive index modulation region.

[0072] A femtosecond laser with an average power P and a beam diameter d is incident from the z-axis and focused onto the initial writing position inside the fiber of the fiber grating to be written. At the same time, the fiber is controlled to move at a constant speed along the y-axis for a set distance l2, thus completing the writing of a single-plane fiber grating with the shape and size of the refractive index modulation region.

[0073] By adjusting the distance between the objective lens and the optical fiber, the boundaries between the fiber core and cladding can be clearly observed by viewing the microscopic images acquired by the CCD camera. The initial writing position is set according to the area to be written on the grating surface, which is a conventional processing method in this field.

[0074] The above embodiments demonstrate that, given a fixed average power of the femtosecond laser, the length l1 of the refractive index modulation region introduced by the femtosecond laser along the z-axis can be controlled by changing the beam diameter d of the femtosecond laser. Therefore, the size of the etched refractive index modulation region is very easy to adjust. Similarly, using the above method, the refractive index of the refractive index modulation region introduced by the femtosecond laser is relatively uniform, whether in the core or cladding region of the fiber. Moreover, compared to the existing fiber grating etched using line splicing, the refractive index modulation region obtained by this invention is more uniform.

[0075] In another embodiment, a method for writing fiber Bragg gratings is provided, comprising:

[0076] Determine the fiber to be inscribed with the fiber Bragg grating, wherein the fiber is placed horizontally along the x-direction;

[0077] The shape of the refractive index modulation region of the fiber grating to be inscribed is determined. The refractive index modulation region is rectangular, with a length of l1 along the z-axis and a length of l2 along the y-axis.

[0078] Determine the beam diameter d of the femtosecond laser;

[0079] When the beam diameter d of a femtosecond laser is constant, the length l1 of the refractive index modulation region introduced by the femtosecond laser along the z-axis is positively correlated with the average power P of the femtosecond laser. By changing the average power P of the femtosecond laser, the length l1 of the refractive index modulation region introduced by the femtosecond laser along the z-axis can be controlled.

[0080] Determine the location of the refractive index modulation region of the fiber optic grating to be inscribed within the fiber, and determine the initial inscription position of the refractive index modulation region.

[0081] A femtosecond laser with an average power P and a beam diameter d is incident from the z-axis and focused onto the initial writing position inside the fiber of the fiber grating to be written. At the same time, the fiber is controlled to move at a constant speed along the y-axis for a set distance l2, thus completing the writing of a single-plane fiber grating with the shape and size of the refractive index modulation region.

[0082] The above embodiments demonstrate that, given a fixed femtosecond laser beam diameter d, the length l1 of the refractive index modulation region introduced by the femtosecond laser along the z-axis can be controlled by changing the average power of the femtosecond laser. Therefore, the size of the etched refractive index modulation region is very easy to adjust. Similarly, using the above method, the refractive index of the refractive index modulation region introduced by the femtosecond laser is relatively uniform, regardless of whether it is in the core or cladding region of the fiber. Moreover, compared to the existing fiber grating etched using line splicing, the refractive index modulation region obtained by this invention is more uniform.

[0083] In another embodiment, a method for writing fiber gratings is provided. Since fiber gratings are often periodically distributed, the following method describes a method for writing fiber gratings by introducing a series of periodically arranged uniform refractive index modulation planes within an optical fiber, including:

[0084] Determine the fiber to be inscribed with the fiber Bragg grating, wherein the fiber is placed horizontally along the x-direction;

[0085] The shape and size of the refractive index modulation region to be inscribed in the fiber grating are determined. The refractive index modulation region is rectangular, with a length of l1 along the z-axis and a length of l2 along the y-axis.

[0086] The average power P of the femtosecond laser is determined based on the length l1 of the refractive index modulation region along the z-axis of the fiber grating to be inscribed.

[0087] The beam diameter d of the femtosecond laser is determined based on the length l1 of the refractive index modulation region of the fiber grating to be inscribed along the z-axis.

[0088] Determine the grating period of the fiber grating to be written, determine the position of the refractive index modulation region of the fiber grating to be written inside the fiber, and determine the initial writing position of the refractive index modulation region.

[0089] The writing of periodic fiber Bragg gratings includes:

[0090] (1) Turn on the femtosecond laser. The femtosecond laser with average power P and beam diameter d emitted by the femtosecond laser is incident from the z-axis direction and focused on the initial writing position inside the fiber 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 l2 to complete the writing of the first single-plane fiber grating with the shape and size of the refractive index modulation region.

[0091] (2) Turn off the femtosecond laser, control the fiber to move at a constant speed along the positive y-axis for a set distance l2, that is, return to the initial position of the fiber, and then control the fiber to move along the x-direction for one grating cycle, return to step (1), until the writing of the required fiber grating is completed.

[0092] Similarly, in the above embodiments, when the average power P of the femtosecond laser is constant, the length l1 of the refractive index modulation region introduced by the femtosecond laser along the z-axis increases as the beam diameter d of the femtosecond laser decreases. Therefore, in the above embodiments, if the average power P of the femtosecond laser is constant, the length l1 of the refractive index modulation region introduced by the femtosecond laser along the z-axis can be controlled by changing the beam diameter d of the femtosecond laser.

[0093] Similarly, in the above embodiments, when the beam diameter d of the femtosecond laser is constant, the length l1 of the refractive index modulation region introduced by the femtosecond laser along the z-axis is positively correlated with the average power P of the femtosecond laser. Therefore, in the above embodiments, if the beam diameter d of the femtosecond laser is constant, the length l1 of the refractive index modulation region introduced by the femtosecond laser along the z-axis can be controlled by changing the average power P of the femtosecond laser.

[0094] Reference Figure 1 A fiber Bragg grating writing apparatus is provided, comprising:

[0095] Femtosecond laser 1 is used to output femtosecond laser. The average power of the femtosecond laser is P. The average power P of the femtosecond laser is determined according to the length l1 of the refractive index modulation region along the z-axis of the fiber grating to be written. The refractive index modulation region is rectangular in shape, with a length l1 along the z-axis and a length l2 along the y-axis.

[0096] Adjustable aperture 2 is used to control the beam diameter d of the femtosecond laser, wherein the beam diameter d of the femtosecond laser is determined according to the length l1 of the refractive index modulation region of the fiber grating to be written along the z-axis.

[0097] Objective lens 3 focuses a femtosecond laser with average power P and beam diameter d onto the initial writing position inside the fiber 5 to be written fiber grating, wherein the position of the refractive index modulation region of the fiber grating to be written is determined inside the fiber.

[0098] The electrically controlled displacement platform 4 is used to set the optical fiber to be inscribed in the fiber grating, and to control the displacement of the optical fiber 5 in the x and y directions when inscribing the fiber grating, wherein the optical fiber is placed horizontally along the x direction.

[0099] By changing the positional relationship between objective lens 3 and fiber 5, the femtosecond laser after passing through objective lens 3 is focused onto the initial writing position inside fiber 5, where the fiber grating to be written. The focusing position of the femtosecond laser after passing through objective lens 3 inside fiber 5 is observed by CCD camera 10.

[0100] The fiber Bragg grating writing device also includes Figure 2 The oil tank 6 shown is mounted on an electrically controlled displacement platform 4. The oil tank 6 contains refractive index matching oil 7. The optical fiber 5 is suspended within the oil tank 6 and completely immersed in the refractive index matching oil 7, meaning the optical fiber is separated from all side walls of the oil tank, including the bottom wall 8, by a certain distance. The two ends of the optical fiber 5 are fixed by clamping or supporting structures at both ends of the oil tank 6. A cover glass 9 is placed above the oil tank 6. The refractive index matching oil 7 is used to reduce the secondary focusing of the laser beam by the cylindrical shape of the optical fiber itself. The requirement for the refractive index matching oil is that its refractive index is the same as or similar to the refractive index of the cladding of the optical fiber to be processed.

[0101] Both optical fiber 5 and oil tank 6 are placed on electrically controlled displacement platform 4. By changing the position between the objective lens and optical fiber 5, the laser beam is focused onto the interior of the optical fiber. The objective lens can also be used as an imaging lens. The area of ​​the optical fiber to be processed can be observed through the CCD camera above the objective lens, thereby determining the laser focusing position.

[0102] Once the average power P of the femtosecond laser is determined, the length l1 of the refractive index modulation region introduced by the femtosecond laser along the z-axis increases as the beam diameter d of the femtosecond laser decreases.

[0103] In one embodiment, when the average power of the femtosecond laser is 8mW, the relationship between the length l1 (unit: μm) of the refractive index modulation region introduced by the femtosecond laser along the z-axis and the beam diameter d (unit: mm) of the femtosecond laser is as follows: Figure 3 As shown, the fitting relationship between the two is:

[0104] l1 = -5.956 * d2 +6.02*d+26.891

[0105] The coefficient of determination R of the fitting result 2 >0.999.

[0106] The adjustable aperture 2 is an aperture with an adjustable aperture. Its specific structure and adjustment method are not limited; any commercially available adjustable aperture can be used. The adjustable aperture 2 adjusts or constrains the beam diameter d of the femtosecond laser. Once the aperture diameter of the adjustable aperture 2 is determined, the beam diameter of the femtosecond laser passing through it is also determined. The length l1 of the refractive index modulation region introduced by the femtosecond laser along the z-axis is positively correlated with the average laser power P, as shown in the following relationship: Figure 4 As shown, the length l1 of the refractive index modulation region introduced by the femtosecond laser along the z-axis can be precisely controlled by changing the average laser power P.

[0107] In one embodiment, when the beam diameter d of the femtosecond laser is 1 mm, the relationship between the length l1 (unit: μm) of the refractive index modulation region introduced by the femtosecond laser along the z-axis and the average power P (unit: mW) of the femtosecond laser is as follows: Figure 4 As shown, the fitting relationship between the two is:

[0108] l1 = 2.377 * P + 8.182

[0109] The coefficient of determination R of the fitting result 2 >0.997.

[0110] In another embodiment, when the beam diameter d of the femtosecond laser is 3 mm, the relationship between the length l1 (unit: μm) of the refractive index modulation region introduced by the femtosecond laser along the z-axis and the average power P (unit: mW) of the femtosecond laser is as follows: Figure 4 As shown, the fitting relationship between the two is:

[0111] l1 = 1.709 * P + 6.056

[0112] The coefficient of determination R of the fitting result 2 >0.995.

[0113] The above-described writing apparatus can be used to write periodic fiber gratings. In one embodiment, the writing is performed in the xy plane according to the following... Figure 5 The inscription path shown is used for periodic fiber gratings, including:

[0114] (1) Turn on the femtosecond laser. The femtosecond laser with average power P and beam diameter d emitted by the femtosecond laser is incident from the z-axis direction and focused on the initial writing position inside the fiber 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 l2 to complete the writing of the first single-plane fiber grating with the shape and size of the refractive index modulation region.

[0115] (2) Turn off the femtosecond laser, control the fiber to move at a constant speed along the positive y-axis for a set distance l2, that is, return to the initial position of the fiber, and then control the fiber to move along the x-direction for one grating cycle, return to step (1), until the writing of the required fiber grating is completed.

[0116] In one embodiment, the diameter of the tunable aperture is set to 0.8 mm, meaning the beam size of the femtosecond laser after passing through the tunable aperture is 0.8 mm. The average laser power after passing through the aperture is set to 8.5 mW. The length of the refractive index modulation region along the z-axis is l1, and the length along the y-axis is l2, where l1 = l2 = 30 μm. A 30*30 μm square uniform refractive index modulation plane is introduced into the fiber using the writing method provided by this invention. Further, based on the periodic fiber grating writing method proposed in this invention, when the grating period Λ is set to 1.115 μm, a high-quality high-reflectivity fiber grating is fabricated in a large-mode-field double-clad fiber with a core and inner cladding diameter of 20 / 400 μm. Its transmission spectrum is as follows... Figure 6 As shown. The fiber grating has a reflectivity >99% and an insertion loss <0.2dB.

[0117] In one embodiment, the diameter of the tunable aperture is set to 1.0 mm, meaning the beam size of the femtosecond laser after passing through the tunable aperture is 1.0 mm. The average laser power after passing through the aperture is set to 7 mW. The length of the refractive index modulation region along the z-axis is l1, and the length along the y-axis is l2, where l1 = 25 μm and l2 = 30 μm. A 25*30 μm rectangular uniform refractive index modulation plane is introduced into the optical fiber using the marking method provided by this invention. The refractive index distribution of the optical fiber after marking the grating surface was tested using a refractive index profiler. The test results are as follows: Figure 7 As shown, it is demonstrated that the refractive index of the refractive index modulation region introduced by the femtosecond laser is relatively uniform in both the core region and the cladding region.

[0118] Matters not covered in this invention are common knowledge.

[0119] 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.

[0120] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. 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 patent application should be determined by the appended claims.

[0121] 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 writing fiber optic gratings, characterized in that, include: The fiber to be inscribed with the fiber grating is determined, and the fiber is placed horizontally along the x-direction. The shape and size of the refractive index modulation region to be inscribed in the fiber grating are determined. The refractive index modulation region is rectangular, and its length along the z-axis is [missing information]. l 1. The length along the y-axis is l 2; The length of the refractive index modulation region of the fiber grating to be inscribed along the z-axis is determined by... l 1. Determine the average power of the femtosecond laser P ; The length of the refractive index modulation region of the fiber grating to be inscribed along the z-axis is determined by... l 1. Determine the beam diameter of the femtosecond laser. d ; Determine the location of the refractive index modulation region of the fiber optic grating to be inscribed within the fiber, and determine the initial inscription position of the refractive index modulation region. Average power P、 Beam diameter d A femtosecond laser is incident from the z-axis and focused onto the initial writing position inside the fiber to be written into the fiber grating, while the fiber is controlled to move at a constant speed along the y-axis for 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.

2. A method for writing fiber optic gratings, characterized in that, include: The fiber to be inscribed with the fiber grating is determined, and the fiber is placed horizontally along the x-direction. The shape of the refractive index modulation region to be inscribed in the fiber grating is determined. The refractive index modulation region is rectangular, and its length along the z-axis is [missing information]. l 1. The length along the y-axis is l 2; Determine the average power of the femtosecond laser P ; Determine the location of the refractive index modulation region of the fiber optic grating to be inscribed within the fiber, and determine the initial inscription position of the refractive index modulation region. 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. With the increase in the beam diameter of femtosecond lasers d The decrease increases by changing the beam diameter of the femtosecond laser. d The length of the refractive index modulation region introduced by the femtosecond laser along the z-axis can be controlled. l 1; Average power P、 Beam diameter d A femtosecond laser is incident from the z-axis and focused onto the initial writing position inside the fiber to be written into the fiber grating, while the fiber is controlled to move at a constant speed along the y-axis for 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.

3. A method for writing fiber optic gratings, characterized in that, include: Determine the fiber to be inscribed with the fiber Bragg grating, wherein the fiber is placed horizontally along the x-direction; The shape of the refractive index modulation region to be inscribed in the fiber grating is determined. The refractive index modulation region is rectangular, and its length along the z-axis is [missing information]. l 1. The length along the y-axis is l 2; Determine the beam diameter of a femtosecond laser d ; femtosecond laser beam diameter d Under certain conditions, the length of the refractive index modulation region introduced by the femtosecond laser along the z-axis is... l 1 is positively correlated with the average power of the femtosecond laser. By changing the average power of the femtosecond laser, the length of the refractive index modulation region introduced by the femtosecond laser along the z-axis can be controlled. l 1; Determine the location of the refractive index modulation region of the fiber optic grating to be inscribed within the fiber, and determine the initial inscription position of the refractive index modulation region. Average power P、 Beam diameter d A femtosecond laser is incident from the z-axis and focused onto the initial writing position inside the fiber to be written into the fiber grating, while the fiber is controlled to move at a constant speed along the y-axis for 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.

4. A method for writing fiber optic gratings, characterized in that, include: Determine the fiber to be inscribed with the fiber Bragg grating, wherein the fiber is placed horizontally along the x-direction; The shape and size of the refractive index modulation region to be inscribed in the fiber grating are determined. The refractive index modulation region is rectangular, and its length along the z-axis is [missing information]. l 1. The length along the y-axis is l 2; The length of the refractive index modulation region of the fiber grating to be inscribed along the z-axis is determined by... l 1. Determine the average power of the femtosecond laser P ; The length of the refractive index modulation region of the fiber grating to be inscribed along the z-axis is determined by... l 1. Determine the beam diameter of the femtosecond laser. d ; Determine the grating period of the fiber grating to be written, determine the position of the refractive index modulation region of the fiber grating to be written inside the fiber, and determine the initial writing position of the refractive index modulation region. The writing of periodic fiber Bragg gratings includes: (1) When the femtosecond laser is turned on, the average power emitted by the femtosecond laser is... P、 Beam diameter d A femtosecond laser is incident from the z-axis and focused onto the initial writing position inside the fiber corresponding to the current grating period, while the fiber is controlled to move uniformly a set distance along the y-axis. l 2. Complete the writing of the first single-plane fiber grating with the shape and size of the refractive index modulation region; (2) 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 2. That is, return to the initial position of the fiber, then control the fiber to move along the x direction for one grating cycle, return to step (1), until the writing of the required fiber grating is completed.

5. The method for writing fiber optic gratings according to claim 4, characterized in that, 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. With the increase in the beam diameter of femtosecond lasers d The decrease increases by changing the beam diameter of the femtosecond laser. d The length of the refractive index modulation region introduced by the femtosecond laser along the z-axis can be controlled. l 1.

6. The method for writing fiber optic gratings according to claim 4, characterized in that, femtosecond laser beam diameter d Under certain conditions, the length of the refractive index modulation region introduced by the femtosecond laser along the z-axis is... l 1 and the average power of femtosecond laser P Positive correlation, by changing the average power of the femtosecond laser P The length of the refractive index modulation region introduced by the femtosecond laser along the z-axis can be controlled. l 1.

7. A fiber grating written by the fiber grating writing method as described in claim 1.

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