FBG (fiber bragg grating) longitudinal line-by-line cold processing preparation method based on femtosecond laser direct writing

Abstract

The invention relates to the technical field of fiber bragg grating preparation, in particular to a method for preparing a fiber bragg grating through longitudinal line-by-line cold machining based on femtosecond laser direct writing, which comprises the following steps of: S10, controlling a femtosecond laser spot to move point by point along the axial direction of an optical fiber by taking a grating period as a step length, and applying a laser pulse at a corresponding position of each grating period, a local refractive index modulation area is formed; the at least m local refractive index modulation areas are periodically arranged in the axial direction of the optical fiber to form a longitudinal modulation track; s20, the femtosecond laser spot is controlled to move by a preset distance in the first direction perpendicular to the axial direction of the optical fiber; and the steps S10 and S20 are repeated until n longitudinal modulation tracks are formed, so that the local refractive index modulation areas located in the same cross section of the optical fiber can form a refractive index modulation plane, m refractive index modulation planes are obtained, and processing of the optical fiber grating is completed. Energy relaxation of the optical fiber material is facilitated, and a refractive index modulation structure is stably formed.

Description

Technical Field

[0001] This invention belongs to the field of fiber grating fabrication technology, and particularly relates to a method for fabricating fiber gratings by longitudinal line-by-line cold processing based on femtosecond laser direct writing. Background Technology

[0002] A fiber grating is a passive optical device in which a periodic refractive index modulation is formed along the axial direction of an optical fiber core. Its basic principle is to achieve wavelength selection, filtering, or feedback functions by generating Bragg reflections of light of a specific wavelength through periodic refractive index changes. Due to its advantages such as compact structure, strong resistance to electromagnetic interference, and direct compatibility with fiber optic systems, fiber gratings have been widely used in fiber optic communication, fiber optic sensing, and fiber lasers.

[0003] With the rapid development of mid-infrared fiber lasers, infrared spectroscopy analysis, and infrared sensing technologies, the demand for mid-infrared fiber optic devices is becoming increasingly urgent. Fluoride fibers possess advantages such as low phonon energy and a wide infrared transmission window, exhibiting significant advantages in mid-infrared fiber lasers and sensing systems. Therefore, fabricating high-performance, stable, and reliable fiber gratings in fluoride fibers has become one of the important research directions in this field.

[0004] Femtosecond lasers, with their extremely short pulse duration and extremely high peak power, can induce localized refractive index changes or microstructure changes within transparent media through nonlinear effects such as multiphoton absorption and avalanche ionization, thereby enabling three-dimensional precision machining of materials. Based on these characteristics, femtosecond laser direct writing technology has become an important method for fabricating fiber gratings in non-photosensitive or weakly photosensitive optical fibers, and is particularly suitable for mid-infrared fiber material systems such as fluoride fibers.

[0005] Among existing femtosecond laser fiber grating fabrication techniques, the most widely used method is the traditional femtosecond laser line-by-line method. This method typically uses a high numerical aperture microscope objective to focus the femtosecond laser onto the fiber core, and controls the relative motion of the fiber or the focused spot through a precision three-dimensional displacement platform.

[0006] In the fabrication of fiber gratings using the traditional femtosecond laser line-by-line method, the femtosecond laser typically operates at a fixed repetition frequency, with adjacent laser pulses output at equal time intervals. For example... Figure 1 As shown, when the laser spot along Figure 1 When the arrows indicated by the dashed lines move continuously to perform line-by-line scanning, the processing time interval t between two adjacent spatial marking points 15 is mainly determined by the laser repetition frequency and the scanning speed, and is usually on the order of milliseconds. For example, when the laser repetition frequency is 1 kHz and the scanning speed matches the pulse spacing, the processing time interval between adjacent marking points is approximately 1 ms.

[0007] During the writing process, the trajectory of the femtosecond laser spot is typically perpendicular to the fiber axis. At each grating period position along the fiber axis, one or more line scans are performed laterally, thereby forming a local refractive index modulation region inside the fiber. Subsequently, according to a predetermined grating period, the above line-by-line scanning process is repeated along the fiber axis, ultimately forming a periodically arranged refractive index modulation structure, i.e., a fiber grating, in the fiber core region. This method enables the formation of regular, repeatable fiber grating structures within fluoride fibers; therefore, the traditional femtosecond laser line-by-line method has become one of the typical implementation schemes for fabricating fluoride fiber gratings and other non-photosensitive fiber gratings.

[0008] However, using the traditional femtosecond laser line-by-line method, it is currently impossible to stably inscribe gratings with high refractive index modulation intensity in fluoride fibers. Therefore, how to stably inscribe gratings with high refractive index modulation intensity in fluoride fibers remains a problem that urgently needs to be solved in this field. Summary of the Invention

[0009] In view of this, the present invention aims to provide a method for fabricating fiber gratings by longitudinal line-by-line cold processing based on femtosecond laser direct writing, so as to significantly improve the reflectivity and overall performance of fluoride fiber gratings without changing the core parameters of the femtosecond laser and other processing conditions.

[0010] To achieve the above objectives, the technical solution created by this invention is implemented as follows: In a first aspect, the present invention provides a method for fabricating fiber gratings by longitudinal line-by-line cold processing based on femtosecond laser direct writing, comprising the following steps: S10. Control the femtosecond laser spot to move point by point along the axial direction of the optical fiber with the grating period as the step size, and apply a laser pulse at the corresponding position of each grating period to form a local refractive index modulation region; at least m of the local refractive index modulation regions are periodically arranged along the axial direction of the optical fiber to form a longitudinal modulation trajectory. S20. Control the femtosecond laser spot to move a preset distance along a first direction, wherein the first direction is perpendicular to the axis of the optical fiber; S30. Repeat steps S10 and S20 until n longitudinal modulation trajectories are formed, so that the local refractive index modulation regions located in the same cross section of the optical fiber can form a refractive index modulation plane, resulting in m refractive index modulation planes, thus completing the fabrication of the fiber grating.

[0011] Furthermore, the local refractive index modulation region is a line segment extending along a second direction on the cross-section of the optical fiber, and in the same cross-section of the optical fiber, the second direction is perpendicular to the first direction.

[0012] Furthermore, in the same cross-section of the optical fiber, the first direction is horizontal and the second direction is vertical.

[0013] Furthermore, the grating period Λ is jointly determined by the writing speed v and the femtosecond laser repetition frequency f, and their relationship is: Λ = v × f - ¹; In step S10, the size of the grating period is adjusted by controlling the writing speed or the laser repetition frequency.

[0014] Furthermore, in step S20, the number n of the longitudinal modulation trajectories is adjusted by controlling the preset distance.

[0015] Furthermore, between step S10 and step S20, or between step S20 and step S30, the method further includes controlling the femtosecond laser spot to pause at a preset time interval.

[0016] Furthermore, the thermal damage threshold of the optical fiber is lower than that of a quartz optical fiber.

[0017] Secondly, the present invention provides a fiber optic grating, which is formed by the method for preparing fiber optic gratings by longitudinal line-by-line cold processing based on femtosecond laser direct writing as described in any embodiment of the present invention.

[0018] In a second aspect, the present invention provides a computer device applied to a femtosecond laser direct-write fiber Bragg grating system, comprising: At least one processor; and A memory communicatively connected to the at least one processor; wherein, The memory stores instructions that can be executed by the at least one processor, which, when executed by the at least one processor, enables the at least one processor to perform the method for fabricating fiber gratings by longitudinal line-by-line cold processing based on femtosecond laser direct writing according to any embodiment of the present invention.

[0019] Thirdly, the present invention provides a non-transient computer-readable storage medium storing computer instructions, the computer instructions being used to cause the computer to execute the method for preparing fiber gratings by longitudinal line-by-line cold processing based on femtosecond laser direct writing as described in any embodiment of the present invention.

[0020] Compared with the prior art, the present invention can achieve the following beneficial effects: (1) The method for preparing fiber gratings by longitudinal line-by-line cold processing based on femtosecond laser direct writing described in this invention achieves cold processing by changing the direction of the laser spot's motion trajectory and the time distribution characteristics of the local refractive index modulation region within the same grating period. This significantly extends the time interval between the formation of adjacent local refractive index modulation regions within the same grating period. This invention provides sufficient conditions for energy relaxation, structural rearrangement and stress release inside the fiber material, thereby promoting the stable formation of the refractive index modulation structure.

[0021] (2) The method for preparing fiber gratings by longitudinal line-by-line cold processing based on femtosecond laser direct writing described in this invention significantly improves the reflectivity of fluoride fiber gratings by cold processing without changing the core parameters such as repetition frequency, pulse energy and focusing conditions of the femtosecond laser and other processing conditions.

[0022] (3) The method for preparing fiber gratings by longitudinal line-by-line cold processing based on femtosecond laser direct writing described in this invention is particularly suitable for fiber materials with low thermal damage threshold, such as fluoride fiber, and has good versatility and application prospects.

[0023] (4) The method created by this invention can be directly implemented in the existing femtosecond laser direct writing system. Only the motion control strategy needs to be adjusted, and the engineering implementation difficulty is low.

[0024] (5) This invention introduces multiple controllable parameters to finely regulate the spatial and temporal distribution of laser action within the same grating period; including: By controlling the femtosecond laser spot to move a preset distance along a first direction perpendicular to the fiber axis, the spatial distribution density of the laser action points within the same grating period is changed, thereby adjusting the number of laser points in each period. By freely controlling the time interval between adjacent longitudinal modulation trajectories on the order of seconds or even minutes, the processing time interval between two adjacent local refractive index modulation regions formed within the same grating period can be effectively adjusted, thereby ensuring that the energy deposition, structural rearrangement, and stress evolution processes caused by the previous laser pulse are fully relaxed before the next laser pulse.

[0025] (6) The method created by this invention is not only applicable to the preparation of fluoride fiber gratings, but can also be extended to the preparation process of fiber gratings of other non-photosensitive or weakly photosensitive fiber materials. Attached Figure Description

[0026] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 A schematic diagram of fiber grating fabrication using the traditional femtosecond laser line-by-line method; Figure 2 A schematic flowchart of the method for fabricating fiber gratings by longitudinal line-by-line cold processing based on femtosecond laser direct writing, as described in an embodiment of the present invention; Figure 3 A schematic diagram of the trajectory of the femtosecond laser spot moving in the XZ plane as observed from the Y-axis direction in the method for fabricating fiber gratings based on longitudinal line-by-line cold processing using femtosecond laser direct writing, as described in the embodiments of the present invention. Figure 4 A schematic diagram of the cross-section of the optical fiber as viewed from the Z-axis direction in the method for fabricating fiber gratings based on longitudinal line-by-line cold processing using femtosecond laser direct writing, as described in the embodiments of the present invention. Figure 5 The image shows the morphology of the fiber grating prepared by the longitudinal line-by-line cold processing method based on femtosecond laser direct writing as described in the embodiments of the present invention. Figure 6 The transmission spectrum of fiber gratings prepared by the traditional line-by-line method; Figure 7 The transmission spectrum of the fiber grating prepared by the longitudinal line-by-line cold processing method based on femtosecond laser direct writing described in the embodiments of the present invention; Figure 8 This is a schematic diagram of a femtosecond laser direct-write fiber Bragg grating system. Figure 9 This is a schematic diagram of the structure of a computer device provided in an embodiment of the present invention.

[0027] Explanation of reference numerals in the attached figures: 10. Optical fiber; 11. Local refractive index modulation region; 12. Longitudinal modulation trajectory; 15. Spatial marking point; 21. Laser emission unit; 22. Beam control unit; 23. Observation and focusing unit; 24. Optical fiber clamping and displacement unit; 25. Spectral monitoring unit; 31. Optical fiber core; 32. Optical fiber cladding; 62. Computer equipment; 64. External equipment; 66. Processing unit; 68. Bus; 70. Network adapter; 72. Input / output (I / O) interface; 74. Display; 78. System memory; 80. Random access memory (RAM); 82. Cache memory; 84. Storage system; 90. Program / utility; 92. Program module. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only for explaining the invention and do not constitute a limitation thereof. Similar elements in different embodiments are referred to by associated similar element reference numerals. In the following embodiments, many details are described to facilitate a better understanding of the invention. However, those skilled in the art will readily recognize that some features may be omitted in different situations, or may be replaced by other elements, materials, or methods. In some cases, some operations related to the invention are not shown or described in the specification. This is to avoid obscuring the core parts of the invention with excessive description. For those skilled in the art, detailed description of these related operations is not necessary; they can fully understand the related operations based on the description in the specification and general technical knowledge in the art.

[0029] It should be noted that, unless otherwise specified, the embodiments and features described in this invention can be combined to form various implementations. Furthermore, the order of the steps or actions in the method description can be changed or adjusted in a manner readily apparent to those skilled in the art. Therefore, the various orders in the specification and drawings are merely for the clear description of a particular embodiment and do not imply a mandatory order, unless otherwise stated that a particular order must be followed.

[0030] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," and "counterclockwise," etc., indicating orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on this invention. The term "based on" should be understood as "at least partially based on." Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, features defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more, and the term "including" means "including but not limited to." Various embodiments of the present invention may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a rigid limitation on the scope of the invention; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within that range; for example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the range referred to.

[0031] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art will understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0032] The invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0033] In the process of fabricating fiber gratings in fluoride fibers using the femtosecond laser line-by-line method, if the same writing parameters (especially the same pulse repetition frequency) are used as those for quartz fibers, the refractive index modulation effect formed in the fluoride fibers is not as expected.

[0034] The applicant discovered that within the same grating period, the processing time interval between the points of action (i.e., spatial marking points) of adjacent laser pulses is extremely short (typically on the order of microseconds to milliseconds). The material has not yet completed energy relaxation, structural rearrangement, or stress release after the previous laser pulse, and then the laser pulse acts on the adjacent location again. This high-time-density energy deposition method is detrimental to the full evolution and stable formation of the refractive index modulation structure. This problem is particularly pronounced in materials with low thermal damage thresholds, such as fluoride fibers, often resulting in insufficient refractive index modulation depth, thus limiting further increases in the refractive index modulation intensity of fiber gratings.

[0035] The existing technology has not recognized the significant impact of the coupling relationship between the aforementioned material properties and the inscription process parameters on the inscription effect.

[0036] This invention provides the material with a sufficient relaxation window by extending the pulse interval between adjacent laser action points within the same grating period, allowing the refractive index modulation to fully evolve and stabilize, thereby achieving stable formation of high refractive index modulation intensity in fluoride optical fibers, which have a lower thermal damage threshold than quartz optical fibers.

[0037] Example 1 like Figure 2 , Figure 3 As shown, this invention provides a method for fabricating fiber gratings using longitudinal line-by-line cold processing based on femtosecond laser direct writing, comprising the following steps: S10. Control the femtosecond laser spot to move point by point along the axial direction of the optical fiber 10 with the grating period as the step size, and apply a laser pulse at the corresponding position of each grating period to form a local refractive index modulation region 11; at least m continuously formed local refractive index modulation regions 11 are periodically arranged along the axial direction of the optical fiber to form a longitudinal modulation trajectory 12.

[0038] The step size is the grating period Λ set according to the Bragg wavelength of the required grating. A laser pulse is applied at each step size position, and each step size position corresponds to a laser action point, on which a local refractive index modulation region 11 is formed.

[0039] In practical implementation, an air-bearing or high-precision linear motor platform can typically be used to carry the optical fiber, and a step-pause-exposure mode can be selected. That is, after the platform moves to the predetermined position, it pauses and triggers a femtosecond laser pulse to write a local refractive index modulation region 11, and then moves to the next step position. This method has extremely high precision but takes a long time. Alternatively, a fly-and-shoot mode can be selected, in which the platform moves at a constant speed and triggers laser pulses in real time according to the encoder signal. This method has extremely strict requirements for the uniformity of platform speed and trigger delay compensation, but it is more efficient and therefore more common.

[0040] The grating period Λ is determined by both the laser writing speed v and the femtosecond laser repetition frequency f, and the relationship is: Λ = v × f - ¹; Thus, by controlling the laser writing speed and / or laser repetition frequency, the size of the grating period can be adjusted to meet the design requirements of different Bragg wavelengths and different grating orders.

[0041] S20. Control the femtosecond laser spot to move a preset distance along a first direction, wherein the first direction is perpendicular to the axis of the optical fiber 10; As can be seen, the first direction can be any possible direction within the cross-section (such as horizontal, vertical, or oblique direction).

[0042] If a coordinate system is established with the axial direction along the fiber 10 as the Z-axis, then the X-axis and Y-axis are located within the cross-section of the fiber 10 and are perpendicular to each other, where the X-axis is horizontal and the Y-axis is vertical. The aforementioned first direction can be the X-axis direction, the Y-axis direction, or any direction within the XY plane.

[0043] In the process of forming multiple longitudinal modulation tracks, after each track is written, the femtosecond laser spot needs to be moved a preset distance along the first direction. During this movement, the Z-axis coordinate must remain constant to change the position of the next longitudinal modulation track within the cross-section of the fiber. There are three ways to control the movement of the femtosecond laser spot along the first direction: (1) Move along the X-axis When the femtosecond laser spot is controlled to move along the X-axis direction, and the Y-axis coordinate remains unchanged during the movement, the preset distance Δd is the distance Δx moved in the X-axis direction.

[0044] (2) Move along the Y-axis When the femtosecond laser spot is controlled to move along the Y-axis, while the X-axis coordinate remains constant during the movement, the preset distance Δd is the distance Δy that the laser moves along the Y-axis.

[0045] (3) XY axis linkage movement When the femtosecond laser spot is controlled to move along a preset angle θ in the XY plane, the moving distance is Δd, which includes the distance Δx moved in the X-axis direction and the distance Δy moved in the Y-axis direction; where Δx = Δd·cosθ and Δy = Δd·sinθ.

[0046] The three movement methods described above can be selected based on the geometry of the target refractive index modulation region. It is important to note that the preset movement distance must be small enough to allow the local refractive index modulation regions located within the same cross-section of the optical fiber to form a refractive index modulation plane.

[0047] S30. Determine whether the number of longitudinal modulation tracks 12 has reached n. If yes, end the process; otherwise, return to step S10. Repeat steps S10 and S20 until n longitudinal modulation tracks 12 are formed; this allows the local refractive index modulation regions 11 located within the same cross-section of the optical fiber to form a refractive index modulation plane, resulting in m refractive index modulation planes, thus completing the fiber grating fabrication. Each refractive index modulation plane contains n local refractive index modulation regions 11, and the distance between adjacent local refractive index modulation regions 11 is the preset distance in step S20. When the distance (preset distance) between adjacent local refractive index modulation regions within the same refractive index modulation plane is sufficiently small, they will physically or optically connect into a whole, forming a refractive index modulation plane. Since the femtosecond laser spot moves point by point with the grating period as the step size, the axial distance between adjacent refractive index modulation planes is equal to the grating period.

[0048] In practical implementation, the number n of longitudinal modulation tracks 12 can be adjusted by controlling the femtosecond laser spot to move along the first direction by a preset distance. The total span of the above n longitudinal modulation tracks in the first direction is fixed. Therefore, when the preset distance is large, the number n of longitudinal modulation tracks 12 is small, and when the preset distance is small, the number n of longitudinal modulation tracks 12 is large.

[0049] Each grating period's boundary corresponds to a refractive index modulation plane of the optical fiber. Within the same refractive index modulation plane of optical fiber 10, the distance between adjacent local refractive index modulation regions is equal to the preset distance mentioned in step S20. Therefore, by controlling the preset distance to adjust the number of local refractive index modulation regions within the same refractive index modulation plane of the optical fiber, the number of local refractive index modulation regions within each grating period can be controlled.

[0050] The proposed method for fabricating fiber gratings using longitudinal line-by-line cold processing based on femtosecond laser direct writing allows for the application of laser pulses on different refractive index modulation planes during the fabrication of a single longitudinal modulation trajectory. In other words, the laser pulses are not applied continuously during the formation of the same refractive index modulation plane (i.e., the same grating period position). Although a single laser pulse remains femtosecond-level, the formation time of a single refractive index modulation plane is extended to the second or even minute level. This effectively avoids excessive temporal concentration of energy in laser pulses applied to the same refractive index modulation plane. This "spatially dispersed, temporally delayed" laser pulse application method provides sufficient relaxation time for fiber intermediate states (such as excited electrons and trapped excitons), as well as ample time for heat conduction, structural rearrangement, or stress release. This is particularly crucial for fibers with low thermal damage thresholds, enabling stable refractive index modulation without damaging the fiber material.

[0051] The method for fabricating fiber gratings based on femtosecond laser direct writing in longitudinal line-by-line cold processing proposed in this invention can be viewed from the equivalent perspective of the processing path as the superposition of multiple parallel longitudinal modulation trajectories 12 in the first direction. Based on this equivalent path, the time interval between laser pulses applied to adjacent local refractive index modulation regions within the same refractive index modulation plane is greatly increased, avoiding local extreme morphologies (such as microcracks, stress concentration, and structural abrupt changes) caused by excessive energy concentration in a short time. This results in a more stable and uniform refractive index modulation structure of the final fiber grating, with higher refractive index modulation intensity. This invention is applicable not only to optical fibers with low thermal damage thresholds such as fluoride fibers, but also to other non-photosensitive or weakly photosensitive optical fiber materials, and even ordinary optical fibers such as silica fibers.

[0052] Example 2 Based on Example 1, the method for fabricating fiber gratings by longitudinal line-by-line cold processing based on femtosecond laser direct writing provided in this example includes the above steps S10 to S30; these will not be repeated here. The difference is that please also refer to... Figure 4 In this embodiment, the local refractive index modulation region is a line segment extending along a second direction on the cross-section of the optical fiber, and the second direction is perpendicular to the first direction in the same cross-section of the optical fiber. More specifically, in step S10, after the femtosecond laser spot is shaped, a line segment-shaped local refractive index modulation region 11 extending along the second direction can be formed on the cross-section of the optical fiber. The line segment-shaped local refractive index modulation region 11 is located within the fiber core 31, and the end of the local refractive index modulation region can extend to the junction of the fiber core 31 and the fiber cladding 32 or directly into the fiber cladding 32, so that the local refractive index modulation region 11 can cover the fiber core 31.

[0053] Optionally, in the same cross-section of the optical fiber, the second direction is the vertical direction (Y-axis). In this case, in step S20, the first direction is the horizontal direction (X-axis). When the femtosecond laser spot moves a preset distance Δx in the X-axis direction each time and forms n longitudinal modulation trajectories 12, the n local refractive index modulation regions 11 in the same refractive index modulation plane will be arranged at equal intervals of Δx in the X-axis direction of the optical fiber cross-section.

[0054] This embodiment maintains the same fiber grating geometry as that obtained by the traditional femtosecond laser line-by-line method, while extending the formation time of a single refractive index modulation plane to any desired duration. This provides sufficient conditions for energy relaxation, structural rearrangement, and stress release within the fiber material, and promotes the stable formation of high refractive index modulation intensity.

[0055] Example 3 Based on Example 1 or Example 2, the method for fabricating fiber gratings by longitudinal line-by-line cold processing based on femtosecond laser direct writing provided in this example includes the above steps S10 to S30; they will not be repeated here.

[0056] In this embodiment, between step S10 and step S20, or between step S20 and step S30, the method further includes controlling the femtosecond laser spot to pause at a preset time interval.

[0057] In specific implementation, when the point-to-point movement is performed in a unidirectional order along the fiber axis, that is, the formation of each longitudinal modulation trajectory 12 is along the same direction, for example, each longitudinal modulation trajectory 12 is formed from left to right. After step S10 and before step S20, the femtosecond laser spot can be controlled to return to the starting position of the longitudinal modulation trajectory 12 first, and then the femtosecond laser spot can be controlled to pause for a preset time interval. Of course, it is also feasible to first control the femtosecond laser spot to pause for a preset time interval, and then control the femtosecond laser spot to return to the starting position of the longitudinal modulation trajectory 12. This invention does not limit this.

[0058] Thus, after the first local refractive index modulation region 11 within the grating period is completed, the second local refractive index modulation region 11 within the same grating period can only be established after the previous longitudinal modulation trajectory 12 is completed. In other words, the time interval between the laser pulses applied to two local refractive index modulation regions formed successively within the same grating period is at least the time required to process a complete longitudinal modulation trajectory. Furthermore, controlling the preset time interval of the femtosecond laser spot pause can allow the time interval between two adjacent local refractive index modulation regions formed successively within the same grating period to reach a second-level or longer time scale, thereby ensuring that the energy deposition, structural rearrangement, and stress evolution processes caused by the previous laser pulse are fully relaxed before the next laser pulse is applied.

[0059] When the point-to-point movement sequence is a reciprocating scan along the fiber axis, that is, the formation of two adjacent longitudinal modulation tracks 12 is in opposite directions. For example, the first longitudinal modulation track 12 is formed from left to right, and the second longitudinal modulation track 12 is formed from right to left. In this case, it is not necessary to control the femtosecond laser spot to return to the starting position of the longitudinal modulation track 12. It is only necessary to control the femtosecond laser spot to pause for a preset time interval after completing one longitudinal modulation track 12. It should be noted that the preset time interval must be set to allow the energy deposition, structural rearrangement, and stress evolution processes caused by the previous laser pulse to fully relax before the next laser pulse is applied.

[0060] Furthermore, the preset time interval for controlling the pause of the femtosecond laser spot can be adjusted according to the specific material properties of the optical fiber being etched by the femtosecond laser. It can be a fixed time interval or a gradually changing time interval. In this embodiment, by setting the preset time interval, the formation time of a single refractive index modulation plane can be extended to any desired duration.

[0061] The method for preparing fiber gratings by longitudinal line-by-line cold processing based on femtosecond laser direct writing provided in any of the above embodiments 1 to 3 can use an optical fiber with a thermal damage threshold lower than that of a quartz optical fiber as the optical fiber to be written by the femtosecond laser, such as a fluoride optical fiber.

[0062] Example 4 To verify the advantages of the method for fabricating fiber gratings by longitudinal line-by-line cold processing based on femtosecond laser direct writing proposed in this invention, fluoride fiber gratings were fabricated under the same experimental conditions using both the traditional femtosecond laser line-by-line method and the method proposed in this invention, and their optical performance was compared and tested.

[0063] The femtosecond laser used in the experiment had a wavelength of 1030 nm, a single pulse energy of 460 nJ, and a laser repetition frequency of 0.5 kHz. The grating used was a second-order fiber grating with a grating length of 10 mm. To control variables, the laser dot density within each grating cycle was set to 10 dots per micrometer in both writing methods, while other processing parameters remained consistent.

[0064] Under the above conditions, 3μm fiber gratings were etched onto fluoride fibers using both methods. The fiber grating etched using the conventional femtosecond laser line-by-line method exhibited a transmission depth of approximately 14 dB in its transmission spectrum. For example... Figure 6 As shown; and the fiber gratings fabricated using the longitudinal line-by-line cold processing method based on femtosecond laser direct writing proposed in this invention are as follows: Figure 5 As shown, its transmission depth is approximately 26 dB, as Figure 7 As shown.

[0065] The above experimental results show that, under the same processing parameters, the method for preparing fiber gratings by longitudinal line-by-line cold processing based on femtosecond laser direct writing provided by this invention can significantly enhance the refractive index modulation intensity of the fiber grating, thereby greatly improving the spectral performance of the fiber grating, which fully verifies the effectiveness and superiority of the technical solution created by this invention.

[0066] Example 5 This embodiment provides a fiber optic grating, which is formed by a method for preparing fiber optic gratings by longitudinal line-by-line cold processing based on femtosecond laser direct writing as provided in any one of the embodiments 1 to 3 above.

[0067] Example 6 This embodiment provides a computer device applied to a femtosecond laser direct-write fiber Bragg grating system, such as... Figure 8 As shown, the femtosecond laser direct-write fiber grating system is an optical micro-nano processing device that uses ultrashort pulse lasers to directly write periodic refractive index modulation structures inside optical fibers. It mainly includes a laser emitting unit 21, a beam control unit 22, an observation and focusing unit 23, an optical fiber clamping and displacement unit 24, a spectral monitoring unit 25, and a computer device 62.

[0068] The laser emitting unit 21 is used to output femtosecond pulsed laser light. The beam control unit 22 includes a mirror, a beam splitter, a power meter, a shutter, a half-wave plate, and a prism. The femtosecond pulsed laser light output by the laser emitting unit 21 is guided into the beam splitter through the mirror. The beam splitter splits a portion of the light to the power meter for real-time power monitoring. The main optical path is then guided into the focusing optical path by the mirror in the observation and focusing unit 23 after passing through the shutter to control the exposure timing, the half-wave plate to control the polarization state, and the prism to shape the light.

[0069] The observation and focusing unit 23 includes a reflector, a dichroic mirror, an objective lens, a CCD camera, and a display. The femtosecond laser passes through the reflector and dichroic mirror and is focused onto the fiber core area by the high numerical aperture objective lens. The CCD camera coaxially acquires images of the fiber and the focal point through the dichroic mirror and displays them in real time on the display, realizing alignment and process monitoring.

[0070] The fiber clamping and displacement unit 24 includes a displacement platform (including X / Y / Z axes) and a fiber clamp. The fiber clamping and displacement unit 24 is used to drive the fiber to move with high precision along a preset path to realize point-by-point direct writing of the fiber grating.

[0071] The spectral monitoring unit 25 includes a broadband light source, an optical fiber, and a spectrometer; the broadband light source is input to the optical fiber, and the spectral monitoring unit 25 can acquire the transmission / reflection spectrum of the fiber grating in real time for online detection of the fabrication quality of the fiber grating.

[0072] Computer device 62 is electrically connected to laser, shutter, displacement platform and spectrometer, and is used to synchronously control laser output, displacement platform movement and spectral data acquisition, so as to realize programmable control of grating period and structure.

[0073] It should be noted that the above-described femtosecond laser direct-write fiber Bragg grating system is merely an example, and this invention does not limit the femtosecond laser direct-write fiber Bragg grating system.

[0074] Please refer to the following at the same time Figure 9 , Figure 9 This is a schematic diagram of the structure of a computer device 62 provided in an embodiment of the present invention. Figure 9A block diagram of an exemplary computer device 62 suitable for implementing embodiments of the present invention is shown. Figure 9 The computer device 62 shown is merely an example and should not impose any limitation on the functionality and scope of use of the embodiments of the present invention.

[0075] like Figure 9 As shown, computer device 62 is represented in the form of a general-purpose computing device. Computer device 62 is intended to represent various forms of digital computers, such as laptop computers, desktop computers, workstations, personal digital assistants, servers, blade servers, mainframe computers, and other suitable computers. Electronic devices may also represent various forms of mobile devices, such as personal digital processors, cellular phones, smartphones, wearable devices, and other similar computing devices. The components shown herein, their connections and relationships, and their functions are merely illustrative and are not intended to limit the implementation of the invention described and / or claimed herein.

[0076] The components of computer device 62 may include, but are not limited to: one or more processors or processing units 66, system memory 78, and bus 68 connecting different system components (including system memory 78 and processing unit 66).

[0077] Bus 68 represents one or more of several bus architectures, including a memory bus or memory controller, a peripheral bus, a graphics acceleration port, a processor, or a local bus using any of the various bus architectures. Examples of these architectures include, but are not limited to, the Industry Standard Architecture (ISA) bus, the Micro Channel Architecture (MAC) bus, the Enhanced ISA bus, the Video Electronics Standards Association (VESA) local bus, and the Peripheral Component Interconnect (PCI) bus.

[0078] Computer device 62 typically includes a variety of computer system readable media. These media can be any available media that can be accessed by computer device 62, including volatile and non-volatile media, removable and non-removable media.

[0079] System memory 78 may include computer system readable media in the form of volatile memory, such as random access memory (RAM) 80 and / or cache memory 82. Computer device 62 may further include other removable / non-removable, volatile / non-volatile computer system storage media. By way of example only, storage system 84 may be used to read and write non-removable, non-volatile magnetic media (…). Figure 9 Not shown; usually referred to as a "hard drive"). Although Figure 9Not shown, a disk drive for reading and writing to a removable non-volatile disk (e.g., a "floppy disk") and an optical disk drive for reading and writing to a removable non-volatile optical disk (e.g., a CD-ROM, DVD-ROM, or other optical media) may be provided. In these cases, each drive may be connected to bus 68 via one or more data media interfaces. System memory 78 may include at least one program product having a set (e.g., at least one) of program modules configured to perform the functions of any of embodiments 1 to 3 of the present invention.

[0080] A program / utility 90 having a set (at least one) of program modules 92 may be stored, for example, in system memory 78. Such program modules 92 include, but are not limited to, an operating system, one or more application programs, other program modules, and program data. Each or some combination of these examples may include an implementation of a network environment. The program modules 92 typically perform the functions and / or methods of any of the embodiments 1 to 3 described in this invention.

[0081] Computer device 62 can also communicate with one or more external devices 64 (e.g., keyboard, pointing device, display 74, etc.), and with one or more devices that enable a user to interact with computer device 62, and / or with any device that enables computer device 62 to communicate with one or more other computing devices (e.g., network card, modem, etc.). This communication can be performed via input / output (I / O) interface 72. Furthermore, computer device 62 can also communicate with one or more networks (e.g., local area network (LAN), wide area network (WAN), and / or public networks, such as the Internet) via network adapter 70. As shown, network adapter 70 communicates with other modules of computer device 62 via bus 68. It should be understood that, although not shown in the figures, other hardware and / or software modules can be used in conjunction with computer device 62, including but not limited to: microcode, device drivers, redundant processing units, external disk drive arrays, RAID systems, tape drives, and data backup storage systems.

[0082] The processing unit 66 executes various functional applications and data processing by running programs stored in the system memory 78, such as implementing the method for preparing fiber gratings by longitudinal line-by-line cold processing based on femtosecond laser direct writing provided in any of the embodiments of the present invention 1 to 3.

[0083] Example 7 This invention also provides a non-transient computer-readable storage medium storing computer instructions, on which a computer program is stored, wherein when the program is executed by a processor, it implements the method for preparing fiber gratings by longitudinal line-by-line cold processing based on femtosecond laser direct writing provided in any of embodiments 1 to 3 of this invention.

[0084] The computer storage medium of this invention can be any combination of one or more computer-readable media. The computer-readable medium can be a computer-readable signal medium or a computer-readable storage medium. More specific examples (a non-exhaustive list) of computer-readable storage media include: electrical connections having one or more wires, portable computer disks, hard disks, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM or flash memory), optical fiber, portable compact disk read-only memory (CD-ROM), optical storage devices, magnetic storage devices, or any suitable combination thereof. In this document, a computer-readable storage medium can be any tangible medium that contains or stores a program that can be used by or in conjunction with an instruction execution system, apparatus, or device.

[0085] Computer-readable signal media may include data signals propagated in baseband or as part of a carrier wave, carrying computer-readable program code. Such propagated data signals may take various forms, including but not limited to electromagnetic signals, optical signals, or any suitable combination thereof. Computer-readable signal media may also be any computer-readable medium other than computer-readable storage media, capable of sending, propagating, or transmitting programs for use by or in connection with an instruction execution system, apparatus, or device.

[0086] The program code contained on the computer-readable medium can be transmitted using any suitable medium, including but not limited to wireless, wired, optical fiber, RF, etc., or any suitable combination thereof. The computer program code for performing the operations of this invention can be written in one or more programming languages ​​or a combination thereof, including object-oriented programming languages ​​such as Java, Smalltalk, and C++, as well as conventional procedural programming languages—such as the "C" language or similar programming languages. The program code can be executed entirely on the user's computer, partially on the user's computer, as a stand-alone software package, partially on the user's computer and partially on a remote computer, or entirely on a remote computer or server. In cases involving a remote computer, the remote computer can be connected to the user's computer via any type of network, including a local area network (LAN) or a wide area network (WAN), or it can be connected to an external computer (e.g., via the Internet using an Internet service provider).

[0087] This invention also provides a computer program product, including a computer program that, when executed by a processor, implements the method described above for fabricating fiber gratings by longitudinal line-by-line cold processing based on femtosecond laser direct writing.

[0088] It should be understood that the various forms of processes shown above can be used to reorder, add, or delete steps. For example, the steps described in this invention disclosure can be executed in parallel, sequentially, or in different orders, as long as the desired result of the technical solution disclosed in this invention can be achieved, and this is not limited herein.

[0089] The specific embodiments described above do not constitute a limitation on the scope of protection of this invention. Those skilled in the art should understand that various modifications, combinations, sub-combinations, and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A method for fabricating fiber gratings by longitudinal line-by-line cold processing based on femtosecond laser direct writing, characterized in that, Including the following steps: S10. Control the femtosecond laser spot to move point by point along the axial direction of the optical fiber with the grating period as the step size, and apply a laser pulse at the corresponding position of each grating period to form a local refractive index modulation region; at least m of the local refractive index modulation regions are periodically arranged along the axial direction of the optical fiber to form a longitudinal modulation trajectory. S20. Control the femtosecond laser spot to move a preset distance along the first direction, wherein the first direction is perpendicular to the axis of the optical fiber; S30. Repeat steps S10 and S20 until n longitudinal modulation trajectories are formed, so that the local refractive index modulation regions located in the same cross section of the optical fiber can form a refractive index modulation plane, resulting in m refractive index modulation planes, thus completing the fabrication of the fiber grating.

2. The method for fabricating fiber gratings by longitudinal line-by-line cold processing based on femtosecond laser direct writing according to claim 1, characterized in that: The local refractive index modulation region is a line segment extending along a second direction on the cross-section of the optical fiber, and in the same cross-section of the optical fiber, the second direction is perpendicular to the first direction.

3. The method for fabricating fiber gratings by longitudinal line-by-line cold processing based on femtosecond laser direct writing according to claim 2, characterized in that: In the same cross-section of the optical fiber, the first direction is horizontal and the second direction is vertical.

4. The method for fabricating fiber gratings by longitudinal line-by-line cold processing based on femtosecond laser direct writing according to claim 1, characterized in that: The grating period Λ is determined by the writing speed v and the femtosecond laser repetition frequency f, and the relationship is: Λ = v × f - ¹; In step S10, the size of the grating period is adjusted by controlling the writing speed and / or the laser repetition frequency.

5. The method for fabricating fiber gratings by longitudinal line-by-line cold processing based on femtosecond laser direct writing according to claim 1, characterized in that: In step S20, the number n of the longitudinal modulation trajectories is adjusted by controlling the preset distance.

6. The method for fabricating fiber gratings by longitudinal line-by-line cold processing based on femtosecond laser direct writing according to claim 1, characterized in that, Between step S10 and step S20, or between step S20 and step S30, the method further includes: controlling the femtosecond laser spot to pause at a preset time interval.

7. The method for fabricating fiber gratings by longitudinal line-by-line cold processing based on femtosecond laser direct writing according to claim 1, characterized in that: The thermal damage threshold of the optical fiber is lower than that of quartz optical fiber.

8. A fiber optic grating, characterized in that: The fiber grating is formed by the method of longitudinal line-by-line cold processing based on femtosecond laser direct writing as described in any one of claims 1 to 7.

9. A computer device used in a femtosecond laser direct-write fiber Bragg grating system, characterized in that, include: At least one processor; as well as A memory communicatively connected to the at least one processor; wherein, The memory stores instructions that can be executed by the at least one processor, which, when executed by the at least one processor, enables the at least one processor to perform the method for fabricating fiber gratings by longitudinal line-by-line cold processing based on femtosecond laser direct writing as described in any one of claims 1 to 7.

10. A non-transitory computer-readable storage medium storing computer instructions, characterized in that, The computer instructions are used to cause the computer to execute the method for fabricating fiber gratings by longitudinal line-by-line cold processing based on femtosecond laser direct writing, as described in any one of claims 1 to 7.

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