A method and apparatus for manufacturing a fiber grating

Abstract

The present application relates to the field of preparation of fiber grating, and discloses a kind of preparation method and preparation device of fiber grating, by the polarization state of femtosecond laser from fixed parameter is promoted to the write-in dimension of active control, make the linear polarization direction of femtosecond laser pulse processing beam in the process of point-by-point processing polarization state space coding type rotation along the optical fiber axial, to prepare the fiber grating of polarization sensitive axis along the optical fiber axial space coding distribution in conventional single-mode optical fiber directly.The fiber grating with space coding polarization sensitive axis prepared by the preparation method of the present application can be directly used as a distributed torsion sensor, when the optical fiber and its internal space coding polarization sensitive axis are physically rotated, it is equivalent to change the relative angle between the incident light polarization direction and the local space coding polarization sensitive axis, so that the reflection intensity or central wavelength of the fiber grating changes significantly, and the torsion sensitivity of the conventional fiber grating can be improved by more than one order of magnitude.

Description

Technical Field

[0001] This invention relates to the field of fiber Bragg grating fabrication, and more particularly to a method and apparatus for fabricating fiber Bragg gratings. Background Technology

[0002] Femtosecond laser point-by-point direct writing technology is currently the core method for fabricating fiber Bragg gratings. This technology uses a high numerical aperture objective lens to focus a femtosecond laser pulse into the fiber core and controls the fiber to move point-by-point along the axis according to a preset grating period. Nonlinear multiphoton absorption induces a localized permanent refractive index change, thereby forming a refractive index modulation structure.

[0003] However, in some specialized applications such as polarization-maintaining gratings, torsion sensors, and robot joint sensors, fiber optic gratings are required to possess a polarization response dimension, high-sensitivity in-situ sensing of torsion states, and spatial polarization encoding capabilities. Currently, fiber optic gratings fabricated using femtosecond laser point-by-point direct writing technology with ordinary optical fibers cannot meet these requirements.

[0004] To address these specific requirements, two main methods are currently employed: one is indirect sensing through complex differential measurement or encapsulation structures; the other is processing using special optical fibers such as polarization-maintaining fibers and spiral fibers. However, the first method struggles to achieve high-sensitivity and direction-identifiable distributed torsion measurements, while the second method relies on the birefringence properties of the special optical fibers themselves, significantly increasing system costs and limiting their compatibility and versatility in ordinary single-mode fiber systems. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method and apparatus for fabricating fiber gratings with polarization correlation filtering, high-sensitivity torsion sensing and spatial polarization coding functions using conventional optical fibers. By writing polarization state spatial coding point by point, a target fiber grating with a spatial coding distribution along the axial direction can be directly fabricated in conventional optical fibers.

[0006] In a first aspect, embodiments of the present invention provide a method for fabricating a fiber Bragg grating, the method comprising the following steps:

[0007] Step S1: Clamp and fix the optical fiber to be processed onto the movable carrier;

[0008] Step S2: Generate a linearly polarized femtosecond laser pulse processing beam;

[0009] Step S3: Control the optical fiber to be processed to step along its axis point by point with a preset scanning step size, and focus the femtosecond laser pulse processing beam on the core of the optical fiber to be processed at each step processing point to irradiate it, forming a local refractive index modulation structure.

[0010] Step S4: During the point-by-point processing, the linear polarization direction of the femtosecond laser pulse processing beam is synchronously controlled to rotate around the fiber axis of the fiber to be processed, so that the linear polarization direction of the next processing point has a preset polarization rotation angle relative to the linear polarization direction of the previous processing point.

[0011] Step S5: Continue the step-by-step processing until the fiber grating of the preset length is completed, and obtain the target fiber grating with a spatially encoded polarization sensitive axis distributed along the fiber axis.

[0012] Compared with existing technologies, this invention improves the polarization state of the femtosecond laser pulse processing beam from a fixed parameter to an actively controlled writing dimension. This allows the linear polarization direction of the femtosecond laser pulse processing beam to rotate spatially encoded along the fiber axis with the processing point during point-by-point processing. As a result, a fiber grating with a spatially encoded polarization sensitive axis distributed along the fiber axis can be directly fabricated in a conventional single-mode fiber. The polarization sensitive axis of this fiber grating exhibits a point-by-point rotating encoded trajectory along the fiber axis, and one typical implementation is a spiral continuous change, thereby generating a highly sensitive optical response to external torsional deformation.

[0013] This invention achieves novel functions such as polarization-dependent filtering, distributed torsion sensing, and spatial polarization encoding without relying on the intrinsic birefringence of special optical fibers such as polarization-maintaining fibers and helical fibers. This significantly reduces system costs and improves its compatibility and versatility in ordinary single-mode fiber systems. The fiber grating fabricated by the method of this invention has a spatially encoded polarization-sensitive axis distributed along its fiber axis. On any cross-section perpendicular to the fiber axis, the micro / nano structure induced by a femtosecond laser has a specific orientation angle. This orientation angle rotates periodically or non-periodically along the fiber axis, and its trajectory and rate are determined by the polarization rotation angle during the writing process. The fiber grating with spatially encoded polarization-sensitive axes fabricated by the method of this invention exhibits a strong selectivity in its reflection spectrum to the polarization state of the incident light, i.e., it exhibits significant polarization-dependent loss characteristics. When the incident light is linearly polarized, its reflectivity changes periodically with the angle between the incident polarization direction and the local polarization-sensitive axis of the fiber grating, and the polarization-dependent loss can reach over 15 dB. Therefore, the fiber grating with spatially encoded polarization-sensitive axes fabricated by the method of this invention can be directly used as a distributed torsion sensor. When the optical fiber and its internal spatially encoded polarization sensitive axis physically rotate, it is equivalent to changing the relative angle between the polarization direction of the incident light and the local spatially encoded polarization sensitive axis, thereby causing a significant change in the reflection intensity or center wavelength of the fiber grating. The torsional sensitivity can be improved by more than an order of magnitude compared to conventional fiber gratings.

[0014] In one feasible implementation, the linear polarization direction of the femtosecond laser pulse processing beam described in step S4 rotates about the fiber axis to satisfy: the linear polarization direction angle θ at the nth step processing point. n =θ0 + n × Δθ, where θ0 is the initial polarization angle and Δθ is a fixed preset polarization rotation angle; preferably, N × Δθ = 360°, where N is a positive integer greater than 2. Theoretically, there is no strict upper limit to the value of N, but in practice, it is limited by the minimum controllable angular resolution and the preset total length of the fiber grating to be processed. As a preferred example, the value of N ranges from 3 to 3600, corresponding to a Δθ range of 120° to 0.1°. The polarization rotation period length L pol =N×Λ, where Λ is the preset scan step size: Λ=m×λ B / (2n eff The scanning step size is the first-order period of the target fiber grating, where m is the diffraction order and λ is the diffraction order. B n is the Bragg wavelength of the target fiber grating. eff The effective refractive index of the optical fiber to be processed is given. At this point, the polarization direction of each step processing point along the fiber axis repeats periodically, meaning the polarization-sensitive axis of the fiber grating exhibits a periodic spatial coding distribution along the axial direction. A typical implementation is a spiral periodic distribution. More preferably, Δθ is 90° or 120°, or any value between 1° and 10°. When Δθ is a small angle, such as any value between 1° and 10°, the polarization-sensitive axis rotates in a quasi-continuous smooth spiral along the fiber axis, which helps reduce the insertion loss of the fiber grating and improve the linearity of the torsional sensing. When Δθ is a large angle, such as 90° or 120°, discrete polarization spatial coding can be achieved, suitable for applications such as fiber optic identification.

[0015] In one feasible implementation, the linear polarization direction of the femtosecond laser pulse processing beam described in step S4, when rotated around the fiber axis of the fiber to be processed, can also satisfy: the linear polarization direction angle θ at the nth step processing point. n =θ0+n×θ(n), where θ0 is the initial polarization angle, n is the sequence number of the step processing point, and θ(n) is the preset polarization rotation angle that changes with the nth step processing point.

[0016] In one feasible implementation, step S5 is followed by step S6, in which the target fiber grating is subjected to thermal annealing at a temperature of 50°C to 450°C for 2 to 6 hours.

[0017] Secondly, embodiments of the present invention provide an apparatus for fabricating fiber Bragg gratings, the apparatus comprising:

[0018] A multi-dimensional precision displacement platform is equipped with clamps for holding and fixing the optical fiber to be processed, and for moving the optical fiber along its axial direction.

[0019] Femtosecond laser source, used to generate femtosecond laser pulse beams;

[0020] The polarization module is used to generate linearly polarized femtosecond laser pulses to process the beam and rotate the direction of linear polarization.

[0021] The focusing objective lens is used to focus the femtosecond laser pulse processing beam onto the stepping point to irradiate the core of the optical fiber to be processed.

[0022] The control device is used to control the multidimensional precision displacement platform to move, control the femtosecond laser source to generate a femtosecond laser pulse beam, control the polarization module to generate a linearly polarized femtosecond laser pulse processing beam, and control the femtosecond laser pulse processing beam to focus and irradiate the fiber core of the optical fiber to be processed at each step processing point, and control the linear polarization direction of the femtosecond laser pulse processing beam to rotate around the fiber axis of the optical fiber to be processed.

[0023] In one feasible implementation, the polarization module consists of a spatial light modulator and a liquid crystal variable polarization rotator. The spatial light modulator shapes the femtosecond laser pulse beam generated by the femtosecond laser source into a linearly polarized femtosecond laser pulse processing beam. The liquid crystal variable polarization rotator rotates the linear polarization direction of the femtosecond laser pulse processing beam according to the instructions of the control device.

[0024] In another feasible implementation, the polarization module comprises a spatial light modulator loaded with a vector beam phase map. The spatial light modulator directly generates a vector beam with an arbitrary polarization state distribution. The control device controls the phase delay of different pixel regions on the spatial light modulator, generating a light field whose polarization direction varies with spatial coordinates before the femtosecond laser pulse processing beam is focused by the focusing objective. This, combined with the movement of the multidimensional precision displacement platform, achieves an equivalent polarization rotation. The basic principle of this technical solution is: the spatial light modulator is a reflective liquid crystal phase modulation device, where each pixel can independently control the phase delay of the incident light. When a vector beam phase map pre-designed by an algorithm is loaded onto the spatial light modulator, pixels at different spatial positions apply different phase modulation amounts to the incident femtosecond laser pulse beam, thereby forming a structured light field, i.e., a vector beam, on the exit surface of the spatial light modulator, where the polarization state continuously varies with spatial coordinates. Typical forms of this vector beam include radially polarized light, cycloid polarized light, and a generalized arbitrary spatial polarization distribution. The control device can change the spatial distribution pattern of polarization states on the cross-section of the beam before focusing by switching or updating the vector beam phase diagram loaded on the spatial light modulator. When the fiber to be processed is stepped point-by-point along the axial direction of the fiber under the drive of a multi-dimensional precision displacement platform, the fiber core sequentially passes through spatial positions with different polarization states in the structured light field. This is equivalent to the local polarization direction "sense" by the fiber core at each step processing point undergoing a preset angle rotation relative to the previous step processing point. Combined with the stepping motion of the displacement platform, a spatially encoded rotation of the linear polarization direction of the femtosecond laser pulse processing beam is achieved, changing with the step processing point. Its macroscopic effect is completely equivalent to the aforementioned implementation using a liquid crystal variable polarization rotator. The advantage of this scheme is that it does not require independent rotating devices, resulting in a more compact system structure. Furthermore, the polarization rotation rate is not limited by the mechanical response time of the devices, but only depends on the refresh rate of the spatial light modulator. The vector beam phase diagram, based on the correspondence between the polarization state distribution and phase distribution of the spatial light field, can be pre-calculated using vector diffraction theory or iterative optimization algorithms (such as the vector Gerchberg-Saxton algorithm).

[0025] In one feasible implementation, the numerical aperture of the focusing objective is ≥0.65. Attached Figure Description

[0026] Figure 1 This is a flowchart illustrating a method for fabricating a fiber Bragg grating according to an embodiment of the present invention.

[0027] Figure 2 This is a structural block diagram of a fiber Bragg grating fabrication apparatus provided in an embodiment of the present invention;

[0028] Figure 3 yes Figure 2 A structural block diagram of the first polarization module in the provided fabrication apparatus;

[0029] Figure 4 yes Figure 2 A structural block diagram of the second polarization module in the provided fabrication apparatus.

[0030] Explanation of reference numerals in the attached figures: 1. Multidimensional precision displacement platform; 2. Femtosecond laser source; 3. Polarization module; 31. Spatial light modulator; 32. Liquid crystal variable polarization rotator; 4. Focusing objective lens; 5. Control device. Detailed Implementation

[0031] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0032] To address the problems in the prior art, this invention provides a method and apparatus for fabricating fiber Bragg gratings.

[0033] The following describes a method for fabricating a fiber Bragg grating provided in an embodiment of the present invention.

[0034] Figure 1 A flowchart illustrating a method for fabricating a fiber Bragg grating according to an embodiment of the present invention is shown. Figure 1 As shown, the method may include the following steps:

[0035] Step S1: Clamp and fix the optical fiber to be processed onto the movable carrier;

[0036] Step S2: Generate a linearly polarized femtosecond laser pulse processing beam, the focal size of which is close to the diffraction limit after focusing;

[0037] Step S3: Control the optical fiber to be processed to step along its axis point by point with a preset scanning step size, and focus the femtosecond laser pulse processing beam on the core of the optical fiber to be processed at each step processing point to irradiate it, forming a local refractive index modulation structure.

[0038] Step S4: During the point-by-point processing, the linear polarization direction of the femtosecond laser pulse processing beam is synchronously controlled to rotate around the fiber axis of the fiber to be processed, so that the linear polarization direction of the next step processing point has a preset polarization rotation angle Δθ relative to the linear polarization direction of the previous step processing point, so that the micro-nano orientation of the refractive index modulation structure at different step processing points along the fiber axis presents a point-by-point rotating spatial coding trajectory.

[0039] Step S5: Continue the step-by-step processing until the fiber grating of the preset length is completed, forming a spatial structure in which the refractive index modulated polarization sensitive axis changes in a spiral shape along the fiber axis, thus obtaining a target fiber grating with a spatially encoded polarization sensitive axis distributed along the fiber axis.

[0040] In the first embodiment, the linear polarization direction of the femtosecond laser pulse processing beam in step S4 rotates around the fiber axis to satisfy: the linear polarization direction angle θ at the nth step processing point. n =θ0+n×Δθ, where θ0 is the initial polarization angle and Δθ is the fixed preset polarization rotation angle between the linear polarization directions of adjacent step processing points.

[0041] To obtain a fiber grating with a periodically spatially encoded polarization-sensitive axis, N×Δθ = 360° can be set, where Δθ is not equal to 180°. Preferably, N×Δθ = 360°, where N is a positive integer greater than 2. Theoretically, there is no strict upper limit to the value of N, but in practice, it is limited by the minimum controllable angular resolution and the preset total length of the fiber grating to be processed. As a preferred example, the value of N ranges from 3 to 3600, corresponding to a Δθ range of 120° to 0.1°. The polarization rotation period length L... pol =N×Λ, where Λ is the preset scan step size: Λ=m×λ B / (2n eff The scanning step size is the first-order period of the fiber grating being written, where m is the diffraction order and λ is the diffraction order. B n is the Bragg wavelength of the target fiber grating. eff Δθ represents the effective refractive index of the optical fiber to be processed. In practical applications, Δθ can be 90° or 120°, or any value between 1° and 10°. When Δθ is a small angle, such as any value between 1° and 10°, the polarization sensing axis rotates in a quasi-continuous smooth spiral along the fiber axis, which helps reduce the insertion loss of the fiber grating and improve the linearity of the torsion sensing. When Δθ is a large angle, such as 90° or 120°, discrete polarization spatial coding can be achieved, which is suitable for applications such as optical fiber identification.

[0042] In the second embodiment, the linear polarization direction of the femtosecond laser pulse processing beam in step S4, when rotated around the fiber axis of the fiber to be processed, can also satisfy: the linear polarization direction angle θ at the nth step processing point. n =θ0+n×θ(n), where θ0 is the initial polarization angle, n is the sequence number of the step processing point, and θ(n) is the preset polarization rotation angle that changes with the nth step processing point.

[0043] In a specific example of the second embodiment, the preset polarization rotation angle θ(n) chirps along the axial direction with the nth step processing point, for example, increasing linearly from 10° to 50°, so that the increment of the polarization rotation angle between adjacent step processing points changes continuously along the fiber axial direction. As a result, the fiber grating has a non-uniform spatially encoded polarization sensitive axis distribution, that is, the rotation rate of the polarization sensitive axis around the fiber axis (the increment of the polarization direction rotation angle per unit length) changes monotonically along the axial direction. This is equivalent to the polarization encoding pitch being gradually distributed along the fiber length direction, rather than a periodic encoding structure with a fixed pitch. This results in a special grating with a non-periodic polarization sensitive axis distribution, which can be used for specific polarization mode conversion or broadband polarization filtering.

[0044] In the third embodiment, after obtaining the target fiber grating in step S5, step S6 can also be implemented to perform thermal annealing on the target fiber grating. The annealing temperature is 50°C to 450°C and the annealing time is 2 to 6 hours.

[0045] exist Figure 2 In one embodiment shown, a fabrication apparatus for fiber Bragg gratings is provided, the apparatus comprising:

[0046] The multi-dimensional precision displacement platform 1 is equipped with a clamp for holding and fixing the optical fiber to be processed, and for moving the optical fiber along its axial direction.

[0047] Femtosecond laser source 2 is used to generate femtosecond laser pulse beams;

[0048] Polarization module 3 is used to generate linearly polarized femtosecond laser pulses for processing beams and to rotate the direction of linear polarization.

[0049] Focusing objective 4 is used to focus the femtosecond laser pulse processing beam onto the stepping processing point. The numerical aperture of focusing objective 4 is ≥0.65.

[0050] The control device 5 can be in different forms such as a programmable logic controller, an industrial computer, or an embedded controller. In this embodiment, a programmable logic controller is used to control the multidimensional precision displacement platform 1 to move, control the femtosecond laser source 2 to generate a femtosecond laser pulse beam, control the polarization module 3 to generate a linearly polarized femtosecond laser pulse processing beam, and control the femtosecond laser pulse processing beam to focus and irradiate the fiber core of the optical fiber to be processed at each step processing point, and control the linear polarization direction of the femtosecond laser pulse processing beam to rotate around the fiber axis of the optical fiber to be processed.

[0051] exist Figure 3In one embodiment shown, the polarization module 3 may consist of a spatial light modulator 31 and a liquid crystal variable polarization rotator 32. The spatial light modulator 31 shapes the femtosecond laser pulse beam generated by the femtosecond laser source 2 into a linearly polarized femtosecond laser pulse processing beam. The liquid crystal variable polarization rotator 32 rotates the linear polarization direction of the femtosecond laser pulse processing beam according to the instructions of the control device 5.

[0052] exist Figure 4 In another embodiment shown, the polarization module 3 can also be composed of a spatial light modulator 31 loaded with a vector beam phase map. The spatial light modulator 31 directly generates a vector beam with arbitrary polarization state distribution. The control device 5 controls the phase delay of different pixel regions on the spatial light modulator 31. Before the femtosecond laser pulse processing beam is focused by the focusing objective lens 4, a light field with polarization direction changing with spatial coordinates is generated. The equivalent polarization rotation is achieved in conjunction with the movement of the multidimensional precision displacement platform 1.

[0053] The following is Figure 3 A specific example of one embodiment is shown.

[0054] In this example, the fiber to be processed is a commercial single-mode fiber of model Corning SMF-28e. Approximately 5 cm of the coating layer was removed using fiber strippers, and then the exposed cladding surface was cleaned with anhydrous ethanol. The processed fiber was placed in a vacuum adsorption fixture on a multi-dimensional precision displacement platform 1, and a slight preload was applied to keep it straight, ensuring that the parallelism between the fiber axis and the X-axis (scanning axis) of the displacement platform was better than 0.1 degrees.

[0055] Next, optical path calibration and polarization initialization are performed. The femtosecond laser source 2 is activated to generate a femtosecond laser pulse beam with typical parameters: center wavelength 800nm, pulse width 120fs, and repetition frequency 1kHz. The femtosecond laser pulse beam is expanded and collimated before being incident on the polarization module 3. The polarization module 3 consists of a spatial light modulator 31 and a transmissive liquid crystal variable polarization rotator 32 connected in series. The spatial light modulator 31 is used to correct optical path aberrations and generate a linearly polarized femtosecond laser pulse processing beam, while the liquid crystal variable polarization rotator 32 is used to perform rapid polarization rotation. The initial angle of the liquid crystal variable polarization rotator 32 is set to 0° by the control device 5. This 0° is used as the initial reference orientation for the polarization direction, and the polarization rotation angle of all subsequent processing points is accumulated based on this initial direction.

[0056] Then, the point-by-point scanning and writing stage begins. Control device 5 sends commands to the multi-dimensional precision displacement platform 1, causing it to move the optical fiber to be processed along the X-axis at a fixed, preset scanning step size Λ. For the Bragg wavelength λ... BThe target fiber grating has an effective refractive index n of 1550nm in a commercial single-mode fiber of model Corning SMF-28e. eff If ≈1.468, then the first-order period of the target fiber grating is Λ=m×λ. B / (2n eff The wavelength is approximately 528 nm. After each step movement is completed and stabilized, the control device 5 sends a trigger signal to the femtosecond laser source 2, outputting a single-pulse femtosecond laser pulse beam with an energy of 80 nJ. The spatial light modulator 31 then generates a linearly polarized femtosecond laser pulse processing beam. The femtosecond laser pulse processing beam is focused by the focusing objective lens 4 (numerical aperture of 0.8) onto the step processing point of the fiber core to be processed, irradiating the fiber core and inducing a refractive index modulation point.

[0057] The key step in this example is that, simultaneously with each pulse trigger, the control device 5 sends a rotation command to the liquid crystal variable polarization rotator 32, causing its polarization direction to rotate relative to the previous pulse by a fixed preset polarization rotation angle Δθ. In this example, θ0 = 0° and Δθ = 30° are selected. This means that at the initial processing point, the polarization direction of the femtosecond laser pulse processing beam is 0°, while at the first step processing point, the linear polarization angle of the femtosecond laser pulse processing beam is 30°, at the second step processing point it is 60°, and so on, until the 12th step processing point, when the linear polarization angle of the femtosecond laser pulse processing beam reaches 360°, completing a full polarization rotation cycle. The spatial length L of this polarization rotation cycle is... pol =12×Λ≈6.34μm. Repeat the above process of moving, rotating, and triggering until the target fiber grating of the required length is written onto the fiber to be processed. For example, a target fiber grating of 6mm in length will contain approximately 11360 grating dots, corresponding to approximately 946 complete polarization rotation cycles. After processing, post-processing is performed. The target fiber grating is removed from the multi-dimensional precision displacement platform 1 and placed in an annealing furnace. For silica fiber, annealing is carried out at 200°C for 4 hours, followed by slow cooling to room temperature at a rate of 10°C / hour. This process effectively eliminates laser-induced residual stress, stabilizes refractive index modulation, and makes the polarization sensitive axis more stable.

[0058] The following is a performance test of the fiber Bragg grating prepared in this example.

[0059] (1) A fiber grating with a spatially encoded polarization sensitive axis, a center wavelength of 1550 nm, a length of 6 mm, and a Δθ=30°, was prepared using the above specific example. It was then used as the fiber grating under test to test its polarization correlation loss when applied to a polarization-dependent reflective filter and its torsional sensing performance when applied to a torsion sensor.

[0060] Polarization-dependent loss testing: A tunable laser (1520~1580nm) and a polarization controller were used as the test light source. The laser was connected to the fiber grating under test (FTBR) via a circulator, and the reflection spectrum was monitored using a spectrometer. When the incident polarization direction was parallel to the polarization-sensitive axis at the starting end of the FTBR (0°), the peak reflectivity reached 98%. When the incident polarization direction was rotated by 90°, the peak reflectivity decreased to less than 15%, and the polarization-dependent loss was greater than 15dB. This result indicates that the FTBR under test can be directly used as a high-performance polarization-dependent filter or an all-fiber polarizer.

[0061] Testing the torsion sensing performance: The fiber Bragg grating under test (FBG) was tightly bonded to the surface of a 5mm diameter aluminum torsion bar, ensuring that the FBG was parallel to the axis of the aluminum torsion bar. One end of the FBG was connected to a broadband light source and a spectrometer via a circulator for monitoring the reflection spectrum, while the other end was an optical terminal (not connected to any device). The entire FBG rotated synchronously with the aluminum torsion bar. A precise torsion angle was applied to the aluminum torsion bar using a precision rotary stage, starting from 0° and increasing by 5° each time until reaching 60°. Within the torsion range of 0° to 30°, the reflected light intensity decreased linearly with the torsion angle, achieving a sensitivity of 0.15dB / °, far exceeding that of conventional uniform Bragg gratings (typically <0.01dB / °). By determining the slope direction of the reflected light intensity change, clockwise or counterclockwise torsion could be distinguished. This result demonstrates that the FBG prepared using this method and apparatus can serve as a highly sensitive, directionally discernible torsion sensor.

[0062] (2) Fiber Bragg gratings with spatially coded polarization sensitive axes are used for optical fiber identification with spatial polarization coding.

[0063] Using the same fabrication apparatus, different polarization rotation coding sequences can be used to write uncopyable identity identifiers into optical fibers.

[0064] For the first type of fiber A, Δθ = 0° (polarization direction does not rotate), corresponding to the encoding sequence "0". For the second type of fiber B, Δθ = 30° (polarization direction rotates at a fixed angle), corresponding to the encoding sequence "1". For the third type of fiber C, a four-step cyclic encoding is set: the first 4 grating points Δθ = 0°, the next 4 grating points Δθ = 30°, the next 4 grating points Δθ = 60°, and the last 4 grating points Δθ = 90°, and this cycle is repeated.

[0065] After fabrication, a handheld polarization analyzer was connected to the fiber optic tail to measure the polarization state distribution of its reflected light. The polarization state of the reflected light from fiber A (type 1) was constant; the polarization state of the reflected light from fiber B (type 2) exhibited a periodic rotational change; and the polarization state of the reflected light from fiber C (type 3) displayed a four-step cyclical characteristic. By reading the polarization state distribution, the unique identity of the fiber can be identified. This scheme provides a novel technical means for anti-counterfeiting and physical layer security authentication of fiber optic network equipment.

[0066] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for fabricating a fiber Bragg grating, characterized in that, Includes the following steps: Step S1: Clamp and fix the optical fiber to be processed onto the movable carrier; Step S2: Generate a linearly polarized femtosecond laser pulse processing beam; Step S3: Control the optical fiber to be processed to step along its axis point by point with a preset scanning step size, and focus the femtosecond laser pulse processing beam on the core of the optical fiber to be processed at each step processing point to irradiate it, forming a local refractive index modulation structure. Step S4: During the point-by-point processing, the linear polarization direction of the femtosecond laser pulse processing beam is synchronously controlled to rotate around the fiber axis of the fiber to be processed, so that the linear polarization direction of the next processing point has a preset polarization rotation angle relative to the linear polarization direction of the previous processing point. Step S5: Continue the step-by-step processing until the fiber grating of the preset length is completed, and obtain the target fiber grating with a spatially encoded polarization sensitive axis distributed along the fiber axis.

2. The method for fabricating a fiber Bragg grating according to claim 1, characterized in that, The linear polarization direction of the femtosecond laser pulse processing beam described in step S4 rotates around the fiber axis to satisfy: the linear polarization direction angle θ at the nth step processing point. n =θ0+n×Δθ, where θ0 is the initial polarization angle, Δθ is a fixed preset polarization rotation angle, and Δθ is not equal to 180°.

3. The method for fabricating a fiber Bragg grating according to claim 2, characterized in that, N×Δθ=360°, where N is a positive integer greater than 2, and L is the polarization rotation period length. pol =N×Λ, where Λ is the preset scan step size: Λ=m×λ B / (2n eff ), where m is the diffraction order, λ B n is the Bragg wavelength of the target fiber grating. eff The effective refractive index of the optical fiber to be processed is given.

4. The method for fabricating a fiber Bragg grating according to claim 1, characterized in that, The linear polarization direction of the femtosecond laser pulse processing beam described in step S4 rotates around the fiber axis of the fiber to be processed, satisfying the following: the linear polarization direction angle θ at the nth step processing point. n =θ0+n×θ(n), where θ0 is the initial polarization angle, n is the sequence number of the step processing point, and θ(n) is the preset polarization rotation angle that changes with the nth step processing point.

5. The method for fabricating a fiber Bragg grating according to claim 1, characterized in that, Step S5 is followed by step S6, in which the target fiber grating is subjected to thermal annealing at a temperature of 50°C to 450°C for 2 to 6 hours.

6. An apparatus for fabricating fiber Bragg gratings, characterized in that, include: A multi-dimensional precision displacement platform is equipped with clamps for holding and fixing the optical fiber to be processed, and for moving the optical fiber along its axial direction. Femtosecond laser source, used to generate femtosecond laser pulse beams; The polarization module is used to generate linearly polarized femtosecond laser pulses to process the beam and rotate the direction of linear polarization. The focusing objective lens is used to focus the femtosecond laser pulse processing beam onto the stepping point to irradiate the core of the optical fiber to be processed. The control device is used to control the multidimensional precision displacement platform to move, control the femtosecond laser source to generate a femtosecond laser pulse beam, control the polarization module to generate a linearly polarized femtosecond laser pulse processing beam, and control the femtosecond laser pulse processing beam to focus and irradiate the fiber core of the optical fiber to be processed at each step processing point, and control the linear polarization direction of the femtosecond laser pulse processing beam to rotate around the fiber axis of the optical fiber to be processed.

7. The apparatus for fabricating a fiber Bragg grating according to claim 6, characterized in that, The polarization module consists of a spatial light modulator and a liquid crystal variable polarization rotator. The spatial light modulator shapes the femtosecond laser pulse beam generated by the femtosecond laser source into a linearly polarized femtosecond laser pulse processing beam. The liquid crystal variable polarization rotator rotates the linear polarization direction of the femtosecond laser pulse processing beam according to the instructions of the control device.

8. The apparatus for fabricating a fiber Bragg grating according to claim 6, characterized in that, The polarization module consists of a spatial light modulator loaded with a vector beam phase map. The spatial light modulator directly generates a vector beam with arbitrary polarization state distribution. The control device controls the phase delay of different pixel regions on the spatial light modulator. Before the femtosecond laser pulse processing beam is focused by the focusing objective, a light field with polarization direction changing with spatial coordinates is generated. This, combined with the movement of the multidimensional precision displacement platform, achieves equivalent polarization rotation.

9. The apparatus for fabricating a fiber Bragg grating according to claim 6, 7, or 8, characterized in that, The numerical aperture of the focusing objective is ≥0.65.

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