A method and device for manufacturing fiber grating and fiber grating

Abstract

The embodiment of the application discloses a preparation method and device of a fiber grating and the fiber grating, converts a pulsed femtosecond laser beam into a femtosecond laser beam of a double-pulse sequence composed of a seed pulse and a processing pulse with a specific time delay, fully utilizes inherent electron relaxation characteristics of a material, pre-excites transient free electrons by the seed pulse below an intrinsic nonlinear ionization threshold of the material and without permanent modification, exposes the material to the processing pulse within an electron energy relaxation time, and significantly reduces an effective processing threshold by using a transient absorption enhancement effect; compared with a traditional technical method, the single pulse energy injection required for stable refractive index modification is reduced by 10%-50%, over-ionization and thermal diffusion are fundamentally inhibited, and the insertion loss of the fiber grating with a reflectivity greater than 95% can be reduced to below 0.1 dB.

Description

Technical Field

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

[0002] Currently, the mainstream fabrication method for fiber gratings is femtosecond laser direct writing, including single-pulse point-by-point processing and equal-energy multi-pulse cumulative processing. In single-pulse processing, to reach the nonlinear ionization threshold, the required pulse energy density is typically high. This leads to excessive ionization within the focusing volume (far exceeding the minimum free electron density required for stable modification), forming a high-density free electron plasma. Subsequently, electron-phonon coupling causes local lattice overheating, resulting in an increase in scattering centers and insertion loss. This contradiction is particularly prominent in transmission-type high-reflectivity gratings. In equal-energy multi-pulse processing, the preceding pulses randomly alter the material's absorption characteristics (e.g., generating defects or localized heating), causing uncontrollable fluctuations in the actual absorbance of subsequent pulses. This results in poor repeatability of the modified region size and refractive index modulation, making it difficult to meet the requirements of precision devices.

[0003] Chinese invention patent application CN117092739A discloses a fabrication scheme for overlapping fiber Bragg gratings. The essence of this scheme is to replace the photomask and sequentially etch two gratings with similar periods at the same location in the fiber, physically superimposing them to form a beat frequency structure. This results in multiple additional equally spaced reflection peaks in addition to their individual Bragg reflection peaks. Therefore, the core purpose of this method is to achieve multi-wavelength filtering using the grating superposition effect, rather than solving the insertion loss or repeatability problems caused by single-pulse or multi-pulse processing. Summary of the Invention

[0004] This invention provides a method, apparatus and a fiber grating for fabricating fiber gratings. The fiber gratings fabricated have significantly reduced insertion loss and can be precisely controlled in terms of refractive index modulation.

[0005] In a first aspect, the present invention provides a method for fabricating a fiber Bragg grating, comprising the following steps: S1: Fix the optical fiber to be processed and make the axis of the optical fiber perpendicular to the propagation direction of the femtosecond laser beam; S2: A femtosecond laser source generates a pulsed femtosecond laser beam with a fixed repetition frequency. The pulsed femtosecond laser beam is converted into a femtosecond laser beam consisting of a seed pulse with a time delay and the same pulse width but different energies and a subsequent processing pulse. The energy densities of the seed pulse and the processing pulse are both lower than the intrinsic nonlinear ionization threshold of the optical fiber to be processed, and the energy density of the processing pulse is higher than the energy density of the seed pulse. S3: Set the time delay between the seed pulse and the processing pulse to be less than the energy relaxation time of the photoexcited free electrons in the optical fiber material to be processed; S4: Focus the femtosecond laser beam of the dual-pulse sequence onto the core of the optical fiber to be processed, and expose each preset processing point of the entire grating along the optical fiber axis according to the preset grating period to complete one round of scanning, so that each preset processing point can enhance nonlinear absorption by means of the seed pulse, thereby generating local refractive index modulation. S5: Repeatedly perform multiple scans on the whole grating as a unit, so that the refractive index modulation of each preset processing point is accumulated round by round until a complete periodic refractive index modulation structure is formed, thereby obtaining the target fiber grating.

[0006] In one feasible implementation, the time delay between the seed pulse and the processing pulse set in step S3 can be 100fs to 10000fs, preferably 500fs.

[0007] In one feasible implementation, when exposing a preset processing point corresponding to each grating period point by point, a femtosecond laser beam with a dual-pulse sequence can be repeatedly applied to the preset processing point by means of multiple reset processing, and the number of repetitions is 1 to 10 times. Preferably, it is 2 times.

[0008] In one feasible implementation, the energy density of the seed pulse can be 30% to 70% of the intrinsic nonlinear ionization threshold of the optical fiber to be processed, and the energy density of the processing pulse can be 50% to 90% of the intrinsic nonlinear ionization threshold of the optical fiber to be processed.

[0009] In one feasible implementation, the center wavelength of the femtosecond laser beam in the dual-pulse sequence is 1030 nm, the single pulse width is 200 fs to 500 fs, and the repetition frequency is 1 kHz to 1 MHz.

[0010] In one feasible implementation, the optical fiber to be processed can be any one of chalcogenide optical fiber, fluoride optical fiber, tellurate optical fiber, rare earth ion doped optical fiber, high germanium doped optical fiber, and pure silica optical fiber.

[0011] In one feasible implementation, the point-by-point exposure in step S4 is achieved by emitting the femtosecond laser beam of the dual-pulse sequence at a repetition frequency f matching the grating period Λ, and moving the optical fiber to be processed at a constant speed v, where v = Λ × f.

[0012] Secondly, the present invention provides a fabrication apparatus for realizing the above-described method for fabricating fiber Bragg gratings, comprising: Femtosecond laser sources are used to generate pulsed femtosecond laser beams with a fixed repetition frequency; The pulse beam splitting and delay module is used to convert the pulsed femtosecond laser beam into a femtosecond laser beam consisting of a pre-sequence seed pulse with time delay, the same pulse width, and different energies, and a subsequent processing pulse. A focusing objective lens is used to focus a femtosecond laser beam of a dual-pulse sequence onto the core of the optical fiber to be processed. The motion and control module is used to fix the optical fiber to be processed. According to the preset grating period and the repetition frequency of the femtosecond laser pulse, it controls the moving speed of the optical fiber to be processed, so that the spacing between the exposure points of adjacent laser pulses on the fiber core is equal to the grating period. According to the preset grating period and the moving speed of the optical fiber, it sends a synchronous trigger signal to the femtosecond laser source to ensure that each double pulse sequence corresponds exactly to a grating period position. After the optical fiber to be processed completes the writing of a whole grating structure, it quickly resets to the starting position and repeats multiple rounds of scanning, so that the refractive index modulation depth of each processing point accumulates round by round until a complete periodic refractive index modulation structure is formed, and the target fiber grating is obtained.

[0013] In one feasible implementation, the pulse beam splitting and delay module comprises a beam splitting module, a first energy adjustment component, a second energy adjustment component, a delay module, and a beam combining module. The beam splitting module splits the pulsed femtosecond laser beam into a seed beam and a processing beam. The seed beam enters the first energy adjustment component for energy adjustment, and the processing beam enters the second energy adjustment component for energy adjustment before entering the delay module for delay. The seed beam emitted from the first energy adjustment component and the processing beam emitted from the delay module respectively enter the beam combining module to obtain a dual-pulse sequence of femtosecond laser beams, which then enter the focusing objective. The focusing objective can be a 60x oil immersion objective with a numerical aperture of 1.42.

[0014] Thirdly, the present invention provides a fiber grating prepared by any of the above-mentioned preparation methods, wherein when the reflectivity is greater than 95%, the insertion loss is less than 0.1 dB, and the refractive index modulation is linearly related to the number of femtosecond laser beams of the double-pulse sequence applied to the processing point corresponding to each grating period.

[0015] Compared with existing technologies, the advantages of this invention are as follows: It utilizes a seed pulse lower than the intrinsic nonlinear ionization threshold of the material and without causing permanent modification to pre-excite transient free electrons (i.e., seed electrons) in the material, and exposes the material with a processing pulse during the electron energy relaxation time. The transient absorption enhancement effect significantly reduces the effective processing threshold, thereby achieving stable and controllable refractive index modulation with lower single-pulse energy injection, reducing over-ionization and scattering losses. In particular, when the time delay is preferably in the range of 500 fs to 1000 fs, the effective processing threshold of the processing pulse can be reduced by 10% to 50%. Under the conditions that the seed pulse energy is 30% to 70% of the threshold, the processing pulse is 50% to 90% of the threshold, the time delay is 500 to 1000 fs, and no more than 3 femtosecond laser beams are applied per grating cycle, a reflectivity >95% and an insertion loss <0.1 dB can be achieved, which is lower than the grating insertion loss obtained by traditional single-pulse processing methods. By performing multiple scans on the entire grating, a femtosecond laser beam with a sequence of 1 to 10 double pulses is applied sequentially to the corresponding processing point of each grating period. This process allows for linear control of the refractive index modulation while maintaining extremely low insertion loss. Since each scan is a unified update of the entire fiber state, rather than a continuous accumulation of pulses at a single point, it effectively avoids the random fluctuations in absorption characteristics caused by applying multiple pulses continuously at a single point. Because each point receives only one set of double pulses in each scan, and the cumulative effect of multiple scans exhibits a good linear relationship, it is easy to control precisely. The fiber grating prepared by this method exhibits good refractive index modulation uniformity, an optimal insertion loss of less than 0.1 dB, and a reflectivity higher than 95%.

[0016] The technical effects achieved by this invention are at least the following aspects: Actively reducing the processing threshold: By converting a single-pulse femtosecond laser beam into a dual-pulse sequence of a seed pulse and a processing pulse with specific time delays, the inherent electronic relaxation properties of the material are fully utilized. The seed pulse pre-excites free electrons, significantly improving the equivalent nonlinear absorption coefficient of the processing pulse. Compared to traditional methods, this reduces the single-pulse energy injection required for stable refractive index modification by 10%–50%. This fundamentally suppresses over-ionization and thermal diffusion, enabling the insertion loss of fiber gratings with reflectivity >95% to be reduced to below 0.1 dB.

[0017] Avoiding random accumulation of multiple pulses: The modification of each grating period is independently accomplished by one or more precisely controllable femtosecond laser beams in a double-pulse sequence, without relying on the random accumulation of equal-energy multiple pulses. The refractive index modulation exhibits a good linear relationship with the number of applied femtosecond laser beams in the double-pulse sequence, with a goodness of fit R. 2 >0.99, significantly improving repeatability.

[0018] Universality: Applicable to a variety of materials such as chalcogenide optical fiber, fluoride optical fiber, tellurate optical fiber, rare earth ion doped optical fiber, highly germanium-doped optical fiber, or pure silica optical fiber. Attached Figure Description

[0019] Figure 1 This is a process flow diagram of a fiber Bragg grating fabrication method provided in an embodiment of the present invention; Figure 2 This is a structural connection block diagram of a preparation device provided in an embodiment of the present invention; Figure 3 The curve showing the relationship between time delay and effective processing threshold in the fiber grating fabrication method provided in Example 1 of this embodiment of the invention was obtained through actual measurement. Figure 4 This is a curve showing the relationship between the refractive index modulation and the number of times the femtosecond laser beam of the applied double-pulse sequence is applied in the fiber grating fabrication method provided in Example 2 of this invention. Figure 5 This describes the relationship between insertion loss and the number of times a femtosecond laser beam with a double-pulse sequence is applied in the fiber grating fabrication method provided in Example 2 of this invention. Figure 6 This describes the relationship between reflectivity and the number of times a femtosecond laser beam with a double-pulse sequence is applied in the fiber grating fabrication method provided in Example 2 of this invention.

[0020] Explanation of icon numbers: 1. Femtosecond laser source; 2. Pulse beam splitting and delay module; 21. Beam splitting module; 22. First energy adjustment component; 23. Second energy adjustment component; 24. Delay module; 25. Beam combining module; 3. Focusing objective lens; 4. Motion and control module. Detailed Implementation

[0021] To address the problems of the prior art, embodiments of the present invention provide a method, apparatus, and grating for fabricating a transmissive high reflectivity fiber grating.

[0022] Below, we will first introduce a method for fabricating a fiber Bragg grating provided in an embodiment of the present invention.

[0023] Figure 1 A schematic 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: S1: Fix the optical fiber to be processed and make the axis of the optical fiber perpendicular to the propagation direction of the femtosecond laser beam; S2: Then, a femtosecond laser source generates a pulsed femtosecond laser beam with a fixed repetition frequency, and converts the pulsed femtosecond laser beam into a double-pulse sequence of femtosecond laser beams consisting of a seed pulse with a time delay and the same pulse width but different energies and a subsequent processing pulse. The energy density of the seed pulse can be 30% to 70% of the intrinsic nonlinear ionization threshold of the fiber material to be processed, and the energy density of the processing pulse can be 50% to 90% of the intrinsic nonlinear ionization threshold of the fiber material to be processed. Both are lower than the intrinsic nonlinear ionization threshold of the fiber material to be processed, and the energy density of the processing pulse is higher than that of the seed pulse. When the seed pulse acts alone, it will not cause a permanent change in the refractive index of the fiber material to be processed. However, the processing pulse, with the assistance of transient free electrons excited by the seed pulse, can cause a permanent change in the refractive index by enhancing nonlinear absorption. The intrinsic nonlinear ionization threshold of the fiber material is an inherent property of the material and can be measured by the femtosecond pump-probe combined plasma extrapolation method in the prior art.

[0024] S3: Set the time delay between the seed pulse and the processing pulse to be less than the energy relaxation time of the photoexcited free electrons in the optical fiber material to be processed. It can be 100fs~10000fs, preferably 500fs, so that the transient free electrons excited by the seed pulse still maintain an effective number density when the processing pulse arrives. S4: Focus the femtosecond laser beam of the dual-pulse sequence onto the core of the optical fiber to be processed, and expose each preset processing point of the entire grating along the fiber axis according to the preset grating period to complete one round of scanning. This allows each preset processing point to enhance nonlinear absorption by means of the seed pulse, thereby generating local refractive index modulation. S5: Repeatedly perform multiple scans on the whole grating as a unit, so that the refractive index modulation of each preset processing point is accumulated round by round until a complete periodic refractive index modulation structure is formed, thereby obtaining the target fiber grating.

[0025] In one embodiment, when exposing each preset processing point corresponding to each grating period point point by point, a femtosecond laser beam with a double pulse sequence is repeatedly applied to the preset processing point by means of multiple reset processing, and the number of repetitions is 1 to 10.

[0026] In one embodiment, the center wavelength of the femtosecond laser beam in the dual-pulse sequence is 1030 nm, the width of a single pulse is 200 fs to 500 fs, and the repetition frequency is 1 kHz to 1 MHz.

[0027] In one embodiment, the optical fiber to be processed can be any one of chalcogenide optical fiber, fluoride optical fiber, tellurate optical fiber, rare earth ion doped optical fiber, high germanium doped optical fiber, and pure silica optical fiber.

[0028] In one embodiment, the point-by-point exposure in step S4 is achieved by emitting a femtosecond laser beam of a dual-pulse sequence at a repetition frequency f that matches the grating period Λ, and moving the optical fiber to be processed at a constant speed v, where v = Λ × f, thereby ensuring that the optical fiber receives exactly one set of femtosecond laser beams of a dual-pulse sequence for each grating period Λ it moves.

[0029] Figure 2 An apparatus for fabricating the above-described fiber Bragg grating according to an embodiment of the present invention is shown, comprising: Femtosecond laser source 1 is used to generate a pulsed femtosecond laser beam with a fixed repetition frequency; The pulse beam splitting and delay module 2 is used to convert the pulsed femtosecond laser beam into a femtosecond laser beam consisting of a seed pulse with time delay and the same pulse width but different energies and a subsequent processing pulse. Focusing objective 3 is used to focus a femtosecond laser beam of a dual-pulse sequence onto the core of the optical fiber to be processed; The motion and control module 4 is used to fix the fiber to be processed. Based on the preset grating period and the repetition frequency of the femtosecond laser pulses, it controls the moving speed of the fiber, ensuring that the spacing between exposure points on the fiber core of adjacent laser pulses equals the grating period. According to the preset grating period and fiber moving speed, it sends a synchronous trigger signal to the femtosecond laser source to ensure that each double-pulse sequence corresponds exactly to a grating period position. After completing the writing of a complete grating structure, the module controls the fiber to be processed to quickly reset to the starting position and repeat multiple scans, accumulating the refractive index modulation depth at each processing point until a complete periodic refractive index modulation structure is formed, thus obtaining the target fiber grating. The motion and control module 4 typically ensures that the repeatability of the motion platform itself is on the order of hundreds of nanometers, which sufficiently guarantees the accuracy of the reset.

[0030] In one embodiment, the pulse beam splitting and delay module 2 may consist of a beam splitting module 21, a first energy adjustment component 22, a second energy adjustment component 23, a delay module 24, and a beam combining module 25. The beam splitting module 21 splits the pulsed femtosecond laser beam into a seed beam and a processing beam. The seed beam enters the first energy adjustment component 22 for energy adjustment, and the processing beam enters the second energy adjustment component 23 for energy adjustment before entering the delay module 24 for delay. The seed beam emitted from the first energy adjustment component 22 and the processing beam emitted from the delay module 24 enter the beam combining module 25 respectively to obtain a dual-pulse sequence of femtosecond laser beams, which enter the focusing objective lens 3. The focusing objective lens 3 may be a 60x oil immersion objective lens with a numerical aperture of 1.42.

[0031] This invention actively reduces the effective processing threshold by converting a single-pulse femtosecond laser beam into a dual-pulse sequence of femtosecond laser beams consisting of a preceding seed pulse with a specific time delay, the same pulse width, and unequal energies. The preceding seed pulse pre-excites transient free electrons in the material. When the time delay is set within the range of 100 fs to 10000 fs, particularly preferably within the range of 500 fs to 1000 fs, the subsequent processing pulse can fully utilize the unrelaxed seed electrons to enhance nonlinear absorption. This achieves a stable and controllable permanent refractive index change at an energy density far below the material's intrinsic nonlinear ionization threshold, with a threshold reduction of 10% to 50%.

[0032] In this invention, the modification of each grating period is independently achieved by one or several precisely controllable femtosecond laser beams in a double-pulse sequence, without relying on the random accumulation of multiple pulses of equal energy. The refractive index modulation exhibits a good linear relationship with the number of applied femtosecond laser beams in the double-pulse sequence, with a goodness of fit R² > 0.99. The refractive index modulation can be linearly controlled by adjusting the number of scanning rounds, significantly improving process repeatability.

[0033] The following are some specific examples of embodiments of the present invention.

[0034] Example 1: The optical fiber to be processed is pure silica fiber, using a femtosecond laser source 1 with a center wavelength of 1030nm, a single pulse width of 300fs, and a repetition frequency of 100kHz. The pulsed femtosecond laser beam generated by the femtosecond laser source 1 is converted into a dual-pulse sequence femtosecond laser beam by a pulse splitting and delay module 2. This sequence consists of a seed pulse with the same pulse width but different energies and a time delay, followed by a processing pulse. The seed pulse energy density is fixed at 50% of the intrinsic nonlinear ionization threshold of the pure silica fiber material, and the processing pulse energy density is fixed at 70% of the intrinsic nonlinear ionization threshold. The time delay is set to a preferred value of 700fs. A 60x oil immersion objective lens with a numerical aperture of 1.42 is used to focus the dual-pulse sequence femtosecond laser beam onto the core of the single-mode pure silica fiber. The motion and control module 4 moves the fiber at a constant speed v = Λ × f, with a grating period Λ = 535nm and a moving speed of 53.5mm / s. After exposing the entire grating point by point to complete one scan, the system is quickly reset and the scan is repeated for a total of two rounds. That is, each grating cycle applies a femtosecond laser beam with a double-pulse sequence twice to the processing point. The resulting fiber grating has a reflectivity of 98.5% and an insertion loss of 0.06 dB.

[0035] Regarding the relationship between time delay and effective processing threshold: To further verify the impact of time delay on the effective processing threshold, while keeping the seed pulse energy density (50% threshold) constant, only the time delay between the two pulses was changed. The percentage of the effective energy density required for the processing pulse to just cause a permanent change in refractive index under each delay was measured relative to the single-pulse threshold. The single-pulse processing threshold without seed pulse assistance was used as the 100% baseline. Figure 3 The measured relationship curve is shown. The horizontal axis of the graph represents the time delay value, and the vertical axis represents the effective processing threshold expressed as a percentage. As can be seen from the graph, the entire delay interval can be divided into three characteristic regions: 1. Near-zero latency region (0~100fs): The effective processing threshold is almost not reduced.

[0036] When the delay is 0 fs, the seed pulse and the processing pulse almost overlap into a single pulse, resulting in an effective processing threshold as high as 100%, which is almost identical to single-pulse processing. The auxiliary absorption effect of the seed electron is not manifested at all, and the processing window is not widened in any way. Even with a delay of 50 fs, the effective processing threshold remains as high as 95%, indicating that the seed electron has not yet formed an effective auxiliary absorption state under extremely short delays.

[0037] 2. Effective processing threshold drop zone (100~500fs): The auxiliary effect is rapidly enhanced.

[0038] As the delay gradually increases, the transient free electrons excited by the seed pulse undergo relaxation processes such as electron-electron scattering, gradually forming a hot electron distribution suitable for assisted enhancement of nonlinear absorption, and the effective processing threshold begins to decrease significantly. At 100 fs, the effective processing threshold drops to 87%, entering the achievable delay range (100 fs~10000 fs). By 500 fs, the effective processing threshold has dropped to 63%, a decrease of 36%.

[0039] 3. Optimal platform zone (500~1000fs): Strongest auxiliary effect.

[0040] Within the preferred time range of 500 fs to 1000 fs, the seed electrons are sufficiently relaxed without significant energy relaxation, resulting in the strongest auxiliary absorption effect. The effective processing threshold decreases to 55%–63%, a reduction of 37%–45%, falling within the optimal range of 10%–50%. The lowest effective processing threshold occurs around 700 fs (55%), indicating that the electron number density and energy distribution of the seed pulse are most ideal at this time delay.

[0041] 4. Relaxation recovery zone (1000fs~10000fs): The auxiliary effect gradually weakens.

[0042] When the delay exceeds 1000 fs, some electrons in the seed pulse begin to relax energy through electron-phonon coupling, resulting in a gradual decrease in effective number density, a weakening of the auxiliary effect, and a slow recovery of the effective processing threshold. In the 1000-1500 fs range, the threshold gradually increases from 63% to 70%. From 1500-5000 fs, the threshold gradually recovers from 70% to 90%. In the 5000-10000 fs range, the recovery rate slows further, and the threshold asymptotically approaches 97% from 90%, nearing the level of a single pulse. The data point encryption clearly demonstrates this asymptotic recovery trend, indicating that the remaining auxiliary effect of the seed pulse electrons continuously decays with increasing delay, essentially exhausting itself by 10000 fs.

[0043] These results demonstrate that only with an appropriate time delay, especially within the preferred range of 500 fs to 1000 fs, can a significant and stable reduction in the effective processing threshold be achieved. Too small a delay results in insufficient establishment of the auxiliary effect, while too large a delay leads to the gradual loss of the auxiliary effect.

[0044] Example 2: Under the preferred conditions of Example 1 (pure silica fiber, seed pulse threshold of 50%, processing pulse threshold of 70%, delay of 700 fs), by changing the total number of scans (i.e., changing the number N of femtosecond laser beams with a double-pulse sequence applied per grating period), the refractive index modulation Δn and the corresponding insertion loss of the resulting fiber grating are measured to verify the linear controllability and loss trade-off characteristics of the present invention. The refractive index modulation Δn can be obtained by measuring the reflection spectrum of the grating and using coupled-mode theory.

[0045] Figure 4 The relationship between the refractive index modulation Δn and the number of pulses applied in a dual-pulse sequence of femtosecond laser beams is shown. Figure 4 The horizontal axis represents the number of refractive indexes, and the vertical axis represents the refractive index modulation degree Δn. Figure 5 The figure shows the relationship between insertion loss and the number of times a femtosecond laser beam with a double-pulse sequence is applied. The horizontal axis represents the number of times the laser beam is applied, and the vertical axis represents the insertion loss. Figure 6 The graph shows the relationship between reflectivity and the number of times a femtosecond laser beam with a double-pulse sequence is applied. The horizontal axis represents the number of pulses, and the vertical axis represents reflectivity.

[0046] analyze Figure 4 , Figure 5 and Figure 6 The following conclusions can be drawn: 1) Performing linear regression on data from N=1 to 8 yields the fitted equation for the refractive index modulation: Δn≈(0.51×10⁻⁶). -4 )×N+0.01×10 -4 Goodness of fit R 2=0.9999. There is an excellent linear relationship between the refractive index modulation Δn and the number of times the femtosecond laser beam in the applied double-pulse sequence is applied; for each additional stroke of the applied femtosecond laser beam in the double-pulse sequence, the refractive index modulation Δn steadily increases by approximately 0.51 × 10⁻⁶. -4 No nonlinear saturation or cumulative deviation occurred throughout the entire process. This numerical range (0.52 × 10⁻⁶) -4 ~4.10×10 -4 This perfectly matches the typical physical order of refractive index change induced by femtosecond lasers in pure quartz optical fibers, typically within 10^6 ppm. -4 Up to 10 -3 Within the range.

[0047] 2) Insertion loss exhibits a significant nonlinear increasing trend with the number of times the femtosecond laser beam of the applied dual-pulse sequence is applied. When N=1~3 times, the loss remains at an extremely low level (≤0.09dB). The reflectivity can be effectively controlled from 75% to >99% by adjusting the number of applications. When N≥4 times, although the refractive index modulation Δn continues to increase linearly, the insertion loss begins to rise significantly (0.18dB to 1.68dB), indicating that excessive pulse energy accumulation begins to induce an increase in scattering centers and enhanced nonlinear absorption, leading to deterioration of transmission loss. N=2 times (preferred value) achieves the optimal performance balance—98.5% reflectivity and only 0.06dB insertion loss, fully meeting the preferred technical specifications of >95% reflectivity and <0.1dB insertion loss. This is the physical basis for the preferred number of repetitions of 2 times in this invention: to obtain a sufficiently high refractive index modulation through the seed pulse-assisted absorption effect to achieve high reflectivity, while avoiding the accumulation of scattering and absorption losses caused by excessive processing.

[0048] Example 3: The difference from Example 1 is that the fiber to be processed is a highly germanium-doped fiber, the femtosecond laser pulse width is 500 fs, and the repetition frequency is 1 MHz. The seed pulse energy density is 60% of the intrinsic nonlinear ionization threshold, the processing pulse energy density is 80% of the intrinsic nonlinear ionization threshold, and the time delay is set to the preferred value of 500 fs. The grating period Λ = 536 nm, and the moving speed is 536 mm / s. Two double-pulse femtosecond laser beams are applied in each grating period, and the entire section is scanned twice. The resulting fiber grating has a reflectivity of 97.8% and an insertion loss of 0.07 dB. At a delay of 500 fs, the effective processing threshold of this material is reduced to about 63%, a reduction of about 37%, which also falls within the preferred range of 10% to 50%. The refractive index modulation deviation of multiple batches of fiber gratings is less than 2%, showing excellent repeatability and verifying the universality of the method of this invention for different fiber materials.

Claims

1. A method for fabricating a fiber Bragg grating, characterized in that, Includes the following steps: S1: Fix the optical fiber to be processed and make the axis of the optical fiber perpendicular to the propagation direction of the femtosecond laser beam; S2: A femtosecond laser source generates a pulsed femtosecond laser beam with a fixed repetition frequency. The pulsed femtosecond laser beam is converted into a femtosecond laser beam consisting of a seed pulse with a time delay and the same pulse width but different energies, and a subsequent processing pulse. The energy densities of the seed pulse and the processing pulse are both lower than the intrinsic nonlinear ionization threshold of the optical fiber to be processed. The energy density of the seed pulse is 30% to 70% of the intrinsic nonlinear ionization threshold of the optical fiber to be processed, and the energy density of the processing pulse is 50% to 90% of the intrinsic nonlinear ionization threshold of the optical fiber to be processed. The energy density of the processing pulse is higher than that of the seed pulse. S3: Set the time delay between the seed pulse and the processing pulse to be less than the energy relaxation time of the photoexcited free electrons in the optical fiber material to be processed; S4: Focus the femtosecond laser beam of the dual-pulse sequence onto the core of the optical fiber to be processed, and expose each preset processing point of the entire grating along the optical fiber axis according to the preset grating period to complete one round of scanning, so that each preset processing point can enhance nonlinear absorption by means of the seed pulse, thereby generating local refractive index modulation. S5: Repeatedly perform multiple scans on the whole grating as a unit, so that the refractive index modulation of each preset processing point is accumulated round by round until a complete periodic refractive index modulation structure is formed, thereby obtaining the target fiber grating.

2. The method for fabricating a fiber Bragg grating according to claim 1, characterized in that, The time delay between the seed pulse and the processing pulse set in step S3 is 100fs~10000fs.

3. The method for fabricating a fiber Bragg grating according to claim 1, characterized in that, When exposing each preset processing point corresponding to each grating period point by point, the preset processing point is repeatedly subjected to a femtosecond laser beam with a double pulse sequence through multiple reset processing, and the number of repetitions is 1 to 10.

4. The method for fabricating a fiber Bragg grating according to claim 1, characterized in that, The center wavelength of the femtosecond laser beam in the dual-pulse sequence is 1030 nm, the width of a single pulse is 200 fs to 500 fs, and the repetition frequency is 1 kHz to 1 MHz.

5. The method for fabricating a fiber Bragg grating according to claim 1, characterized in that, The optical fiber to be processed is any one of chalcogenide optical fiber, fluoride optical fiber, tellurate optical fiber, rare earth ion doped optical fiber, high germanium doped optical fiber, and pure silica optical fiber.

6. The method for fabricating a fiber Bragg grating according to claim 1, characterized in that, The point-by-point exposure in step S4 is achieved in the following way: the femtosecond laser beam of the dual-pulse sequence is emitted at a repetition frequency f that matches the grating period Λ, and the optical fiber to be processed moves at a constant speed v, where v = Λ × f.

7. A fabrication apparatus for implementing the fabrication method of the fiber Bragg grating according to claim 1, characterized in that, include: Femtosecond laser sources are used to generate pulsed femtosecond laser beams with a fixed repetition frequency; The pulse beam splitting and delay module is used to convert the pulsed femtosecond laser beam into a femtosecond laser beam consisting of a pre-sequence seed pulse with time delay, the same pulse width, and different energies, and a subsequent processing pulse. A focusing objective lens is used to focus a femtosecond laser beam of a dual-pulse sequence onto the core of the optical fiber to be processed. The motion and control module is used to fix the optical fiber to be processed. According to the preset grating period and the repetition frequency of the femtosecond laser pulse, it controls the moving speed of the optical fiber to be processed, so that the distance between the exposure points of adjacent laser pulses on the fiber core is equal to the grating period. According to the preset grating period and the moving speed of the optical fiber, it sends a synchronous trigger signal to the femtosecond laser source to ensure that each double pulse sequence corresponds exactly to a grating period position. After the optical fiber to be processed completes the writing of a whole grating structure, it quickly resets to the starting position and repeats multiple rounds of scanning, so that the refractive index modulation depth of each preset processing point is accumulated round by round until a complete periodic refractive index modulation structure is formed, and the target fiber grating is obtained.

8. The preparation apparatus according to claim 7, characterized in that, The pulse beam splitting and delay module consists of a beam splitting module, a first energy adjustment component, a second energy adjustment component, a delay module, and a beam combining module. The beam splitting module splits the pulsed femtosecond laser beam into a seed beam and a processing beam. The seed beam enters the first energy adjustment component for energy adjustment, and the processing beam enters the second energy adjustment component for energy adjustment before entering the delay module for delay. The seed beam emitted from the first energy adjustment component and the processing beam emitted from the delay module enter the beam combining module respectively to obtain a dual-pulse sequence of femtosecond laser beams, which then enter the focusing objective. The focusing objective is a 60x oil immersion objective with a numerical aperture of 1.

42.

9. A fiber grating fabricated by the method described in any one of claims 1 to 6, characterized in that, When the reflectivity is greater than 95%, the insertion loss is less than 0.1dB, and the refractive index modulation is linearly related to the number of femtosecond laser beams of the double-pulse sequence applied to the processing point corresponding to each grating period.

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