A method and device for manufacturing a soft glass fiber grating

Abstract

The embodiment of the application provides a preparation method and a preparation device of a soft glass fiber grating, relates to the technical field of fiber grating preparation, and fully utilizes the difference of the interaction mechanism of different photon energies and glass network according to the characteristic that there is an essential difference between the nonlinear ionization mechanism and the energy relaxation channel of femtosecond laser pulse beams of different wavelengths excited by the soft glass fiber, the dynamics process of refractive index modulation is artificially decomposed into a controllable slow process through the time sequence combination of infrared softening, visible densification and ultraviolet curing, the stress cracking and thermal crystallization risks caused by single large energy injection are avoided from the mechanism level, and a refractive index grating structure with low loss and high modulation depth is constructed through step-by-step irradiation.

Description

Technical Field

[0001] This invention relates to a method for fabricating fiber Bragg gratings, and more particularly to a method and apparatus for fabricating soft glass fiber Bragg gratings. Background Technology

[0002] Soft glass fiber is a special type of optical fiber with fluorides, chalcogenides, or heavy metal oxides as matrix materials. Due to its extremely low phonon energy, wide transparency window covering the mid- and far-infrared bands, and high nonlinear coefficient, it has become a core carrier for realizing mid-infrared fiber lasers, supercontinuum light sources, gas sensing, and molecular spectroscopy applications. Compared with traditional silica fiber, soft glass fiber has a looser glass network structure, lower chemical bond strength, and a smaller cation migration barrier. While these intrinsic properties endow it with superior infrared optical performance, they also lead to a significantly low damage threshold and a tendency for brittle fracture when exposed to high-energy-density femtosecond laser irradiation. How to accurately construct permanent, low-scattering-loss periodic refractive index modulation structures within such a fragile material system is a key scientific problem and technological challenge currently facing the field of fiber grating fabrication.

[0003] The core strategy of existing femtosecond laser direct-write fiber grating technology is to utilize a single-wavelength femtosecond laser pulse in the near-infrared band, focusing it on the fiber core to generate multiphoton ionization, micro-explosions, or thermal densification effects to directly create a refractive index abrupt change. However, when this strategy is applied to soft glass fiber, it encounters serious negative effects due to insufficient intrinsic material tolerance. Specifically, if the laser pulse energy is set to a level sufficient to induce a significant refractive index change, the soft glass fiber core is highly susceptible to generating uncontrollable microcrack networks, thermally induced crystallization particles, or cavitation bubbles at the focal point. These structural defects constitute strong Mie scattering centers and Fresnel reflection interfaces, causing a sharp increase in the insertion loss of the grating device, with background loss reaching tens of decibels per centimeter, completely rendering it impractical. On the other hand, attempting to avoid macroscopic material disintegration by simply reducing the laser pulse energy would result in insufficient energy deposition to overcome the activation energy barrier of the soft glass network rearrangement. This would lead to only reversible color centers or metastable thermal effects, making it difficult to form a stable, thermodynamically durable refractive index modulation structure. Specifically, this manifests as extremely low grating reflectivity, broadened spectral resonance peaks, and significant thermal relaxation decay of the constructed refractive index modulation at room temperature, causing the grating performance to degrade rapidly over time. This contradiction between grating formation efficiency and structural integrity, constrained by the control of single-wavelength, single-pulse energy parameters, constitutes a long-standing obstacle in the fabrication process of soft glass fiber gratings. Summary of the Invention

[0004] This invention provides a method and apparatus for fabricating soft glass fiber gratings, which can accurately construct permanent, low-scattering-loss periodic refractive index modulation structures within the material system.

[0005] In a first aspect, the present invention provides a method for fabricating a soft glass fiber Bragg grating, the method comprising: Step S1: Fix the soft glass optical fiber to be processed onto a movable carrier; Step S2: Select the first femtosecond laser pulse beam in the infrared band as the femtosecond laser pulse processing beam to focus and irradiate the soft glass fiber to be processed. The focusing and irradiation is specifically as follows: focus the femtosecond laser pulse processing beam on the preset grating writing area in the core of the soft glass fiber to be processed, and irradiate the soft glass fiber to be processed; then move the soft glass fiber to be processed along the fiber axis through a movable carrier, so that the femtosecond laser pulse processing beam scans along the preset spatial trajectory in the core of the soft glass fiber to be processed, forming a metastable structure perturbation lattice distributed along the preset grating period. Step S3: Switch the second femtosecond laser pulse beam in the visible light band as the femtosecond laser pulse processing beam to focus and irradiate the soft glass fiber to be processed. The focusing and irradiation is specifically as follows: focus the femtosecond laser pulse processing beam on the preset grating writing area in the core of the soft glass fiber to be processed, and irradiate the soft glass fiber to be processed; then move the soft glass fiber to be processed through a movable carrier, and scan the metastable structure perturbation lattice with the same preset spatial trajectory as in step S2 to construct a refractive index modulation prototype structure. Step S4: Switch the third femtosecond laser pulse beam in the ultraviolet band as the femtosecond laser pulse processing beam to focus and irradiate the soft glass fiber to be processed. The focusing and irradiation is specifically as follows: focus the femtosecond laser pulse processing beam on the preset grating writing area in the core of the soft glass fiber to be processed to irradiate the soft glass fiber to be processed; then move the soft glass fiber to be processed through a movable carrier and scan the refractive index modulation prototype structure with the same preset spatial trajectory as in step S2 to form a refractive index modulation grating structure. Among them, the bulk damage threshold of the soft glass fiber to be processed is greater than the single pulse energy of the femtosecond laser pulse processing beam selected in step S2, which is greater than the single pulse energy of the femtosecond laser pulse processing beam selected in step S3, which is greater than the single pulse energy of the femtosecond laser pulse processing beam selected in step S4.

[0006] In this invention, the bulk damage threshold of soft glass fiber is defined as: the lowest single pulse energy corresponding to the first appearance of an irreversible scattering center or refractive index mutation when the core material of the soft glass fiber is subjected to single pulse irradiation under a specific pulse width and repetition frequency, as observed by a phase contrast microscope or dark field scattering imaging system. The refractive index mutation is manifested as a continuous and stable contrast difference with the surrounding unirradiated area. The measurement steps for the bulk damage threshold are as follows: Take a section of soft glass fiber from the same batch as the soft glass fiber to be processed, remove the coating layer, and fix it on a multi-dimensional displacement platform. Position the focal point of the focusing objective at the center of the soft glass fiber core. Using the single-pulse irradiation method, starting with a laser single-pulse energy of 20 nJ and gradually increasing in increments of 5 nJ, select at least 10 irradiation points along the fiber axis within the soft glass fiber core at equal intervals of 50 μm for single-pulse irradiation. After irradiating all points, remove the soft glass fiber and observe it under a phase-contrast microscope at a magnification of no less than 500x. The first clearly identifiable irreversible scattering center or refractive index abrupt change is used as the damage judgment criterion. Record the lowest single-pulse energy value at which damage occurs. Repeat the above test at least three times for the same batch of soft glass fiber, and take the average value as the bulk damage threshold of that batch of soft glass fiber. The specific value of this bulk damage threshold is calibrated on the same batch of soft glass fiber before formally writing the grating, serving as a direct basis for setting the single-pulse energy of each process. In an exemplary, and not limiting, embodiment of the present invention, the measurement conditions for the volume damage threshold are a femtosecond laser pulse width of 220 fs, a repetition rate of 1 kHz, and a focusing objective numerical aperture of 0.65. Those skilled in the art should understand that the specific value of the volume damage threshold will change accordingly when the measurement conditions change, but the above-described definition method and measurement steps remain applicable.

[0007] Compared with existing technologies, the advantages of this invention are as follows: It utilizes the inherent differences in the nonlinear ionization mechanisms and energy relaxation channels excited by femtosecond laser pulses of different wavelengths in soft glass optical fibers. It fully leverages the different interaction mechanisms between different photon energies and the glass network. Through a sequential combination of infrared softening, visible densification, and ultraviolet curing, the soft glass optical fiber to be processed is first focused and irradiated with a first femtosecond laser pulse beam in the infrared band. Infrared photons dominate tunneling ionization and lattice heating, resulting in structural softening in the core of the soft glass optical fiber to form metastable structures. The process involves perturbing the lattice structure; then focusing and irradiating the soft glass fiber to be processed with a second femtosecond laser pulse beam in the visible light band. Visible photons enhance multiphoton absorption and ion migration, driving the directional migration of cations and micro-densification within the softened region, thus constructing a preliminary refractive index modulation structure. Finally, focusing and irradiating the soft glass fiber to be processed with a third femtosecond laser pulse beam in the ultraviolet band, ultraviolet photons directly act on valence band electrons, inducing permanent chemical bond reconstruction and filling of deep-level electron traps, forming a permanent, low-loss refractive index modulation grating structure. This invention artificially decomposes the dynamic process of refractive index modulation into a controllable slow process, avoiding the risks of stress cracking and thermal crystallization caused by a single large-energy injection at the mechanistic level. Through step-by-step and sequential irradiation, a low-loss, high-modulation-depth refractive index modulation grating structure is constructed.

[0008] The soft glass fiber gratings fabricated by the method of this invention exhibit extremely low insertion loss. Because the irradiation energy of each femtosecond laser pulse processing beam is far below the material damage threshold, and the first-stage infrared pre-irradiation effectively releases the residual stress accumulated during subsequent processing, the fabricated grating region maintains a density and uniformity in its microstructure consistent with the background of the soft glass fiber, introducing almost no additional scattering sources, and the background loss approaches the inherent material absorption limit of the soft glass fiber. This method is particularly suitable for the high-quality fabrication of non-quartz matrix infrared fiber gratings with low damage thresholds, such as fluoride and chalcogenide glass fibers, and can further be extended to the fabrication of other soft glass fiber gratings, such as tellurate glass fibers or heavy metal oxide glass fibers.

[0009] In the preparation method of this invention, the changes in the optical fiber transmission spectrum are all slow and monotonic, which makes it easy to accurately control the irradiation dose in real time through the spectrometer, and avoids the overexposure or underexposure phenomenon that often occurs in traditional single-wavelength direct writing.

[0010] In one feasible implementation, the first femtosecond laser pulse beam is directly obtained from an infrared-band fundamental femtosecond laser pulse beam generated by a fundamental femtosecond laser source, with a wavelength range of 1000nm to 1100nm; the second femtosecond laser pulse beam is generated by frequency doubling of the fundamental femtosecond laser pulse beam, with a wavelength range of 500nm to 550nm; and the third femtosecond laser pulse beam is generated by third harmonicization of the fundamental femtosecond laser pulse beam, with a wavelength range of 330nm to 370nm.

[0011] In one feasible implementation, the single-pulse energy of the femtosecond laser pulse processing beam is less than 60% of the bulk damage threshold of the soft glass fiber to be processed.

[0012] The soft glass optical fiber to be processed can be a fluoride glass optical fiber or a chalcogenide glass optical fiber.

[0013] In one feasible implementation, the soft glass fiber to be processed is a fluoride glass fiber, the single pulse energy of the first femtosecond laser pulse beam is 60nJ to 90nJ, the single pulse energy of the second femtosecond laser pulse beam is 50nJ to 70nJ, and the single pulse energy of the third femtosecond laser pulse beam is 15nJ to 30nJ.

[0014] In one feasible implementation, the soft glass fiber to be processed is a chalcogenide glass fiber, the single pulse energy of the first femtosecond laser pulse beam is 30nJ to 50nJ, the single pulse energy of the second femtosecond laser pulse beam is 20nJ to 40nJ, and the single pulse energy of the third femtosecond laser pulse beam is 5nJ to 15nJ.

[0015] Secondly, the present invention provides an apparatus for fabricating a soft glass fiber Bragg grating, the apparatus comprising: A fundamental frequency femtosecond laser source is used to generate a fundamental frequency femtosecond laser pulse beam in the infrared band. The frequency conversion module converts the fundamental frequency femtosecond laser pulse beam in the infrared band into a first femtosecond laser pulse beam in the infrared band, a second femtosecond laser pulse beam in the visible light band, and a third femtosecond laser pulse beam in the ultraviolet band, respectively. The processing control component is used to select the first femtosecond laser pulse beam in the infrared band, the second femtosecond laser pulse beam in the visible light band, or the third femtosecond laser pulse beam in the ultraviolet band as the femtosecond laser pulse processing beam output and control the irradiation time during processing. An energy control component is used to adjust the single-pulse energy of the femtosecond laser pulse processing beam; The focusing objective lens focuses the energy-adjusted femtosecond laser pulse processing beam onto a preset grating writing area within the core of the soft glass fiber to be processed. The multidimensional displacement platform is equipped with fiber clamps for fixing the soft glass fiber to be processed. The soft glass fiber to be processed is moved along the fiber axis by the multidimensional displacement platform, so that the femtosecond laser pulse processing beam scans along a preset spatial trajectory in the core of the soft glass fiber to be processed. The control device is used to control the fundamental frequency femtosecond laser source to generate a fundamental frequency femtosecond laser pulse beam in the infrared band, control the processing control component to select the femtosecond laser pulse processing beam and control the irradiation time during processing, control the energy regulation component to set the single pulse energy of the femtosecond laser pulse processing beam, and control the movement of the multidimensional displacement platform to complete the scanning of the femtosecond laser pulse processing beam along a preset spatial trajectory within the core of the soft glass optical fiber to be processed.

[0016] The fabrication apparatus of the present invention, through a frequency conversion module and an energy regulation component, can generate femtosecond laser pulse beams of different wavelengths and adjust the energy of each femtosecond laser pulse beam. It does not depend on hydrogen sensitization or special doping processes of specific optical fibers, nor does it require complex wavefront shaping components. At the same time, the processing control component selects the output femtosecond laser pulse processing beam and controls the irradiation time during processing.

[0017] In one feasible implementation, the frequency conversion module includes a beam splitting module, a second harmonic module, and a third harmonic module; the processing control component includes a first electric shutter, a second electric shutter, a third electric shutter, and a beam combining module; and the energy control component includes a first energy adjustable attenuation module, a second energy adjustable attenuation module, and a third energy adjustable attenuation module. The beam splitting module receives the fundamental frequency femtosecond laser pulse beam generated by the fundamental frequency femtosecond laser source and splits it into three beams: the first beam is a first femtosecond laser pulse beam in the infrared band directly obtained from the fundamental frequency femtosecond laser pulse beam; the second beam is converted into a second femtosecond laser pulse beam in the visible light band after passing through the second harmonic module; and the third beam is converted into a third femtosecond laser pulse beam in the ultraviolet band after passing through the third harmonic module. The first electric shutter receives the first femtosecond laser pulse beam, the second electric shutter receives the second femtosecond laser pulse beam, and the third electric shutter receives the third femtosecond laser pulse beam. In the first-level processing step, the control device synchronously controls the opening of the first electric shutter, the closing of the second electric shutter, and the closing of the third electric shutter to select the first femtosecond laser pulse beam as the femtosecond laser pulse processing beam, and controls the irradiation time during processing by controlling the opening time of the first electric shutter. The first energy adjustable attenuation module adjusts the single pulse energy of the femtosecond laser pulse processing beam output by the first electric shutter to be lower than the volume damage threshold of the soft glass fiber to be processed, and then inputs it to the beam combining module. In the second-stage processing step, the control device synchronously controls the opening of the second electric shutter, the closing of the first electric shutter, and the closing of the third electric shutter to select the second femtosecond laser pulse beam as the femtosecond laser pulse processing beam. The irradiation time during processing is controlled by controlling the opening time of the second electric shutter. The second energy adjustable attenuation module adjusts the single pulse energy of the femtosecond laser pulse processing beam output by the second electric shutter to be lower than the bulk damage threshold of the soft glass fiber to be processed before inputting it to the beam combining module. In the third-level processing step, the control device synchronously controls the opening of the third electric shutter, the closing of the first electric shutter, and the closing of the second electric shutter to select the third femtosecond laser pulse beam as the femtosecond laser pulse processing beam, and controls the irradiation time during processing by controlling the opening time of the third electric shutter. The third energy adjustable attenuation module adjusts the single pulse energy of the femtosecond laser pulse processing beam output by the third electric shutter to be lower than the volume damage threshold of the soft glass fiber to be processed, and then inputs it to the beam combining module. The single pulse energy of the femtosecond laser pulse processing beam output by the first electric shutter after energy adjustment is greater than that of the femtosecond laser pulse processing beam output by the second electric shutter after energy adjustment, which in turn is greater than that of the femtosecond laser pulse processing beam output by the third electric shutter after energy adjustment. The beam combining module then inputs the energy-adjusted femtosecond laser pulse processing beam into the focusing objective.

[0018] In another possible implementation, the frequency conversion module includes a second harmonic module and a third harmonic module, the processing control component includes an electric switching module and a fourth electric shutter, the energy regulation component includes a fourth energy adjustable attenuation module, the second harmonic module and the third harmonic module are disposed on the electric switching module, and the electric switching module is also provided with an empty space. The electric switching module selects the first femtosecond laser pulse beam that generates the infrared band as the femtosecond laser pulse processing beam by cutting into the optical path where the empty space is located and then incident it onto the fourth electric shutter. The electric switching module selects the second femtosecond laser pulse beam that generates the visible light band as the femtosecond laser pulse processing beam and directs it onto the fourth electric shutter by switching the frequency doubling module into the optical path. The electric switching module selects the third femtosecond laser pulse beam that generates the ultraviolet band as the femtosecond laser pulse processing beam by switching the third frequency harmonic module into the optical path and then incident it onto the fourth electric shutter. The control device controls the opening of the fourth electric shutter, inputs the selected femtosecond laser pulse processing beam into the fourth energy adjustable attenuation module, and controls the irradiation time during processing by controlling the opening time of the fourth electric shutter; the fourth energy adjustable attenuation module adjusts the single pulse energy of the femtosecond laser pulse processing beam to be lower than the volume damage threshold of the soft glass fiber to be processed before inputting it into the focusing objective lens; the single pulse energy of the first femtosecond laser pulse beam after energy adjustment is greater than the single pulse energy of the second femtosecond laser pulse beam after energy adjustment, which is greater than the single pulse energy of the third femtosecond laser pulse beam after energy adjustment. Attached Figure Description

[0019] Figure 1 A flowchart illustrating a method for fabricating a soft glass fiber Bragg grating according to an embodiment of the present invention; Figure 2 A basic structural block diagram of the fabrication apparatus used in a method for fabricating a soft glass fiber Bragg grating provided in an embodiment of the present invention; Figure 3 This is a structural block diagram of the preparation apparatus according to the first specific embodiment of the present invention; Figure 4 This is a structural block diagram of the preparation apparatus according to the second specific embodiment of the present invention.

[0020] Explanation of reference numerals in the attached figures: 1. Fundamental frequency femtosecond laser source; 2. Frequency conversion module; 20. Empty space; 21. Beam splitting module; 22. Second harmonic module; 23. Third harmonic module; 3. Processing control components; 31. First electric shutter; 32. Second electric shutter; 33. Third electric shutter; 34. Beam combining module; 35. Fourth electric shutter; 4. Energy regulation components; 41. First energy adjustable attenuation module; 42. Second energy adjustable attenuation module; 43. Third energy adjustable attenuation module; 44. Fourth energy adjustable attenuation module; 5. Focusing objective; 6. Multidimensional displacement platform; 7. Control device; 8. Electric switching module. Detailed Implementation

[0021] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0022] To address the problems in the prior art, this invention provides a method and apparatus for fabricating a soft glass fiber grating.

[0023] Below, we will first introduce a method for fabricating a soft glass fiber grating provided in the embodiments of the present invention.

[0024] Figure 1 This diagram illustrates a process flow of a method for fabricating a soft glass fiber Bragg grating according to an embodiment of the present invention. Figure 1 As shown, the method includes the following steps: Step S1: Fix the soft glass fiber to be processed on a movable carrier. The soft glass fiber to be processed can be a fluoride glass fiber or a chalcogenide glass fiber. Step S2: Select a first femtosecond laser pulse beam in the infrared band with a wavelength range of 1000nm to 1100nm as the femtosecond laser pulse processing beam to focus and irradiate the soft glass fiber to be processed. The first femtosecond laser pulse beam is directly obtained from the fundamental frequency femtosecond laser pulse beam in the infrared band generated by the fundamental frequency femtosecond laser source. The focusing irradiation is as follows: focus the femtosecond laser pulse processing beam on the preset grating writing area in the core of the soft glass fiber to be processed, irradiate the soft glass fiber to be processed, and then move the soft glass fiber to be processed along the fiber axis through a movable carrier so that the femtosecond laser pulse processing beam scans along the preset spatial trajectory in the core of the soft glass fiber to be processed, forming a metastable structure perturbation lattice distributed along the preset grating period. Step S3: Switch the second femtosecond laser pulse beam with a wavelength range of 500nm to 550nm in the visible light band as the femtosecond laser pulse processing beam to focus and irradiate the soft glass fiber to be processed. The second femtosecond laser pulse beam is generated by frequency doubling of the fundamental frequency femtosecond laser pulse beam. The focusing irradiation is as follows: focus the femtosecond laser pulse processing beam on the preset grating writing area in the core of the soft glass fiber to be processed, irradiate the soft glass fiber to be processed, and then move the soft glass fiber to be processed through a movable carrier to scan the metastable structure perturbation lattice with the same preset spatial trajectory as in step S2 to construct a refractive index modulation prototype structure. Step S4: Switch the third femtosecond laser pulse beam with a wavelength range of 330nm to 370nm in the ultraviolet band as the femtosecond laser pulse processing beam to focus and irradiate the soft glass fiber to be processed. The third femtosecond laser pulse beam is generated by third harmonicization of the fundamental frequency femtosecond laser pulse beam. The focusing irradiation is as follows: focus the femtosecond laser pulse processing beam on the preset grating writing area in the core of the soft glass fiber to be processed to irradiate the soft glass fiber to be processed. Then move the soft glass fiber to be processed through a movable carrier and scan the refractive index modulation prototype structure with the same preset spatial trajectory as in step S2 to form a refractive index modulation grating structure. Among them, 60% of the bulk damage threshold of the soft glass fiber to be processed > the single pulse energy of the femtosecond laser pulse processing beam selected in step S2 > the single pulse energy of the femtosecond laser pulse processing beam selected in step S3 > the single pulse energy of the femtosecond laser pulse processing beam selected in step S4.

[0025] In one embodiment, the soft glass fiber to be processed is a fluoride glass fiber, the single pulse energy of the first femtosecond laser pulse beam is 60nJ to 90nJ, the single pulse energy of the second femtosecond laser pulse beam is 50nJ to 70nJ, and the single pulse energy of the first femtosecond laser pulse beam is greater than that of the second femtosecond laser pulse beam, and the single pulse energy of the third femtosecond laser pulse beam is 15nJ to 30nJ.

[0026] In one embodiment, the soft glass fiber to be processed is a chalcogenide glass fiber, the single pulse energy of the first femtosecond laser pulse beam is 30nJ to 50nJ, the single pulse energy of the second femtosecond laser pulse beam is 20nJ to 40nJ, and the single pulse energy of the first femtosecond laser pulse beam is greater than that of the second femtosecond laser pulse beam, and the single pulse energy of the third femtosecond laser pulse beam is 5nJ to 15nJ.

[0027] Figure 2 This invention illustrates a fabrication apparatus for a flexible glass fiber Bragg grating according to an embodiment of the present invention. The fabrication apparatus includes: Fundamental frequency femtosecond laser source 1, used to generate a fundamental frequency femtosecond laser pulse beam in the infrared band; Frequency conversion module 2 converts the fundamental frequency femtosecond laser pulse beam in the infrared band into a first femtosecond laser pulse beam in the infrared band, a second femtosecond laser pulse beam in the visible light band, and a third femtosecond laser pulse beam in the ultraviolet band, respectively. The processing control component 3 is used to select the first femtosecond laser pulse beam in the infrared band, the second femtosecond laser pulse beam in the visible light band, or the third femtosecond laser pulse beam in the ultraviolet band as the femtosecond laser pulse processing beam output and control the irradiation time during processing. Energy control component 4 is used to adjust the single-pulse energy of the femtosecond laser pulse processing beam; Focusing objective 5 focuses the energy-adjusted femtosecond laser pulse processing beam onto the preset grating writing area in the core of the soft glass fiber to be processed. The multidimensional displacement platform 6 is equipped with an optical fiber clamp for fixing the soft glass optical fiber to be processed. The soft glass optical fiber to be processed is moved along the optical fiber axis by the multidimensional displacement platform 6, so that the femtosecond laser pulse processing beam scans along a preset spatial trajectory in the core of the soft glass optical fiber to be processed. The control device 7 can be in different forms such as PLC, industrial computer, or embedded controller. In this embodiment, a PLC is used to control the fundamental frequency femtosecond laser source 1 to generate a fundamental frequency femtosecond laser pulse beam in the infrared band, control the processing control component 3 to select the femtosecond laser pulse processing beam and control the irradiation time during processing, control the energy regulation component 4 to set the single pulse energy of the femtosecond laser pulse processing beam, and control the movement of the multidimensional displacement platform 6 to complete the scanning of the femtosecond laser pulse processing beam along a preset spatial trajectory within the core of the soft glass optical fiber to be processed.

[0028] exist Figure 3 In the first specific embodiment shown, the frequency conversion module 2 consists of a beam splitting module 21, a second harmonic module 22, and a third harmonic module 23. The processing control component 3 consists of a first electric shutter 31, a second electric shutter 32, a third electric shutter 33, and a beam combining module 34. The energy regulation component 4 consists of a first energy adjustable attenuation module 41, a second energy adjustable attenuation module 42, and a third energy adjustable attenuation module 43. The first energy adjustable attenuation module 41, the second energy adjustable attenuation module 42, and the third energy adjustable attenuation module 43 are conventional structures, each consisting of a continuous energy adjustment unit and an attenuation wheel. The continuous energy adjustment unit can be composed of a half-wave plate and a Glan-Taylor prism, and the attenuation wheel can be a set of neutral density filters.

[0029] After receiving the fundamental frequency femtosecond laser pulse beam generated by the fundamental frequency femtosecond laser source 1, the beam splitting module 21 splits it into three beams. The first beam is the first femtosecond laser pulse beam in the infrared band, which is obtained by the fundamental frequency femtosecond laser pulse beam directly reaching the first motorized shutter 31. The second beam, after being converted by the frequency doubling module 22, reaches the second motorized shutter 32 and is the second femtosecond laser pulse beam in the visible light band. The third beam, after being converted by the frequency tripling module 23, reaches the third motorized shutter 33 and is the third femtosecond laser pulse beam in the ultraviolet band.

[0030] When a first femtosecond laser pulse beam in the infrared band is selected as the femtosecond laser pulse processing beam, the control device 7 synchronously controls the opening of the first electric shutter 31, the closing of the second electric shutter 32, and the closing of the third electric shutter 33. The first femtosecond laser pulse beam in the infrared band is input to the first energy adjustable attenuation module 41 as the femtosecond laser pulse processing beam. After the first energy adjustable attenuation module 41 adjusts the single pulse energy to below 60% of the bulk damage threshold of the soft glass fiber to be processed, it is output to the focusing objective lens 5 through the beam combining module 34. At this time, the irradiation time during processing can be controlled by controlling the opening time of the first electric shutter 31. When a second femtosecond laser pulse beam in the visible light band is selected as the femtosecond laser pulse processing beam, the control device 7 synchronously controls the opening of the second electric shutter 32, the closing of the first electric shutter 31, and the closing of the third electric shutter 33. The second femtosecond laser pulse beam in the visible light band is input to the first energy adjustable attenuation module 41. The second energy adjustable attenuation module 42 adjusts the single pulse energy to below 60% of the bulk damage threshold of the soft glass fiber to be processed, and then outputs it to the focusing objective lens 5 via the beam combining module 34. At this time, the irradiation time during processing can be controlled by controlling the opening time of the second electric shutter 32. When the third femtosecond laser pulse beam in the ultraviolet band is selected as the femtosecond laser pulse processing beam, the control device 7 synchronously controls the opening of the third electric shutter 33, the closing of the second electric shutter 32, and the closing of the first electric shutter 31. The third femtosecond laser pulse beam in the ultraviolet band is input to the third energy adjustable attenuation module 43 as the femtosecond laser pulse processing beam. After the third energy adjustable attenuation module 43 adjusts the single pulse energy to below 60% of the bulk damage threshold of the soft glass fiber to be processed, it is output to the focusing objective lens 5 via the beam combining module 34. At this time, the irradiation time during processing can be controlled by controlling the opening time of the third electric shutter 33.

[0031] The single pulse energy of the femtosecond laser pulse processing beam output by the first electric shutter 31 after energy adjustment is greater than the single pulse energy of the femtosecond laser pulse processing beam output by the second electric shutter 32 after energy adjustment, which is greater than the single pulse energy of the femtosecond laser pulse processing beam output by the third electric shutter 33 after energy adjustment.

[0032] exist Figure 4 In the second specific embodiment shown, the frequency conversion module 2 includes a second harmonic module 22 and a third harmonic module 23, the processing control component 3 includes an electric switching module 8 and a fourth electric shutter 35, and the energy regulation component 4 includes a fourth energy adjustable attenuation module 44. The second harmonic module 22 and the third harmonic module 23 are disposed on the electric switching module 8, and the electric switching module 8 is also provided with a slot 20.

[0033] The electric switching module 8 selects a first femtosecond laser pulse beam that generates an infrared band as the femtosecond laser pulse processing beam and directs it onto the fourth electric shutter 35 by switching the empty slot 20 into the optical path; the electric switching module 8 selects a second femtosecond laser pulse beam that generates a visible light band as the femtosecond laser pulse processing beam and directs it onto the fourth electric shutter 35 by switching the frequency-doubled module 22 into the optical path; the electric switching module 8 selects a third femtosecond laser pulse beam that generates an ultraviolet band as the femtosecond laser pulse processing beam and directs it onto the fourth electric shutter 35 by switching the frequency-thirdd module 23 into the optical path. The control device 7 controls the opening of the fourth electric shutter 35, inputs the selected femtosecond laser pulse processing beam into the fourth energy adjustable attenuation module 44, and controls the irradiation time during processing by controlling the opening time of the fourth electric shutter 35; the fourth energy adjustable attenuation module 44 adjusts the single pulse energy of the femtosecond laser pulse processing beam to be lower than 60% of the bulk damage threshold of the soft glass fiber to be processed before inputting it into the focusing objective lens 5; the single pulse energy of the first femtosecond laser pulse beam after energy adjustment is greater than the single pulse energy of the second femtosecond laser pulse beam after energy adjustment is greater than the single pulse energy of the third femtosecond laser pulse beam after energy adjustment.

[0034] The following are some specific examples of embodiments of the present invention.

[0035] Example 1: This example uses ZBLAN fluoride fiber (ZBLAN is a standard fluoride fiber composition, its composition is ZrF4-BaF2-LaF3-AlF3-NaF), a typical low-damage-threshold soft glass fiber, as the soft glass fiber to be processed. This ZBLAN fluoride fiber has an outer diameter of 125 μm, a core diameter of 8 μm, a numerical aperture of 0.21, and exhibits extremely low background loss in the mid-infrared 3 μm band.

[0036] In this example one, such as Figure 4The fabrication apparatus shown is as follows. The fundamental frequency femtosecond laser source 1 is a ytterbium-doped solid-state femtosecond laser amplifier. The output center wavelength of the fundamental frequency femtosecond laser pulse beam is 1030 nm, the pulse width is 220 fs, and the repetition frequency is set to 1 kHz. The fundamental frequency femtosecond laser pulse beam first reaches the frequency conversion module 2 mounted on the electric switching module 8. The electric switching module 8 is a precision electric displacement stage. The frequency conversion module 2 includes a second harmonic module 22 composed of a barium borate crystal and a third harmonic module 23 composed of a lithium triborate crystal. The electric switching module 8 also has a vacancy 20. The femtosecond laser pulse processing beam output from the frequency conversion module 2 is incident on the fourth electric shutter 35. The femtosecond laser pulse processing beam output from the fourth electric shutter 35 is finely set in terms of light energy by a fourth energy adjustable attenuation module 44, which consists of a continuous energy adjustment unit and an attenuation wheel. The continuous energy adjustment unit consists of a half-wave plate and a Glan-Taylor prism, and the attenuation wheel is a set of neutral density filters. A focusing objective 5 with a long working distance and a numerical aperture of 0.65 is set after the fourth energy-adjustable attenuation module 44. The focusing objective 5 focuses the waist of the femtosecond laser pulse processing beam with adjusted energy emitted from the fourth energy-adjustable attenuation module 44 into the core of the ZBLAN fluoride fiber fixed on the multidimensional displacement platform 6. The multidimensional displacement platform 6 uses a three-dimensional air-bearing nano-displacement platform. One end of the ZBLAN fluoride fiber is connected to a supercontinuum broadband light source covering the 1.5μm to 3.6μm band via a single-mode fiber jumper, and the other end is connected to a cooled indium antimony arsenide fiber spectrometer to monitor the evolution of the grating resonance peak in the transmission spectrum in real time.

[0037] Once all preparations are complete, the control device 7 is activated to initiate the first-stage infrared softening pretreatment process. The control device 7, via the electric switching module 8, moves the empty slot 20 into the optical path. At this time, the frequency conversion module 2 is in direct-flow mode, outputting a 1030nm wavelength femtosecond laser pulse processing beam, i.e., the first femtosecond laser pulse beam in the infrared band. The fourth electric shutter 35 is opened, and the half-wave plate and attenuator wheel are adjusted. Calibration is performed using an optical power meter placed in front of the focusing objective lens 5, precisely setting the single-pulse energy of the femtosecond laser pulse processing beam acting on the ZBLAN fluoride fiber core to 75nJ. Based on previous damage tests, this energy value... The damage threshold of this batch of ZBLAN fluoride optical fibers is approximately 45%. The ZBLAN fluoride optical fiber is irradiated with a femtosecond laser pulse processing beam with adjusted energy. A multi-dimensional displacement platform 6 is controlled to move the ZBLAN fluoride optical fiber along the fiber axis at a constant speed of 0.4 mm / s, allowing the femtosecond laser pulse processing beam to scan the core of the soft glass optical fiber along a preset spatial trajectory. Simultaneously, the opening time of the fourth motorized shutter 35 is controlled according to the preset Bragg grating period (1.07 μm in this example, corresponding to a third-order grating). After completing a full-area scan of the preset length, the fourth motorized shutter 35 is closed. At this point, the spectrometer shows a flat transmission spectrum without characteristic depressions. However, lateral micro-fluorescence imaging reveals a weak green photoluminescent lattice arranged along the fiber core axis, proving that the metastable color center precursor has been successfully implanted, forming a metastable perturbation lattice distributed along the preset grating period. The second stage of visible light densification construction is then performed. Control device 7 moves the frequency-doubler module 22 into the optical path via the electric switching module 8. At this time, the frequency conversion module 2 outputs a 515nm wavelength green femtosecond laser pulse processing beam, which is the second femtosecond laser pulse beam in the visible light band. The fourth electric shutter 35 is opened, and the half-wave plate and attenuator wheel are readjusted to precisely set the single-pulse energy of the 515nm wavelength femtosecond laser pulse processing beam, which is focused and acts on the ZBLAN fluoride fiber core, to 60nJ, irradiating the ZBLAN fluoride fiber. The multi-dimensional displacement platform 6 is controlled at 0.4... A constant speed of mm / s drives the ZBLAN fluoride fiber to move along the fiber axis, allowing the femtosecond laser pulse processing beam to scan the fiber core along the same scanning path, replicating the preset spatial trajectory in the first-stage infrared softening pretreatment process. At this time, a slowly deepening transmission depression begins to appear on the spectrometer, with a center wavelength of approximately 1950 nm. The gradual formation of this transmission depression reflects the coordinated migration of barium and zirconium ions in the softened region induced by green photons, which promotes a continuous increase in local density and refractive index, and initially constructs the spatial outline of the refractive index modulation prototype structure.

[0038] The third stage, UV curing and locking, then begins. Control device 7 moves the third-harmonic generation module 23 into the optical path via the electric switching module 8. At this point, the frequency conversion module 2 outputs a 343nm wavelength femtosecond laser pulse processing beam, i.e., the third femtosecond laser pulse beam in the UV band. The fourth electric shutter 35 is opened, and the half-wave plate and attenuator wheel are readjusted. Due to the significantly increased absorption coefficient of the UV band material, the energy input needs to be further reduced, attenuating the single-pulse energy of the femtosecond laser pulse processing beam to an extremely small level of 20nJ. The ZBLAN fluoride fiber is irradiated, and the multi-dimensional displacement platform 6 is controlled to move the ZBLAN fluoride fiber along the fiber axis at the same speed as the previous two stages, allowing the femtosecond laser pulse processing beam to perform a third scan within the fiber core along the same preset spatial trajectory. At this point, the depth of the transmission depression on the spectrometer rapidly increases from a few decibels to over 25dB, and the baseline transmittance on both sides of the transmission depression does not decrease significantly, indicating that the background loss of the grating structure is extremely low. During the third-level ultraviolet irradiation process, high-energy photons selectively act on the non-bridging oxygen defect sites in the already compacted region, completing valence state compensation and structural relaxation, permanently locking the refractive index modulation structure, and forming a refractive index modulation grating structure.

[0039] After processing, the ZBLAN fluoride fiber was removed from the multidimensional displacement platform 6 and encapsulated. The insertion loss of the ZBLAN fluoride fiber grating was precisely measured using the truncation method. The results showed that the single-end insertion loss of the ZBLAN fluoride fiber grating at the resonant wavelength was only 0.25 dB, with a reflectivity greater than 99%, and no additional broadband loss introduced by the writing was observed throughout the entire mid-infrared monitoring band. In contrast, gratings written on the same fiber using a traditional single 1030 nm femtosecond laser with the lowest effective grating energy generally had insertion losses higher than 1.5 dB. The insertion loss of the grating was measured using the truncation method, and the test method followed the international standard distributed by the International Electrotechnical Commission: IEC 60793-1-40:2024 (full title: "Fiber Optics Part 1-40: Methods of Measurement and Test Procedures—Attenuation"), and the single-end insertion loss of the grating at the resonant wavelength was measured to be only 0.25 dB. Grating reflectivity was calculated using the resonant peak depth of the transmission spectrum; a resonant peak depth exceeding 25 dB corresponded to reflectivity greater than 99%. No additional broadband loss introduced by the writing process was observed across the entire mid-infrared monitoring band. In contrast, gratings written on the same fiber using a conventional single 1030 nm femtosecond laser at the lowest effective grating energy generally exhibited insertion losses higher than 1.5 dB when measured using the same method.

[0040] Example 2: This Example 2 further demonstrates the applicability of the method of the present invention to chalcogenide glass optical fibers. The soft glass optical fiber to be processed is As2S3 chalcogenide glass optical fiber with a core diameter of 6μm. It has a low glass transition temperature and a more sensitive damage threshold than ZBLAN fluoride optical fiber.

[0041] In this example two, using as follows Figure 4 The fabrication apparatus is shown. As2S3 chalcogenide glass fiber is mounted on the fiber clamp of the multidimensional displacement platform 6. In the first-stage infrared softening pretreatment process, the single-pulse energy of the 1030nm wavelength infrared femtosecond laser pulse processing beam is further reduced to 40nJ to avoid excessive softening due to potential heat accumulation in the As2S3 chalcogenide glass fiber in the infrared band. In the second-stage visible light densification construction process, the single-pulse energy of the 515nm wavelength green femtosecond laser pulse processing beam is set to 30nJ, utilizing the appropriate penetration depth of green light in the chalcogenide glass to activate the vibrational relaxation of the sulfur-arsenic bonds. In the third-stage UV curing and locking process, since the penetration depth of UV light in chalcogenide glass is relatively shallow, a multi-layer scanning strategy is adopted. That is, while keeping the axial grating writing trajectory unchanged and maintaining the preset spatial trajectory, the focal position is moved layer by layer along the radial direction of the optical fiber in a step of 1μm, so that the UV laser beam waist irradiates the upper, middle and lower layers of the fiber core in sequence, to ensure that the refractive index modulation prototype structure in the entire guided mode section is fully locked and cured in valence state. In this process, the single pulse energy of the 343nm wavelength UV femtosecond laser pulse processing beam is strictly limited to below 10nJ. After three-stage irradiation, a Bragg grating with a center wavelength of 3.2 μm was successfully written into As2S3 chalcogenide glass fiber, with a transmission recess depth of 18 dB and an insertion loss of only 0.6 dB. In contrast, the insertion loss of As2S3 chalcogenide glass fiber gratings with the same parameters, which are directly written using a single 1030 nm femtosecond laser, is usually greater than 4 dB. This further confirms the universality and effectiveness of the method of the present invention for various low-damage-threshold soft glass fibers.

[0042] The mechanism is explained as follows: This invention employs the microscopic evolution mechanism of the interaction between photon energy and the core material of soft glass optical fiber during a three-stage irradiation process. When using femtosecond laser pulses in the infrared band to process the beam, due to its long wavelength and low photon energy, low-density plasma is mainly generated through tunneling ionization. The energy released during this recombination process leads to a mild increase in local temperature, generating structural relaxation and metastable defect enrichment regions in the glass network, thus lowering the activation energy barrier for subsequent modification. When using femtosecond laser pulses in the visible light band to process the beam, its enhanced two-photon absorption cross-section allows its photon energy to better match the activation energy of cation migration in the soft glass, effectively driving the diffusion of modified ions from high free energy regions to low free energy regions, resulting in micro-area densification and the initial establishment of refractive index contrast. When using femtosecond laser pulses in the ultraviolet band to process the beam, its photon energy is close to or exceeds the binding energy of some chemical bonds in the glass matrix, directly inducing valence band electron transitions and triggering site-specific bonding reconstruction and deep-level trap filling for specific defect sites. Since the first two processes have fully activated the target area and maintained the low temperature of the overall structure, the UV curing behavior of the third process is gentle and precise, ultimately forming a grating structure with both high refractive index contrast and low interface roughness in the fiber core.

[0043] The above embodiments are only used to help understand the method and core idea of ​​the present invention, and are not intended to limit the scope of the present invention. For those skilled in the art, without departing from the principles of the present invention, several adjustments and improvements can be made to the process parameters for soft glass optical fibers with different compositions, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for fabricating a soft glass fiber grating, characterized in that, The method includes: Step S1: Fix the soft glass optical fiber to be processed onto a movable carrier; Step S2: Select the first femtosecond laser pulse beam in the infrared band as the femtosecond laser pulse processing beam, focus and irradiate the soft glass fiber to be processed, and then move the soft glass fiber to be processed along the fiber axis through a movable carrier so that the femtosecond laser pulse processing beam scans along a preset spatial trajectory in the core of the soft glass fiber to be processed, forming a metastable structure perturbation lattice distributed along a preset grating period. Step S3: Switch the second femtosecond laser pulse beam in the visible light band as the femtosecond laser pulse processing beam, focus and irradiate the soft glass fiber to be processed, and then move the soft glass fiber to be processed through a movable carrier to scan the metastable structure perturbation lattice with the same preset spatial trajectory as in step S2 to construct a refractive index modulation prototype structure. Step S4: Switch the third femtosecond laser pulse beam in the ultraviolet band as the femtosecond laser pulse processing beam, focus and irradiate the soft glass fiber to be processed, and then move the soft glass fiber to be processed through a movable carrier to scan the refractive index modulation prototype structure with the same preset spatial trajectory as in step S2 to form a refractive index modulation grating structure. Among them, the bulk damage threshold of the soft glass fiber to be processed is greater than the single pulse energy of the femtosecond laser pulse processing beam selected in step S2, which is greater than the single pulse energy of the femtosecond laser pulse processing beam selected in step S3, which is greater than the single pulse energy of the femtosecond laser pulse processing beam selected in step S4.

2. The method for fabricating a soft glass fiber grating as described in claim 1, characterized in that, The first femtosecond laser pulse beam is directly obtained from the infrared band of the fundamental frequency femtosecond laser pulse beam generated by the fundamental frequency femtosecond laser source, with a wavelength range of 1000nm to 1100nm; the second femtosecond laser pulse beam is generated by frequency doubling of the fundamental frequency femtosecond laser pulse beam, with a wavelength range of 500nm to 550nm; the third femtosecond laser pulse beam is generated by third harmonicization of the fundamental frequency femtosecond laser pulse beam, with a wavelength range of 330nm to 370nm.

3. The method for fabricating a soft glass fiber grating as described in claim 1, characterized in that, The single-pulse energy of the femtosecond laser pulse processing beam is less than 60% of the bulk damage threshold of the soft glass fiber to be processed.

4. The method for fabricating a soft glass fiber grating as described in claim 1, characterized in that, The soft glass optical fiber to be processed is a fluoride glass optical fiber or a chalcogenide glass optical fiber.

5. The method for fabricating a soft glass fiber grating as described in claim 4, characterized in that, The soft glass fiber to be processed is a fluoride glass fiber. The single pulse energy of the first femtosecond laser pulse beam is 60nJ to 90nJ, the single pulse energy of the second femtosecond laser pulse beam is 50nJ to 70nJ, and the single pulse energy of the third femtosecond laser pulse beam is 15nJ to 30nJ.

6. The method for fabricating a soft glass fiber grating as described in claim 4, characterized in that, The soft glass fiber to be processed is a chalcogenide glass fiber. The single pulse energy of the first femtosecond laser pulse beam is 30nJ to 50nJ, the single pulse energy of the second femtosecond laser pulse beam is 20nJ to 40nJ, and the single pulse energy of the third femtosecond laser pulse beam is 5nJ to 15nJ.

7. An apparatus for fabricating a soft glass fiber Bragg grating, characterized in that, The preparation apparatus includes: A fundamental frequency femtosecond laser source is used to generate a fundamental frequency femtosecond laser pulse beam in the infrared band. The frequency conversion module converts the fundamental frequency femtosecond laser pulse beam in the infrared band into a first femtosecond laser pulse beam in the infrared band, a second femtosecond laser pulse beam in the visible light band, and a third femtosecond laser pulse beam in the ultraviolet band, respectively. The processing control component is used to sequentially select the first femtosecond laser pulse beam in the infrared band, the second femtosecond laser pulse beam in the visible light band, and the third femtosecond laser pulse beam in the ultraviolet band as the femtosecond laser pulse processing beam output and control the irradiation time during processing. An energy control component is used to adjust the single-pulse energy of the femtosecond laser pulse processing beam so that the bulk damage threshold of the soft glass fiber to be processed is greater than the single-pulse energy of the first femtosecond laser pulse beam after energy adjustment, which is greater than the single-pulse energy of the second femtosecond laser pulse beam after energy adjustment, which is greater than the single-pulse energy of the third femtosecond laser pulse beam after energy adjustment. The focusing objective lens focuses the energy-adjusted femtosecond laser pulse processing beam onto the preset grating writing area within the core of the soft glass fiber to be processed. The multidimensional displacement platform is equipped with fiber clamps for fixing the soft glass fiber to be processed. The soft glass fiber to be processed is moved along the fiber axis by the multidimensional displacement platform, so that the femtosecond laser pulse processing beam scans along a preset spatial trajectory in the core of the soft glass fiber to be processed. The control device is used to control the fundamental frequency femtosecond laser source to generate a fundamental frequency femtosecond laser pulse beam in the infrared band, control the processing control component to select the femtosecond laser pulse processing beam and control the irradiation time during processing, control the energy regulation component to set the single pulse energy of the femtosecond laser pulse processing beam, and control the movement of the multidimensional displacement platform to complete the scanning of the femtosecond laser pulse processing beam along a preset spatial trajectory within the core of the soft glass optical fiber to be processed.

8. The apparatus for fabricating a soft glass fiber grating as described in claim 7, characterized in that, The frequency conversion module includes a beam splitting module, a second harmonic module, and a third harmonic module. The processing control components include a first electric shutter, a second electric shutter, a third electric shutter, and a beam combining module. The energy regulation components include a first energy adjustable attenuation module, a second energy adjustable attenuation module, and a third energy adjustable attenuation module. The beam splitting module receives the fundamental frequency femtosecond laser pulse beam generated by the fundamental frequency femtosecond laser source and splits it into three beams. The first beam is a first femtosecond laser pulse beam in the infrared band directly obtained from the fundamental frequency femtosecond laser pulse beam. The second beam is converted into a second femtosecond laser pulse beam in the visible light band after passing through the second harmonic module. The third beam is converted into a third femtosecond laser pulse beam in the ultraviolet band after passing through the third harmonic module. The first electric shutter receives the first femtosecond laser pulse beam, the second electric shutter receives the second femtosecond laser pulse beam, and the third electric shutter receives the third femtosecond laser pulse beam. In the first-level processing step, the control device synchronously controls the opening of the first electric shutter, the closing of the second electric shutter, and the closing of the third electric shutter to select the first femtosecond laser pulse beam as the femtosecond laser pulse processing beam, and controls the irradiation time during processing by controlling the opening time of the first electric shutter. The first energy adjustable attenuation module adjusts the single pulse energy of the femtosecond laser pulse processing beam output by the first electric shutter to be lower than the volume damage threshold of the soft glass fiber to be processed, and then inputs it to the beam combining module. In the second-stage processing step, the control device synchronously controls the opening of the second electric shutter, the closing of the first electric shutter, and the closing of the third electric shutter to select the second femtosecond laser pulse beam as the femtosecond laser pulse processing beam. The irradiation time during processing is controlled by controlling the opening time of the second electric shutter. The second energy adjustable attenuation module adjusts the single pulse energy of the femtosecond laser pulse processing beam output by the second electric shutter to be lower than the volume damage threshold of the soft glass fiber to be processed before inputting it to the beam combining module. In the third-level processing step, the control device synchronously controls the opening of the third electric shutter, the closing of the first electric shutter, and the closing of the second electric shutter to select the third femtosecond laser pulse beam as the femtosecond laser pulse processing beam, and controls the irradiation time during processing by controlling the opening time of the third electric shutter. The third energy adjustable attenuation module adjusts the single pulse energy of the femtosecond laser pulse processing beam output by the third electric shutter to be lower than the volume damage threshold of the soft glass fiber to be processed, and then inputs it to the beam combining module. The beam combining module inputs the energy-adjusted femtosecond laser pulse processing beam into the focusing objective.

9. The apparatus for fabricating a soft glass fiber grating as described in claim 7, characterized in that, The frequency conversion module includes a second harmonic module and a third harmonic module; the processing control component includes an electric switching module and a fourth electric shutter; the energy regulation component includes a fourth energy adjustable attenuation module; the second harmonic module and the third harmonic module are disposed on the electric switching module; and the electric switching module is also provided with an empty space. The electric switching module selects the first femtosecond laser pulse beam that generates the infrared band as the femtosecond laser pulse processing beam by cutting into the optical path where the empty space is located and then incident it onto the fourth electric shutter. The electric switching module selects the second femtosecond laser pulse beam that generates the visible light band as the femtosecond laser pulse processing beam by switching the frequency doubling module into the optical path and then incident it onto the fourth electric shutter. The electric switching module selects the third femtosecond laser pulse beam that generates the ultraviolet band as the femtosecond laser pulse processing beam by switching the third frequency harmonic module into the optical path and then incident it onto the fourth electric shutter. The control device controls the opening of the fourth electric shutter, inputs the selected femtosecond laser pulse processing beam into the fourth energy adjustable attenuation module, and controls the irradiation time during processing by controlling the opening time of the fourth electric shutter; the fourth energy adjustable attenuation module adjusts the single pulse energy of the femtosecond laser pulse processing beam to be lower than the bulk damage threshold of the soft glass fiber to be processed before inputting it into the focusing objective.

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