Femtosecond laser phase mask FBG inscribing system based on off-axis parabolic cylindrical mirror

Abstract

The invention relates to the technical field of fiber bragg grating preparation, and provides a femtosecond laser phase mask FBG inscribing system based on an off-axis parabolic cylindrical mirror, which comprises a grating inscribing light path system, an off-axis parabolic cylindrical mirror, a phase mask plate and a glass slide which are sequentially arranged along the light path direction, and is characterized in that the grating inscribing light path system emits laser into one end of the cylindrical surface of the off-axis parabolic cylindrical mirror; one end of the off-axis parabolic cylinder of the off-axis parabolic cylinder mirror focuses the laser and emits the laser into the phase mask plate, and the phase mask plate is arranged above the off-axis parabolic cylinder mirror and can generate interference fringes for inscribing the optical fiber; an optical fiber inscribing area is formed on the upper surface of the glass slide, optical fiber refractive index matching liquid is arranged on the optical fiber inscribing area, an optical fiber to be inscribed is attached to the optical fiber inscribing area of the glass slide and placed in the refractive index matching liquid, and FBG inscribing is conducted on the optical fiber to be inscribed through interference fringes generated by the phase mask plate. According to the invention, the energy density of the focusing light spot is obviously improved, and the laser beam can be accurately focused on the optical fiber core.

Description

Technical Field

[0001] The present invention relates to the technical field of fiber Bragg grating fabrication, and particularly to a femtosecond laser phase mask FBG writing system based on an off-axis parabolic cylinder mirror. Background Art

[0002] Fiber Bragg grating (FBG) sensors, with their advantages of small size, light weight, high sensitivity, and excellent electromagnetic interference resistance, have become a mainstream sensitive structure form in the field of fiber optic sensors and are widely used in many key fields such as aerospace, civil engineering, and life health. Femtosecond lasers, with their extremely short pulse width and extremely high peak power, have become an ideal tool for fabricating FBGs. When the energy of the femtosecond laser exceeds the damage threshold of the fiber material, the fiber material in the laser irradiation area will experience a process of melting and rapid solidification and shrinkage, thereby forming a permanent refractive index modulation inside the fiber. Among the mainstream methods of femtosecond laser writing gratings, the phase mask method and the direct writing method each have their own characteristics. The direct writing method focuses the femtosecond laser inside the fiber through a microscope objective lens and constructs the periodic structure of the grating one by one by precisely moving the fiber. Although it can flexibly control parameters such as the grating period, length, and refractive index modulation degree, it requires high equipment stability and precision, and the high-precision displacement platform is expensive. In contrast, the phase mask method uses a cylindrical lens to focus the femtosecond laser and then incident it on the phase mask, and the grating is written on the fiber through the interference of the diffracted lights of each order generated by the mask. This method has lower requirements for the coherence of the light source, good stability, strong repeatability, high processing efficiency, and low loss, and is very suitable for mass production.

[0003] Currently, the traditional phase mask method generally uses a device combining a cylindrical lens and a phase mask. Although the cylindrical lens can meet the basic focusing requirements, due to its inherent limitations in shape and structure, it is difficult to focus the femtosecond laser with high quality and high energy through the coating layer into the core of the fiber, and at the same time, it is easy to cause the temperature rise problem of the fiber coating layer. Summary of the Invention

[0004] In order to solve the problem that it is difficult to focus the femtosecond laser with high quality and high energy through the coating layer into the core of the fiber in the phase mask grating writing of the prior art, and at the same time, it is easy to cause the temperature rise of the fiber coating layer, the present invention provides a femtosecond laser phase mask FBG writing system based on an off-axis parabolic cylinder mirror.

[0005] The present invention provides a femtosecond laser phase mask FBG writing system based on an off-axis parabolic cylinder mirror. The system includes: a grating writing optical path system, an off-axis parabolic cylinder mirror, a phase mask plate, and a glass slide, which are sequentially arranged along the optical path direction. The grating writing optical path system injects laser light into one end of the cylindrical surface of the off-axis parabolic cylinder mirror, and the off-axis parabolic cylinder end of the off-axis parabolic cylinder mirror focuses the laser light and injects it into the phase mask plate. The phase mask plate is arranged above the off-axis parabolic cylinder mirror and can generate interference fringes for writing optical fibers. A fiber writing area is formed on the upper surface of the glass slide. An optical fiber refractive index matching liquid is arranged on the fiber writing area. The optical fiber to be written is attached to the fiber writing area of the glass slide and placed in the refractive index matching liquid, and the interference fringes generated by the phase mask plate are used to perform FBG writing on the optical fiber to be written.

[0006] Further, the system further includes a gantry. An accommodation space for the off-axis parabolic cylinder mirror is formed between the gantries. A single-axis displacement device is arranged in the accommodation space, and the off-axis parabolic cylinder mirror is installed on the single-axis displacement device to adjust the position of the off-axis parabolic cylinder mirror through the single-axis displacement device. A displacement platform is arranged on the gantry. A phase mask plate fixture for clamping the phase mask plate, a glass slide slot for installing the glass slide, and an optical fiber fixture for fixing the optical fiber to be written are arranged on the displacement platform. The displacement platform can drive the phase mask plate fixture, the glass slide slot, and the optical fiber fixture to move as a whole to adjust the positions of the phase mask plate, the glass slide, and the optical fiber.

[0007] Further, the position of the glass slide slot is relatively fixed with respect to the phase mask plate fixture to define the relative positions between the phase mask plate and the glass slide.

[0008] Further, the grating writing optical path system includes a femtosecond laser, a half-wave plate, a polarizer, a beam splitter, a power detector, and a reflector, which are sequentially arranged along the laser optical path. The femtosecond laser is used to emit laser light. The half-wave plate is used to adjust the polarization state of the laser light. The polarizer is used to eliminate the interference of stray light in the laser light. The beam splitter is used to split the laser beam into the power detector and the reflector. The power detector is used to monitor the optical power of the laser light in real time. The reflector is used to reflect part of the laser light into the off-axis parabolic cylinder mirror.

[0009] Further, the grating writing optical path system further includes an optical shutter, which is arranged between the femtosecond laser and the half-wave plate. When the optical shutter is opened, the femtosecond laser light is injected into the half-wave plate, and when the optical shutter is closed, the femtosecond laser light is blocked from being injected into the half-wave plate.

[0010] Further, the system further includes a circulator connected to the optical fiber, and an ASE light source and a spectral analyzer connected to the circulator. The ASE light source and the spectral analyzer are used to online monitor the transmission spectrum and reflection spectrum of the written FBG.

[0011] The femtosecond laser phase mask FBG writing system based on an off-axis parabolic cylinder mirror provided by the present invention includes a grating writing optical path system, an off-axis parabolic cylinder mirror, a phase mask plate, and a glass slide arranged in sequence along the optical path direction. The grating writing optical path system injects laser light into one cylindrical end of the off-axis parabolic cylinder mirror, and the off-axis parabolic cylindrical end of the off-axis parabolic cylinder mirror refracts the laser light into a straight line and injects it upward into the phase mask plate. The phase mask plate generates interference fringes for writing the optical fiber. The glass slide is close to the phase mask plate and is arranged above the phase mask plate. An optical fiber writing area is formed on the upper surface of the glass slide, and an optical fiber refractive index matching liquid is injected into the optical fiber writing area. The optical fiber to be written is placed in the refractive index matching liquid, and the interference fringes generated by the phase mask plate are used to perform FBG writing on the optical fiber. The present invention uses an off-axis parabolic cylinder mirror and utilizes its unique achromatic focusing characteristic to achieve efficient tight focusing of the femtosecond laser beam, improve the energy density of the focused light spot, ensure that the laser beam can be accurately focused on the optical fiber core, effectively reduce the temperature rise of the optical fiber coating layer, and thus greatly improve the quality and stability of grating writing. In addition, the present invention is provided with a glass slide above the phase mask plate, and the optical fiber refractive index matching liquid is dropped on the glass slide, so that the optical fiber adheres to the glass slide and is immersed in the refractive index matching liquid. The combined design of the glass slide and the refractive index matching liquid significantly improves the accuracy and stability of grating writing. Description of the Drawings

[0012] By reading the following detailed description of the preferred embodiments, various other advantages and benefits will become clear to those of ordinary skill in the art. The drawings are only for the purpose of showing the preferred embodiments and are not considered to be a limitation of the present invention. Moreover, throughout the drawings, the same reference numerals are used to represent the same components. In the drawings:

[0013] Figure 1 is the off-axis parabolic cylinder mirror focusing femtosecond laser phase mask writing system provided by the embodiment of the present invention;

[0014] Figure 2 is the off-axis parabolic cylinder mirror (90° off-axis) and its optical path simulation diagram;

[0015] Figure 3 is the three-dimensional view of the off-axis parabolic cylinder mirror focusing femtosecond laser phase mask writing system provided by the embodiment of the present invention;

[0016] Figure 4 is the simplified schematic diagram of the glass slide and the optical fiber refractive index matching liquid provided by the embodiment of the present invention;

[0017] Figure 5 is the three-dimensional view of the fixing fixture of the phase mask plate provided by the embodiment of the present invention.

[0018] Figure 6The FBG reflection spectrum and transmission spectrum written through the coating layer according to the embodiments of the present invention. Specific embodiments

[0019] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0020] Aiming at the problems that it is difficult to focus femtosecond laser with high quality and high energy through the coating layer into the fiber core by the current phase mask method, and at the same time, it is easy to cause the temperature rise of the fiber coating layer. The present invention proposes an innovative solution, that is, using an off-axis parabolic cylinder mirror 8 to focus the femtosecond laser mask FBG writing system. This system uses the off-axis parabolic cylinder mirror 8 to focus the femtosecond laser into a thin line, generates interference fringes through the phase mask plate, forms a periodic refractive index change on the fiber core, and writes the fiber grating. Utilizing the achromatic aberration characteristics of the off-axis mirror, the laser is tightly focused, the energy density is increased, and it accurately reaches the fiber core, reducing the temperature rise of the coating layer. At the same time, the optical shutter 2 periodically controls the light output to ensure that a high-quality fiber grating is written without damage when penetrating the coating layer.

[0021] Specifically, as Figures 1 - 3 shown, the femtosecond laser phase mask FBG writing system based on the off-axis parabolic cylinder mirror 8 according to the embodiments of the present invention includes a titanium sapphire femtosecond laser 1, an optical shutter 2, a half-wave plate 3, a polarizer 4, a beam splitter 5, a power detector 6, a reflector 7, an off-axis parabolic cylinder mirror 8, a phase mask plate fixture 9, a phase mask plate 10, a gantry 11, a glass slide 12, a fiber fixture 13, a fiber fixture knob 14, a displacement stage knob 15, a circulator 18, an ASE light source 19, and a spectrum analyzer 20.

[0022] Among them, the titanium sapphire femtosecond laser 1, the optical shutter 2 switch, the half-wave plate 3, the polarizer 4, the beam splitter 5, the power detector 6, and the reflector 7 constitute the grating writing optical path system of the embodiments of the present invention. The grating writing optical path system injects the laser into the cylindrical end of the off-axis parabolic cylinder mirror 8, and the off-axis parabolic cylindrical end of the off-axis parabolic cylinder mirror 8 refracts the laser into a straight line and injects it upward into the phase mask plate 10. The present invention converts the grating writing optical path into injecting upward into the phase mask plate 10 by the off-axis parabolic cylinder mirror 8, which is convenient for injecting into the fiber from below the fiber for grating writing.

[0023] Further, a femtosecond laser 1, a half-wave plate 3, a polarizing mirror 4, a beam splitter 5, a power detector 6, and a mirror 7 sequentially arranged along the laser optical path constitute the basic optical path system of the embodiment of the present invention. The femtosecond laser 1 is used to emit laser light, the half-wave plate 3 is used to adjust the polarization state of the laser light, the polarizing mirror 4 is used to eliminate the stray light interference in the laser light, and the beam splitter 5 is used to split the laser beam into the power detector 6 and the mirror 7. Specifically, part of the laser light can be reflected into the power detector 6, and the other part of the laser light can be refracted into the mirror 7. Furthermore, the optical power of the laser light can be monitored in real time through the power detector, and part of the laser light can be reflected into the off-axis parabolic cylinder mirror 8 through the mirror 7.

[0024] Further, in a specific embodiment of the present invention, the femtosecond laser 1(1) used for writing has a wavelength of 800 nm, a pulse width of 35 fs, and a repetition frequency of 1 KHz. The pulse energy of the femtosecond laser can be attenuated by rotating the half-wave plate 3(3) in front of the polarizing mirror 4(4).

[0025] Further, an optical shutter 2 is also arranged between the femtosecond laser 1 and the half-wave plate 3 in the embodiment of the present invention. When the optical shutter 2 is opened, the femtosecond laser light is incident on the half-wave plate 3, and when the optical shutter 2 is closed, the femtosecond laser light is blocked from entering the half-wave plate 3. Through the host computer program, the opening and closing of the optical shutter 2 can be precisely controlled, thereby periodically modulating the output of the incident light. This design further controls the temperature rise of the fiber coating layer and improves the stability of grating writing.

[0026] Further, in the grating writing optical path system of the embodiment of the present invention, the laser light is incident on the cylindrical end of the off-axis parabolic cylinder mirror 8, and the off-axis parabolic cylinder end of the off-axis parabolic cylinder mirror 8 refracts the laser light into a straight line to be focused upward and incident on the phase mask 10. Specifically, Figure 2 is the optical path focusing simulation diagram of the 90° off-axis parabolic cylinder mirror 8. The off-axis parabolic cylinder mirror 8 is composed of a cylinder and an off-axis parabolic cylinder. The characteristic of "off-axis" means that the optical axes of the cylinder and the off-axis parabolic cylinder are parallel but not coincident. This unique design makes the off-axis parabolic cylinder mirror 8 have special application value in the optical system. In the present invention, the included angle between the focusing axis and the optical axis is 90°. This special off-axis angle setting makes the focus located outside the optical path, providing more flexibility for the design of the optical system. From Figure 2 it can be clearly seen that the beam parallel to the Y-axis direction passes through the XZ plane, and after the reflection and focusing of the off-axis parabolic cylinder mirror 8, it is finally focused into a point on the XY plane. This process fully demonstrates the unique ability of the off-axis parabolic cylinder mirror 8 in optical path focusing. The focal length of the off-axis parabolic cylinder mirror 8 of the present invention is selected to be 15 mm.

[0027] In view of the chromatic aberration problem and insufficient spot focusing intensity existing in the traditional cylindrical lens solution, the present invention innovatively adopts an off-axis parabolic cylinder mirror 8, and utilizes its unique achromatic focusing characteristic to achieve efficient tight focusing of the femtosecond laser beam. This improvement significantly enhances the energy density of the focused spot, ensures that the laser beam can be accurately focused on the fiber core, effectively reduces the temperature rise of the fiber coating layer, and thus greatly improves the quality and stability of grating writing.

[0028] Furthermore, in the embodiment of the present invention, the phase mask 10 and the glass slide 12 are arranged close to each other, the glass slide 12 is located above the phase mask 10, the phase mask 10 can generate interference fringes for writing on the fiber, a fiber writing area is formed on the glass slide 12, a fiber refractive index matching liquid 17 is injected into the fiber writing area, the fiber to be written is attached to the fiber writing area of the glass slide and placed in the refractive index matching liquid, and the interference fringes generated by the phase mask 10 are used for FBG writing on the fiber to be written.

[0029] Specifically, the fiber is placed closely adjacent to the glass slide 12, and the fiber refractive index matching liquid 17 can be stably injected into the fiber writing area on the upper surface of the glass slide 12. As Figure 4 shown in the simplified relationship diagram of the glass slide 12 and the single-mode fiber, the fiber refractive index matching liquid 17 can eliminate the cylindrical lens effect between the fiber and the surrounding environment (such as air) during the grating writing process, making the laser energy act more concentratedly on the core; it can also conduct heat to reduce the temperature rise of the fiber coating layer; and it can also isolate oxygen in the air to play a role in protecting the fiber. The glass slide 12 can form a more compact optical contact interface between the writing surface of the single-mode fiber and the fiber refractive index matching liquid 17, reduce the reflection and scattering of light on the interface, and improve the transmission efficiency of light.

[0030] The femtosecond laser phase mask FBG writing system based on the off-axis parabolic cylinder mirror 8 provided by the embodiment of the present invention is different from the traditional phase mask 10 writing. It makes the light path enter the fiber from the bottom upwards, and a refractive index matching liquid can be injected on the glass slide 12. The refractive index matching liquid here is used for heat conduction and eliminating the fiber lens effect. The combined design of the glass slide 12 and the refractive index matching liquid significantly improves the accuracy and stability of grating writing.

[0031] Furthermore, to implement the optical path system of the embodiments of the present invention, the femtosecond laser phase mask FBG writing system based on the off-axis parabolic cylinder mirror 8 of the embodiments of the present invention further includes a gantry 11. An accommodation space for the off-axis parabolic cylinder mirror 8 is formed between the gantries 11. A single-axis displacement device is arranged in the accommodation space, and the off-axis parabolic cylinder mirror 8 is installed on the single-axis displacement device to adjust the position of the off-axis parabolic cylinder mirror 8 through the single-axis displacement device; a displacement platform is arranged on the gantry 11. A phase mask plate fixture 9, a slide glass 12 card slot and an optical fiber fixture 13 are arranged on the displacement platform. The phase mask plate fixture 9 clamps the phase mask plate 10. The slide glass 12 is installed in the slide glass 12 card slot. The optical fiber fixture 13 clamps the optical fiber to be written. The displacement platform can drive the phase mask plate fixture 9, the slide glass 12 card slot and the optical fiber fixture 13 to move as a whole to adjust the positions of the phase mask plate 10, the slide glass 12 and the optical fiber.

[0032] In the present invention, the gantry 11 is specifically used to design the beam direction passing through the off-axis cylindrical mirror upward, aiming to leave an accommodation space for the focusing optical path of the off-axis cylindrical mirror. Secondly, the writing displacement stage, the phase mask plate and the slide glass 12 can be fixed more firmly above the focusing optical path. The optical fiber is placed next to the slide glass 12, and the optical fiber refractive index matching liquid 17 can be stably injected into the optical fiber writing area on the upper surface of the slide glass 12.

[0033] Furthermore, there are optical fiber fixture knobs 14 on both sides of the optical fiber fixture 13 for adjusting the distance between the optical fiber fixtures 13; the lower parts of the optical fiber fixtures 13 are respectively fixed on the displacement stage, and the micrometer knob of the displacement stage can adjust the height of the displacement stage. Among them, Figure 5 It is a three-dimensional view of the fixed fixture for the phase mask plate.

[0034] First, when grating writing is performed on the optical fiber, the single-mode optical fiber with a polyimide coating layer is wiped clean with alcohol and lint-free paper and clamped on the optical fiber fixture 13 with a pre-tightening force. Then, the femtosecond laser pulse is focused by using a writing system composed of an off-axis parabolic cylinder mirror 8 and a phase mask plate. The off-axis parabolic cylinder mirror 8 is regulated by a single-axis displacement stage to ensure that the femtosecond laser can be accurately focused into the core of the optical fiber. After the single-axis displacement stage adjusts the focal length, it is locked and does not move during the grating writing process. Then, one end of the single-mode optical fiber is connected to the circulator 18 and connected to the spectral analyzer 20. The off-axis parabolic cylinder mirror 8 focuses the laser beam onto the core, thereby increasing the energy density of the focused light spot and effectively reducing the temperature rise of the optical fiber coating layer.

[0035] Through the control software interface of the femtosecond laser shutter 2 of the host computer, the optical shutter 2 and the power detector 6 are connected to the host computer, enabling real-time monitoring of the power value and setting the exposure time for grating writing (based on the length of the written grating). The optical shutter 2 is opened, and it is controlled to periodically modulate the output of the incident light into the fiber core to complete FBG writing. The periodic closing of the optical shutter 2 further controls the temperature rise of the fiber coating layer.

[0036] During the preparation of the FBG, the ASE broadband light source and the spectral analyzer 20 are used to online monitor the transmission spectrum and reflection spectrum of the grating and record the spectral data. Figure 6 The reflection spectrum and transmission spectrum for writing the grating through the polyimide coating layer.

[0037] The femtosecond laser phase mask FBG writing system based on the off-axis parabolic cylinder mirror 8 provided by the present invention includes a grating writing optical path system, an off-axis parabolic cylinder mirror 8, a phase mask plate 10, and a glass slide 12 arranged in sequence along the optical path direction. The grating writing optical path system injects the laser into the planar end of the off-axis parabolic cylinder mirror 8, and the off-axis parabolic cylinder end of the off-axis parabolic cylinder mirror 8 refracts the laser into a straight line and injects it upward into the phase mask plate 10. The phase mask plate 10 generates interference fringes for writing the optical fiber. The glass slide 12 is close to the phase mask plate 10 and is arranged above the phase mask plate 10. An optical fiber writing area is formed on the upper surface of the glass slide 12. The optical fiber refractive index matching liquid 17 is injected into the optical fiber writing area. The optical fiber to be written is placed in the refractive index matching liquid, and the interference fringes generated by the phase mask plate 10 are used to perform FBG writing on the optical fiber. The present invention adopts the off-axis parabolic cylinder mirror 8 and utilizes its unique achromatic focusing characteristic to achieve the high-efficiency tight focusing of the femtosecond laser beam, improve the energy density of the focused light spot, ensure that the laser beam can be accurately focused on the fiber core, effectively reduce the temperature rise of the fiber coating layer, and thus greatly improve the quality and stability of grating writing. In addition, the present invention is provided with a glass slide 12 above the phase mask plate. The optical fiber refractive index matching liquid 17 is dropped on the glass slide 12, enabling the optical fiber to adhere to the glass slide 12 and be immersed in the refractive index matching liquid. The combined design of the glass slide 12 and the refractive index matching liquid significantly improves the accuracy and stability of grating writing.

[0038] In addition, those skilled in the art can understand that although some embodiments herein include certain features included in other embodiments rather than other features, the combination of the features of different embodiments means that it is within the scope of the present invention and forms different embodiments. For example, any one of the claimed embodiments can be used in any combination.

[0039] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A femtosecond laser phase mask FBG writing system based on an off-axis parabolic mirror, characterized in that: The system comprises: a grating optical path system, an off-axis parabolic mirror, a phase mask and a glass slide which are sequentially arranged along the optical path direction; the grating optical path system injects laser into one end of the cylindrical surface of the off-axis parabolic mirror, and focuses the laser through one end of the off-axis parabolic surface of the off-axis parabolic mirror and injects the laser into the phase mask; the phase mask is arranged above the off-axis parabolic mirror and can generate interference fringes for writing optical fibers; an optical fiber writing area is formed on the upper surface of the glass slide, an optical fiber refractive index matching liquid is arranged on the optical fiber writing area, an optical fiber to be written is attached to the optical fiber writing area of ​​the glass slide and placed in the refractive index matching liquid, and the interference fringes generated by the phase mask are used to perform FBG writing on the optical fiber to be written.

2. The system according to claim 1, characterized in that The system also includes a gantry, a housing space for an off-axis parabolic mirror is formed between the gantries, a uniaxial displacement device is arranged in the housing space, the off-axis parabolic mirror is mounted on the uniaxial displacement device, so that the position of the off-axis parabolic mirror can be adjusted by the uniaxial displacement device; a displacement platform is arranged on the gantry, a phase mask template clamp for clamping a phase mask template, a glass slide slot for mounting a glass slide, and an optical fiber clamp for fixing an optical fiber to be engraved are arranged on the displacement platform, and the displacement platform can drive the phase mask template clamp, the glass slide slot and the optical fiber clamp to move as a whole, so as to adjust the positions of the phase mask template, the glass slide and the optical fiber.

3. The system according to claim 2, characterized in that The positions of the glass slide slot and the phase mask clamp are relatively fixed to define the relative position between the phase mask and the glass slide.

4. The system according to claim 2, characterized in that The grating optical path system includes a femtosecond laser, a half-wave plate, a polarizer, a spectrometer, a power detector and a reflector which are arranged in sequence along the laser optical path. The femtosecond laser is used to emit laser light, the half-wave plate is used to adjust the polarization state of the laser, the polarizer is used to eliminate stray light interference in the laser, the spectrometer is used to split the laser beam into the power detector and the reflector, the power detector is used to monitor the optical power of the laser in real time, and the reflector is used to reflect part of the laser light into the off-axis parabolic mirror.

5. The system according to claim 4, characterized in that The grating optical path system also includes an optical shutter, which is arranged between the femtosecond laser and the half-wave plate. When the optical shutter is opened, the femtosecond laser is allowed to enter the half-wave plate, and when the optical shutter is closed, the femtosecond laser is prevented from entering the half-wave plate.

6. The system according to claim 2, characterized in that The system also includes a circulator connected to the optical fiber, and an ASE light source and a spectrum analyzer connected to the circulator. The ASE light source and the spectrum analyzer are used for online monitoring of the transmission spectrum and reflection spectrum of the written FBG.

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