Femtosecond laser inscribing system for manufacturing apodized fiber bragg grating

Abstract

The invention provides a femtosecond laser inscribing system used for manufacturing an apodized fiber bragg grating, which is applied to the technical field of fiber bragg grating preparation, and comprises a femtosecond laser used for outputting laser, the laser is used for adjusting the refractive index of an optical fiber, and the number of laser pulses corresponding to each refractive index modulation point in the optical fiber is different; wherein the laser pulse number is determined based on a preset apodization envelope function; the adjusting module is used for adjusting the propagation direction and the laser parameters of the output laser; the displacement platform is used for moving the optical fiber to enable the laser to accurately irradiate a corresponding refractive index modulation point in the optical fiber so as to modulate the refractive index and obtain an apodized fiber grating; and the control module is used for determining the number of laser pulses corresponding to each refractive index modulation point based on a preset apodization envelope function, adjusting the laser output parameters of the femtosecond laser and controlling the movement parameters of the displacement platform.

Description

Technical Field

[0001] The present disclosure relates to the technical field of fiber grating preparation, and more specifically, to a femtosecond laser writing system for making apodized fiber gratings. Background Art

[0002] When making fiber Bragg gratings, the refractive index modulation depth of the uniform fiber Bragg grating changes suddenly at the boundaries of both ends of the grating, resulting in the "Fabry-Perot" effect, which causes large side lobes on both sides of the spectrum. Excessive side lobes can cause many problems such as signal crosstalk, system instability, and line width broadening in practical applications such as wavelength division multiplexing systems, Q-switched lasers, and high-performance fiber lasers, limiting the further development and application of fiber Bragg gratings. In order to improve the side mode suppression ratio of fiber Bragg gratings, the toe-cutting technology can be used to effectively suppress side lobes, which is of great significance to promoting the widespread application of fiber Bragg gratings.

[0003] Traditional apodization methods all rely on phase masks, mainly including apodization phase mask method, apodization plate method and scanning writing method. The apodization type and grating length of the first two are limited by the phase mask and beam apodization plate, and the apodization quality of the latter is limited by the precision of the piezoelectric ceramic itself and the execution of the motor movement, resulting in low preparation efficiency and poor flexibility. The femtosecond laser direct writing technology has the advantages of high flexibility, no need for phase mask and good repeatability, making the preparation of apodized fiber gratings more efficient and convenient.

[0004] There are studies on the use of femtosecond laser direct writing technology to prepare apodized fiber Bragg gratings. There are four commonly used methods: tilted apodization, femtosecond laser energy adjustment, line-by-line method, and surface-by-surface method. The existing femtosecond direct writing apodized fiber Bragg grating preparation technology mainly relies on devices such as half-wave plates and stepper motors to control laser energy, or achieves apodization through multi-dimensional synchronization and continuous change of the translation stage, making the apodization effect extremely susceptible to the influence of writing process and environmental errors. In addition, the interaction between femtosecond lasers and transparent materials is highly nonlinear. The existing technology is limited by the energy control accuracy and the refractive index modulation envelope accuracy, and cannot accurately control the change of the refractive index modulation amplitude, which limits the apodization effect.

[0005] High-precision control of the refractive index modulation depth distribution of fiber Bragg grating according to the change of the toe-cutting envelope is the key to improving the toe-cutting performance of fiber Bragg grating. However, it is difficult to achieve this with existing technologies, which limits the toe-cutting effect and makes it difficult to further improve the side mode suppression ratio. Summary of the invention

[0006] 1. Technical issues to be resolved

[0007] The present disclosure provides a femtosecond laser writing system for producing apodized fiber gratings, which is used to at least partially solve one of the above-mentioned technical problems.

[0008] (II) Technical solution

[0009] The present invention provides a femtosecond laser writing system for making an apodized fiber grating, the system comprising: a femtosecond laser, for outputting laser light, the laser light being used to adjust the refractive index of the optical fiber, the number of laser pulses corresponding to each refractive index modulation point in the optical fiber being different; wherein the number of laser pulses is determined based on a preset apodization envelope function; an adjustment module, for adjusting the propagation direction and laser parameters of the output laser light; a displacement platform, for moving the optical fiber so that the laser light can be accurately irradiated to the corresponding refractive index modulation point in the optical fiber to perform refractive index modulation and obtain the apodized fiber grating; and a control module, for determining the number of laser pulses corresponding to each refractive index modulation point based on a preset apodization envelope function, adjusting the laser output parameters of the femtosecond laser, and controlling the movement parameters of the displacement platform.

[0010] According to an embodiment of the present disclosure, the adjustment module includes: a first transmission module, used to change the propagation direction of the laser and expand the diameter of the laser beam; a first adjustment module, used to adjust the power and polarization state of the laser to obtain a first laser; a second adjustment module, used to adjust the diameter and shape of the first laser beam, and focus the adjusted first laser beam on the corresponding refractive index modulation point in the optical fiber.

[0011] According to an embodiment of the present disclosure, the first transmission module includes: a high-reflection mirror for changing the propagation direction of the laser emitted by the femtosecond laser and reflecting the laser to the beam expander; and a beam expander for expanding the diameter of the laser beam.

[0012] According to an embodiment of the present disclosure, the first adjustment module includes: an electric laser power attenuator for adjusting the laser power; and a polarizer group for regulating the polarization state of the laser to obtain the first laser.

[0013] According to an embodiment of the present disclosure, the second adjustment module includes: an aperture beam limiter for limiting the diameter and shape of the first laser beam; and an objective lens for focusing the first laser beam to a refractive index modulation point in the optical fiber.

[0014] According to an embodiment of the present disclosure, the energy of a single laser pulse output by a femtosecond laser is kept constant during the apodization process, and the number of laser pulses varies based on a preset apodization envelope function.

[0015] According to an embodiment of the present disclosure, the preset apodization envelope functions include: Gaussian function, Hamming function, Blackman function, raised cosine function, Tanh function, Cauchy function, and sinc function.

[0016] According to an embodiment of the present disclosure, the structure of the apodized fiber grating is written based on a femtosecond laser direct writing method, which includes a point-by-point method, a line-by-line method, and a plane-by-plane method.

[0017] According to an embodiment of the present disclosure, the system further comprises a spectrometer, a broadband light source and a circulator, wherein the spectrometer, the broadband light source and the circulator are used to monitor the spectrum changes caused by the apodized fiber grating structure in real time.

[0018] According to an embodiment of the present disclosure, the system further includes: an LED light source, disposed below the displacement platform, for illuminating the displacement platform; and an image sensor, for capturing in real time an image of the cut fiber grating structure on the displacement platform for real-time monitoring by the control module.

[0019] (III) Beneficial effects

[0020] The femtosecond laser writing system for making apodized fiber gratings provided by the present disclosure includes at least the following beneficial effects:

[0021] The preset program in the control module can be used to control the high-precision change of the laser pulse number and accurately control the refractive index modulation depth according to the toe-cutting envelope, thereby realizing the fine regulation and arbitrary toe-cutting of the refractive index modulation depth of the fiber Bragg grating, which can significantly improve the toe-cutting effect and reduce the high-precision requirements for laser energy regulation and displacement platform movement. It has better consistency and flexibility, and provides an effective new way for the preparation of high-performance fiber Bragg gratings. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The above and other objects, features and advantages of the present disclosure will become more apparent through the following description of the embodiments of the present disclosure with reference to the accompanying drawings, in which:

[0023] Figure 1 The structure diagram of a femtosecond laser writing system for manufacturing an apodized fiber grating according to an embodiment of the present disclosure is schematically shown;

[0024] Figure 2 A schematic diagram of a uniformly apodized fiber grating junction structure fabricated based on a femtosecond laser point-by-point method according to an embodiment of the present disclosure is shown;

[0025] Figure 3 A schematic diagram of a uniformly apodized fiber Bragg grating structure fabricated based on a laser line-by-line method according to an embodiment of the present disclosure is shown;

[0026] Figure 4 A schematic diagram of a tilted apodized fiber Bragg grating structure fabricated based on a laser line-by-line method according to an embodiment of the present disclosure is shown;

[0027] Figure 5 A schematic diagram of a uniformly apodized fiber grating structure manufactured based on a femtosecond laser face-by-face method according to an embodiment of the present disclosure is shown. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical solutions and advantages of the present disclosure more clearly understood, the present disclosure is further described in detail below in combination with specific embodiments and with reference to the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present disclosure, rather than all of the embodiments. Based on the embodiments in the present disclosure, all other embodiments obtained by ordinary technicians in the field without creative work are within the scope of protection of the present disclosure.

[0029] The terms used herein are only for describing specific embodiments and are not intended to limit the present disclosure. The terms "comprise", "include", etc. used herein indicate the existence of the features, steps, operations and / or components, but do not exclude the existence or addition of one or more other features, steps, operations or components.

[0030] In the present disclosure, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection, an electrical connection, or can communicate with each other; it can be a direct connection, or it can be indirectly connected through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present disclosure can be understood according to specific circumstances.

[0031] In the description of the present disclosure, it is necessary to understand that the terms "longitudinal", "length", "circumferential", "front", "rear", "left", "right", "top", "bottom", "inside", "outside", etc., indicating the orientation or positional relationship, are based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing the present disclosure and simplifying the description, and do not indicate or imply that the subsystem or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present disclosure.

[0032] Throughout the drawings, the same elements are represented by the same or similar reference numerals. Conventional structures or configurations will be omitted when they may cause confusion in the understanding of the present disclosure. The shapes, sizes, and positional relationships of the components in the drawings do not reflect the actual size, proportion, and actual positional relationship. In addition, in the claims, any reference symbol between brackets shall not be construed as a limitation to the claims.

[0033] Similarly, in order to simplify the present disclosure and help understand one or more of the various disclosed aspects, in the above description of the exemplary embodiments of the present disclosure, the various features of the present disclosure are sometimes grouped together into a single embodiment, figure, or description thereof. The description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" and the like means that the specific features, structures, materials, or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present disclosure. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in any one or more embodiments or examples in a suitable manner.

[0034] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the feature. In the description of the present disclosure, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

[0035] The embodiment of the present disclosure provides a femtosecond laser writing system for manufacturing apodized fiber gratings. Before introducing the technical solution provided by the embodiment of the present disclosure, the related technologies involved in the present disclosure are first described.

[0036] There are four commonly used methods for preparing apodized fiber Bragg gratings using femtosecond laser direct writing technology: tilted apodization, femtosecond laser energy adjustment, line-by-line and surface-by-surface methods. The tilted apodization method achieves apodization by controlling the uniform oblique distribution of laser pulses in the fiber core, but its effective modulation length is limited by the core size, the reflectivity is low under the same writing energy, and the writing flexibility is poor; the femtosecond laser energy adjustment method achieves the distribution of laser pulse energy along the fiber direction in the form of an apodization function by precisely adjusting the speed of the stepper motor and mechanically rotating the position of the half-wave plate. This method relies on the stepper motor and has very high requirements for the vibration isolation and laser energy attenuation accuracy of the displacement platform. It is difficult to accurately control the refractive index modulation amplitude, and the apodization effect is limited; the apodized fiber Bragg grating is prepared based on the line-by-line method or the surface-by-surface method. This method requires controlling the continuous change of laser pulses at different lateral positions, durations and writing depths. It has very high requirements for the accuracy of laser focusing and displacement platforms. It is difficult to ensure consistency and symmetry in the grating structure. It often cannot achieve the expected effect due to technical limitations, and the writing flexibility and consistency are limited.

[0037] In addition, there is currently a patent that discloses a rotating aperture amplitude toe-cutting method, which realizes the preparation of various toe-cutting gratings by controlling the rotation speed of the aperture and the pulse number of the excimer laser. However, the grating length and type are limited by the aperture itself, and the flexibility is very low and the scope of application is limited. In addition, this method controls the output of the number of laser pulses by adjusting the rotation of another aperture, which requires the synchronous composite operation of mechanical movement and software control, increases the difficulty and complexity of the preset toe-cutting function, and the matching difficulty between different apertures is also high. The overall operation is complex and the accuracy is low.

[0038] Figure 1 The structure of a femtosecond laser writing system for producing apodized fiber gratings according to an embodiment of the present disclosure is schematically shown.

[0039] like Figure 1 As shown, the femtosecond laser writing system for making apodized fiber gratings includes: a femtosecond laser, an adjustment module, a displacement platform, and a control module.

[0040] A femtosecond laser is used to output a laser, which is used to adjust the refractive index of an optical fiber, and the number of laser pulses corresponding to each refractive index modulation point in the optical fiber is different. The laser output parameters of the femtosecond laser are controlled by a control module, and the energy of the single laser pulse output by the femtosecond laser remains constant during the apodization process to ensure the consistency and repeatability of the fiber grating quality. The number of laser pulses changes based on a preset apodization envelope function. Among them, the preset apodization envelope function includes: Gaussian function, Hamming function, Blackman function, raised cosine function, Tanh function, Cauchy function, sinc function. The laser output parameters of the femtosecond laser are controlled by a control module to achieve precise control of the number of laser pulses at each refractive index modulation point in the optical fiber, so that the number of laser pulses gradually decreases from the center of the grating to both ends of the grating, and changes precisely according to the apodization envelope, thereby effectively changing the distribution of the refractive index modulation depth and achieving the effect of apodization compensation.

[0041] The adjustment module is used to adjust the propagation direction and laser parameters of the output laser. The adjustment module may include, for example, a first transmission module, a first adjustment module, and a second adjustment module. The first transmission module is used to change the propagation direction of the laser and expand the diameter of the laser beam; the first adjustment module is used to adjust the power and polarization state of the laser to obtain the first laser; the second adjustment module is used to adjust the diameter and shape of the first laser beam and focus the adjusted first laser beam on the corresponding refractive index modulation point in the optical fiber.

[0042] In some embodiments, the first transmission module includes: a high-reflection mirror and a beam expander. The high-reflection mirror is used to change the propagation direction of the laser emitted by the femtosecond laser. The high-reflection mirror has a high reflectivity and can efficiently reflect the laser emitted by the femtosecond laser to the next optical element. Using a high-reflection mirror in the optical path can ensure that the energy loss of the laser during the transmission process is minimized. The laser is reflected to the beam expander by the high-reflection mirror, and the beam expander expands the diameter of the laser beam while reducing the divergence angle of the beam, thereby obtaining a smaller focused spot to increase the power density of the laser, thereby producing a stronger refractive index modulation effect in the optical fiber.

[0043] The first adjustment module includes: an electric laser power attenuator and a polarizer group. The electric laser power attenuator can adjust the power of the laser as needed. When making a toe-cut fiber grating, the power of the laser needs to be precisely controlled to ensure the depth and uniformity of the refractive index modulation. The polarizer group includes a polarization controller and a polarization converter, which are used to adjust the polarization state of the laser to obtain the first laser. The polarization state of the laser has an important influence on its interaction with the optical fiber material. By adjusting the angle and configuration of the polarizer, the interaction conditions between the laser and the optical fiber can be optimized, thereby improving the efficiency and quality of the refractive index modulation.

[0044] The second adjustment module includes: an aperture beam limiter and an objective lens. The aperture beam limiter is used to limit the diameter and shape of the first laser beam. By adjusting the size and position of the aperture, it can ensure that the laser beam is irradiated on the optical fiber with a suitable size and shape, thereby avoiding unnecessary energy loss and spot distortion. The objective lens is used to focus the first laser beam to the refractive index modulation point in the optical fiber. By accurately adjusting the focal length and position of the objective lens, it can ensure that the laser beam is irradiated on the optical fiber with extremely high power density, thereby generating a strong refractive index modulation effect in the optical fiber.

[0045] The displacement platform is used to move the optical fiber to ensure that the laser can accurately irradiate the corresponding refractive index modulation point in the optical fiber to perform refractive index modulation and obtain an apodized fiber grating. Specifically, the optical fiber is placed on the optical fiber clamping unit on the displacement platform, and an apodized fiber grating structure is formed in the optical fiber by femtosecond laser direct writing.

[0046] By controlling the moving speed and distance of the displacement platform, the interaction time and area between the laser and the optical fiber can be precisely controlled, thereby producing a grating structure with a specific refractive index distribution from the optical fiber.

[0047] The control module is used to control the femtosecond laser and the displacement platform to improve the accuracy of making the apodized fiber grating. Specifically, the control module can be used to design the structure to be engraved, set the grating parameters and the parameters related to the motion during the processing, and set the single pulse laser energy and pulse variation parameters.

[0048] The control module is used to design the structure to be engraved, including: setting the processing position and path through the software control module, ensuring that the laser engraving point is located in the middle of the fiber core, and that the engraving points are evenly arranged along the fiber axis with equal spacing. It should be noted that the uniform fiber grating engraving must ensure a constant engraving speed.

[0049] Setting the grating parameters and the parameters related to the motion in the processing process through the control module includes: setting the processing speed of the translation stage, the grating structure parameters, such as the central wavelength, the grating length, the grating period, and the refractive index modulation depth. Setting the single pulse laser energy and pulse variation parameters through the control module includes: selecting the type of grating to be written and the pulse variation parameters with the apodization function, and the control module adjusts the distribution of the number of laser pulses according to the preset envelope function to achieve any change in the refractive index modulation depth in the desired apodized fiber grating, and different types of apodized gratings can be obtained with different apodization function variation curves.

[0050] The control module is used to design the structure to be engraved, and the grating parameters and parameters related to the movement of the displacement platform during the processing are set. For example, the processing speed of the displacement platform, the grating structure parameters, and the laser pulse number change curve can be set through the control module. The grating type to be engraved, the pulse change parameters with the apodization function, and the single-pulse laser energy are selected. The control module accurately adjusts the distribution of the laser pulse according to the writing requirements and the preset envelope function, and realizes the fine control and arbitrary change of the refractive index modulation depth in the fiber grating, thereby achieving the expected spectral characteristics.

[0051] The femtosecond laser writing system provided by the present invention sets a reasonable laser energy attenuation value and a maximum single pulse energy through a control module, selects a suitable single pulse laser energy, a grating type to be written, and a pulse change parameter with a cut-off function, so that the femtosecond system can accurately adjust the distribution of the number of laser pulses according to a preset envelope function, realize fine control and arbitrary change of the refractive index modulation depth in the fiber grating, and thus achieve the expected spectral characteristics.

[0052] In addition, the femtosecond laser writing system provided by the present disclosure may also include, for example, a spectrometer, a broadband light source, and a circulator for real-time monitoring of spectral changes caused by the apodized fiber grating structure.

[0053] And, an LED light source and an image sensor, wherein the LED light source is arranged below the displacement platform for illuminating the displacement platform, and the image sensor is used to capture the image of the apodized fiber grating structure on the displacement platform in real time for real-time monitoring by the control module.

[0054] The following combination Figure 1 The process of making apodized fiber gratings based on the femtosecond laser writing system provided by the present invention is further introduced.

[0055] In some embodiments, the laser emitted by the femtosecond laser is reflected by a high-reflection mirror to a beam expander, attenuated by an electric laser power attenuator, and then transmitted to two polarizers via a high-reflection mirror to regulate the laser polarization state. The regulated laser beam is reflected again by the high-reflection mirror, limited by an aperture and focused by an objective lens, and then irradiated onto the optical fiber to perform refractive index modulation.

[0056] By adjusting the LED illumination light source under the displacement platform, the image transmitted by the image sensor can be monitored in real time. At the same time, the spectral changes caused by the apodized fiber grating structure can be monitored in real time through the spectrometer, broadband light source and circulator.

[0057] Before making apodized fiber Bragg gratings, the preparation work for making apodized fiber Bragg gratings is first carried out: the structure to be written, grating parameters (such as central wavelength, grating length, grating period, refractive index modulation depth, etc.), motion-related parameters during processing (such as processing speed of the displacement platform, etc.), laser energy attenuation value and maximum single pulse energy are determined based on the control module. After wiping the optical fiber with alcohol cotton, place it on the optical fiber clamping unit on the XY displacement platform, and then immerse the area to be processed in the refractive index matching liquid to reduce the refractive index difference of different layers of the optical fiber and the influence of the resulting cylindrical lens effect.

[0058] After the preparation work is completed, the fiber Bragg grating is engraved. The grating structure and reflection spectrum are monitored in real time through image sensors and spectrometers to check whether the performance of the fiber Bragg grating meets the requirements and ensure that the quality of the produced fiber Bragg grating is controllable. After the engraving is completed, the engraved apodized fiber Bragg grating is removed.

[0059] Figure 2-Figure 5 The schematic diagram shows the structure of a uniformly apodized fiber grating manufactured by a femtosecond laser writing system provided by some embodiments of the present disclosure.

[0060] Figure 2 A schematic diagram of a uniformly apodized fiber grating junction structure fabricated based on a femtosecond laser point-by-point method according to an embodiment of the present disclosure is shown schematically.

[0061] like Figure 2 As shown, the fabricated apodized fiber Bragg grating structure includes a fiber core 1 and a cladding 2, and the number of femtosecond laser point-by-point pulses 3 is set to change according to the apodized envelope, thereby obtaining a uniform apodized fiber Bragg grating structure. Figure 2 It can be seen that the grating period 4 of the uniformly cut fiber grating obtained by the femtosecond laser writing system provided by the embodiment of the present disclosure remains unchanged, indicating that the femtosecond laser processing speed remains constant.

[0062] Figure 3 A schematic diagram of a uniformly apodized fiber Bragg grating structure fabricated based on a laser line-by-line method according to an embodiment of the present disclosure is shown; Figure 4A schematic diagram of a tilted apodized fiber Bragg grating structure fabricated based on a laser line-by-line method according to an embodiment of the present disclosure is shown; Figure 5 A schematic diagram of a uniformly apodized fiber grating structure produced by a femtosecond laser face-by-face method according to an embodiment of the present disclosure is shown.

[0063] As shown in the figure above, the processed structure 5 on the cross section of the optical fiber is distributed along the axial direction of the optical fiber, and the pulse number of the obtained apodized fiber grating gradually decreases from the center of the grating to the two ends of the optical fiber. The refractive index modulation depth is the largest in the central area of ​​the grating, and the refractive index modulation amplitude is the smallest at both ends of the grating. Through the above steps, the required apodized fiber grating based on the femtosecond laser system can be accurately produced to meet the application needs of different fields, with significant technical advantages and market potential.

[0064] The embodiments of the present disclosure are described above. However, these embodiments are only for illustrative purposes and are not intended to limit the scope of the present disclosure. Although the embodiments are described above separately, this does not mean that the measures in the various embodiments cannot be used in combination to advantage. Without departing from the scope of the present disclosure, those skilled in the art may make a variety of substitutions and modifications, which should all be included in the scope of protection of the present disclosure.

Claims

1. A femtosecond laser writing system for making apodized fiber Bragg gratings, characterized in that: The system comprises: A femtosecond laser, used to output laser light, wherein the laser light is used to adjust the refractive index of the optical fiber, and the number of laser pulses corresponding to each refractive index modulation point in the optical fiber is different; wherein the number of laser pulses is determined based on a preset apodization envelope function; An adjustment module, used for adjusting the propagation direction and laser parameters of the output laser; The displacement platform is used to move the optical fiber so that the laser can accurately irradiate the corresponding refractive index modulation point in the optical fiber to perform refractive index modulation and obtain an apodized fiber grating; The control module is used to determine the number of laser pulses corresponding to each refractive index modulation point based on a preset apodization envelope function, adjust the laser output parameters of the femtosecond laser, and control the movement parameters of the displacement platform.

2. The femtosecond laser writing system according to claim 1, characterized in that: The adjustment module comprises: A first transmission module, used to change the propagation direction of the laser and expand the diameter of the laser beam; A first adjustment module, used for adjusting the power and polarization state of the laser to obtain a first laser; The second adjustment module is used to adjust the diameter and shape of the first laser beam and focus the adjusted first laser beam on a corresponding refractive index modulation point in the optical fiber.

3. The femtosecond laser writing system according to claim 2, characterized in that: The first transmission module includes: A high-reflection mirror, used to change the propagation direction of the laser emitted by the femtosecond laser and reflect the laser to the beam expander; Beam expanders are used to expand the diameter of a laser beam.

4. The femtosecond laser writing system according to claim 2, characterized in that: The first adjustment module includes: Electric laser power attenuator, used to adjust the laser power; The polarizer group is used to adjust the polarization state of the laser to obtain the first laser.

5. The femtosecond laser writing system according to claim 2, characterized in that: The second adjustment module includes: An aperture beam limiter, used for limiting the diameter and shape of the first laser beam; An objective lens is used to focus the first laser beam to a refractive index modulation point in the optical fiber.

6. The femtosecond laser writing system according to claim 1, characterized in that: The energy of a single laser pulse output by the femtosecond laser is kept constant during the apodization process, and the number of laser pulses varies based on a preset apodization envelope function.

7. The femtosecond laser writing system according to claim 6, characterized in that: The preset apodization envelope functions include: Gaussian function, Hamming function, Blackman function, raised cosine function, Tanh function, Cauchy function, and sinc function.

8. The femtosecond laser writing system according to claim 6, characterized in that: The structure of the apodized fiber grating is written based on a femtosecond laser direct writing method, which includes a point-by-point method, a line-by-line method, and a plane-by-plane method.

9. The femtosecond laser writing system according to claim 1, characterized in that: The system also includes a spectrometer, a broadband light source and a circulator, wherein the spectrometer, the broadband light source and the circulator are used to monitor the spectrum changes caused by the apodized fiber grating structure in real time.

10. The femtosecond laser writing system according to claim 1, characterized in that: The system further comprises: An LED light source is disposed below the displacement platform and is used to illuminate the displacement platform; The image sensor is used to capture the image of the apodized fiber grating structure on the displacement platform in real time so as to provide real-time monitoring for the control module.

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