CN211603608U - Femtosecond laser direct-writing fiber grating preparation device based on machine learning image recognition - Google Patents

Abstract

The utility model discloses a femto second laser directly writes fiber grating preparation facilities based on machine learning image recognition, directly write optical path system including femto second laser, the real-time imaging system of visible light, orientation module and image recognition program control module, femto second laser directly writes optical path system, the real-time imaging system of visible light and orientation module all with image recognition program control module electric connection, according to waiting to carve the picture of carving of optic fibre, image recognition program control module treats through orientation module that carves optic fibre and fixes a position, and directly write optical path system through femto second laser and carve the optic fibre of carving after fixing a position, observe the imaging picture of carving the optic fibre of waiting to carve after carving through the real-time imaging system of visible light simultaneously. The utility model discloses can fix a position optic fibre Bragg grating orbit, realize by the micron order of magnitude to the fiber grating of the arbitrary length in the meter level scope, also can realize the Bragg fiber grating of arbitrary type.

Description

Fabrication device for femtosecond laser direct writing fiber grating based on machine learning image recognition

technical field

The utility model relates to the technical field of femtosecond laser direct writing fiber Bragg grating preparation, in particular to a femtosecond laser direct writing fiber grating preparation device based on machine learning image recognition.

Background technique

Fiber Bragg gratings have the characteristics of small size, good wavelength selectivity, easy coupling and integration with optical fiber devices and systems, and easy use and maintenance. The preparation methods of fiber gratings mainly include two-beam interference method and phase mask preparation method based on ultraviolet band laser, and direct writing method and phase mask preparation method based on near-infrared femtosecond laser. The former requires the fiber core to have a certain photosensitivity, while the latter overcomes the limitation of the former in terms of optical fibers, and can prepare fiber gratings in any type of silica glass fiber. In addition, the length and type of the fiber grating obtained by the phase mask manufacturing method depend on the length and type of the phase mask used, and the manufacturing cost is relatively high. However, the direct writing method based on the near-infrared femtosecond laser avoids the limitation of the phase mask. By precisely controlling the moving speed of the optical fiber or the femtosecond laser focusing beam and the repetition frequency of the femtosecond laser, different lengths can be flexibly prepared. and any type of fiber grating, and the cost is low. Therefore, in recent years, the preparation technology of near-infrared femtosecond laser direct writing fiber grating has attracted more and more attention and favor from domestic and foreign industrial and academic circles.

The femtosecond laser direct writing method to fabricate fiber grating is to focus the incident femtosecond laser beam into the fiber core and expose it point by point along the fiber axis, so that the refractive index of the fiber core forms a periodically distributed grating structure along the axis. Using visible light imaging technology and high-precision XYZ three-dimensional displacement platform control technology, the focused beam can be accurately focused to the center of the fiber core, and at the same time, by moving the laser beam or fiber with high precision, a grating structure that follows a certain trajectory is formed in the fiber core. When the grating track coincides with the central axis of the fiber, the coupling efficiency of the grating is the highest and the backlight loss is the lowest. When the grating track deviates from the central axis of the fiber, it will cause problems such as low coupling efficiency of the grating, unnecessary cladding modes and grating insertion loss. Therefore, the precise alignment of grating tracks is an important prerequisite for the preparation of high-quality fiber gratings by femtosecond laser direct writing.

Utility model content

Purpose of the utility model: For the preparation of fiber gratings by the femtosecond laser direct writing method, the actual trajectory of the fiber obtained by the existing two-point fixation method is nonlinear, resulting in low coupling efficiency and poor repeatability of the grating, and it is even impossible to prepare a certain length of fiber grating. To solve the problem of fiber grating, the utility model proposes a femtosecond laser direct writing fiber grating preparation device based on machine learning image recognition.

Technical scheme: in order to realize the purpose of the present utility model, the technical scheme adopted by the present utility model is:

A femtosecond laser direct writing fiber grating preparation device based on machine learning image recognition, the femtosecond laser direct writing fiber grating preparation device includes a femtosecond laser direct writing optical path system, a visible light real-time imaging system, a positioning module and an image recognition program The control module, the femtosecond laser direct writing optical path system, the visible light real-time imaging system and the positioning module are all electrically connected with the image recognition program control module. The optical fiber to be inscribed is positioned, and the positioned optical fiber to be inscribed is inscribed by the femtosecond laser direct writing optical path system, and the imaging picture of the inscribed optical fiber to be inscribed is observed by the visible light real-time imaging system.

Further, the optical fiber to be engraved includes a core, a cladding layer and a coating layer, the core is arranged in the center of the optical fiber to be engraved, the cladding layer is arranged outside the core, and the coating layer is arranged On the outside of the cladding, both the core and the cladding have a refractive index range of 1.44-2.5.

Further, the femtosecond laser direct writing optical path system includes a near-infrared femtosecond laser or amplifier, an electronic shutter, a combination lens, a dichroic mirror and a focusing lens, and the near-infrared femtosecond laser or amplifier emits a femtosecond laser. The femtosecond laser beam is sent to a dichroic mirror through an electronic shutter and a combined lens for high reflection, and the high reflection femtosecond laser beam is focused on the fiber to be engraved through a focusing lens, and the Optical fiber for writing.

Further, the combined lens includes a half-wave plate I, a Glan laser wave plate and a half glass plate II, the Glan laser wave plate is arranged in the middle of the half-wave plate I and the half glass plate II, and the The focal centers of half-wave plate I, Glan laser wave plate and half-glass plate II are on the same horizontal plane.

Further, the visible light real-time imaging system includes a visible light source and a CCD camera, the visible light source emits a visible light beam, the visible light beam passes through the optical fiber to be engraved into a focusing lens, and is focused on a dichroic by the focusing lens. At the same time, it enters the CCD camera through the high transmittance of the dichroic mirror, and the imaging picture of the fiber to be engraved is obtained in the CCD camera.

Further, the positioning module includes a three-dimensional displacement platform, on the three-dimensional displacement platform, a U-shaped groove assembly is fixed by a clamp of a three-dimensional adjustment frame, and the optical fiber to be engraved is arranged in the U-shaped groove assembly by an optical fiber clamp. inside of the piece.

Further, the U-shaped groove assembly includes a U-shaped groove, a cover glass and a refractive index matching liquid, the optical fiber to be engraved is placed in the middle of the U-shaped groove, and the refractive index matching liquid is arranged on the optical fiber to be engraved. Between the inner side of the U-shaped groove, the cover glass is arranged on the upper surface of the U-shaped groove.

Further, the image recognition program control module includes a computer and a data line, and the computer is electrically connected to the electronic shutter, the CCD camera and the three-dimensional displacement platform through the data line.

The working principle of the present utility model:

S1: Through the femtosecond laser direct writing optical path system and the visible light real-time imaging system, the femtosecond laser incident optical path and the visible light imaging optical path are coaxial and confocal;

S2: Open the electronic shutter, the near-infrared femtosecond laser or amplifier emits a femtosecond laser beam, and simultaneously open the imaging software in the CCD camera through the computer, the CCD camera enables the real-time capture mode, and adjusts the near-infrared femtosecond laser beam. The power of the infrared femtosecond laser or amplifier until a bright spot is seen in the imaging software, and the position of the bright spot is marked on the display screen of the computer;

S3: Close the electronic shutter (3), and adjust the position of the three-dimensional displacement platform (16), so that when the visible light source (9) emits a visible light beam (10), an image is obtained in the CCD camera (11) Picture, and confirm whether the marked bright spot position is located in the center of the fiber core (12.1) of the imaging picture, if so, keep it unchanged, if not, move the three-dimensional displacement platform (16) until the The marked bright spot is located at the center of the fiber core (12.1) of the imaging picture;

S4: Find the optical fiber central axis track of the fiber grating region to be engraved;

S5: Return the X-axis, Y-axis and Z-axis of the three-dimensional displacement platform to the starting position of the fiber grating to be engraved, and set the X-axis movement speed of the three-dimensional displacement platform at the same time, and according to the optical fiber center axis trajectory, Set the Y-axis and Z-axis displacement of the three-dimensional displacement platform;

S6: Set the output power of the near-infrared femtosecond laser or amplifier, open the electronic shutter, start the writing program, and close the electronic shutter until the X-axis of the three-dimensional displacement platform reaches the end position of the fiber grating to be inscribed. Finish the preparation of the fiber grating to be engraved.

Further, in the step S1, the incident light path of the femtosecond laser and the visible light imaging light path are made coaxial and confocal, as follows:

S1.1: Pass the femtosecond laser beam vertically through the center of the electronic shutter and the combined lens, and at the same time enter the center of the dichroic mirror at 45°, the femtosecond laser beam passes through the high reflection of the dichroic mirror and enters vertically Three-dimensional displacement platform, and normal incidence focusing lens;

S1.2: After the visible light beam passes through a focusing lens and a dichroic mirror, it is vertically incident into the center of the imaging chip of the CCD camera.

Beneficial effects: Compared with the prior art, the technical solution of the present utility model has the following beneficial technical effects:

(1) The present utility model can precisely locate the fiber Bragg grating trajectory, and can be customized according to the moving range and actual needs of the displacement platform, and can realize the fiber grating of any length in the range from the micron level to the meter level, and can also control the fly by the program. The repetition frequency, movement rate and position distribution of the second pulse in the fiber can realize uniform fiber Bragg grating, chirped fiber Bragg grating, apodized fiber Bragg grating, phase-shifted fiber Bragg grating and sampled fiber Bragg grating, that is, any type of fiber Bragg grating can be realized. fiber Bragg grating;

(2) The size, material and type of the optical fiber of the present invention are not limited, no pretreatment is required, and the coating layer of the optical fiber is also not limited, and can be written with the coating layer, thereby enhancing the robustness of the fiber Bragg grating It also improves the efficiency and repeatability of optical fiber preparation, and reduces the input cost.

Description of drawings

1 is a diagram of a femtosecond laser direct writing fiber grating preparation device based on machine learning image recognition of the present utility model;

Fig. 2 is the structural schematic diagram of the fiber grating to be engraved in the three-dimensional displacement platform of the present invention;

3 is a schematic cross-sectional view of the optical fiber to be engraved of the present invention placed in a U-shaped groove;

Fig. 4 is the microscope picture after the utility model writes grating on the central axis of G652D and UHNA3 optical fibers;

5 is a 50 mm long G652D fiber uniform Bragg grating transmission spectrum diagram of the present invention;

6 is a 50 mm long UHNA3 fiber uniform Bragg grating transmission spectrum diagram of the present invention;

The label in the figure corresponds to the name of the part:

1. Near-infrared femtosecond laser or amplifier; 2. Femtosecond laser beam; 3. Electronic shutter; 4. Half-wave plate; 5. Glan laser wave plate; 6. Half-glass; 7. Dichroic mirror; 8 , focusing lens; 9, visible light source; 10, visible light beam; 11, CCD camera; 12, fiber to be engraved; 12.1, fiber core; 12.2, cladding; 12.3, coating layer; 13, fiber fixture; 14, U-shaped Tank assembly; 14.1, U-shaped groove; 14.2, Cover glass; 14.3, Refractive index matching liquid; 15, Fixture of 3D adjustment frame; 16, 3D displacement platform; 17, Computer; 18, Data cable; 19, To be engraved fiber grating.

Detailed ways

In order to make the purposes, technical solutions and advantages of the embodiments of the present utility model more clear, the technical solutions in the embodiments of the present utility model will be described clearly and completely below with reference to the accompanying drawings in the embodiments of the present utility model. Among them, the described embodiments are part of the embodiments of the present invention, but not all of the embodiments. Accordingly, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the invention as claimed, but is merely representative of selected embodiments of the invention.

Example 1

Referring to FIG. 1, this embodiment provides a femtosecond laser direct writing fiber grating preparation device based on machine learning image recognition. The femtosecond laser direct writing fiber grating preparation device is composed of a femtosecond laser direct writing optical path system and a visible light real-time imaging system. It consists of four parts: positioning module and image recognition program control module. Among them, the femtosecond laser direct writing optical path system, the visible light real-time imaging system and the positioning module are all electrically connected with the image recognition program control module. Position, and use the femtosecond laser direct writing optical path system to inscribe the positioned optical fiber 12 to be inscribed, and observe the imaging picture of the inscribed optical fiber 12 to be inscribed through the visible light real-time imaging system.

2 , the optical fiber 12 to be engraved includes a core 12.1, a cladding 12.2 and a coating layer 12.3. In this embodiment, the optical fiber 12 to be engraved refers to a solid core optical fiber with a centrally symmetric cross section. The core 12.1 is arranged in the center of the optical fiber 12 to be engraved, and the outer surface of the core 12.1 is surrounded by a cladding layer 12.2, and the outer surface of the cladding layer 12.2 is surrounded by a coating layer 12.3, that is, the diameter of the coating layer 12.3 is larger than that of the cladding layer 12.2. , the diameter of the cladding 12.2 is greater than the diameter of the core 12.1. Specifically, the number of layers of the core 12.1 is 1, the number of layers of the cladding layer 12.2 is 1 or 2, and the number of layers of the coating layer 12.3 is 0-3. When the number of layers of the coating layer 12.3 is 0, then It means that the optical fiber 12 to be engraved does not contain the coating layer 12.3.

At the same time, the refractive index range of the core 12.1 and the cladding 12.2 is 1.44-2.5, which are based on quartz glass, fluoride glass, silicate glass, phosphate glass, borosilicate glass, sulfide glass, telluric acid Salt glasses, and optical fibers made from mixtures of these glass matrices. The diameter of the core 12.1 is in the range of 0.5 microns to 100 microns, and the diameter of the cladding 12.2 is in the range of 50 microns to 2 mm.

In the femtosecond laser direct writing optical path system, the femtosecond laser direct writing optical path system includes a near-infrared femtosecond laser or amplifier 1, an electronic shutter 3, a combined lens, a dichroic mirror 7 and a focusing lens 8, wherein the combined lens includes a Half wave plate I4, Glan laser wave plate 5 and half glass plate II6. Specifically, the near-infrared femtosecond laser or amplifier 1 is a femtosecond laser light source, which emits a femtosecond laser beam 2, and the center wavelength range of the femtosecond laser light source is: 780 nm-1080 nm, and the pulse width range is: 25 femtoseconds Second-220 femtoseconds, repetition frequency range: 1Hz-100MHz. The electronic shutter 3 is used to control the on and off of the femtosecond laser pulse when the grating is written, the combined lens is used to adjust the power and polarization state of the femtosecond laser, and the dichroic mirror 7 is used to achieve high reflection of the incident femtosecond laser, and to the incident femtosecond laser. The visible light achieves high transparency, and the focusing lens 8 is used to focus the converging light spot to the micron level, which is a microlens or oil lens of 10X, 20X, 30X, 40X, 50X, 60X, 70X, 80X, 90X and 100X.

After the femtosecond laser beam 2 passes through the electronic shutter 3, the half-wave plate I4, the Glan laser wave plate 5 and the half-glass plate II6 in sequence, it enters the dichroic mirror 7 for high reflection, and the high-reflection femtosecond laser beam 2 The optical fiber 12 to be inscribed is focused on the optical fiber 12 to be inscribed by the focusing lens 8, and the optical fiber 12 to be inscribed is inscribed. It is worth noting that the Glan laser wave plate 5 is arranged in the middle of the half wave plate I4 and the half glass plate II6, and the focal centers of the half wave plate I4, the Glan laser wave plate 5 and the half glass plate II6 are on the same horizontal plane.

In the visible light real-time imaging system, the visible light real-time imaging system includes a visible light source 9 and a CCD camera 11, wherein the visible light source 9 emits a visible light beam 10 that is close to parallel light, and the visible light source 9 can be a visible light LED or a broad-spectrum visible light source , for example: halogen light sources. The CCD camera 11 can be an industrial-grade or scientific-grade high-resolution color or black-and-white camera.

The visible light beam 10 will pass through the optical fiber 12 to be engraved into the focusing lens 8, through the focusing of the focusing lens 8, the visible light beam 10 will be focused in the dichroic mirror 7, and enter the CCD camera 11 through the high transmittance of the dichroic mirror 7, The imaging picture of the optical fiber 12 to be engraved is obtained in the CCD camera 11 .

Referring to FIG. 3, in the positioning module, the positioning module includes a three-dimensional displacement platform 16, and the three-dimensional displacement platform 16 is provided with an optical fiber clamp 13 and a U-shaped groove assembly 14, wherein the U-shaped groove assembly 14 is composed of The U-shaped groove 14.1, the cover glass 14.2 and the refractive index matching liquid 14.3 are formed. Specifically, the U-shaped groove assembly 14 is fixed on the three-dimensional displacement platform 16 by the clamp 15 of the three-dimensional adjustment frame, and the optical fiber 12 to be engraved is placed in the middle of the U-shaped groove 14.1 by the optical fiber clamp 13. The inner side of the U-shaped groove 14.1 The structure can be a U-shaped structure, a V-shaped structure, or other structures, the inner diameter of which is not less than the outer diameter of the optical fiber 12 to be engraved, and a refraction is provided between the optical fiber 12 to be engraved and the inner side of the U-shaped groove 14.1. The rate matching solution 14.3 is provided with a cover glass 14.2 on the upper surface of the U-shaped groove 14.1, and its thickness ranges from 80 microns to 150 microns. The refractive index range of the refractive index matching liquid 14.3 is: 1.33-2.5, which can be edible oil, refractive index matching oil for commercial use or scientific research, and the like. The control precision of the three-dimensional displacement platform 16 is 1 nanometer, and the moving speed range is: 0.0001 mm/s-10 m/s.

In the image recognition program control module, the image recognition program control module includes a computer 17 and a data line 18 , and the computer 17 is electrically connected to the electronic shutter 3 , the CCD camera 11 and the three-dimensional displacement platform 16 through the data line 18 .

This embodiment also provides a preparation method of a femtosecond laser direct writing fiber grating preparation device based on machine learning image recognition, and the preparation method specifically includes the following steps:

Step S1: Complete the preparation for the femtosecond laser incident optical path and the visible light imaging optical path, that is, through the femtosecond laser direct writing optical path system and the visible light real-time imaging system, make the femtosecond laser incident optical path and the visible light imaging optical path coaxial and confocal, as follows:

Step S1.1: The femtosecond laser beam 2 vertically passes through the center of the electronic shutter 3 and the combined lens, and enters the center of the dichroic mirror 7 at a 45° angle. After the femtosecond laser beam 2 passes through the dichroic mirror 7, the Entering the XY plane of the three-dimensional displacement stage 16 perpendicularly, and entering the focusing lens 8 perpendicularly.

Step S1.2: After the visible light beam 10 passes through the focusing lens 8 and the dichroic mirror 7, it is vertically incident into the center of the imaging chip of the CCD camera 11.

Step S2: Open the electronic shutter 3, the near-infrared femtosecond laser or the amplifier 1 emits the femtosecond laser beam 2, and at the same time, the imaging software in the CCD camera 11 is opened through the computer 17, and the real-time capture mode is turned on, and the near-infrared flying is adjusted from small to large. The power of the second laser or amplifier 1, until a bright spot is seen in the imaging software, and the position of the bright spot is marked on the display screen of the computer 17, and the mark indicates that the femtosecond laser beam 2 passes through the focusing lens Imaging position of the focus after 8.

Step S3: close the electronic shutter 3, adjust the position of the three-dimensional displacement platform 16, so that when the visible light source 9 emits the visible light beam 10, the imaging picture is obtained in the CCD camera 11, and confirm whether the bright spot position marked in step S2 is located in the imaging picture. The core center, if located, remains unchanged.

If it is not located, move the Y axis of the three-dimensional displacement platform 16 with an accuracy of 1 nanometer until the marked bright spot is located at the center of the fiber core 12.1 of the imaging picture, and then obtain the image of the fiber to be engraved 12 in the center of the fiber core 12.1. its characteristic curve.

Step S4: Find the optical fiber central axis track in the area of the fiber grating 19 to be engraved.

Step S5: Return the X-axis, Y-axis and Z-axis of the three-dimensional displacement platform 16 to the starting position of the fiber grating 19 to be engraved, and set the X-axis movement speed of the three-dimensional displacement platform 16 at the same time, and according to the optical fiber obtained in step S4 The trajectory of the central axis is used to set the displacement amounts of the Y-axis and Z-axis of the three-dimensional displacement platform 16 .

In this embodiment, the characteristic formula of the fiber Bragg grating is specifically:

m·λ _FBG =2·n _eff ·Λ

Where: m is the order of the fiber Bragg grating, λ _FBG is the center wavelength of the fiber Bragg grating, n _eff is the effective refractive index of the fiber to be engraved, and Λ is the period of the fiber Bragg grating.

Specifically, the fiber Bragg grating period Λ is specifically:

Λ=v/f

Among them: Λ is the fiber Bragg grating period, v is the moving speed of the fiber to be engraved along the X axis of the three-dimensional displacement platform, and f is the repetition frequency of the femtosecond laser.

Step S6: set the output power of the near-infrared femtosecond laser or amplifier 1, and open the electronic shutter 3, start the writing program, until the X-axis of the three-dimensional displacement platform 16 reaches the end position of the fiber grating 19 to be engraved, close all The electronic shutter 3 is finished, and the preparation of the fiber grating 19 to be engraved is completed.

After the preparation of the fiber grating is completed, the reflection and transmission spectra of the prepared fiber grating are tested by a spectrometer, and the morphology and trajectory of the prepared fiber grating in the fiber core are detected by an optical microscope to complete the characterization of the fiber grating.

Referring to Figure 4, the incident femtosecond laser pulse energy of G652D fiber grating is 200nJ, the incident femtosecond laser pulse energy of UHNA3 fiber grating is 86nJ, and the X-axis moving speed is v=1.08 mm/s. Under these conditions, a uniform fiber Bragg grating with a length of 50 mm was prepared, and the transmission spectrum was measured with a wavelength accuracy of 0.02 nm, as shown in Figures 5 and 6. It can be seen from the figures that the center wavelength of the G652D fiber grating is 1556.82 nm, and the grating The reflectivity at the center wavelength is 99.98%, the center wavelength of the UHNA3 fiber grating is 1564.85nm, and the reflectivity at the center wavelength of the grating is 99.76%.

The present invention and its embodiments have been schematically described above, and the description is not restrictive, and what is shown in the accompanying drawings is only one of the embodiments of the present invention, and the actual structure and method are not limited thereto. Therefore, if those of ordinary skill in the art are inspired by it, without departing from the purpose of creation of the present utility model, without creative design of the structural mode and embodiment similar to the technical solution, all belong to the protection of the present utility model. scope.

Claims (8)

1. a femtosecond laser direct writing fiber grating preparation device based on machine learning image recognition, is characterized in that, described femtosecond laser direct writing fiber grating preparation device includes femtosecond laser direct writing optical path system, visible light real-time imaging system, The positioning module and the image recognition program control module, the femtosecond laser direct writing optical path system, the visible light real-time imaging system and the positioning module are all electrically connected with the image recognition program control module, and according to the to-be-engraved picture of the optical fiber (12) to be engraved, all the The image recognition program control module locates the optical fiber (12) to be engraved through the positioning module, and inscribes the optical fiber (12) to be engraved after the positioning through the femtosecond laser direct writing optical path system, and simultaneously passes through the visible light real-time imaging system. Observe the imaging picture of the optical fiber (12) to be inscribed after inscription. 2. A device for preparing femtosecond laser direct writing fiber grating based on machine learning image recognition according to claim 1, wherein the optical fiber (12) to be engraved comprises a fiber core (12.1), a cladding (12.1) 12.2) and a coating layer (12.3), the core (12.1) is arranged in the center of the optical fiber (12) to be engraved, the cladding layer (12.2) is arranged outside the core (12.1), the coating layer (12.3) is arranged on the outside of the cladding (12.2), and the refractive indices of the core (12.1) and the cladding (12.2) are both in the range of 1.44-2.5. 3. a kind of femtosecond laser direct writing fiber grating preparation device based on machine learning image recognition according to claim 1 or 2, is characterized in that, described femtosecond laser direct writing optical path system comprises a near-infrared femtosecond laser or An amplifier (1), an electronic shutter (3), a combination lens, a dichroic mirror (7) and a focusing lens (8), the near-infrared femtosecond laser or amplifier (1) emits a femtosecond laser beam (2), so The femtosecond laser beam (2) is sent to a dichroic mirror (7) through an electronic shutter (3) and a combined lens for high reflection, and the high reflection femtosecond laser beam (2) passes through a focusing lens (8) Focusing on the optical fiber (12) to be inscribed, and inscription on the optical fiber (12) to be inscribed. 4. A device for preparing femtosecond laser direct writing fiber grating based on machine learning image recognition according to claim 3, wherein the combined lens comprises a half-wave plate I (4), a Glan laser wave plate (5) and half glass plate II (6), the Glan laser wave plate (5) is arranged in the middle of half wave plate I (4) and half glass plate II (6), while the half wave plate I ( 4) The focal centers of the Glan laser wave plate (5) and the half glass plate II (6) are on the same horizontal plane. 5. A femtosecond laser direct writing fiber grating preparation device based on machine learning image recognition according to claim 3, wherein the visible light real-time imaging system comprises a visible light source (9) and a CCD camera (11) , the visible light source (9) emits a visible light beam (10), and the visible light beam (10) passes through the optical fiber (12) to be engraved and enters the focusing lens (8), and is focused on the two-way through the focusing lens (8). In the chromatic mirror (7), the high transmittance of the dichroic mirror (7) enters the CCD camera (11) at the same time, and the imaging picture of the optical fiber (12) to be engraved is obtained in the CCD camera (11). 6. A femtosecond laser direct writing fiber grating preparation device based on machine learning image recognition according to claim 5, wherein the positioning module comprises a three-dimensional displacement platform (16), and the three-dimensional displacement platform On (16), the U-shaped groove assembly (14) is fixed by the clamp (15) of the three-dimensional adjustment frame, and the optical fiber (12) to be engraved is arranged on the U-shaped groove assembly (14) by the fiber clamp (13). internal. 7. The device for preparing femtosecond laser direct writing fiber grating based on machine learning image recognition according to claim 6, wherein the U-shaped groove assembly (14) comprises a U-shaped groove (14.1), A cover glass (14.2) and a refractive index matching liquid (14.3), the optical fiber (12) to be engraved is placed in the middle of the U-shaped groove (14.1), and the refractive index matching liquid (14.3) is arranged on the optical fiber (12) to be engraved ) and the inner side of the U-shaped groove (14.1), the cover glass (14.2) is arranged on the upper surface of the U-shaped groove (14.1). 8. A femtosecond laser direct writing fiber grating preparation device based on machine learning image recognition according to claim 6, wherein the image recognition program control module comprises a computer (17) and a data line (18) , the computer (17) is electrically connected to the electronic shutter (3), the CCD camera (11) and the three-dimensional displacement platform (16) through a data line (18).

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