Double-clad square chalcogenide fiber and fiber grating preparation method based on double-clad square chalcogenide fiber

Abstract

The invention discloses a double-clad square chalcogenide fiber and a fiber grating preparation method based on the double-clad square chalcogenide fiber, and belongs to the technical field of intermediate infrared fiber devices. According to the double-cladding square chalcogenide optical fiber disclosed by the invention, the core layer is sequentially coated with the inner cladding and the outer cladding, the overall section shape is designed to be square, each side face of the square optical fiber is a plane, the core layer is sequentially coated with the inner cladding and the outer cladding, and the overall section shape is designed to be square, so that the double-cladding square chalcogenide optical fiber is obtained. Each side surface of the square optical fiber is a plane; based on the double-cladding square chalcogenide fiber, precise regulation and control of a laser focus in the fiber core can be realized, and the efficiency and quality of fiber grating inscribing are improved. Meanwhile, the plane structure on the other side of the optical fiber is utilized, real-time online monitoring can be carried out on the inscribing process, and the consistency and the performance stability of the grating are ensured; the invention has important application value in the technical fields of mid-infrared optical fiber sensing, nonlinear optics and laser.

Description

Technical Field

[0001] The invention belongs to the technical field of mid-infrared optical fiber devices, and in particular relates to a double-clad square chalcogenide optical fiber and a method for preparing an optical fiber grating based on the double-clad square chalcogenide optical fiber. Background Art

[0002] With the rapid development of mid-infrared band technology, sulfur-based optical fibers have attracted extensive attention in fiber-optic sensing, nonlinear optics and laser applications due to their high nonlinear coefficient, wide light transmission window and excellent infrared transmission characteristics. As a key device, fiber Bragg gratings have extremely high measurement accuracy and can accurately detect small changes, making them suitable for high-precision measurement and monitoring. At the same time, fiber Bragg gratings are not affected by electromagnetic fields and are suitable for measurement and sensing applications in strong electromagnetic environments. In addition, fiber Bragg gratings have long-term stability and reliability and can maintain stable performance in harsh environments, making them suitable for long-term monitoring and measurement tasks. Fiber Bragg gratings are widely used in structural health monitoring, temperature measurement, pressure sensing, vibration monitoring and other fields due to their excellent characteristics. They can replace traditional sensors and improve system performance and reliability.

[0003] However, the writing of fiber Bragg gratings on existing highly nonlinear chalcogenide fibers faces the following technical bottlenecks: 1) Self-focusing effect: Highly nonlinear chalcogenide fibers are prone to self-focusing effect under the action of high peak power femtosecond lasers, which makes it impossible to effectively focus the laser on the core area of ​​the highly nonlinear chalcogenide fibers, greatly limiting the efficiency and accuracy of fiber Bragg grating writing; 2) Structural limitations: The circular cross-section of traditional highly nonlinear chalcogenide fibers has a curvature arc surface, which will cause laser focus deviation during fiber Bragg grating writing; in addition, the arc surface structure makes it difficult to observe the fiber core and cannot be used for online writing monitoring. Summary of the invention

[0004] The purpose of the present invention is to provide a double-clad square chalcogenide fiber and a fiber grating based thereon, as well as a preparation and monitoring method, so as to solve the technical problem that the fiber curvature effect of the existing chalcogenide fiber interferes with the laser focusing, resulting in poor efficiency and accuracy of femtosecond laser writing gratings.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions: The present invention discloses a double-clad square chalcogenide optical fiber, which comprises a core layer, an inner cladding layer and an outer cladding layer; the inner cladding layer and the outer cladding layer are sequentially coated on the outside of the core layer; the cross sections of the core layer and the inner cladding layer are circular; the cross section of the outer cladding layer is square; The raw materials of the core layer, the inner cladding layer and the outer cladding layer are chalcogenide glass raw materials.

[0006] The present invention also discloses a method for preparing the above-mentioned double-clad square chalcogenide optical fiber, comprising the following steps: The raw materials of the chalcogenide glass are pre-treated, and then the core layer material, the inner cladding material and the outer cladding material are prepared respectively by a vacuum high-temperature melting method; The core layer material, the inner cladding material and the outer cladding material are mechanically processed and then sequentially placed into an extrusion die to prepare a double-clad preform rod by extrusion; The double-clad preform rod is drawn to obtain a core layer, an inner cladding layer and an outer cladding layer through drawing of the core layer material, the inner cladding layer material and the outer cladding layer material, and finally a double-clad square chalcogenide optical fiber is obtained.

[0007] Furthermore, the chalcogenide glass raw material includes As 40 S 60 、As 38 S 62 and As 36 S 64 ; The chalcogenide glass raw material of the core material is As 40 S 60； The raw material of the chalcogenide glass of the inner cladding material is As 38 S 62 ; The raw material of the chalcogenide glass of the outer cladding material is As 36 S 64 .

[0008] Further, the pretreatment of the chalcogenide glass raw material includes polishing and cleaning treatments performed in sequence; The process parameters of the extrusion method are: extrusion temperature is 280-330° C., protective gas is helium or argon, and extrusion speed is 0.1-0.4 mm / min.

[0009] Furthermore, the process parameters of the drawing process are: drawing temperature of 330-380° C., rod feeding speed of 0.2-0.6 mm / min, drawing speed of 2-10 m / min, and protective gas of helium or argon.

[0010] The present invention also discloses a method for preparing a fiber grating based on the above double-clad square chalcogenide fiber, comprising the following steps: The double-clad square chalcogenide fiber is installed in a femtosecond laser direct writing device, and the plane of the double-clad square chalcogenide fiber is adjusted to be perpendicular to the laser incident direction; The focus of the femtosecond laser is adjusted to be positioned at the core region of the double-clad square chalcogenide fiber; The fiber Bragg grating is obtained by writing the grating structure point by point using a femtosecond laser.

[0011] Furthermore, the femtosecond laser has a central wavelength of 800 nm, a pulse width of 91 fs, and a repetition rate of 0.5-1 kHz.

[0012] Furthermore, the grating period when the grating structure is written point by point using a femtosecond laser is 1292-2200nm.

[0013] Furthermore, the monitoring method during the preparation of the above-mentioned fiber Bragg grating comprises the following steps: On the side perpendicular to the plane of the double-clad square chalcogenide fiber perpendicular to the laser incident direction, the relative position of the focus of the femtosecond laser and the core of the double-clad square chalcogenide fiber during the writing process is observed in real time through a microscopic optical system to ensure the writing quality and consistency.

[0014] Furthermore, the microscope optical system includes a magnifying glass and a CCD imaging system; the magnifying glass and the CCD imaging system are electrically connected.

[0015] Compared with the prior art, the present invention has the following beneficial effects: The present invention discloses a double-clad square chalcogenide optical fiber, which is formed by sequentially coating an inner cladding and an outer cladding on the outside of a core layer, and designing the overall cross-sectional shape to be square, and each side of the square optical fiber is a plane. The design of the plane surface eliminates the focusing effect of a chalcogenide glass lens, thereby reducing the power density of a writing femtosecond laser, thereby eliminating the interference of a traditional optical fiber curvature effect on laser focusing, further effectively suppressing the self-focusing effect, and improving the efficiency and accuracy of femtosecond laser writing gratings.

[0016] The present invention also discloses a method for preparing a fiber grating based on the above-mentioned double-clad square chalcogenide fiber, which uses femtosecond laser direct writing technology to write a grating on the fiber core. Due to the planar design of the double-clad square chalcogenide fiber, the interference of nonlinear effects on laser focusing is reduced, and precise control of the laser focus in the fiber core can be achieved, thereby improving the efficiency and quality of fiber grating writing.

[0017] The present invention also discloses a monitoring method for the preparation method of a fiber grating, which utilizes the planar structure on the other side of the optical fiber to monitor the writing process in real time online, ensuring the consistency and performance stability of the grating, and solving the problem that the circular cross section of the traditional optical fiber has a curved surface of curvature, which causes the laser focus to shift during grating writing. In addition, the curved surface structure makes it difficult to observe the fiber core and cannot perform online writing monitoring. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a schematic structural diagram of a double-clad square chalcogenide optical fiber of the present invention; Where: 1-core layer; 2-inner cladding; 3-outer cladding; a-overall structure; b-cross section; Figure 2 A schematic diagram of the process of preparing the fiber grating of the present invention; Figure 3This is a graph showing the morphology test results of the fiber grating obtained by the present invention; Among them: a-main perspective; b-side perspective; Figure 4 This is a test chart of the transmission / reflection spectrum of the fiber grating prepared by the present invention. DETAILED DESCRIPTION

[0019] In order to enable those skilled in the art to understand the characteristics and effects of the present invention, the following is a general description and definition of the terms and expressions mentioned in the specification and claims. Unless otherwise specified, all technical and scientific terms used in the text are the common meanings understood by those skilled in the art for the present invention. In the event of a conflict, the definition in this specification shall prevail.

[0020] The theories or mechanisms described and disclosed herein, whether correct or incorrect, should not limit the scope of the present invention in any way, that is, the present invention can be implemented without being limited by any specific theory or mechanism.

[0021] In this article, all features such as values, quantities, contents and concentrations defined in the form of numerical ranges or percentage ranges are for simplicity and convenience only. Accordingly, the description of numerical ranges or percentage ranges should be considered to have included and specifically disclosed all possible secondary ranges and individual values ​​within the range (including integers and fractions).

[0022] In this document, unless otherwise specified, “includes,” “including,” “contains,” “has,” or similar terms cover the meanings of “consisting of” and “mainly consisting of,” for example, “A includes a” covers the meanings of “A includes a and other” and “A only includes a.”

[0023] In this document, in order to make the description concise, not all possible combinations of various technical features in various embodiments or examples are described. Therefore, as long as there is no contradiction in the combination of these technical features, the various technical features in various embodiments or examples can be combined arbitrarily, and all possible combinations should be considered to be within the scope of this specification.

[0024] like Figure 1 As shown, the first aspect of the present invention provides a double-clad square chalcogenide optical fiber, which optimizes the circular cross-section design of a traditional optical fiber into a square structure in which the four side surfaces are flat, thereby eliminating the focusing problem caused by the curvature effect in the traditional optical fiber, and effectively suppressing the self-focusing effect of the femtosecond laser writing process in the highly nonlinear optical fiber. The double-clad square chalcogenide optical fiber includes a core layer 1, an inner cladding 2 and an outer cladding 3.

[0025] The second aspect of the present invention discloses a method for preparing the above-mentioned double-clad square chalcogenide optical fiber, using high-purity chalcogenide glass material as the basic material, and adopting a vacuum high-temperature melting method to prepare high-performance outer cladding 3, inner cladding 2, and core layer 1 respectively.

[0026] The third aspect of the present invention discloses a method for preparing a fiber Bragg grating based on the above double-clad square chalcogenide fiber, such as Figure 2 As shown, the following steps are included: Step 1: Equipment debugging The double-clad square chalcogenide fiber is installed in the fixing fixture of the femtosecond laser direct writing device, and the plane of the double-clad square chalcogenide fiber is adjusted to be perpendicular to the laser incident direction; a short optical fiber with the same size as the optical fiber is placed on the glass slide to cushion the cover glass so that it is always placed in a horizontal state, and a matching liquid with a refractive index of 1.8 is filled in the glass slide and the cover glass to eliminate the aberration caused by the large refractive index difference as much as possible to affect the writing; Step 2: Focus Positioning By adjusting the size of the aperture in the femtosecond laser optical path to determine whether the shape of the laser spot meets the circular and collimation requirements, ensure that the focal position is the same as the reference point position on the imaging system; adjust the laser focus through the optical system to accurately focus on the core area of ​​the double-clad square chalcogenide fiber; during the writing process, by exposing in the fiber core, use the fiber rotator to rotate the fiber, observe the position of the laser focus relative to the core, and ensure that the insertion position of the grating area is distributed in the middle of the core as much as possible to ensure sufficient refractive index modulation depth; Step 3: Writing process The fiber Bragg grating is obtained by writing the grating structure point by point using a femtosecond laser. The focusing objective lens used for writing is an Olympus PLN40x objective lens. The central wavelength of the femtosecond laser is ~800nm, the pulse width is ~91fs, the repetition rate is 1KHz, and the writing power is about 80μW. Based on the laser repetition rate of 1KHz, the pulse energy is 80nJ, and the grating period setting value is 1292nm (central wavelength ~1550nm, m=4).

[0027] The fourth aspect of the present invention discloses a monitoring method for a preparation method of a fiber Bragg grating, which utilizes the planar structure on the other side of the optical fiber to observe the writing quality in real time through an online monitoring device.

[0028] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall within the scope limited by the appended claims of the application equally.

[0029] The following examples use conventional instruments and equipment in the art. The experimental methods in the following examples that do not specify specific conditions are usually carried out under conventional conditions or under conditions recommended by the manufacturer. The various raw materials used in the following examples, unless otherwise specified, are conventional commercially available products, and their specifications are conventional specifications in the art. In the specification of the present invention and the following examples, unless otherwise specified, "%" means weight percentage, "part" means weight part, and ratio means weight ratio.

[0030] Example 1 A method for preparing a double-clad square chalcogenide optical fiber comprises the following steps: 1) The raw materials of chalcogenide glass were pre-treated, and then the core layer materials (As 40 S 60 ), inner cladding material (As 38 S 62 ) and outer cladding material (As 36 S 64 ); 2) The core material, the inner cladding material and the outer cladding material are mechanically polished and then sequentially placed into an extrusion die to prepare a double-clad preform rod by extrusion. The process parameters of the extrusion method are: extrusion temperature is 280°C, protective gas is helium, and extrusion speed is 0.1 mm / min; 3) The double-clad preform rod is placed in a fiber drawing tower for drawing treatment to obtain a double-clad square chalcogenide optical fiber with an outer diameter of 125 μm; the process parameters of the drawing treatment are: drawing temperature of 330°C, rod feeding speed of 0.2 mm / min, drawing speed of 2 m / min, and protective gas type of helium.

[0031] Example 2 A method for preparing a double-clad square chalcogenide optical fiber comprises the following steps: 1) The raw materials of chalcogenide glass were pre-treated, and then the core layer materials (As 40 S 60 ), inner cladding material (As 38 S 62 ) and outer cladding material (As 36 S 64 ); 2) The core layer material, the inner cladding material and the outer cladding material are mechanically polished and then sequentially placed into an extrusion die to prepare a double-clad preform rod by extrusion. The process parameters of the extrusion method are: extrusion temperature is 305°C, protective gas is argon, and extrusion speed is 0.25 mm / min; 3) The double-clad preform rod is placed in a fiber drawing tower for drawing treatment to obtain a double-clad square chalcogenide optical fiber with an outer diameter of 125 μm. The process parameters of the drawing treatment are: drawing temperature of 350°C, rod feeding speed of 0.4 mm / min, drawing speed of 6 m / min, and protective gas type of helium.

[0032] Example 3 A method for preparing a double-clad square chalcogenide optical fiber comprises the following steps: 1) The raw materials of chalcogenide glass were pre-treated, and then the core layer materials (As 40 S 60 ), inner cladding material (As 38 S 62 ) and outer cladding material (As 36 S 64 ); 2) The core material, the inner cladding material and the outer cladding material are mechanically polished and then sequentially placed into an extrusion die to prepare a double-clad preform rod by extrusion. The process parameters of the extrusion method are: extrusion temperature is 330°C, protective gas is helium, and extrusion speed is 0.4 mm / min; 3) The double-clad preform rod is placed in a fiber drawing tower for drawing treatment to obtain a double-clad square chalcogenide optical fiber with an outer diameter of 125 μm. The process parameters of the drawing treatment are: drawing temperature of 380°C, rod feeding speed of 0.6 mm / min, drawing speed of 10 m / min, and protective gas type of helium.

[0033] Example 4 A method for preparing a fiber grating comprises the following steps: 1) The raw materials of chalcogenide glass were pre-treated, and then the core layer materials (As 40 S 60 ), inner cladding material (As 38 S 62 ) and outer cladding material (As 36 S 64 ); 2) The core layer material, the inner cladding material and the outer cladding material are mechanically polished and then sequentially placed into an extrusion die to prepare a double-clad preform rod by an extrusion method. The process parameters of the extrusion method are: extrusion temperature is 330°C, protective gas is argon, and extrusion speed is 0.4 mm / min; 3) The double-clad preform rod is placed in a fiber drawing tower for drawing treatment to obtain a double-clad square chalcogenide optical fiber with an outer diameter of 125 μm. The process parameters of the drawing treatment are: drawing temperature of 380°C, rod feeding speed of 0.6 mm / min, drawing speed of 10 m / min, and protective gas type of argon.

[0034] Example 5 A method for preparing a fiber grating comprises the following steps: 1) Equipment debugging The double-clad square chalcogenide fiber prepared in Example 1 is installed in a fixing fixture of a femtosecond laser direct writing device, and the plane of the double-clad square chalcogenide fiber is adjusted to be perpendicular to the laser incident direction; a short optical fiber with the same size as the optical fiber is placed on a glass slide to cushion the cover glass so that it is always placed in a horizontal state, and a matching liquid with a refractive index of 1.8 is filled in the glass slide and the cover glass to eliminate the aberration caused by the large refractive index difference as much as possible to affect the writing; 2) Focus Positioning By adjusting the size of the aperture in the femtosecond laser optical path to determine whether the shape of the laser spot meets the circular and collimation requirements, ensure that the focal position is the same as the reference point position on the imaging system; adjust the laser focus through the optical system to accurately focus on the core area of ​​the double-clad square chalcogenide fiber; during the writing process, by exposing in the fiber core, use the fiber rotator to rotate the fiber, observe the position of the laser focus relative to the core, and ensure that the insertion position of the grating area is distributed in the middle of the core as much as possible to ensure sufficient refractive index modulation depth; 3) Writing process The fiber Bragg grating is obtained by writing the grating structure point by point using a femtosecond laser. The focusing lens used for writing is an Olympus PLN40x objective lens. The central wavelength of the femtosecond laser is 800nm, the pulse width is 91fs, the repetition rate is 1KHz, the writing power is about 80μW, and the grating period is set to 1292nm (central wavelength ~1550nm, m=4).

[0035] Example 6 A method for preparing a fiber grating comprises the following steps: 1) Equipment debugging The double-clad square chalcogenide fiber prepared in Example 2 is installed in a fixing fixture of a femtosecond laser direct writing device, and the plane of the double-clad square chalcogenide fiber is adjusted to be perpendicular to the laser incident direction; a short optical fiber with the same size as the optical fiber is placed on a glass slide to cushion the cover glass so that it is always placed in a horizontal state, and a matching liquid with a refractive index of 1.8 is filled in the glass slide and the cover glass to eliminate the aberration caused by the large refractive index difference as much as possible to affect the writing; 2) Focus Positioning By adjusting the size of the aperture in the femtosecond laser optical path to determine whether the shape of the laser spot meets the circular and collimation requirements, ensure that the focal position is the same as the reference point position on the imaging system; adjust the laser focus through the optical system to accurately focus on the core area of ​​the double-clad square chalcogenide fiber; during the writing process, by exposing in the fiber core, use the fiber rotator to rotate the fiber, observe the position of the laser focus relative to the core, and ensure that the insertion position of the grating area is distributed in the middle of the core as much as possible to ensure sufficient refractive index modulation depth; 3) Writing process The fiber Bragg grating is obtained by writing the grating structure point by point using a femtosecond laser. The focusing lens used for writing is an Olympus PLN40x objective lens, the central wavelength of the femtosecond laser is 800nm, the pulse width is 91fs, the repetition rate is 0.5KHz, the writing power is about 80μW, and the grating period setting value is 2200nm.

[0036] Example 7 A method for preparing a fiber grating comprises the following steps: 1) Equipment debugging The double-clad square chalcogenide fiber prepared in Example 3 is installed in a fixing fixture of a femtosecond laser direct writing device, and the plane of the double-clad square chalcogenide fiber is adjusted to be perpendicular to the laser incident direction; a short optical fiber with the same size as the optical fiber is placed on a glass slide to cushion the cover glass so that it is always placed in a horizontal state, and a matching liquid with a refractive index of 1.8 is filled in the glass slide and the cover glass to eliminate the aberration caused by the large refractive index difference as much as possible to affect the writing; 2) Focus Positioning By adjusting the size of the aperture in the femtosecond laser optical path to determine whether the shape of the laser spot meets the circular and collimation requirements, ensure that the focal position is the same as the reference point position on the imaging system; adjust the laser focus through the optical system to accurately focus on the core area of ​​the double-clad square chalcogenide fiber; during the writing process, by exposing in the fiber core, use the fiber rotator to rotate the fiber, observe the position of the laser focus relative to the core, and ensure that the insertion position of the grating area is distributed in the middle of the core as much as possible to ensure sufficient refractive index modulation depth; 3) Writing process The fiber Bragg grating is obtained by writing the grating structure point by point using a femtosecond laser. The focusing lens used for writing is an Olympus PLN40x objective lens, the central wavelength of the femtosecond laser is 800nm, the pulse width is 91fs, the repetition rate is 0.75KHz, the writing power is about 80μW, and the grating period setting value is 1600nm.

[0037] Figure 3 This is a graph showing the morphology test result of the fiber grating obtained by the present invention. From the graph, it can be seen that the grating is evenly written into the core of the double-clad square chalcogenide fiber.

[0038] The prepared fiber Bragg grating (FBG) was tested for performance, and its reflectivity, central wavelength, bandwidth and other properties were characterized to verify that it met the design requirements. The test light source was the ElECTRO MIR 4.8 mid-infrared supercontinuum light source (800nm-4800nm) of LEUKOS, and the spectrometer was the YOKOGAWA AQ6377 (1900nm-5500nm) with a maximum resolution of 0.2nm. Two magnifying glasses placed in two dimensions were combined with the CCD image output to the display screen, and the docking effect of the light source output pigtail and the FBG fiber was observed by direct docking coupling. The test optical path was the light source butt-coupled to the FBG fiber, and the tail end of the FBG fiber was connected to the spectrometer for measurement through a bare fiber adapter. Figure 4 The figure shows the transmission / reflection spectrum test diagram of the fiber grating prepared by the present invention. From the figure, it can be obtained that the central wavelength of the grating is 1535.4nm, and the grating reflectivity is greater than 95%, achieving the successful writing of the high-reflection grating.

[0039] The above contents are only for explaining the technical idea of ​​the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the claims of the present invention.

Claims

1. A double-clad square chalcogenide optical fiber, characterized in that: The double-clad square chalcogenide optical fiber comprises a core layer (1), an inner cladding layer (2) and an outer cladding layer (3); the inner cladding layer (2) and the outer cladding layer (3) are sequentially coated on the outside of the core layer (1); the cross-sections of the core layer (1) and the inner cladding layer (2) are circular; and the cross-section of the outer cladding layer (3) is square; The raw materials of the core layer (1), the inner cladding layer (2) and the outer cladding layer (3) are chalcogenide glass raw materials.

2. A method for preparing a double-clad square chalcogenide optical fiber, characterized in that: The following steps are involved: The raw materials of the chalcogenide glass are pre-treated, and then the core layer material, the inner cladding material and the outer cladding material are prepared respectively by a vacuum high-temperature melting method; The core layer material, the inner cladding material and the outer cladding material are mechanically processed and then sequentially placed into an extrusion die to prepare a double-clad preform rod by extrusion; The double-clad preform rod is drawn to obtain a core layer (1), an inner cladding layer (2) and an outer cladding layer (3) through drawing of the core layer material, the inner cladding layer material and the outer cladding layer material, and finally the double-clad square chalcogenide optical fiber according to claim 1 is obtained.

3. The method for preparing a double-clad square chalcogenide optical fiber according to claim 2, characterized in that: The chalcogenide glass raw materials include As 40 S 60 、As 38 S 62 and As 36 S 64 ; The chalcogenide glass raw material of the core material is As 40 S 60； The raw material of the chalcogenide glass of the inner cladding material is As 38 S 62 ; The raw material of the chalcogenide glass of the outer cladding material is As 36 S 64 .

4. The method for preparing a double-clad square chalcogenide optical fiber according to claim 2, characterized in that: The pretreatment of the chalcogenide glass raw material includes polishing and cleaning processes performed in sequence; The process parameters of the extrusion method are: extrusion temperature is 280-330° C., protective gas is helium or argon, and extrusion speed is 0.1-0.4 mm / min.

5. The method for preparing a double-clad square chalcogenide optical fiber according to claim 2, characterized in that: The process parameters of the drawing process are: drawing temperature of 330-380° C., rod feeding speed of 0.2-0.6 mm / min, drawing speed of 2-10 m / min, and protective gas of helium or argon.

6. A method for preparing a fiber Bragg grating, characterized in that: The following steps are involved: Installing the double-clad square chalcogenide optical fiber according to claim 1 in a femtosecond laser direct writing device, and adjusting the plane of the double-clad square chalcogenide optical fiber to be perpendicular to the laser incident direction; The focus of the femtosecond laser is adjusted to be positioned at the core region of the double-clad square chalcogenide fiber; The fiber Bragg grating is obtained by writing the grating structure point by point using a femtosecond laser.

7. The method for preparing a fiber Bragg grating according to claim 6, characterized in that: The femtosecond laser has a central wavelength of 800nm, a pulse width of 91fs, and a repetition rate of 0.5-1kHz.

8. The method for preparing a fiber Bragg grating according to claim 6, characterized in that: The grating period when the grating structure is written point by point using a femtosecond laser is 1292-2200nm.

9. The method for preparing a fiber Bragg grating according to claim 6, characterized in that: The following steps are also included: On the side perpendicular to the plane of the double-clad square chalcogenide fiber perpendicular to the laser incident direction, the relative position of the focus of the femtosecond laser and the core of the double-clad square chalcogenide fiber during the writing process is observed in real time through a microscopic optical system to ensure the writing quality and consistency.

10. The method for preparing a fiber Bragg grating according to claim 9, characterized in that: The microscope optical system comprises a magnifying glass and a CCD imaging system; the magnifying glass and the CCD imaging system are electrically connected.

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