Preparation method of equivalent large-refractive-index-difference mid-infrared gradient optical fiber based on alternate refractive index structure

Abstract

The invention discloses a preparation method of an equivalent large-refractive-index-difference mid-infrared gradient optical fiber based on an alternating refractive index structure, which is characterized by comprising the following steps of: designing the thickness of chalcogenide glass with different refractive indexes: forming a first periodic structure by a low-refractive-index material and a high-refractive-index material on the adjacent outer side of the low-refractive-index material from the original point of a fiber core to the outside in sequence; n + 1 periodic structures are designed outwards in sequence, the outward thickness increment of the low-refractive-index material is xi, i = 1, 2, 3... n, and xi-xi-1 is progressively increased according to the linearity and the power exponent; carrying out precision polishing and grinding on the single chalcogenide glass, carrying out ultrasonic cleaning, stacking and extruding to prepare an all-solid-state optical fiber preform; and finally, inserting into a pre-drilled cladding glass tube for assembling, introducing a protective atmosphere, performing thermal extension processing, and drawing to obtain the equivalent large-refractive-index-difference intermediate infrared gradient optical fiber, which has the advantages that the large refractive index difference and the accurate and flexible regulation and control of a refractive index profile curve can be realized.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of infrared imaging and high-power laser transmission, and particularly relates to a preparation method of a mid-infrared gradient optical fiber with an equivalent large refractive index difference based on an alternating refractive index structure. BACKGROUND

[0002] Graded index (GRIN) optical fiber has been widely used in the field of optical coherence tomography (OCT) due to its small volume, light weight and excellent beam control performance. OCT probe is a new imaging technology based on the principle of low coherence light interference, which has the advantages of non-invasiveness, no ionizing radiation and real-time imaging. With the miniaturization development of OCT probe, the outer diameter of the optical fiber OCT probe with quartz GRIN optical fiber as the focusing element has been reduced to 125 mu m. GRIN optical fiber not only realizes the high integration of core devices, but also significantly improves the mechanical stability and spatial adaptability of the system, providing an innovative solution for imaging detection under complex conditions.

[0003] Chalcogenide glass has an ultra-wide light transmission window (0.5-20 mu m) and an ultra-large refractive index control range (2.0-3.1). By adjusting its composition and structure, the refractive index can be precisely and significantly controlled, thereby realizing large refractive index distribution and numerical aperture NA of large optical fiber. At present, the structure of traditional chalcogenide GRIN optical fiber mainly includes the following parts: a core region composed of a refractive index gradient matrix and a cladding region composed of a refractive index uniform matrix.

[0004] The preparation of chalcogenide GRIN materials has been extensively studied at home and abroad. Currently, the main preparation techniques include ion diffusion method and stacking diffusion method. In terms of ion diffusion method, in 2021, a research team from the University of Rennes in France introduced alkali halide into the GeSe2-Ga2Se3 chalcogenide glass system and performed ion exchange using a potassium ion-containing solution, successfully controlling the refractive index of the material. This system exhibits excellent ion diffusion performance, with a diffusion length of 2 mm and a refractive index difference of 0.045. However, research has found that long-term halide solution diffusion can cause significant reduction in the near-infrared transmittance of chalcogenide glass and result in asymmetric refractive index profile. In terms of stacking diffusion method, researchers from the US Naval Research Laboratory successfully prepared optical glass with axial refractive index gradient distribution by adjusting 24 different component chalcogenide glass systems and using heat to make atoms migrate from high-concentration areas to low-concentration areas. In 2016, the team further used a multiple sleeve method combined with thermal diffusion process to prepare GRIN glass with radial refractive index gradient distribution, with a diffusion range of more than 4 mm, a refractive index difference of 0.2, and a numerical aperture of 0.43. However, the preparation process of stacking diffusion method is complex and requires precise control of glass thickness, diffusion temperature, and time, etc. Otherwise, it is easy to cause asymmetric or stepped distribution of refractive index profile. For example, long diffusion time can cause refractive index distortion in areas with fast diffusion rate, while short diffusion time can form stepped distribution in areas with slow diffusion rate, which greatly increases the difficulty of preparing GRIN glass.

[0005] Currently, the stacking and drawing method is the main technology for preparing chalcogenide GRIN optical fibers. The core principle of this technology is based on the effective refractive index theory, which realizes the control of the gradient refractive index distribution of the optical fiber by accurately designing the number ratio and spatial arrangement of two different refractive index glass rods. In 2017, Siwicki et al. used the AsSe-GeAsSe rod system, combined with microstructure optical fiber design theory and effective medium theory, to successfully obtain the gradient refractive index distribution curve. The study showed that by adjusting the filling ratio of low refractive index GeAsSe optical fiber, the radial refractive index distribution of the optical fiber can be effectively controlled, with a maximum refractive index difference of 0.2. However, this method requires as many as 7550 glass rods with different refractive indices, and the excessive interfaces lead to a significant increase in scattering loss. In 2021, a research team from the University of Rennes in France successfully prepared mid-infrared GeAsSe core microstructure GRIN optical fiber using the stacking and drawing method. Based on the analysis of the effective medium theory, the refractive index difference of the optical fiber is 0.15, the core diameter is 40 μm, and the numerical aperture NA is 0.41. However, experimental results show that the transmission loss of the prepared GRIN optical fiber increases significantly to more than 20 dB / m, which is two orders of magnitude higher than the original GeAsSe optical fiber. This phenomenon is mainly attributed to the cumulative effect of interface scattering defects in the multiple stacking and drawing process.

[0006] Limited by the problems existing in the above-mentioned technologies, in general, the current sulfur-based GRIN optical fiber is faced with the technical problem that low loss, large refractive index difference and accurate refractive index profile control are difficult to be considered at the same time on the micron scale. The existing mid-infrared GRIN optical fiber structure has the problem that it is difficult to realize large refractive index difference and flexible control of refractive index profile curve in the micron size range. SUMMARY

[0007] The technical problem to be solved by the present application is to provide a preparation method of an equivalent large refractive index difference mid-infrared gradient optical fiber based on an alternating refractive index structure, which can realize large refractive index difference and accurate and flexible control of refractive index profile curve.

[0008] The technical scheme adopted by the present application to solve the above technical problem is: a preparation method of an equivalent large refractive index difference mid-infrared gradient optical fiber based on an alternating refractive index structure, comprising the following steps: Step 1, process sulfur-based glass with different refractive indices into the required thickness, and the thickness is designed as follows: the first periodic structure is composed of low refractive index material and its adjacent high refractive index material outward from the core origin, and n+1 periodic structures are designed outward in turn, and the outward thickness increment of the low refractive index material is designed as x i , wherein i =1, 2, 3…n, x i - x i-1 The linear and power index increasing mode is used for increasing; Step 2, precisely polish and grind the single piece of sulfur-based glass with different periodic thicknesses T j or the same periodic thickness T obtained in step 1 to strictly match the size to the optical design requirements; Step 3, ultrasonic clean the single piece of sulfur-based glass obtained in step 2, and stack the preform sulfur-based glasses with different thicknesses in turn according to the refractive index; Step 4, place the stacked sulfur-based glass pieces obtained in step 3 in an extrusion mold, apply vertical pressure to the center region of the stack, induce the material to collapse downward, and extrude into the mold to form a full solid optical fiber preform rod with a periodic structure; Step 5, precisely insert the full solid optical fiber preform rod obtained in step 4 into a pre-drilled cladding glass tube for assembly to form a preform rod; Step 6, process the preform rod obtained in step 5, pass a protective atmosphere, and perform hot stretching processing at 280-300 DEG C to finally draw the equivalent large refractive index difference mid-infrared gradient optical fiber based on the alternating refractive index structure.

[0009] Further, in step 1, the two different refractive index chalcogenide glasses have a large refractive index difference (△ n ), wherein the high refractive index chalcogenide glass and the low refractive index chalcogenide glass have similar glass transition temperatures Tg, with a difference of 10℃ or less.

[0010] Further, the high refractive index chalcogenide glass is Ge-As-Se, and the low refractive index chalcogenide glass is As-S.

[0011] Further, in step 1, the diameter of the core is d 5-20μm.

[0012] Further, in step 1, the thickness increment x i - x i-1 = x , x 2 or x 4 .

[0013] Further, in step 3, when alternately stacking, the chalcogenide glass stacked at the bottom is the low refractive index chalcogenide glass, and the chalcogenide glass stacked at the top is the high refractive index chalcogenide glass.

[0014] Further, in step 5, the refractive index of the pre-drilled cladding glass tube is less than the refractive index of the low refractive index chalcogenide glass.

[0015] Compared with the prior art, the present application has the following advantages: Firstly, by using chalcogenide glass as the base material and combining with the design of subwavelength multilayer periodic structure, the present application successfully realizes a large refractive index difference and a large numerical aperture. Not only does it solve the problem of multi-phonon absorption of quartz material in the mid-infrared waveband, but also it uses effective medium theory (EMT) to equivalent the non-uniform refractive index distribution as a multilayer uniform material superposition, thereby accurately controlling the refractive index gradient on the micron scale to meet the demand for high resolution and miniaturization of mid-infrared optical systems.

[0016] Secondly, the optical fiber designed in the present application can realize highly concentrated light field distribution and periodic self-focusing effect. This feature makes it have important application value in optical coherence tomography (OCT) endoscope probes, and can significantly improve the imaging resolution and signal-to-noise ratio. In addition, the optical fiber supports low-loss transmission in the mid-infrared waveband (2-12μm), and is suitable for micro-crack detection and early cancer screening of strongly scattering materials (such as biological tissues and industrial ceramics). By adjusting the layer thickness distribution function (T(x)) x , x 2 , x4 ), and can also flexibly optimize the refractive index profile to adapt to the needs of different application scenarios.

[0017] Finally, the present application uses mature melt quenching and stacking extrusion process to prepare the optical fiber preform, and the multilayer periodic structure is realized by controlling the thickness and arrangement of the glass sheet, effectively avoiding the deformation and eccentricity of the preform. Compared with the traditional vapor deposition technology, this method is simple, low in cost and easy to scale up.

[0018] In summary, the present application is a preparation method of an equivalent large refractive index difference mid-infrared gradient optical fiber based on an alternating refractive index structure. The optical fiber uses high and low refractive index materials and follows a set arrangement. By changing the filling rate of the low refractive index material, it can achieve precise and flexible control of the refractive index difference and the refractive index profile curve on the micron scale. It has large refractive index difference, mid-infrared compatibility and excellent optical performance. At the same time, the simplified preparation process provides reliable technical support for practical application, and has broad application prospects in biomedical imaging, industrial detection and infrared optical systems. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 (A) and thermal property diagram (B) of the stacked chalcogenide glass; Figure 2 (A) and thermal property diagram (B) of the stacked chalcogenide glass; x , x 2 , x 4 function distribution; T 1j distribution; Figure 3 (A) and thermal property diagram (B) of the stacked chalcogenide glass; x , x 2 , x 4 equivalent refractive index distribution diagram when the thickness distribution function is Figure 4 Detailed step diagram for manufacturing a preform rod of a full solid structure; Figure 5 (A) and thermal property diagram (B) of the stacked chalcogenide glass; x 4 (A) and thermal property diagram (B) of the stacked chalcogenide glass; DETAILED DESCRIPTION

[0020] The present application will be further described in detail below with reference to the accompanying drawings.

[0021] Specific embodiment one, the equivalent large refractive index difference mid-infrared gradient optical fiber is prepared when the thickness distribution function is x .

[0022] The selected chalcogenide glasses were high-refractive-index Ge-As-Se and low-refractive-index As-S. Ge1As was prepared using the conventional melt-quenching method. 39 Se 60 The process for making As2S3 glass is as follows: Pretreated 5N high-purity raw material is loaded into a low-impurity quartz tube, and the mixture is heated under a vacuum of less than 10... -4 At Pa, the quartz tube containing the raw material is sealed and placed in a swing furnace for high-temperature melting for 12 hours. After quenching in air, it is then melted at the glass transition temperature (T). g ) near Ge1As 39 Se 60 The glass was annealed with As2S3 glass for 15 hours, then gradually cooled to room temperature, resulting in a glass sample with a diameter of 46 mm. The properties of the glass sample are as follows: Figure 1 As shown in Figure A, impurities present in the raw materials cause the glass to exhibit distinct impurity absorption peaks. These include, for example, SH peaks, Se-H peaks, and H₂O peaks. The glass exhibits a transmittance exceeding 50% in the 2-12 μm wavelength range, which is crucial for the preparation of Ge1As. 39 Se 60 Similar to As2S3, T g ,like Figure 1 As shown in B.

[0023] The equivalent mid-infrared gradient fiber with a large refractive index difference is fabricated by the following steps: Step 1: Process the Ge-As-Se and As-S matrix glasses to the required thickness. The thickness design is as follows: The first periodic structure is formed by the low-refractive-index material and its adjacent high-refractive-index material sequentially outward from the fiber core origin. A total of 5 periods are designed outward, with the thickness increment of the low-refractive-index material designed outward. x i ,in i =1, 2, 3...n, so that they increase linearly; Specifically, the thickness needs to meet the following requirements: core diameter is... d The first periodic structure consists of a first low-refractive-index material and its adjacent high-refractive-index material arranged sequentially outward from the origin of the fiber core. The periodic thickness is [missing information]. T The thickness of the low refractive index material is T 1 The thickness of the high refractive index material is T - T 1 The filler content of low refractive index materials is f 1j The thickness of the second periodic structure is T The thickness of the low refractive index material is T 1+x 1. The thickness of the high refractive index material is T -( T 1 +x 1) The thickness of the third periodic structure is T The thickness of the low refractive index material is T 1 +x 2. The thickness of the high refractive index material is T -( T 1 +x 2). Following this pattern, the thickness of each cycle is [missing information - likely a number] from the fiber core origin outwards. T By designing the thickness increment of low refractive index materials x i ( i =1, 2, 3…), so that it increases linearly (e.g., x 2- x 1=x), thereby enabling precise control and flexible design of the gradient fiber refractive index distribution profile.

[0024] The theoretical basis for the above design is as follows: EMT theory is used to study the relationship between thickness distribution and equivalent refractive index distribution, and Rsoft simulation software is used to study the optical transmission characteristics and mode control of the equivalent GRIN fiber, as detailed below: Based on EMT theory, a multilayer periodic fiber structure composed of alternating high-refractive-index and low-refractive-index materials was designed using the Maxwell-Garnett model. Its equivalent refractive index... n j It can be calculated using formula (1) (1) in n 1 is the refractive index of low-refractive-index materials. n 2 represents the refractive index of high-refractive-index materials. f 1j This represents the volume fraction of low-refractive-index materials. The calculation formula is as follows: f 1j = T 1j / T j ,in, T 1j The thickness occupied by the low-refractive material within one period. T j The total thickness of each discrete layer (i.e., a periodic layer). By controlling the thickness distribution, the filling rate of the low-refractive-index material is adjusted, which in turn affects the refractive index distribution of the optical fiber.

[0025] To investigate the intrinsic correlation between refractive index profile distribution and optical parameters of graded-index fiber, considering scattering loss and our focus on the mid-infrared band (>2.5 μm), we assumed different period thicknesses... T j Down, T 1j According to x The type of equivalent refractive index distribution of the fiber core derived from the function distribution, such as Figure 2 The black lines in the text are shown ( T j The refractive index values ​​satisfy the subwavelength dimension, and we only consider 5 periodic discrete layers (for later practical implementation). Further polynomial fitting is performed on the above refractive index distribution curves to derive the refractive index distribution expression. At this point, according to EMT theory, the multi-period alternating refractive index structure can be equivalent to a gradient refractive index fiber with a large refractive difference, as shown in the figure. Figure 3 As shown by the black lines in the diagram. Using BPM simulations of the propagation of a Gaussian beam in a GRIN fiber, it was found that the light field propagates periodically with a sinusoidal pattern, and in each cycle... L Two focusing operations are completed within the fiber core. The effective mode field diameter varies periodically with propagation distance, indicating the confinement and diffusion of the light field within the fiber core. (The last sentence appears to be incomplete and possibly refers to a specific thickness distribution.) x In the case of a function, the core diameter was investigated. d The influence of (5μm, 10μm, 15μm, 20μm) on optical transmission characteristics was studied. It was found that the fiber period varies with the core diameter. d The effective mode field diameter increases with the core diameter. d The proportion of the fundamental mode increases with the core diameter. d Increases and decreases.

[0026] Step 2: Apply the thickness from Step 1 according to different cycles. T j Or the same period thickness T The pre-processed monolithic chalcogenide glass sheets are required to undergo precision polishing and grinding to ensure their dimensions strictly match the optical design requirements; the stacking thickness distribution is based on... x The function distribution is equivalent to different gradient refractive index distribution patterns; Step 3: Perform ultrasonic cleaning on the single piece of chalcogenide glass after precision polishing and grinding. Stack the pre-made chalcogenide glass of different thicknesses according to their refractive index, ensuring that the refractive index alternates sequentially during stacking. It should be noted that when the polished and ground chalcogenide glasses are stacked, the composition of the bottommost chalcogenide glass sheet is As2S3, and the composition of the topmost chalcogenide glass sheet is Ge1As. 39 Se 60In this example, five cycles are stacked, with one cycle consisting of a low refractive index and the other of a high refractive index.

[0027] Step 4: Place the stacked chalcogenide glass sheets into the extrusion mold. Through the stacking and extrusion process, apply vertical pressure to the central region of the stack to induce the material to collapse downward and be extruded into the designed mold, thus forming an all-solid-state optical fiber preform with a periodic structure. Step 5: Precisely insert the all-solid-state optical fiber preform with a periodic structure prepared in Step 4 into the pre-drilled cladding glass tube As. 38 S 62 The components are assembled to form the final preform; it should be noted that detailed illustrations of steps 3-6 are provided below. Figure 4 As shown; Step 6: Process the assembled preform by introducing a protective atmosphere and performing thermal stretching at 280-300℃ to finally draw it into an equivalent large refractive index difference mid-infrared gradient fiber based on an alternating refractive index structure.

[0028] Specific embodiment two, the thickness distribution pattern is as follows: x 2 Fabrication of mid-infrared gradient optical fiber with equivalent large refractive index difference of function.

[0029] Similar to the above specific embodiment one, the difference lies in: in step 1, the Ge-As-Se and As-S matrix glasses are processed to the required thickness, and the thickness design is as follows: the first periodic structure is formed by the low refractive index material and the adjacent high refractive index material from the fiber core origin outwards, and a total of 5 periods are designed outwards, with the thickness increment of the low refractive index material designed outwards. x i ,in i =1, 2, 3…n, so that they follow the order of n. x 2 Increasing sequentially.

[0030] Stack thickness distribution is based on x 2 The function distribution is equivalent to different gradient refractive index distribution patterns. We assume different period thicknesses. T j Down, T 1j According to x 2 The type of equivalent refractive index distribution of the fiber core derived from the function distribution, such as Figure 2 As shown by the red line in the diagram. Next, we perform polynomial fitting on the above refractive index distribution curve to derive the refractive index distribution expression. At this point, according to EMT theory, the multi-period alternating refractive index structure can be equivalent to a gradient refractive index fiber with a large refractive difference. For example... Figure 3As shown by the red lines in the image, it can be found that based on x 2 The fiber refractive index derived from the function distribution is closest to a perfect parabolic distribution. At this point, using BPM to simulate the propagation of a Gaussian beam in a GRIN fiber, it was found that the light field propagates periodically with a sinusoidal pattern, and in each cycle... L Two focusing operations are completed within the period, and the period length is... L satisfy .

[0031] This further proves that the thickness distribution is based on x 2 The function distribution is closest to a perfect parabolic distribution. The effective mode field diameter varies periodically with propagation distance, indicating the confinement and diffusion of the light field within the fiber core. The thickness distribution is based on... x 2 The core diameter was investigated under the condition of functional distribution. d The influence of (5μm, 10μm, 15μm, 20μm) on optical transmission characteristics was studied. It was found that the fiber period varies with the core diameter. d The effective mode field diameter increases with increasing propagation distance and also with increasing fiber diameter. d Increase as it grows.

[0032] It should be noted that, compared with the thickness distribution based on x Compared to the function distribution, the average effective mode diameter increases, indicating a more dispersed light field distribution and a weakened ability to confine light. Compared to the distribution based on... x Compared to the functional distribution, the effective refractive index increases. As the core diameter increases, the proportion of the fundamental mode decreases. This is because the normalized frequency increases. Since the normalized frequency is proportional to the core radius, according to mode theory, a larger normalized frequency value means the fiber can support more transmission modes. These additional modes are mainly higher-order modes, which will propagate alongside the fundamental mode within the core, thus leading to a decrease in the proportion of the fundamental mode.

[0033] Specific embodiment three, thickness distribution pattern is as follows: x 4 Fabrication of mid-infrared gradient optical fiber with equivalent large refractive index difference of function.

[0034] Similar to the above specific embodiment one, the difference lies in: in step 1, the Ge-As-Se and As-S matrix glasses are processed to the required thickness, and the thickness design is as follows: the first periodic structure is formed by the low refractive index material and the adjacent high refractive index material from the fiber core origin outwards, and a total of 5 periods are designed outwards, with the thickness increment of the low refractive index material designed outwards. x i ,in i =1, 2, 3…n, so that they follow the order of n.x 4 Increasing sequentially.

[0035] Stack thickness distribution is based on x 4 The function distribution is equivalent to different gradient refractive index distribution patterns. We assume different period thicknesses. T j Down, T 1j According to x 4 The type of equivalent refractive index distribution of the fiber core derived from the function distribution, such as Figure 2 The blue line in the figure shows the refractive index distribution curve. Next, we perform a polynomial fitting on the above refractive index distribution curve to derive the refractive index distribution expression. According to EMT theory, a multi-period alternating refractive index structure can be equivalent to a gradient refractive index fiber with a large refractive difference, as shown in the figure. Figure 3 As shown by the blue lines in the diagram. At this point, using BPM to simulate the propagation of a Gaussian beam in a GRIN fiber, it was found that the optical field propagates periodically with a sinusoidal pattern in each cycle. L Two focusing operations are completed within the core. The effective mode field diameter varies periodically with propagation distance, indicating the confinement and diffusion of the light field within the fiber core. The thickness distribution is based on... x 4 The core diameter was investigated under the condition of functional distribution. d The influence of (5μm, 10μm, 15μm, 20μm) on optical transmission characteristics was studied. It was found that the fiber period varies with the core diameter. d The effective mode field diameter increases with increasing propagation distance and also increases with increasing diameter. d Increases with increasing thickness. (Based on thickness distribution) x 2 Compared to the case of the function distribution, the average effective mode diameter increases, indicating that the optical field distribution is relatively more dispersed and the ability to confine light is weakened. Furthermore, a smaller core diameter leads to stronger confinement of the optical field within the fiber, making the propagation path of light within the core more restricted.

[0036] It should be noted that the thickness distribution is based on x 2 Compared to the function distribution, the effective refractive index increases. As the core diameter increases, the proportion of the fundamental mode decreases. This is because the normalized frequency increases. Since the normalized frequency is proportional to the core radius, according to mode theory, a larger normalized frequency value means the fiber can support more transmission modes. These additional modes are mainly higher-order modes, which will propagate alongside the fundamental mode within the core, thus leading to a decrease in the proportion of the fundamental mode.

[0037] The final drawn fiber end face is as follows Figure 5As shown, the periodic structural features of the fiber core end face are clearly visible.

[0038] In summary, this invention, by designing and alternating stacked material layers with different refractive indices, utilizes effective medium theory to achieve an equivalent gradient refractive index distribution, enabling flexible control of the refractive index curve at the micrometer scale to meet diverse optical requirements. Its core advantage lies in simultaneously achieving a large numerical aperture and low transmission loss, overcoming the limitations of traditional mid-infrared optical fibers. This structure not only enhances the light field confinement capability but also reduces scattering loss through subwavelength size design, ensuring efficient transmission in the mid-infrared band. Furthermore, the thermally matched material combination avoids interface defects during fabrication, ensuring structural stability. This technology provides superior optical performance for applications such as optical coherence tomography and industrial inspection, demonstrating unique value, particularly in scenarios requiring high resolution and strong signal penetration.

[0039] The foregoing description is not intended to limit the invention, nor is the invention limited to the examples given. Any changes, modifications, additions, or substitutions made by those skilled in the art within the scope of the invention should also be considered within the protection scope of the invention.

Claims

1. A method for fabricating an equivalent large refractive index difference mid-infrared gradient optical fiber based on an alternating refractive index structure, characterized in that... Includes the following steps: Step 1: Process chalcogenide glasses with different refractive indices to the required thickness. The thickness design is as follows: The first periodic structure is formed by sequentially adding low-refractive-index materials and adjacent high-refractive-index materials from the fiber core origin outwards. A total of n+1 periodic structures are designed outwards. The thickness increment of the low-refractive-index material outwards is designed as follows: x i ,in i =1, 2, 3…n, x i - x i-1 Increment in a linear or power-law manner; Step 2: Calculate the different period thicknesses obtained in Step 1. T j Or the same period thickness T Precision polishing and grinding of single-layer chalcogenide glass; Step 3: Perform ultrasonic cleaning on the single piece of chalcogenide glass obtained in Step 2, and stack pre-made chalcogenide glass of different thicknesses alternately according to their refractive index. Step 4: Place the stacked chalcogenide glass sheets obtained in Step 3 into an extrusion mold. Apply vertical pressure to the central region of the stack through the stacking extrusion process to induce the material to collapse downward and be extruded into the mold to form an all-solid-state optical fiber preform with a periodic structure. Step 5: Precisely insert the all-solid-state optical fiber preform obtained in Step 4 into the pre-drilled cladding glass tube for assembly to form the preform. Step 6: Process the preform obtained in Step 5 by introducing a protective atmosphere and performing thermal stretching at 280-300℃ to finally draw it into an equivalent large refractive index difference mid-infrared gradient fiber based on an alternating refractive index structure.

2. The method for fabricating an equivalent large refractive index difference mid-infrared gradient optical fiber based on an alternating refractive index structure according to claim 1, characterized in that: In step 1, the two chalcogenide glasses with different refractive indices have a large refractive index difference, wherein the high-refractive-index chalcogenide glass and the low-refractive-index chalcogenide glass have similar glass transition temperatures Tg, with a Tg difference of less than 10℃.

3. The method for fabricating an equivalent large refractive index difference mid-infrared gradient optical fiber based on an alternating refractive index structure according to claim 2, characterized in that: The high-refractive-index chalcogenide glass is Ge-As-Se, and the low-refractive-index chalcogenide glass is As-S.

4. The method for fabricating an equivalent large refractive index difference mid-infrared gradient optical fiber based on an alternating refractive index structure according to claim 1, characterized in that: In step 1, the diameter of the fiber core... d It is 5-20μm.

5. The method for fabricating an equivalent large refractive index difference mid-infrared gradient optical fiber based on an alternating refractive index structure according to claim 1, characterized in that: In step 1, the thickness increment x i - x i-1 = x , x 2 or x 4 .

6. The method for fabricating an equivalent large refractive index difference mid-infrared gradient optical fiber based on an alternating refractive index structure according to claim 1, characterized in that: In step 3, when stacking alternately, the chalcogenide glass at the bottom is a low-refractive-index chalcogenide glass, and the chalcogenide glass at the top is a high-refractive-index chalcogenide glass.

7. The method for fabricating an equivalent large refractive index difference mid-infrared gradient optical fiber based on an alternating refractive index structure according to claim 2, characterized in that: In step 5, the refractive index of the pre-drilled cladding glass tube is less than that of the low-refractive-index chalcogenide glass.

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