Fiber core glass as well as preparation method and application thereof

Abstract

The invention provides fiber core glass as well as a preparation method and application thereof. The fiber core glass comprises the following components in percentage by mole: 31-43% of SiO2, 3-5% of Al2O3, 14-20% of B2O3 and 6-10% of CaO; the ceramic is prepared from the following components in percentage by weight: 2 to 4 percent of ZnO, 3 to 7 percent of BaO, 5 to 7 percent of Bi2O3, 2 to 4 percent of Li2O, 1 to 3 percent of Y2O3, 5 to 8 percent of TiO2, 7 to 10 percent of ZrO2, 3 to 5 percent of P2O5, 3 to 5 percent of Ta2O5, 2 to 4 percent of Nb2O5 and 1 to 3 percent of PbO. The technical problem to be solved is how to prepare the fiber core glass and use the fiber core glass for preparing the glass with the gradient refractive index, so that the fiber core glass has more advantages compared with existing MCVD and other vapor deposition processes, complex gas flow and deposition temperature real-time regulation and control are not needed, and the glass with the gradient refractive index can be prepared only through fiber core glass component design, high-temperature ion diffusion, multi-stage wire drawing and other processes. The continuous and smooth gradient refractive index structure can be formed by using the method, the process is simpler, the preparation period is shorter, and the process uniformity is easier to control; and the prepared glass with the gradient refractive index can reduce optical loss and crosstalk, improve the imaging resolution, can be stably made into devices such as an optical fiber panel and the like, and meets the requirements of multiple fields, so that the glass is more suitable and practical.

Description

Technical Field

[0001] This invention relates to the field of optical materials and preparation technology, and in particular to a core glass, its preparation method and application. Background Technology

[0002] Fiber optic imaging arrays are core optical devices in fields such as low-light imaging, medical endoscopy, industrial inspection, and high-energy physics detection. They are composed of tens of millions of independent optical fibers arranged closely together. Each fiber relies on a high-refractive-index core and low-refractive-index cladding structure to achieve constrained transmission of light signals through total internal reflection or gradient refraction. Each fiber corresponds to an independent pixel channel, which can accurately maintain the spatial information of the image. In military and scientific research scenarios, they are often used in conjunction with microchannel plates and photocathodes to build high-performance imaging systems.

[0003] Based on refractive index distribution, fiber optic imaging arrays are classified into step-index and graded-index types. Step-index arrays, due to the abrupt change in refractive index between the core and cladding, are simple to fabricate, but their reliance on total internal reflection for light transmission and insufficient focusing constraints limit resolution, making them unsuitable for high-resolution applications. Graded-index arrays, by varying the refractive index of the core radially, theoretically reduce intermodal dispersion and improve focusing performance, and represent the mainstream development direction.

[0004] However, most existing fiber-core glasses are simple SiO2-based or B2O3-based systems, lacking multi-component synergistic design, with insufficient ion diffusion capacity or limited refractive index adjustment range, making it difficult to form a continuous and smooth refractive index gradient, thus exacerbating optical losses. At the same time, improper addition of some oxides may lead to glass network instability, mismatch with the thermal expansion coefficient of the cladding, easy cracking during annealing, or increased crystallization tendency, affecting optical uniformity. Crosstalk rate is usually higher than 0.05%, and resolution is generally lower than 150 lp / mm. Relying on vapor deposition processes such as MCVD, precise control of multiple parameters is required to compensate for component defects, which is complex, costly, and time-consuming, making it difficult to achieve large-scale mass production. Summary of the Invention

[0005] The main objective of this invention is to provide a core glass, its preparation method, and its applications. The technical problem to be solved is how to prepare a core glass and use it to prepare graded refractive index glass, making it more advantageous than existing vapor deposition processes such as MCVD. This invention does not require complex real-time control of gas flow and deposition temperature. It can form a continuous and smooth graded refractive index structure simply by designing the core glass composition and using processes such as high-temperature ion diffusion and multi-stage drawing. The process is simpler, the preparation cycle is shorter, and the control of process uniformity is easier to achieve. Moreover, the prepared graded refractive index glass can reduce light loss and crosstalk, improve imaging resolution, and can be stably made into optical fiber panels and other devices, adapting to the needs of multiple fields, thus making it more suitable for practical use.

[0006] The objective of this invention and the technical problem it solves are achieved by the following technical solution. According to this invention, a fiber core glass comprises, by molar percentage of oxides, the following raw materials: 31-43% SiO2, 3-5% Al2O3, 14-20% B2O3, 6-10% CaO; 2-4% ZnO, 3-7% BaO, 5-7% Bi2O3, 2-4% Li2O, 1-3% Y2O3, 5-8% TiO2, 7-10% ZrO2, 3-5% P2O5, 3-5% Ta2O5, 2-4% Nb2O5, and 1-3% PbO.

[0007] The objective of this invention and the technical problem it solves are achieved by the following technical solution. A method for preparing fiber-core glass according to this invention includes the following steps: S1 Weigh the raw materials and mix them evenly; the raw materials include, by the molar percentage of oxides: 31~43% SiO2, 3~5% Al2O3, 14~20% B2O3, 6~10% CaO; 2~4% ZnO, 3~7% BaO, 5~7% Bi2O3, 2~4% Li2O, 1~3% Y2O3, 5~8% TiO2, 7~10% ZrO2, 3~5% P2O5, 3~5% Ta2O5, 2~4% Nb2O5, 1~3% PbO; S2 is melted, homogenized, extruded and shaped, and annealed to obtain fiber core glass.

[0008] The objectives of this invention and the technical problems it addresses can be further achieved by the following technical measures.

[0009] Preferably, in the preparation method, the melting involves placing the mixed raw materials in a platinum crucible and heating them at 1400~1650℃ for 6~9 hours; the annealing involves placing the molded part in an annealing furnace and holding it at 630~670℃ for 2~4 hours.

[0010] The objective of this invention and the technical problem it solves are achieved through the following technical solution. A method for preparing graded refractive index glass according to this invention includes the following steps: S1 The aforementioned fiber core glass is made into a core glass rod according to the aforementioned preparation method; S2 assembles the core glass rod and the outer glass tube in a coaxial nesting manner to form a rod-tube assembly; wherein, the coefficient of thermal expansion of the outer glass tube is 3×10 lower than that of the core glass. -7 ~7×10 -7 / ℃; the softening point of the glass tube is 60~110℃ lower than that of the core glass; the outer diameter of the glass tube is 36~39mm, and the inner diameter is 20~26mm; the diameter of the core glass rod is 18~24mm. S3 involves drawing the rod-tube assembly into a single filament, arranging the single filaments into a primary multifilament rod, drawing the primary multifilament rod into a primary multifilament, arranging the primary multifilament into a secondary multifilament rod, and drawing the secondary multifilament rod into a secondary multifilament, so that the ratio of the fiber core diameter to the cladding thickness of each image transmission unit is 4.5~6.5:1, thus obtaining a gradient refractive index glass.

[0011] The objectives of this invention and the technical problems it addresses can be further achieved by the following technical measures.

[0012] Preferably, in the preparation method, the step of drawing the rod-tube assembly into a monofilament involves holding the rod-tube assembly at 830~855℃ for 25~30 minutes and then drawing it into a monofilament with a diameter of 3.5~5.0 mm; the drawing time is 80~180 minutes.

[0013] Preferably, in the preparation method, the primary multifilament rod is drawn into a primary multifilament by holding the primary multifilament rod at 805~845℃ for 25~35min and then drawing it into a primary multifilament; the drawing time is 50~230min.

[0014] Preferably, in the preparation method, the secondary multifilament rod is drawn into a secondary multifilament by holding the secondary multifilament rod at 825~845℃ for 18~38min and then drawing it into a secondary multifilament; the drawing time is 90~240min.

[0015] The objective of this invention and the technical problem it solves are achieved by the following technical solution. According to this invention, a gradient refractive index glass comprises a plurality of image-transferring units arranged in an array; each image-transferring unit includes a fiber core and a cladding covering the outside of the fiber core; the fiber core is made of the aforementioned fiber core glass; the thermal expansion coefficient of the cladding is 3 × 10⁻⁶ lower than that of the fiber core. -7 ~7×10 -7 / ℃; the cladding softening point is 60~110℃ lower than the core softening point; the ratio of cladding thickness to core diameter is 1:4.5~6.5.

[0016] The objectives of this invention and the technical problems it addresses can be further achieved by the following technical measures.

[0017] Preferably, the gradient refractive index glass is prepared according to the aforementioned preparation method.

[0018] The objective of this invention and the technical problem it solves are achieved by the following technical solution. According to this invention, an application of the aforementioned graded refractive index glass in the field of optical materials and their preparation is proposed, wherein the graded refractive index glass is used to prepare fiber optic panels, fiber optic image converters, or optical cones.

[0019] By employing the above technical solutions, the fiber core glass, its preparation method, and its application proposed in this invention have at least the following beneficial effects: This invention proposes a fiber core glass, its preparation method, and its applications. Targeting the high-resolution application requirements of fiber optic imaging arrays, it optimizes the material composition of the fiber core glass. By clearly defining the molar percentage range of each oxide, it ensures precise raw material proportions. This allows for the construction of a stable glass framework using SiO2, the role of B2O3 as a flux and refractive index adjuster, and the synergistic optimization of optical performance and structural stability through other components. Ultimately, this results in a fiber core glass with uniform composition and optical performance suitable for graded refractive index glass preparation. The core steps are raw material mixing, melt homogenization, die forming, and annealing, with a clear process and strong operability. This method transforms precisely proportioned raw materials into structurally stable fiber core glass: the melt homogenization step ensures full fusion of components, avoiding undissolved particles or phase separation defects; the annealing step eliminates internal thermal stress generated during forming, reducing the risk of cracking during subsequent processing or use, thereby stably producing core glass rods that meet the requirements for graded refractive index glass preparation.

[0020] The fiber-core glass, its preparation method, and its application proposed in this invention involve using the aforementioned fiber-core glass to make a core glass rod, then coaxially nesting the core glass rod with a cladding glass tube, and finally employing a multi-stage drawing process (monofilament → primary multifilament → secondary multifilament) as the core flow. This process clearly defines the thermal expansion coefficients, softening points, and dimensional matching relationships between the cladding glass tube and the core glass rod, while simultaneously controlling the ratio of core diameter to cladding thickness. This method eliminates the need for complex real-time control of gas flow rate and deposition temperature in existing MCVD vapor deposition processes. By simply matching materials and performing multi-stage drawing, it promotes ion diffusion between the core and cladding glass, forming a continuous and smooth gradient refractive index structure, thus simplifying the process. The invention streamlines the manufacturing process, shortens the cycle, and reduces the difficulty of controlling process uniformity, making it easier to achieve large-scale production. The graded refractive index glass of this invention, through a structural design of several array-type image transmission units, each unit containing a fiber core and cladding, with a core diameter to cladding thickness ratio of 4.5~6.5:1, allows for smooth constraint of light transmission from the fiber core to the cladding. This structure effectively mitigates the steep refractive index transition problem at the fiber core-cladding interface in traditional fiber arrays, reducing interface reflection loss and mode dispersion, thereby lowering crosstalk during light transmission and improving image clarity and resolution to meet high-resolution imaging requirements. The graded refractive index glass of this invention can be used to fabricate fiber optic panels, fiber optic image inverters, or optical cones, adapting to the mainstream structural forms of fiber optic image transmission arrays. Combining the low-loss and high-resolution performance advantages of graded refractive index glass, the fabricated fiber optic panels, image inverters, or optical cones can meet the needs of multiple fields such as low-light imaging, medical endoscopes, and industrial inspection, filling the performance gaps of traditional devices in high-resolution application scenarios and expanding the applicability of optical imaging devices.

[0021] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it in accordance with the contents of the specification, the preferred embodiments of the present invention are described in detail below with reference to the accompanying drawings. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the structure of a primary multifilament prepared in one embodiment; Figure 2 This is a cross-sectional schematic diagram and refractive index distribution diagram of a primary multifilament prepared in one embodiment. Detailed Implementation

[0023] To further illustrate the technical means and effects adopted by the present invention to achieve its intended purpose, the following, in conjunction with the appended tables and preferred embodiments, details the specific implementation methods and effects of a fiber-core glass, its preparation method, and its application according to the present invention. In the following description, different "embodiments" or "embodiments" do not necessarily refer to the same embodiment. Furthermore, the results of one or more embodiments can be combined in any suitable manner. These embodiments are provided to make the invention thorough and complete, and to fully express the scope of the invention to those skilled in the art. It should be noted that, unless otherwise specifically stated, the relative arrangement of components and steps, material composition, numerical expressions, and values ​​described in these embodiments should be interpreted as merely exemplary and not as limiting.

[0024] This invention proposes a core glass and its preparation method. The raw materials, based on the molar percentage of oxides, include: 31-43% SiO2, 3-5% Al2O3, 14-20% B2O3, 6-10% CaO; 2-4% ZnO, 3-7% BaO, 5-7% Bi2O3, 2-4% Li2O, 1-3% Y2O3, 5-8% TiO2, 7-10% ZrO2, 3-5% P2O5, 3-5% Ta2O5, 2-4% Nb2O5, and 1-3% PbO.

[0025] During glass preparation, the raw materials are accurately weighed according to the designed component ratio, converted into corresponding mass parts, and then the various measured raw materials are mixed evenly, melted, homogenized, extruded and shaped, and annealed to obtain fiber core glass.

[0026] In some specific embodiments of the present invention, the uniformly mixed raw materials are placed in a platinum crucible and heated to melt at 1400~1650℃, and held at this temperature for 6~9 hours to ensure that all components are fully melted and that the composition is homogenized. When the heating temperature is below 1400℃, some raw materials are difficult to melt completely, which can easily lead to the formation of undissolved particles or phase separation defects in the glass, resulting in a decrease in optical performance. When the temperature exceeds 1650℃, it may cause component volatilization loss, accelerated crucible corrosion, and deviation of the glass composition from the design value, thereby affecting the refractive index control. If the holding time is less than 6 hours, the melt homogenization is insufficient, which can easily lead to uneven refractive index distribution. If it exceeds 9 hours, it will increase energy consumption and aggravate component volatilization, which is not conducive to maintaining the stability of the glass composition.

[0027] After melting and homogenization, the molten glass is guided through a bottom drain to form a preliminary glass rod. The resulting glass rod is then transferred to an annealing furnace and held at 630–670°C for 2–4 hours to eliminate internal thermal stress and improve the material's structural stability. When the annealing temperature is below 630°C, the internal stress of the glass is difficult to fully release, potentially leading to cracks during subsequent processing or use; while temperatures above 670°C may cause localized softening or deformation of the glass, affecting the finished product's dimensions and optical uniformity. Similarly, annealing for less than 2 hours will not completely eliminate stress, while annealing for more than 4 hours offers no significant improvement and instead reduces process efficiency. After annealing, the glass rod is allowed to cool naturally to room temperature in the furnace to avoid thermal cracking or residual stress caused by rapid cooling, thereby obtaining a fiber-core glass with uniform composition, stable structure, and excellent optical properties.

[0028] In the above technical solution, silicon dioxide (SiO2) serves as the main component of the glass framework and plays a crucial role. If the SiO2 content is too low (e.g., less than 31%), the chemical stability of the glass will decrease. When the SiO2 content is too high (e.g., greater than 43%), the high-temperature viscosity of the glass will increase significantly, resulting in excessively high glass melting temperatures.

[0029] Aluminum oxide (Al₂O₃) is an intermediate oxide in glass production. 3+ Al₂O₃ exists in two coordination states: tetrahedral and octahedral. When oxygen is abundant in the glass, aluminum-oxygen tetrahedra [AlO₄] are formed, which can form a continuous network with silicon-oxygen tetrahedra, enhancing the stability of the glass structure. When oxygen is insufficient in the glass, aluminum-oxygen octaheddra [AlO₆] are formed, existing as network exogenous bodies in the vacancies of the silicon-oxygen network. Within a certain content range, Al₂O₃ can synergistically form the main body of the glass network with SiO₂. When the Al₂O₃ content is too high (e.g., greater than 5%), the glass melting temperature becomes too high.

[0030] Boron oxide (B₂O₃) is a glass-forming oxide and a component of the glass framework. It is also an effective flux that reduces the viscosity of molten glass. Its structural components are boron-oxygen trigonal [BO₃] and boron-oxygen tetrahedron [BO₄]. Under different conditions, boron may exist in the form of trigonal [BO₃] or boron-oxygen tetrahedron [BO₄]. Under high-temperature melting conditions, it is generally difficult to form boron-oxygen tetrahedra, and it mostly exists in trihedral form. However, at low temperatures, under specific conditions, B³⁺ tends to capture free oxygen to form tetrahedra, making the structure more compact and thus increasing the low-temperature viscosity of the glass. Because B₂O₃ has the property of reducing glass viscosity at high temperatures and increasing it at low temperatures, it is also a major component for reducing the refractive index of glass, which determines its relatively narrow content range. If the content is too high (e.g., greater than 20%), the tendency for phase separation in the glass increases. When the B₂O₃ content is too low (e.g., less than 14%), the stability of the glass network structure decreases, leading to excessively high high-temperature viscosity and difficulty in melting.

[0031] Calcium oxide (CaO) is an oxide on the outer layer of the glass network. If the CaO content is too high (e.g., greater than 10%), the glass's chemical stability decreases, and its tendency to crystallize increases. If the CaO content is too low (e.g., below 6%), the glass network structure becomes unstable, leading to reduced strength and hardness.

[0032] Zirconia (ZrO2) is used to improve the refractive index and transmittance of glass, thus adjusting optical properties and improving chemical resistance. A ZrO2 content greater than 10% will increase the melting temperature and crystallization tendency of the glass.

[0033] Zinc oxide (ZnO) is an oxide used to regulate the melting temperature and crystallization properties of glass. If the ZnO content is too high (e.g., greater than 4%), the chemical stability of the glass decreases, while the tendency for crystallization increases. Conversely, if the ZnO content is too low (e.g., less than 2%), it causes instability in the glass network structure.

[0034] Barium oxide (BaO) is an outer oxide of the glass structure network that can effectively improve the refractive index of glass. A BaO content greater than 7% will increase the crystallization temperature of the glass, increase the tendency of the glass to crystallize, and at the same time significantly increase the density of the glass.

[0035] Bismuth oxide (Bi₂O₃) readily polarizes and deforms its cations, making it easy to insert into the glass network and act as a glass forging, effectively improving the chemical stability and refractive index of the glass. However, if the Bi₂O₃ content is too low (e.g., less than 5%), the mid-temperature stability of the core glass deteriorates, leading to severe deformation during the melting and pressing process, making it impossible to maintain a circular shape, reducing structural stability, and exacerbating network defects. Conversely, if the Bi₂O₃ content is too high (e.g., greater than 7%), it reduces the viscoelastic matching of the core and skin glass materials at high temperatures, increasing thermal stress and potentially causing breakage.

[0036] Yttrium oxide (Y2O3) is a lanthanide rare earth oxide that can improve the refractive index of glass and is also used to adjust the crystallization properties of glass. When the Y2O3 content is greater than 3%, it will cause the coefficient of thermal expansion of glass to increase.

[0037] Titanium oxide (TiO2) is used to adjust the chemical stability and crystallinity of glass. When the TiO2 content is too high (e.g., greater than 8%), the chemical resistance of the glass decreases and the tendency to crystallize increases. When the TiO2 content is too low (e.g., less than 5%), the glass's resistance to crystallization may be insufficient, leading to easier crystal precipitation.

[0038] Tantalum oxide (Ta2O5) is a rare earth oxide that can increase the refractive index of glass. When the Ta2O5 content is too high (greater than 5%), it will cause the density and coefficient of thermal expansion of the glass to increase.

[0039] Niobium pentoxide (Nb2O5) is a rare earth oxide that can increase the refractive index of glass. When the Nb2O5 content is too high (greater than 4%), it will cause the density and coefficient of thermal expansion of the glass to increase.

[0040] The Pb-O bonds in lead oxide (PbO) are covalent, readily entering the network at low coordination levels, promoting glass formation, acting as a flux, and improving various glass properties. When the PbO content is too low (below 1%), the glass network structure is difficult to form, leading to difficulties in forming, decreased optical properties, and reduced chemical stability. When the PbO content is too high (greater than 3%), the glass is prone to crystallization and its thermal stability decreases, potentially causing optical loss and increased processing difficulty.

[0041] Lithium oxide (Li2O) is also a network exooxide. When its content is 2-4%, it can improve the water resistance of glass, reduce the melting temperature of glass, and improve the yield and quality of glass. When the content of Li2O is too high (greater than 4%), it is easy to increase the crystallization tendency. When the content of Li2O is too low (below 2%), it is difficult to effectively reduce the melting temperature, resulting in an excessively high glass melting temperature and increasing the difficulty of preparing the core glass rod.

[0042] Phosphorus pentoxide (P2O5) is a typical glass network agglomerant that can improve the refractive index and optical uniformity, which is beneficial to the light transmission performance of fiber core glass. When the P2O5 content is too high (greater than 5%), it is easy to cause glass crystallization or phase separation, affecting optical stability. When the P2O5 content is too low (below 3%), the glass network is not dense enough, the refractive index decreases, and the optical performance is difficult to meet the requirements of fiber core glass.

[0043] This invention also proposes a graded refractive index glass and its preparation method. The graded refractive index glass comprises a plurality of image-transferring units arranged in an array; each image-transferring unit comprises a core and a cladding covering the outside of the core; the ratio of the core diameter to the cladding thickness is 4.5~6.5:1; its preparation includes the following steps: First, the core glass is made into a core glass rod according to the aforementioned preparation method. Then, the core glass rod and the outer glass tube are assembled in a coaxial nesting manner, so that the core glass rod is accurately positioned in the center of the outer glass tube. At the same time, the physical compatibility of the core material and the outer material must be fully considered, such as the coefficient of thermal expansion and softening point. The coefficient of thermal expansion of the outer glass tube is 3×10⁻⁶ lower than that of the core glass rod. -7 ~7×10 -7 (1 / ℃), the softening point of the outer glass tube is 60~110℃ lower than that of the core glass. The outer diameter of the outer glass tube is controlled at 36~39mm and the inner diameter is controlled at 20~26mm. The diameter of the core glass rod is 18~24mm.

[0044] After assembly, the rod and tube assembly is placed in a high-temperature wire drawing furnace and heated in the range of 830~855℃ for 25~30 minutes to ensure that the glass is completely softened and achieves good interfacial fusion bonding, while promoting ion diffusion between the core and skin glass, thereby forming a gradient refractive index structure.

[0045] Compared with traditional processes, this process appropriately increases the heating temperature and extends the holding time in order to enhance the interdiffusion of components at the interface, making the refractive index transition smoother, which is beneficial to reducing light scattering loss and improving optical uniformity.

[0046] If the temperature is below 830℃, the glass will not soften sufficiently, the interface will not melt adequately, and the core-skin glass interface will only achieve mechanical contact, resulting in insufficient ion diffusion and an excessively steep refractive index interface. This can easily lead to optical discontinuities, increasing mode coupling and scattering losses. If the temperature is above 855℃, the glass will flow excessively, its viscosity will decrease sharply, and excessive diffusion of components may cause the refractive index gradient to become overly smooth or even homogenized, weakening the optical focusing characteristics of the design. If the holding time is less than 25 minutes, the glass will not soften completely, the interface will not bond tightly, and ion diffusion will be insufficient. If the holding time exceeds 30 minutes, the glass will melt excessively, potentially introducing the risk of bubbles.

[0047] Subsequently, a drawing process is used to draw the single filament for 80-180 minutes. This time range ensures that the core-skin glass continues to undergo appropriate component diffusion under tension while avoiding structural instability. If the drawing time is less than 80 minutes, the drawing speed is too fast, the diameter control is unstable, and uneven thickness or excessive internal stress may occur. If the drawing time exceeds 180 minutes, the drawing speed is too slow, and the glass remains at high temperature for too long, which may lead to impurity precipitation or a decrease in optical performance. The final single filament diameter is controlled between 3.5 and 5 mm.

[0048] The prepared monofilaments are arranged in a close-packed hexagonal pattern, forming a regular two-dimensional array structure of the core units in the cross-section. To improve the overall mechanical stability and optical performance of the array, gap fibers are inserted into the gaps between the monofilaments. These gap fibers are used to fill the gaps, improve the structural compactness of the composite rod, and reduce uneven deformation during the subsequent drawing process.

[0049] The composite rod formed after arrangement and filling is then drawn. It is placed in a high-temperature drawing furnace and heated to 805-845℃ for 25-35 minutes to fully soften the composite rod and ensure uniform fusion of the multi-filament interfaces. If the heating temperature is below 805℃ or the holding time is less than 25 minutes, the composite rod will not soften sufficiently, resulting in insufficient interface fusion and low diffusion between the core and skin. This leads to an indistinct refractive index gradient or even abrupt interface changes, increasing scattering loss and crosstalk risk. Conversely, if the temperature exceeds 845℃ or the holding time exceeds 35 minutes, although diffusion deepens, excessive ion migration may weaken or even homogenize the designed refractive index distribution, causing loss of the gradient refractive index effect. It may also lead to bubble formation and decreased optical uniformity.

[0050] The drawing process then takes 50-230 minutes to complete, resulting in a single multifilament, as shown in the attached diagram. Figure 1 The diagram shows a single-filament multifilament, where 1 represents the cladding glass, 2 represents the core glass, and 3 represents the monofilament; see attached diagram. Figure 2 The diagram shows a cross-sectional view and refractive index distribution along the plane containing the diameter of a row of optical fibers in a single-pass multifilament. The vertical axis 'n' at the top of the diagram represents the refractive index at that location. (See attached diagram.) Figure 2 It is evident that the refractive index of each imaging unit gradually increases from the cladding to the fiber core center, without any abrupt change in refractive index. If the drawing time in this step is less than 50 minutes, diffusion will be insufficient and the stretching process will be unstable; if it exceeds 230 minutes, problems such as uneven diameter and excessive structural relaxation may occur.

[0051] The obtained primary composite wires are then arranged again in a close-packed hexagonal pattern to form a secondary composite rod with a more regular structure.

[0052] After the secondary composite rods are arranged, they are placed in a high-temperature drawing furnace and heated in the range of 825~845℃ for 18~38 minutes to ensure that the internal multifilaments soften uniformly and bond tightly. If the heating temperature is below 825℃ or the holding time is less than 18 minutes, the multifilaments will not soften sufficiently, the diffusion between the core and the skin will be limited, and the refractive index gradient will not be formed sufficiently, resulting in a steep interface and increased light scattering and mode coupling loss. At the same time, problems such as loose structure and unstable diameter control are likely to occur during the drawing process. If the temperature exceeds 845℃ or the holding time exceeds 38 minutes, although the diffusion is more sufficient, excessive ion migration may weaken or even homogenize the designed refractive index gradient, causing the gradient refractive index effect to be lost. In addition, high-temperature long-term treatment may also cause bubble formation or a decrease in optical uniformity, affecting the optical performance and mechanical strength of the finished product. The secondary multifilament is then drawn through a drawing process of 90 to 240 minutes. If the drawing time is less than 90 minutes, the multifilament will not spread sufficiently during the stretching process and the diameter will be difficult to control evenly. If it exceeds 240 minutes, it may cause excessive structural relaxation, uneven diameter, or surface optical defects.

[0053] Through the aforementioned steps and process control, the ratio of core diameter to cladding thickness is ultimately achieved between 4.5 and 6.5:1, forming a step-gradient refractive index distribution structure between the core and cladding. This geometric and refractive index design smoothly confines light during the transition from the core to the cladding, thereby reducing mode dispersion and improving optical field focusing characteristics. Simultaneously, the appropriate cladding thickness not only enhances mode confinement in optical performance but also improves the structural strength and thermal stability of the fiber, ensuring the integrity and long-term stability of the array during multiple fiber drawing processes and subsequent applications.

[0054] This invention also proposes an application of the aforementioned graded refractive index glass in the field of optical materials and their fabrication, wherein the graded refractive index glass is used to fabricate fiber optic panels, fiber optic image converters, or optical cones. Specific details are as follows: The secondary multifilaments are cut into lengths of 80-125mm and bundled into fused fiber rods using wire. These rods are then placed in a hot press mold for forming. It should be noted that the term "fused fiber rod" here does not mean the fiber rod itself is already molten, but rather emphasizes the subsequent hot pressing and fusion bonding process. The hot pressing temperature is controlled between 525-590℃, and the hot pressing time is controlled between 160-260 minutes to ensure that the plate segment is fully densified under pressure and internal micropores are eliminated. If the hot pressing temperature is below 525℃ or the hot pressing time is less than 160 minutes, the fibers will not soften sufficiently, the gradient refractive index will not form adequately, and the interface refractive index will abruptly change, leading to poor mode coupling, increased optical scattering loss, and the possibility of residual internal micropores, reducing structural density and mechanical strength. If the temperature exceeds 590℃ or the hot pressing time exceeds 260 minutes, although diffusion is more complete, it may lead to an excessively smooth refractive index gradient, and the core-skin interface will tend to be homogenized, weakening the gradient refractive index design effect. Furthermore, prolonged high-temperature exposure may increase the risk of crystallization, bubble formation, and thermal stress accumulation. During the forming process, the pressure range is set to 60~120N to achieve tight bonding between fibers and surface smoothing. When the applied pressure is below 60N, the bonding between fibers is not tight, and the plate segment is prone to deformation in subsequent operations. If the pressure exceeds 120N, the fibers may be over-compressed, leading to local stress concentration or structural displacement, affecting the uniformity of the graded refractive index and the overall stability of the composite rod. After this process, a preliminary optical fiber blank is obtained, providing a stable intermediate for subsequent annealing and finishing processes.

[0055] After annealing, the molten fiber rod is rolled into a round shape and then cut into fiber blanks with a length of 17-30mm. The blanks are then placed in an annealing furnace to relieve stress. The annealing temperature program is set as follows: from room temperature, the temperature is increased to 540-560℃ in 350-400min, then increased to 550-600℃ in 50-70min, and held for 200-900min. If the holding time is less than 200min, the core-skin glass diffusion is insufficient, the refractive index gradient is not fully formed, and the interface changes abruptly, resulting in increased optical scattering and mode coupling loss. If the holding time is more than 900min, although the core-skin diffusion is more sufficient, excessive diffusion may cause the refractive index gradient to be too smooth or even tend to be uniform, weakening the designed focusing effect. After that, the temperature is naturally cooled to 60℃.

[0056] The annealed fiber blank end face undergoes precision grinding (1.2~3.2h) and polishing (1.5~4.5h) to ultimately obtain a fiber optic panel with excellent surface optical properties and capable of high-fidelity transmission. If the grinding time is less than 1.2h, the end face flatness is insufficient, and minor surface roughness or residual burrs will lead to increased optical scattering and decreased imaging contrast. If the grinding time exceeds 3.2h, excessive thinning of the end face or microstructural deformation may occur, affecting the geometric ratio of the fiber core and cladding, leading to local failure of the refractive index distribution. If the polishing time is less than 1.5h, the surface roughness will not meet optical requirements, reducing optical transmission efficiency. If it exceeds 4.5h, minor deformation or stress concentration may occur, affecting the long-term stability and imaging consistency of the fiber optic panel.

[0057] The annealed fiber blank is placed in a high-temperature torsion furnace for heat treatment. The torsion temperature is set at 720~810℃; the torsion time is controlled at 40~120min to ensure that the fiber array rotates stably at high temperature, thereby reversing the image transmission direction. If the temperature is below 720℃ or the time is less than 40min, the fiber will lack plasticity at high temperature, the array will not rotate sufficiently, the image reversal effect will be incomplete, and optical torsion distortion will occur. If the temperature exceeds 810℃ or the time exceeds 120min, the fiber will soften excessively, which may lead to local deformation of the array and widening of the gaps, affecting the optical uniformity and mechanical stability of the image reverser. After the heat torsion is completed, the end face is precision ground (1.4~3.2h) and polished (1.5~4.5h) to finally obtain a fiber optic image reverser with excellent surface optical properties and capable of high-fidelity image reversal transmission.

[0058] The annealed fiber blank is placed in a high-temperature stretching furnace and heated at 710~750℃ to soften it before controlled stretching. The stretching time is controlled between 40~180 min to obtain a tapered structure with gradually changing dimensions along the length direction. If the temperature is below 710℃ or the stretching time is less than 40 min, the fiber is not sufficiently softened, the tapered structure is not fully formed, and the dimensional transition is discontinuous, resulting in poor focusing at the end face of the optical cone. If the temperature exceeds 750℃ or the stretching time exceeds 180 min, the fiber is over-softened or the stretching time is too long, which may cause uneven diameter and stress concentration, thus affecting the optical performance of the cone. After stretching, the blank is symmetrically divided into two parts along the length direction to form the initial optical cone blank. Subsequently, the fiber cone blank is precision ground (1.4~3.2 h) and polished (1.5~4.5 h), which can be used to realize the magnification or reduction of image transmission.

[0059] The crosstalk rate of the fiber optic panel, image reversor, and optical cone was tested using a contrast meter, the transmittance of the fiber optic panel, image reversor, and optical cone was tested using a transmittance meter, and the resolution of the fiber optic panel, image reversor, and optical cone was tested using a USAF1951 resolution board.

[0060] The present invention will be further described below with reference to specific embodiments, but this should not be construed as a limitation on the scope of protection of the present invention. Some non-essential improvements and adjustments made to the present invention by those skilled in the art based on the above description of the present invention still fall within the scope of protection of the present invention.

[0061] Unless otherwise specified, all materials and reagents mentioned below are commercially available products well known to those skilled in the art; unless otherwise specified, all methods described are methods known in the art. Unless otherwise defined, the technical or scientific terms used should have the ordinary meaning understood by those skilled in the art to which this invention pertains.

[0062] Examples 1-12 and Comparative Examples 1) Preparation of fiber core glass In the preparation of fiber core glass, the present invention first follows the designed component ratio, expressed as a molar percentage of oxides, as shown in Table 1. The raw materials are then accurately weighed after being converted to the corresponding mass ratio based on the molar percentage. After thorough and uniform mixing, the raw materials are placed in a platinum crucible and heated to 1550°C for melting, and held at this temperature for 7 hours to ensure complete melting of all components and achieve homogenization of the composition.

[0063] After melting and homogenization, the molten glass is guided through the bottom drain to form a preliminary glass rod. The resulting glass rod is then transferred to an annealing furnace and held at 650°C for 3.5 hours to eliminate internal thermal stress and improve the structural stability of the material. After annealing, the glass rod is allowed to cool naturally to room temperature in the furnace to avoid thermal cracking or residual stress caused by rapid cooling, thereby obtaining a fiber-core glass with uniform composition, stable structure, and excellent optical properties.

[0064] The outer glass tube and the core glass rod are assembled in a coaxial nesting manner, ensuring that the core glass is accurately centered in the outer glass tube. The coefficient of thermal expansion of the outer glass tube is 4 × 10⁻⁶ lower than that of the core glass rod. -7 (1 / ℃), the softening point of the outer glass tube is 75℃ lower than that of the core glass. The outer diameter of the outer glass tube is controlled at 37mm, and the inner diameter at 25mm; the diameter of the core glass rod is 23mm. After assembly, the combined structure is placed in a high-temperature drawing furnace, heated at 835℃ and held for 27 minutes. Subsequently, the single filament is drawn in 100 minutes using a drawing process, and the final single filament diameter is controlled at 4mm, with a core diameter to cladding glass thickness ratio of 6.2:1.

[0065] The prepared monofilaments are arranged in a close-packed hexagonal pattern, forming a regular two-dimensional array structure of the core units in cross-section. Gap fibers are inserted into the gaps between the monofilaments. After arrangement and filling, a primary composite rod is formed, which is then placed in a high-temperature drawing furnace and heated to 828°C for 23 minutes. The drawing process is then completed after 110 minutes to obtain a primary multifilament.

[0066] The obtained primary multifilaments are arranged again in a close-packed hexagonal pattern to form a secondary composite rod with a more regular structure. After the arrangement is completed, the secondary composite rod is placed in a high-temperature drawing furnace and heated at 830°C for 22 minutes to ensure that the internal multifilaments soften uniformly and bond tightly. Then, the secondary multifilaments are drawn through a 130-minute drawing process.

[0067] The secondary multifilaments are cut to lengths of 80-125mm and bundled into molten fiber rods using wire. These rods are then placed in a hot press mold for forming. The hot pressing temperature is controlled between 540℃ and 180 minutes. The pressure range during forming is set to 65N.

[0068] After annealing, the molten fiber rod is rolled into a round shape and then cut into fiber blanks with a length of 25mm. The blanks are placed in an annealing furnace to relieve stress. The annealing temperature program is set as follows: the temperature is increased from room temperature to 540℃ in 380 minutes, then increased to 600℃ in 55 minutes, held for 500 minutes, and then naturally cooled to 60℃.

[0069] The end face of the annealed optical fiber blank is precision ground (1.6h) and polished (2.6h) to finally obtain an optical fiber panel with excellent surface optical properties and capable of high-fidelity transmission.

[0070] The annealed fiber blank was placed in a high-temperature torsion furnace for heat treatment. The torsion temperature was set at 731℃ and the torsion time was controlled at 90 minutes to ensure that the fiber array rotated stably as a whole at high temperature. After the heat torsion was completed, the end face was precision ground (1.6h) and polished (2.6h) to finally obtain a fiber optic image inverter with excellent surface optical properties and capable of high-fidelity image inversion transmission.

[0071] The annealed fiber blank was placed in a high-temperature stretching furnace and heated to 729°C to soften it before controlled stretching. The stretching time was controlled at 110 minutes to obtain a tapered structure with gradually changing dimensions along its length. After stretching, the blank was symmetrically divided into two parts along its length to form the initial fiber cone blank. Subsequently, the fiber cone blank underwent precision grinding (1.6 hours) and polishing (2.6 hours), which can be used to realize the magnification or reduction of image transmission.

[0072] The crosstalk rate of the fiber optic panel, image reversor, and optical cone was tested using a contrast meter, the transmittance of the fiber optic panel, image reversor, and optical cone was tested using a transmittance meter, and the resolution of the fiber optic panel, image reversor, and optical cone was tested using a USAF1951 resolution board. The test results are shown in Table 2.

[0073] Table 1. Mole percentage of components (mol%) of the core glass in Examples 1-7 and Comparative Examples Continued from Table 1: Molar percentage (mol%) of the core glass components in Examples 8-12 Table 2. Performance test results of fiber optic image transmission arrays in Examples 1-12 and comparative examples. As can be seen from the above examples and comparative data, Examples 1-3, 4-5, and 6-7 characterize the influence of each component on the spatial distribution of graded refractive index in the optical fiber imaging array by increasing the molar percentage content of B2O3, Li2O, and P2O5, respectively. Since B2O3 can reduce the rigidity of the glass network and adjust the refractive index distribution, Li2O, as an alkali metal oxide, can introduce non-bridging oxygen ions to improve ion mobility and optical uniformity of the glass, and P2O5, as a network precursor, can enhance the network-forming ability of the glass structure and help form a more stable refractive index gradient, the changes in the content of these three components directly affect the self-focusing effect of light in the optical fiber.

[0074] Comparative test results show that, compared to Comparative Example 1 (where the molar percentages of B2O3, Li2O, and P2O5 were all reduced to the lowest values), Examples 1-7, by increasing the molar percentages of the above components within a reasonable range, significantly improved the resolution of the fiber optic image transmission arrays (including fiber optic panels, fiber optic image converters, and fiber optic tapers), and effectively reduced the crosstalk rate. This is mainly attributed to the fact that a more reasonable refractive index gradient distribution can suppress light leakage and crosstalk between fiber cores, thereby improving the clarity of image transmission.

[0075] In Example 8, the molar percentages of B2O3, Li2O, and P2O5 were set to the lowest possible values ​​within the controllable range. In Examples 11 and 12, the molar percentage of P2O5 was set to the lowest possible value within the controllable range, and the contents of other components were also at low levels. These three key components still played a fundamental regulatory role and successfully constructed a gradient refractive index distribution. However, due to insufficient concentration of the functional components, the resulting refractive index gradient was relatively gentle, and the self-focusing constraint on the beam was relatively weak. Therefore, when testing the optical performance of the final fiber element, although its resolution, crosstalk rate, and transmittance met the basic imaging requirements, their performance was relatively poor compared to other examples. Conversely, in Example 9, the molar percentage of P2O5 was set to the highest possible value within the controllable range, and in Example 10, the molar percentage of B2O3 was set to the highest possible value within the controllable range. Combined with optimized settings of other components, the high concentration of functional oxides significantly enhanced the ion exchange capacity and structural stability of the glass network, forming a more advantageous steep refractive index gradient. This optimized gradient distribution greatly improves the light transmission efficiency and imaging quality of the optical fiber, resulting in optimal resolution, crosstalk rate, and transmittance.

[0076] However, the introduction of excessive functional components can also increase the non-bridging oxygen content in the glass network, leading to enhanced scattering and absorption, thus reducing transmittance. Nevertheless, these fiber optic imaging elements optimized for high resolution can still play an important role in applications requiring high resolution, such as high-precision imaging, photoelectric detection, and military optical systems.

[0077] The technical features in the claims and / or specification of this invention can be combined, and the combination is not limited to the combinations obtained through reference in the claims. Technical solutions obtained by combining the technical features in the claims and / or specification are also within the scope of protection of this invention.

[0078] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A core glass characterized in that, The raw materials include, in terms of mole percentage of oxides: 31-43% SiO2, 3-5% Al2O3, 14-20% B2O3, 6-10% CaO; 2-4% ZnO, 3-7% BaO, 5-7% Bi2O3, 2-4% Li2O, 1-3% Y2O3, 5-8% TiO2, 7-10% ZrO2, 3-5% P2O5, 3-5% Ta2O5, 2-4% Nb2O5, 1-3% PbO.

2. A method of making a core glass, characterized by, It comprises the following steps: S1 weighing raw materials and mixing them evenly; The raw materials include, in terms of mole percentage of oxides: 31-43% SiO2, 3-5% Al2O3, 14-20% B2O3, 6-10% CaO; 2-4% ZnO, 3-7% BaO, 5-7% Bi2O3, 2-4% Li2O, 1-3% Y2O3, 5-8% TiO2, 7-10% ZrO2, 3-5% P2O5, 3-5% Ta2O5, 2-4% Nb2O5, 1-3% PbO; S2 melting, homogenizing, forming by leakage, annealing to obtain the core glass.

3. The preparation method according to claim 2, characterized in that, The melting is heating and melting the mixed raw materials in a platinum crucible at 1400-1650℃ for 6-9 hours; the annealing is placing the formed piece in an annealing furnace and keeping it at 630-670℃ for 2-4 hours.

4. A method of making a graded index glass, characterized by, It comprises the following steps: S1 preparing the core glass rod according to claim 1 by the preparation method according to claim 2 or 3; S2, the core glass rod and the skin glass tube are assembled in coaxial nesting mode to form a rod-tube assembly; wherein the skin glass tube has a thermal expansion coefficient lower than the thermal expansion coefficient of the core glass by 3x10 -7 7x10 -7 / ℃; the skin glass tube has a softening point lower than the softening point of the core glass by 60~110℃; the skin glass tube has an outer diameter of 36~39mm and an inner diameter of 20~26mm; the core glass rod has a diameter of 18~24mm; S3 drawing the rod tube assembly into a single filament, arranging the single filaments into a primary multifilament rod, drawing the primary multifilament rod into a primary multifilament, arranging the primary multifilaments into a secondary multifilament rod, and drawing the secondary multifilament rod into a secondary multifilament, so that the ratio of the core diameter to the cladding thickness of each image transmission unit is 4.5-6.5:1, to obtain the graded refractive index glass.

5. The preparation method according to claim 4, characterized in that, The drawing of the rod tube assembly into a single filament is keeping the rod tube assembly at 830-855℃ for 25-30min, and then drawing it into a single filament with a diameter of 3.5-5.0mm; the drawing time is 80-180min.

6. The preparation method according to claim 4, characterized in that, The drawing of the primary multifilament rod into a primary multifilament is keeping the primary multifilament rod at 805-845℃ for 25-35min, and then drawing it into a primary multifilament; the drawing time is 50-230min.

7. The preparation method according to claim 4, characterized in that, The drawing of the secondary multifilament rod into a secondary multifilament is keeping the secondary multifilament rod at 825-845℃ for 18-38min, and then drawing it into a secondary multifilament; the drawing time is 90-240min.

8. A graded index glass, characterized by, It comprises several image transmission units arranged in an array; each image transmission unit includes a fiber core and a cladding covering the outside of the fiber core; the fiber core is made of the fiber core glass as described in claim 1; the coefficient of thermal expansion of the cladding is 3 × 10⁻⁶ lower than that of the fiber core. -7 ~7×10 -7 / ℃; the cladding softening point is 60~110℃ lower than the core softening point; the core diameter to cladding thickness ratio is 4.5~6.5:

1.

9. The graded index glass of claim 8 wherein, It is prepared according to the preparation method of any one of claims 4 to 7.

10. Use of the graded-index glass according to claim 8 or 9 in the field of optical materials and their production, characterized in that The graded refractive index glass is used to prepare a fiber panel, a fiber inverter or a light cone.

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