Graded refractive index optical fiber image transmission array and preparation method and application thereof

Abstract

The invention provides a gradient refractive index optical fiber image transmission array and a preparation method and application thereof. The gradient refractive index optical fiber image transmission array comprises a plurality of image transmission units which are arranged in an array mode. Each image transmitting unit comprises a fiber core and a wrapping layer wrapping the outer side of the fiber core; a parabola type gradient refractive index distribution structure with a downward opening is arranged in each image transmitting unit along the diameter direction of the image transmitting unit; and the ratio of the diameter of the fiber core to the thickness of the cladding is (4.5-6.5): 1. The technical problem to be solved is how to prepare an optical fiber image transmitting array, so that a continuous and smooth gradient refractive index structure is formed only through core-skin glass coaxial nesting and accurate control of technological parameters such as multi-stage wire drawing, hot pressing and annealing, the process is simple, the preparation period is short, and the technological uniformity is easier to control. And the manufactured graded-index optical fiber image transmission array can reduce optical loss and crosstalk, improve the imaging resolution, can be stably manufactured into devices such as an optical fiber panel, an optical fiber image inverter and a light cone, and meets the requirements of multiple fields such as low-illumination imaging and medical endoscopes.

Description

Technical Field

[0001] This invention relates to the field of optical materials and fabrication technology, and in particular to a graded refractive index fiber imaging array, its fabrication method, and its application. Background Technology

[0002] Fiber optic imaging arrays are optical transmission devices composed of tens of millions of independent optical fibers arranged closely together. Each fiber consists of a core and a cladding. The optical signal is constrained and transmitted by relying on the characteristic that the refractive index of the core is higher than that of the cladding. Each fiber corresponds to an independent pixel channel, which can maintain the pixel correspondence in the image space and realize high-fidelity transmission of two-dimensional images. They are widely used in low-light imaging, night vision devices, medical endoscopes, industrial inspection and high-energy physics detection. In military and scientific research scenarios, they are often used in conjunction with microchannel plates, photocathodes or fluorescent screens to form high-performance imaging and detection systems.

[0003] Currently, most individual fibers in fiber optic imaging arrays employ a step-index refractive index distribution structure, which exhibits a significant abrupt change in refractive index at the core-cladding interface. In practical applications, step-index fiber optic imaging arrays have significant drawbacks: spatial resolution is difficult to improve, failing to meet the detail requirements of high-precision imaging; light is prone to leakage through the cladding and coupling with adjacent fibers during transmission, leading to increased crosstalk and affecting image sharpness and contrast; furthermore, modal dispersion exists, and the limited numerical aperture further restricts the transmission quality of high-resolution images.

[0004] To address the shortcomings of step-index structures, while there have been attempts at graded-index fiber optic imaging arrays, the fabrication of existing graded-index fiber optic imaging arrays typically relies on vapor deposition processes such as MCVD. This method achieves a gradient distribution of different components along the cross-section in the fiber core region by precisely controlling the flow rate and deposition temperature of the gas feedstock in real time during deposition, ensuring a continuous refractive index transition between deposition layers. However, such processes are complex, have long fabrication cycles, and require extremely high process uniformity. Summary of the Invention

[0005] The main objective of this invention is to provide a graded refractive index fiber optic imaging array, its fabrication method, and its applications. The technical problem to be solved is how to fabricate a graded refractive index fiber optic imaging array that is more advantageous than existing vapor deposition processes such as MCVD. This invention does not require complex real-time control of gas flow rate and deposition temperature. Instead, it precisely controls process parameters such as core-skin glass coaxial nesting and multi-stage drawing, hot pressing, and annealing to form a continuous and smooth graded refractive index structure. The process is simpler, the fabrication cycle is shorter, and the control of process uniformity is easier to achieve. Furthermore, the fabricated graded refractive index fiber optic imaging array can reduce optical loss and crosstalk, improve imaging resolution, and can be stably manufactured into fiber optic panels, fiber optic image inverters, optical cones, and other devices, adapting to the needs of low-light imaging, medical endoscopes, and other 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 graded-index fiber imaging array comprises several arrayed imaging units; each imaging unit includes a fiber core and a cladding covering the outside of the fiber core; each imaging unit has a parabolic graded-index distribution structure with an opening downwards along its diameter direction; the ratio of the fiber core diameter to the cladding thickness is 4.5~6.5:1.

[0007] The objectives of this invention and the technical problems it addresses can be further achieved by the following technical measures.

[0008] Preferably, in the graded-index fiber imaging array, the coefficient of thermal expansion of the cladding glass is 3 × 10⁻ lower than that of the core glass. 7 ~7×10⁻ 7 / ℃; the softening point of the cladding glass is 60-95℃ lower than that of the core glass.

[0009] The objective of this invention and the technical problem it solves are achieved through the following technical solution. A method for fabricating a graded-index fiber optic imaging array according to this invention includes the following steps: S1 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 60-95℃ lower than that of the core glass; the outer diameter of the glass tube is 36-39mm, and the inner diameter is 19-27mm; the diameter of the core glass rod is 17-25mm. S2 holds the rod-tube assembly at 820–860°C for 20–30 minutes, and then draws it into a monofilament after 70–200 minutes; the diameter of the monofilament is 3.5–5 mm, and the ratio of the core diameter to the cladding thickness is 4.5–6.5:1; S3 arranges the monofilaments in a close-packed hexagonal pattern, inserts gap wires into the gaps between the monofilaments to form a primary multifilament bar; draws the primary multifilament bar into a primary multifilament; arranges the primary multifilaments into a secondary multifilament bar; draws the secondary multifilament bar into a secondary multifilament. S4 cuts and bundles the secondary multifilaments, and then hot-presses them at 530-600℃ and 50-120N for 180-270 minutes. S5 annealing allows for full diffusion of the core and sheath, resulting in a graded-index fiber imaging array.

[0010] The objectives of this invention and the technical problems it addresses can be further achieved by the following technical measures.

[0011] Preferably, in the preparation method, the primary multifilament rod is drawn into a primary multifilament by holding the primary multifilament rod at 810-840°C for 20-35 minutes and then drawing it into a primary multifilament for 70-250 minutes.

[0012] Preferably, in the preparation method, the secondary multifilament rod is drawn into a secondary multifilament by holding the secondary multifilament rod at 830-850°C for 20-38 minutes and then drawing it into a secondary multifilament for 100-250 minutes.

[0013] Preferably, in the preparation method, the annealing procedure is as follows: heating from room temperature to 540-560°C for 350-400 min; heating to 550-600°C for 50-70 min; and holding at that temperature for 300-1000 min.

[0014] The objective of this invention and the technical problem it solves are achieved by the following technical solution. This invention provides an application of a graded-index fiber optic imaging array based on the aforementioned method and the graded-index fiber optic imaging array prepared according to the aforementioned method in the field of optical materials.

[0015] The objectives of this invention and the technical problems it addresses can be further achieved by the following technical measures.

[0016] Preferably, in the aforementioned application, the graded-index fiber optic imaging array is precision ground and polished to obtain a fiber optic panel with a resolution ≥181 lp / mm, crosstalk ≤0.01%, and transmittance ≥31%.

[0017] Preferably, in the aforementioned application, the graded refractive index fiber image transmission array is twisted at 730–810°C for 50–120 min; then the end face is precisely ground and polished to obtain a fiber image inverter; its resolution is ≥181 lp / mm, crosstalk rate is ≤0.02%, and transmittance is ≥29%.

[0018] Preferably, in the aforementioned application, the graded refractive index fiber imaging array is stretched at 720–750°C for 50–180 min; symmetrically cut into two parts along the radial cross section; and then the end face is precisely ground and polished to obtain a light cone; its resolution is ≥228 lp / mm, crosstalk rate is ≤0.02%, and transmittance is ≥26%.

[0019] By employing the above technical solutions, the graded refractive index fiber imaging array, its fabrication method, and its application proposed in this invention have at least the following beneficial effects: The graded-index fiber imaging array proposed in this invention achieves a smooth refractive index transition from the core to the cladding through a downward-opening parabolic graded-index distribution. This enhances the confinement and focusing capabilities of light, significantly improves imaging resolution, and reduces mode dispersion and signal distortion. The ratio of core diameter to cladding glass thickness is controlled at 4.5~6.5:1. The thicker cladding effectively increases the physical isolation distance between adjacent fibers, suppresses light leakage and coupling between adjacent fibers, reduces signal crosstalk, and improves imaging clarity and contrast. The rational core-cladding structure design balances optical performance and mechanical stability, improves the structural strength and thermal stability of the fiber array, and ensures integrity during multiple processing and long-term use.

[0020] The proposed method for fabricating graded-index fiber optic imaging arrays eliminates the need for complex real-time control of gas flow and deposition temperature in vapor deposition processes such as MCVD. Instead, it simplifies the process and shortens the fabrication cycle by employing coaxial nesting of core and skin glass, multi-stage fiber drawing, and high-temperature ion diffusion. This method facilitates process uniformity control and is suitable for large-scale production. Inserting gap wires during single-filament arrangement enhances the density of the composite rod structure, reduces deformation unevenness during subsequent fiber drawing, and ensures the regularity of the array structure. Precise matching of heating, heat preservation, and fiber drawing parameters at each stage promotes sufficient and appropriate component diffusion in the core and skin glass, forming a continuous and smooth graded-index structure and reducing light scattering loss. Precise matching of the thermal expansion coefficient, softening point, and dimensions of the core and skin glass, combined with hot pressing and annealing processes, effectively eliminates internal stress and micropores, preventing excessive smoothing or abrupt changes in the refractive index gradient and improving product performance consistency and stability.

[0021] The graded-index fiber optic imaging array proposed in this invention is compatible with the fabrication requirements of mainstream fiber optic imaging devices such as fiber optic panels, fiber optic image inverters, and optical cones. The fabricated devices possess comprehensive optical performance with high resolution, low crosstalk, and high transmittance, enabling high-fidelity transmission of two-dimensional images. The device performance meets the high-precision application requirements of multiple fields such as low-light imaging, medical endoscopy, industrial inspection, and high-energy physics detection, filling the gaps of traditional devices in high-fidelity, low-noise transmission scenarios. It expands the application boundaries of graded-index fiber optic imaging arrays, which can be combined with devices such as microchannel plates and photocathodes to build high-performance imaging and detection systems, adapting to the needs of multiple scenarios such as military, scientific research, and civilian applications.

[0022] 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

[0023] Figure 1 This is a schematic diagram of the structure of a graded refractive index fiber imaging array prepared in one embodiment; Figure 2 This is a cross-sectional schematic diagram and refractive index distribution diagram of a graded refractive index fiber imaging array prepared according to one embodiment. Detailed Implementation

[0024] To further illustrate the technical means and effects adopted by the present invention to achieve its intended purpose, the following, in conjunction with the appendix and preferred embodiments, details the specific implementation methods and effects of a graded refractive index fiber imaging array, its fabrication 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.

[0025] This invention proposes a graded-index fiber optic imaging array and its fabrication method, as shown in the appendix. Figure 1 and attached Figure 2 As shown, the graded-index fiber imaging array includes several array-arranged imaging units 3; each imaging unit includes a fiber core 2 and a cladding 1 covering the outside of the fiber core; each imaging unit has a parabolic graded-index distribution structure with an opening downwards along its diameter; the vertical coordinate n shown at the top of the figure represents the refractive index corresponding to that position; from the attached... 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 changes in refractive index. Furthermore, its gradual refractive index change trend exhibits characteristics of a parabolic distribution.

[0026] In some specific embodiments of the present invention, the glass tube and the core glass rod are assembled in a coaxial nesting manner, so that the core glass is accurately located at the center of the glass tube, forming a rod-tube assembly. For better performance of the final array, the compositions of the core glass and the glass tube used for assembly in this invention must be compatible. This invention does not specifically limit the specific composition of the core material and the skin material, but it does have specific limitations on the compatibility of their components. This invention preferably emphasizes the physical compatibility of the core material and the skin material, such as their coefficients of thermal expansion and softening points, wherein the coefficient of thermal expansion of the glass tube is 3 × 10⁻⁶ lower than that of the core glass rod. -7 ~7×10 -7 / ℃, the softening point of the outer glass tube is 60-95℃ lower than that of the core glass, the outer diameter of the outer glass tube is controlled at 36-39 mm, and the inner diameter is controlled at 19-27 mm; the diameter of the core glass rod is 17-25 mm.

[0027] After assembly, the rod and tube assembly is placed in a high-temperature wire drawing furnace and heated in the range of 820~860℃ for 20~30 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.

[0028] 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.

[0029] If the temperature is below 820℃, 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 overly steep refractive index interface. This can easily lead to optical discontinuities, increasing mode coupling and scattering losses. If the temperature is above 860℃, 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 20 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.

[0030] Subsequently, a drawing process is used to draw the single filament, completing the process in 70-200 minutes. This time range ensures that the core-skin glass continues to undergo appropriate component diffusion under tension while preventing structural instability. If the drawing time is less than 70 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 200 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.

[0031] 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.

[0032] The composite rod formed after arrangement and filling is then drawn. It is placed in a high-temperature drawing furnace and heated to 810-840℃ for 20-35 minutes to fully soften the composite rod and ensure uniform fusion of the multi-filament interfaces. If the heating temperature is below 810℃ or the holding time is less than 20 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 840℃ or the holding time exceeds 35 minutes, although diffusion is deepened, 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.

[0033] The drawing process then takes 70 to 250 minutes to complete, resulting in a single multifilament. If the drawing time is less than 70 minutes, the diffusion will be insufficient and the stretching process will be unstable. If it exceeds 250 minutes, problems such as uneven diameter and excessive structural relaxation may occur.

[0034] 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.

[0035] After the secondary composite rods are arranged, they are placed in a high-temperature drawing furnace and heated in the range of 830~850℃ for 20~38 minutes to ensure that the internal multifilaments soften uniformly and bond tightly. If the heating temperature is below 830℃ or the holding time is less than 20 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 850℃ 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 100-250 minutes. If the drawing time is less than 100 minutes, the multifilament will not spread sufficiently during the stretching process and the diameter will be difficult to control evenly. If it exceeds 250 minutes, it may cause excessive structural relaxation, uneven diameter, or surface optical defects.

[0036] Through the above steps and process control, the ratio of core diameter to cladding thickness is ultimately achieved between 4.5 and 6.5:1, forming a gradient refractive index distribution structure between the core and cladding. Testing shows that the refractive index gradient of the imaging unit formed according to this process conforms to a parabolic structure. 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.

[0037] Next, the secondary multifilaments are cut into lengths of 80-125mm and bundled with wire to form fused fiber rods, which are then placed in a hot press mold for molding. 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 fusion bonding process. The hot-pressing temperature is controlled between 530-600℃, and the hot-pressing time is controlled between 180-270 minutes to ensure that the board segment is fully densified under pressure and internal micropores are eliminated. If the hot-pressing temperature is below 530℃ or the hot-pressing time is less than 180 minutes, the fibers are not sufficiently softened, the gradient refractive index is not fully formed, and the interface refractive index changes abruptly, 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 600℃ or the hot-pressing time exceeds 270 minutes, although diffusion is more complete, it may lead to an overly smooth refractive index gradient, and the core-skin interface tends to be homogenized, weakening the gradient refractive index design effect. Simultaneously, high temperature and long-term exposure may increase the risk of bubble formation and thermal stress accumulation. During the forming process, the pressure range is set to 50~120N to achieve tight bonding between fibers and surface smoothing. When the applied pressure is below 50N, 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.

[0038] Finally, the annealed molten fiber rod is rolled into a round shape and cut into fiber blanks with a length of 17-30 mm. The blanks are then placed in an annealing furnace for stress relief. The ultimate goal of the annealing process is to ensure sufficient diffusion of the core and skin without excessive diffusion. The present invention preferably sets the annealing temperature program as follows: from room temperature to 540-560°C in 350-400 min, then to 550-600°C in 50-70 min, and held for 300-1000 min. If the holding time is less than 300 min, the core-skin glass diffusion is insufficient, the refractive index gradient is not fully formed, and the interface changes abruptly, leading to increased optical scattering and mode coupling loss. If the holding time is more than 1000 min, although the core-skin diffusion is more sufficient, excessive diffusion may cause the refractive index gradient to become too smooth or even homogenized, weakening the designed focusing effect. Afterward, the temperature is naturally cooled to 60°C.

[0039] The present invention also proposes an application of the graded refractive index fiber imaging array based on the aforementioned method and the graded refractive index fiber imaging array prepared according to the aforementioned method in the field of optical materials.

[0040] One application involves using graded-index fiber optic imaging arrays as fiber optic panels. This is achieved by precision grinding the end face of the annealed graded-index fiber optic imaging array for 1.2–3.2 hours and polishing for 1.5–4.5 hours, ultimately resulting in a fiber optic panel with excellent surface optical properties and high-fidelity transmission. If the grinding time is less than 1.2 hours, insufficient end face flatness, minor surface roughness, or residual burrs can lead to increased optical scattering and decreased imaging contrast. If the grinding time exceeds 3.2 hours, excessive thinning of the end face or microstructural deformation may occur, affecting the core-cladding geometry and causing localized failure of the refractive index distribution. Polishing time less than 1.5 hours will result in insufficient surface roughness to meet optical requirements, reducing optical transmission efficiency. Polishing time exceeding 4.5 hours may cause minor deformation or stress concentration, affecting the long-term stability and imaging consistency of the fiber optic panel. Testing shows that the graded-index fiber optic panel prepared by this method has a resolution ≥181 lp / mm, crosstalk ≤0.01%, and transmittance ≥31%.

[0041] One application involves fabricating graded-index fiber optic image transmission arrays into fiber optic image reversers. Specifically, the annealed graded-index fiber optic image transmission array is placed in a high-temperature torsion furnace for heat treatment. The torsion temperature is set between 730 and 810°C, and the torsion time is controlled between 50 and 120 minutes to ensure the fiber array achieves stable overall rotation at high temperatures, thereby reversing the image transmission direction. If the temperature is below 730°C or the time is less than 50 minutes, the fiber lacks sufficient plasticity at high temperatures, resulting in insufficient array rotation, incomplete image reversal, and optical torsion distortion. If the temperature exceeds 810°C or the time exceeds 120 minutes, the fiber softens excessively, potentially leading to localized array deformation and widened gaps, affecting the optical uniformity and mechanical stability of the image reverser. After the hot twisting is completed, the end face is precision ground for 1.4~3.2h and polished for 1.5~4.5h to finally obtain an optical fiber image inverter with excellent surface optical properties and capable of high-fidelity image inversion transmission. According to the test, the resolution of the graded refractive index optical fiber image inverter prepared by the method of the present invention is ≥181lp / mm, the crosstalk rate is ≤0.02%, and the transmittance is ≥29%.

[0042] One application involves fabricating graded-index fiber imaging arrays into optical cones. Specifically, the annealed graded-index fiber imaging array is placed in a high-temperature stretching furnace and heated to 720-750°C to soften it before controlled stretching. The stretching time is controlled between 50 and 180 minutes to obtain a tapered structure with gradually changing dimensions along its length. If the temperature is below 720°C or the stretching time is less than 50 minutes, the fiber is not sufficiently softened, resulting in an incomplete tapered structure and discontinuous dimensional transitions, leading to poor focusing at the cone's end face. If the temperature exceeds 750°C or the stretching time exceeds 180 minutes, the fiber is over-softened, or the stretching time is too long, potentially causing uneven diameter and stress concentration, thus affecting the cone's optical performance. After stretching, the blank is symmetrically divided into two parts along its length; specifically, the end face is cut into a radial section, thus forming the initial optical cone blank. The fiber optic cone blank is then precision ground for 1.4–3.2 hours and polished for 1.5–4.5 hours to obtain a graded refractive index optical cone, which can be used to achieve image magnification or reduction transmission. Testing shows that the graded refractive index optical cone prepared by the method of this invention has a resolution ≥228 lp / mm, crosstalk ≤0.02%, and transmittance ≥26%.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] Example 1 The coefficient of thermal expansion of the core glass is 82×10⁻⁶. -7 (1 / ℃), softening point is 675℃, and the coefficient of thermal expansion of the glass tube is 78×10. -7 (1 / ℃), softening point is 610℃. Specific process details are as follows: 1) Assemble the outer glass tube and the core glass rod in a coaxial nesting manner, so that the core glass is accurately located in the center of the outer glass tube. The outer diameter of the outer glass tube is controlled at 37 mm and the inner diameter is controlled at 25 mm; the diameter of the core glass rod is 23 mm.

[0047] 2) After assembly, the combined structure is placed in a high-temperature wire drawing furnace and heated and held at 830℃ for 22 minutes. Then, the single filament is drawn at a wire drawing speed of 2.3 r / min for 120 minutes, and the diameter of the obtained single filament is controlled at 4 mm. The ratio of the core diameter to the cladding glass thickness is measured to be 6.2:1.

[0048] 3) The prepared monofilaments are arranged in a close-packed hexagonal pattern to form a regular two-dimensional array structure in the cross-section of the core unit. The gaps between the monofilaments are filled with gap wires of the same material as the cladding; after the monofilaments and gap wires are arranged and filled, a primary composite rod is formed.

[0049] 4) Place the primary composite rod in a high-temperature drawing furnace, heat it at 830℃ and hold it for 25 minutes. Then, complete the drawing process at a drawing speed of 2.1 r / min for 130 minutes to obtain the primary composite wire.

[0050] 5) The obtained primary composite wires are arranged again in a close-packed hexagonal pattern to form a secondary composite rod with a more regular structure.

[0051] 6) After the arrangement is completed, the secondary composite rod is placed in a high-temperature drawing furnace and heated in the range of 835℃ for 25 minutes to ensure that the internal multifilaments are softened uniformly and tightly bonded. Then, the secondary multifilaments are drawn by drawing at a drawing speed of 1.9r / min for 160 minutes.

[0052] 7) Cut the secondary multifilament into lengths of 80-125mm and bind them into molten fiber rods using wire. Place these rods in a hot press mold for forming. Control the hot pressing temperature at 550℃ and the hot pressing time at 200 minutes. Set the pressure range to 70 N during the forming process.

[0053] 8) After the annealing is completed, the molten fiber rod is rolled into a round shape and cut into fiber blanks with a length of 25mm. The blanks are placed in the annealing furnace to relieve stress. The annealing temperature program is set as follows: the temperature is increased from room temperature to 540℃ in 380min, then increased to 600℃ in 55min, held for 400min, and then cooled naturally to 60℃.

[0054] 9) Take a portion of the annealed fiber blank and perform precision grinding on the end face of the blank for 1.6 h and polishing for 2.6 h to obtain a fiber optic panel with excellent surface optical properties and capable of high-fidelity transmission.

[0055] 10) Take a portion of the annealed fiber blank and place it in a high-temperature torsion furnace for heat treatment. The torsion temperature is set at 736℃; the torsion speed is 9° / min; and the torsion time is controlled at 100 min to ensure that the fiber array rotates stably as a whole at high temperature. After the heat torsion is completed, the end face is precision ground for 1.6 h and polished for 2.6 h to obtain a fiber optic image inverter with excellent surface optical properties and capable of high-fidelity image inversion transmission.

[0056] 11) Take a portion of the annealed fiber optic blank and place it in a high-temperature stretching furnace. Heat it at 735℃ to soften it, then perform controlled stretching. The stretching speed is 1.17 mm / min; the stretching time is controlled at 120 min to obtain a tapered structure with gradually changing dimensions along the length direction. After stretching, the blank is symmetrically divided into two parts along the central radial section to form the initial fiber optic cone blank. Subsequently, the end face of the fiber optic cone blank is precision ground for 1.6 h and polished for 2.6 h, which can be used to realize the magnification or reduction of image transmission.

[0057] 12) Performance testing: 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 USAF 1951 resolution board. The test results are shown in Table 1.

[0058] Example 2 Same as Example 1, except that the heat preservation time in step 2) is 24 min, and the test results are shown in Table 1.

[0059] Example 3 Same as Example 1, except that the heat preservation time in step 2) is 27 min, and the test results are shown in Table 1.

[0060] Example 4 Same as Example 1, except that the wire drawing time in step 2) is 150 min. The test results are shown in Table 1.

[0061] Example 5 Same as Example 1, except that the wire drawing time in step 2) is 180 min. The test results are shown in Table 1.

[0062] Example 6 Same as Example 1, except that the hot pressing time in step 7) is 210 min, and the test results are shown in Table 1.

[0063] Example 7 Same as Example 1, except that the hot pressing time in step 7) is 220 min, and the test results are shown in Table 1.

[0064] Example 8 Same as Example 1, except that the heat preservation time in step 8) is 500 min, and the test results are shown in Table 1.

[0065] Example 9 Same as Example 1, except that the heat preservation time in step 8) is 600 min, and the test results are shown in Table 1.

[0066] Example 10 Same as Example 1, except that in step 1), the inner diameter of the cladding glass tube is controlled at 23 mm; the diameter of the core glass rod is 21 mm; and in step 2), the ratio of the core diameter to the cladding glass thickness is 6.0:1. The test results are shown in Table 1.

[0067] Example 11 Same as Example 1, except that in step 1), the inner diameter of the outer glass tube is controlled at 21 mm; the diameter of the core glass rod is 19 mm; and in step 2), the ratio of the fiber core diameter to the cladding glass thickness is 5.7:1. The test results are shown in Table 1.

[0068] Example 12 Same as Example 1, except that in step 1), the inner diameter of the cladding glass tube is controlled at 19 mm; the diameter of the core glass rod is 17 mm; and in step 2), the ratio of the core diameter to the cladding glass thickness is 4.5:1. The test results are shown in Table 1.

[0069] Example 13 Same as Example 1, except that in step 1), the inner diameter of the outer glass tube is controlled at 27 mm; the diameter of the core glass rod is 25 mm; and in step 2), the ratio of the fiber core diameter to the cladding glass thickness is 6.5:1. The test results are shown in Table 1.

[0070] Example 14 Same as Example 1, except that the coefficient of thermal expansion of the core glass is 81 × 10⁻⁶. -7 (1 / ℃), softening point is 670℃, and the coefficient of thermal expansion of the glass tube is 78×10. -7 (1 / ℃), softening point is 610℃, and the test results are shown in Table 1.

[0071] Example 15 Same as Example 1, except that the coefficient of thermal expansion of the core glass is 84 × 10⁻⁶. -7 (1 / ℃), softening point is 705℃, and the coefficient of thermal expansion of the glass tube is 78×10. -7 (1 / ℃), softening point is 610℃, and the test results are shown in Table 1.

[0072] Example 16 Same as Example 1, except that the wire drawing temperature in step 2) is 820℃, and the test results are shown in Table 1.

[0073] Example 17 Same as Example 1, except that the wire drawing temperature in step 2) is 860℃, and the test results are shown in Table 1.

[0074] Example 18 Same as Example 1, except that the heat preservation time in step 2) is 20 min, and the test results are shown in Table 1.

[0075] Example 19 Same as Example 1, except that the heat preservation time in step 2) is 30 minutes. The test results are shown in Table 1.

[0076] Example 20 Same as Example 1, except that the wire drawing time in step 2) is 70 min. The test results are shown in Table 1.

[0077] Example 21 Same as Example 1, except that the wire drawing time in step 2) is 200 min. The test results are shown in Table 1.

[0078] Example 22 Same as Example 1, except that the hot pressing temperature in step 7) is 530℃, and the test results are shown in Table 1.

[0079] Example 23 Same as Example 1, except that the hot pressing temperature in step 7) is 600℃, and the test results are shown in Table 1.

[0080] Example 24 Same as Example 1, except that the hot pressing time in step 7) is 180 min, and the test results are shown in Table 1.

[0081] Example 25 Same as Example 1, except that the hot pressing time in step 7) is 270 min, and the test results are shown in Table 1.

[0082] Example 26 Same as Example 1, except that the hot pressing pressure in step 7) is 50N, and the test results are shown in Table 1.

[0083] Example 27 Same as Example 1, except that the hot pressing pressure in step 7) is 120N, and the test results are shown in Table 1.

[0084] Example 28 Same as Example 1, except that the heat preservation time in step 8) is 300 min, and the test results are shown in Table 1.

[0085] Example 29 Same as Example 1, except that the heat preservation time in step 8) is 1000 min, and the test results are shown in Table 1.

[0086] Comparative Example 1 Same as Example 1, except that the heat preservation time in step 2) is 35 min and the wire drawing time is 150 min. The test results are shown in Table 1.

[0087] Comparative Example 2 Same as Example 1, except that the wire drawing time in step 2) is 260 min. The test results are shown in Table 1.

[0088] Comparative Example 3 Same as Example 1, except that the hot pressing time in step 7) is 280 min, and the test results are shown in Table 1.

[0089] Comparative Example 4 Same as Example 1, except that the heat preservation time in step 8) is 1100 min, and the test results are shown in Table 1.

[0090] Comparative Example 5 Same as Example 1, except that in step 1), the inner diameter of the cladding glass tube is controlled at 30 mm; the diameter of the core glass rod is 29 mm; and in step 2), the ratio of the core diameter to the cladding glass thickness is 7.6:1. The test results are shown in Table 1.

[0091] Table 1. Test results of the examples and comparative examples. As can be seen from the test data of the examples and comparative examples, in Examples 1-3 and Examples 18 and 19, during the monofilament drawing process, by adjusting the holding time from 22 to 30 minutes, the component diffusion at the core-cladding interface is more complete, forming a smoother refractive index gradient structure. This reduces optical scattering and mode coupling loss, and the resolution is gradually improved. The present invention designs the core diameter to cladding glass thickness ratio to be 4.5 to 6.5:1. The cladding glass accounts for a large proportion of the overall structure, effectively suppressing crosstalk between adjacent cores and reducing the crosstalk rate. However, at the same time, the proportion of the core glass in the cross-section is relatively reduced, resulting in a decrease in the light transmission capacity per unit cross-section and thus a reduction in overall transmittance. In Comparative Example 1, the holding time was set to 35 minutes. Because the holding time significantly exceeded the optimal process range, the component diffusion at the core-cladding interface was excessive. The originally designed gradient refractive index distribution gradually became homogenized, the refractive index difference between the core and cladding decreased, the light confinement ability decreased, the imaging resolution was severely reduced, and the image boundaries became blurred. At the same time, the effective transmission area of ​​the fiber core glass is further weakened, resulting in a significant decrease in the overall transmittance of the optical fiber.

[0092] In Examples 1, 4, 5, 20, and 21, adjusting the single-filament drawing time from 120 to 200 minutes resulted in improved resolution while maintaining a relatively constant crosstalk rate. However, the overall transmittance of the fiber optic imaging array decreased. In Examples 16, 1, and 17, the increased single-filament drawing temperature exacerbated component diffusion between the fiber core and cladding glass. In Comparative Example 2, the single-filament drawing time was extended to 260 minutes. Due to the prolonged drawing process, the glass remained in a softened and viscous state at high temperatures for an extended period, intensifying component diffusion at the core-cladding interface. This caused the originally designed refractive index gradient to gradually flatten. Furthermore, prolonged heating caused localized microstructural inhomogeneities and bubble formation, weakening light transmission. While the crosstalk rate between the fiber cores remained relatively constant, the effective focusing ability of the optical mode decreased, significantly reducing the resolution of the fiber optic imaging array. Simultaneously, the increased interface diffusion and structural defects also weakened light transmittance.

[0093] In Examples 1, 6, 7, 24, and 25, adjusting the melting and pressing time of the molten fiber rod from 200 to 270 minutes resulted in increased resolution with essentially unchanged crosstalk rate. However, the overall transmittance of the fiber optic imaging array decreased. Furthermore, in Examples 22, 1, and 23, increasing the melting and pressing temperature, and in Examples 26 and 27, increasing the hot-pressing pressure, all enhanced the diffusion capacity between the fiber core and cladding glass. In Comparative Example 3, the hot-pressing time of the molten fiber rod was extended to 280 minutes. Due to the excessively long hot-pressing time, excessive component diffusion occurred between the fiber core and cladding glass under high temperature and pressure, causing the originally designed refractive index gradient to gradually homogenize. Prolonged hot-pressing led to localized structural stress relaxation and the expansion of interface micro-defects in the glass under pressure, even introducing bubbles or micro-cracks, thereby weakening the effective light transmittance. While the crosstalk rate of the fiber optic imaging array did not change significantly, the excessive smoothing of the refractive index and the generation of interface defects weakened the optical mode focusing, resulting in a significant decrease in the resolution and transmittance of the fiber optic imaging array.

[0094] In Examples 1, 8, 9, 28, and 29, by adjusting the annealing holding time from 400 to 1000 min, the resolution was improved while the crosstalk rate remained essentially unchanged. Simultaneously, the overall transmittance of the fiber optic imaging array decreased. In Comparative Example 4, extending the annealing holding time to 1100 min further released residual stress within the glass. However, the excessively long high-temperature annealing caused excessive diffusion of ions between the fiber core and cladding, even approaching a uniform refractive index structure. While the crosstalk rate remained essentially unchanged, the optical mode resolution decreased and the imaging quality weakened due to the more uniform refractive index distribution and the introduction of material defects. Simultaneously, the light transmission transmittance significantly decreased.

[0095] In Examples 1 and 10-13, by appropriately reducing the inner diameter of the cladding glass tube and correspondingly decreasing the diameter of the core glass rod, the ratio of core diameter to cladding thickness gradually decreases, effectively increasing the thickness of the cladding glass layer. The thicker cladding not only enhances component diffusion at the core-skin interface during high-temperature fiber drawing, resulting in a smoother gradient refractive index distribution, but also effectively limits light leakage to adjacent fibers, significantly reducing crosstalk. Simultaneously, although the reduced effective light-transmitting area of ​​the single filament due to the decreased core diameter ratio leads to a slight decrease in overall transmittance, the optical wave mode is well constrained, and the resolution remains at a high level, ensuring the accuracy of optical imaging. In contrast, in Comparative Example 5, the ratio of core glass to cladding thickness is further reduced to 1:7.6, resulting in an excessively small cladding thickness ratio. Under the same process conditions, insufficient component diffusion between the core and skin results in a refractive index distribution that tends towards a step-like structure, failing to form an ideal gradient refractive index transition. In Examples 1, 14, and 15, increasing the softening point temperature of the fiber core glass slows down ion diffusion between the core and cladding components, resulting in a slight decrease in resolution, a reduction in crosstalk rate, and an increase in transmittance. Although this structure can still reduce crosstalk rate to some extent at the geometric level, its mode confinement capability is weakened, leading to a significant decrease in the resolution of the fiber optic imaging array. However, the relatively increased core proportion and larger light-transmitting area result in an overall increase in transmittance.

[0096] 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.

[0097] 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 graded index fiber image relay array, comprising: It comprises several arrayed image transmission units; each image transmission unit comprises a core and a cladding layer covering the outside of the core; each image transmission unit has a parabolic graded refractive index distribution structure opening downward along its diameter direction; the ratio of the core diameter to the cladding layer thickness is 4.5-6.5:

1.

2. The graded index fiber image guide array of claim 1 wherein, The thermal expansion coefficient of the cladding glass is 3 x 10⁻ 7 ~ 7 x 10⁻ 7 / °C lower than that of the core glass; and the softening point of the cladding glass is 60 ~ 95°C lower than that of the core glass.

3. A method of making a graded index fiber image relay array, comprising: It comprises the following steps: S1 assembles the core glass rod and the skin glass tube 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 3*10 -7 7*10 -7 / ℃; the skin glass tube has a softening point lower than the softening point of the core glass by 60~95℃; the skin glass tube has an outer diameter of 36~39mm and an inner diameter of 19~27mm; the core glass rod has a diameter of 17~25mm; S2: the rod tube assembly is kept at 820-860℃ for 20-30min, and is drawn into a single filament for 70-200min; the diameter of the single filament is 3.5-5mm, and the ratio of the core diameter to the cladding layer thickness is 4.5-6.5:1; S3: the single filaments are arranged in a regular hexagonal closest packing mode, and a gap filament is inserted into the gap between the single filaments to form a primary multifilament rod; the primary multifilament rod is drawn into a primary multifilament; the primary multifilament is arranged into a secondary multifilament rod; and the secondary multifilament rod is drawn into a secondary multifilament; S4: the secondary multifilament is cut and baled, and is hot-pressed at 530-600℃ and 50-120N for 180-270min; S5: annealing is performed to make the core and the sheath fully diffuse, so as to obtain a graded refractive index optical fiber image transmission array.

4. The production method according to claim 3, characterized by, The primary multifilament rod is drawn into a primary multifilament by keeping the primary multifilament rod at 810-840℃ for 20-35min, and drawing for 70-250min.

5. The preparation method according to claim 3, characterized in that, The secondary multifilament rod is drawn into a secondary multifilament by keeping the secondary multifilament rod at 830-850℃ for 20-38min, and drawing for 100-250min.

6. The preparation method according to claim 3, characterized in that, The annealing program is as follows: from room temperature, it is raised to 540-560℃ for 350-400min; it is raised to 550-600℃ for 50-70min, and is kept for 300-1000min.

7. Application of the graded refractive index optical fiber image transmission array according to claim 1 or 2, and the graded refractive index optical fiber image transmission array prepared by the preparation method according to any one of claims 3-6 in the field of optical materials.

8. The application according to claim 7, characterized in that, The graded refractive index optical fiber image transmission array is precisely ground and polished to obtain a fiber panel; The resolution thereof is ≥181lp / mm, the crosstalk rate is ≤0.01%, and the transmittance is ≥31%.

9. Use according to claim 7, characterized in that, The graded refractive index optical fiber image transmission array is twisted at 730-810℃ for 50-120min; and the end face is precisely ground and polished to obtain a fiber inverter; the resolution thereof is ≥181lp / mm, the crosstalk rate is ≤0.02%, and the transmittance is ≥29%.

10. Use according to claim 7, characterized in that, The graded refractive index optical fiber image transmission array is stretched at 720-750℃ for 50-180min; it is cut into two parts along the radial cross section; and the end face is precisely ground and polished to obtain a fiber taper; the resolution thereof is ≥228lp / mm, the crosstalk rate is ≤0.02%, and the transmittance is ≥26%.

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