A method of making a square core optical fiber preform, product and apparatus

Abstract

The application discloses a preparation method, product and device of a square core optical fiber preform. The preparation method comprises the following steps: (1) making the rotating central axis of a Leilo triangular drill bit coaxial with the geometric central axis of a circular cavity cladding sleeve, and drilling a rounded square with the width d as the side length by using the Leilo triangular drill bit to obtain a square cavity cladding sleeve; (2) sleeving a square core rod with the width d' into the square cavity cladding sleeve, and stretching or fusing the square core rod into a circular solid rod; and (3) manufacturing the square core optical fiber preform according to the designed shape size and refractive index profile. The Leilo triangular drill bit is designed according to the width fixing principle of the Leilo triangular structure, and can be used for square hole processing of brittle materials such as quartz glass. The optical fiber prepared by using the square core rod and the square hole sleeve combination drawing reduces the probability of uncontrollable core-cladding interface air line occurrence caused by the original square core rod and circular sleeve combination drawing, and guarantees the reliability of the optical fiber.

Description

Technical Field

[0001] This invention belongs to the field of optical fiber technology, and more specifically, relates to a method, product, and apparatus for preparing a square core optical fiber preform. Background Technology

[0002] Square fiber cores have unique applications in laser technology, such as laser beam shaping. Laser beam shaping is the process of converting a laser beam into a specific pattern, shape, or intensity distribution to optimize its performance for specific applications. Gaussian beams can be directly converted into flat-topped, square, ring-shaped, or other custom shapes, altering the pulse energy distribution and achieving higher laser processing efficiency and quality. In laser applications, compared to circular cores, square-core fibers produce near-square output beams. This shape homogenizes the output beam energy, making it particularly suitable for high-efficiency welding heat treatments such as laser cleaning, laser cladding, 3D printing, and photovoltaics, showing great application potential. Furthermore, this square-core structure is highly advantageous for coupling light emitted from square diode lasers into square-core fibers.

[0003] However, the compatibility of square optical fibers with large-scale industrial production and application is very limited. The main reason is that quartz optical fibers are brittle materials, and it is very difficult to manufacture such non-circular optical fibers that do not have polar centrifugal symmetry. The concentricity and non-circularity of the core and cladding are more prone to deviation than those of circular optical fibers, resulting in large fiber splicing loss and severe heat generation. Especially in the field of high-power laser technology, this deviation makes the device packaging stress change ability weak, causing more severe thermal effects and burning out the optical fiber and device. Summary of the Invention

[0004] To address the aforementioned shortcomings of existing square-core fiber fabrication technologies, this invention provides a stable and reliable fabrication process. Its core key lies in utilizing the fixed-width principle of the Reilly triangle to design a Reilly triangle drill bit capable of drilling square holes in brittle quartz glass. By changing the side length of the Reilly triangle drill bit, square holes of different sizes can be fabricated. Furthermore, by matching and assembling different square core rods, arbitrary combinations of square fibers with core-cladding ratios can be freely achieved. This method is particularly suitable for fabricating fibers with fluorine-doped cladding also exhibiting a square structure. This invention solves the problems of existing square-core fiber fabrication processes being complex, time-consuming, and having high core-cladding concentricity, making them unsuitable for high-power laser output.

[0005] To achieve the above objectives, according to one aspect of the present invention, a method for preparing a square-core optical fiber preform is provided, comprising the following steps:

[0006] (1) Make the rotation axis of the Reilly triangular drill bit coaxial with the geometric axis of the circular cavity cladding casing; use the Reilly triangular drill bit to drill a rounded square with a side length of width d to obtain a square cavity cladding casing;

[0007] (2) Insert the square core rod with a width of d' into the square cavity cladding sleeve obtained in step (1), and stretch or shrink it into a round solid rod;

[0008] (3) Using the solid rod obtained in step (2), a square core fiber preform is made according to the designed shape, size and refractive index profile.

[0009] Preferably, in the method for preparing the square core optical fiber preform, the difference between the side length d' of the square core rod and the width d of the Reilly triangular drill bit, |d-d'|, is between 1.0 and 4.0 mm; the square core rod has a rounded square cross-section.

[0010] Preferably, in the method for preparing the square-core optical fiber preform, the ratio of the inner diameter of the cladding sleeve of the circular cavity to the width of the Reilly 3D drill bit is [value missing]. The Leroy triangular drill bit drills with a width d less than or equal to one-third of the outer diameter D of the cladding casing.

[0011] Preferably, in the preparation method of the square core optical fiber preform, step (1) involves introducing grinding oil through a drill bit into the cladding sleeve.

[0012] Preferably, in the method for preparing the square core optical fiber preform, when preparing an optical fiber preform with a circular outer cladding and one or more square cladding layers between the outer cladding and the square core, the outer circle of the solid rod prepared in step (3) can be ground into a square shape or a square cavity sleeve and a square core rod can be stretched or fused into a square solid rod as a new square core rod. Steps (1) to (3) are repeated to prepare multiple square cladding layers from the inside out until a circular outer cladding is formed.

[0013] When preparing a preform with a circular outer cladding and one or more circular cladding optical fibers between the outer cladding and the square core, the cladding sleeve can be designed as multiple layers according to the corresponding refractive index profile, as the circular cavity cladding sleeve in step (1).

[0014] According to another aspect of the present invention, a square core optical fiber preform is provided, which is prepared according to the preparation method of the square core optical fiber preform provided by the present invention.

[0015] According to another aspect of the present invention, a square-core optical fiber is provided, characterized in that it is drawn from an optical fiber preform provided by the present invention.

[0016] Preferably, the square core optical fiber has a side-to-side length of 50~1000 μm, the ratio of the adjacent cladding diameter to the side-to-side length of the square core is between 1.5 and 2.5, and the concentricity is less than 1.0% of the cladding diameter.

[0017] Preferably, the outer cladding of the square core optical fiber is circular with a non-circularity of less than 0.5%.

[0018] According to another aspect of the present invention, a square core optical fiber preform processing apparatus is provided, which includes a Reilly triangular drill bit and a cladding sleeve clamp;

[0019] The rotation axis of the drill bit is coaxial with the central axis of the cladding casing clamp, so that after the cladding casing is clamped, the rotation axis of the Reilly triangular drill bit is coaxial with the geometric central axis of the hollow cladding casing.

[0020] Preferably, in the square core optical fiber preform processing device, the Leylow triangular drill bit has a hollow cavity with a grinding oil supply port inside.

[0021] In summary, compared with the prior art, the above-described technical solutions conceived by this invention can achieve the following beneficial effects:

[0022] This invention utilizes the Reilly triangle's fixed width principle to design a Reilly triangle structure drill bit, which can process square holes in brittle materials including quartz glass. The optical fiber prepared by combining a square core rod with a square hole sleeve for fiber drawing reduces the probability of uncontrollable gas lines at the core-cladding interface caused by the original combination of square core rod and circular sleeve drawing, thus ensuring the reliability of the optical fiber.

[0023] The preferred solution uses a Leroy triangular structure drill bit, and is matched with a grinding oil supply device that can simultaneously supply grinding oil to achieve heat conduction and heat dissipation and glass slag removal. This can significantly increase the depth and edge-to-edge width of square holes in brittle quartz glass materials, ensuring sufficient length of square holes, thereby indirectly increasing the investment in fiber preform drawing, reducing fiber manufacturing costs, and improving product consistency.

[0024] The square-core optical fiber provided by this invention is used as a power transmission fiber for beam shaping. It adopts a square core design and is matched with a circular or square fluorine-doped cladding. This changes the characteristics of the original circular core energy transmission Gaussian distribution, realizing beam energy uniformity and square beam output. It has obvious advantages for processing applications that require flat-top light energy distribution, especially for semiconductor lasers in terms of mode matching and transmission coupling. The square core structure is closer to flat-top energy output.

[0025] The method for fabricating square-core optical fibers provided by this invention eliminates the problem of non-circular outer diameter caused by the non-uniform gap combination of existing square core rods and fluorinated cladding tubes with circular inner holes. The proposed Reilly triangle drill bit design enables the processing of square holes in brittle quartz glass. Because the square core rod and the fluorinated square-hole cladding tube are combined with the same gap, the resulting square-core optical fiber exhibits superior geometric performance, simplifies the fabrication process, and shortens the rod-making production flow. Attached Figure Description

[0026] Figure 1 This is a schematic cross-sectional view of the square core optical fiber preform processing device provided in Embodiment 1 of the present invention.

[0027] Figure 2 This is a schematic diagram of the assembly cross-section of the square core optical fiber preform processing device provided in Embodiment 1 of the present invention;

[0028] Figure 3 This is a side view of the Reilly triangular drill bit provided in Embodiment 1 of the present invention;

[0029] Figure 4 This is a Reilly triangular drill bit end face view provided in Embodiment 1 of the present invention;

[0030] Figure 5 These are schematic diagrams of the square core optical fiber end face structure provided in embodiments 2 to 7 of the present invention;

[0031] Figure 6 These are the test results of the square core optical fiber spot provided in Embodiments 2 to 7 of the present invention;

[0032] Figure 7 This is a schematic diagram of the square core optical fiber end face structure provided in Embodiment 8 of the present invention;

[0033] Figure 8 This is a schematic diagram of the square core optical fiber end face structure provided in Embodiment 9 of the present invention.

[0034] In all the accompanying drawings, the same reference numerals are used to denote the same elements or structures, wherein: 1 is a Leylow triangular drill bit, 11 is a cutting edge, 12 is a grinding oil supply port, 2 is a cladding sleeve, 3 is a square fiber core, 4 is the cladding adjacent to the square fiber core, and 5 is the outer cladding. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0036] Unless otherwise specified, the core-cladding ratio in this invention refers to the ratio of the fluorine-doped cladding with optical cladding function to the side-to-side length of the square fiber core. If the optical cladding is circular, it is the ratio of the cladding diameter to the side-to-side length of the square core; if the optical cladding is square, the core-cladding ratio is the ratio of the side-to-side length of the square cladding to the side-to-side length of the square fiber core.

[0037] Experimental findings indicate that the high splicing loss and severe heat generation in square-core optical fibers are caused by poor core-cladding concentricity and batch-to-batch consistency. Repeated experimental research has revealed that these problems are closely related to existing square-core optical fiber manufacturing processes.

[0038] The fabrication of square-core optical fibers typically involves creating a circular core rod using deposition processes such as PCVD, MCVD, or VAD. This circular core rod is then ground into a square shape and assembled with a fluorine-doped tube containing a circular hollow core, which provides the optical cladding. The assembly is then fused or stretched into a solid core rod. Due to the structural difference between the square core and the circular tube (outer circle, inner square), their gaps are not the same. Therefore, the solid core rod, after fusion or stretching, suffers from an inherent defect in its rod-forming process: a non-circular diameter. To eliminate this non-circular diameter caused by geometric differences, the rod surface is typically smoothed and rounded by external grinding. However, this cold-working process easily deteriorates the concentricity of the core and cladding, especially for square cores, significantly increasing the difficulty of center positioning and resulting in poor batch-to-batch concentricity consistency.

[0039] As the core diameter of optical fiber is the core channel that bears high power, variations in its core-cladding concentricity and cladding non-circularity directly affect the compatibility and matching of optical devices. The concentricity of square-core optical fibers is typically above 2.0 μm, significantly larger than the less than 1.0 μm concentricity of traditional round-core optical fibers. This can easily lead to high splice loss, severe heat generation, and weaker resistance to changes in device encapsulation stress, resulting in more severe thermal effects and potentially burning out the fiber and optical devices.

[0040] This invention addresses the problems of non-circularity and large concentricity deviations in existing square-core optical fiber cladding fabrication methods. It modifies the inner hole of the fluorine-doped cladding tube to a square structure, combining it with the square core rod in the same shape, and then stretching or melting it into a solid rod. This process ensures that the square core rod and the square-hole fluorine-doped tube can collapse and fuse uniformly and synchronously around their perimeter, avoiding the non-circularity of the optical rod caused by asymmetrical melting of the square and circular parts in the cross-section. This also eliminates the problem of large concentricity deviations in the core-cladding caused by the need for grinding to remove the non-circular surface.

[0041] The method for preparing a square-core optical fiber preform provided by the present invention includes the following steps:

[0042] (1) Make the rotation axis of the Reilly triangular drill bit coaxial with the geometric axis of the cladding sleeve with a circular cavity; use the Reilly triangular drill bit to drill a rounded square with a side length of width d to obtain a square cavity cladding sleeve; the sleeve with a circular cavity is a commonly used material in the industrial production of optical fibers, with rich size specifications, flexible refractive index design, mature technology and low cost. Using the sleeve with a circular cavity to make a square cavity cladding sleeve can reduce costs by utilizing the mature circular cavity sleeve; in the preferred embodiment, the ratio of the inner diameter of the cladding sleeve with the width of the Reilly triangular drill bit is 1. By cleverly utilizing the processing principle of the Reilly duct, the machining process is minimized while allowing the Reilly duct drill bit to be inserted into the casing to begin machining. This aligns the rotation axis of the Reilly duct drill bit with the geometric axis of the casing, thereby simultaneously improving machining accuracy and reducing processing costs. In the fabrication of active optical fibers, the cladding is often a low-refractive-index cladding, with fluorine-doped cladding casings being the most common.

[0043] The preferred solution is to drill with a Reilly triangular drill bit such that the width d is less than or equal to one-third of the outer diameter D of the fluorine-doped casing. Otherwise, if the side-to-side length d of the square hole is too large, it will cause the drilled casing to crack and increase the risk of cracking.

[0044] The preferred method involves introducing grinding oil through the drill bit and channeling it into the cladding casing. The grinding oil can dissipate heat, flush away glass shards, and prevent overall fracture caused by improper grinding and drilling of brittle materials such as quartz glass.

[0045] (2) Insert the square mandrel with width d' into the square cavity cladding sleeve obtained in step (1), and stretch or shrink it into a round solid rod; the difference between the side length d' of the square mandrel and the width d of the Reilly triangular drill bit, |d-d'|, is between 1.0 and 4.0 mm to facilitate the insertion of the square mandrel. In the preferred embodiment, the square mandrel has a rounded rectangular cross-section, which facilitates insertion and allows |d-d'| to be applied to a smaller value, reducing the deformation of the round solid rod and reducing the roundness.

[0046] The non-circularity of the fiber cladding is defined as follows:

[0047]

[0048] D max To measure the maximum diameter of a specific cross-section of an optical fiber in a single measurement, D min To measure the minimum diameter of the fiber cross-section in a single measurement, D avg The fitted circle diameter value is used for the same cross-section in a single measurement.

[0049] Core-cladding concentricity is defined as the center distance between a square core and a circular or square fluorinated cladding.

[0050] (3) Using the solid rod obtained in step (2), a square core fiber preform is made according to the designed shape, size and refractive index profile.

[0051] Using the above method, single-clad square-core fiber preforms can be prepared, typically such as single-fluorine-doped clad square-core fibers, whose circular cladding can be cold-processed by external cylindrical grinding as needed; multi-clad square-core fiber preforms can also be prepared, specifically:

[0052] When preparing a preform of an optical fiber with a circular outer cladding and one or more square cladding layers between the outer cladding and the square core, the outer circle of the solid rod prepared in step (3) can be ground into a square shape or a square cavity sleeve and a square core rod can be stretched or fused into a square solid rod as a new square core rod. According to the method for preparing a square core optical fiber preform provided by this invention, multiple square cladding layers are made from the inside out until a circular outer cladding is formed.

[0053] When preparing a preform of an optical fiber with a circular outer cladding and one or more circular cladding layers between the outer cladding and the square core, the cladding sleeve can be designed to be multi-layered according to the corresponding refractive index profile, and then the square core optical fiber preform prepared by the present invention can be prepared using a Reilly caliper.

[0054] The fabrication of the square optical fiber utilizes the Leylow triangle's fixed width principle to design a Leylow triangle structure drill bit. The middle portion of the drill bit is modified into a wedge-shaped hollow design to increase the supply of grinding oil. Figure 2 As shown, this reduces the contact area between the solid drill bit and the glass material, lowers frictional resistance, and allows the grinding oil to conduct away heat and flush away glass shards, thus preventing overall breakage caused by improper grinding and drilling of brittle materials such as quartz glass.

[0055] The square-core optical fiber prepared according to the method for preparing the square-core optical fiber preform provided by this invention exhibits significantly improved concentricity, which is less than 1.0% of the cladding diameter; the non-circularity is also improved, being less than 0.5%, fully meeting the current processing requirements of lasers for beam shaping and high-power matching applications. By drawing the preform, a square-core optical fiber with a corresponding refractive index profile can be obtained. The square core of the optical fiber formed by the square core rod has a side-to-side length of 50~1000 μm; the cladding tube forms a cladding adjacent to the square core, and its ratio to the side-to-side length of the square core is between 1.5 and 2.5. For the square cladding, its diameter refers to the side-to-side length of the fluorine-doped cladding.

[0056] The square core region can be pure quartz or doped quartz, such as germanium-doped, fluorine-doped, phosphorus-doped, or aluminum-doped, and its numerical aperture (NA) relative to the surrounding fluorine-doped cladding can be 0.10~0.24. The fluorine-doped quartz cladding surrounding the square core can be circular or square. If the fluorine-doped quartz cladding is square, a circular pure quartz cladding can be matched on the outside to ensure better fiber coating and wetting performance.

[0057] Different core edge lengths and different core doping NAs reflect the distribution of different transmission modes within the core region. A design with too low an NA will result in large fluctuations in the beam spot due to external disturbances, leading to unstable output. Conversely, a design with too high an NA will result in Gaussian energy distribution, making it difficult to achieve the ideal square flat-top beam output. If the square core is too small, there will be fewer modes in waveguide transmission, making it difficult to demonstrate the beam homogenization effect of the square core. If the square core is too large, it will result in poor splicing compatibility with other fiber types, which is detrimental to high-power coupled transmission.

[0058] The core-to-wall ratio is a key indicator for confined energy transmission and is crucial for reducing power exposure. The core-to-wall ratio is designed to be between 1.5 and 2.5. If the core-to-wall ratio is too small, insufficient to exceed the diagonal length ratio of the square core corners, the optical cladding transmission function cannot be guaranteed, leading to a risk of light leakage in curved applications. If the core-to-wall ratio is too large, it poses significant challenges to the fabrication of fluorine-doped clad tubes.

[0059] The square core optical fiber preform processing device provided by the present invention includes a Leroy triangular drill bit and a cladding sleeve clamp;

[0060] The rotation axis of the drill bit is coaxial with the central axis of the cladding casing clamp, so that after the cladding casing is clamped, the rotation axis of the Reilly triangular drill bit is coaxial with the geometric central axis of the hollow cladding casing.

[0061] In a preferred embodiment, the Leroy triangular drill bit has a wedge-shaped hollow cavity with a grinding oil supply port inside.

[0062] In summary, the beam shaping process for square-core optical fiber provided by this invention uses a combination of square core rod and square-hole sleeve to achieve same-gap fusion, avoiding the probability of uncontrollable core-cladding interface gas lines caused by non-same-gap collapse in the original combination of square core rod and inner-hole circular sleeve, thus ensuring the reliability of the optical fiber.

[0063] The following is an example:

[0064] Example 1

[0065] The cross-sectional view of the square core optical fiber preform processing device provided in this embodiment is as follows. Figure 1 As shown, the cross-sectional view is as follows Figure 2 As shown, it includes a Reilly triangular drill bit and a cladding casing clamp (not shown in the figure); the rotation axis of the drill bit is coaxial with the central axis of the cladding casing clamp, such that after the cladding casing is clamped, the rotation axis of the Reilly triangular drill bit is coaxial with the geometric axis of the hollow cladding casing. The structure is as follows... Figure 3 , Figure 4 As shown, the side has a cutting edge, and the Leroy triangular drill bit has a wedge-shaped hollow cavity with a grinding oil supply port inside. This reduces the contact area between the original solid drill bit and the glass material, thus lowering frictional resistance.

[0066] Examples 2 to 7:

[0067] Using the square-core optical fiber preform provided in Example 1, the end-face structure of the fiber core is designed as follows: Figure 5 As shown, it has a square fiber core and an adjacent circular cladding.

[0068] The preform preparation method includes the following steps:

[0069] (1) Make the rotation axis of the Reilly triangular drill bit coaxial with the geometric axis of the fluorine-doped cladding casing with a circular cavity; use the Reilly triangular drill bit to drill a rounded square with a side length of width d to obtain a square cavity cladding casing; the ratio of the inner diameter of the circular cavity cladding casing to the width of the Reilly triangular drill bit is 1 / 2. Grinding oil is introduced through the drill bit and directed into the cladding casing.

[0070] (2) Insert the square mandrel with width d' into the square cavity cladding sleeve obtained in step (1), and stretch or shrink it into a round solid rod; the difference between the side length d' of the square mandrel and the width d of the Leroy triangle drill bit, |d-d'|, is between 1.0 and 4.0 mm, and the cross-section of the square mandrel is a rounded rectangle.

[0071] (3) Using the solid rod obtained in step (2), a square core fiber preform is made according to the designed shape, size and refractive index profile.

[0072] The optical fiber obtained by drawing preforms is shown in Example 2 of Table 1 below. A square core optical fiber with a core side-to-side length of 40 μm was prepared. Although the numerical aperture NA was taken as the minimum value of 0.10 required for application, due to the small core diameter, no matter how the test end was coiled or the test distance was increased to filter the mode, the power output of the square spot showed that the energy in the core region was relatively dispersed, and the flat-top homogenization effect was not achieved, as shown below. Figure 6 As shown. In addition, because the ratio of the side length of the square perforation to the diameter of the fluorine-doped perforated sleeve was not greater than 3:1, the expected perforation length of 500 m was not achieved, and brittle fracture occurred midway, with the actual effective usable length being only 200 mm.

[0073] The square-core optical fibers prepared in Examples 3, 4, 6, and 7 meet the requirements for geometrical accuracy and flat-top uniformity of the optical spot, such as... Figure 6 Intermediate test image. Different square fiber sizes are suitable for various laser beam shaping applications, such as bridging or QBH (Quick Beam Height).

[0074] In Embodiment 5 of this invention, a high-NA fluorine-doped cladding tube was used for perforation. The set value of 0.26 was not significantly larger than the actual application requirement of 0.22, and a 200 μm edge-to-edge square core size, which is currently in high demand in the industry, was used. However, when the optical fiber's output energy was measured, the core region's optical energy did not achieve the ideal flat-top homogenization, but rather concentrated towards the center. Figure 4 The rightmost image shows a beam homogenization effect that, in practice, cannot meet the requirements for beam shaping, especially since such large-size optical fibers are commonly used in high-power laser welding processes.

[0075] Table 1 Parameters and test results of Examples 2 to 7

[0076]

[0077] Example 8

[0078] Using the square-core optical fiber preform provided in Example 1, the end-face structure of the fiber core is designed as follows: Figure 7 As shown, it has a square fiber core and an adjacent circular cladding, as well as an outer cladding.

[0079] The preform preparation method includes the following steps:

[0080] (1) The rotation axis of the Reilly triangular drill bit is coaxial with the geometric axis of the fluorine-doped cladding sleeve with a circular cavity. The fluorine-doped cladding sleeve has a double-layer refractive index structure and can be fabricated by in-tube deposition. A rounded square with a side length of width d is drilled using the Reilly triangular drill bit to obtain a square cavity cladding sleeve. The ratio of the inner diameter of the circular cavity cladding sleeve to the width of the Reilly triangular drill bit is [value missing]. Grinding oil is introduced through the drill bit and directed into the cladding casing.

[0081] (2) Insert the square mandrel with width d' into the square cavity cladding sleeve obtained in step (1), and stretch or shrink it into a round solid rod; the difference between the side length d' of the square mandrel and the width d of the Leroy triangle drill bit, |d-d'|, is between 1.0 and 4.0 mm, and the cross-section of the square mandrel is a rounded rectangle.

[0082] (3) Using the solid rod obtained in step (2), a square core fiber preform is made according to the designed shape, size and refractive index profile.

[0083] Optical fibers are produced by drawing preforms into fibers.

[0084] Example 9

[0085] Using the square-core optical fiber preform provided in Example 1, the end-face structure of the fiber core is designed as follows: Figure 8 As shown, it has a square fiber core and an adjacent square cladding, as well as a circular outer cladding.

[0086] Its preparation method includes the following steps:

[0087] (1) Make the rotation axis of the Reilly triangular drill bit coaxial with the geometric axis of the fluorine-doped cladding casing with a circular cavity; use the Reilly triangular drill bit to drill a rounded square with a side length of width d to obtain a square cavity cladding casing; the ratio of the inner diameter of the circular cavity cladding casing to the width of the Reilly triangular drill bit is 1 / 2. Grinding oil is introduced through the drill bit and directed into the cladding casing.

[0088] (2) Insert the square mandrel with width d' into the square cavity cladding sleeve obtained in step (1), and stretch or shrink it into a round solid rod; the difference between the side length d' of the square mandrel and the width d of the Leroy triangle drill bit, |d-d'|, is between 1.0 and 4.0 mm, and the cross-section of the square mandrel is a rounded rectangle.

[0089] (3) Using the solid rod obtained in step (2), grind the outer circle to a square shape to obtain a square clad solid rod.

[0090] (4) Make the rotation axis of the Reilly triangular drill bit coaxial with the geometric axis of the outer cladding sleeve with a circular cavity; use another Reilly triangular drill bit to drill a rounded square with its width as the side length to obtain a square cavity cladding sleeve; the ratio of the inner diameter of the circular cavity cladding sleeve to the width of the Reilly triangular drill bit is 1 / 3. Grinding oil is introduced through the drill bit and directed into the cladding casing.

[0091] (5) Insert the square cladding solid rod obtained in step (3) into the square cavity cladding sleeve obtained in step (4), stretch or shrink it into a round solid rod to obtain an optical fiber preform.

[0092] Optical fibers are produced by drawing preforms into fibers.

[0093] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a square-core optical fiber preform, characterized in that, Includes the following steps: (1) Make the rotation axis of the Reilly triangular drill bit coaxial with the geometric axis of the circular cavity cladding casing; A rounded square with a width d as its side length is drilled using a Leylow triangular drill bit to obtain a square cavity cladding casing; (2) Insert the square core rod with side length d' into the square cavity cladding sleeve obtained in step (1), and stretch or shrink it into a round solid rod; (3) Using the solid rod obtained in step (2), a square core fiber preform is made according to the designed shape, size and refractive index profile.

2. The method for preparing a square core optical fiber preform as described in claim 1, characterized in that, The difference between the side length d' of the square mandrel and the width d of the Leylow triangular drill bit, |d-d'|, is between 1.0 and 4.0 mm; the square mandrel has a rounded square cross-section.

3. The method for preparing a square core optical fiber preform as described in claim 1, characterized in that, The ratio of the inner diameter of the cladding sleeve of the circular cavity to the width of the Leylow triangular drill bit is [value missing]. The width d of the Leylow triangular drill bit is less than or equal to one-third of the outer diameter D of the cladding casing.

4. The method for preparing a square core optical fiber preform as described in claim 1, characterized in that, Step (1) introduce grinding oil through the drill bit and guide it into the cladding casing.

5. The method for preparing a square core optical fiber preform as described in claim 1, characterized in that, When preparing a preform of an optical fiber with a circular outer cladding and one or more square cladding layers between the outer cladding and the square core, the outer circle of the solid rod prepared in step (3) is ground into a square shape or a square cavity sleeve and a square core rod are stretched or fused into a square solid rod as a new square core rod. Steps (1) to (3) are repeated to prepare multiple square cladding layers from the inside out until a circular outer cladding is formed. When preparing a preform with a circular outer cladding and one or more circular cladding optical fibers between the outer cladding and the square core, the cladding sleeve is designed as multiple layers according to the corresponding refractive index profile, serving as the circular cavity cladding sleeve in step (1).

6. A square-core optical fiber preform, characterized in that, Prepared according to the preparation method of the square core optical fiber preform as described in any one of claims 1 to 5.

7. A square-core optical fiber, characterized in that, It is formed by drawing the optical fiber preform as described in claim 6.

8. The square-core optical fiber as described in claim 7, characterized in that, The side length of the square core is 50~1000 μm, the ratio of the diameter of the adjacent cladding to the side length of the square core is between 1.5 and 2.5, and the core-cladding concentricity is less than 1.0% of the cladding diameter; the core-cladding concentricity is defined as the center distance between the square core and the cladding.

9. The square-core optical fiber as described in claim 7, characterized in that, When fabricating a preform with a circular outer cladding and one or more square cladding optical fibers between the outer cladding and the square core, the non-circularity is less than 0.5%. The non-circularity of the fiber cladding is defined as follows: ； D max To measure the maximum diameter of a specific cross-section of an optical fiber in a single measurement, D min To measure the minimum diameter of the fiber cross-section in a single measurement, D avg The fitted circle diameter value is used for the same cross-section in a single measurement.

10. A square-core optical fiber preform processing apparatus for manufacturing the optical fiber preform as described in claim 6, characterized in that, Includes Leylow triangular drill bits and cladding casing clamps; The rotation axis of the drill bit is coaxial with the central axis of the cladding casing clamp, so that after the cladding casing is clamped, the rotation axis of the Reilly triangular drill bit is coaxial with the geometric central axis of the hollow cladding casing.

11. The square core optical fiber preform processing apparatus as described in claim 10, characterized in that, The Lelo triangular drill bit has a hollow cavity with a grinding oil supply port inside.

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