Reducing geometric distortion in multicore fiber matrix materials

Abstract

A process for manufacturing an MCF preform having core rods disposed within the core holes and a common cladding covering each of the core rods. A cylinder having an outer diameter of at least about 200 mm is provided, which forms the cladding and may have a central core hole. Peripheral core holes are formed in the cylinder. Each of a plurality of core rods is inserted into a respective peripheral core hole. The cylinder with the core rods inserted is heated, thereby collapsing the cylinder onto the core rods to form the preform. A gap between the peripheral core rods and the peripheral holes is maintained within a range of about 0.2 mm to 4 mm during the step of forming the plurality of peripheral core holes, an average radial temperature gradient is maintained between about 0.5° K / mm to 4° K / mm during the step of heating the cylinder, or both.

Description

Technical Field

[0001] The present disclosure generally relates to the field of optical fiber technology, and more particularly to a base material for multi-core fiber (MCF). These fibers enable an increase in bit rate capacity in optical cables for telecommunications by spatial division multiplexing.

Background Art

[0002] As disclosed in U.S. Patent Application Publication No. 2018 / 0145752 (and related European Patent No. 3323791) entitled "Upward Collapse Process and Apparatus for Making Glass Preforms" and U.S. Patent Application Publication No. 2020 / 0223737 entitled "Automated Large Outside Diameter Preform Tipping Process and Resulting Glass Preforms", filed by Heraeus Quarzglas GmbH & Co. KG, the assignee of the present application, the field of applied science and engineering related to the design and application of optical fibers is known as optical fibers. An optical fiber is a flexible, transparent fiber made by stretching glass (silica) to a diameter slightly thicker than that of a human hair. Optical fibers are most frequently used to transmit light between two ends of the fiber and are widely used in optical fiber communication, enabling transmission at high bandwidths (data rates) over longer distances than wire cables. The fiber is used instead of metal wires because signals travel along the fiber with reduced loss and high capacity. In addition, the fiber is also not affected by electromagnetic interference, which is a problem that plagues metal wires. The fiber is also used for lighting and can be bundled to carry images, thus enabling observation in a limited space, as in the case of a fiber optic scope. Specially designed fibers are also used for various other applications such as optical fiber sensors and fiber lasers.

[0003] Optical fibers typically contain a transparent core surrounded by a transparent cladding material with a lower refractive index. Light is retained within the core by total internal reflection, which allows the fiber to function as a waveguide. Fibers that support many propagation paths or transverse modes are called multimode fibers. Fibers that support a single mode are called single-mode fibers. Optical fibers are generally manufactured by heating a pre-fabricated matrix in a furnace and stretching the matrix into an optical fiber. A single matrix can potentially produce an optical fiber of approximately 7,000 to 8,000 km.

[0004] Today, stringent optical fiber cutoff wavelength specifications must be met, and yield losses to achieve these specifications are unacceptable. The cutoff wavelength can be defined as the wavelength below which a single-mode optical fiber functions as a multimode optical fiber. In other words, the cutoff wavelength can be defined as the wavelength above which single-mode operation is guaranteed in a single-mode optical fiber. Many network planners now recognize that the cable cutoff wavelength is one of the most important parameters to define when creating optical fiber cable specifications.

[0005] Multicore fiber (MCF) transmission technology has been widely studied as the simplest form of spatial multiplexing (SM) or spatial division multiplexing (SDM), and as a response to the increasing demand for bandwidth. SDM refers to the use of the lateral dimension of the fiber to separate channels in optical fiber communication systems. MCF technology contains multiple cores within a single cladding. Each core in the MCF can be single-mode or multi-mode, depending on the SDM method used. Typically, an MCF has 4 to 8 cores, but other numbers of cores are also possible. When the cores are relatively far apart, the overlap of their individual modes is negligible, and the multicore fiber behaves like a bundle of single-mode fibers. However, when the cores are closely spaced, the mode overlap is not negligible.

[0006] Japanese companies are particularly active in the development of MCF technology. For example, Furukawa Electric Co., Ltd. has filed a Japanese Patent Application No. 2016-191693 (issued as Patent No. 6560178) entitled "Method for manufacturing a multicore fiber matrix and a method for manufacturing a multicore fiber." This patented method for manufacturing a multicore fiber matrix includes preparing a clad matrix having a plurality of through holes extending in the longitudinal direction of a cylindrical glass matrix, connecting a cylindrical member coaxially with the clad matrix to one end of the clad matrix, and inserting a core matrix into each of the plurality of through holes in the clad matrix. The inner diameter of the cylindrical member is smaller than the diameter of the circumscribed circle of the through hole located on the outermost periphery of the plurality of through holes formed in the clad matrix, and in the preparation process, the clad matrix has a communication structure formed to communicate the through holes that overlap at least a portion of the cylindrical member when viewed from above with the interior of the cylindrical member.

[0007] Sumitomo Electric Industries, Ltd. has obtained U.S. Patent No. 9,604,868 (claiming priority to Japanese Patent Application No. 2013-030890) entitled “Preform Manufacturing Method”. This manufacturing method comprises a hole-forming step in which multiple holes are formed in a glass body to produce a glass pipe, and a heating and integration step in which the glass pipe is heated with a core rod including a core portion inserted into each hole to integrate the core rod and the glass pipe. In the hole-forming step, the peripheral holes among the holes formed in the glass body are formed at positions determined considering the positional variation of the core portion before and after integration. More specifically, multiple core rods are formed using a porous glass pipe / cylinder. Collapse The peripheral core shift relative to the center of the MCF base material is estimated during this process. A lower limit of 0.15 mm has been proposed for the gap between the radius R of the peripheral hole and the radius r of the peripheral core rod. The peripheral hole is required to be located in a position that satisfies the relationship between the center position of the peripheral core and the central axis of the glass body.

[0008] Sumitomo Electric Industries, Ltd. also holds U.S. Patent No. 10,520,668, entitled "Method for Manufacturing Multicore Optical Fiber and Multicore Optical Fiber" (claiming priority to Japanese Patent Application No. 2018-61331). This patent describes a method for manufacturing MCF with a non-circular cross-section, which utilizes deformation of the cross-sectional area (CSA) of the base material due to holes and gaps within the cladding. Specifically, the patent discloses forming a common cladding tube by creating recess-forming holes at different locations from multiple core rod insertion holes. Thus, the common cladding tube and core rods are integrated, and the recess-forming holes are... Collapse This process forms a core-clad composite with a non-circular cross-sectional shape, which includes multiple cores and a common cladding.

[0009] concave-shaped hole CollapseThe deformation of the MCF matrix CSA from a circular shape is used to help fabricate MCF matrix materials having a non-circular CSA. This patent disclosure is not intended to prevent or reduce the non-circularity of the matrix cladding or the ellipticity of the core. Rather, the goal is to use the intentionally generated non-circularity of the matrix CSA. The goal is to fabricate MCFs with a non-circular cross-sectional shape to facilitate easier rotational alignment during MCF connection. This disclosure does not address the detrimental effects on MCF fiber performance, such as polarization mode dispersion (PMD), caused by geometric distortions of the matrix cladding and core ellipticity.

[0010] Summary of the Invention To address problems inherent in conventional MCF matrix manufacturing processes, the objective of the disclosed MCF matrix manufacturing process is to minimize (if not eliminate) the geometric distortions of the matrix cladding's non-circularity and core ellipticity. Another objective is to manufacture a matrix that minimizes such geometric distortions. A related objective is to provide a process that yields an MCF with improved PMD performance. Furthermore, an objective of the present invention is to provide a matrix manufacturing process that enables the relatively easy and efficient manufacture of MCF matrix with minimized geometric distortions. A related objective is to manufacture a matrix with high precision in terms of the circularity of the core and cladding.

[0011] To achieve these and other objectives, and with regard to those objectives, the Disclosure provides a process for manufacturing an MCF base material having a central longitudinal axis, a plurality of core rods, each positioned within a respective core bore and extending along the axis, and a common cladding covering each of the plurality of core rods. The process comprises the following steps: A cylinder is provided having an outer diameter of at least about 200 mm and which may have a central core bore, forming the cladding of the base material. Peripheral core bores extending along the longitudinal axis are formed within the cylinder. Each of the plurality of core rods is inserted into its respective peripheral core bore. The cylinder having the core rods inserted into each core bore is heated by exposing the cylinder and core rods to a heating element of a furnace, thereby heating the cylinder on the plurality of core rods. Collapse This is done to form the base material. The gap (g) between the peripheral core rod and the peripheral hole is maintained in the range of approximately 0.2 mm to 4 mm during the step of forming multiple peripheral core holes, and the average radial temperature gradient is maintained between the core rod and the cylinder during the step of heating the cylinder. Collapse The temperature is maintained at approximately 0.5°K / mm to 4°K / mm in the plane where it begins, or both.

[0012] Please understand that the general descriptions above and the detailed descriptions below are illustrative and not limiting to this disclosure. [Brief explanation of the drawing]

[0013] This disclosure will be best understood from the following detailed description when read in conjunction with the attached drawings. In accordance with common practice, it should be emphasized that various features in the drawings are not to scale. Conversely, the dimensions of various features have been arbitrarily enlarged or reduced for clarity. The drawings include the following figures: [Figure 1] This is a schematic diagram showing the main components of the equipment used in the upward collapse process for forming long glass components. [Figure 2] This is a perspective side view of a glass body used in the manufacture of optical components. [Figure 3]This figure shows a cross-section of a multicore glass fiber having two core regions and a common outer cladding region. [Figure 4A] Figures 4A (7-core MCF) and 4B (4-core MCF) show two examples of MCF matrix cross-sectional images obtained by tomographic refractive index profile measurements, exhibiting both cladding non-circularity and core ellipticity, respectively. [Figure 4B] Figures 4A (7-core MCF) and 4B (4-core MCF) show two examples of MCF matrix cross-sectional images obtained by tomographic refractive index profile measurements, exhibiting both cladding non-circularity and core ellipticity, respectively. [Figure 5] Figure 4A shows a finite element model of the 7-core MCF matrix material with only 2D collapse. [Figure 6] Figure 4A shows the field of the magnitude of the glass flow velocity in a perforated cylinder collapse under vacuum for the 7-core MCF base material shown. [Figure 7A] This is a graph of a relatively large radial temperature gradient (average 5.2°K / mm) used as the first case for modeling the deformation of a perforated cylinder. [Figure 7B] Figure 7A shows the deformation of a perforated cylinder modeled against a relatively large temperature gradient, illustrating not only the hexagonal shape of the cladding, which is very similar to the actual 7-core MCF matrix example shown in Figure 4A, but also the severe peripheral core ellipticity. [Figure 8A] This is a graph of a relatively large radial temperature gradient (average 6.3°K / mm) used as a second case for modeling the deformation of a perforated cylinder. [Figure 8B] Figure 8A shows the deformation of a perforated cylinder modeled against a relatively large temperature gradient, illustrating not only the hexagonal shape of the cladding, which is very similar to the actual 7-core MCF matrix example shown in Figure 4A, but also the severe peripheral core ellipticity. [Figure 9A] This is a graph of a relatively large radial temperature gradient (average 7.9°K / mm) used as a third case for modeling the deformation of a perforated cylinder. [Figure 9B]It shows the deformation of the perforated cylinder modeled for a relatively large temperature gradient in FIG. 9A, and is a figure showing not only a hexagonal shape of the clad that is very close to the example of the actual 7-core MCF base material shown in FIG. 4A, but also showing severe peripheral core ellipticity. [Figure 10A] It is a graph of the radial temperature gradient (average 1.1 °K / mm) used as the first case for modeling the deformation of the perforated cylinder after significantly reducing the temperature gradient during the manufacturing process of the present disclosure. [Figure 10B] It shows the deformation of the perforated cylinder modeled for the significantly reduced temperature gradient in FIG. 10A, and is a figure showing that the circular shapes of both the peripheral holes and the entire clad are much better preserved. [Figure 11A] It is a graph of the radial temperature gradient (average 1.6 °K / mm) used as the second case for modeling the deformation of the perforated cylinder after significantly reducing the temperature gradient during the manufacturing process of the present disclosure. [Figure 11B] It shows the deformation of the perforated cylinder modeled for the significantly reduced temperature gradient in FIG. 11A, and is a figure showing that the circular shapes of both the peripheral holes and the entire clad are much better preserved [Figure 12A] It is a graph of the radial temperature gradient (average 2.0 °K / mm) used as the third case for modeling the deformation of the perforated cylinder after significantly reducing the temperature gradient during the manufacturing process of the present disclosure. [Figure 12B] It shows the deformation of the perforated cylinder modeled for the significantly reduced temperature gradient in FIG. 12A, and is a figure showing that the circular shapes of both the peripheral holes and the entire clad are much better preserved. [Figure 13] It is a figure showing the influence of the central hole sizes of 24 mm (FIG. 13), 33 mm (FIG. 14), 35.92 mm (FIG. 15), 49.5 mm (FIG. 16) and 51 mm (FIG. 17) on the performance of the disclosed process by FEM simulation for relatively low radial temperature gradients. [Figure 14]Diagram showing the influence of central hole sizes of 24 mm (Figure 13), 33 mm (Figure 14), 35.92 mm (Figure 15), 42.5 mm (; Figure 16), and 51 mm (Figure 17) on the performance of the disclosed process by FEM simulation for a relatively low radial temperature gradient. [Figure 15] Diagram showing the influence of central hole sizes of 24 mm (Figure 13), 33 mm (Figure 14), 35.92 mm (Figure 15), 42.5 mm (; Figure 16), and 51 mm (Figure 17) on the performance of the disclosed process by FEM simulation for a relatively low radial temperature gradient. [Figure 16] Diagram showing the influence of central hole sizes of 24 mm (Figure 13), 33 mm (Figure 14), 35.92 mm (Figure 15), 42.5 mm (; Figure 16), and 51 mm (Figure 17) on the performance of the disclosed process by FEM simulation for a relatively low radial temperature gradient. [Figure 17] Diagram showing the influence of central hole sizes of 24 mm (Figure 13), 33 mm (Figure 14), 35.92 mm (Figure 15), 42.5 mm (; Figure 16), and 51 mm (Figure 17) on the performance of the disclosed process by FEM simulation for a relatively low radial temperature gradient. [Figure 18] Diagram showing the influence of central hole sizes of 24 mm (Figure 18), 33 mm (Figure 19), 42.5 mm (Figure 20), 45 mm (Figure 21), 48 mm (Figure 22), and 51 mm (Figure 23) on the performance of the disclosed process by FEM simulation for a relatively high radial temperature gradient. [Figure 19] Diagram showing the influence of central hole sizes of 24 mm (Figure 18), 33 mm (Figure 19), 42.5 mm (Figure 20), 45 mm (Figure 21), 48 mm (Figure 22), and 51 mm (Figure 23) on the performance of the disclosed process by FEM simulation for a relatively high radial temperature gradient. [Figure 20]This figure shows the effect of central hole sizes of 24 mm (Figure 18), 33 mm (Figure 19), 42.5 mm (Figure 20), 45 mm (Figure 21), 48 mm (Figure 22), and 51 mm (Figure 23) on the performance of the disclosed process, based on FEM simulations for relatively high radial temperature gradients. [Figure 21] This figure shows the effect of central hole sizes of 24 mm (Figure 18), 33 mm (Figure 19), 42.5 mm (Figure 20), 45 mm (Figure 21), 48 mm (Figure 22), and 51 mm (Figure 23) on the performance of the disclosed process, based on FEM simulations for relatively high radial temperature gradients. [Figure 22] This figure shows the effect of central hole sizes of 24 mm (Figure 18), 33 mm (Figure 19), 42.5 mm (Figure 20), 45 mm (Figure 21), 48 mm (Figure 22), and 51 mm (Figure 23) on the performance of the disclosed process, based on FEM simulations for relatively high radial temperature gradients. [Figure 23] This figure shows the effect of central hole sizes of 24 mm (Figure 18), 33 mm (Figure 19), 42.5 mm (Figure 20), 45 mm (Figure 21), 48 mm (Figure 22), and 51 mm (Figure 23) on the performance of the disclosed process, based on FEM simulations for relatively high radial temperature gradients. [Figure 24] This figure shows the effect of cladding sizes of 134 mm (Figure 24), 167 mm (Figure 25), and 200 mm (Figure 26) on the performance of the disclosed process, based on FEM simulations for relatively low radial temperature gradients. [Figure 25] This figure shows the effect of cladding sizes of 134 mm (Figure 24), 167 mm (Figure 25), and 200 mm (Figure 26) on the performance of the disclosed process, based on FEM simulations for relatively low radial temperature gradients. [Figure 26] This figure shows the effect of cladding sizes of 134 mm (Figure 24), 167 mm (Figure 25), and 200 mm (Figure 26) on the performance of the disclosed process, based on FEM simulations for relatively low radial temperature gradients. [Figure 27] This figure shows the effects of temperature gradients of 6°K / mm (Figure 27), 2°K / mm (Figure 28), and 1.2°K / mm (Figure 29) on the performance of the disclosed process for a 4-hole base material example without a central hole, as determined by FEM simulation. [Figure 28] This figure shows the effects of temperature gradients of 6°K / mm (Figure 27), 2°K / mm (Figure 28), and 1.2°K / mm (Figure 29) on the performance of the disclosed process for a 4-hole base material example without a central hole, as determined by FEM simulation. [Figure 29] This figure shows the effects of temperature gradients of 6°K / mm (Figure 27), 2°K / mm (Figure 28), and 1.2°K / mm (Figure 29) on the performance of the disclosed process for a 4-hole base material example without a central hole, as determined by FEM simulation. [Figure 30] Figures 30 (without central hole) and 31 (with central hole) show the effect of the central hole on the performance of the disclosed process for a 4-hole matrix example, as determined by FEM simulation. [Figure 31] Figures 30 (without central hole) and 31 (with central hole) show the effect of the central hole on the performance of the disclosed process for a 4-hole matrix example, as determined by FEM simulation. [Figure 32] This figure shows the effect of 150 mm (Figure 32) and 200 mm (Figure 33) cladding sizes on the performance of the disclosed process for a 4-hole base material example without a central hole, as determined by FEM simulation. [Figure 33] This figure shows the effect of 150 mm (Figure 32) and 200 mm (Figure 33) cladding sizes on the performance of the disclosed process for a 4-hole base material example without a central hole, as determined by FEM simulation. [Figure 34] This is a flowchart summarizing the steps of one embodiment of the disclosed process. [Modes for carrying out the invention]

[0014] In this specification and the following claims, several terms are used, and these terms are defined as having the following meanings:

[0015] Terms such as "include," "includes," "including," "have," "has," "having," "comprise," "comprises," and "comprising" all mean to include but not limit, that is, to be comprehensive and not exclusive.

[0016] The term “approximately” means that quantities, sizes, formulations, parameters, and other quantities and characteristics are not, and do not need to be, exact, but may be approximate and / or greater or less, to reflect tolerances, conversion factors, rounding, measurement errors, and other factors known to those skilled in the art, as desired. Where a value is stated to be approximately equal to a particular number, or if it is approximately equal to a particular number, that value is within ±10% of that number. For example, a value that is approximately 10 refers to a value between 9 and 11 (including 9 and 11). Where the term “approximately” is used to describe the endpoint of a value or range, this disclosure should be understood to include a particular value or endpoint. Regardless of whether the endpoint of a numerical value or range in this specification states “approximately,” the numerical values ​​or endpoints of a range are intended to include two embodiments: those modified by “approximately” and those not modified by “approximately.” It will be further understood that each endpoint of a range is significant both in relation to the other endpoints and independently of the other endpoints.

[0017] The term “approximately” further refers to all terms within the scope unless otherwise specified. For example, approximately 1, 2, or 3 is equivalent to approximately 1, approximately 2, or approximately 3, and further includes approximately 1–3, approximately 1–2, and approximately 2–3. The specific and preferred values ​​disclosed for compositions, components, ingredients, additives, and similar embodiments, and their scopes, are for illustrative purposes only. They do not exclude other defined values ​​or other values ​​within the defined scope. The compositions and processes of this disclosure include any values, or any combination of values, specific values, more specific values, and preferred values ​​described herein.

[0018] As used in this disclosure, the indefinite articles "a" or "an" and their corresponding definite articles "the" mean at least one or more, unless otherwise specified.

[0019] The directional terms used in this disclosure (e.g., up, down, right, left, front, back, top, bottom) are used only in relation to the depicted figures and the coordinate axes given to those figures, and are not intended to imply absolute orientation.

[0020] "Contact" refers to direct or indirect contact. Direct contact refers to contact in the absence of intervening material, while indirect contact refers to contact mediated by one or more intervening materials. Elements in direct contact are touching each other. Elements in indirect contact are not touching each other but are touching an intervening material or a series of intervening materials, with at least one of the intervening material or series of intervening materials touching the other. In contact elements may be firmly or loosely joined. "To bring into contact" refers to bringing two elements into direct or indirect contact. Elements in direct (indirect) contact can be said to be in direct (indirect) contact with each other.

[0021] An “optical fiber” refers to a waveguide having a glass portion surrounded by a cladding. The glass portion includes a core and a cladding, and is referred to in this disclosure as a “glass fiber.” A multicore fiber is an optical fiber having a glass fiber, which includes two or more cores surrounded by a common cladding. The glass fiber functions as a waveguide.

[0022] "Radial position," "radius," or radial coordinate "r" refers to the radial position relative to the centerline (r=0) of the cores of a multicore fiber. Each of the two or more cores of a multicore fiber has a centerline and a separate radial coordinate r. "Radial position," "radius," or radial coordinate "R" refers to the radial position relative to the centerline (R=0) of a multicore fiber. A multicore fiber has a single centerline.

[0023] Optical fibers are generally manufactured through two separate processes. First, a core rod is prepared, and then a matrix is ​​manufactured by a rod-in-tube (RIT) or rod-in-cylinder (RIC) process, or by another overcladding process such as an outside vapor deposition (OVD) process. Second, the manufactured matrix is ​​heated in a furnace and stretched into an optical fiber. Conventional processes and equipment for manufacturing optical fiber matrix, completing the first of the two processes, can include providing optical fiber RIT overcladding equipment.

[0024] The overcladding apparatus includes a vertical lathe, chucks mounted at each end of the vertical lathe, a carriage within the vertical lathe for moving up and down between the ends of the vertical lathe, an oxygen-hydrogen burner mounted on the carriage, a furnace mounted on the carriage, a vacuum pump provided at one end of the vertical lathe, a coupler connecting the vacuum pump to the other end of the vertical lathe, and an external controller for controlling the vertical movement of the carriage, the flow rate of the oxygen-hydrogen burner, and the rotation of the chucks. The furnace preheats or heats the glass tubes for overcladding the core rod with glass tubes.

[0025] In practice, the outer diameter of the base material is limited to 90 mm or less in conventional RIT overcladding equipment. This limitation is imposed by inefficient heating with an oxygen-hydrogen burner. Furthermore, the handle must be welded to a single core rod (of the same length as the RIT overcladding tube) to provide separate support for the core rod weight from the upper end. This results in two disadvantages: (1) waste of core rod material as short core rods cannot be used effectively; and (2) welding the handle to the core rod, especially using an oxygen-hydrogen torch, leads to the incorporation of surface hydroxide (OH) on the surface of the core rod, which, if not etched off (additional cost for the process), can increase fiber attenuation, particularly at 1,383 nm, due to OH absorption.

[0026] More recently, quartz glass tubes, rods, or Collapse The base material for an offline rod-in-cylinder (ORIC) is a quartz glass component (e.g., cylinder, ingot, or CollapseThe material (RIC) is manufactured by introducing it in a vertical orientation into a device including a heating zone (e.g., a furnace) so that its lower end begins to soften and form a strand. The strand is then placed in a tensioning device including one or more sets of tensioning wheels. The stretching rate of the strand is controlled by the speed of the tensioning wheels, which can apply either a downward or upward force depending on the temperature or viscosity and weight of the formation zone of the strand supported by the wheels. Forming is achieved without the help of a die. Thus, the strand dimensions are controlled by the supply rate of the quartz glass component, the temperature of the heating zone, and the speed of the tensioning wheels.

[0027] In the conventional ORIC process, a cylinder made of synthetic high-purity glass (typically 3m long and about 200mm in outer diameter) is placed on a high-purity glass core rod. Collapse Then, heat and vacuum are used to form the optical fiber preform in the interfacial gap. The preform is usually continuously stretched downward to a diameter significantly smaller than the original diameter of the cylinder. Collapse To facilitate this and to support the weight of the core rod through the softened glass, a sufficient vacuum must be applied to the gap between the cylinder and the core rod. The vacuum is essential to prevent the core rod from moving relative to the cylinder. Otherwise, the cladding-to-core ratio of the resulting matrix will be distorted, and the fiber drawn from the matrix will not be able to meet the required waveguide specifications (such as cutoff wavelength). Complex and expensive matrix outer diameter measurement and feedback control are also necessary for downward Collapse In the stretching and drawing processes, however, even with such control, achieving precise base material shape (including low base material warp or curvature and diameter variation) and waveguide characteristics free from cladding versus core distortion is difficult. The effect of this inherent waveguide distortion in the downward drawing process is largely due to gravity and vacuum forces acting on the molten glass and the unmounted core rod in the furnace, as the outer cladding glass is hotter and flows downward faster than the inner core rod glass.

[0028] Using conventional downward stretching systems and processes, it is considerably difficult to produce the largest base material with an outer diameter close to the original cylinder or cladding size. If the geometry and waveguide characteristics of the base material deviate significantly from the required specifications in terms of parameters such as geometry, cladding-to-core ratio, core eccentricity, and warpage, a considerable amount of good base glass is wasted at the beginning and end of the process. Therefore, conventional base material systems and processes have clear drawbacks.

[0029] According to U.S. Patent Application Publication No. 2018 / 0145752, a device for producing a base material having the maximum known outer diameter and length (i.e., an outer diameter of approximately 200 mm and a length of approximately 3 m, compared to approximately 150 mm for conventional outer diameters, or approximately the same size as the original cylinder or cladding) with minimal waveguide (cladding vs. core) distortion and significantly reduced waste and costs, and for upward Collapse The process is provided. Conventional optical fiber preforms have an outer diameter of 90mm to 150mm. Streamlined upward Collapse In the process, the stacked core rods within the ORIC cladding are supported from below (therefore the core rods are, Collapse (Does not move relative to the cladding during the process), the entire ORIC assembly moves upward relative to the furnace, and therefore the base material moves continuously as shown in Figure 1 and described below. Collapse It is then stretched upward. The device and upward Collapse The process involves (1) using the largest known overclad cylinder Collapse(1) It can be manufactured in a single process, thus producing the largest known matrix; (2) reducing costs due to a streamlined and simplified process (e.g., no need for online measurement or feedback control), including a nearly 100% overcladding and finished (chipped) matrix yield (almost no waste) and an integrated online matrix chipping process (saving processing time and heating steps); (3) improving waveguide quality for inherently low waveguide (cladding vs. core) distortion with variable and arbitrary length, fixed, stacked, and supported core rods; and (4) allowing the application of reactive gases (such as SF6) to the interface up to about 1 atmosphere (i.e., no vacuum required) for an improved interface and a lower core-rod D / d ratio (interface closer to the waveguide core).

[0030] The core-rod D / d ratio is the ratio of the outer diameter of the core rod to the diameter of the waveguide core (through which light propagates), where "D" is the outer diameter of the core rod and "d" is the diameter of the waveguide core. This ratio is crucial for those manufacturing optical fibers using RIT or RIC matrix materials, as it defines core capacity expansion. As the core-rod D / d ratio decreases, the interface moves closer to the waveguide core, which means a decrease in the relative amount of glass required in the core rod (while the amount of glass in the cladding needs to increase). This, in turn, means that the ability to manufacture core rods (or the equivalent capacity of optical fiber cores) using the same core-rod manufacturing equipment is approximately proportional to the square of D / d (e.g., doubling the core capacity by reducing D / d from 3.3 to 2.3). However, reducing the core-rod D / d presents significant challenges to the purity of the overcladding material and interface quality due to the exponentially increasing optical power propagation therein. Therefore, more aggressive gas etching, cleaning, and drying processes at the interface (e.g., with SF6) will be required at lower core-rod D / d ratios. In short, lower D / d ratios (i.e., the interface is closer to the core) allow matrix manufacturers to (a) easily expand core capacity without expensive investment, and (b) realize more complex and advanced optical fiber designs with refractive index characteristics closer to the core.

[0031] Referring to Figure 1, an apparatus 10 for manufacturing optical fiber preforms is shown. The apparatus 10 includes a vertically positioned frame 12. From bottom to top, the frame 12 has a lower open end, a preheating zone or lower insulation zone 14, a heating zone 16, a post-heating or upper insulation zone 17, a post-heating cooling, annealing, and oven gas purge zone 18, and an upper open end opposite the lower open end. The heating zone 16 can be heated by a heating element (typically an oven or furnace) to a temperature preferably about 500°C to 2,300°C, more preferably about 1,000°C to 2,300°C, and most preferably about 1,500°C to 2,300°C. More specifically, the heating element is preferably in an annular configuration. The heating element is preferably positioned inside or around the frame 12 to form the heating zone 16 of the frame 12. An inert gas is injected into the heating element at a high temperature to prevent oxidation of the heating element.

[0032] Referring to Figure 2, the glass body 20 is used to manufacture the optical fiber preform. The glass body 20 has a cylindrical or tubular structure. The glass body 20 has a length L extending from a first end or upper end 22 to a second end or lower end 24 on the opposite side. The vertical axis X extends between the first end 22 and the second end 24 on the opposite side. Preferably, both the first end 22 and the second end 24 of the glass body 20 are ends cut at a right angle.

[0033] The glass body 20 preferably comprises a glass core or core rod 30 containing a waveguide optical fiber core, and a glass cladding 32 surrounding the core rod 30. More specifically, the core rod 30 is preferably formed at the geometric center of the glass body 20 and extends along the length L of the glass body 20. The cladding 32 is preferably formed on the core rod 30 so as to radially surround the core rod 30 along the length L of the glass body 20. The cladding 32 surrounds the core rod 30 in a coaxial arrangement aligned along a common centerline. A gap 31 initially exists between the core rod 30 and the cladding 32. The cladding 32 has an outer diameter "OD".

[0034] The cladding 32 may be pure quartz glass or doped quartz glass. However, it is preferable that the cladding 32 be the highest purity synthetic silica, whether doped or not (e.g., with fluorine). The core rod 30 is preferably mostly the highest purity quartz glass, with doped and undoped regions to achieve a suitable refractive index profile. The cladding 32 and the core rod 30 may each be formed by fused silica or any preferred process such as one or more types of chemical vapor deposition (CVD) (including internal deposition, external deposition, and axial deposition). The core material at the center of the core rod 30 typically has a refractive index greater than that of the surrounding cladding 32 material to allow internal reflection of the optical signal passing through the fiber drawn from the matrix, resulting in an effective waveguide.

[0035] Returning to Figure 1, the first collar or upper collar 40 is fixed to the top of the clad 32. Other mechanisms can be used to attach the upper collar 40 to the clad 32, but the upper weld 42 is suitable. The outer diameter of the upper collar 40 is approximately the same as or smaller than the outer diameter of the clad 32. The second collar or bottom collar 44 is fixed to the bottom of the clad 32. Other mechanisms can be used to attach the bottom collar 44 to the clad 32, but the bottom weld 46 is suitable. The outer diameter of the bottom collar 44 is either smaller than or approximately the same as the outer diameter of the clad 32. Both the upper collar 40 and the lower collar 44 are hollow tubular components.

[0036] The stacked core rods 30 are positioned inside the cladding 32 and placed on any short spacers 48, which are then placed on long spacers 50. The long spacers 50 are supported by a bottom collar holder and vacuum unit 52 positioned beneath them. The bottom collar holder and vacuum unit 52 also holds and supports the bottom collar 44, as its name suggests. The rod-in-cylinder or RIC assembly (including the stacked core rods 30 and cladding 32 of the glass body 20, along with upper collars 40 and lower collars 44 fixed to the cladding 32) and the lower collar holder and vacuum unit 52 are first loaded onto the upper collar holder and vacuum unit 54 located above the oven gas purge zone 18. (The bottom collar holder and vacuum unit 52 and the top collar holder and vacuum unit 54 allow the apparatus 10 to remove gas from the apparatus 10, i.e., create a vacuum, or introduce gas into the apparatus 10 at either end of the apparatus 10. The top collar holder and vacuum unit 54, as its name suggests, holds and supports the top collar 40.) Next, the glass body 20 is positioned relative to the heating zone 16, more specifically relative to the heating elements of the heating zone 16, and moved upward through the heating elements. The bottom collar holder and vacuum unit 52 is gripped and supported below the heating zone 16. The top collar holder and vacuum unit 54 is gripped and supported above the heating zone 16. Before the heating step begins, the top weld 42 (and thus the top of the cladding 32) is first positioned at a predetermined distance below the center of the heating elements to avoid thermal shock to the top weld 42. ("Predetermined" means determined in advance; therefore, the predetermined characteristics must be determined prior to some event, i.e., selected, or at least known.) For example, this distance may be approximately 350 mm.

[0037] Refer to Figure 1, and the apparatus 10 is used to manufacture the base material above CollapseThe process is described below. The glass body 20 passes through the frame 12, where it is heated, softened, and stretched to form an optical component such as an optical fiber matrix. More specifically, the lower end 24 of the glass body 20 is preferably stably positioned within the frame 12 at the start of the process, and then the glass body 20 moves upward (i.e., opposite to the conventional downward direction) through the frame 12. In the frame 12, the glass body 20 is heated zone by zone in the heating zone 16. The matrix is ​​then subjected to the overcladding gap 31. Collapse The core rod 30 is continuously produced by melt deformation that fuses it to the overclad cylinder or clad 32 (optionally, the base material may be stretched / elongated or shortened / compressed by either tensile or compressive forces applied during the process by the upper collar holder and vacuum unit 54 and the bottom collar holder and vacuum unit 52).

[0038] In one embodiment, the glass body 20 is a coaxial assembly of two separate glass components, namely, a stacked core rod 30 and a cladding 32. More specifically, the core rod 30 is in the form of a solid cylindrical rod, and the cladding 32 is in the form of a hollow overcladding cylinder surrounding the stacked core rod 30 (i.e., a rod-in-cylinder assembly). In the coaxial assembly, the stacked core rod 30 and cladding 32 are not fused together before the glass assembly enters the heating zone 16.

[0039] As the coaxial assembly of the glass body 20 in this embodiment moves upward through the frame 12, the core rod 30 and cladding 32 are heated to a predetermined temperature and time sufficient to soften the two glass components and fuse them together to form a single solidified glass body 20. "Single solidified" means a complete single piece or single member without additional pieces, i.e., the member is a single monolithic piece formed as one unit together with another member. More specifically, as the continuous portion of the two-piece glass body 20 approaches the heating zone 16 and is heated within the heating zone 16, the cladding 32 and core rod 30 soften, and the softened cladding 32 fuses with the core rod 30. Collapse The core rod 30 is then fused with it. The stretched-ready matrix can then be stretched directly from the resulting monolithic glass body 20 into a fiber.

[0040] Preferably, the coaxial arrangement of the glass body 20 in this embodiment is heated to a temperature of about 500°C to 2,300°C, more preferably about 1,000°C to 2,300°C, and most preferably about 1,500°C to 2,300°C. More preferably, the cladding 32 on the core rod 30 is softened and Collapse This occurs at a temperature of approximately 1,000°C to 2,200°C, more preferably approximately 1,300°C to 2,000°C, and most preferably approximately 1,600°C to 1,800°C. Collapse The fusion of the clad 32 and the softened core rod 30 is preferably carried out at a temperature of about 1,000°C to 2,200°C, more preferably about 1,300°C to 2,200°C, and most preferably about 1,600°C to 2,200°C. However, as those skilled in the art will know, other factors such as the glass material composition and throughput also affect the fusion of the clad 32 on the core rod 30. Collapse This will help you understand how it affects the temperature at which it fuses with the core rod 30.

[0041] The fusion interface between the core rod 30 and the cladding 32 is reliably cleaned by several components of the apparatus 10. For example, the bottom collar holder and vacuum unit 52 and the upper collar holder and vacuum unit 54 are both sealed and upward Collapse This allows the process to operate in a vacuum. The bottom collar holder and vacuum unit 52 and the top collar holder and vacuum unit 54 also isolate the base material assembly (particularly the interface) from heating elements (e.g., furnaces) and potential contaminants in the external environment. Furnaces and the external environment are typical sources of contamination during conventional processes, particularly vacuum initiation processes where it is difficult to avoid the intrusion of contaminants into the interface. Furthermore, the interface can be etched, cleaned, and dried using reactive interface treatment gases.

[0042] A typical recipe used to heat the heating elements in heating zone 16 involves heating at approximately 50 kW for 30 minutes, then at approximately 100 kW for 10 minutes, at approximately 150 kW for 10 minutes, at approximately 200 kW for 10 minutes, and finally reaching a steady state of the process at approximately 220 kW (or a somewhat lower maximum power, e.g., 212 kW). The bottom collar holder and vacuum unit 52, located at the bottom of the apparatus 10, move at speed V1, and the top collar holder and vacuum unit 54, located at the top of the apparatus 10, move at speed V2. Typically, at the start of the process, V1 = V2. In a typical recipe, after reaching 100 kW in 2 minutes, V1 = V2 = approximately 13.5 mm / min for 6 minutes. The assembly is then stopped for approximately 4 minutes. After the 4-minute pause, the assembly rises again at approximately 13.5 mm / min until the upper weld 42 reaches the center of the heating element. Once the upper weld 42 reaches the center of the heating element, the assembly is stopped for approximately 6 minutes. Next, the assembly enters a steady state. Collapse Therefore, V1=V2, causing it to rise again.

[0043] When the upper weld 42 is approximately 110 mm to 135 mm above the center of the heating zone 16, the vacuum pump of the bottom collar holder and vacuum unit 52 is activated (i.e., turned on). This activation draws a vacuum in the direction of arrow 56, and the pressure inside the upper collar 40 begins to decrease. When the pressure inside the upper collar 40 stops decreasing, the top of the cladding 32 Collapse Then, the gap 31 closes, and the cladding 32 seals or fuses with the core rod 30. At this time, the vacuum is maintained in the bottom collar holder and vacuum unit 52 while refilling the upper collar 40 with gas (e.g., nitrogen gas N2) until the pressure reaches approximately 1 atmosphere. Next, the upper collar 40 is connected to the air.

[0044] The vacuum pumps of the upper collar holder and vacuum unit 54 can be activated (i.e., turned on) to draw a vacuum in the direction of arrow 58. Similarly, purging of the gas used in the heating elements of the heating zone 16 (typically an inert gas such as argon, helium, or most typically nitrogen) can be achieved by introducing the gas into the heating elements in the direction of arrow 60. Gas purging is performed between the outer surface of the glass body 20 and the surface of the heating elements to prevent soot formation on the outer surface of the glass body 20 and oxidation of the heating elements. Gas purging at the top of the heating elements is typically turned on from the start of the process. It is important to identify an appropriate purging rate (e.g., about 9 m³ / h) so that no soot or other deposits form on the surface of the base material during or after the process.

[0045] When the bottom weld 46 is at a predetermined distance (e.g., about 500 mm) below the center of the heating zone 16, the power of the heating element begins to decrease linearly. When the bottom weld 46 reaches the center of the heating zone 16, the power of the heating element should be at a predetermined final power value (e.g., about 150 kW to about 160 kW). While maintaining this final power, the assembly should continue to move upward over a short distance (e.g., about 50 mm). This process step suppresses the temperature rise in the final stage and avoids overheating and slumping of the glass near the bottom.

[0046] The process is complete when the bottom weld 46 is at a short distance (e.g., about 50 mm) above the center of the heating element. At this position, the power to the heating element is completely turned off, and at the same time, the movement of the assembly is stopped. The cladding 32 to the lower end 24 of the glass body 20 is complete. Collapse To ensure this, vacuum pumping can be maintained for a short time (e.g., about 1-2 minutes) after the process has stopped. However, if the final heating recipe is 100% correct, there is no need to maintain the vacuum, and maintaining the vacuum for an extra time may also carry the risk of deforming the bottom collar 44.

[0047] The load cell 68 is used to measure the total weight supported by the bottom collar 44. When a slight, constant vibration perturbation is superimposed on the velocity V2 of the upper collar and vacuum unit 54, and the velocity V1 of the bottom collar holder and vacuum unit 52 is kept constant, a "ripple" appears on the load cell reading curve. The larger the amplitude of the "ripple," the lower the process temperature. This is because in lower temperature processes, the softened glass at the center of the heating element is more rigid and can transmit the vibrational force more effectively to the bottom of the assembly. With a constant heating element power setting, this information indicates whether the process is slightly on the hotter or slightly colder side, due to the actual state of the heating element. Based on this knowledge, the termination power of the process can be determined; that is, the lower the process temperature, the higher the termination power needs to be. This "ripple" amplitude is essentially a true viscosity measurement of the glass body 20 at the center of the heating element and is far more reliable than any glass surface temperature measurement using a pyrometer.

[0048] Therefore, the apparatus 10 and related upward CollapseThe process enables viscosity measurement of the glass body 20 at the center of the heating element by imposing vibrational motion. Small vibrations are applied to the upper position of the base material assembly. In parallel, the weight of the base material assembly is measured by a load cell 68. The measurement by the load cell 68 provides an indirect measurement of the viscosity of the glass body 20 at the center of the heating element. This information can be used, for example, to control the temperature / heating power of the heating zone 16 using a controller 88 (described later).

[0049] A clear difference from conventional downward stretching processes is that the stacked core rods 30 are supported by spacers 48 at the bottom of the stacked core rods 30, instead of being supported by vacuum, which essentially fixes the position of the core rods 30 relative to the cladding 32 during the overcladding and stretching processes. In other words, upward Collapse The process does not require a vacuum to prevent core rod movement, and core rod movement can lead to cladding-to-core waveguide distortion and thus fiber cutoff wavelength problems. Furthermore, in contrast to conventional downward stretching processes, the weight of both the upper and lower glass of the molten glass in the heating zone 16 is well supported by the upper collar 40 and bottom collar 44 in the upward stretching process, which essentially eliminates the effects of cladding-to-core waveguide distortion that was conventionally caused in the heating zone 16 by gravity and vacuum forces. This difference means that the heating element or Collapse When operating at a lower temperature, upward Collapse The process becomes much more tolerant (because the glass is not soft enough to convert the pressure difference from the vacuum and support the core rod 30).

[0050] Since a vacuum is not required to support the weight of the core rod 30, upward Collapse The process also allows for partial pressure in the gap 31 between the core rod 30 and the cladding 32 (up to atmospheric pressure or slightly higher, typically around 1,100 mbar). Therefore, high temperature CollapseIn the interface, a reactive interface treatment gas such as sulfur hexafluoride (SF6, which is safe to handle at room temperature) can be freely applied in the direction of the interface treatment gas arrow 62 to etch away potential interface contaminants such as metal particles or surface hydroxides (OH). In addition to sulfur hexafluoride, other suitable reactive interface treatment gases include oxygen (O2), chlorine (Cl2), fluorine (F2), nitrogen trifluoride (NF3), silicon tetrafluoride (SiF4), carbon tetrafluoride (CF4), and fluoroform (CHF3), although safety concerns may arise. By etching, cleaning, and drying the matrix interface using a reactive interface treatment gas, the interface can be improved, the quality of the optical fiber can be enhanced (reducing fiber breakage, bubbles, loss, or airlines), and the core-rod D / d ratio can be reduced.

[0051] As mentioned in the previous paragraph, the stacked core rods 30 are supported from below by the spacers 48, and the weight of both the upper and lower glass in the heating zone 16 (where the glass is softened) is also supported, so upward Collapse The process is far less susceptible to the effects of core-clad glass flow differences or waveguide distortion. Such support eliminates the problem of uncontrolled glass flow and distortion. Therefore, there is a natural advantage in processing low-viscosity glass materials (such as high-concentration F-doped clads 32) without risking excessive heating or clad-core waveguide distortion due to gravity and vacuum forces. This provides a significant processing advantage for certain classes of fiber designs with F-doped clad 32 material.

[0052] Returning to Figure 1, a glass disc 70 having an outer diameter slightly smaller than the inner diameter of the upper collar 40 (i.e., typically about 126 mm) is positioned on top of the core rod 30 and cladding 32, and inside the upper collar 40. The disc 70 may be about 5 cm thick. During the process startup, after a 6-minute residence time of the upper weld 42 at the center of the heating element, vacuum is applied from both the bottom collar holder and vacuum unit 52 and the upper collar holder and vacuum unit 54. The vacuum presses the upper collar 40 onto the disc 70. CollapseTo allow this to happen, by setting V2 > V1, the tip will remain in the steady state of the remaining cladding 32 where V2 = V1. Collapse It is pulled onto the top of clad 32 beforehand. The result is a low-cost, high-yield online matrix chipping process, which provides the easiest and most efficient matrix for subsequent fiber stretching. The integrated online matrix chipping process saves a considerable amount of both labor and cost compared to conventional offline chipping processes (e.g., by saving an extra heating step).

[0053] interface Collapse When it starts and continues, if V2 is set higher than V1, then Collapse The process can also stretch or elongate the base material upward, resulting in a diameter significantly smaller than the original diameter of the clad 32. The diameter of the stretched (or compressed) base material can be precisely controlled by precisely setting the linear-vertical velocities V1 and V2 through the law of conservation of mass. However, the starting loss of good base material glass is greater in the upward stretching process than in the conventional downward stretching process. Collapse The process involves far fewer steps, and therefore can result in significant cost reductions for the drawn base material.

[0054] The weight of the glass both above and below the heating zone 16 (where the glass is softened) is supported by the upper collar 40 and the bottom collar 44, while the outer surface of the base material itself is not in contact, so the upward Collapse The process is also completely non-contact with the finished base material. By avoiding contact with the base material and therefore avoiding any lateral or transverse forces, unlike conventional downward stretching processes in which the puller wheel is always in contact with the base material and applies force to the base material throughout the process, both an extremely clean base material surface and a base material with almost no warping are achieved.

[0055] In many conventional downward stretching processes, a small contact area exists between the puller wheel and the outer circumference of the base material. Such contact can introduce impurities or contaminants onto the base material surface. In addition, the puller wheel exerts lateral forces during the downward stretching process, which can cause warping of the base material (this worsens with longer base materials; i.e., warping increases as the square of the base material length in the case of simple curvature). The amount of contact force that can be applied to the base material by the puller wheel is limited because excessive pressure can damage the glass surface of the base material. Therefore, for large base materials requiring greater tensile force than can be applied by a single set of puller wheels, multiple sets of puller wheels can be applied to the base material at different levels to achieve the total vertical (frictional) force required to support the weight of the base material. However, multiple sets of puller wheels increase both the height and cost of the equipment. Furthermore, minimizing warping of the base material can only be achieved with multiple sets of puller wheels if the sets of puller wheels are precisely aligned, and precise alignment is difficult to achieve in practice. Collapse The process results in very little warping of the base material because no lateral forces are applied to it.

[0056] Returning to Figure 1, the apparatus 10 may optionally include a gripper system 80 mounted on the frame 12. A preferred gripper system 80 is fully described in U.S. Patent No. 10,590,022, which claims priority to International Patent Application No. PCT / US2015 / 012471, “Formation Of Elongated Glass Components With Low Bow Using A Gripper Device,” filed on 22 January 2015 by Heraeus Quarzglas GmbH&Co.KG, the assignee of this application. In one embodiment, the gripper system 80 is included in the apparatus 10 by mounting the gripper system 80 to the frame 12.

[0057] The gripper system 80 includes a clamping element 82 and a mounting element 84 for attaching the clamping element 82 to the gripper system 80. The gripper system 80 can move perpendicular to the length of the frame 12 (defined as the Z direction in Figure 1). The mounting element 84 allows for the translational motion of the clamping element 82 in the X and Y directions (i.e., to any position in the XY plane). (Although not required or preferred, a chuck system that allows rotation may also be used, particularly if a torch rather than a furnace provides the heating element). In one embodiment, the mounting element 84 is an XY table including a pair of arms mounted on linear bearings or linear rails and a motor, e.g., manual or servo motor drive, for controlling the movement of the arms. The mounting element 84 is an even lower friction device, so that a force applied to the clamping element 82 by an external object causes the clamping element 82 to flex along the mounting element 84 rather than the clamping element 82 applying a resistive force to the external object.

[0058] Once the base material is formed, the gripper system 80 may be mounted by moving the clamping element 82 into contact with the bottom collar 44 or (as shown in Figure 1) the bottom collar holder and vacuum unit 52. Preferably, the clamping element 82 should not be in contact with the base material. The clamping element 82 may be sized to have a convex region having the opposite shape to the bottom collar 44 so that the clamping element 82 fits securely around the bottom collar 44 without damaging the bottom collar 44. The clamping element 82 may be in contact with all or (as shown in Figure 1) only a portion of the outer surface of the bottom collar 44 or the bottom collar holder and vacuum unit 52. In exemplary embodiments, the clamping element 82 may be made from a high-temperature compressible material such as calcium silicate, asbestos, compressed glass, or ceramic fibers (e.g., rock wool) or high-temperature rubber (e.g., silicone or fluoropolymer elastomer).

[0059] The clamp element 82 is first aligned with the center of the bottom collar 44 or bottom collar holder and vacuum unit 52 by first determining the center of the bottom collar 44 or bottom collar holder and vacuum unit 52, and then moving the clamp element 82 so as to align with the center in the X direction. In some embodiments, the clamp element 82 may be aligned with the estimated center of the bottom collar 44 or bottom collar holder and vacuum unit 52, for example, the expected center based on a desired movement path. In other embodiments, in order to more accurately align the clamp element 82 with the bottom collar 44 or bottom collar holder and vacuum unit 52, the apparatus 10 may further include a sensing element capable of locating the center of the bottom collar 44 or bottom collar holder and vacuum unit 52, and a computer for determining the center from the output of the sensing element. The sensing element may include one or more laser devices, camera / visual systems, or mechanical contact (dial indicator) systems. In exemplary embodiments, the sensing element may be attached to the gripper system 80 or outside the gripper system 80, for example, attached to the frame 12. In another embodiment, the sensing element may be outside both the gripper system 80 and the frame 12 (e.g., a camera). The gripper system 80 does not need to be perfectly aligned with the center of the bottom collar 44 or the bottom collar holder and the vacuum unit 52, as it includes further elements to prevent misalignment.

[0060] When the clamp element 82 is aligned, it comes into contact with the bottom collar 44 or bottom collar holder and vacuum unit 52 as the mounting element 84 moves in the X direction. The mounting element 84 can be moved by any suitable mechanism, for example, by a motor used to control a pair of arms on an XY table. Since the mounting element 84 is a low-friction device, if the clamp element 82 is not properly aligned with the center when attempting to attach it to the bottom collar 44 or bottom collar holder and vacuum unit 52, the force of the bottom collar 44 or bottom collar holder and vacuum unit 52 pushing the clamp element 82 will move the clamp element 82 to the aligned position instead of the bottom collar 44 or bottom collar holder and vacuum unit 52 moving. The mounting element 84 may further include a locking mechanism that can be engaged and disengaged to prevent the clamp element 82 from moving once the clamp element 82 is attached to the bottom collar 44 or bottom collar holder and vacuum unit 52. While the clamp element 82 is being moved to a predetermined position, the locking mechanism is released, and as a result, the clamp element 82 can be moved by the motor, while still being displaced by any additional force applied to the clamp element 82. When the clamp element 82 comes into contact with the bottom collar 44 or the bottom collar holder and vacuum unit 52, the locking mechanism engages to prevent further movement of the clamp element 82 in the XY plane.

[0061] To detect misalignment, in one embodiment, the gripper system 80 further includes a force sensing device, such as a load cell, to sense and measure the reaction force generated during the process of attaching the clamp element 82 to the bottom collar 44 or the bottom collar holder and vacuum unit 52. The load cell is a transducer that converts the force applied to the clamp element 82 into an electrical signal relative to a strain gauge (not shown) of each load cell. The electrical signal can then be measured and correlated with the force applied to the strain gauge. Exemplary load cells include hydraulic load cells, pneumatic load cells, and strain gauge load cells. If the clamp element 82 is not properly aligned with the center of the bottom collar 44 or the bottom collar holder and vacuum unit 52, the reaction force will be greater than if the clamp element 82 were properly aligned. By measuring the reaction force using a force sensing device, misalignment can be detected and corrected before the clamp element 82 applies enough force to the bottom collar 44 or the bottom collar holder and vacuum unit 52 to cause movement of the bottom collar 44 or the bottom collar holder and vacuum unit 52. In one embodiment, the force sensing device may be used in conjunction with a low-friction mounting element 84, and the speed at which the clamping element 82 is attached to the bottom collar 44 or bottom collar holder and vacuum unit 52 is reduced in response to a larger-than-expected reaction force to allow the clamping element 82 to move to the alignment position on the mounting element 84. In an exemplary embodiment, the clamping element 82 can move toward the bottom collar 44 or bottom collar holder and vacuum unit 52 at a speed in the range of approximately 50 mm / min to approximately 100 mm / min while no misalignment is detected by the force sensing device, and the speed can be reduced to approximately 10 mm / min to approximately 25 mm / min if misalignment is detected. In other applications, the clamping speed may exceed these ranges.

[0062] In summary, the gripper system 80 helps support the weight of the base material assembly (which may be approximately 350 kg or more) and replaces conventional all-contact puller wheel systems. The gripper system 80 enables floating positioning of the glass body 20 used to fabricate the base material in the horizontal (XY) plane, as well as precise linear movement in the vertical (Z) direction for accurate alignment and control of the base material shape and the base material chipping process. In particular, when the gripper system 80 is incorporated, the apparatus 10 can avoid lateral or transverse forces acting on the base material, thereby minimizing, and possibly eliminating, warping of the base material, and can monitor the glass behavior during heating using load cells, and can use physics (conservation of mass) to precisely control dimensions (eliminating the costs of conventional online measurement and feedback control).

[0063] Furthermore, the apparatus 10 and related upward Collapse The process can be used in combination with a matrix measurement device. A suitable matrix measurement device is fully described in U.S. Patent No. 10,378,996, which claims priority to International Patent Application No. PCT / US2014 / 050368, “Methods And Apparatus For Determining Geometric Properties Of Optical Fiber Preforms,” filed by the assignee of this application on August 8, 2014.

[0064] A controller is a hardware device or software program that manages or directs (i.e., facilitates communication) the data flow between two components. Apparatus 10 includes a controller 88. The controller 88 provides, for example, the ability to acquire data from load cells 68, gripper systems 80, upper and lower collar holders and vacuum units 52, 54, and vacuum and processing gas systems, which use the data to control other components of apparatus 10 and related upward CollapseThe controller controls the process. The controller 88 is programmed with preset control programs or routines to efficiently ensure an optimal heating and transfer process recipe, in a manner well known to those skilled in the art. More specifically, the controller 88 can define, for example, speeds V1 and V2, gas flow rate, and vacuum pump pressure. The controller 88 helps ensure a robust and repeatable "one-button" automated process for manufacturing.

[0065] Upward Collapse A key advantage of the process is the minimization, and possibly elimination, of waveguide (cladding versus core) distortion. Waveguide distortion is caused by gravity and vacuum forces on the core rod and molten glass, which are inherent in conventional processes, and upward Collapse It is eliminated by the process. Waveguide distortion is a problem that is rarely addressed in the RIT / RIC field, even if it exists. The reason the problem is not recognized may be that past optical fiber performance requirements were not very stringent, and therefore, in this field, there was a tendency to treat the optical matrix like a simple glass rod without worrying about the effects of actual waveguide (cladding vs. core) distortion, which can, for example, cause fiber cutoff wavelength interference.

[0066] Globally connected devices, cloud services, 5G (fifth-generation mobile networks or fifth-generation wireless systems, which represent a major stage in mobile telecommunications standards), and Industry 4.0 (or the Fourth Industrial Revolution, the current trend of automation and data exchange in manufacturing technologies, including cyber-physical systems, the Internet of Things, and cloud computing), as well as other advancements, are driving exponentially increasing demand for bandwidth. Therefore, optical fiber manufacturers must increase their output and productivity. For next-generation optical fiber manufacturing, very large matrix materials that are stretched at high speeds are required. CollapseThe process results in a "stretch-ready" solid matrix that can withstand multi-day uninterrupted optical fiber stretching, increasing productivity and optical fiber output while reducing costs and achieving improved fiber yield for matrix users.

[0067] Upward Collapse The process naturally involves upward stretching (and optionally, stretching or compressing), as well as the upper collar 40 or bottom collar 44. Collapse This includes low-cost upward online chipping by doing so and by matching the outer diameter of the upper collar 40 to the outer diameter of the clad 32. Collapse These additional features of the process can be implemented far more accurately and inexpensively than conventional downward stretching processes, through precise physics of mass and glass flow preservation. Upward stretching / extension and tip features Collapse The process can also achieve nearly 100% lead-tipped Collapse It is worth noting that using this process results in a good base glass yield of nearly 100% without the wasteful sacrificial starting material used to initiate the conventional downward stretching process. Furthermore, the consumption of material used for the upper color 40 and bottom color 44 is also reduced. Collapse It is minimal in the process.

[0068] Above a single overclad Collapse The process is described above. However, this process can be applied to multiple overclad "gap" jacketed tubes or cylinders by applying minor modifications such as increasing the outer diameter of the spacer 48 and slightly adjusting the maximum heating power and termination power. Furthermore, since the weight of the stacked core rods 30 is fully supported from below, upward CollapseThe process can also accommodate cladding 32 that is twice (or even three times or more) in length, with the clear advantage that it does not require matching of the cladding inner diameters (i.e., a smaller inner diameter of the lower cladding to support the core rod inside the upper cladding, as in the case of conventional downward stretching processes). Collapse The process can also be applied to produce MCF.

[0069] MCFs are used in telecommunications and fiber sensors. Figure 3 shows a multicore fiber 100 having two cores. In each core, the core region is directly adjacent to a common outer cladding region. The first core includes a core region 120 and a common outer cladding 136. The core region 120 has a radius r1 indicated by 124. The second core includes a core region 130 and a common outer cladding 136. The core region 130 has a radius r2 indicated by 134. The common outer cladding region 136 has a radius R indicated by 114, and the distance between the centerlines of the core regions 120 and 130 is indicated by 116.

[0070] Polarization mode dispersion (PMD) is a problem for MCF. PMD is a form of mode dispersion in which two different polarizations of light in a waveguide, which normally travel at the same speed, travel at different speeds due to random imperfections, asymmetries, and geometric distortions, causing random diffusion of the light pulse. Difficult to prevent, but unless prevented (or at least minimized) or compensated for, PMD ultimately limits the rate at which data can be transmitted through the fiber.

[0071] In an ideal optical fiber, the core has a perfectly circular cross-section. Ideally, the fundamental mode has two orthogonal polarizations (directions of electric fields) propagating at the same speed. The signal transmitted through the fiber is randomly polarized (i.e., a random superposition of these two polarizations), but this is not a problem in an ideal fiber because the two polarizations propagate identically (degenerate). However, in real fibers, there are random imperfections that break the circular symmetry, causing the two polarizations to propagate at different speeds. In reality, the two polarization components of the signal slowly separate, spreading and overlapping the pulses. Since the imperfections are random, the pulse-spreading effect corresponds to a random walk and therefore has an average polarization-dependent time-derivative Δτ (also called differential group delay) proportional to the square root of the propagation distance L.

number

number

[0072] The random imperfections in symmetry fracture that cause PMDs fall into several categories. Firstly, there is stress-induced material birefringence, where the refractive index itself depends on the polarization. Secondly, there are geometric distortions or asymmetries, such as cladding noncircularity and core ellipticity (particularly with respect to the peripheral core) with respect to the cross-sectional shape of the MCF matrix. Both of these effects can be attributed to either imperfections in manufacturing (never perfect or lacking stress) or thermal and mechanical stresses applied to the fibers in situ (furthermore, the latter stresses generally change over time). The manufacturing processes of this disclosure focus on reducing geometric distortions.

[0073] The problems related to the non-circularity of fiber cladding are (1) alignment errors and bonding losses during fiber bonding, and (2) its negative effect on the ellipticity of the fiber core. Fiber core ellipticity, particularly the ellipticity of the peripheral core in MCF, is strongly correlated with an increase in PMD of the fiber. Therefore, it is extremely important to minimize core ellipticity and cladding non-circularity in MCF matrix materials with a circular cross-sectional shape.

[0074] Figures 4A (7-core MCF) and 4B (4-core MCF) show two examples of MCF matrix cross-sectional images obtained by tomographic refractive index profile measurements. The example of the 7-core MCF in Figure 4A clearly shows both the cladding non-circularity of the matrix outer diameter (i.e., the hexagonal shape) and the core ellipticity of the six peripheral cores. The cladding non-circularity (NC) is approximately 0.24%. Table I below summarizes the core ellipticity (indicated as "internal core NC%") for each of the seven cores.

[0075] [Table 1]

[0076] In the example of a 7-core MCF, the outer diameter profile of the base material is close to hexagonal, and the peripheral core ellipticity reaches about 20%, which is very detrimental to the PMD performance of the fiber.

[0077] The geometric distortion in the 4-core MCF example in Figure 4B is not as severe as in the 7-core MCF example. The cladding noncircularity is approximately 0.12%. Table II below summarizes the core ellipticity for each of the four cores.

[0078] [Table 2]

[0079] The core ellipticity of an example 4-core MCF is in the range of 3%–4%. This range is acceptable with the help of a PMD compensation device, such as the coherent 40 Gbit / s dual-polarization orthogonal phase-shift keying (DP-QPSK) transceiver available from Nortel (Ottawa, Canada). See C. Laperle et al., "WDM Performance and PMD Tolerance of a Coherent 40-Gbit / s Dual-Polarization QPSK Transceiver," Journal of Lightwave Technology, Vol.26, No.1 (Jan.1, 2008). PMD compensation devices use a polarization controller to compensate for PMDs in the fiber. Essentially, the device splits the fiber output into two principal polarizations (typically polarizations without a first-order variation in time delay with frequency) and applies a differential delay to resynchronize them. Since PMD effects are random and time-dependent, this compensation requires an active device that responds to feedback over time. Such requirements make compensation devices relatively expensive and complex. Combined with the fact that PMD is still not a limiting factor at the lower data rates commonly used, this means that PMD compensation devices have seen limited deployment in large-scale communication systems. Therefore, it is highly desirable to minimize non-circular or elliptic geometric distortions in the MCF matrix so that fiber PMD and bandwidth performance are maximized.

[0080] The objective of the disclosed process is to prevent, or at least minimize, two types of geometric distortions, namely, the non-circularity of the base material and the ellipticity of the core. In other words, the objective is to achieve a circular MCF base material with minimal core ellipticity, particularly for the peripheral core. For a given MCF design, these distortions in non-circularity and ellipticity are minimized during the disclosed base material manufacturing process. This process has two main aspects: (1) by appropriately designing the gap between the core rod and the hole in the perforated cylinder and optimizing the diameter of the perforated cylinder, and (2) by the core rod and cladding Collapse Non-circularity and ellipticity are minimized by optimizing heat treatment conditions based on a precise understanding and modeling of the temperature gradient, viscosity gradient, and glass flow during the process. The descriptions of each of these two aspects are as follows:

[0081] A. Optimization of geometric parameters Three different manufacturing processes can be distinguished from each other when producing MCF. The three alternative processes are the "stack-and-draw" process, the "grain-in-tube" process, and the perforation process. Each of the three processes will be described in turn below.

[0082] In the "stack-and-draw" process, individual cores and filler rods are stacked in a hexagonal shape, melted together in a cladding tube under vacuum, and simultaneously stretched or drawn into a fiber. The stack-and-draw process is the subject of European Patent Publication No. 1002249(A1) (Corning Inc.) and European Patent Publication No. 2320256(A1) (Furukawa Co. Ltd.). The advantages of this process are that it is fast and does not require additional mechanical processing. The disadvantages are reduced design versatility (hexagonal packing) and higher crosstalk and optical loss due to scattering (numerous distorted internal interfaces). Volume scaling is also difficult to achieve using the stack-and-draw process.

[0083] The second process is the grain-in-tube process (also known as the powder-in-tube process or sand process). The sand process is the subject of Japanese Patent Publication No. 2019-081681A (Furukawa Corporation) and Japanese Patent No. 5995298B1 (Nippon Telegraph and Telephone Corporation). Individual cores are placed inside the jacketed tube according to design specifications, and the remaining cavity is filled with quartz glass powder. Controlling the geometric positioning accuracy of the cores on the one hand, and controlling the production of a defect-free cladding using the sand process on the other hand, is extremely difficult.

[0084] The third process (drilling process) involves a clad cylinder that is drilled without limitation according to the desired design specifications, and then the hole is filled with a core rod. In a subsequent step, the cylinder is placed on the core rod. Collapse It is then (optionally) stretched to the appropriate size, or directly stretched into a fiber. Collapse The stepping and stretching steps can also be performed in a single step. Eliminating surface defects and geometric artifacts is a challenge in the direct fiber stretching step of the drilling process.

[0085] From the geometric aspects of the fiber composition, essential factors are the absolute radial position and absolute azimuthal position of the core, as well as the variation of the core along the fiber. Therefore, the manufacture of a high-precision base material is highly desirable with respect to core size, position, and shape, as well as cladding circularity. The disclosed process can establish a robust overcladding process that fabricates precisely machined MCF cylinders and fuses the MCF cylinders with core rods to form a base material with high geometric accuracy. The perforation process used to fabricate porous cylinders is currently the most promising process for manufacturing high-precision multicore fibers with the greatest potential for volumetric scaling. In this disclosure, the largest cylinder bodies (having a diameter up to approximately 250 mm) are machined such that the maximum accuracy of the MCF core and cladding circularity can be achieved for the final MCF base material or fiber with the same absolute machining accuracy.

[0086] Therefore, the process involves drilling to produce a porous or perforated cylinder (with or without a central hole). The disclosed process begins by providing a glass cylinder (forming the cladding of the base material) having an OD in the range of about 150 mm to 250 mm, preferably about 200 mm or more. The relatively large size of the cylinder makes it easy to accurately drill holes in the cylinder. The error in hole position is less than a percentage of the larger cylinder's outer diameter (OD) size. The glass cylinder may be solid or may have a central hole (a central hole exists if a mandrel was used to make the glass cylinder). Peripheral or side holes (to receive the core rod) are carefully and very precisely formed (e.g., drilled) within the cylinder.

[0087] Once the perforated cylinder is obtained, the next step in the disclosed process is to fabricate a base material having the perforated MCF cylinder. A glass rod is inserted into the hole to fabricate a peripheral core (and optionally a central core). The MCF base material is then constructed by placing the perforated cylinder on a plurality of core rods. Collapse It is manufactured by optionally stretching this Collapse The step is either the conventional downward stretching process or the upward stretching process described above. Collapse The process can be completed using any of the following methods. To minimize waveguide (clad vs. core) distortion in the base material manufacturing process, it is preferable that the process incorporates an upward stretching process step (as described above) so as to minimize the difference in axial flow between the clad glass and core glass due to gravity.

[0088] In the upward stretching process, the freely moving core rods and the self-supporting core rods are conveniently stacked and supported from the bottom of the cylinder. Collapse The axial extensions are orthogonal to each other, providing complete and precise control of both radial and axial glass flow. Any desired matrix OD can be achieved with minimal strain in the cladding-to-core waveguide ratio and maximum precision in the radius / azimuth core position.

[0089] To prepare the base material, an MCF cylinder is placed on the core rod inside the drilled hole. Collapse The steps can be performed with or without stretching. The accuracy of the different geometric features from the MCF cylinder (ellipticity, warp, siding, eccentricity, and hole position, etc.) can be perfect without the stretching operation. Collapse This allows for maximum maintenance in the final base material. However, the OD of the base material is limited to being slightly smaller than the OD of the MCF cylinder (in the absence of stretching).

[0090] Therefore, the MCF cylinder, which has a core rod inside the drilled hole, is heated to produce the base material while Collapse And it is preferable that it be stretched. Collapse The stretching operation provides greater flexibility, enabling the production of a matrix OD within a range that can be adapted to the stretching capabilities of the fiber manufacturer. (Typically, the matrix is ​​later stretched into an optical fiber.) The stretching step reduces the OD of the cylinder. The step of forming holes in the MCF cylinder is configured to form peripheral holes in place, taking into account the positional variations of the core before and after the step of heating the core rod and glass cylinder. As a result, this operation makes it possible to easily produce an MCF matrix in which multiple cores are precisely positioned in place as designed in the specifications.

[0091] The ellipticity (or non-circularity) of the peripheral core within the matrix can be calculated using one of four alternative equations. Specifically, ellipticity = (1) (maximum diameter - minimum diameter) ÷ average diameter, or (2) change in the major axis + change in the minor axis. Ellipticity is also (3) the approximate sum of the effects of the azimuthal and radially inward glass flow. This third equation calls the cylindrical coordinate system a three-dimensional coordinate system in which the point position is specified by the distance from a selected reference axis, the direction from the axis relative to a selected reference direction, and the distance from a selected reference plane perpendicular to the axis. The latter distance is given as a positive or negative number depending on which side of the reference plane faces the point. The distance from the axis is sometimes called the radial distance or radius, and the angular coordinate is sometimes called the angular position or azimuthal angle.

[0092] Finally, the ellipticity is (4) approximately α(g / d) + β(G / D), where α and β are proportionality constants that depend on the process conditions and the MCF geometric design, D is the diameter of the cylinder, G is the gap between the central core rod and the central hole, d is the diameter of the peripheral hole, and g is the gap between the peripheral core rod and the peripheral hole. Thus, from the standpoint of preparing the core rod and the holy cylinder, it is clear that the following steps directed at the geometric parameters should help reduce the peripheral core ellipticity: (a) reduce the total overclad gap G and the peripheral hole spacing g, (b) increase the diameter D of the perforated cylinder, (c) reduce the effect g / d of the "crushing" or "flattening" orientation glass flow of the core rod relative to the main axis, and (d) reduce the effect G / D of the inward radial flow of the core rod glass relative to the minor axis.

[0093] In these steps, the process maintains parameters g and G below approximately 4 mm, but increases them above a threshold (e.g., 0.2 mm) to make core rod insertion into the hole more practical. Thus, parameters g and G may be in the range of approximately 0.2 mm to 4 mm, more preferably approximately 0.2 mm to 3 mm, even more preferably approximately 0.2 mm to 2 mm, and most preferably approximately 0.2 mm to 1 mm. Parameters g and G may further be in the range of approximately 0.3 mm to 4 mm, more preferably approximately 0.3 mm to 3 mm, even more preferably approximately 0.3 mm to 2 mm, and most preferably approximately 0.3 mm to 1 mm. Parameters g and G may further be in the range of approximately 0.5 mm to 4 mm, more preferably approximately 0.5 mm to 3 mm, even more preferably approximately 0.5 mm to 2 mm, and most preferably approximately 0.5 mm to 1 mm. Parameters g and G may further be in the range of approximately 0.6 mm to 1 mm, and more preferably approximately 0.6 mm to 0.8 mm. The diameter (D) of the perforated cylinder should be as large as possible, at least about 200 mm or larger.

[0094] Therefore, the disclosed process minimizes non-circularity and ellipticity by optimizing geometric parameters. Specifically, the gap between the core rod and the hole in the perforated cylinder and the diameter of the perforated cylinder are optimized. In a second main embodiment, the disclosed process involves the core rod and cladding Collapse Non-circularity and ellipticity are minimized by optimizing heat treatment conditions based on a precise understanding and modeling of temperature and viscosity gradients, as well as glass flow, during the process.

[0095] B. Optimization of heat treatment conditions Another way to minimize the non-circularity of the cladding and the ellipticity of the core for a given design of the base material assembly (perforated cylinder + core rod) is to place a holy cylinder on the core rod. Collapse This involves understanding the precise glass flow during the process and therefore optimizing the heat treatment recipe or conditions accordingly. To understand and verify the complex glass flow, a perforated cylinder is used. CollapseFurthermore, multiphysics FEM (finite element modeling) has been used for the stretching process. Using the 7-core MCF matrix in Figure 4A as an example, which has severe peripheral core ellipticity and hexagonal cladding, as shown in Figure 5, 2D Collapse A model was created for only that.

[0096] By utilizing the symmetry of the base material, only half of the geometric shape is modeled. A constant oven temperature (e.g., 2,200°K, 2,300°K) is assumed to provide radiant heating to the outer surface of the perforated cylinder. As a method for simulating throughput effects, a specific initial temperature is assumed for the entire perforated cylinder to form a specific radial temperature gradient (higher throughput results in a higher radial temperature gradient). The radial temperature is measured at the edge of the base material at time t = 100 seconds, as shown in Figure 5. For all simulated examples, at t = 100 seconds, under vacuum... Collapse To simulate this, a decompression of 950 mbar is applied within 1 second.

[0097] Approximates the peripheral core shape of Figure 4A Collapse From random cases of the model resulting in the peripheral hole shape and the overall cladding shape approaching a hexagon, a field of glass flow velocity magnitudes as shown in Figure 6 is observed. It is clear that the maximum glass flow velocity occurs between the peripheral hole and the outer surface of the perforated cylinder, which is precisely why the peripheral hole and core are crushed. This observation indicates that the viscosity of the glass between the peripheral hole and the outer surface of the cylinder is much lower than the viscosity of the inner portion of the cylinder, particularly the viscosity of the portion between the central hole and the peripheral hole. This is because, CollapseThis indicates that the radial temperature gradient within the perforated cylinder is too large, resulting in a large radial viscosity gradient. In practice, to reduce this radial temperature gradient, it is necessary to reduce the glass throughput, i.e., the mass flow rate through the furnace, so that the process has more time for heat to transfer radially and thus better homogenizes the radial temperature field. This insight leads to the use of different initial temperature levels in the perforated cylinder, as described in the model setting above, but under vacuum. Collapse The process is Collapse To simulate different levels of radial temperature gradients within the system, and as a way to approximate the effects of different levels of throughput, the process is initiated at a fixed time (t=100 seconds). [Examples]

[0098] To further illustrate the overall nature of this disclosure, the following examples are provided. These examples are illustrative and not limiting to this disclosure. FEM is used to develop the examples through simulation.

[0099] A large difference between the oven temperature and the initial temperature of the perforated cylinder is used to obtain a relatively large radial temperature gradient and to simulate a higher throughput effect. In the three cases shown in Figures 7-9, the oven temperature is varied from 2,400°K to 2,300°K and then to 2,200°K, while maintaining the same difference (800°K) between the oven temperature and the initial cylinder temperature.

[0100] Figure 7A is a graph of the radial temperature gradient, and Figure 7B shows the deformation of the perforated cylinder in the first case. The conditions for the first case are oven temperature = 2,400°K, initial cylinder temperature = 1,600°K, and vacuum start at t = 100 seconds. The average radial temperature gradient is 5.2°K / mm.

[0101] Figure 8A is a graph of the radial temperature gradient, and Figure 8B shows the deformation of the perforated cylinder in the second case. The conditions for the second case are oven temperature = 2,300°K, initial cylinder temperature = 1,500°K, and vacuum start at t = 100 seconds. The average radial temperature gradient is 6.3°K / mm.

[0102] Figure 9A is a graph of the radial temperature gradient, and Figure 9B shows the deformation of the perforated cylinder in the third case. The conditions for the third case are oven temperature = 2,200°K, initial cylinder temperature = 1,400°K, and vacuum start at t = 100 seconds. The average radial temperature gradient is 7.9°K / mm.

[0103] Therefore, from Figures 7 to 9, it is observed that at t=100 seconds when vacuum is applied, the average temperature gradients are 5.2°K / mm, 6.3°K / mm, and 7.9°K / mm, respectively. These relatively large temperature gradients result in severe peripheral core ellipticity, as well as the hexagonal shape of the cladding, which is very close to the actual 7-core MCF matrix example shown in Figure 4A, under different oven temperature levels.

[0104] Since the model can reproduce the deformation of the surrounding core and the entire cladding during actual operation, the model can be used to demonstrate the advantages achieved by significantly reducing the temperature gradient during the manufacturing process of this disclosure. Figures 10 to 12 show three cases illustrating these advantages. Each of these cases is described below.

[0105] Figure 10A is a graph of the radial temperature gradient, and Figure 10B shows the deformation of the perforated cylinder in the first case. The conditions for the first case are oven temperature = 2,500°K, initial cylinder temperature = 2,200°K, and vacuum start at t = 100 seconds. The average radial temperature gradient is 1.1°K / mm.

[0106] Figure 11A is a graph of the radial temperature gradient, and Figure 11B shows the deformation of the perforated cylinder in the second case. The conditions for the second case are oven temperature = 2,300°K, initial cylinder temperature = 2,000°K, and vacuum start at t = 100 seconds. The average radial temperature gradient is 1.6°K / mm.

[0107] Figure 12A is a graph of the radial temperature gradient, and Figure 12B shows the deformation of the perforated cylinder in the third case. The conditions for the third case are oven temperature = 2,200°K, initial cylinder temperature = 1,900°K, and vacuum start at t = 100 seconds. The average radial temperature gradient is 2.0°K / mm.

[0108] As shown in Figures 10-12, under different oven temperatures, the difference between the oven temperature and the initial oven temperature was kept constant at 300°K, resulting in temperature gradients of 1.1°K / mm, 1.6°K / mm, and 2.0°K / mm, respectively. It is observed that these significantly low radial temperature gradients greatly reduce the radial "crushing" effect of the periphery holes. Thus, the circular shape of both the periphery holes and the entire cladding is preserved much better. However, it should be noted that if the oven temperature is too high, i.e., 2,500°K, even under the low radial temperature gradient as shown in Figure 10A, the periphery holes may be compressed in the azimuthal direction instead of being crushed radially. Figures 12A and 12B show the case approximating the best condition in which the periphery hole shape is most circular among the three cases with small temperature gradients.

[0109] Therefore, in order to minimize the non-circularity of the cladding within the MCF base material and the ellipticity of the holes and core (especially the peripheral holes and core), the process of the present disclosure involves placing a cylinder onto a core rod. CollapseIn addition, the radial temperature gradient of the perforated cylinder is kept at a low level. Reducing glass throughput is a good practical way to reduce this radial temperature gradient. According to FEM results, in the case of the 7 core in Figure 4A, the radial temperature gradient that causes deformation of the peripheral core needs to be reduced by about 70% in order to preserve the circular shape of the cladding and peripheral core. Furthermore, a moderate oven temperature (e.g., about 2,200°K) or the lowest possible oven temperature is preferred, as this helps to prevent the peripheral holes from being compressed / crushed in the azimuthal direction and to achieve the best results.

[0110] As mentioned above, certain base material designs have a central hole; other designs do not. To investigate the effect of central hole size on the performance of the disclosed process, additional FEM simulations were completed for seven hole examples (one central hole and six peripheral holes). Figures 13–17 show the effect of central hole size for relatively low radial temperature gradients. The conditions for Figures 13–17 are oven temperature = 2,200°K, initial cylinder temperature = 1,900°K, and vacuum start at t = 100 seconds. The central hole diameters simulated were 24 mm (Figure 13), 33 mm (Figure 14), 35.92 mm (equal radial thickness on two sides of the peripheral core, Figure 15), 42.5 mm (original value; Figure 16), and 51 mm (Figure 17). Observations regarding cladding non-circularity and peripheral hole ellipticity for each simulation are summarized in Table III.

[0111] [Table 3]

[0112] Figures 18 to 23 show the influence of the central hole size on a relatively high radial temperature gradient. The conditions of Figures 18 to 23 are an oven temperature = 2,400°K, an initial cylinder temperature = 1,600°K, and a vacuum start at t = 100 seconds. Central hole diameters of 24 mm (Figure 18), 33 mm (Figure 19), 42.5 mm (original value, Figure 20), 45 mm (Figure 21), 48 mm (Figure 22), and 51 mm (Figure 23) were simulated. The observation results regarding clad non-circularity and peripheral hole ellipticity for each simulation are summarized in Table IV.

[0113]

Table 4

[0114] Many conclusions can be drawn from the data shown in Figures 13 to 23. First, the influence of the central hole size on the deformation of the peripheral holes is significant. When the central hole is relatively small and thus the thickness (t1) between the central hole and the peripheral holes is much larger than the thickness (t2) between the peripheral holes and the outer surface of the clad, the peripheral holes tend to be radially crushed. Under a relatively low radial temperature gradient, the scenario that best maintains the circular shape of the peripheral holes is when the thickness (t1) between the central hole and the peripheral holes is equal to or slightly smaller than the thickness (t2) between the peripheral holes and the outer surface of the clad. The best scenario is estimated as 0.7*t2 < t1 ≦ t2 under a relatively low temperature gradient. When t1 is much smaller than t2, i.e., when the central hole is much larger than the peripheral holes, the peripheral holes tend to be compressed in the azimuthal direction. On the other hand, when t1 is much larger than t2, the peripheral core tends to be radially crushed. Finally, when the gap (diameter difference) between the peripheral holes / peripheral cores is 1.5% of the clad OD (i.e., 2 mm / 134 mm), the peripheral core ellipticity can be maintained at ≦ 0.2% using the best recipe combining a low temperature gradient and 0.7*t2 < t1 ≦ t2.

[0115] To investigate the effect of cladding size on the performance of the disclosed process, additional FEM simulations were completed for seven hole examples (one central hole and six peripheral holes). Figures 24–26 show the effect of cladding size on relatively low radial temperature gradients. The conditions and observations for Figures 24–26 are summarized below.

[0116] The simulation conditions shown in Figure 24 are oven temperature = 2,200°K, initial cylinder temperature = 1,900°K, and vacuum start at t = 100 seconds. The diameter of the central bore is 42.5 mm (original value), and the cladding OD is 134 mm. Figure 24 shows that the peripheral core radial compression effect is significantly reduced, and the cladding circular shape is well preserved.

[0117] The simulation conditions shown in Figure 25 are oven temperature = 2,200°K, initial cylinder temperature = 1,900°K, and vacuum start at t = 120 seconds (to allow time to achieve a radial temperature gradient of approximately 2.0°K / mm). The 134mm clad OD design is proportionally scaled up to a 167mm OD design. Figure 25 shows that the peripheral core radial compression effect is significantly reduced and the clad circular shape is well preserved.

[0118] The simulation conditions shown in Figure 26 are oven temperature = 2,200°K, initial cylinder temperature = 1,900°K, and vacuum start at t = 145 seconds (to allow time to achieve a radial temperature gradient of approximately 2.0°K / mm). The 134mm clad OD design is proportionally scaled up to a 200mm OD design. Figure 26 shows that the peripheral core radial compression effect is significantly reduced and the clad circular shape is well preserved.

[0119] Important conclusions can be drawn from the data shown in Figures 24 to 26. For example, when a perforated cylinder design is proportionally scaled from an OD of 134 mm to an OD of 200 mm, there is essentially no change in the circularity of the cladding or the ellipticity of the peripheral holes, as long as the radial temperature gradient remains the same.

[0120] To investigate the effect of temperature gradients on the performance of the disclosed process, additional FEM simulations were completed for a 4-hole example without a central hole. Figures 27–29 show the effect of relatively low radial temperature gradients. The conditions and observations for Figures 27–29 are summarized below.

[0121] The simulation conditions shown in Figure 27 are: oven temperature = 2,400°K, initial cylinder temperature = 1,600°K, and vacuum start at t = 65 seconds. The mean radial temperature gradient is 6.0°K / mm. The peripheral hole diameter is 43 mm, the distance to the center of the cladding is 33.94 mm, and the cladding OD is 150 mm. Figure 27 shows that the radial compression effect of the peripheral core is significant, and the circular shape of the cladding deforms into a square shape.

[0122] The simulation conditions shown in Figure 28 are oven temperature = 2,200°K, initial cylinder temperature = 1,900°K, and vacuum start at t = 65 seconds. The mean radial temperature gradient is 2.0°K / mm. The peripheral hole diameter is 43 mm, the distance to the center of the cladding is 33.94 mm, and the cladding OD is 150 mm. Figure 28 shows that the peripheral core radial compression effect is not very significant, the flattened edges are instead near the center of the cladding, and the cladding circular shape is well preserved.

[0123] The simulation conditions shown in Figure 29 are oven temperature = 2,200°K, initial cylinder temperature = 2,000°K, and vacuum start at t = 65 seconds. The mean radial temperature gradient is 1.2°K / mm. The peripheral hole diameter is 43 mm, the distance to the center of the cladding is 33.94 mm, and the cladding OD is 150 mm. Figure 29 shows that the peripheral core radial compression effect is not very significant, the flattened edges are instead near the center of the cladding, and the cladding circular shape is well preserved.

[0124] Several conclusions can be drawn from the data presented in Figures 27–29 for a 4-core design without a central hole. Firstly, when the radial temperature gradient is relatively high, the peripheral holes are radially compressed toward the outer surface of the cladding at their flattened edges, and the circular shape of the cladding deforms into a square shape. Secondly, when the radial temperature gradient is relatively low, the peripheral holes are radially compressed toward the center of the cladding at their flattened edges, and the circular shape of the cladding is well preserved. This was the preferred scenario and was the case in actual operation.

[0125] To investigate the effect of the central hole on the performance of the disclosed process, additional FEM simulations were completed for a 4-hole example. Figures 30 and 31 show the effect of the central hole for a relatively low radial temperature gradient. The conditions for Figures 30 and 31 are oven temperature = 2,200°K, initial cylinder temperature = 2,000°K, and vacuum start at t = 65 seconds. The mean radial temperature gradient is 1.2°K / mm. The peripheral hole diameter is 43 mm, the distance to the center of the cladding is 33.94 mm, and the cladding OD is 150 mm. The design in Figure 30 does not have a central hole. Figure 30 shows that the peripheral core radial crushing effect is not very significant, the flattened edges are instead near the center of the cladding, and the cladding circular shape is well preserved. The design in Figure 31 has a central hole with a diameter of 20 mm. Figure 31 shows that adding a central hole much smaller than the peripheral hole in this case does not help much in reducing the peripheral hole ellipticity.

[0126] To investigate the effect of cladding size on the performance of the disclosed process, additional FEM simulations were completed for a 4-hole example without a central hole. Figures 32 and 33 show the effect of cladding size for relatively low radial temperature gradients. Common conditions for Figures 32 and 33 are oven temperature = 2,200°K, initial cylinder temperature = 2,000°K, mean radial temperature gradient = 1.2°K / mm, peripheral hole diameter = 43 mm, and distance to cladding center = 33.94 mm. The design in Figure 32 starts vacuuming at t = 65 seconds and has a cladding OD = 150 mm. The design in Figure 33 starts vacuuming at t = 85 seconds and has a cladding OD = 200 mm. Both Figures 32 and 33 show that the peripheral core radial crushing effect is not very significant, the flattened edges are instead near the center of the cladding, and the cladding circular shape is well preserved.

[0127] Important conclusions can be drawn from the data shown in Figures 32 and 33. For example, if the perforated cylinder design is proportionally scaled from an OD of 150 mm to an OD of 200 mm, there is essentially no change in the circularity of the cladding or the ellipticity of the peripheral holes, as long as the radial temperature gradient remains the same.

[0128] Figure 34 is a flowchart summarizing the steps of one embodiment of the disclosed process 500. In the first step 510, a glass cylinder having an OD of at least about 200 mm is provided. In the second step 520, a perforated cylinder is fabricated by drilling holes in the glass cylinder. The holes include peripheral holes and optionally a central hole. Step 520 is completed while maintaining a gap (g) between the peripheral core rod and the peripheral hole in the range of about 0.2 mm to 4 mm. In the third step 530, a glass rod is inserted into the hole to fabricate a peripheral core (and optionally a central core). In the fourth step 540, the perforated (i.e., porous) cylinder is fabricated on multiple core rods. Collapse (and optionally extended). Step 540 is between the core rod and the cylinder Collapse The process is completed in the plane where it begins, while maintaining the mean radial temperature gradient within a specific range.

[0129] The radial temperature gradient may be in the range of approximately 0.5°K / mm to 4°K / mm, more preferably approximately 0.5°K / mm to 3°K / mm, even more preferably approximately 0.5°K / mm to 2°K / mm, and most preferably approximately 0.5°K / mm to 1°K / mm. The radial temperature gradient may further be in the range of approximately 1°K / mm to 4°K / mm, more preferably approximately 1°K / mm to 3°K / mm, and even more preferably approximately 1°K / mm to 2°K / mm. The radial temperature gradient may further be in the range of approximately 2°K / mm to 4°K / mm, more preferably approximately 2°K / mm to 3°K / mm. The radial temperature gradient may also be in the range of approximately 3°K / mm to 4°K / mm.

[0130] The manufacturing process disclosed above is reproducible, accurate, and cost- and time-efficient for industrial-scale multicore matrix materials and fibers. It is intended for the manufacture of MCF matrix materials and fibers for telecommunications applications.

[0131] The above description of preferred embodiments should be interpreted as illustrative rather than limiting the invention as defined by the claims. As is readily apparent, numerous variations and combinations of the features described above can be utilized without departing from the invention as defined in the claims. Such variations are not considered departures from the spirit and scope of the invention, and all such variations are intended to be included within the following claims. For example, all broad scopes described herein are expressly intended to include all narrow scopes contained within the broad scope. As will be understood by those skilled in the art, certain steps included in the process may be omitted. Certain additional steps may be added, and the order of the steps may be changed from the specific order described.

Claims

1. A process for manufacturing a multicore optical fiber matrix having a central vertical axis, a plurality of core rods each positioned within a core hole and extending along the vertical axis, and a common cladding covering each of the plurality of core rods, wherein the process is: The steps include providing a cylinder that forms the cladding of the base material, The steps include forming a plurality of peripheral core holes extending along the vertical axis within the cylinder, The steps include inserting each of the multiple core rods into the respective periphery core holes of the cylinder, The steps include: exposing the cylinder and the core rods to a heating element, thereby heating the cylinder into which the plurality of core rods are inserted into the respective core holes, thereby causing the cylinder to collapse onto the plurality of core rods and forming the base material; During the step of forming the plurality of peripheral core holes, the plurality of peripheral core holes are formed such that the gap (g) between the core rod and the peripheral core holes after insertion is maintained in the range of approximately 0.2 mm to 4 mm, and the mean radial temperature gradient is maintained at approximately 0.5 K / mm to 4 K / mm in the plane where the collapse between the core rod and the cylinder begins during the step of heating the cylinder.

2. The process according to claim 1, wherein the cylinder has an outer diameter of at least about 200 mm.

3. The process according to claim 2, wherein the cylinder has an outer diameter in the range of approximately 200 mm to 250 mm.

4. The process according to claim 1, wherein the gap (g) is maintained in the range of about 0.3 mm to 1 mm during the step of forming the plurality of peripheral core holes.

5. The process according to claim 1, wherein the mean radial temperature gradient is maintained at approximately 1 K / mm to 2 K / mm in the plane where the collapse between the core rod and the cylinder begins during the step of heating the cylinder.

6. The process according to claim 1, wherein the step of forming the plurality of peripheral core holes includes drilling the plurality of peripheral core holes.

7. The process according to claim 1, wherein the step of heating the cylinder includes simultaneously stretching the cylinder and the core rod while collapsing the cylinder onto the core rod.

8. The process according to claim 1, wherein the step of heating the cylinder includes maintaining the heating element at a temperature of less than about 2,500 K.

9. The process according to claim 1, wherein the cylinder has a central core hole, a thickness (t1) between the central core hole and the peripheral core hole, and a thickness (t2) between the peripheral core hole and the outer diameter of the cladding, and the cylinder satisfies the following relationship: 0.7 × t2 < t1 ≤ t2.

10. The step of heating the cylinder is performed as part of an upward stretching process in which the cylinder collapses onto the core rod in the core hole, and the upward stretching process is Both the cylinder and the core rod are supported from below, the weight of the cylinder and the core rod is completely supported from below, and the core rod does not move vertically relative to the cylinder when the cylinder collapses onto the core rod. This includes moving the cylinder and the core rod upward relative to the heating element so that the cylinder continuously collapses on the core rod as the cylinder and the core rod move upward, The process according to claim 1, wherein the flow rate difference between the cylinder and the core rod along the vertical axis due to gravity is minimized.

11. The process according to claim 10, wherein the upward stretching process includes simultaneously stretching the cylinder and the core rod while collapsing the cylinder on the core rod.

12. A process for manufacturing a multicore optical fiber matrix having a central vertical axis, a plurality of core rods each positioned within a core hole and extending along the vertical axis, and a common cladding covering each of the plurality of core rods, wherein the process is: The steps include providing a cylinder that forms the cladding of the base material, The steps include forming a plurality of peripheral core holes extending along the vertical axis within the cylinder, The steps include inserting each of the multiple core rods into the respective periphery core holes of the cylinder, A process comprising the steps of exposing the cylinder and the core rods to a heating element, thereby heating the cylinder with the plurality of core rods inserted into the respective core holes, thereby causing the cylinder to collapse on the plurality of core rods and forming the base material, wherein the mean radial temperature gradient is maintained at approximately 0.5 K / mm to 4 K / mm in the plane where the collapse between the core rods and the cylinder begins.

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