Reducing geometric distortion of multicore fiber preforms

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 relates generally to the field of optical fiber technology, and more particularly to preforms for multi-core fibers (MCFs), which enable increased bit-rate capacity in optical cables for telecommunications through space division multiplexing. [Background technology]

[0002] As disclosed in U.S. Patent Application Publication No. 2018 / 0145752 (and related European Patent Application 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 the assignee of the present application, Heraeus Quarzglas GmbH & Co. KG, the field of applied science and engineering related to the design and application of optical fibers is known as optical fiber technology. Optical fibers are flexible, transparent fibers made by stretching glass (silica) to a diameter slightly thicker than that of a human hair. Optical fibers are most often used to transmit light between two ends of the fiber and are widely used in optical fiber communications, allowing for transmission at high bandwidths (data rates) over longer distances than wire cables. Fibers are used in place of metal wires because signals travel along the fiber with reduced loss and higher capacity. In addition, fibers are also immune to electromagnetic interference, a problem that plagues metal wires. Fibers are also used for illumination and are wrapped in bundles so that they can be used to carry images, thus enabling observation in confined spaces, as in the case of fiberscopes. Specially designed fibers are also used in a variety of other applications, such as fiber optic 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 the phenomenon of 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 prefabricated blank in a furnace and stretching the blank into optical fiber. A single blank can potentially produce optical fiber lengths of 7,000 to 8,000 km.

[0004] Today, stringent optical fiber cutoff wavelength specifications must be met, and no yield loss can be tolerated to achieve these specifications. The cutoff wavelength can be defined as the wavelength below which a single-mode optical fiber behaves like a multimode fiber. Alternatively, 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 an answer to the increasing demand for bandwidth. SDM refers to using the transverse dimensions of the fiber to separate channels in optical fiber communication systems. MCF technology contains multiple cores within a single cladding. Each core of an MCF can support single or multiple modes, depending on the SDM method used. Typically, MCFs have four to eight cores, although other numbers are possible. When the cores are relatively far apart, the overlap of their individual modes is negligible, and the multicore fiber behaves as a bundle of single-mode fibers. However, when the cores are closely spaced, the overlap of modes cannot be ignored.

[0006] Japanese companies have been particularly active in the development of MCF technology. For example, Furukawa Electric Co., Ltd. has filed Patent Application No. 2016-191693 (issued as Patent No. 6560178) entitled "Method for Manufacturing a Multicore Fiber Preform and a Method for Manufacturing a Multicore Fiber." This patented method for manufacturing a multicore fiber preform includes preparing a cladding preform having a cylindrical glass preform with a plurality of through holes extending in the longitudinal direction of the glass preform; connecting a cylindrical member to one end of the cladding preform so as to be coaxial with the cladding preform; and inserting core preforms into each of the plurality of through holes in the cladding preform. The inner diameter of the cylindrical member is smaller than the diameter of the circumscribed circle of the outermost through hole among the plurality of through holes formed in the cladding preform. During the preparation process, the cladding preform has a communication structure formed to communicate the interior of the cylindrical member with the through hole among the plurality of through holes that overlaps at least a portion of the cylindrical member in a top view.

[0007] Sumitomo Electric Industries, Ltd. has been granted 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 a glass pipe is fabricated by forming multiple holes in a glass body, and a heating and integration step in which core rods containing core portions are inserted into each hole and the glass pipe is heated to integrate the core rods and the glass pipe. In the hole-forming step, peripheral holes among the holes formed in the glass body are formed at positions determined in consideration of variations in the positions of the core portions before and after integration. More specifically, a multi-hole glass pipe / cylinder is used to form multiple core rods. Collapse The peripheral core shift relative to the center of the MCF preform is estimated when the MCF is rotated. 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 must be located on a line that connects the center of the peripheral core portion and the central axis of the glass body.

[0008] Sumitomo Electric Industries, Ltd. has also been granted U.S. Patent No. 10,520,668 (claiming priority to Patent Application No. 2018-61331) entitled "Method for Manufacturing Multi-Core Optical Fiber and Multi-Core Optical Fiber." This patent describes a method for manufacturing an MCF with a non-circular cross section, in which deformation of the cross-sectional area (CSA) of the preform is used by holes and gaps in the cladding. Specifically, the patent discloses forming recess-forming holes at positions different from the multiple core rod insertion holes to form a common cladding tube. Thus, the common cladding tube and the core rods are integrated, and the recess-forming holes are formed. Collapse This results in the formation of a core-clad composite having a non-circular cross section that includes a plurality of cores and a common clad.

[0009] Concave forming hole CollapseThe deformation of the MCF preform CSA from a circular shape due to the preform cladding is used to aid in the creation of MCF preforms with noncircular CSAs. This patent disclosure does not intend to prevent or reduce the noncircularity of the preform cladding or the ellipticality of the core. Rather, the goal is to use intentionally generated noncircularity of the preform CSA. The goal is to create MCFs with noncircular cross-sectional shapes to facilitate easier rotational alignment during MCF splicing. This disclosure does not address the detrimental effects on MCF fiber performance, such as polarization mode dispersion (PMD), caused by geometric distortion of the preform cladding noncircularity and core ellipticity.

[0010] Summary of the Invention To address the problems inherent in conventional MCF preform manufacturing processes, an objective of the disclosed MCF preform manufacturing process is to minimize (if not eliminate) geometric distortions of the preform cladding non-circularity and core ovality. Another objective is to manufacture a preform that minimizes such geometric distortions. A related objective is to provide a process that results in MCFs with improved PMD performance. Another objective is to provide a preform manufacturing process that allows for relatively easy and efficient production of MCF preforms with minimized geometric distortions. A related objective is to manufacture a preform with high precision with respect to core and cladding circularity.

[0011] To achieve these and other objects, and with that object in mind, the present disclosure provides a process for manufacturing an MCF preform having a central longitudinal axis, a plurality of core rods extending along the axis, each disposed within a respective core hole, and a common cladding covering each of the plurality of core rods. The process includes the following steps: A cylinder is provided that forms the cladding of the preform, the cylinder having an outer diameter of at least about 200 mm and may have a central core hole; Peripheral core holes extending along the longitudinal axis are formed in the cylinder; Each of the plurality of core rods is inserted into a respective peripheral core hole; The cylinder with the core rods inserted in their respective core holes is heated by exposing the cylinder and core rods to heating elements of a furnace, thereby heating the cylinder over the plurality of core rods. Collapse The gap (g) between the peripheral core rod and the peripheral holes is maintained in the range of about 0.2 mm to 4 mm during the step of forming the plurality of peripheral core holes, and the average radial temperature gradient is maintained in the range of about 0.2 mm to 4 mm during the step of heating the cylinder. Collapse is maintained at approximately 0.5°K / mm to 4°K / mm in the plane where it begins, or both.

[0012] It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the present disclosure. [Brief explanation of the drawings]

[0013] The present disclosure is best understood from the following detailed description when read in connection with the accompanying drawings. It is emphasized that, according to common practice, the various features of the drawings are not to scale. Conversely, dimensions of the various features have been arbitrarily increased or reduced for clarity. The drawings include the following figures: [Figure 1] FIG. 1 is a schematic diagram showing the main components of the apparatus used in the upward collapse process for forming elongated glass pieces. [Figure 2] FIG. 1 is a perspective side view of a glass body used to manufacture an optical component. [Figure 3]FIG. 1 shows a cross section of a multicore glass fiber having two core regions and a common outer cladding region. [Figure 4A] Figure 4A (7-core MCF) and Figure 4B (4-core MCF) show two example MCF preform cross-section images obtained by tomographic refractive index profile measurements, each showing both cladding non-circularity and core ellipticity. [Figure 4B] Figure 4A (7-core MCF) and Figure 4B (4-core MCF) show two example MCF preform cross-section images obtained by tomographic refractive index profile measurements, each showing both cladding non-circularity and core ellipticity. [Figure 5] FIG. 4B shows a 2D collapse-only finite element model of the 7-core MCF preform shown in FIG. 4A. [Figure 6] FIG. 4B shows the velocity magnitude field of glass flow during perforated cylinder collapse under vacuum for the 7-core MCF preform shown in FIG. 4A. [Figure 7A] 1 is a graph of a relatively large radial temperature gradient (average 5.2°K / mm) used as a first case to model the deformation of a perforated cylinder. [Figure 7B] Figure 7A shows the deformation of a modeled perforated cylinder for a relatively large temperature gradient, illustrating the severe peripheral core ovality as well as the hexagonal-like shape of the cladding, which closely resembles the actual 7-core MCF preform example shown in Figure 4A. [Figure 8A] 1 is a graph of a relatively large radial temperature gradient (average 6.3°K / mm) used as a second case to model the deformation of a perforated cylinder. [Figure 8B] Figure 8A shows the deformation of a modeled perforated cylinder for a relatively large temperature gradient, illustrating the severe peripheral core ellipticity as well as the hexagonal-like shape of the cladding, which closely resembles the actual 7-core MCF preform example shown in Figure 4A. [Figure 9A] 10 is a graph of a relatively large radial temperature gradient (average 7.9°K / mm) used as a third case to model the deformation of a perforated cylinder. [Figure 9B]Figure 9A shows the deformation of a modeled perforated cylinder for a relatively large temperature gradient, illustrating the severe peripheral core ovality as well as the hexagonal-like shape of the cladding, which closely resembles the actual 7-core MCF preform example shown in Figure 4A. [Figure 10A] 1 is a graph of the radial temperature gradient (average 1.1°K / mm) used as a first case to model the deformation of a perforated cylinder after significantly reducing the temperature gradient during the manufacturing process of the present disclosure. [Figure 10B] 10B shows the deformation of the perforated cylinder modeled for the significantly reduced temperature gradient of FIG. 10A, showing that the circular shape of both the peripheral holes and the overall cladding is much better preserved. [Figure 11A] 10 is a graph of the radial temperature gradient (average 1.6°K / mm) used as a second case to model the deformation of a perforated cylinder after significantly reducing the temperature gradient during the manufacturing process of the present disclosure. [Figure 11B] FIG. 11B shows the deformation of the perforated cylinder modeled for the significantly reduced temperature gradient of FIG. 11A, showing that the circular shape of both the peripheral holes and the overall cladding is much better preserved. [Figure 12A] 10 is a graph of the radial temperature gradient (average 2.0°K / mm) used as a third case to model the deformation of a perforated cylinder after significantly reducing the temperature gradient during the manufacturing process of the present disclosure. [Figure 12B] FIG. 12B shows the deformation of the perforated cylinder modeled for the significantly reduced temperature gradient of FIG. 12A, showing that the circular shape of both the peripheral holes and the overall cladding is much better preserved. [Figure 13] 16A and 16B show the effect of center hole sizes of 24 mm (FIG. 13), 33 mm (FIG. 14), 35.92 mm (FIG. 15), 42.5 mm (FIG. 16) and 51 mm (FIG. 17) on the performance of the disclosed process, according to FEM simulations, for relatively low radial temperature gradients. [Figure 14]16A-16D show the effect of center hole sizes of 24 mm (FIG. 13), 33 mm (FIG. 14), 35.92 mm (FIG. 15), 42.5 mm (FIG. 16) and 51 mm (FIG. 17) on the performance of the disclosed process, according to FEM simulations, for relatively low radial temperature gradients. [Figure 15] 16A-16D show the effect of center hole sizes of 24 mm (FIG. 13), 33 mm (FIG. 14), 35.92 mm (FIG. 15), 42.5 mm (FIG. 16) and 51 mm (FIG. 17) on the performance of the disclosed process, according to FEM simulations, for relatively low radial temperature gradients. [Figure 16] 16A-16D show the effect of center hole sizes of 24 mm (FIG. 13), 33 mm (FIG. 14), 35.92 mm (FIG. 15), 42.5 mm (FIG. 16) and 51 mm (FIG. 17) on the performance of the disclosed process, according to FEM simulations, for relatively low radial temperature gradients. [Figure 17] 16A-16D show the effect of center hole sizes of 24 mm (FIG. 13), 33 mm (FIG. 14), 35.92 mm (FIG. 15), 42.5 mm (FIG. 16) and 51 mm (FIG. 17) on the performance of the disclosed process, according to FEM simulations, for relatively low radial temperature gradients. [Figure 18] 20A-20D show the effect of center hole sizes of 24 mm (FIG. 18), 33 mm (FIG. 19), 42.5 mm (FIG. 20), 45 mm (FIG. 21), 48 mm (FIG. 22), and 51 mm (FIG. 23) on the performance of the disclosed process, according to FEM simulations, for relatively high radial temperature gradients. [Figure 19] 20A-20D show the effect of center hole sizes of 24 mm (FIG. 18), 33 mm (FIG. 19), 42.5 mm (FIG. 20), 45 mm (FIG. 21), 48 mm (FIG. 22), and 51 mm (FIG. 23) on the performance of the disclosed process, according to FEM simulations, for relatively high radial temperature gradients. [Figure 20]20A-20D show the effect of center hole sizes of 24 mm (FIG. 18), 33 mm (FIG. 19), 42.5 mm (FIG. 20), 45 mm (FIG. 21), 48 mm (FIG. 22), and 51 mm (FIG. 23) on the performance of the disclosed process, according to FEM simulations, for relatively high radial temperature gradients. [Figure 21] 20A-20D show the effect of center hole sizes of 24 mm (FIG. 18), 33 mm (FIG. 19), 42.5 mm (FIG. 20), 45 mm (FIG. 21), 48 mm (FIG. 22), and 51 mm (FIG. 23) on the performance of the disclosed process, according to FEM simulations, for relatively high radial temperature gradients. [Figure 22] 20A-20D show the effect of center hole sizes of 24 mm (FIG. 18), 33 mm (FIG. 19), 42.5 mm (FIG. 20), 45 mm (FIG. 21), 48 mm (FIG. 22), and 51 mm (FIG. 23) on the performance of the disclosed process, according to FEM simulations, for relatively high radial temperature gradients. [Figure 23] 20A-20D show the effect of center hole sizes of 24 mm (FIG. 18), 33 mm (FIG. 19), 42.5 mm (FIG. 20), 45 mm (FIG. 21), 48 mm (FIG. 22), and 51 mm (FIG. 23) on the performance of the disclosed process, according to FEM simulations, for relatively high radial temperature gradients. [Figure 24] 26 shows the effect of cladding sizes of 134 mm (FIG. 24), 167 mm (FIG. 25), and 200 mm (FIG. 26) on the performance of the disclosed process, according to FEM simulations, for relatively low radial temperature gradients. [Figure 25] 26 shows the effect of cladding sizes of 134 mm (FIG. 24), 167 mm (FIG. 25), and 200 mm (FIG. 26) on the performance of the disclosed process, according to FEM simulations, for relatively low radial temperature gradients. [Figure 26] 26 shows the effect of cladding sizes of 134 mm (FIG. 24), 167 mm (FIG. 25), and 200 mm (FIG. 26) on the performance of the disclosed process, according to FEM simulations, for relatively low radial temperature gradients. [Figure 27] 27A-27C show the effect of temperature gradients of 6°K / mm (FIG. 27), 2°K / mm (FIG. 28), and 1.2°K / mm (FIG. 29) on the performance of the disclosed process for an example four-hole preform without a center hole, via FEM simulation. [Figure 28] 27A-27C show the effect of temperature gradients of 6°K / mm (FIG. 27), 2°K / mm (FIG. 28), and 1.2°K / mm (FIG. 29) on the performance of the disclosed process for an example four-hole preform without a center hole, via FEM simulation. [Figure 29] 27A-27C show the effect of temperature gradients of 6°K / mm (FIG. 27), 2°K / mm (FIG. 28), and 1.2°K / mm (FIG. 29) on the performance of the disclosed process for an example four-hole preform without a center hole, via FEM simulation. [Figure 30] FIG. 30 (no center hole) and FIG. 31 (center hole) show the effect of a center hole on the performance of the disclosed process for an example four-hole preform, via FEM simulation. [Figure 31] FIG. 30 (no center hole) and FIG. 31 (center hole) show the effect of a center hole on the performance of the disclosed process for an example four-hole preform, via FEM simulation. [Figure 32] 32 and 33 show the effect of cladding sizes of 150 mm (FIG. 32) and 200 mm (FIG. 33) on the performance of the disclosed process for an example four-hole preform without a center hole, according to FEM simulations. [Figure 33] 32 and 33 show the effect of cladding sizes of 150 mm (FIG. 32) and 200 mm (FIG. 33) on the performance of the disclosed process for an example four-hole preform without a center hole, according to FEM simulations. [Figure 34] FIG. 1 is a flow diagram summarizing the steps of one embodiment of the disclosed process. DETAILED DESCRIPTION OF THE INVENTION

[0014] In this specification and in the claims that follow, reference will be made to a number of terms that shall be defined to have the following meanings ascribed to them.

[0015] The terms "include," "includes," "including," "have," "has," "having," "comprise," "comprises," "comprising," and the like mean including but not limited, i.e., inclusive and not exclusive.

[0016] The term "about" means that amounts, sizes, formulations, parameters, and other quantities and characteristics are not and need not be exact, but may be approximate and / or larger or smaller, as desired, to reflect tolerances, conversion factors, rounding, measurement error, and the like, as well as other factors known to those of ordinary skill in the art. When a value is described as approximately equal to or is approximately equal to a particular number, the value is within ±10% of that number. For example, a value of about 10 refers to a value between 9 and 11, inclusive. When the term "about" is used in describing values ​​or endpoints of a range, the disclosure is to be understood to include the particular value or endpoint. Regardless of whether a numerical value or an endpoint of a range herein is described as "about," the numerical value or endpoint of the range is intended to include two embodiments: those modified by "about" and those not modified by "about." It will be further understood that each endpoint of a range is significant both in relation to the other endpoint and independently of the other endpoint.

[0017] The term "about" also refers to all terms within a range unless otherwise specified. For example, about 1, 2, or 3 is equivalent to about 1, about 2, or about 3, further including about 1-3, about 1-2, and about 2-3. Specific and preferred values ​​disclosed for compositions, components, ingredients, additives, and similar embodiments, as well as ranges thereof, are for illustrative purposes only. They do not exclude other defined values ​​or other values ​​within defined ranges. The compositions and processes of the present disclosure include those having any recited value, or any combination of values, particular values, more specific values, and preferred values.

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

[0019] Directional terms used in this disclosure (e.g., up, down, right, left, front, back, top, bottom) are used only with reference to the figures depicted and the coordinate axes given in 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 an intervening material, while indirect contact refers to contact through one or more intervening materials. Elements in direct contact are in contact with each other. Elements in indirect contact are not in contact with each other, but are in contact with an intervening material or series of intervening materials, with at least one of the intervening material or series of intervening materials touching the other. Elements in contact may be firmly or non-firmly joined. "Contacting" 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] "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 glass fibers that include two or more cores surrounded by a cladding common to the two or more cores. The glass fibers function as waveguides.

[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 distinct 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 fiber is generally manufactured in two separate processes. First, a core rod is prepared, and then a preform 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 preform is heated in a furnace and drawn into an optical fiber. Conventional processes and equipment for manufacturing optical fiber preforms that complete the first of the two processes can include providing an optical fiber RIT overcladding apparatus.

[0024] The overclad device includes a vertical lathe, chucks installed at each end of the vertical lathe, a carriage in the vertical lathe for moving up and down between both ends of the vertical lathe, an oxygen-hydrogen burner installed on the carriage, a furnace installed on the carriage, a vacuum pump installed at one end of the vertical lathe, a coupler connecting the vacuum pump to one end of the vertical lathe, and a controller outside the vertical lathe for controlling the up and down movement of the carriage, the flow rate of the oxygen-hydrogen burner, and the rotation of the chuck. The furnace preheats or heats the glass tube in order to overclad the core rod with the glass tube.

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

[0026] More recently, quartz glass tubes, rods, or Collapse The base material for offline rod-in-cylinder (ORC) is a quartz glass component (e.g., a cylinder, ingot, or CollapseIt is produced by introducing quartz glass (unprocessed RIC) into an apparatus containing a heated zone (e.g., a furnace) in a vertical orientation so that the bottom end begins to soften and form a strand. The strand is then placed in a pulling device containing one or more pulling wheels. The rate at which the strand is drawn is controlled by the speed of the pulling wheels, which can apply either a downward or upward force depending on the forming zone temperature or the viscosity and weight of the strand supported by the wheels. Forming is achieved without the aid of a die. Thus, the strand dimensions are controlled by the feed rate of the quartz glass components, the temperature of the heated zone, and the speed of the pulling wheels.

[0027] In the conventional ORIC process, a cylinder made of synthetic high-purity glass (typically 3 m long and approximately 200 mm in outer diameter) is placed on a high-purity glass core rod. Collapse The optical fiber preform is then formed using heat and vacuum at the interface gap. The preform is then continuously drawn downward, typically to a diameter significantly smaller than the original diameter of the cylinder. Collapse Sufficient vacuum must be applied to the gap between the cylinder and the core rod to facilitate the core rod's movement and support the weight of the core rod through the softened glass. The vacuum is essential to prevent movement of the core rod relative to the cylinder. Otherwise, the cladding-to-core ratio of the resulting preform will be distorted, and the fiber drawn from the preform will not meet the required waveguide specifications (such as cutoff wavelength). Complex and expensive preform outer diameter measurement and feedback control are also required to prevent downward movement. Collapse Even with such control, it is difficult to achieve precise preform shape (including low preform bow or curvature and diameter variation) and waveguide properties without cladding-to-core distortion. This inherent waveguide distortion effect in the downward drawing process is due in large part to gravity and vacuum forces acting on the molten glass and the unattached core rod in the furnace, with the outer cladding glass being hotter and flowing downward faster than the inner core rod glass.

[0028] Using conventional down-draw systems and processes, there are significant difficulties in producing the largest preforms with outer diameters approaching the original cylinder or cladding size. If the preform geometry and waveguide properties are far from the required specifications with respect to parameters such as geometry, cladding-to-core ratio, core eccentricity, and warpage, a significant amount of good preform glass is wasted at the beginning and end of the process. Thus, conventional preform systems and processes have distinct drawbacks.

[0029] According to U.S. Patent Application Publication No. 2018 / 0145752, a device and an upwards process are described that produce preforms having the largest outer diameters and lengths known to exist (i.e., outer diameters of about 200 mm and lengths of about 3 m, or approximately the same size as the original cylinder or cladding, as opposed to conventional outer diameters that are limited to about 150 mm) with little waveguide (cladding vs. core) distortion, and with significantly reduced waste and cost. Collapse The conventional optical fiber preform has an outer diameter of 90 mm to 150 mm. Collapse In the process, the stacked core rods in the ORIC cladding are supported from below (thus the core rods Collapse The entire ORIC assembly moves upward relative to the furnace, so that the base material is continuously heated, as shown in Figure 1 and described below. Collapse The device and the upward stretch Collapse The process consisted of: (1) using the largest known overclad cylinder; Collapse(2) reduce costs due to a streamlined and simplified process (e.g., no need for online measurements or feedback control) including nearly 100% overclad and finished (chipped) preform yield (almost no waste) and an integrated online preform chipping process (saving processing time and heating steps); (3) improve waveguide quality due to inherently low waveguide (cladding to core) distortion due to variable and arbitrary length, fixed, stacked, and supported core rods; and (4) allow reactive gases (such as SF6) to be applied to the interface up to about 1 atmosphere (i.e., without the need for a vacuum) for improved interface and lower core rod D / d ratio (interface closer to the waveguide core).

[0030] The D / d ratio of a core rod 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 very important for those who manufacture optical fiber using RIT or RIC preforms when specifying core capacity expansion. As the D / d ratio of the core rod decreases, the interface moves closer to the waveguide core, which means that the relative amount of glass required in the core rod decreases (while the amount of glass in the cladding needs to increase). This, in turn, means that using the same core rod manufacturing equipment, its capacity to produce core rods (or equivalent capacity of optical fiber cores) 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 overclad material purity and interface quality due to the exponentially increasing optical power propagating there. 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. In short, a lower D / d ratio (i.e., the interface closer to the core) allows preform manufacturers to (a) easily expand core capacity without expensive investments, and (b) realize more complex and advanced optical fiber designs with refractive index characteristics closer to the core.

[0031] Referring to FIG. 1, an apparatus 10 for manufacturing an optical fiber preform is shown. The apparatus 10 includes a vertically arranged frame 12. From bottom to top, the frame 12 has a lower open end, a preheating 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 between about 500°C and 2,300°C, more preferably between about 1,000°C and 2,300°C, and most preferably between about 1,500°C and 2,300°C. More specifically, the heating element is preferably annular in configuration. The heating element is preferably positioned within 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 an elevated temperature to prevent oxidation of the heating element.

[0032] Referring to FIG. 2, a glass body 20 is used to manufacture an optical fiber preform. The glass body 20 is cylindrical or tubular in configuration. The glass body 20 has a length L extending from a first or upper end 22 to an opposite second or lower end 24. A longitudinal axis X extends between the opposite first and second ends 22, 24. Preferably, both the first and second ends 22, 24 of the glass body 20 are square-cut ends.

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

[0034] The cladding 32 may be pure silica glass or doped silica glass. However, the cladding 32 is preferably the highest purity synthetic silica, whether undoped or doped (e.g., with fluorine). The core rod 30 is preferably predominantly the highest purity silica glass, with doped and undoped regions to achieve the appropriate refractive index profile. The cladding 32 and core rod 30 may each be formed by any suitable process, such as fused silica or one or more types of chemical vapor deposition (CVD) (including internal, external, and axial deposition). The central core material 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 preform, resulting in an effective waveguide.

[0035] Returning to FIG. 1 , a first or top collar 40 is secured to the top of the cladding 32. While other mechanisms can be used to attach the top collar 40 to the cladding 32, a top weld 42 is suitable. The outer diameter of the top collar 40 is approximately the same as or smaller than the outer diameter of the cladding 32. A second or bottom collar 44 is secured to the bottom of the cladding 32. Although other mechanisms can be used to attach the bottom collar 44 to the cladding 32, a 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 cladding 32. Both the top collar 40 and the bottom collar 44 are hollow tubular components.

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

[0037] Referring to FIG. 1, an apparatus 10 is used to manufacture a preform. CollapseThe process will now be described. A glass body 20 passes through a frame 12 where it is heated, softened, and stretched to form an optical component, such as an optical fiber preform. 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 the glass body 20 then advances upward (i.e., as opposed to the conventional downward direction) through the frame 12. Within the frame 12, the glass body 20 is heated zone by zone in the heating zones 16. The preform passes through the overcladding gap 31. Collapse and continuously produced by melt deformation that fuses the core rod 30 into the overclad cylinder or cladding 32 (optionally, the base material can be stretched / extended or shortened / compressed by either tensile or compressive forces applied by the top collar holder and vacuum unit 54 and the bottom collar holder and vacuum unit 52 during the process).

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

[0039] As this embodiment of the coaxial assembly of glass body 20 advances upward through frame 12, core rod 30 and cladding 32 are heated to a predetermined temperature and time sufficient to soften and fuse the two glass components together to form a one-piece consolidated glass body 20. By "one-piece" is meant a single piece or member that is complete in itself, without additional pieces; i.e., the member is one monolithic piece formed as a unit with another member. More specifically, as successive portions of two-piece glass body 20 approach and are heated within heating zone 16, cladding 32 and core rod 30 soften, and the softened cladding 32 melts onto core rod 30. Collapse and fuses with the core rod 30. At least one, and more preferably multiple, "draw-ready" preforms can then be drawn directly into fiber from the resulting monolithic glass body 20.

[0040] Preferably, this embodiment of the coaxial arrangement of glass bodies 20 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 softening and Collapse This occurs at temperatures of about 1,000°C to 2,200°C, more preferably about 1,300°C to 2,000°C, and most preferably about 1,600°C to 1,800°C. Collapse The fusion of the softened cladding 32 with 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, one skilled in the art will appreciate that other factors, such as glass material composition and throughput, also affect the temperature at which the cladding 32 is fused onto the core rod 30. Collapse It will be appreciated that this will affect the temperature at which the core rod 30 fuses with the metal.

[0041] The fusion interface between the core rod 30 and the cladding 32 is ensured to be clean by several components of the apparatus 10. For example, the bottom collar holder and vacuum unit 52 and the top collar holder and vacuum unit 54 are both sealed and have an upward Collapse 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 potential contaminants in the heating elements (e.g., furnace) and the external environment, which are typical sources of contamination during conventional processes, especially vacuum-start processes, where it is difficult to avoid introducing contaminants to the interface. Additionally, reactive interface treatment gases can be used to etch, clean, and dry the interface.

[0042] A typical recipe used to heat the heating elements in heating zone 16 is approximately 50 kW for 30 minutes, approximately 100 kW for 10 minutes, approximately 150 kW for 10 minutes, approximately 200 kW for 10 minutes, and a process steady state of approximately 220 kW (or some lower maximum power, e.g., 212 kW). The bottom collar holder and vacuum unit 52, located at the bottom of the apparatus 10, moves at a speed V1, while the top collar holder and vacuum unit 54, located at the top of the apparatus 10, moves at a speed V2. Typically, at the start of the process, V1 = V2. A typical recipe reaches 100 kW in 2 minutes, and then 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 is raised again at approximately 13.5 mm / min until the top weld 42 reaches the center of the heating element. Once the top weld 42 reaches the center of the heating element, the assembly is stopped for approximately 6 minutes. The assembly is then placed in a steady state Collapse Therefore, V1 is raised again to V2.

[0043] When the top weld 42 is about 110 mm to about 135 mm above the center of the heating zone 16, the bottom collar holder and the vacuum pump of the vacuum unit 52 are activated (i.e., turned on). Such activation draws a vacuum in the direction of arrow 56, and the pressure within the top collar 40 begins to decrease. When the pressure within the top collar 40 stops decreasing, the top of the cladding 32 Collapse The gap 31 closes and the cladding 32 is sealed or fused to the core rod 30. At this time, a vacuum is maintained pumping on the bottom collar holder and vacuum unit 52 while the top collar 40 is backfilled with gas (e.g., nitrogen gas N) until a pressure of approximately 1 atmosphere is reached. The top collar 40 is then connected to air.

[0044] The vacuum pump of the upper collar holder and vacuum unit 54 can be activated (i.e., turned on) to pull a vacuum in the direction of arrow 58. Similarly, purging of the gas (typically an inert gas such as argon, helium, or most typically nitrogen) used in the heating elements of the heating zone 16 can be achieved by introducing the gas into the heating elements in the direction of arrow 60. The gas purge occurs between the outer surface of the glass body 20 and the surface of the heating elements to prevent soot generation on the outer surface of the glass body 20 and oxidation of the heating elements. The gas purge at the top of the heating elements is typically on from the start of the process. It is important to identify an appropriate purge rate (e.g., about 9 m3 / h) to prevent soot or other deposits from forming on the surface of the substrate during or after the process.

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

[0046] The process is complete when the bottom weld 46 is a short distance (e.g., about 50 mm) above the center of the heating element. At this point, power to the heating element is turned off completely, and movement of the assembly is simultaneously stopped. Complete sintering of the cladding 32 to the bottom edge 24 of the glass body 20 is completed. 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, maintaining the vacuum is not necessary, and maintaining the vacuum for an excessive amount of time may also risk deforming the bottom collar 44.

[0047] The load cell 68 is used to measure the total weight supported by the bottom collar 44. If a small, constant vibration perturbation is superimposed on the velocity V2 of the top collar and vacuum unit 54 while the velocity V1 of the bottom collar holder and vacuum unit 52 is kept constant, a "ripple" will appear on the load cell reading curve. The larger the amplitude of the "ripple," the colder the process. This is because at a colder process, the softened glass at the center of the heating element is more rigid and can transmit the vibration force further to the bottom of the assembly. For a given heating element power setting, this information indicates whether the process is slightly on the hotter or colder side due to the actual state of the heating element. Based on this knowledge, the end power of the process can be determined; i.e., the colder the process, the higher the end power required. This "ripple" amplitude is essentially a true viscosity measurement of the glass body 20 at the center of the heating element, and is much more reliable than any glass surface temperature measurement using a pyrometer.

[0048] Therefore, the device 10 and the associated upward CollapseThe process allows for viscosity measurement of the glass body 20 at the center of the heating element by imposing a vibrational motion. A small vibration is applied to a location on top of the matrix assembly. In parallel, the weight of the matrix assembly is measured by the load cell 68. The load cell 68 measurement provides an indirect measurement of the viscosity of the glass body 20 at the center of the heating element. This information can be used to control the temperature / heating power of the heating zone 16, for example, using a controller 88 (described below).

[0049] In a distinct departure from conventional downward drawing processes, the stacked core rods 30, instead of being supported by a vacuum, are supported by spacers 48 at the bottom of the stacked core rods 30, essentially fixing the position of the core rods 30 relative to the cladding 32 during the overcladding and drawing process. Collapse The process does not require a vacuum to prevent core rod movement, which can result in cladding-to-core waveguide distortion and therefore fiber cutoff wavelength issues. Furthermore, in contrast to conventional downward-draw processes, the weight of the glass both above and below the molten glass in the heating zone 16 is well supported by the top collar 40 and bottom collar 44 in the upward-draw process, which essentially eliminates the effects of cladding-to-core waveguide distortion conventionally caused in the heating zone 16 by gravity and vacuum forces. This difference allows for the heating element or Collapse When operating at low temperatures, Collapse The process is much more forgiving (because the glass is not softened enough to translate the pressure difference from the vacuum and support the core rod 30).

[0050] Since no vacuum is required to support the weight of the core rod 30, Collapse The process also allows for partial pressure (up to atmospheric pressure or slightly higher, typically about 1,100 mbar) in the gap 31 between the core rod 30 and the cladding 32. CollapseDuring the process, a reactive interface treatment gas such as sulfur hexafluoride (SF6, which is safe to handle at room temperature) can be applied liberally in the direction of 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. Using a reactive interface treatment gas to etch, clean, and dry the preform interface can improve the interface, resulting in improved optical fiber quality (reduced fiber breaks, bubbles, loss, or air bubbles) and a lower core rod D / d ratio.

[0051] As mentioned in the previous paragraph, the stacked core rods 30 are supported from below by the spacers 48, which also support the weight of the glass both above and below the heating zone 16 (where the glass is softened). Collapse The process is much less susceptible to 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 to processing low viscosity glass materials (such as highly F-doped cladding 32) without risking excessive heating or cladding-to-core waveguide distortion due to gravity and vacuum forces. This provides an important processing advantage for certain classes of fiber designs with F-doped cladding 32 materials.

[0052] Returning to Figure 1, a glass disk 70 having an outer diameter slightly smaller than the inner diameter of the top collar 40 (i.e., typically about 126 mm) is placed on top of the core rod 30 and cladding 32 and inside the top collar 40. The disk 70 may be about 5 cm thick. During start-up of the process, after a 6 minute dwell time of the top weld 42 in the center of the heating element, a vacuum is applied from both the bottom collar holder and vacuum unit 52 and the top collar holder and vacuum unit 54. The vacuum presses the top collar 40 onto the disk 70. CollapseBy setting V2>V1, the tip is forced to the rest of the steady state of the cladding 32 where V2=V1. Collapse The preform is pulled on top of the cladding 32 before being drawn. The result is a low-cost, high-yield online preform chipping process that results in the easiest and most efficient preform for subsequent fiber drawing. The integrated online preform chipping process saves a significant amount of both labor and cost (e.g., saves an extra heating step) over conventional offline chipping processes.

[0053] interface Collapse When V2 is set higher than V1, the upward Collapse The process can also draw or stretch a preform upward having a diameter significantly smaller than the original diameter of the cladding 32. The diameter of the stretched (or even compressed) preform can be precisely controlled by precisely setting the linear vertical velocities V1 and V2 through the law of conservation of mass. However, starting losses of good preform glass are greater in the upward stretching process than in the conventional downward stretching process. Collapse The process is much less and therefore can result in significant cost savings for the drawn blank.

[0054] The weight of the glass both above and below the heating zone 16 (where the glass is softened) is supported by the top collar 40 and bottom collar 44, while the outer surface of the base material itself is not contacted, so that the weight of the glass upwards Collapse The process is also a completely contactless process for the finished workpiece. Avoiding workpiece contact, and therefore any lateral or transverse forces, results in both a very clean workpiece surface and a workpiece with little to no warping, unlike traditional downward stretching processes where the puller wheels are in constant contact with and apply force to the workpiece throughout the process.

[0055] In many conventional downward stretching processes, a small contact area exists between the puller wheels and the outer periphery of the workpiece. Such contact can introduce impurities or contaminants onto the workpiece surface. In addition, the puller wheels exert lateral forces during the downward stretching process, potentially causing warpage of the workpiece (this is exacerbated for longer workpieces; warpage increases as the square of the workpiece length in the case of simple curvature). The amount of contact force that can be applied to the workpiece by the puller wheels is limited because excessive pressure can damage the glass surface of the workpiece. Therefore, for large workpieces requiring greater pulling forces than can be applied by a single set of puller wheels, multiple sets of puller wheels can be applied to the workpiece at different levels to achieve the total normal (friction) force required to support the workpiece weight. However, multiple sets of puller wheels increase both the height and cost of the equipment. Furthermore, reducing workpiece warpage can only be achieved with multiple sets of puller wheels if the sets of puller wheels are precisely aligned, which is difficult to achieve in practice. Non-contact upward stretching Collapse The process results in very little bowing of the workpiece because there are no lateral forces applied to the workpiece.

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

[0057] Gripper system 80 includes a clamping element 82 and a mounting element 84 that mounts clamping element 82 to gripper system 80. Gripper system 80 can move vertically parallel to the length of frame 12 (defined as the Z direction in FIG. 1 ). Mounting element 84 allows translational movement of clamping element 82 in the X and Y directions (i.e., to any position in the XY plane). (Though not necessary or preferred, a chuck system that allows rotation can also be used, particularly if a torch rather than a furnace provides the heating element.) In one embodiment, mounting element 84 is an XY table that includes a pair of arms mounted on linear bearings or linear rails and motors, e.g., manual or servo motor drives, to control arm movement. Mounting element 84 is also a low-friction device, so that a force applied to clamping element 82 by an external object causes clamping element 82 to deflect along mounting element 84 rather than clamping element 82 applying a resistive force to the external object.

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

[0059] The clamping element 82 is 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 clamping element 82 so that it is aligned with the center in the X direction. In some embodiments, the clamping element 82 may be aligned with an estimated center of the bottom collar 44 or bottom collar holder and vacuum unit 52, for example, an expected center based on a desired path of movement. In other embodiments, to more precisely align the clamping 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, a camera / vision system, or a mechanical contact (dial indicator) system. In an exemplary embodiment, the sensing element may be attached to gripper system 80 or may be external to gripper system 80, for example, attached to frame 12. In another embodiment, the sensing element may be external to both gripper system 80 and frame 12 (e.g., a camera). Gripper system 80 does not need to be perfectly aligned with the center of bottom collar 44 or bottom collar holder and vacuum unit 52, as gripper system 80 includes additional elements to prevent misalignment.

[0060] Once the clamping element 82 is aligned, it contacts the bottom collar 44 or the bottom collar holder and vacuum unit 52 by movement of the attachment element 84 in the X direction. The attachment element 84 can be moved by any suitable mechanism, such as a motor used to control a pair of arms of an XY table. Because the attachment element 84 is a low-friction device, if the clamping element 82 is not properly aligned and centered when attempting to attach it to the bottom collar 44 or the bottom collar holder and vacuum unit 52, the force of the bottom collar 44 or the bottom collar holder and vacuum unit 52 pressing on the clamping element 82 will move it into the aligned position instead of moving the bottom collar 44 or the bottom collar holder and vacuum unit 52. The attachment element 84 can further include a locking mechanism that can be engaged and disengaged to prevent movement of the clamping element 82 once the clamping element 82 is attached to the bottom collar 44 or the bottom collar holder and vacuum unit 52. While the clamping element 82 is being moved into position, the locking mechanism is unlocked so that the clamping element 82 can be moved by the motor while still being displaced by any additional force applied to the clamping element 82. When the clamping element 82 contacts the bottom collar 44 or the bottom collar holder and vacuum unit 52, the locking mechanism engages, preventing further movement of the clamping 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 clamping elements 82 to the bottom collar 44 or bottom collar holder and vacuum unit 52. The load cell is a transducer that converts the force applied to the clamping elements 82 into an electrical signal relative to the strain gauge (not shown) of each load cell. The electrical signal can then be measured and correlated to the force applied to the strain gauge. Exemplary load cells include hydraulic load cells, pneumatic load cells, and strain gauge load cells. If the clamping elements 82 are not properly aligned with the center of the bottom collar 44 or bottom collar holder and vacuum unit 52, the reaction force will be greater than if the clamping elements 82 are properly aligned. By measuring the reaction force with the force-sensing device, misalignment can be detected and corrected before the clamping elements 82 apply sufficient force to the bottom collar 44 or bottom collar holder and vacuum unit 52 to cause movement of the bottom collar 44 or bottom collar holder and vacuum unit 52. In one embodiment, a force-sensing device may be used in conjunction with a low-friction mounting element 84, and the rate at which the clamping element 82 is attached to the bottom collar 44 or bottom collar holder and vacuum unit 52 is slowed down in response to a greater than expected reaction force to allow the clamping element 82 to move into alignment on the mounting element 84. In an exemplary embodiment, the clamping element 82 may be moved toward the bottom collar 44 or bottom collar holder and vacuum unit 52 at a speed ranging from about 50 mm / min to about 100 mm / min while no misalignment is detected by the force-sensing device, and the speed may be reduced to about 10 mm / min to about 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 preform assembly (which can be approximately 350 kg or more) and replaces conventional full-contact puller wheel systems. The gripper system 80 enables floating positioning in the horizontal (XY) plane of the glass body 20 used to create the preform, as well as precise linear movement in the vertical (Z) direction for precise alignment and control of the preform shape and preform chipping process. In particular, when the gripper system 80 is incorporated, the apparatus 10 avoids lateral or transverse forces on the preform, thereby minimizing and possibly eliminating preform warping, can use load cells to monitor glass behavior during heating, and enables the use of physics (conservation of mass) to precisely control dimensions (eliminating the expense of conventional online measurement and feedback control).

[0063] Furthermore, the device 10 and associated upward Collapse The process can be used in combination with a preform measurement device, a suitable preform measurement device being more fully described in U.S. Patent No. 10,378,996, which claims priority to International Patent Application No. PCT / US2014 / 050368, entitled "Methods And Apparatus For Determining Geometric Properties Of Optical Fiber Preforms," ​​filed August 8, 2014, by the assignee of the present application.

[0064] A controller is a hardware device or software program that manages or directs (i.e., facilitates communication with) the flow of data between two components. Apparatus 10 includes a controller 88. Controller 88 provides the ability to acquire data from, for example, load cells 68, gripper system 80, and controls other components of apparatus 10 and associated upward movements, including top and bottom collar holder and vacuum units 52, 54, and vacuum and process gas systems. CollapseThe controller 88 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, the speeds V1 and V2, the flow rates of gases, and the pressure of the vacuum pump. The controller 88 helps ensure a robust, 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 vs. core) distortion, which is caused by gravity and vacuum forces on the core rod and molten glass inherent in conventional processes, and the upward force. Collapse Waveguide distortion is an issue that is rarely, if ever, addressed in the RIT / RIC field. This lack of recognition may be due to the fact that optical fiber performance requirements in the past were less stringent, and therefore the field has tended to treat optical preforms like simple glass rods without worrying about the effects of actual waveguide (cladding vs. core) distortion, which can result in, among other things, fiber cutoff wavelength failures.

[0066] Globally connected devices, cloud services, 5G (5th Generation Mobile Networks or 5th Generation Wireless Systems, which represents a major step 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 an exponentially increasing demand for bandwidth. Therefore, optical fiber manufacturers must increase their output and productivity. For next-generation optical fiber manufacturing, very large preforms that are drawn at high speeds are required. CollapseThe result of the process is a "draw-ready" solid preform that can withstand multiple days of uninterrupted optical fiber drawing, increasing productivity and optical fiber output while reducing costs and achieving improved fiber yields for users of the preform.

[0067] Upward Collapse The process, of course, involves upward stretching (and, optionally, stretching or compressing) and the removal of the top collar 40 or bottom collar 44. Collapse This includes low-cost upward on-line chipping by chipping the upper collar 40 and by matching the outer diameter of the upper collar 40 to the outer diameter of the cladding 32. Collapse These additional features of the process can be performed much more accurately and cheaply than the traditional downward drawing process through the precise physics of conservation of mass and glass flow. Collapse The process can also achieve near 100% tip-on blank yield (i.e., better glass) with minimal waveguide distortion at the blank end. Collapse It is worth pointing out that the process results in nearly 100% good parent glass yield without the wasteful sacrificial starting material used to initiate the conventional downward drawing process. And the consumption of material used for the top collar 40 and bottom collar 44 is also reduced by the upward drawing process. Collapse It is minimal in the process.

[0068] Above the single overclad Collapse The process is described above. However, this process can be applied to multiple overclad "gap" jacket tubes or cylinders with minor modifications such as increasing the outer diameter of the spacers 48 and slightly adjusting the maximum heating power and end power. Furthermore, since the weight of the stacked core rods 30 is fully supported from below, the upward force is reduced. CollapseThe process can also accommodate double (or even triple or more) lengths of cladding 32, with the distinct advantage of not requiring matching of the cladding inner diameter (i.e., a smaller inner diameter of the lower cladding to support the core rod inside the upper cladding, as in conventional down-draw processes). Collapse The process can also be applied to produce MCFs.

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

[0070] Polarization mode dispersion (PMD) is a problem for MCFs. PMD is a form of mode dispersion in which two different polarizations of light in a waveguide that normally travel at the same speed travel at different speeds due to random imperfections, asymmetries, and geometric distortions, causing random spreading of the light pulse. Unless prevented (or at least minimized) or compensated for, which is difficult, 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. In the ideal case, the fundamental mode has two orthogonal polarizations (electric field orientations) traveling 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, random imperfections exist that destroy the circular symmetry, causing the two polarizations to propagate at different speeds. In real cases, the two polarization components of the signal slowly separate, spreading the pulses and causing them to overlap. Because 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 the differential group delay) that is proportional to the square root of the propagation distance L.

number

number

[0072] Symmetry-breaking random imperfections that cause PMD fall into several categories. First, there is stress-induced material birefringence, where the refractive index itself is polarization-dependent. Second, there are geometric distortions or asymmetries, such as cladding noncircularity and core ellipticity (especially relative to the surrounding core) relative to the cross-sectional shape of the MCF preform. Both of these effects can result from either imperfections in manufacturing (which are never perfect or stress-free) or thermal and mechanical stresses applied to the fiber in the field (and the latter stresses generally change over time). The manufacturing process of this disclosure focuses on reducing geometric distortions.

[0073] The problems associated with fiber cladding non-circularity are (1) alignment errors and splice losses during fiber splicing, and (2) its negative impact on the ellipticity of the fiber core. Fiber core ellipticity, especially the ellipticity of the peripheral core in MCFs, has a strong correlation with the increase in fiber PMD. Therefore, it is very important to minimize core ellipticity and cladding non-circularity in circular cross-sectional MCF preforms.

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

[0075] [Table 1]

[0076] In the example of a 7-core MCF, the preform outer diameter profile is also 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 of the 4-core MCF example in Figure 4B is much less severe than the 7-core MCF example. The cladding non-circularity is about 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 four-core MCF is in the 3% to 4% range. This range can be tolerated with the help of a PMD compensation device, such as the coherent 40 Gbit / s dual-polarization quadrature 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 polarization controllers to compensate for PMD in the fiber. Essentially, the device splits the fiber's output into two primary polarizations (usually polarizations with no first-order variation in time delay with frequency) and applies a differential delay to resynchronize them. Because PMD effects are random and time-dependent, this compensation requires an active device that responds to feedback over time. This requirement makes 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 elliptical geometric distortions in MCF preforms so that fiber PMD and bandwidth performance are maximized.

[0080] The goal of the disclosed process is to prevent or at least minimize two types of geometric distortions: preform non-circularity and core ovality. Stated another way, the goal is to achieve a circular-shaped MCF preform with minimal core ovality, especially for the peripheral core. For a given MCF design, these distortions in non-circularity and ovality are minimized during the disclosed preform manufacturing process. This process has two main aspects: (1) by properly designing the gap between the core rod and the holes in the perforated cylinder and optimizing the diameter of the perforated cylinder, and (2) by optimizing the core rod and cladding. Collapse Non-circularity and ovality are minimized by optimizing heat treatment conditions based on accurate understanding and modeling of temperature and viscosity gradients and glass flow during the process. A description of each of these two aspects follows.

[0081] A. Optimization of geometric parameters A distinction can be made between three different manufacturing processes used to make MCFs. 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 core and filler rods are stacked in a hexagonal configuration, fused together under vacuum within a cladding tube, and simultaneously drawn or stretched into a fiber. The stack and draw process is the subject of European Patent Application Publication No. 1002249(A1) (Corning Inc.) and European Patent Application 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 expected 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 JP 2019-081681A (Furukawa Corporation) and JP 5995298B1 (Nippon Telegraph and Telephone Corporation). Individual cores are placed in a jacket tube according to design specifications, and the remaining cavity is filled with silica glass powder. It is very difficult to control the geometric positioning accuracy of the cores on the one hand and the production of defect-free cladding using the sand process on the other hand.

[0084] The third process (the drilling process) involves a cladding cylinder that is drilled according to the desired design specifications without restriction, and then the hole is filled with a core rod. In a subsequent step, the cylinder is placed over the core rod. Collapse The fiber is then (optionally) drawn to size or directly into fiber. Collapse The step and drawing step can also be performed in one step. Elimination of surface defects and geometric artifacts is a challenge in the direct fiber drawing step of the drilling process.

[0085] From the perspective of the geometric aspects of fiber composition, essential factors are the absolute radial and azimuthal positions of the cores, as well as their variation along the fiber. Therefore, the fabrication of high-precision preforms with respect to core size, position, and shape, as well as cladding circularity, is highly desirable. The disclosed process can create precisely machined MCF cylinders and establish a robust overcladding process that fuses the MCF cylinder with a core rod to form a preform with high geometric precision. The drilling process used to create the perforated cylinders is currently the most promising process for fabricating high-precision multicore fibers with the greatest potential for volume scaling. In this disclosure, the largest cylinder bodies (with diameters up to approximately 250 mm) are machined so that, with the same absolute machining precision, the highest precision of MCF core and cladding circularity can be achieved for the final MCF preform or fiber.

[0086] Thus, the process involves drilling holes to produce a porous or perforated cylinder (with or without a central hole). The disclosed process begins by providing a glass cylinder (forming the preform cladding) with an OD ranging from about 150 mm to 250 mm, preferably about 200 mm or greater. The relatively large size of the cylinder facilitates accurate drilling of holes in the cylinder. The hole location error is less than a percentage of the outer diameter (OD) size of the larger cylinder. The glass cylinder may be solid or may have a central hole (a central hole is present if a mandrel was used to create the glass cylinder). Peripheral or side holes (to accommodate the core rod) are carefully and very precisely formed (e.g., drilled) in the cylinder.

[0087] Once the perforated cylinders are obtained, the next step in the disclosed process is to create a preform with perforated MCF cylinders. Glass rods are inserted into the holes to create the peripheral cores (and optionally the central core). The MCF preform is then molded to form the perforated cylinders on multiple core rods. Collapse and optionally stretching the film. Collapse The steps may be performed using a conventional downward stretching process or the upward stretching process described above. Collapse To minimize distortion of the waveguide (cladding vs. core) during the preform fabrication process, the process preferably incorporates an upward drawing process step (as described above) so that the difference in axial flow of the cladding glass and the core glass due to gravity is minimized.

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

[0089] To prepare the base material, an MCF cylinder was placed on the core rod inside the drilled hole. Collapse The drawing step can be performed with or without drawing. The accuracy of different geometric features (ovality, warping, siding, eccentricity, and hole location, etc.) from the MCF cylinder can be completely achieved without the drawing operation. Collapse can be used to maximize the retention in the final preform, however the OD of the preform is limited to be slightly smaller than the OD of the MCF cylinder (in the absence of stretching).

[0090] Therefore, the MCF cylinder with the core rod inside the drilled holes is heated to create the base material. Collapse It is preferable that the film is stretched. Collapse The drawing operation provides great flexibility, allowing for the production of a range of preform ODs that can be matched to the drawing capabilities of the fiber manufacturer. (Typically, the preform is later drawn into optical fiber.) The drawing step reduces the OD of the cylinder. The step of forming holes in the MCF cylinder is configured to form peripheral holes at predetermined locations, taking into account variations in the position of the cores before and after the step of heating the core rod and glass cylinder. As a result, this operation facilitates the production of MCF preforms with multiple cores precisely positioned at predetermined locations as designed.

[0091] The ovality (or non-circularity) of the surrounding core within the matrix can be calculated using one of four alternative equations. Specifically, ovality = (1) (maximum diameter - minimum diameter) ÷ average diameter, or (2) change in major axis + change in minor axis. Ovality is also approximately the sum of the effects of (3) azimuthal and radially inward glass flow. This third equation invokes the cylindrical coordinate system, a three-dimensional coordinate system that specifies point locations by their distance from a selected reference axis, their direction from the axis relative to the selected reference direction, and their 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 referred to as the radial distance or radius, and the angular coordinate is sometimes referred to as the angular position or azimuthal angle.

[0092] Finally, the ovality is (4) approximately α(g / d) + β(G / D), where α and β are proportionality constants that depend on the process conditions and MCF geometric design, the diameter of the cylinder is D, the gap between the central core rod and the central hole is G, the diameter of the peripheral hole is d, and the gap between the peripheral core rod and the peripheral holes is g. Therefore, from the perspective of the preparation of the core rod and holy cylinder, it is clear that the following steps directed at geometric parameters should help reduce the peripheral core ovality: (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 of the "squashing" or "flattening" azimuthal glass flow of the core rod relative to the major axis, g / d; and (d) reduce the effect of the inward radial flow of the core rod glass relative to the minor axis, G / D.

[0093] In these steps, the process maintains parameters g and G at or below about 4 mm, but above a threshold (e.g., 0.2 mm) so that core rod insertion into the hole is more practical. Thus, parameters g and G may range from about 0.2 mm to 4 mm, more preferably from about 0.2 mm to 3 mm, even more preferably from about 0.2 mm to 2 mm, and most preferably from about 0.2 mm to 1 mm. Parameters g and G may further range from about 0.3 mm to 4 mm, more preferably from about 0.3 mm to 3 mm, even more preferably from about 0.3 mm to 2 mm, and most preferably from about 0.3 mm to 1 mm. Parameters g and G may further range from about 0.5 mm to 4 mm, more preferably from about 0.5 mm to 3 mm, even more preferably from about 0.5 mm to 2 mm, and most preferably from about 0.5 mm to 1 mm. Parameters g and G may further range from about 0.6 mm to 1 mm, more preferably from about 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 more.

[0094] Thus, the disclosed process minimizes non-circularity and ovality by optimizing geometric parameters. Specifically, the gap between the core rod and the perforated cylinder and the diameter of the perforated cylinder are optimized. In a second main aspect, the disclosed process minimizes non-circularity and ovality by optimizing the geometric parameters. Specifically, the gap between the core rod and the perforated cylinder and the diameter of the perforated cylinder are optimized. Collapse Non-circularity and ovality are minimized by optimizing heat treatment conditions based on accurate understanding and modeling of temperature and viscosity gradients and glass flow during the process.

[0095] B. Optimization of heat treatment conditions Another way to minimize cladding non-circularity and core ovality for a given design of preform assembly (perforated cylinder + core rod) is to mount the holy cylinder on the core rod. Collapse The key to understanding and validating the complex glass flow is to understand the exact glass flow during the process and therefore optimize the heat treatment recipe or conditions accordingly. CollapseMultiphysics finite element modeling (FEM) of the drawing and stretching process has been used. Using the 7-core MCF preform of FIG. 4A as an example, which has severe peripheral core ellipticity and a hexagonal cladding, the 2D MCF preform is shown in FIG. 5. Collapse Only one model was created.

[0096] Taking advantage of the symmetry of the base material, only half of the geometry is modeled. A constant oven temperature (2,200°K, 2,300°K, etc.) is assumed to provide radiant heating to the outer surface of the perforated cylinder. As a way to simulate throughput effects, a specific initial temperature across the perforated cylinder is assumed to create 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, the temperature under vacuum at t = 100 seconds is Collapse To simulate this, a vacuum of 950 mbar is applied within 1 second.

[0097] Similar to the peripheral core shape in Figure 4A Collapse From the random case of the model, which results in a convex peripheral hole shape and an overall cladding shape approaching a hexagon, we observe a glass flow velocity magnitude field as shown in Figure 6. 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 part of the cylinder, especially the part between the central hole and the peripheral hole. This is because CollapseThis indicates that the radial temperature gradient within the perforated cylinder during the process is too large, resulting in a large radial viscosity gradient. In practice, to reduce this radial temperature gradient, the glass throughput, i.e., the mass flow rate through the furnace, must be reduced, resulting in a process that allows more time for the heat to transfer radially and thus better homogenizes the radial temperature field. This insight allows different initial temperature levels for the perforated cylinder to be used, as explained in the model setup above, but without vacuum. Collapse The process is Collapse The temperature gradient is started at a fixed time (t=100 seconds) to mimic different levels of radial temperature gradients in the chamber and as a way to approximate the effect of different levels of throughput. [Example]

[0098] The following examples are provided to more clearly illustrate the overall nature of the present disclosure. These examples are intended to illustrate, but not limit, the present disclosure. FEM is used to develop 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 higher throughput effects. For the three cases shown in Figures 7-9, the oven temperature is varied from 2,400°K to 2,300°K to 2,200°K while maintaining the same difference (800°K) between the oven temperature and the initial temperature of the cylinder.

[0100] Figure 7A is a graph of the radial temperature gradient, and Figure 7B shows the deformation of the perforated cylinder for Case 1. Conditions for Case 1 are oven temperature = 2,400°K, initial cylinder temperature = 1,600°K, and vacuum initiation 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 for the second case. The conditions for the second case are oven temperature = 2,300°K, initial cylinder temperature = 1,500°K, and vacuum initiation 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 for Case 3. The conditions for Case 3 are oven temperature = 2,200°K, initial cylinder temperature = 1,400°K, and vacuum initiation at t = 100 seconds. The average radial temperature gradient is 7.9°K / mm.

[0103] 7-9, it can be observed that at t=100 seconds when the 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 a hexagonal-like shape of the cladding under different oven temperature levels, which closely resembles the actual 7-core MCF preform example shown in FIG. 4A.

[0104] Because the model can reproduce the deformation of the surrounding core and the entire cladding during actual operation, it can be used to demonstrate the benefits achieved by significantly reducing the temperature gradient during the manufacturing process of the present disclosure. Figures 10-12 show three cases that demonstrate these benefits. Each case is described below.

[0105] Figure 10A is a graph of the radial temperature gradient, and Figure 10B shows the deformation of the perforated cylinder for Case 1. Conditions for Case 1 are oven temperature = 2,500°K, initial cylinder temperature = 2,200°K, and vacuum initiation 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 for the second case. The conditions for the second case are oven temperature = 2,300°K, initial cylinder temperature = 2,000°K, and vacuum initiation 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 for Case 3. The conditions for Case 3 are oven temperature = 2,200°K, initial cylinder temperature = 1,900°K, and vacuum initiation 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 lower radial temperature gradients significantly reduce the radial "squashing" effect of the peripheral holes. Therefore, the circular shapes of both the peripheral holes and the entire cladding are much better preserved. However, it is also noteworthy that if the oven temperature is too high, i.e., 2,500°K, even under the low radial temperature gradient shown in Figure 10A, the peripheral holes may be azimuthal compressed instead of being radially crushed. Figures 12A and 12B show the best case, where the peripheral hole shape is the most circular, among the three cases with the lowest temperature gradients.

[0109] Therefore, to minimize the non-circularity of the cladding and the ovality of the holes and cores (especially the peripheral holes and cores) within the MCF preform, the process of the present disclosure involves the formation of a cylinder on a core rod. CollapseDuring the process, the radial temperature gradient of the perforated cylinder is maintained at a low level. Reducing the glass throughput is a good practical way to reduce this radial temperature gradient. FEM results estimate that the radial temperature gradient that causes peripheral core deformation in the seven-core case of Figure 4A needs to be reduced by approximately 70% to preserve the circular shape of the cladding and peripheral core. Furthermore, a moderate oven temperature (such as approximately 2,200°K) or the lowest possible oven temperature is preferred, as this helps prevent the peripheral holes from being compressed / crushed in the azimuthal direction and achieve the best results.

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

[0111] [Table 3]

[0112] Figures 18 to 23 show the influence of the central hole size for a relatively high radial temperature gradient. The conditions for 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 observations regarding clad non-circularity and peripheral hole ellipticity for each of the simulations 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 clad surface, 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 clad surface. The best scenario is estimated to be 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 core 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 a seven-hole example (one central hole and six peripheral holes). Figures 24-26 show the effect of cladding size for 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 central hole diameter is 42.5 mm (original value), and the cladding OD is 134 mm. Figure 24 shows that the peripheral core radial crushing 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 for the radial temperature gradient to achieve approximately the same 2.0°K / mm). The 134 mm cladding OD design is scaled proportionally to a 167 mm OD design. Figure 25 shows that the peripheral core radial crushing effect is significantly reduced and the cladding 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 for the radial temperature gradient to achieve approximately the same 2.0°K / mm). The 134 mm cladding OD design is scaled proportionally to a 200 mm OD design. Figure 26 shows that the peripheral core radial crushing effect is significantly reduced and the cladding circular shape is well preserved.

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

[0120] To investigate the effect of temperature gradients on the performance of the disclosed process, additional FEM simulations were completed for a four-hole example without a center hole. Figures 27-29 show the effect of a relatively low radial temperature gradient. 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 average 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 peripheral-core radial crushing effect is significant, and the cladding circular shape 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 average 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 crushing effect is not very significant, with the flattened edge instead being 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 average 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 crushing effect is not very significant, with the flattened edge instead being 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 the four-core design without a central hole. First, when the radial temperature gradient is relatively high, the peripheral holes are crushed radially with their flattened ends toward the outer surface of the cladding, and the circular cladding shape is deformed into a square shape. Second, when the radial temperature gradient is relatively low, the peripheral holes are crushed radially with their flattened ends toward the center of the cladding, and the circular cladding shape is well preserved. This is the favorable 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 the four-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 average 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 edge is 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, which is much smaller than the peripheral hole in this case, does not significantly reduce the peripheral hole ovality.

[0126] To investigate the effect of cladding size on the performance of the disclosed process, additional FEM simulations were completed for a four-hole example without a central hole. Figures 32 and 33 show the effect of cladding size for a relatively low radial temperature gradient. The common conditions for Figures 32 and 33 are oven temperature = 2,200°K, initial cylinder temperature = 2,000°K, average radial temperature gradient = 1.2°K / mm, peripheral hole diameter = 43 mm, and distance to the center of the cladding = 33.94 mm. The design in Figure 32 starts evacuation at t = 65 seconds and has a cladding OD = 150 mm. The design in Figure 33 starts evacuation 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 edge is 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 scaled proportionally from 150 mm OD to 200 mm OD, there is essentially no change in cladding circularity or peripheral hole ellipticity as long as the radial temperature gradient is kept the same.

[0128] FIG. 34 is a flow diagram summarizing the steps of one embodiment of the disclosed process 500. In a first step 510, a glass cylinder having an OD of at least about 200 mm is provided. In a second step 520, the glass cylinder is drilled to create a perforated cylinder. The holes include peripheral holes and, optionally, a central hole. Step 520 is completed while maintaining a gap (g) between the peripheral core rods and the peripheral holes in the range of about 0.2 mm to 4 mm. In a third step 530, glass rods are inserted into the holes to create the peripheral core (and optionally the central core). In a fourth step 540, a perforated (i.e., perforated) cylinder is formed by laminating a plurality of core rods onto the glass cylinder. Collapse Step 540 is performed to separate the core rod and the cylinder. Collapse is completed while maintaining the average radial temperature gradient within a specified range at the plane where it begins.

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

[0130] The manufacturing process disclosed above is repeatable, accurate, cost- and time-effective for industrial-scale multicore preforms and fibers, intended for the production of MCF preforms and fibers for telecommunications applications.

[0131] The foregoing description of preferred embodiments should be construed as illustrative, not limiting, of the present invention as defined by the claims. As will be readily understood, numerous variations and combinations of the features described above can be utilized without departing from the present invention as defined by the claims. Such variations are not considered a departure from the spirit and scope of the present invention, and all such variations are intended to be included within the scope of the following claims. For example, all broad ranges described herein are expressly intended to also include within their scope all narrower ranges that are included within the broad ranges. As will be understood by those skilled in the art, it is also expressly intended that certain steps within a process may be omitted. Certain additional steps may be added, and the order of steps may be changed from the specific order described.

Claims

1. 1. A process for manufacturing a multi-core optical fiber preform having a central longitudinal axis, a plurality of core rods each positioned within a respective core hole and extending along said longitudinal axis, and a common cladding covering each of said plurality of core rods, said process comprising: providing a cylinder that forms the cladding of the base material; forming a plurality of peripheral core holes in the cylinder extending along the longitudinal axis; inserting each of a plurality of core rods into a respective peripheral core hole of the cylinder; heating the cylinder with the core rods inserted into the respective core holes by exposing the cylinder and the core rods to a heating element, thereby collapsing the cylinder onto the core rods and forming the preform; During the step of forming the plurality of peripheral core holes, the plurality of peripheral core holes are formed such that a gap (g) between the core rod and the peripheral core holes after insertion is maintained in a range of about 0.2 mm to 4 mm, and during the step of heating the cylinder, an average radial temperature gradient is maintained at a plane where collapse between the core rod and the cylinder begins, of about 0.5 K / mm to 4 K / mm.

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

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

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

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

6. The process of claim 1 , wherein forming the plurality of peripheral core holes comprises drilling the plurality of peripheral core holes.

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

8. The process of claim 1 , wherein the step of heating the cylinder comprises maintaining the heating element at a temperature below about 2,500 K.

9. 2. The process of claim 1, wherein the cylinder has a central core hole, a thickness (t1) between the central core hole and the peripheral core holes, 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 x t2 < t1 ≦ t2.

10. The step of heating the cylinder is performed as part of an upward drawing process in which the cylinder is collapsed onto the core rod within the core bore, the upward drawing process comprising: supporting both the cylinder and the core rod from below so that the weight of the cylinder and the core rod is fully supported from below and the core rod does not move vertically relative to the cylinder when the cylinder collapses onto the core rod; moving the cylinder and the core rod upward relative to the heating element such that the cylinder continuously collapses onto the core rod as the cylinder and the core rod move upward; The process of claim 1 , wherein the flow rate difference between the cylinder and the core rod along the longitudinal axis due to gravity is minimized.

11. The process of claim 10 , wherein the upward drawing process comprises simultaneously stretching the cylinder and the core rod while collapsing the cylinder onto the core rod.

12. 1. A process for manufacturing a multi-core optical fiber preform having a central longitudinal axis, a plurality of core rods each positioned within a respective core hole and extending along said longitudinal axis, and a common cladding covering each of said plurality of core rods, said process comprising: providing a cylinder that forms the cladding of the base material; forming a plurality of peripheral core holes in the cylinder extending along the longitudinal axis; inserting each of a plurality of core rods into a respective peripheral core hole of the cylinder; heating the cylinder with the core rods inserted into the respective core holes by exposing the cylinder and the core rods to a heating element, thereby collapsing the cylinder onto the core rods and forming the preform; During the step of forming the plurality of peripheral core holes, the plurality of peripheral core holes are formed such that a gap (g) between the core rod and the peripheral core hole after insertion is maintained in a range of about 0.2 mm to 4 mm.

13. 1. A process for manufacturing a multi-core optical fiber preform having a central longitudinal axis, a plurality of core rods each positioned within a respective core hole and extending along said longitudinal axis, and a common cladding covering each of said plurality of core rods, said process comprising: providing a cylinder that forms the cladding of the base material; forming a plurality of peripheral core holes in the cylinder extending along the longitudinal axis; inserting each of a plurality of core rods into a respective peripheral core hole of the cylinder; heating the cylinder with the multiple core rods inserted into the respective core holes by exposing the cylinder and the core rod to a heating element, thereby collapsing the cylinder onto the multiple core rods and forming the preform, wherein an average radial temperature gradient is maintained between the core rod and the cylinder at the plane of initiation of collapse of about 0.5 K / mm to 4 K / mm.

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