Method for fabricating alkali-doped multi-core optical fiber preforms with reduced devitrification

Abstract

1. A method for making a multi-core optical fiber preform, the method comprising: bonding a preform assembly to form a multi-core optical fiber preform; the preform assembly comprising a plurality of core canes, each core cane disposed within an axial bore of a sleeve; each core cane comprising a core section of alkali-doped silica glass, the silica glass having a maximum alkali concentration of about 0.10% to about 10% by weight; the core section of each core cane being surrounded by a sleeve along the height of the core cane and by covers disposed on first and second axial ends of the core section, the covers comprising silica glass having a chlorine concentration of about 0.05% or less by weight.

Description

[Technical Field]

[0001] Alkali-doped multi-core optical fiber with reduced devitrification This application claims the benefit of priority under 35 U.S.C. § 120 of U.S. Provisional Application No. 63 / 442,511, filed February 1, 2023, the entire contents of which are relied upon and incorporated herein by reference.

[0002] The present disclosure is directed generally to alkali-doped multi-core optical fibers, and more particularly to cane-based multi-core optical fibers including alkali-doped cores and having reduced devitrification, and methods for forming the same. [Background technology]

[0003] Multicore optical fibers have increased transmission capacity in communication systems over single-core optical fibers. In multicore optical fibers, multiple cores are surrounded by a single cladding so that light propagates through each core. To fabricate multicore optical fibers, an all-glass process can be used, which uses a bulk cladding glass with one or more precisely shaped axial holes. Each hole houses a core cane, which shapes the core of the multicore optical fiber.

[0004] To convert soot to glass, an all-glass process may be preferred over deposition-based processes (e.g., outside vapor deposition (OVD) processes) that involve soot layering, sintering, and solidification. In an all-glass process, the cladding glass can be precisely ground to a selected diameter, which provides both precision and flexibility in selecting various spacings, shapes, and arrangements of one or more axial holes when shaping the glass preform. In some multicore optical fiber applications (e.g., undersea cables), the cores of the multicore optical fibers contain alkali metals to obtain low attenuation in these fibers.

[0005] However, the all-glass process is relatively expensive and time-consuming. Precision drilling is time-consuming, and one or more core canes must be shaped to define the selected refractive index profile and then added to the cladding glass, with the entire structure being clamped in a furnace to form a solid glass preform. To create a glass preform of sufficient length, separate glass cladding sections may need to be axially combined, which involves precise alignment of the axial holes. Furthermore, such separate glass cladding sections are prone to forming nucleation sites when the glass is heated, which ultimately causes devitrification or crystallization in the multicore optical fiber preform and drawn optical fiber. Such devitrification or crystallization can make the optical fiber glass preform difficult to process and / or increase attenuation in the optical fiber, which can result in lower glass yield or suboptimal transmission characteristics in the optical fiber. Summary of the Invention

[0006] Because the inclusion of alkali dopants causes the glass to have a lower viscosity, alkali-doped glasses are particularly vulnerable to the formation of nucleation sites. Nucleation sites are locations on the glass where crystals form. These sites further grow and mature into crystallization or devitrification in the glass, which can spread through a significant portion of the glass or throughout the entire glass. Once devitrification has spread in the glass, it cannot be repaired, further downstream processing of the glass is not possible, and the glass must be discarded. Embodiments of the present disclosure are directed to processes for reducing and / or preventing the formation of crystallization or devitrification in alkali-doped glasses.

[0007] An aspect of the present disclosure includes a method of making a multi-core optical fiber preform, the method including bonding a preform assembly to form a multi-core optical fiber preform, the preform assembly including a plurality of core canes, each core cane disposed within an axial bore of a sleeve, each core cane including a core section made of alkali-doped silica glass, the silica glass having a maximum alkali concentration of about 0.10 wt % to about 10 wt %, the core section of each core cane being surrounded by a sleeve along the height of the core cane and by covers disposed on first and second axial ends of the core section, the covers including silica glass having a chlorine concentration of about 0.05 wt % or less.

[0008] An aspect of the present disclosure is a method of making a multi-core optical fiber preform, the method comprising exposing a preform assembly to a treatment for a time t to form a multi-core optical fiber preform, the preform assembly comprising a plurality of core canes, each core cane disposed within an axial bore of a sleeve, each core cane comprising a core section comprised of alkali-doped silica glass, the silica glass having a maximum alkali concentration of about 0.50 wt % to about 10 wt %, and the time t (seconds) is greater than or equal to t <tcかつ

number

[0009] An aspect of the present disclosure is directed to a multi-core optical fiber preform including a plurality of core canes, each core cane disposed within an axial bore of a sleeve, each core cane including a core section made of alkali-doped silica glass, the silica glass having a maximum alkali concentration of about 0.10 wt % to about 10 wt %, the core section of each core cane being surrounded by a sleeve along the height of the core cane and by covers disposed at first and second axial ends of the core section, the covers including silica glass having a chlorine concentration of about 0.05 wt % or less.

[0010] Although many different embodiments are listed, the embodiments may exist individually or in any combination where possible. Exemplary embodiments are shown and described below. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a process for producing a multi-core optical fiber according to an embodiment of the present disclosure. [Figure 2] 1 illustrates a sleeve having one or more axial through holes according to an embodiment of the present disclosure. [Figure 3A] 10 illustrates a process of inserting a core cane into a sleeve according to an embodiment of the present disclosure. [Figure 3B] 10 illustrates a process of inserting a core cane into a sleeve according to an embodiment of the present disclosure. [Figure 4A] 1 illustrates a core cane having an alkali-doped core according to an embodiment of the present disclosure. [Figure 4B] 4B illustrates a cross-sectional view of the core cane of FIG. 4A through line AA, according to an embodiment of the present disclosure. [Figure 4C] 1 illustrates a cover disposed over the end of an alkali-doped core according to an embodiment of the present disclosure. [Figure 4D] 1 illustrates a cover disposed over the end of an alkali-doped core according to an embodiment of the present disclosure. [Figure 4E]1 illustrates a cover disposed over the end of an alkali-doped core according to an embodiment of the present disclosure. [Figure 4F] 1 illustrates a cover disposed over the end of an alkali-doped core according to an embodiment of the present disclosure. [Figure 5A] 10 illustrates a process of disposing a cover on the end of an alkali-doped core according to an embodiment of the present disclosure. [Figure 5B] 10 illustrates a process of disposing a cover on the end of an alkali-doped core according to an embodiment of the present disclosure. [Figure 6] 1 illustrates a preform assembly according to an embodiment of the present disclosure. [Figure 7] 1 illustrates a preform assembly in a draw tower furnace and connected to a vacuum system, according to an embodiment of the present disclosure. [Figure 8] 1 shows a plot of exposure temperature versus nucleation forming rate according to an embodiment of the present disclosure. [Figure 9] 1 shows a plot of exposure temperature versus devitrification growth rate according to an embodiment of the present disclosure. [Figure 10] 1 shows a plot of crystalline volume fraction for potassium-doped glasses at a concentration of 0.5 mol % (=0.78 wt %) as a function of exposure temperature versus exposure time, according to an embodiment of the present disclosure. [Figure 11] 1 shows a plot of crystalline volume fraction for potassium-doped glasses at a concentration of 1 mol% (=1.56 wt%) as a function of exposure temperature versus exposure time, according to an embodiment of the present disclosure. [Figure 12] FIG. 1 is a schematic diagram illustrating an exemplary drawing system, according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0012] Additional features and advantages of the present disclosure will be set forth in the following detailed description, and will become apparent to those skilled in the art from the description, or may be learned by practicing the present disclosure as set forth in the following description, taken in conjunction with the claims and accompanying drawings.

[0013] As used herein, the term "and / or," when used in a list of two or more items, means that any one of the listed items can be used alone, or any combination of two or more of the listed items can be used. For example, if a composition is described as containing components A, B, and / or C, the composition can contain A only, B only, C only, A and B in combination, A and C in combination, B and C in combination, or A, B, and C in combination.

[0014] In this document, relationship terms such as first and second, top and bottom, etc. are used only to distinguish one entity or operation from another entity or operation and do not necessarily require or imply any actual such relationship or order between such entities or operations.

[0015] As used herein, the word "comprises" includes the term "consists of" as a special case, so that, for example, the phrase "A comprises B and C" is understood to include the case where "A consists of B and C."

[0016] In the case of an assembly made of different glass components that are not bonded to one another, the term "bonded" as the term is used herein means heating the assembly so that the glass components flow and bond or seal to one another to maintain the general overall configuration of the glass components, i.e., the glass components can form a unified glass component that does not substantially change its basic shape.

[0017] The term "axial bore" or "axial through bore" means a bore that runs parallel to the axial direction, i.e., parallel to the central axis or centerline.

[0018] As used herein, the term "cylindrical" means a three-dimensional shape formed by taking a two-dimensional shape and projecting it along a third dimension perpendicular to the plane of the two-dimensional shape. Thus, as the term is used herein, a cylinder may have a cross-sectional shape other than circular.

[0019] "Refractive index" refers to the refractive index at a wavelength of 1550 nm, unless otherwise specified.

[0020] A "refractive index profile" is the relationship between refractive index or relative refractive index and radius. For relative refractive index profiles depicted herein as having step boundaries between adjacent core and / or cladding regions, normal variations in processing conditions may prevent obtaining sharp step boundaries at the interfaces of the adjacent regions. While the boundaries of a refractive index profile may be depicted herein as step changes in refractive index, it is understood that in practice the boundaries may be rounded or may otherwise deviate from a perfect step function characteristic. It is further understood that the value of the relative refractive index may vary with radial position within either the core and / or cladding regions. When the relative refractive index varies with radial position in a particular region of the fiber (e.g., either the core and / or cladding regions), it may be expressed in terms of its actual or approximate functional dependence, or in terms of its value at a particular location within the region, or in terms of an average value applicable to the region as a whole. Unless otherwise specified, when the relative refractive index of a region (e.g., either the core region and / or the cladding region) is expressed as a single value or as a parameter applicable to the region as a whole (e.g., Δ or Δ%), it is understood that the relative refractive index in the region is constant or nearly constant and corresponds to the single value, or that the single value or parameter represents an average value of the non-constant relative refractive index dependence with radial position in the region. For example, if "i" is a region of a glass fiber, then the parameter Δ irefers to the average value of the relative refractive index in a region as defined below, unless otherwise specified. Whether by design or as a result of normal manufacturing variability, the dependence of the relative refractive index on radial position may be sloped, curved, or otherwise non-stationary.

[0021] "Relative refractive index," as used herein, is defined by equation (1):

number

[0022] The average relative refractive index (Δ ave ) is determined from equation (2) below:

number

[0023] The refractive index profile of an optical fiber can be measured using commercially available devices such as the IFA-100 Fiber Index Profiler (Interfiber Analysis LLC, Sharon, MA USA) or the S14 Refractive Index Profiler (Photon Kinetics, Inc., Beaverton, OR USA). These devices measure the reference refractive index, n(r)-n meas Measure the refractive index relative to the reference refractive index n meas is typically a calibrated refractive index matching oil or pure silica glass. The measurement wavelength can be 632.5 nm, 654 nm, 677.2 nm, 654 nm, 702.3 nm, 729.6 nm, 759.2 nm, 791.3 nm, 826.3 nm, 864.1 nm, 905.2 nm, 949.6 nm, 997.7 nm, 1050 nm, or any wavelength in between. The absolute refractive index, n(r), is then used to calculate the relative refractive index, defined above.

[0024] It will be understood by those skilled in the art that the structures of the disclosures and other components described are not limited to any particular material. Other exemplary embodiments of the disclosures disclosed herein may be molded from a wide variety of materials, unless otherwise described herein.

[0025] It is also important to note that the structure and arrangement of elements of the present disclosure as shown in the exemplary embodiments are merely illustrative. While only a few embodiments have been described in detail in this disclosure, those skilled in the art who review this disclosure will readily appreciate that numerous modifications (e.g., the size, dimensions, structure, shape, and proportions of various elements, the values ​​of parameters, mounting arrangements, use of materials, color, orientation, etc.) are possible without materially departing from the novel and obvious teachings and advantages of the recited subject matter. For example, elements shown as integrally molded may be constructed of multiple pieces, or elements shown as multiple pieces may be integrally molded, interface operation may be reversed or otherwise modified, the structure and / or length or width of members, connectors, or other elements of the system may be changed, and the nature or number of adjustment positions provided between elements may be altered. It should be noted that the elements and / or assemblies of the system may be constructed from any of a wide variety of materials that provide sufficient strength or durability in any of a variety of colors, textures, and combinations. Accordingly, all such modifications are intended to be within the scope of the present disclosure. Other substitutions, modifications, changes, and omissions may be made in the desired design, operating conditions, and arrangements, as well as other exemplary embodiments, without departing from the spirit of this disclosure.

[0026] Reference will now be made in detail to the preferred embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

[0027] Referring now to FIG. 1 , an exemplary process 1 for producing a multi-core optical fiber according to an embodiment of the present disclosure is shown. However, it is contemplated that process 1 may also be used to produce a single-core optical fiber. As shown in FIG. 1 , step 10 of process 1 involves forming holes in a glass sleeve. The holes are each sized to accommodate a core cane. The core cane may include an alkali-metal-doped core. In step 20, as discussed below, a cover is disposed on the core cane to prevent devitrification of the core cane. In an embodiment, the cover is welded to the core cane. In step 30, the core cane is inserted into the hole in the glass sleeve. A vacuum holding process is performed in step 40, followed by a bonding process in step 50. During bonding in step 50, the fiber is heated in a bonding furnace at an optimized temperature and exposure time compared to conventional bonding processes. The fiber is then drawn in step 60. In some embodiments, the drawn fiber is a submarine fiber. Each of the steps of Process 1 is discussed in further detail below.

[0028] The core cane disclosed herein may include an alkali metal-doped core. However, when the glass is heated at relatively high temperatures, such as about 730°C to about 1630°C (about 1000 Kelvin (K) to about 1900K), the alkali metal dopant causes the glass to become more susceptible to devitrification. Devitrification is a process in which the glass undergoes structural changes and forms crystalline solids. These crystalline solids are undesirable in optical fibers because they create a "haze" defect in the translucent glass and therefore increase Rayleigh scattering and attenuation in the glass. A significant amount of devitrification can render the glass unprocessable downstream. Heating the glass at too high a temperature for too long a time can cause crystal nucleation and growth, resulting in devitrification of the glass. Inclusions can also cause the formation of nucleation sites in the glass. Alkali-doped glasses are known to reduce the viscosity of the glass, making it more susceptible to devitrification. By reducing the viscosity of the glass, nucleation sites tend to form more readily when the glass is heated.

[0029] The alkali metal-doped core of the core cane disclosed herein tends to form nucleation sites when the core cane is heated, such as during the vacuum hold of step 40 and / or during the bonding of step 50. Therefore, the alkali metal-doped core of the embodiments disclosed herein is capped with a cover to prevent and / or reduce the formation of nucleation sites. Additionally, the core cane (including the alkali metal-doped core) is exposed to a heat treatment (e.g., a bonding step) with an optimized temperature and exposure time to also prevent and / or reduce devitrification of the glass. These embodiments are further disclosed below.

[0030] Referring again to Process 1, and as shown in FIG. 2, a hole 110 is formed in sleeve 100 in step 10. In some embodiments, hole 110 is formed by a precision drill, such as, for example, a diamond-ground core drill and / or an ultrasonically assisted core drill. Sleeve 100 is a cylindrical glass body having a top surface 102 and a bottom surface 104. Furthermore, sleeve 100 is composed of silica (e.g., pure silica or doped silica). As shown in FIG. 2, sleeve 100 has a diameter D S and height H S In some embodiments, the diameter D S is in the range of about 25 mm to about 200 mm, or about 50 mm to about 125 mm, and the height H S In one exemplary embodiment, the diameter D S is about 70 mm, and the height H S is approximately 110 mm. Other diameters and heights are contemplated, as will be apparent to one of ordinary skill in the art.

[0031] The holes 110 are each axial through-holes molded within the interior volume of the sleeve 100. While FIG. 2 shows four holes 110, it is contemplated that the sleeve 100 may include more or fewer holes 110. For example, the sleeve 100 may include one or more holes, two or more holes, four or more holes, six or more holes, eight or more holes, ten or more holes, or twelve or more holes. Furthermore, the holes 110 may have a circular cross-sectional shape, as shown in FIG. 2. It is also contemplated that the holes 110 may include other cross-sectional shapes, and that one or more holes may have a different cross-sectional shape than one or more other holes.

[0032] The holes 110 have a top end that opens to the top surface 102 of the sleeve 100 and a bottom end that opens to the bottom surface 104 of the sleeve 100. Thus, each hole 110 is a continuous opening from the top surface 102 to the bottom surface 104. In some embodiments, the holes 110 each have a diameter of about 2 mm to about 60 mm, or about 5 mm to about 45 mm, or about 10 mm to about 30 mm. It is also contemplated that one or more holes 110 may have a different diameter than one or more other holes.

[0033] The holes 110 may be equally spaced from one another. Furthermore, the holes 110 may be arranged in any configuration and layout known in the art.

[0034] After the bore 110 is formed in the sleeve 100, one or more surfaces of the sleeve 100 may be polished or finely ground. For example, the outer surface of the sleeve 100 may be polished or finely ground to a precise diameter D S and / or the exact height H S Additionally or alternatively, the interior surfaces of the holes 110 may be polished or fine ground to achieve a flatness of about 2 microns or less. Additionally or alternatively, the interior surfaces of the holes 110 may be polished or fine ground. It is also contemplated that the top surface 102 and / or the bottom surface 104 may be polished or fine ground to achieve precise flatness. In some embodiments, the top surface 102 and the bottom surface 104 are fine ground to achieve a surface roughness (RMS) of about 2 microns or less, or about 1 micron or less.

[0035] In step 20 of process 1, a cover is disposed on the core cane (this is discussed further below). In step 30 of process 1, the core cane is inserted into bore 110 of sleeve 100. The cover may be disposed on the core cane either before or after the core cane is inserted into bore 110 of sleeve 100. Thus, in embodiments, step 20 of process 1 may be performed before or after step 30. FIG. 3A shows core cane 140 operably disposed relative to bore 110 of sleeve 100 for insertion of core cane 140 into sleeve 100. FIG. 3B shows core cane 140 positioned within bore 110 of sleeve 100, thus forming an exemplary assembled cane-clad assembly 150. Cane-clad assembly 150 is then connected to a nosecone and handle to form a preform assembly.

[0036] As shown in Figure 4A, each core cane 140 is formed with a glass body including a core section 142 surrounded by an inner cladding section 144. Figure 4A shows the core cane before a cover is disposed over the peripheral end of the core cane, and Figure 4B shows a cross-sectional view of the core cane 140 through line AA in Figure 4A. However, it is contemplated that in embodiments, the glass body of the core cane 140 may not include the inner cladding section 144. In these embodiments, the glass body of the core cane 140 consists only of the core section 142.

[0037] In the final drawn optical fiber produced by the processes disclosed herein, core section 142 forms the core within the fiber, inner cladding section 144 forms the inner cladding within the fiber, and sleeve 100 forms the outer cladding within the fiber.

[0038] The core section 142 may include a top-doped silica glass, and the inner cladding section 144 may include a top-doped silica glass, a bottom-doped silica glass, or an undoped silica glass. Top-doped silica glasses include, for example, silica glasses doped with germanium (e.g., GeO), phosphorus (e.g., P2O5), aluminum (e.g., Al2O3), chlorine (Cl), and / or alkali metals, as discussed further below. In embodiments, the core section 142 may have a higher refractive index than the inner cladding section 144 and the sleeve 100. Therefore, the refractive index (Δ 1,max %), the refractive index of the inner cladding section 144 (Δ2%), and the refractive index of the sleeve 100 (Δ3%) are 1,max %>Δ2% and Δ 1,max %>Δ3%. Furthermore, in some embodiments, Δ 1,max % > Δ3% > Δ2%. In some embodiments, the inner cladding section 144 has a discernible core-cladding boundary with the core section 142 (in a drawn optical fiber). However, it is contemplated that the inner cladding section 144 may lack a clear boundary with the core section 142. Similarly, the inner cladding section 144 may have a discernible core-cladding boundary with the sleeve 100 (in a drawn optical fiber), or may lack a clear boundary with the sleeve 100. It is also contemplated that the inner cladding section 144 may include one or more cladding portions having a different refractive index than one or more other portions of the inner cladding section 144. For example, one or more portions of the inner cladding section 144 may include a trench region (depressed-index cladding region).

[0039] In embodiments disclosed herein, the core section 142 may be doped with an alkali metal, including, for example, potassium, sodium, rubidium lithium, cesium, or a combination thereof. The alkali metal dopant may be a metal oxide of these alkali metals, such as KO, NaO, RbO, LiO, CsO, or a combination thereof. The core section 142 can include one or more alkali metal dopants such that the maximum concentration of alkali metal dopants in the core section 142 is between about 0.10% and about 10% by weight, or between about 0.25% and about 8% by weight, or between about 0.50% and about 10% by weight, or between about 0.50% and about 6% by weight, or between about 0.40% and about 5% by weight, or between about 0.75% and about 4% by weight, or between about 0.75% and about 5% by weight, or between about 0.78% and about 4% by weight, or between about 0.78% and about 5% by weight, or between about 1% and about 2% by weight, or between about 1.56% and about 6% by weight, or between about 1.56% and about 4% by weight. Alkali concentrations less than 0.10% by weight do not sufficiently reduce attenuation in the drawn optical fiber, and alkali concentrations greater than 10% by weight are not cost-effective to produce. The maximum alkali concentration disclosed herein is obtained at the centerline of the core section 142. The concentration of alkali metal dopant decreases radially outward from the centerline of the core section 142, with the concentration highest at the centerline. Furthermore, the maximum alkali concentration disclosed herein is obtained by Electron Probe Microanalysis (EPMA) prior to the stretching of step 60 of Process 1. For such EPMA measurements, the core sections 142 are each polished, and a conductive carbon coating is applied to the polished surface. EPMA analysis is then performed on a JEOL 8500F Hyperprobe (2008) electron microprobe analyzer. Line scan analysis is performed using a focused beam, stepped at 1-micrometer step intervals across the diameter of each core section 142, traversing the core center. Typical beam parameters used for the analysis are a beam current of 50-100 nA and an accelerating potential of 15 keV, with an on-peak count time ranging from 10 to 30 seconds.

[0040] The core section 142 may further include other dopants in addition to the alkali metal dopant. For example, the core section 142 may include a halide dopant, such as fluorine. In embodiments, only the radially central portion of the core section 142 includes the alkali metal dopant. Therefore, the radially outer portion of the core section 142 does not include the alkali metal dopant. In these embodiments, the radially central portion of the core section 142 (including the alkali metal dopant) should be chlorine-free or contain only minimal amounts of chlorine. For example, the radially central portion of the core section may include about 0.05% chlorine by weight or less, or about 0.04% chlorine by weight or less, or about 0.03% chlorine by weight or less, or about 0.02% chlorine by weight or less, or about 0.01% chlorine by weight or less, or about 0.00% chlorine by weight. It should also be noted that in these embodiments, the radially outer portion of the core section 142 (not including the alkali metal dopant) may include a greater amount of chlorine.

[0041] It is also contemplated that the inner cladding section 144 and / or the sleeve 100 may be doped with one or more dopants, such as a halide dopant. In some embodiments, the halide is fluorine.

[0042] 4A, the alkali-metal-doped core section 142 is radially surrounded by an inner cladding section 144. More specifically, the inner cladding section 144 has a height H CThe alkali-doped glass surrounds the core section 142 along the axial direction. However, the first and second axial ends 141, 143 (i.e., top and bottom) of the core section 142 are not surrounded by the inner cladding section 144 and are therefore prone to devitrification when the core section 142 is heated. As discussed above, the alkali-metal-doped core section 142 has a relatively low viscosity, which can lead to the formation of nucleation sites as the glass is heated. With further heat treatment, the nucleation sites undergo crystal growth and contribute to crystallization and devitrification in the optical fiber preform and / or drawn fiber. The inventors of the present disclosure have found that such nucleation sites are more easily formed in the exposed portions of the alkali-doped glass compared to the bulk of the glass. Thus, referring to FIG. 4A , when the core cane 140 is heated to a sufficiently high temperature, nucleation sites tend to form at the exposed ends 141, 143 of the core section 142 (not surrounded by the inner cladding section 144). These nucleation sites mature into devitrification throughout the glass structure. C Note that this portion of the glass along is enclosed by inner cladding section 144 (and sleeve 100) and is not exposed, so is not prone to forming nucleation sites.

[0043] Therefore, embodiments of the present disclosure include capping the ends 141, 143 of the core section 142 with a cover 146 so that the ends 141, 143 are no longer exposed when the core cane 140 is heated. With such a cover 146 disposed on the ends 141, 143, the alkali-doped core section 142 no longer has exposed portions that tend to form nucleation sites. Thus, the capped core section 142 can be heated without devitrifying the glass. As shown in FIG. 4A, the length (i.e., diameter) of the ends 141, 143 is greater than the height H of the core section 142. C In the embodiment disclosed herein, it is the length of the ends 141, 143 that is capped with the cover 146.

[0044] More specifically, in the embodiments disclosed herein, the covers 146 are disposed over at least the ends 141, 143 of the core section 142. FIG. 4C illustrates an embodiment in which covers 146 are disposed on both ends of the core cane 140, so that they each cover the entire end of the core cane 140. Thus, each cover 146 is disposed over the ends 141, 143 of the core section 142, as well as the end of the inner cladding section 144. In this embodiment, the covers 146 form end caps that sandwich the core section 142 and the inner cladding section 144. Furthermore, in the embodiment of FIG. 4C, the covers 146 are in direct contact with both the core section 142 and the inner cladding section 144. As disclosed herein, the covers 146 are each devitrification inhibitors that prevent devitrification of the glass. The covers 146 may also be referred to herein as end caps, barriers, bodies, block bodies, or barricades. The core section 142 has a cover 146 at the ends 141, 143, and a height H C It should be noted that the presence of the inner cladding section 144 covering the core section 142 along the length of the cover 146 means that the core section 142 is completely enveloped and encapsulated by the glass of the cover 146 and the inner cladding section 144 .

[0045] The cover 146 may be composed of top-doped silica glass, bottom-doped silica glass, or undoped silica glass. More specifically, the cover 146 may include the same materials as those disclosed above for the core section 142, inner cladding section 144, and / or sleeve 100. However, the cover 146 should each contain no more than about 0.05% chlorine by weight, or no more than about 0.04% chlorine by weight, or no more than about 0.03% chlorine by weight, or no more than about 0.02% chlorine by weight, or no more than about 0.01% chlorine by weight, or no more than about 0.00% chlorine by weight. Alkali dopants (e.g., potassium) in the core section 142 react with the chlorine to form, for example, potassium chloride (KCl), which forms cristobalite in the resulting preform. Such preforms cannot be drawn into optical fiber and are instead discarded. Therefore, the cover 146 should be chlorine-free or essentially chlorine-free to prevent the formation of potassium chloride. In some embodiments, cover 146 is doped with fluorine such that cover 146 has a fluorine concentration of about 0.0% to about 2.0% by weight, or about 0.1% to about 1.8% by weight, or about 0.2% to about 1.6% by weight, or about 0.5% to about 1.4% by weight, or about 0.8% to about 1.2% by weight, or about 1.0% to about 1.5% by weight. In embodiments, the fluorine concentration in cover 146 is selected so that the viscosity of cover 146 is substantially the same as the viscosity of core section 142.

[0046] 4D shows core cane 140 with covers 146 disposed on its ends inserted into sleeve 100 to form core-clad assembly 150. In the embodiment of FIG. 4D, covers 146 are disposed over the ends of both core section 142 and inner cladding section 144 (as shown in FIG. 4C).

[0047] 4E shows a second embodiment in which a cover 146 is disposed over the ends 141, 143 of the core section 142. However, in this embodiment, the cover 146 is not disposed over the end of the inner cladding section 144. In this embodiment, the cover 146 forms an end cap that sandwiches the core section 142. Furthermore, in the embodiment of FIG. 4E, the cover 146 is in direct contact with the core section 142, and the inner cladding section 144 is exposed at the end of the core cane 140.

[0048] 4E , cover 146 is disposed over the entirety of each of ends 141, 143. In some other embodiments, cover 146 may be disposed over less than the entirety of each of ends 141, 143, so long as cover 146 is disposed over the alkali-containing portions of ends 141, 143. As discussed above, in some embodiments, only a radially central portion of core section 142 includes an alkali metal dopant, and a radially outer portion of core section 142 does not include an alkali metal dopant. Thus, it is contemplated that cover 146 may be disposed on the radially central portions of ends 141, 143, but not on the radially outer portions of ends 141, 143.

[0049] 4F shows yet another embodiment in which cover 146 is disposed over not only the ends of core section 142 and inner cladding section 144, but also the end of sleeve 100. Thus, cover 146 is disposed over the entire end of core-clad assembly 150. In the embodiment of FIG. 4F, cover 146 is in direct contact with core section 142, inner cladding section 144, and sleeve 100.

[0050] Cover 146 has a height H (shown in FIG. 4F ) of about 0.1 mm or more, or about 0.2 mm or more, or about 0.3 mm or more, or about 0.4 mm or more, or about 0.5 mm or more, or about 0.6 mm or more, or about 0.7 mm or more, or about 0.8 mm or more, or about 0.9 mm or more, or about 1.0 mm or more, or about 0.1 mm to about 1.0 mm, or about 0.2 mm to about 0.9 mm, or about 0.3 mm to about 0.8 mm, or about 0.4 mm to about 0.7 mm, or about 0.5 mm to about 0.6 mm, O The height H O are referenced with respect to FIG. 4F, it should be noted that any of the embodiments disclosed herein may include a cover 146 having these disclosed heights.

[0051] FIG. 5A illustrates the process of attaching a cover 146 to the end of a core cane 140. In this example, the cover 146 is attached to the end 141 of the core section 142. In embodiments disclosed herein, the core cane 140 and the cover 146 are heated by a flame 160, and then the core cane 140 and the cover 146 are brought into contact to weld and / or fuse the two components together. As shown in FIG. 5B, in some embodiments, the core cane 140 is first heated to create a protruding tip 164 on the core cane 140, which helps adhere the components together. When bonding the cover 146 to the core cane 140, it is important to keep the temperature of the core cane 140 above the melting point of cristobalite (which is approximately 1700° C.) to maintain the stability of the glass so that it does not bend to one side or the other. In some preferred embodiments, the temperature is maintained at a temperature of at least about 1800°C, or at least about 1900°C, or at least about 2000°C, or at least about 2100°C, or at least about 2200°C, or at least about 2300°C, or at least about 2400°C, or at least about 2500°C. The maximum temperature of the core cane 140 (when joining the core cane 140 to the cover 146) should be about 3000°C, or about 2900°C, or about 2800°C, or about 2700°C, or about 2600°C. It should also be noted that similar welding and / or fusing processes can be used to attach the cover 146 to the inner cladding section 144 and sleeve 100 in embodiments in which the cover 146 extends beyond the boundary of the core section 142 at the peripheral edges.

[0052] Once the core cane 140 (with the cover 146 disposed thereon) is inserted into the bore 110 of the sleeve 100 to form the cane-clad assembly 150, a nose cone and handle are attached to the assembly to form the preform assembly. FIG. 6 shows an assembled preform assembly 170 including multiple stacked sleeves 100. In the exemplary embodiment of FIG. 6, the preform assembly 170 includes four sleeves 100. However, it should be noted that the preform assembly 170 may include only one sleeve 100. In embodiments using multiple sleeves 100, each core cane 140 extends the length of the combined sleeves. Thus, each core cane 140 is disposed within the axial bore of each stacked sleeve 100. In these embodiments using multiple sleeves 100, the cover 146 is still disposed on the end of the core cane 140.

[0053] Further, preform assembly 170 includes a handle 172 and a nosecone 174, each molded from glass. As discussed further below, these components are held together during a vacuum holding process (step 40 of Process 1). When in the stacked arrangement shown in FIG. 6 , handle 172 is disposed vertically above sleeve(s) 100, which are disposed vertically above nosecone 174. When in the stacked arrangement shown in FIG. 6 , handle 172 is in direct contact with the top sleeve 100, and each sleeve 100 is in direct contact with an adjacent sleeve 100. Furthermore, the bottom-most sleeve 100 is in direct contact with nosecone 174. Note also that top cover 146 is in direct contact with handle 172, and bottom cover 146 is in direct contact with nosecone 174. The handle 172, sleeve(s) 100, and nosecone 174 are cleaned (eg, acid washed and then rinsed with deionized water) before being assembled into a stacked formation.

[0054] As discussed further below, the handle 172 and the nosecone 174 are held together with the sleeve(s) 100 using vacuum pressure. While the handle 172 and the nosecone 174 are secured to the sleeve(s) 100 with vacuum pressure, these components are not sealed together. Because these components are not sealed together, the ends 141, 143 of the core section 142 are still exposed to the surrounding atmosphere when the glass is heated, which tends to cause the alkali-doped core section 142 to form nucleation sites. Therefore, to prevent the formation of nucleation sites (as discussed above), a cover 146 is disposed over the alkali-doped core section 142.

[0055] 7, the stacked preform assemblies 170 are connected to a vacuum system 180, and together the stacked preform assemblies 170 and vacuum system 180 comprise a preform system 200. The vacuum system 180 supports the preform assemblies 170 and further comprises a conduit 185 that provides an airflow connection between the vacuum system 180 and the preform assemblies 170. As discussed further below, the vacuum system 180 uses air pressure to hold the handle 172, sleeve(s) 100, and nosecone 174 together when in a stacked arrangement of the preform assemblies 170.

[0056] Upon activation, the vacuum system 180 draws air from a channel in the sleeve(s) 100 (formed by the gap between the core cane 140 and the sleeve 100) into the vacuum system 180. The vacuum pull creates a substantial pressure difference ΔP between the channel and the ambient environment 190 surrounding the preform system 200. This pressure difference ΔP causes the handle 172, sleeve(s) 100, and nosecone 174 to hold together when oriented vertically in a stacked arrangement. These components are therefore held together against gravity. In one example, the pressure difference ΔP between the vacuum system 180 and normal ambient pressure at sea level is 100 / 122 mm diameter DS axial compression force of 98.5 kg for a typical assembly having a pressure differential ΔP. In other embodiments, the pressure differential ΔP may be on the order of about 100 kg, with the exact value depending on the weight of the various components of the preform assembly 200, as would be apparent to one skilled in the art. The vacuum tension of the vacuum system 180 shapes the vacuum-held together ("vacuum-held") preform assembly 170.

[0057] After the vacuum hold process of Process 1, step 40, the preform assembly 170 may be bonded in a bonding furnace (Process 1, step 50). In some embodiments, the preform assembly 170 is bonded in the same furnace in which the vacuum hold process disclosed above was performed. In yet some further embodiments, this same furnace is also used to draw the preform into optical fiber. Thus, in these embodiments, the bonding furnace is also a draw tower furnace. Note that the bonding of step 50 is performed after the cover 146 is welded / fused to the ends 141, 143 of the core section 142. Because the glass of the preform assembly 170 is heated during the bonding of step 50, the cover 146 is required to prevent the formation of nucleation sites in the alkali-doped core section 142.

[0058] In addition to providing the cover 146 on the core section 142 to prevent the formation of nucleation sites, the temperature and duration of the bonding and downstream processing of the preform in step 50 are optimized to prevent devitrification in the optical fiber preform and / or drawn optical fiber. It should be noted that even with the cover 146 disposed on the alkali-doped core section 142, devitrification can still form in the bulk of the glass if the glass is heated at a sufficient temperature for a sufficient time. Accordingly, embodiments of the present disclosure include not only positioning the cover 146 on the end of the core section 142 (to prevent the formation of nucleation sites at the exposed ends of the glass), but also managing and optimizing the bonding and exposure heating conditions (to reduce and / or prevent devitrification growth in the bulk of the glass). To manage and optimize the bonding and exposure heating conditions, the inventors of the present disclosure discovered the relationship between crystal nucleation and growth (in alkali-doped glass) and the temperature of the glass, with the crystallization rate correlating with both the nucleation and growth rate and the devitrification growth rate of the alkali-doped glass. By determining this relationship, the inventors of the present disclosure were then able to optimize it to prevent and / or reduce crystal growth, as discussed below.

[0059] To determine the optimized relationship between crystallization rate and temperature, the relationship between nucleation rate and glass temperature was first determined. This relationship is shown in equation (3):

number

number

[0060] The thermodynamic free energy barrier to nucleation, W*, in equation (3) above is the barrier in glass to nucleation forming in glass. Thus, the higher W*, the less prone the glass is to nucleation forming. The value of W* is calculated using equation (5):

number

number

[0061] The viscosity η of the alkali-doped glass in equation (3) above is calculated using equation (7),

number

[0062] Using equation (3) above, the inventors of the present disclosure plotted the nucleation forming rate versus temperature for glasses doped with varying amounts of alkali, as shown in FIG. 8. More specifically, the plot in FIG. 8 includes glasses doped with 1 mol% (=1.56 wt%) potassium, 0.5 mol% (=0.78 wt%) potassium, and 0 mol% potassium. As shown in FIG. 8, the nucleation forming rate in the glass is greatest at about 1350 K (1077°C) and decreases as the temperature of the glass increases or decreases from 1350 K. Furthermore, as shown in FIG. 8, the nucleation forming rate increases as the potassium concentration in the glass increases. This again supports the conclusion that alkali-doped glasses are more prone to nucleation forming than undoped glasses due to the reduced viscosity of the alkali-doped glasses.

[0063] To determine the optimized relationship between crystal growth and temperature (as discussed above), the relationship between devitrification growth rate and glass temperature was also determined. This relationship is shown by equation (8):

number

[0064] Using equation (8) above, the inventors of the present disclosure plotted the devitrification growth rate versus temperature for glasses doped with varying amounts of alkali, as shown in FIG. 9. More specifically, the plot in FIG. 9 includes glasses doped with 1 mol % (=1.56 wt %) potassium, 0.5 mol % (=0.78 wt %) potassium, and 0 mol % potassium. As shown in FIG. 9, the devitrification growth rate in the glass increases as the temperature of the glass increases. Although not shown in FIG. 9, it should be noted that the devitrification growth rate in the glass only increases as the temperature increases up to the glass liquidus temperature, beyond which the growth rate is zero. Furthermore, as shown in FIG. 9, the devitrification growth rate increases as the potassium concentration increases.

[0065] Comparing Figures 8 and 9, the nucleation forming rate in Figure 8 decreases as the temperature increases, and therefore, a higher temperature is favored to reduce nucleation forming. However, the devitrification growth rate in Figure 9 increases as the temperature increases, and therefore, a lower temperature is favored to reduce devitrification growth. Therefore, the inventors of the present disclosure have discovered an optimized exposure temperature for reducing the overall crystallization rate, which depends on both the nucleation forming rate and the devitrification growth rate, as discussed below with reference to Equation (9) and Equation (10) and Figures 10 and 11.

[0066] The following equation (9) provides a calculation of the overall crystal growth as a function of both the nucleation forming rate and the devitrification growth rate for alkali-doped glasses and is provided as follows:

number

number

[0067] The calculation of the nucleation formation rate and devitrification growth rate disclosed above to derive the disclosed crystalline volume fractions is further disclosed in the following references, each of which is incorporated herein by reference: DRUhlmann,'A Kinetic Treatment of Glass Formation,'Journal of Non-Crystalline Solids,vol.7,pg.337-348(1972), Chih-Yao Fang,'A Kinetic Treatment of Glass Formation.VIII:Critical cooling rates for Na2O-SiO2and K2O-SiO2glasses,'Journal of Non-Crystalline Solids,vol.7,pg.465-471(1983), Kazumasa Matusita,'Rate of Homogeneous Nucleation in Alkali Disilicate Glasses,'Journal of Non-Crystalline Solids,vol.11,pg.471-484(1973), and Michael C. Weinberg,'Critical Cooling Rate Calculations for Glass Formation,'Journal of Non-Crystalline Solids, vol. 123, pg. 90-96 (1990).

[0068] Figure 10 is a time-temperature transformation (TTT) plot showing the crystalline volume fraction curve for potassium-doped silica glass with a potassium concentration of 0.5 mol % (=0.78 wt %). Using equation (9) above, the crystalline volume fraction values ​​in Figure 10 were calculated, and the modeled crystalline volume fraction curves in Figure 10 were plotted as a function of exposure time versus exposure temperature. In particular, Figure 10 shows combinations of exposure temperature and time corresponding to three crystalline volume curves (A, B, and C), each with a different crystallized volume fraction. Curves A, B, and C in Figure 10 each show the minimum time required for a given volume fraction to crystallize as a function of exposure temperature.

[0069] The inventors of the present disclosure have discovered that to achieve a crystalline volume fraction below a threshold crystalline volume fraction, which may result in unacceptable attenuation increases or may adversely affect downstream processing of the glass, the exposure temperature and exposure time must be below the desired curves shown in FIG. 10. More specifically, curves A, B, and C in FIG. 10 each represent a threshold crystalline volume fraction. To produce a glass having a crystalline volume fraction below one of these curves, the exposure temperature and exposure time must be below that particular curve. For example, to achieve a desired crystalline volume fraction of 0.01 ppm (curve C) or less in a 0.5 mol % (=0.78 wt %) potassium-doped silica glass, the bonding temperature and exposure time must be below curve C, which is within region X. Thus, region X provides the exposure time and corresponding exposure temperature for achieving a crystalline volume fraction below the desired threshold of 0.01 ppm in a 0.5 mol % (=0.78 wt %) potassium-doped silica glass. Note that any corresponding exposure times and temperatures that fall within Region X should achieve at or below the desired volume fraction of crystallization of Curve C. Also shown in Figure 10, Region X, in embodiments, falls within temperatures of about 1000 K (about 730°C) and about 1925 K (about 1652°C).

[0070] Furthermore, as considered above, the overall crystal volume fraction in the glass depends on both the crystal nucleation forming and the devitrification growth rate. As shown in FIG. 10, the combined effect of nucleation forming and devitrification growth expands at an exposure temperature of about 1550 K (1277° C.), which is the temperature corresponding to the minimum exposure time to reach the desired crystal volume fraction. Therefore, an exposure temperature of about 1550 K requires the most stringent exposure time to reach the desired crystal volume fraction. This process is more tolerant with respect to the exposure time at temperatures above or below 1550 K.

[0071] Based on the results of FIG. 10, the inventors of the present disclosure further calculated that the exposure treatment time of potassium-doped glass having a potassium concentration of 0.5 mol % (= 0.78 wt %) can be calculated using the following equation (11). Note that equation (11) is the cumulative best-fit curve of curves A, B, and C. Also note that the exposure treatment time calculated using the following equation (11) is applicable to the heat treatment of the glass to produce glass with a crystal volume fraction less than the threshold crystal volume fraction. The exposure treatment can be, for example, the fixing in step 50 of process 1 or the vacuum holding in step 40. Equation (11) is tc = 10 ^ (1.86×10 -10 T 4 -9.69×10 -7 T 3 +1.91×10 -3 T 2 [[ID=]))-1.68T + 571.9)(11)[[ID=]] is provided, where tc is the time (seconds) for the glass to crystallize to a volume fraction of 1 ppm, and T is the exposure temperature (K). To achieve a crystal volume fraction less than the 1 ppm threshold, the exposure time (t) of the glass needs to be less than tc (such that t < tc). Therefore, for example, when heating the glass during the fixing in step 50 of process 1, the fixing exposure time must be less than the time tc. In an embodiment, the best-fit curve of equation 11 is at a temperature of about 1000 K (about 730° C.) to about 1925 K (about 1652° C).

[0072] Figure 11 is a time-temperature transformation (TTT) plot showing the crystalline volume fraction curve for potassium-doped silica glass with a potassium concentration of 1 mol% (=1.56 wt%). Using equation (9) above, the crystalline volume fraction values ​​in Figure 11 were calculated, and the modeled crystalline volume fraction curves in Figure 11 were plotted as a function of exposure time versus exposure temperature. Figure 11 shows combinations of exposure temperature and time corresponding to three crystallization volume curves (A', B', C'), each with a different crystallization volume fraction. Curves A', B', and C' in Figure 11 each show the minimum exposure time required for a given volume fraction to crystallize as a function of exposure temperature.

[0073] The inventors of the present disclosure have discovered that to achieve a desired crystalline volume fraction below a threshold crystalline volume fraction that may result in unacceptable attenuation increases or may adversely affect downstream processing of the glass, the exposure temperature and exposure time must be below the desired curves shown in FIG. 11. More specifically, curves A', B', and C' in FIG. 11 each represent a threshold crystalline volume fraction. To produce a glass having a crystalline volume fraction below one of these curves, the exposure temperature and exposure time must be below that particular curve. For example, to achieve a desired crystalline volume fraction of 0.01 ppm or less (curve C') in a 1 mol% (=1.56 wt%) potassium-doped silica glass, the bonding temperature and exposure time must be below curve C', which is within region Y. Thus, region Y provides the exposure time and corresponding exposure temperature for achieving a desired crystalline volume fraction of 0.01 ppm or less in a 1 mol% (=1.56 wt%) potassium-doped silica glass. Note that any corresponding exposure times and temperatures that fall within region Y should achieve the desired crystalline volume fraction or less than curve C'. As also shown in Figure 11, region Y, in embodiments, falls within temperatures of about 1000 K (about 730°C) and about 1925 K (about 1652°C).

[0074] Furthermore, as considered above, the overall crystal volume fraction in the glass depends on both the crystal nucleation forming and the devitrification growth rate. As shown in FIG. 11, the combined effect of nucleation forming and devitrification growth expands at an exposure temperature of about 1550 K (1277 °C), which is the temperature corresponding to the minimum exposure time to reach the desired crystal volume fraction. Therefore, an exposure temperature of about 1550 K requires the most stringent exposure time to reach the desired crystal volume fraction. This process is more tolerant with respect to the exposure time at temperatures above or below 1550 K.

[0075] Based on the results of FIG. 11, the inventors of the present disclosure further calculated that the fixed exposure treatment time of potassium-doped glass having a potassium concentration of 1 mol% (= 1.56 wt%) can be calculated using the following equation (12). Note that equation (12) is the cumulative best-fit curve of curves A’, B’, and C’. Also note that the exposure treatment time calculated using the following equation (12) is applicable to the heat treatment of the glass to produce glass with a crystal volume fraction less than the threshold crystal volume fraction. The exposure treatment can be, for example, the fixing in step 50 of process 1 or the vacuum holding in step 40. Equation (12) is tc = 10 ^ (1.67×10 -10 T 4 - 8.68×10 -7 T 3 + 1.7×10 -3 T 2 - 1.5T + 506)(12) is provided, where tc is the time (seconds) for the glass to crystallize to a volume fraction of 1 ppm, and T is the exposure temperature (K). To achieve a crystal volume fraction less than the 1 ppm threshold, the exposure time (t) of the glass needs to be less than tc (such that t < tc). Therefore, for example, when heating the glass during the fixing in step 50 of process 1, the fixing exposure time must be less than the time tc. In an embodiment, the best-fit curve of equation 12 is at a temperature of about 1000 K (about 730 °C) to about 1925 K (about 1625 °C).

[0076] Comparing FIGS. 10 and 11, it should be noted that curves A', B', and C' in FIG. 11, which have a relatively high potassium concentration of 1 mol% (= 1.56 wt%), have a more severe exposure time at the same corresponding exposure temperature than curves A, B, and C in FIG. 10, which have a relatively low potassium concentration of 0.5 mol% (= 0.78 wt%). Thus, meeting the exposure time and exposure temperature of curves A', B', and C' in FIG. 11 not only achieves a crystal volume fraction in silica glass with a potassium concentration less than the threshold crystal volume fraction of 1 mol% (= 1.56 wt%) of potassium, but also in silica glass with a potassium concentration of 0.5 mol% (= 0.78 wt%) of potassium. More specifically, the exposure time and temperature of curves A', B', and C' achieve a desired crystal volume fraction in silica glass having 0.5 mol% (= 0.78 wt%) to 1 mol% (= 1.56 wt%) of potassium. Stated in another way, the exposure time (t) should be less than the time (tc) for the glass to crystallize to a volume fraction of 1 ppm (t < tc), and achieve a desired crystal volume fraction in silica glass having 0.5 mol% (= 0.78 wt%) to 1 mol% (= 0.78 wt%) of potassium.

[0077] Referring again to FIG. 7, in the embodiments disclosed herein, the preform assembly 170 is disposed within the furnace 220 (the furnace can be, for example, a fusing furnace, a re-draw furnace, or a fusing and draw tower furnace). During fusing, the furnace 220 is heated by a lower heater 210 that creates a hot zone 215 within the furnace. The hot zone 215 has a maximum temperature within the range of about 1200 °C to about 2100 °C, or about 1300 °C to about 1800 °C, or about 1400 °C to about 1600 °C.

[0078] Once disposed within furnace 220, preform assembly 170 slowly lowers toward lower heater 210 as it is consumed and drawn into optical fiber. Additionally, in some embodiments, the vacuum hold process disclosed above occurs simultaneously with preform assembly 170 slowly lowering toward lower heater 210. Thus, a vacuum pull from vacuum system 180 occurs while preform assembly 170 is consumed and drawn into optical fiber. Furnace 220 may contain one or more inert gases in addition to preform assembly 170.

[0079] 12 illustrates a drawing system 300 for drawing optical fiber (step 60 of Process 1) according to an embodiment disclosed herein. The exemplary drawing system 300 includes the furnace 220 discussed above. Additionally, the drawing system 300 includes non-contact measurement sensors 310, 315 for measuring the size (e.g., diameter control) of the drawn (bare) fiber 320 exiting the furnace 320. A cooling station 330 is downstream of the measurement sensors 310, 315 and configured to cool the bare fiber 320. A coating station 340 is downstream of the cooling station 330 and configured to deposit a protective coating material 345 on the bare fiber 320 to form a coated fiber 325. A tensioner 350 is downstream of the coating station 340. The tensioner 350 has a surface 355 that tensions (draws) the coated fiber 325. A set of guide wheels 360, each having a surface 365, resides downstream of the tensioner 350. The guide wheels 360 serve to guide the coated fiber 325 to a fiber intake spool (“spool”) 370 for storing the coated fiber 325. Embodiments of the present disclosure can be used to mold single-core optical fibers or multi-core optical fibers.

[0080] The process disclosed herein produces preforms with devitrification levels of about 1.00 ppm or less, or about 0.80 ppm or less, or about 0.75 ppm or less, or about 0.50 ppm or less, or about 0.30 ppm or less, or about 0.25 ppm or less, or about 0.20 ppm or less, or about 0.15 ppm or less, or about 0.10 ppm or less, or about 0.08 ppm or less, or about 0.05 ppm or less, or about 0.02 ppm or less, or about 0.01 ppm or less, or about 0.00 ppm. For purposes of this disclosure, the devitrification levels disclosed herein were measured using X-ray diffraction analysis (XRD). In particular, to measure the devitrification levels disclosed herein, crystallized samples of the glasses were ground into fine powders and run on a diffractometer. X-ray diffraction patterns were obtained for each composition and several crystallization temperatures. The disclosed method of determining devitrification levels using XRD is also disclosed in G.W.Scherer and D.R.Uhlmann, "Diffusion Controlled Crystal-Growth in K2O-SiO2 Compositions," Journal of Non-Crystalline Solids, 23, 59-80 (1977), which is incorporated herein by reference.

[0081] While various embodiments have been described herein, they are presented by way of example only, and not limitation. Accordingly, it should be apparent that adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. It will be apparent to those skilled in the art that various changes in form and detail can be made to the embodiments disclosed herein without departing from the spirit and scope of the disclosure. Elements of the embodiments presented herein are not necessarily mutually exclusive, but can be interchanged to meet various needs, as understood by those skilled in the art.

[0082] It is to be understood that the terms or phrases used herein are for purposes of description, and not of limitation. The breadth and scope of the present disclosure should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

Claims

1. 1. A method for making a multi-core optical fiber preform, the method comprising: and bonding the preform assembly to form the multi-core optical fiber preform. the preform assembly includes a plurality of core canes, each core cane being disposed within an axial bore of a sleeve; each core cane includes a core section comprised of alkali-doped silica glass, said silica glass having a maximum alkali concentration of about 0.10 wt. % to about 10 wt. %; the core section of each core cane is surrounded by the sleeve along the height of the core cane and by covers disposed at first and second axial ends of the core section; The method, wherein the cover comprises silica glass having a chlorine concentration of about 0.05% by weight or less.

2. 10. The method of claim 1, wherein the alkali is sodium, potassium, rubidium, cesium, or a combination thereof.

3. 3. The method of claim 2, wherein the alkali is potassium.

4. The method of any one of claims 1 to 3, wherein the maximum alkali concentration in the alkali-doped core section is from about 0.4 wt% to about 5.0 wt%.

5. the maximum alkali concentration is from about 0.50% to about 10% by weight; Fixing the preform assembly includes subjecting the preform assembly to a temperature T (K) for a time t (seconds); t<tc and [Equation 1] 5. The method of claim 1, comprising exposing the glass to a temperature of 1000° C. for 10 seconds, where tc is the time in seconds for the glass to crystallize.

6. 6. The method of claim 5, wherein the temperature T is between about 1000 K and about 1925 K.

7. 7. The method of claim 5, wherein the maximum alkali concentration is from about 0.75% to about 4% by weight.

8. 8. The method of claim 7, wherein the maximum alkali concentration is from about 0.78% to about 4% by weight.

9. the maximum alkali concentration is from about 0.50% to about 10% by weight; The method includes exposing the preform assembly to a temperature T (K) for a time t (seconds); t<tc and [Equation 2] 5. The method of claim 1, further comprising exposing the glass such that: tc = 0.05; ...

10. 10. The method of claim 9, wherein the temperature T is between about 1000 K and about 1925 K.

11. 11. The method of claim 9 or 10, wherein the maximum alkali concentration is from about 0.75% to about 4% by weight.

12. 12. The method of claim 11, wherein the maximum alkali concentration is from about 0.78% to about 4% by weight.

13. the maximum alkali concentration is from about 1.56% to about 4% by weight; Fixing the preform assembly includes subjecting the preform assembly to a temperature T (K) for a time t (seconds); t<tc and [Equation 3] 5. The method of any one of claims 1 to 4, comprising exposing such that:

14. 14. The method of claim 13, wherein the temperature T is between about 1000 K and about 1925 K.

15. the maximum alkali concentration is from about 1.56% to about 4% by weight; The method includes exposing the preform assembly to a temperature T (K) for a time t (seconds); t<tc and [Equation 4] 5. The method of claim 1, further comprising exposing such that: tc=tc / tc / tc+tc / tc / tc+tc / tc+tc / tc.

16. 16. The method of claim 15, wherein the temperature T is between about 1000 K and about 1925 K.

17. 17. The method of any one of claims 1 to 16, wherein the diameter of the first axial end is perpendicular to the height of the alkali-doped core section and the diameter of the second axial end is perpendicular to the height of the alkali-doped core section.

18. The method of any one of claims 1 to 17, wherein each core cane includes an inner cladding section disposed radially outward of the alkali-doped core section.

19. 20. The method of claim 18, wherein the relative refractive index of the alkali-doped core section is higher than the relative refractive index of the inner cladding section.

20. The method of any one of claims 1 to 19, wherein the relative refractive index of the alkali-doped core is higher than the relative refractive index of the sleeve.

21. 1. A method for making a multi-core optical fiber preform, the method comprising: exposing the preform assembly to a treatment for a time t to form the multi-core optical fiber preform; the preform assembly includes a plurality of core canes, each core cane being disposed within an axial bore of a sleeve; each core cane including a core section comprised of alkali-doped silica glass, said silica glass having a maximum alkali concentration of about 0.50 wt. % to about 10 wt. %; The time t (seconds) is t<tc and [Equation 5] where tc is the time (seconds) for the glass to crystallize to a volume fraction of 1 ppm and T is the exposure temperature (K).

22. 22. The method of claim 21, wherein the exposure temperature is between 1000K and 1925K.

23. 23. The method of claim 21 or 22, wherein the maximum alkali concentration is from about 0.75% to about 10% by weight.

24. The method of any one of claims 21 to 23, wherein exposing the preform to the treatment comprises bonding the preform assembly.

25. The method of any one of claims 21 to 24, further comprising drawing air through holes molded between the plurality of core canes and the sleeve to mold the preform assembly.

26. 26. The method of any one of claims 21 to 25, wherein the alkali is sodium, potassium, rubidium, cesium, or a combination thereof.

27. 27. The method of claim 26, wherein the alkali is potassium.

28. The method of any one of claims 21 to 27, wherein the multi-core optical fiber preform has a devitrification level of about 1.00 ppm or less.

29. 30. The method of claim 28, wherein the devitrification level is about 0.80 ppm or less.

30. 30. The method of claim 29, wherein the devitrification level is about 0.50 ppm or less.

31. A multi-core optical fiber preform, a plurality of core canes, each core cane disposed within an axial bore of a sleeve; each core cane includes a core section comprised of alkali-doped silica glass, said silica glass having a maximum alkali concentration of about 0.10 wt. % to about 10 wt. %; the core section of each core cane is surrounded by the sleeve along the height of the core cane and by covers disposed at first and second axial ends of the core section; The multi-core optical fiber preform, wherein the cover comprises silica glass having a chlorine concentration of about 0.05% by weight or less.

32. 32. The multi-core optical fiber preform according to claim 31, wherein the alkali is sodium, potassium, rubidium, cesium, or a combination thereof.

33. 33. The multi-core optical fiber preform according to claim 32, wherein the alkali is potassium.

34. 34. The multi-core optical fiber preform according to claim 31, wherein a diameter of the first axial end is perpendicular to a height of the alkali-doped core section, and a diameter of the second axial end is perpendicular to a height of the alkali-doped core section.

35. The multi-core optical fiber preform according to any one of claims 31 to 34, wherein each core cane includes an inner cladding section disposed radially outward of the alkali-doped core section.

36. 36. The method of claim 35, wherein the relative refractive index of the alkali-doped core section is higher than the relative refractive index of the inner cladding section.

37. A method according to any one of claims 31 to 36, wherein the relative refractive index of the alkali-doped core section is higher than the relative refractive index of the sleeve.

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