Method for fabricating a hollow-core optical fiber preform

Abstract

1. A method of manufacturing a preform, the method including: positioning at least one glass tube within a glass outer cladding to form a preform predecessor, the glass tube having a first open end and a second open end; forming a preform from the preform predecessor; and heat treating at least one of the preform predecessor and the preform. The heat treating includes sealing the first open end and the second open end of the glass tube to form a closed tube; and heating and / or cooling the glass tube to manipulate gas pressure within the closed glass tube.

Description

[Technical Field]

[0001] This application claims the benefit of priority to U.S. Provisional Application No. 63 / 423,560, filed November 8, 2022, the contents of which are relied upon and incorporated by reference herein in their entirety.

[0002] FIELD OF THE DISCLOSURE The present disclosure is directed generally to hollow core optical fibers, and more particularly to anti-resonant hollow core optical fibers, including methods for making the same. [Background technology]

[0003] An anti-resonant hollow-core optical fiber conventionally consists of a hollow outer cladding within which are disposed multiple structural tubes. The structural tubes have a smaller diameter and thickness than the outer cladding. Each structural tube is bonded to the inner surface of the outer cladding such that the structural tube is disposed around the inner circumference of the outer cladding. Furthermore, each structural tube runs parallel to the length of the outer cladding. The central portion of the outer cladding around which the structural tubes are disposed remains empty as an air-filled void. The resulting anti-resonant fiber guides light through the empty central portion of the core. Such an anti-resonant fiber can reduce optical losses of the signal.

[0004] The structural tube must have specific dimensions to transmit an optical signal within the air-filled cavity formed by the outer cladding. If the structural tube is not fabricated with these specific dimensions, it will not function as an anti-resonant member at a given wavelength. However, producing a structural tube with precise dimensions in a hollow-core optical fiber is extremely difficult. Summary of the Invention

[0005] Exemplary approaches to solving the object are set forth in the independent claims. Various embodiments are defined by the dependent claims.

[0006] The present disclosure is directed to hollow-core optical fibers and methods for making the same. According to aspects of the present disclosure, a structural tube is fabricated to have precise dimensions. More specifically, the structural tube is fabricated so that the internal capillary formed by the structural tube has a specific size, and as a result, the wall thickness of the structural tube has a specific size. This allows the structural tube to be tuned to provide an anti-resonance at a predetermined wavelength. In some embodiments, the structural tube is specifically sized so that the structural tube provides an anti-resonance for a wavelength of about 1550 nm.

[0007] A structural tube can be formed with such precise size dimensions by first forming a glass tube. The glass tube is positioned around the inner cladding of the precursor to the hollow-core optical fiber. The glass tube is then heated to manipulate the gas pressure within the glass tube. More specifically, an increase in temperature increases the gas pressure, and thus a structural tube is formed with a relatively larger capillary size and a relatively smaller wall thickness. One or more glass tubes can be manipulated differently, separate from one or more other glass tubes. The resulting hollow-core optical fiber can be formed with precise dimensions to provide excellent transmission of optical signals.

[0008] According to a first aspect, there is provided a method of manufacturing a preform, the method comprising: positioning at least one glass tube within a glass outer cladding to form a preform predecessor, the glass tube having a first open end and a second open end; forming a preform from the preform predecessor; and heat treating at least one of the preform predecessor and the preform; the heat treating comprising sealing the first open end and the second open end of the glass tube to form a closed tube; and heating and / or cooling the glass tube to manipulate gas pressure within the closed glass tube.

[0009] According to a second aspect, there is provided a method of manufacturing a preform, the method including: positioning at least one glass tube within a glass outer cladding to form a preform predecessor, the glass tube having a first open end and a second open end; forming a preform from the preform predecessor; and heat treating at least one of the preform predecessor and the preform; sealing the first and second open ends of the glass tube to form a closed tube; and heating the glass tube after forming the closed tube to increase gas pressure within the closed glass tube.

[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 1A] 1 illustrates a cross-sectional view of an exemplary hollow-core optical fiber, according to an embodiment of the present disclosure. [Figure 1B] 1 illustrates a cross-sectional view of another exemplary hollow-core optical fiber, according to an embodiment of the present disclosure. [Figure 1C] 1B illustrates an enlarged view of a portion of the exemplary hollow-core optical fiber of FIG. 1A, in accordance with an embodiment of the present disclosure. [Figure 2] 1 illustrates a process for producing a hollow-core optical fiber according to an embodiment of the present disclosure. [Figure 3A] 1 illustrates a cross-sectional view of an exemplary preform precursor to a hollow-core optical fiber, according to an embodiment of the present disclosure. [Figure 3B] 1 illustrates a cross-sectional view of an exemplary preform for a hollow-core optical fiber, according to an embodiment of the present disclosure. [Figure 3C] 3B illustrates radially outward pressure exerted on the wall of the glass tube of the preform precursor of FIG. 3A, according to an embodiment of the present disclosure. [Figure 4A]1 illustrates a process for producing a hollow-core optical fiber according to an embodiment of the present disclosure. [Figure 4B] 1 illustrates a process for producing a hollow-core optical fiber according to an embodiment of the present disclosure. [Figure 4C] 1 illustrates a process for producing a hollow-core optical fiber according to an embodiment of the present disclosure. [Figure 5A] 1 illustrates a redraw process for producing a hollow-core optical fiber according to an embodiment of the present disclosure. [Figure 5B] 5B illustrates the change in dimensions before and after the re-stretching process of FIG. 5A according to an embodiment of the present disclosure. [Figure 5C] 5B illustrates the change in dimensions before and after the re-stretching process of FIG. 5A according to an embodiment of the present disclosure. [Figure 6] 1 illustrates a fiber drawing system according to an embodiment of the present disclosure. [Figure 7A] 1A-1C illustrate cross-sectional views of exemplary examples of a preform precursor and a preform before and after a heat treatment step, according to an embodiment of the present disclosure. [Figure 7B] 1A-1C illustrate cross-sectional views of exemplary examples of a preform precursor and a preform before and after a heat treatment step, according to an embodiment of the present disclosure. [Figure 8A] 1A-1C illustrate cross-sectional views of exemplary examples of a preform precursor and a preform before and after a heat treatment step, according to an embodiment of the present disclosure. [Figure 8B] 1A-1C illustrate cross-sectional views of exemplary examples of a preform precursor and a preform before and after a heat treatment step, according to an embodiment of the present disclosure. [Figure 9] 1 illustrates a cross-sectional view of an exemplary embodiment of a preform for producing a hollow-core optical fiber, according to an embodiment of the present disclosure. [Figure 10A] 1 illustrates an exemplary implementation for producing a preform for a hollow-core optical fiber, according to an embodiment of the present disclosure. [Figure 10B] 1 illustrates an exemplary implementation for producing a preform for a hollow-core optical fiber, according to an embodiment of the present disclosure. [Figure 10C] 1 illustrates an exemplary implementation for producing a preform for a hollow-core optical fiber, 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 employed alone, or any combination of two or more of the listed items can be employed. 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, relational terms such as first and second, upper and lower, etc. are used solely to distinguish one entity or action from another, without necessarily requiring or implying any such actual relationship or order between such entities or actions.

[0015] 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 formed from a wide variety of materials, unless otherwise described herein.

[0016] 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., size, dimensions, structure, shape, and proportions of various elements, parameter values, mounting configurations, 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 formed may be constructed of multiple pieces, or elements shown as multiple pieces may be integrally formed, 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.

[0017] 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.

[0018] As is known in the art, the behavior of gases is governed by gas laws, such as the ideal gas law. PV=nRT, where P is the pressure of the gas, V is the volume of the gas, n is the number of moles of the gas, R is the ideal gas constant, and T is the temperature of the gas. The ideal gas law describes the relationships between temperature, pressure, volume, and number of moles of gas. Aspects of the present disclosure utilize these relationships to efficiently produce hollow-core optical fibers with specific dimensions. In some aspects of the present disclosure, the relationships between temperature, pressure, volume, and number of moles of gas are utilized to efficiently produce hollow-core optical fibers. According to embodiments of the present disclosure, hollow-core optical fibers can be easily produced to have specific dimensions, including specific diameter sizes and wall thickness values.

[0019] 1A, there is shown an exemplary cross-sectional view of a hollow-core optical fiber 10. The fiber 10 comprises an outer cladding 20, one or more structural tubes 30, and a hollow core 40. The structural tubes 30 are disposed radially around the hollow core 40, and the outer cladding 20 is disposed radially around the structural tubes 30.

[0020] The outer cladding 20 is a hollow cylindrical member formed of glass. As such, the outer cladding 20 has a hollow interior and forms a ring-like donut shape in cross section (as shown in FIG. 1A). In some embodiments, the outer cladding 20 is formed of doped or undoped silica glass. The outer cladding 20 can have a length of about 10 cm to about 2 m, or about 25 cm to about 1.5 m, or about 50 cm to about 1 m.

[0021] The structural tubes 30 are glass tubes disposed within the hollow interior of the outer cladding 20. Like the outer cladding 20, the structural tubes 30 are hollow, cylindrical members formed of glass. Thus, the structural tubes 30 each form a ring-like donut shape in cross section (as shown in FIG. 1A). In some embodiments, the structural tubes 30 are formed of doped or undoped silica glass. The structural tubes 30 can each have a length that extends the length (or substantially the length) of the outer cladding 20. Thus, the structural tubes 30 and the outer cladding 20 can have the same length.

[0022] 1A, the hollow interior of each structural tube 30 forms a capillary 35 through which a gas, such as ambient air or nitrogen gas, can flow. As used herein, a capillary 35 is a tubular cavity formed by the walls of the structural tube 30. The outer diameter of each capillary 35 is defined by the inner diameter of each structural tube 30. The wall thickness of the structural tube 30 is carefully selected to optimize anti-resonance conditions in the hollow core 40.

[0023] The hollow core 40 may be formed and defined by the exterior profile of the structural tube 30. The exterior surface of the structural tube 30 therefore forms the outer diameter of the hollow core 40. During use, light is guided along the fiber 10 through the air within the hollow core 40. The structural tube 30 helps to maintain the light within the hollow core 40. This provides for transmission of the optical signal along the fiber 10 with reduced transmission loss.

[0024] In some embodiments, the hollow-core optical fiber 10 is an anti-resonant hollow-core optical fiber. As is known in the art, an anti-resonant fiber is one type of hollow-core fiber. There are three types of hollow-core fibers. The first type is a Bragg hollow-core fiber, in which the cladding is a Bragg structure of concentric periodic dielectric multilayers that confine light in the hollow (air) region. The second type is a photonic bandgap hollow-core fiber, which uses a two-dimensional photonic crystal structure with periodically arranged air holes that confine light in the hollow-core region. The third type is an anti-resonant hollow-core fiber, in which the fiber includes one or more layers of a thin glass structural tube to prevent light from escaping from the air core.

[0025] It should also be noted that the structural tubes 30 can be positioned in various configurations around the inner diameter of the outer cladding 20. FIG. 1A illustrates a first embodiment in which the fiber 10 includes six structural tubes 30 evenly spaced around the inner diameter of the outer cladding 20. However, the fiber 10 can include more or fewer structural tubes 30. For example, the fiber 10 can include two, three, four, five, seven, eight, nine, ten, eleven, twelve, or more structural tubes 30. Furthermore, the structural tubes 30 can be evenly spaced from one another, or the structural tubes 30 can be inconsistently spaced from one another. Note that in some embodiments, one or more structural tubes 30 can contact adjacent structural tubes 30 at a contact point. However, generally, adjacent structural tubes 30 are typically separated by a gap to avoid the formation of a waveguide at the contact point. More specifically, such a contact point between two structural tubes can form an increased wall thickness at the contact point (due to the additional wall thickness of the two structural tubes), which creates a waveguiding region, which reduces the attenuation of the optical fiber.

[0026] 1B illustrates an embodiment in which fiber 10B comprises nested structural tubes 30B. More specifically, structural tube 30B comprises outer tube 32B (first structural tube) and inner tube 34B (second structural tube), such that inner tube 34B is nested within outer tube 32B. Note also that fiber 10B comprises outer cladding 20B and hollow core 40B, similar to the embodiment of FIG. 1A. Furthermore, the present disclosure is not limited to the exemplary arrangements disclosed herein. Other embodiments and arrangements of structural tubes are also contemplated.

[0027] Each structural tube 30 can be formed to have a variety of sizes, including various inner and outer diameters and various wall thickness values. As shown in FIG. 1C, structural tube 30 has a wall thickness dimension T defined by the structural tube's inner and outer diameters. The size of structural tube 30 (including the inner and outer diameters and wall thickness) affects the wavelength at which the fiber is anti-resonant. For example, a wall thickness of about 300 nm to about 450 nm within a hollow-core optical fiber has been found to provide zero transmission through the structural tube's walls (such that the fiber is anti-resonant) at wavelengths from about 1200 nm to about 1600 nm. The thickness of the hollow-core optical fiber can then be varied beyond or beyond the 300 nm to 450 nm range to vary the wavelength window beyond or beyond the 1200 nm to 1600 nm window. Embodiments of the present disclosure are directed to processes and methods for controlling the size of structural tubes, including the inner and outer diameter dimensions of the structural tubes and their wall thicknesses. Such dimensions also affect the size of the capillary 35 formed by structural tube 30. The processes and methods disclosed herein for controlling the size of the structural tube can be implemented during different process steps of producing the hollow-core optical fiber, as discussed below.

[0028] FIG. 2 provides an exemplary process 100 for producing a hollow-core optical fiber. Step 110 of process 100 includes forming a hollow-core preform predecessor, which is a precursor to the hollow-core preform. As discussed further below, in embodiments, the preform predecessor forms a preform after a bonding step, and the preform may then be subjected to a re-drawing step before being drawn into a hollow-core optical fiber. In some embodiments, step 110 of process 100 specifically includes inserting one or more glass tubes into a glass cladding tube to form a preform predecessor. FIG. 3A shows an exemplary embodiment of a glass tube 230 inserted into a glass cladding tube 220 within preform predecessor 200. As shown in FIG. 3A, glass tube 230 forms an inner capillary 235. Glass tube 230 becomes structural tube 30 within the drawn optical fiber, and glass cladding tube 220 becomes outer cladding 20 within the drawn optical fiber. The preform predecessor (e.g., preform predecessor 200) formed in step 110 may be a predecessor of preform 300 of FIG. 3B. Note that FIG. 3A illustrates an exemplary preform predecessor 200, and FIG. 3B illustrates an exemplary preform 300. However, embodiments of the present disclosure, including steps of process 100, may be used with preform predecessors and preforms having configurations other than those shown in FIGS. 3A and 3B. References to the specific embodiments of FIGS. 3A and 3B for steps of process 100 are used for illustrative purposes only.

[0029] 2, in step 120 of process 100, preform predecessor 200 is heated to a temperature above the softening point of the glass, causing preform predecessor 200 to elongate and contract into a smaller diameter preform predecessor, thereby causing inner capillary 235 and glass tube 230 to extend in length.

[0030] In step 130 of process 100, preform predecessor 200 is heat treated to control the dimensions of structural tubes 30 in the resulting preform 300 and the optical fiber drawn from preform 300. As discussed further below, heat treating preform predecessor 200 includes heating and / or cooling the preform predecessor.

[0031] In step 140 of process 100, preform precursor 200 is heated to bond the glass. During bonding, preform precursor 200 may be placed inside a furnace and heated to a temperature above the sintering temperature of the glass, which may be about 1400°C to about 2000°C, or about 1500°C to about 1900°C, or about 1600°C to about 1800°C, or about 1675°C to about 1800°C, or about 1800°C to about 1950°C, or about 1700°C. The glass may also be exposed to chlorine gas to remove impurities and reduce the moisture content of the glass. An additional layer of soot (soot formation) may also be applied to preform precursor 200 followed by an additional bonding step. After the final bonding step, a preform is formed. FIG. 3B shows an exemplary preform 300 comprising a glass tube 330 disposed within a glass cladding tube 320. Glass tube 330 forms inner capillary 335. Furthermore, glass tube 330 becomes structural tube 30 within the drawn optical fiber, and glass cladding tube 320 becomes outer cladding 20 within the drawn optical fiber. In an embodiment, the outer diameter of preform 300 is less than the outer diameter of preform precursor 200.

[0032] It should be noted that in the embodiment, the heat treatment of step 130 occurs before the adhesion process of step 140. Thus, the heat treatment of step 130 occurs before the soot formation of adhesion of step 140.

[0033] Step 150 of process 100 includes heat treating preform 300 to control the dimensions of glass tube 330 within preform 300 and to control the dimensions of structural tube 30 within the optical fiber drawn from preform 300. As discussed further below, heat treating preform 300 includes heating and / or cooling the preform. Note that the heat treating of step 150, in embodiments, occurs after the bonding process of step 140. In particular, the heat treating of step 150 occurs after the soot formation of the bonding of step 140.

[0034] In step 160 of process 100, preform 300 is subjected to a redraw step before the preform is drawn into optical fiber. More specifically, in step 160, preform 300 is heated to a temperature above the softening point of the glass to shrink the preform into a smaller diameter preform. At the end of step 160, the preform may be ready to be drawn into optical fiber. Note that the heat treatment of step 150, in embodiments, precedes the redraw of step 160.

[0035] In step 170, preform 300 is drawn into an optical fiber. More specifically, preform 300 is drawn into a hollow-core optical fiber. Also, note that in some embodiments, process 100 may include more or fewer steps than those shown in FIG. 2 . For example, process 100 may not include heat treatment step 130 or heat treatment step 150. In some embodiments, process 100 includes heat treatment step 130 but not heat treatment step 150. In yet some further embodiments, process 100 includes heat treatment step 130 but not heat treatment step 150, and does not include re-draw step 160. In embodiments in which process 100 does not include re-draw step 160, the preform may be drawn into an optical fiber without being subjected to a re-draw step. In yet some other embodiments, process 100 does not include stretching the preform in step 120 and / or bonding in step 140.

[0036] It should also be noted that in some embodiments, the heat treatment of step 130 may be performed during the stretching of step 120. In particular, heating the preform precursor during step 120 may also include the heat treatment of step 130. Furthermore, in some embodiments, the heat treatment of step 130 and / or step 150 may be performed during the bonding of step 140. Yet in some other embodiments, the heat treatment of step 150 may be performed during the re-stretching of step 160.

[0037] As discussed above, embodiments of the present disclosure include heat-treating the preform predecessor 200 and / or preform 300 to control the dimensions of the glass tube 330 within the preform 300, which in turn controls the dimensions of the structural tube 30 within the drawn optical fiber. In particular, in embodiments, the heat-treating (step 130) precedes soot formation during preform predecessor bonding, and / or the heat-treating (step 150) follows soot formation during preform predecessor bonding. During such heat-treatment(s), the preform predecessor 200 and / or preform 300 are heat-treated to control capillary size and structural tube wall thickness within the drawn optical fiber. As discussed further below, heat-treating the preform predecessor 200 and / or preform 300 includes modifying the gas pressure of the capillaries 235 / 335 within the glass tube 230 / 330.

[0038] The gas pressure within the capillary 235 in the preform precursor 200 and / or within the capillary 330 in the preform 300 can be modified to increase or decrease the size of the capillary. Such a change in gas pressure can increase or decrease the size of the capillary and, therefore, the wall thickness of the glass tube 230 / 330. For example, increasing the gas pressure within the capillary 235 / 335 expands the size of the capillary and, therefore, decreases the wall thickness of the glass tube 230 / 330. This, in turn, causes the capillary 35 in the drawn optical fiber 10 to have a relatively larger outer diameter and a relatively smaller wall thickness relative to the structural tube 30 in the drawn optical fiber 10.

[0039] As an example, FIG. 3C depicts the radially outward pressure on the wall of the glass tube 230 from increased gas pressure within the capillary 235. As shown in FIG. 3C, the radially outward pressure causes the capillary 235 to expand and grow in size, similar to a balloon, while the wall of the glass tube 230 stretches and shrinks to accommodate the larger internal capillary size. Due to the relatively thinner wall of the glass tube 230, less effort is required to expand the glass tube 230 during the fiber drawing process. Therefore, during the fiber drawing process, the glass tube 230 can be more easily expanded to produce a structural tube 30 with a relatively larger diameter. Thus, after the final drawing process, the resulting structural tube 30 is of a relatively larger size with a relatively larger capillary 35.

[0040] In some embodiments, the gas pressure within the capillary 235 / 335 is increased by increasing the temperature of the gas within the capillary. Additionally, as discussed below, in embodiments, the ends of the glass tube 230 / 330 are sealed to facilitate changes in pressure within the capillary.

[0041] FIG. 4A illustrates a process 400 for heat-treating the preform precursor 200 and / or the preform 300. Thus, the process 400 provides a more detailed representation of step 130 and / or step 150 of the process 100. The process 400 may be used when heating a gas within a capillary to expand the capillary. In step 410 of the process 400, the ends of the glass tube 230 / 330 are sealed to close the capillary 235 / 335. For example, the top and / or bottom of the glass tube 230 / 330 may be sealed through welding, heating, or any other means for sealing the ends of a glass tube. The ends of the glass tube 230 / 330 may be sealed while the gas inside the glass tube is at room temperature (approximately 25° C.). After the sealing process, the glass tube 230 / 330 forms a closed tube.

[0042] The sealing of the glass tube 230 / 330 helps to maintain a constant volume and number of moles of gas within the capillary 235 / 335. With such a constant gas volume and moles, the gas pressure within the capillary can be more easily adjusted and controlled as the temperature increases.

[0043] Step 420 of process 400 involves heating gas within glass tube 230 / 330. More specifically, step 420 involves heating gas within one or more glass tubes 230 within preform predecessor 200 and / or one or more glass tubes 330 within preform 300. During step 420, glass tube 230 / 330 may be heated with, for example, a heated sleeve, a ring burner, a heated air blower, an induction coil, a torch, an isothermal heating element, a laser, including a CO or CO laser, a plasma heater, a furnace, or any other heating element known in the art.

[0044] It should also be noted that one or more glass tubes may not be exposed to the heating of step 420. Thus, in one exemplary embodiment, a first glass tube in a preform (or preform precursor) is heated to increase the capillary size of that glass tube, while an adjacent second glass tube is not exposed to the heating of heat treatment steps 130 / 150. Thus, these pre-drawing steps do not change the capillary size of the second glass tube.

[0045] In some particular embodiments, step 420 includes heating one or more nested glass tubes (e.g., inner tube 34B) differently from one or more other glass tubes (e.g., outer tube 32B) during heat-treating steps 130 / 150. For example, a predecessor to inner tube 34B may be heated to a higher temperature than a predecessor to outer tube 32B during heat-treating steps 130 and / or 150, such that the predecessor to inner tube 34B has a relatively greater expansion rate than the predecessor to outer tube 32B. In embodiments in which one or more nested glass tubes (e.g., inner tube 34B of FIG. 1B) are heated differently from one or more other glass tubes (e.g., outer tube 32B of FIG. 1B), it is contemplated that one or more glass tubes may be sealed while one or more glass tubes are not sealed. For example, a predecessor to inner tube 34B may be sealed and heat-treated (during steps 130 and / or 150), while a predecessor to outer tube 32B is not sealed and / or heat-treated.

[0046] In some embodiments, the glass tube is heated uniformly along its length. In other embodiments, the glass tube is heated inconsistently along its length. For example, it is contemplated that the glass tube may be heated such that the gas within the glass tube is heated to a first temperature in a first portion of the glass tube, a second temperature in a second portion of the glass tube, and a third temperature in a third portion of the glass tube, the first temperature being higher than the second temperature, and the second temperature being higher than the third temperature, such that the first, second, and third portions are disposed longitudinally along the length of the glass tube. In yet other embodiments, the glass tube may be heated inconsistently radially along the axial length of the glass tube. In these embodiments, for example, the glass tube may be heated such that the gas within the glass tube is heated to a relatively higher temperature in a radially central portion of the glass tube and a relatively lower temperature in a radially peripheral portion of the glass tube. In another embodiment, the glass tube can be heated such that the gas within the glass tube is heated to a relatively high temperature on a first radial side (e.g., the left side) of the glass tube and to a relatively low temperature on a second radial side (e.g., the right side) of the glass tube.

[0047] During step 420 of process 400, the glass tube 230 / 330 may be heated to a temperature above the strain point of the glass (approximately 1100°C or higher). In some embodiments, the glass tube 230 / 330 is heated to a temperature above the softening point of the glass (approximately 1100°C or higher, or approximately 1200°C or higher, or approximately 1300°C or higher). Such heating of the glass allows the glass tube 230 / 330 to expand and contract (due to increased pressure within the glass tube), resulting in thinner walls and an increase in the size of the capillary 235 / 335. Because the ends of the glass tube 230 / 330 are sealed (thus the volume and number of moles of gas within the glass tube remain constant), heating the glass tube 230 / 330 increases the gas pressure within the capillary 235 / 335. Such an increase in gas pressure causes the capillary 235 / 335 to expand to a larger size while the wall thickness of the glass tube 230 / 330 decreases. In some embodiments, the glass tube 230 / 330 (and also the gas within the tube) is heated to a temperature of about 1100°C to about 2200°C, or about 1500°C to about 2100°C, or about 1600°C to about 2000°C, or about 1700°C to about 1900°C, or about 1800°C to about 2000°C, or about 1100°C to about 1700°C, or about 1300°C to about 1600°C, or about 1400°C to about 1500°C, or about 1450°C to about 1550°C during step 420 of process 400. The glass tube 230 / 330 may be heated for a period of about 0.01 seconds to about 12 hours, or about 1 second to about 10 hours, or about 5 seconds to about 8 hours, or about 30 seconds to about 6 hours, or about 1 minute to about 4 hours, or about 10 minutes to about 2 hours, or about 30 minutes to about 8 hours, or about 30 minutes to about 4 hours, or about 30 minutes to about 2 hours, or about 1 hour to about 4 hours. As discussed above, heating the glass tube 230 / 330 in step 420 causes the structural tube 30 in the drawn optical fiber to have a larger capillary diameter and thinner walls.

[0048] As discussed above, the steps of process 400 may be performed during either steps 130 and / or 150 of process 100. Thus, the steps of process 400 are performed before drawing the optical fiber (before step 170 of process 100).

[0049] As discussed above, the heat treatment of steps 130 and / or 150 may be separate and distinct from the stretching of step 120, the fixing of step 140, and the re-stretching of step 160. However, in other embodiments, the heat treatment of steps 130 and / or 150 is part of and encompasses at least one of the stretching of step 120, the fixing of step 140, and / or the re-stretching of step 160. For example, heating the preform precursor to stretch it during the stretching of step 120 may also include the heat treatment of step 130 (in conjunction with sealing the ends of the preform precursor). Thus, the heating is sufficient not only to stretch the preform precursor (as discussed above), but also to heat-treat the preform precursor to expand the capillaries. As another example, heating the preform to fix it during the fixing of step 140 may also include the heat treatment of step 130 (in conjunction with sealing the ends of the preform precursor). Thus, the heating is sufficient not only to bond the preform precursor (as discussed above), but also to heat-treat the preform precursor, causing the capillaries to expand. As another example, heating the preform to re-stretch it during re-stretching of step 160 may also include the heat-treating of step 150 (in conjunction with sealing the ends of the preform). Thus, the heating is sufficient not only to re-stretch the preform (as discussed above), but also to heat-treat the preform, causing the capillaries to expand. Note also that the heat-treating process affects the amount of expansion within structural tube 30. More specifically, when heating the glass tube during the heat-treating of steps 130 and / or 150, a higher change in temperature causes a greater amount of expansion compared to a lower change in temperature.

[0050] In some exemplary embodiments of the processes disclosed herein, during step 410 of process 400, glass tube 230 is sealed while the gas within the tube is at room temperature (approximately 25° C.). Glass tube 230 is then heated prior to glass tube bonding. Thus, in these exemplary embodiments, glass tube 230 is heated during step 130 of process 100. Specifically, glass tube 230 (and the gas within the tube) is heated to a temperature of approximately 1750° C., which increases the gas pressure within glass tube 230 to approximately 100 psi. After glass tube bonding (step 140), the resulting preform glass tube has a relatively larger diameter and a relatively smaller wall thickness. The resulting preform is then drawn into an optical fiber such that structural tube 30 also has a relatively larger diameter. In these illustrative examples, due to the increase in temperature of glass tube 230 from room temperature to approximately 1750°C during step 130, structural tube 30 may experience approximately twice the expansion from a structural tube in a similar fiber not produced by the heat treatment of step 130.

[0051] In other exemplary embodiments, the glass tube 230 (and the gas within the tube) is heated to a first temperature. In one particular embodiment, the first temperature is approximately 100°C. After heating to the first temperature, the ends of the glass tube 230 are sealed in step 410 of process 400 so that the glass tube 230 is a closed tube. The glass tube 230 is then heated prior to glass tube bonding. Thus, in these exemplary embodiments, the glass tube 230 is heated during step 130 of process 100. Specifically, the glass tube 230 (and the gas within the tube) is heated to a temperature of approximately 1750°C. After glass tube bonding (step 140), the resulting preform glass tube has a relatively larger diameter and a relatively smaller wall thickness. The resulting preform is then drawn into an optical fiber such that the structural tube 30 also has a relatively larger diameter. In these exemplary embodiments, due to the increase in temperature from the first temperature (100°C) to the second temperature (1750°C), the structural tube 30 may only experience approximately 1.5 times the expansion of a structural tube in a similar fiber not produced by the heat treatment of step 130. Note that these second exemplary embodiments experienced a lower expansion rate than the previous exemplary embodiments (1.5 times vs. 2 times) due to the smaller increase in temperature from the first temperature to the second temperature, which is a result of the higher first temperature in these second exemplary embodiments (100°C vs. room temperature). Furthermore, these second exemplary embodiments expanded at a slower, more controlled rate than the previous exemplary embodiments due to the sealing of the glass tube 230 at the elevated temperature of 100°C.

[0052] 4B, process 400′ illustrates another process for heat treating preform precursor 200 and / or preform 300. Thus, like process 400, process 400′ also provides more detailed process steps of step 130 and / or step 150 of process 100. Process 400′ may be used when heating a gas within a capillary to cause the capillary to expand. However, as discussed further below, the steps of process 400′ result in greater expansion of the glass tube than the steps of process 400.

[0053] In step 410' of process 400', a first end of glass tube 230 / 330 is sealed. The first end can be the top end of the glass tube. Note that the second end (e.g., the bottom end) of the glass tube remains open during step 410'. In step 420' of process 400', the gas within glass tube 230 / 330 is cooled. For example, the gas can be cooled to a temperature of about room temperature (about 25°C) or below, or about 10°C or below, or about 5°C or below, or about 0°C or below, or about -5°C or below, or about -10°C or below, or about -15°C or below, or about -25°C or below, or about -35°C or below, or about -50°C or below, or about -75°C or below, or about -100°C or below. In embodiments, the glass tube 230 / 330 is cooled to a temperature of about 25°C to about -100°C, or about 0°C to about -75°C, or about 0°C to about -50°C. Cooling the glass tube in step 420' causes the gas molecules to contract so that they are packed more tightly within the glass tube. This provides room for more gas molecules to enter the glass tube. Thus, the volume occupied by the gas remains constant, but the number of moles of gas increases (due to the addition of more gas molecules into the glass tube).

[0054] In step 430' of process 400', the second end of the glass tube 230 / 330 is sealed, thus forming a closed glass tube. The second end may be the bottom end of the glass tube. Note that the second end of the glass tube is sealed while the gas is at the cooling temperature.

[0055] During step 440' of process 400', the glass tube 230 / 330 may be heated to a temperature above the strain point of the glass (approximately 1100°C or higher). In some embodiments, the glass tube 230 / 330 is heated to a temperature above the softening point of the glass (approximately 1100°C or higher, or approximately 1200°C or higher, or approximately 1300°C or higher). Such heating of the glass allows the glass tube 230 / 330 to expand and contract (due to increased pressure within the glass tube), resulting in thinner walls and an increase in the size of the capillary 235 / 335. Because the ends of the glass tube 230 / 330 are now sealed, heating the glass tube 230 / 330 causes an increase in the temperature of the gas within the glass tube 230 / 330, increasing the gas pressure within the capillary 235 / 335. As discussed above, such an increase in gas pressure causes the capillary 235 / 335 to expand to a larger size while the wall thickness of the glass tube 230 / 330 decreases. In some embodiments, the glass tube 230 / 330 (and also the gas therein) is heated to a temperature of about 1100°C to about 2200°C, or about 1500°C to about 2100°C, or about 1600°C to about 2000°C, or about 1700°C to about 1900°C, or about 1800°C to about 2000°C, or about 1100°C to about 1700°C, or about 1300°C to about 1600°C, or about 1400°C to about 1500°C, or about 1450°C to about 1550°C during step 420 of process 400, as discussed above with reference to process 400. The glass tube 230 / 330 may be heated for a period of about 0.01 seconds to about 12 hours, or about 1 second to about 10 hours, or about 5 seconds to about 8 hours, or about 30 seconds to about 6 hours, or about 1 minute to about 4 hours, or about 10 minutes to about 2 hours, or about 30 minutes to about 8 hours, or about 30 minutes to about 4 hours, or about 30 minutes to about 2 hours, or about 1 hour to about 4 hours, as discussed above with reference to process 400.

[0056] Because glass tube 230 / 330 was first cooled in step 420' of process 400', a greater number of moles of gas were introduced into the glass tube compared to process 400 (which did not include such a cooling step). Thus, process 400' involves not only increasing the temperature of the gas within the glass tube (step 440'), which increases the pressure within the glass tube, but also increasing the number of moles of gas within the glass tube (step 420'). Because the number of moles of gas increased during step 420', more gas molecules were heated during step 440'. Thus, the pressure within glass tube 230 / 330 increased by a greater amount compared to process 400.

[0057] FIG. 4C illustrates a second embodiment in which the glass tube 230 / 330 is cooled during the heat treatment of step 130 and / or step 150. In this embodiment, the gas within the glass tube is cooled such that the gas has a reduced temperature, which causes a decrease in the gas pressure within the glass tube. In particular, because the ends of the glass tube are sealed (thus the number of moles of gas within the glass tube remains constant), cooling the glass tube reduces the gas pressure within the capillary 235 / 335. This decrease in gas pressure causes the capillary 235 / 335 to collapse, resulting in the capillary 235 / 335 becoming relatively smaller in size while the wall thickness of the glass tube increases. Due to the increased wall thickness, more effort is required to expand the glass tube 230 / 330 during the fiber drawing process. Therefore, the glass tube 230 / 330 cannot be easily expanded during the fiber drawing process, thereby producing a structural tube 30 with a relatively smaller diameter. Thus, after the final drawing step, the resulting structural tube 30 is of a relatively smaller size, with relatively smaller capillaries 35 .

[0058] As shown in process 400-2 of FIG. 4C , the heat treatment of step 130 and / or step 150 of process 100 may include sealing both ends of glass tube 230 / 330 (step 410-2) to form a closed tube. In embodiments, the gas within glass tube 230 / 330 is at room temperature during step 410-2. In step 420-2 of process 400-2, glass tube 230 / 330 may be heated to a temperature above the strain point of glass (approximately 1100°C or higher). In some embodiments, glass tube 230 / 330 may be heated to a temperature above the softening point of glass (approximately 1100°C or higher, or approximately 1200°C or higher, or approximately 1300°C or higher). Such heating of the glass tube may be performed as disclosed above with respect to process 400 and process 400′. Thus, in embodiments, glass tube 230 / 330 (and also the gas therein) is heated during step 420-2 to a temperature of about 1100°C to about 2200°C, or about 1500°C to about 2100°C, or about 1600°C to about 2000°C, or about 1700°C to about 1900°C, or about 1800°C to about 2000°C, or about 1100°C to about 1700°C, or about 1300°C to about 1600°C, or about 1400°C to about 1500°C, or about 1450°C to about 1550°C. The glass tube 230 / 330 may be heated for a period of about 0.01 seconds to about 12 hours, or about 1 second to about 10 hours, or about 5 seconds to about 8 hours, or about 30 seconds to about 6 hours, or about 1 minute to about 4 hours, or about 10 minutes to about 2 hours, or about 30 minutes to about 8 hours, or about 30 minutes to about 4 hours, or about 30 minutes to about 2 hours, or about 1 hour to about 4 hours, as discussed above with reference to process 400.

[0059] In step 430-2 of process 400-2, the gas within glass tube 230 / 330 is cooled. For example, the gas may be cooled to a temperature of about room temperature (about 25° C.) or below, or about 10° C. or below, or about 5° C. or below, or about 0° C. or below, or about −5° C. or below, or about −10° C. or below, or about −15° C. or below, or about −25° C. or below, or about −35° C. or below, or about −50° C. or below, or about −75° C. or below, or about −100° C. or below. In embodiments, glass tube 230 / 330 is cooled to a temperature of about 25° C. to about −100° C., or about 0° C. to about −75° C., or about 0° C. to about −50° C. In other embodiments, the gas within glass tube 230 / 330 is cooled such that the temperature of the glass tube remains above the strain point of the glass or above the softening point of the glass. Cooling the glass tube in step 430-2 causes the gas to contract so that the glass tube collapses inward.

[0060] As discussed above, cooling of glass tube 230 / 330 in step 430-2 causes structural tube 30 within the drawn optical fiber to have a smaller capillary diameter and thicker walls. Further, as discussed above, the steps of process 400-2 can be performed during either step 130 and / or step 150 of process 100. Thus, the steps of process 400-2 are performed prior to drawing the optical fiber (before step 170 of process 100).

[0061] In some exemplary embodiments, the glass tube 230 (and the gas within the tube) is initially at an initial temperature of approximately 25° C. when both ends of the glass tube 230 are sealed. The glass tube 230 is then heated in step 420-2 of process 400-2 and then cooled in step 430-2. In particular, in these embodiments, the glass tube 230 (and the gas within the tube) is cooled to a temperature of approximately −50° C. during step 430-2 so that the gas within the glass tube is at a pressure of 14.7 psi. After cooling of the glass tube 230, the glass tube is bonded in step 140 of process 100. After the bonding step, the resulting preform glass tube has a relatively smaller diameter and a relatively larger wall thickness. The resulting preform is then drawn into an optical fiber such that the structural tube 30 also has a relatively smaller diameter.

[0062] It should be noted that embodiments of the present disclosure encompass combining one or more steps of processes 400, 400', and 400-2. For example, a first glass tube within a preform (or predecessor) may be heated by steps of process 400 or process 400', while a second glass tube within the same preform (or within the same preform predecessor) may be cooled by steps of process 400-2. This may be advantageous, for example, when it is desired to increase the capillary size of the first glass tube and decrease the capillary size of the second glass tube.

[0063] During step 420' of process 400' and / or during step 430-2 of process 400-2, glass tube 230 / 330 may be cooled, for example, with a cooling gas or liquid (such as helium or nitrogen), dry ice, a refrigerator, or any other cooling element known in the art.

[0064] Referring again to the embodiment in which the glass tube is heated to increase capillary size, and with reference to FIGS. 5A-5C, an exemplary embodiment will be described in which the preform 300 is heat-treated in step 150 of the process 100 prior to the re-draw in step 160. FIG. 5A shows the dimensions of the preform 300 during the exemplary re-draw in step 160 and after the heat treatment of the preform 300. As shown in FIG. 5A, the preform 300 is disposed in a re-draw furnace having a heating element 510. The sealed end of the glass tube 330 within the preform 300 is depicted in FIG. 5A with reference numeral 337. Due to the heat from the heating element 510, the preform 300 can be stretched into a preform having a relatively smaller outer diameter. The heat from the heating element 510 (together with the sealed end of the glass tube 330) also reduces the wall thickness of the glass tube 330.

[0065] 5B and 5C more clearly show the decrease in wall thickness of glass tube 330 and the increase in size of capillary 335 during the redrawing of preform 300 (and after the heat treatment of step 150). As shown in FIG. 5B, before the redrawing step, glass tube 330 has a wall thickness T c and outer diameter D c Furthermore, as shown in FIG. 5C, after the redrawing process, the glass tube 330 has a wall thickness t c , and outer diameter d c According to embodiments disclosed herein, T c >t c and D c >d c In some embodiments, T c is about 0.25 mm to about 4.0 mm, or about 0.5 mm to about 3.5 mm, or about 1.0 mm to about 3.0 mm, or about 1.5 mm to about 2.5 mm, or about 2.0 mm to about 3.0 mm. In some embodiments, t c is about 1.00 mm or less, or about 0.75 mm or less, or about 0.50 mm or less, or about 0.25 mm or less, or about 0.20 mm or less, or about 0.10 mm or less, or in the range of about 0.10 mm to about 1.00 mm, or about 0.20 mm to about 0.75 mm, or about 0.25 mm to about 0.50 mm. Further, in some embodiments, Dc In some embodiments, d is in the range of about 2 nm to about 45 nm, or about 5 nm to about 30 nm, or about 8 nm to about 25 nm, or about 10 nm to about 20 nm. c is about 0.25 mm to about 10 mm, or about 0.50 mm to about 8 mm, or about 1 mm to about 5 mm, or about 2 mm to about 4 mm. c :t c and D c :d c is about 1.5 or more, or about 2 or more, or about 3 or more, or about 4 or more, or about 5 or more, or about 6 or more, or in the range of about 1.5 to about 6, respectively.

[0066] 5B and 5C also show that the wall thickness of the glass clad tube 320 is the same as that of the preform 300 before redrawing (T s ) to the preform 300 after re-stretching (t s ) before the preform 300 is re-stretched. s ) to the preform 300 after re-stretching (D s ) and therefore T s >t s and D s >d s In some embodiments, T s is about 5 mm to about 50 mm, or about 10 mm to about 40 mm, or about 20 mm to about 30 mm. s is about 0.5 mm to about 3.0 mm, or about 0.75 mm to about 2.5 mm, or about 1.0 mm to about 2.25 mm, or about 1.25 mm to about 2.0 mm, or about 1.5 mm to about 1.75 mm, or about 1.5 mm to about 2.0 mm. s is about 30 mm to about 150 mm, or about 50 mm to about 125 mm, or about 75 mm to about 100 mm, or about 80 mm to about 90 mm. s is about 10 mm to about 80 mm, or about 12.5 mm to about 75 mm, or about 15 mm to about 60 mm, or about 20 mm to about 50 mm, or about 12.5 mm to about 75 mm, or about 15 mm to about 20 mm.

[0067] As discussed above, in step 170 of process 100, preform 300 is drawn into optical fiber. Figure 6 depicts an exemplary fiber drawing system 600 comprising a draw furnace 610 having a heating element 620 and a muffle 614. Preform 300 is disposed vertically within draw furnace 600, and heating element 620 of draw furnace 610 applies heat to at least a lower portion of preform 300. Optical fiber 10 (in the form of bare, uncoated optical fiber) is then drawn from the heated preform 300.

[0068] To draw the fiber 10, the root portion of the preform 300 is pulled by a tractor 650 and wound onto a spoon or reel 660. The system 600 may include additional components, such as a monitor 630 for monitoring the draw rate of the optical fiber 10. Additionally, the system 100 may further include a coating device 640. The optical fiber 10 is a bare, uncoated fiber until it reaches the coating device 640, which may apply a polymer-based coating to the outer surface of the bare optical fiber. The coated fiber may then pass through a coating curing device (not shown) before being wound onto the reel 660.

[0069] Illustrative Examples In a first exemplary embodiment, a hollow-core preform precursor was formed by positioning six glass tubes within a glass-clad tube. The glass tubes were evenly spaced around the inner diameter of the glass-clad tube, with adjacent tubes separated by gaps. The preform precursors were then secured together before being subjected to a heat-treatment process. Prior to the heat-treatment process, the glass tubes had an average inner diameter of 0.31 mm and an average outer diameter of 0.47 mm. The glass-clad tubes had an inner diameter of 2.52 mm and an outer diameter of 12.42 mm. During heat treatment, the glass tubes were first sealed at both the top and bottom ends to form a closed tube while the gas inside the tube was at 25°C. The preforms were then heated in a furnace at atmospheric pressure. The furnace was heated to 1475°C at a rate of 10°C / min and then held at 1475°C for 30 minutes.

[0070] Figure 7A shows the middle section of the preform after the bonding step and before the heat treatment step. Figure 7B shows the same middle section, but after the bonding step and the heat treatment step. As shown in Figure 7B, after the heat treatment step, the glass tubes of the resulting preforms had an average inner diameter of 0.49 mm and an average outer diameter of 0.61 mm. Thus, as discussed above, due to the heat treatment of the preform precursor to 1475°C, the inner diameter of each glass tube increased while the wall thickness decreased.

[0071] Figure 8A shows the bottom portion of the same preform after the bonding step and before the heat treatment step. Figure 8B shows the same bottom portion, but after the bonding step and the heat treatment step. As shown in Figure 8B, after the heat treatment step, the glass tubes of the resulting preforms had an average inner diameter of 0.0.51 mm and an average outer diameter of 0.63 mm. Thus, as discussed above, due to the heat treatment of the preform precursor to 1475°C, the inner diameter of each glass tube increased while the wall thickness decreased.

[0072] FIG. 9 illustrates an exemplary preform fabricated using an embodiment of the present disclosure. Specifically, in preform 900 of FIG. 9, glass tube 910 was sealed, but glass tube 920 was not, prior to the bonding process. Furthermore, after sealing glass tube 910, preform 900 was subjected to a heat treatment process in which the preform was heated to a temperature of 1350° C. for 4-6 hours prior to bonding. Both glass tubes 910 and 920 were then subjected to the same bonding process. Because glass tube 910 was sealed during the heat treatment process, the pressure within this tube increased, causing an increase in the capillary size of the glass tube (as shown in FIG. 9) and a decrease in the wall thickness of the glass tube. However, glass tube 920 was not sealed and therefore did not experience the same change in size.

[0073] 10A-10C show another exemplary embodiment in which the glass tubes within the preform are sealed before the redraw process. FIG. 10A shows a preform 1000 with a glass tube 1010 before the redraw process. In this exemplary embodiment, glass tubes 1010-2 and 1010-3 were sealed before the redraw process and then heat-treated during the redraw of the preform. However, in this exemplary embodiment, glass tube 1010-1 was not sealed but was subjected to the same heat treatment as the other glass tubes during the redraw of the preform. In this exemplary embodiment, glass tube 1010 was heated to a temperature of 1780°C during the redraw process. FIG. 10B shows the sealed ends 1015 of glass tubes 1010-2 and 1010-3 before the redraw process. FIG. 10C shows the preform 1000 after the redraw process (and therefore after the heat-treatment process). 10C, glass tubes 1010-2 and 1010-3 have larger capillary sizes and reduced wall thickness values ​​compared to glass tube 1010-1. Table 1 below provides the dimensions of preform 1000 before and after the redraw process. [Table 1]

[0074] As shown above in Table 1, structural tube 1010-1 was not sealed during the heat treatment process and therefore had the largest wall thickness and smallest diameter after re-stretching. Additionally, structural tube 1010-1 also had the smaller expansion ratio of the three structural tubes.

[0075] As discussed above, embodiments of the present disclosure are directed to methods of producing hollow-core optical fibers such that the fibers have specific size dimensions. More specifically, the structural tube of the fiber is fabricated such that the internal capillary formed by the structural tube has a specific size, and consequently the wall thickness of the structural tube has a specific size. This allows the structural tube to be tuned to provide an antiresonance at a predetermined wavelength. In some embodiments, the structural tube is specifically sized such that the structural tube provides an antiresonance for a wavelength of about 1550 nm.

[0076] 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 will be understood by those skilled in the art.

[0077] It is to be understood that the phraseology or terminology used herein is for the purpose 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 of manufacturing a preform, said method comprising: positioning at least one glass tube within the glass outer cladding to form a preform precursor, the glass tube having a first open end and a second open end; forming a preform from the preform precursor; and heat treating at least one of the preform precursor and the preform, wherein the heat treating comprises: sealing the first open end and the second open end of the glass tube to form a closed tube; and heating and / or cooling the closed glass tube to manipulate the gas pressure within the glass tube.

2. The method of claim 1, wherein the heat treating the glass tube comprises heating the glass tube to a temperature of about 1100°C to about 2200°C.

3. The method of claim 2, wherein said heat treating said glass tube comprises heating said glass tube to a temperature of about 1100°C to about 1700°C.

4. The method of claim 1 , wherein the heat treating the glass tube comprises cooling the glass tube to a temperature of about 0° C. or less.

5. 5. The method of claim 1, further comprising bonding the preform predecessor to form the preform, and wherein the heat treating of the glass tube is before depositing soot on the preform predecessor during the bonding of the preform predecessor.

6. 6. The method of claim 1, further comprising heat treating a second glass tube, wherein the second glass tube is a nested tube disposed radially inside the glass tube.

7. 7. The method of claim 6, wherein during the heat treatment of the glass tube and the heat treatment of the second glass tube, the glass tube is heated to a different temperature than the second glass tube.

8. 10. The method of claim 1, wherein the heat treating the glass tube comprises heating a gas in a radially central portion of the glass tube to a higher temperature than a gas in a radially peripheral portion of the glass tube.

9. 9. The method of claim 1, further comprising bonding the preform precursor to form the preform, and wherein the heat treating of the glass tube occurs after depositing soot on the preform precursor during the bonding of the preform precursor.

10. 10. The method of claim 9, wherein the glass tube undergoing the heat treatment is the glass tube of the preform.

11. 10. The method of claim 9, further comprising re-drawing the preform before drawing the preform and after heat treating the glass tube.

12. 12. The method of claim 11, wherein the ratio of the wall thickness of the glass tube before redrawing to the wall thickness of the glass tube after redrawing is about 1.5 or greater.

13. 12. The method of claim 11, wherein the outer diameter of the glass tube after the re-drawing is from about 0.25 mm to about 10 mm.

14. The method of claim 13, wherein the outer diameter is from about 0.50 mm to about 8 mm.

15. The method of claim 1 , wherein the heat treating the glass tube comprises heating the glass tube and increasing a gas pressure within the glass tube.

16. 16. The method of claim 15, wherein the glass tube comprises an inner capillary, and heating the glass tube increases the outer diameter of the inner capillary and decreases the wall thickness of the glass tube.

17. The method of claim 1 , wherein the heat treating the glass tube comprises cooling the glass tube and reducing the gas pressure within the glass tube.

18. 18. The method of claim 17, wherein the glass tube comprises an inner capillary, and cooling the glass tube reduces the outer diameter of the inner capillary and increases the wall thickness of the glass tube.

19. 19. The method of any one of claims 1 to 18, wherein the at least one glass tube comprises a second glass tube, and wherein the heat-treating the glass tube comprises heating the glass tube to increase a gas pressure within the glass tube, and the method further comprises cooling the second glass tube and sealing both ends of the second glass tube to form a second closed tube and reducing the gas pressure within the second glass tube.

20. 19. The method of any one of claims 1 to 18, wherein the at least one glass tube comprises a second glass tube, and wherein the heat-treating the glass tube comprises heating the glass tube to a first temperature to increase a gas pressure within the glass tube, and the method further comprises sealing both ends of the second glass tube to form a second closed tube, and heating the second glass tube to a second temperature to increase a gas pressure within the second glass tube.

21. 21. The method of claim 20, wherein the first temperature is greater than the second temperature.

22. 21. The method of claim 20, wherein the first temperature is substantially equal to the second temperature.

23. The method of claim 1 , wherein the heat treating comprises sealing the first open end and the second open end of the glass tube before heating the glass tube.

24. The method of claim 1 , wherein the heat treating comprises sealing both ends of the glass tube and then cooling the gas tube.

25. A method according to any one of the preceding claims, wherein the step of heat treating occurs during the step of bonding the preform precursor.

26. The method of any one of claims 1 to 25, further comprising bonding the preform precursors to form the preform.

27. The method of claim 1 , wherein the heat treating comprises first cooling the glass tube and then heating the glass tube.

28. 1. A method of manufacturing a preform, said method comprising: positioning at least one glass tube within the glass outer cladding to form a preform precursor, the glass tube having a first open end and a second open end; forming a preform from the preform precursor; and heat treating at least one of the preform precursor and the preform, wherein the heat treating comprises: sealing the first open end and the second open end of the glass tube to form a closed tube; and after forming the closed tube, heating the glass tube to increase gas pressure within the closed glass tube.

29. 30. The method of claim 28, wherein the heat treating the glass tube comprises heating the glass tube to a temperature above the softening point of the glass.

30. 30. The method of claim 28, wherein said heat treating said glass tube comprises heating said glass tube to a temperature of about 1100°C to about 2200°C.

31. 30. The method of claim 28, wherein said heat treating said glass tube comprises heating said glass tube to a temperature of about 1100°C to about 1700°C.

32. 32. The method of any one of claims 28 to 31, further comprising bonding the preform predecessor to form the preform, and wherein the heat treating the glass tube occurs before depositing soot on the preform predecessor during the bonding of the preform predecessor.

33. 33. The method of any one of claims 28 to 32, further comprising heat treating a second glass tube, wherein the second glass tube is a nested tube disposed radially inside the glass tube.

34. 34. The method of claim 33, wherein during the heat treatment of the glass tube and the heat treatment of the second glass tube, the glass tube is heated to a different temperature than the second glass tube.

35. 35. The method of any one of claims 28 to 34, wherein the heat treating the glass tube comprises heating a gas in a radially central portion of the glass tube to a higher temperature than a gas in a radially peripheral portion of the glass tube.

36. 36. The method of any one of claims 28 to 35, further comprising bonding the preform predecessor to form the preform, and wherein the heat treating of the glass tube occurs after depositing soot on the preform predecessor during the bonding of the preform predecessor.

37. 37. The method of claim 36, wherein the glass tube that undergoes the heat treatment is the glass tube of the preform.

38. 38. The method of claim 37, further comprising re-drawing the preform before drawing the preform and after heat treating the glass tube.

39. 39. The method of claim 38, wherein the ratio of the wall thickness of the glass tube before redrawing to the wall thickness of the glass tube after redrawing is about 1.5 or greater.

40. 39. The method of claim 38, wherein the outer diameter of the glass tube after the re-drawing is between about 0.25 mm and about 10 mm.

41. 41. The method of claim 40, wherein the outer diameter is from about 0.50 mm to about 8 mm.

42. 42. The method of any one of claims 28 to 41, wherein the glass tube comprises an inner capillary, and heating the glass tube increases the outer diameter of the inner capillary and decreases the wall thickness of the glass tube.

43. 43. The method of any one of claims 28 to 42, wherein the at least one glass tube comprises a second glass tube, and wherein the heat-treating the glass tube comprises heating the glass tube to a first temperature to increase a gas pressure within the glass tube, and the method further comprises sealing both ends of the second glass tube to form a second closed tube, and heating the second glass tube to a second temperature to increase a gas pressure within the second glass tube.

44. 44. The method of claim 43, wherein the first temperature is greater than the second temperature.

45. 44. The method of claim 43, wherein the first temperature is substantially equal to the second temperature.

46. 46. ​​The method of any one of claims 28 to 45, further comprising bonding the preform precursors to form the preform, and wherein the step of heat treating occurs during the step of bonding the preform precursors.

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