Method for fabricating hollow-core optical fibers

Abstract

1. A method for manufacturing a hollow-core optical fiber, 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; and forming a preform from the preform predecessor. 2. The method further comprises: drawing the preform into a hollow-core optical fiber; and, while drawing the preform, heat-treating the preform to manipulate the gas pressure within the glass tube by at least one of (i) heating at least a portion of the preform to increase the gas pressure within the glass tube, and (ii) cooling at least a portion of the preform to decrease the gas pressure within the glass tube.

Description

[Technical Field]

[0001] This application claims the benefit of priority to U.S. Provisional Application No. 63 / 423,562, filed November 8, 2022, the contents of which are relied upon and incorporated herein by reference 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 either heated or cooled to manipulate the gas pressure within the glass tube. 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. A decrease in temperature decreases the gas pressure, and thus a structural tube is formed with a relatively smaller capillary size and a relatively larger 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]

[0006] Embodiments of the present disclosure are directed to a method of manufacturing a hollow-core optical fiber, 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; and forming a preform from the preform predecessor. The method further includes drawing the preform into a hollow-core optical fiber, and while drawing the preform, heat-treating the preform to manipulate the gas pressure within the glass tube by at least one of (i) heating at least a portion of the preform to increase the gas pressure within the glass tube, and (ii) cooling at least a portion of the preform to decrease the gas pressure within the glass tube.

[0009] 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]

[0010] [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] 3C illustrates the radially outward pressure exerted on the wall of the glass tube of the preform of FIG. 3B, according to an embodiment of the present disclosure. [Figure 4] 1 illustrates a preform for producing a hollow-core optical fiber, according to an embodiment of the present disclosure. [Figure 5] 1 illustrates a process for producing a hollow-core optical fiber according to an embodiment of the present disclosure. [Figure 6A] 1 illustrates a fiber drawing system according to an embodiment of the present disclosure. [Figure 6B] 1 illustrates a fiber drawing system according to an embodiment of the present disclosure. [Figure 7A] 1 illustrates a cross-sectional view of an exemplary embodiment of a hollow-core optical fiber, according to an embodiment of the present disclosure. [Figure 7B] 1 illustrates a cross-sectional view of an exemplary embodiment of a hollow-core optical fiber, according to an embodiment of the present disclosure. [Figure 7C] 1 illustrates a cross-sectional view of an exemplary embodiment of a hollow-core optical fiber, according to an embodiment of the present disclosure. [Figure 7D] 1 illustrates a cross-sectional view of an exemplary embodiment of a hollow-core optical fiber, according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0011] 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.

[0012] 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.

[0013] 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.

[0014] 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.

[0015] 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.

[0016] 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.

[0017] 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 pressure, volume, temperature, and the number of moles of gas. Aspects of the present disclosure utilize these relationships to efficiently produce hollow-core optical fibers having specific dimensions. In some aspects of the present disclosure, the relationships between pressure, volume, temperature, and the 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.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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) is important for reducing attenuation and shifting the transmission window to a desired wavelength. Therefore, embodiments of the present disclosure are directed to processes and methods for controlling the size of structural tubes, including the structural tube's inner and outer diameter dimensions and their wall thickness. 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 structural tubes can be implemented during preform drawing into optical fiber, as discussed below.

[0027] 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 includes 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.

[0028] 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.

[0029] In step 130 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 may be applied to the preform precursor 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 comprises an inner capillary 335. Additionally, 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.

[0030] Step 140 of process 100 involves subjecting preform 300 to a redrawing step before the preform is drawn into optical fiber. More specifically, in step 140, preform 300 is heated to a temperature above the softening point of the glass, causing preform 300 to shrink into a smaller diameter preform. At the end of step 140, preform 300 may be ready for drawing into optical fiber.

[0031] Drawing preform 300 into optical fiber begins at step 150a of process 100. This involves heating the lower portion of preform 300 to the softening point of the glass (approximately 200°C or higher) while applying tension to the glass, causing the glass to be drawn downward into a smaller diameter fiber. As is known in the art, drawing preform 300 into optical fiber can occur in a drawing furnace. Note that in conventional drawing processes, the lower portion of the preform is heated by a lower heater in the drawing furnace to draw the glass downward, while the upper portion of the preform is not separately heated (except for circulating heat from the lower heater). Thus, in conventional drawing processes, the upper portion of the preform is typically maintained at a temperature below the strain point of the glass and below 200°C.

[0032] Referring again to process 100 of FIG. 2 , after the initiation of the drawing process, the preform 300 is then heat-treated in step 150b to control the dimensions of the structural tube 30 in the optical fiber drawn from the preform 300. As discussed further below, the heat-treating of the preform 300 includes cooling and / or heating an upper portion of the preform 300. Note that the heat-treating of step 150b occurs after the initiation of the drawing process in step 150a. Thus, the heat-treating of step 150b occurs during the process of drawing the preform 300 into optical fiber (and, more specifically, during the process of drawing the preform 300 into a hollow-core optical fiber). Furthermore, the heat-treating of step 150b can occur simultaneously while the preform 300 is being heated by a bottom heater and drawn downward into optical fiber. It should be noted that the heat treatment of step 150b may occur during the remainder of the fiber drawing process or for only a reduced, temporary period during the fiber drawing process.

[0033] In step 150c of process 100, the fiber drawing process is completed, which may occur when all of the required or specified amount of preform has been drawn into optical fiber.

[0034] It should also be noted that in some embodiments, process 100 may include more or fewer steps than those shown in Figure 2. For example, in some embodiments, process 100 does not include redraw step 140. Thus, in these embodiments, the preform may be drawn into optical fiber without being subjected to a redraw step. Yet in some other embodiments, process 100 does not include stretching the preform in step 120 and / or bonding in step 130.

[0035] As discussed above, embodiments of the present disclosure include heat treating preform 300 to control the dimensions of structural tube 30 within the drawn optical fiber. In particular, the upper portion of preform 300 is heat treated (during step 150b of process 100) to control capillary size and structural tube wall thickness within the drawn optical fiber. As discussed further below, heat treating preform 300 includes modifying the gas pressure of capillary 335 within glass tube 330.

[0036] FIG. 4 illustrates the upper (first section 350) and lower (second section 360) portions of preform 300. Notably, second section 360 of preform 300 may be referred to as the root-forming portion. First section 350 may be any portion(s) of preform 300 above the root-forming portion. As discussed above, during drawing of preform 300 into optical fiber, second section 360 is heated to a temperature above the softening point of the glass to draw the glass downward within this section of the preform. In embodiments, second section 360 is heated to a temperature of about 200° C. or greater. Additionally, the heat treatment of step 150b includes heating and / or cooling first section 350 of preform 300 to control capillary size and structural tube wall thickness within the drawn optical fiber. In particular, the gas within the capillary 335 in the first section 350 of the preform 300 is heated and / or cooled during the heat treatment process. Because the ends of the glass tube 330 are sealed, the volume and number of moles of gas within the capillary 335 remain constant during the heat treatment process. Due to the constant volume and number of moles of gas within the capillary, heating or cooling the gas within the capillary causes the capillary to expand or contract, respectively.

[0037] In particular, heating the gas within capillary 335 increases the gas pressure within the capillary (due to the relationship defined by the equation PV=nRT). The increase in gas pressure within capillary 335 causes the capillary to expand in size, and therefore decreases the wall thickness of glass tube 330. This further causes the capillary within preform 300 to have a relatively larger outer diameter and a relatively smaller wall thickness relative to glass tube 330. Thus, it further causes the capillary within drawn optical fiber 10 to have a relatively larger outer diameter and a relatively smaller wall thickness relative to structural tube 30 within drawn optical fiber 10.

[0038] As an example, FIG. 3C depicts the radially outward pressure on the wall of the glass tube 330 from increased gas pressure within the capillary 335. As shown in FIG. 3C, the radially outward pressure causes the capillary 335 to expand and grow in size, similar to a balloon, while the wall of the glass tube 330 stretches and shrinks to accommodate the larger internal capillary size. Due to the relatively thinner wall of the glass tube 330, less effort is required to expand the glass tube 330 during the fiber drawing process. Therefore, during the fiber drawing process, the glass tube 330 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.

[0039] Conversely, cooling the gas within capillary 335 reduces the gas pressure within the capillary (due to the relationship defined by the equation PV=nRT). The reduction in gas pressure within capillary 335 reduces the size of the capillary and therefore increases the wall thickness of glass tube 330. This further causes the capillary within preform 300 to have a relatively smaller outer diameter and a relatively larger wall thickness relative to glass tube 300. Thus, it also causes the capillary within drawn optical fiber 10 to have a relatively smaller outer diameter and a relatively larger wall thickness relative to structural tube 30 within drawn optical fiber 10. More specifically, due to the reduction in gas pressure within capillary 335 of preform 300, capillary 335 collapses and is of a relatively smaller size, while the wall thickness of glass tube 330 increases. Due to the increased wall thickness, more effort is required to expand glass tube 330 during the fiber drawing process. Therefore, during the fiber drawing process, the glass tube 330 cannot be easily expanded, thus producing a structural tube 30 with a relatively smaller diameter. Thus, after the final drawing process, the produced structural tube 30 is of a relatively smaller size, with relatively smaller capillaries 35.

[0040] As discussed above, in some embodiments, the gas pressure in the capillary 335 is increased by increasing the temperature of the gas in the capillary. In other embodiments, the gas pressure is reduced by decreasing the temperature of the gas in the capillary. Additionally, as discussed below, the ends of the glass tube 330 are sealed to facilitate changes in pressure in the capillary.

[0041] FIG. 5 illustrates a process 500 for heat-treating preform 300. Process 500 thus provides more detailed process steps than step 150b of process 100. In step 510 of process 500, the upper ends of glass tubes 330 are sealed. The upper end of each glass tube 330 may be the upper end of the first section 350, such as upper end 355 shown in FIG. 4. Thus, the upper end may be disposed on the opposite side of preform 300 from the root-forming portion of the preform. Sealing the upper end may be performed by welding, heating, or any other means known in the art. Furthermore, the lower end of preform 300 may be effectively sealed during the initiation of the drawing process (during step 150a of process 100). The root-forming portion (second section 360) of preform 300 may be tapered to a small diameter such that gas cannot enter or leave the preform at the lower end during drawing of the preform. Thus, the bottom end is effectively sealed. The bottom end may be the end of the preform 300 at the root-forming portion, such as bottom end 365, as shown in FIG.

[0042] The top end 335 of the preform 300 may be sealed prior to drawing the preform (before step 150a of process 100). In some embodiments, the top end 335 is sealed while the gas within the capillary 335 is at room temperature (i.e., 25°C). In other embodiments, the top end 335 is sealed after step 150a, i.e., after the start of the drawing process.

[0043] Glass tube 330 is a closed glass tube due to the seals at first end 355 and second end 365. The seals at ends 355, 365 of glass tube 330 maintain a constant gas volume and number of gas moles within capillary 335. With such a constant gas volume and number of gas moles, the gas pressure within the capillary is easily adjusted and controlled by increasing or decreasing the temperature.

[0044] Step 520 of process 500 involves heating or cooling gas within one or more glass tubes 330 within preform 300. As discussed above, the heating or cooling is applied to the first section 350 of the preform. Note that embodiments of the present disclosure involve heating a first glass tube within the first section 350 of preform 300 and cooling a second glass tube within the same first section 350 of preform 300. 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. It is also contemplated that during step 520, a single glass tube 330 may be heated in a first portion of first section 350 and cooled in a second portion of first section 350. Thus, in this embodiment, the single glass tube 330 is subjected to both cooling and heating during the heat treatment of step 520.

[0045] During step 520, glass tube 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. The heating of glass tube 330 during step 520 is different and distinct from the heating of preform 300 to draw it into optical fiber. During step 520, glass tube 330 may be cooled with, for example, a cooling gas or liquid (such as helium or nitrogen), dry ice, a refrigerator, or any other cooling element known in the art.

[0046] It should also be noted that one or more glass tubes may not be exposed to the heating / cooling of step 520. Thus, in one exemplary embodiment, a first glass tube within preform 300 is heated to increase the capillary size of that glass tube, while an adjacent second glass tube is not heated or cooled. Thus, the capillary size of the second glass tube does not change due to the heat treatment process disclosed herein.

[0047] In some particular embodiments, step 520 includes heating or cooling one or more nested glass tubes (e.g., inner tube 34B) that are different from one or more other glass tubes (e.g., outer tube 32B). For example, the predecessor to inner tube 34B may be heated to a higher temperature during heat treatment step 150b than the predecessor to outer tube 32B, such that the predecessor to inner tube 34B has a relatively greater expansion rate than the predecessor to outer tube 32B.

[0048] In embodiments in which one or more nested glass tubes (e.g., inner tube 34B of FIG. 1B) are heated or cooled differently than 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 at their upper ends while one or more glass tubes are not sealed at their upper ends. For example, the predecessor to inner tube 34B may be sealed and heat-treated (during step 150b), while the predecessor to outer tube 32B is not sealed and / or heat-treated.

[0049] In some embodiments, the glass tube is heated or cooled uniformly along its length. In other embodiments, the glass tube is heated or cooled 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 first section 350, heated to a second temperature in a second portion of the first section 350, and cooled to a third temperature in a third portion of the third section 350, 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 still other embodiments, the glass tube may be heated or cooled inconsistently radially along the axial length of the glass tube in the first section 350. 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 to 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., left side) of the glass tube in the first section 350 and to a relatively low temperature on a second radial side (e.g., right side) of the glass tube in the first section 350.

[0050] During step 520 of process 500, the first section 350 of the glass tube 330 may be heated to a temperature of about 200°C or higher, or about 225°C or higher, or about 250°C or higher, or about 275°C or higher, or about 300°C or higher, or about 325°C or higher, or about 350°C or higher, or about 375°C or higher, or about 400°C or higher, or about 425°C or higher, or about 450°C or higher, or about 475°C or higher, or about 500°C or higher. Additionally or alternatively, during step 520 of process 500, first section 350 of glass tube 330 can be heated to a temperature of about 500° C. or less, or about 475° C. or less, or about 450° C. or less, or about 425° C. or less, or about 400° C. or less, or about 375° C. or less, or about 350° C. or less, or about 325° C. or less, or about 300° C. or less, or about 275° C. or less, or about 250° C. or less, or about 225° C. or less, or about 200° C. or less. In embodiments, first section 350 is heated to a temperature of about 200° C. to about 500° C., or about 200° C. to about 450° C., or about 200° C. to about 400° C., or about 250° C. to about 450° C., or about 300° C. to about 450° C. In embodiments, first section 350 is heated to a temperature below the strain point of the glass. During step 520 of process 500, glass tube 330 may be heated for the entire duration of drawing. In some embodiments, during step 520, glass tube 330 may be heated for about 99% or less, or about 95% or less, or about 90% or less, or about 85% or less, or about 80% or less, or about 75% or less, or about 70%, or about 65% or less, or about 60% or less, or about 55% or less, or about 50% or less, or about 45% or less of the duration of drawing. As discussed above, heating glass tube 330 during step 520 causes structural tube 30 within the drawn optical fiber to have a larger capillary diameter and thinner walls.

[0051] Because the ends of glass tube 330 are sealed (thus the volume of gas and number of moles of gas in the glass tube remain constant), heating glass tube 330 causes an increase in the temperature of the gas in glass tube 330, increasing the gas pressure in capillary 335. In embodiments, the gas pressure may increase by about 0.05 psi or more, or about 0.15 psi or more, or about 0.25 psi or more, or about 0.50 psi or more, or about 0.75 psi or more, or about 1.00 psi or more, or about 1.05 psi or more, or about 1.15 psi or more, or about 1.25 psi or more, or about 1.50 psi or more, or about 1.75 psi or more, or about 2.00 psi or more, or about 2.05 psi or more, or about 2.15 psi or more, or about 2.25 psi or more, or about 2.50 psi or more, or about 2.75 psi or more, or about 3.00 psi or more. In embodiments, the gas pressure may be increased from about 0.05 psi to about 3.00 psi, or from about 0.15 psi to about 2.75 psi, or from about 0.25 psi to about 2.50 psi, or from about 0.50 psi to about 2.50 psi, or from about 1.00 psi to about 2.50 psi.

[0052] During step 520 of process 500, first section 350 of glass tube 330 may be cooled to a temperature of about 50° C. or less, or about 25° C. or less, or about 0° C. or less, or about −5° C. or less, or about −10° C. or less, or about −15° C. or less, or about −25° C. or less, or about −50° C. or less, or about −75° C. or less, or about −80° C. or less, or about −100° C. or less. Additionally or alternatively, during step 520 of process 500, first section 350 of glass tube 330 may be cooled to a temperature of about −100° C. or more, or about −80° C. or more, or about −75° C. or more, or about −50° C. or more, or about −25° C. or more, or about −15° C. or more, or about −10° C. or more, or about −5° C. or more, or about 0° C. or more, or about 25° C. or more, or about 50° C. or more. In embodiments, the first section 350 of the glass tube 330 is cooled to a temperature of about −100° C. to about 50° C., or about −75° C. to about 25° C., or about −50° C. to about 0° C., or about −50° C. to about −10° C. The first section 350 of the glass tube 330 is cooled to a temperature below the strain point of the glass. During step 520 of the process 500, the glass tube 330 may be cooled for the entire duration of drawing. In some embodiments, during step 520, the glass tube 330 may be cooled for about 99% or less, or about 95% or less, or about 90% or less, or about 85% or less, or about 80% or less, or about 75% or less, or about 70%, or about 65% or less, or about 60% or less, or about 55% or less, or about 50% or less, or about 45% or less of the duration of drawing. As discussed above, cooling of the glass tube 330 in step 520 causes the structural tube 30 within the drawn optical fiber to have a smaller capillary diameter and thicker walls.

[0053] Because the ends of glass tube 330 are sealed (thus the volume of gas and number of moles of gas in the glass tube remain constant), cooling glass tube 330 causes a decrease in the temperature of the gas in glass tube 330, decreasing the gas pressure in capillary 335. In embodiments, the gas pressure may decrease by about 0.05 psi or more, or about 0.15 psi or more, or about 0.25 psi or more, or about 0.50 psi or more, or about 0.75 psi or more, or about 1.00 psi or more, or about 1.05 psi or more, or about 1.15 psi or more, or about 1.25 psi or more, or about 1.50 psi or more, or about 1.75 psi or more, or about 2.00 psi or more, or about 2.05 psi or more, or about 2.15 psi or more, or about 2.25 psi or more, or about 2.50 psi or more, or about 2.75 psi or more, or about 3.00 psi or more. In embodiments, the gas pressure may be reduced to between about 0.05 psi and about 3.00 psi, or between about 0.15 psi and about 2.75 psi, or between about 0.25 psi and about 2.50 psi, or between about 0.50 psi and about 2.50 psi, or between about 1.00 psi and about 2.50 psi.

[0054] It should also be noted that the heat treatment process affects the amount of expansion or contraction within the structural tube 30. More specifically, a higher change in temperature (when heating the glass tube during the heat treatment of step 150b) causes a greater amount of expansion than a lower change in temperature. Similarly, a higher change in temperature (when cooling the glass tube during the heat treatment of step 150b) causes a greater amount of contraction than a lower change in temperature. Furthermore, the duration of the heat treatment can also affect the amount of expansion or contraction within the structural tube 30. For example, in the first example, the upper end of the first glass tube was sealed prior to drawing the preform, and the first glass tube had an inner diameter of 490 micrometers prior to drawing. During drawing, the lower end of the first glass tube was sealed due to the drawing process itself. Furthermore, during drawing, the first glass tube was subjected to a heat treatment step in which the first glass tube was heated to 500°C for approximately 10 minutes and the gas pressure within the first glass tube was increased to 0.126 psi. After the drawing process, the glass tube had an inner diameter of 13.3 micrometers. Conversely, in the second example, the upper end of the second glass tube was sealed before drawing the preform, and the second glass tube also had an inner diameter of 490 micrometers before drawing. During drawing, the lower end of the second glass tube was sealed due to the drawing process itself. Furthermore, during drawing, the second glass tube was subjected to a heat treatment process in which the second glass tube was heated to a temperature of approximately 500°C for approximately 50 minutes, and the gas pressure within the second glass tube was increased to 0.155 psi. After the drawing process, the second glass tube had an inner diameter of 19.9 micrometers. The second glass tube was subjected to the heated temperature for a longer period of time and therefore had a higher gas pressure during the heat treatment process than the first glass tube.

[0055] 6A depicts a fiber drawing system 600 comprising a draw furnace 620 having a heating element 622 (e.g., a bottom heater) and a muffle 624. The preform 300 is disposed vertically within the draw furnace 620, and the heating element 622 of the draw furnace 620 applies heat to the second portion 360 of the preform 300. The optical fiber 10 (in the form of bare, uncoated optical fiber) is then drawn from the heated preform 300.

[0056] To draw the fiber 10, the second portion 360 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 600 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.

[0057] 6A , system 600 may further include a heat-treatment unit 610 above draw furnace 620. Thus, first section 350 of preform 300 may be subjected to heat-treatment unit 610, while second section 360 of preform 300 is heated by heating elements 622 of draw furnace 620. In some embodiments, heat-treatment unit 610 applies heat to first section 350 of preform 300 to increase the gas pressure within glass tube 330 (as discussed above with reference to step 150b of process 100). In other embodiments, heat-treatment unit 610 cools first section 350 of preform 300 to reduce the gas pressure within glass tube 330 (as discussed above with reference to step 150b of process 100).

[0058] 6B shows a second embodiment of the system 600 in which the furnace 620 and the thermal treatment unit 610 are one integral part. However, in this second embodiment of the system 600, the thermal treatment unit 610 is still disposed above and upstream of the heating element 622.

[0059] In some embodiments, the heat-treating unit 610 is a heated sleeve, ring burner, hot air blower, induction coil, torch, isothermal heating element, laser including CO or CO laser, plasma heater, draw furnace heater, or any other heating element known in the art. Additionally or alternatively, the heat-treating unit 610 is a spray mechanism that sprays cooling gas or liquid (such as helium or nitrogen gas or liquid), dry ice, refrigerator, or any other cooling element known in the art. In embodiments, the heat-treating unit 610 applies heating / cooling functions to only a specific number of glass tubes 330 (e.g., one, two, three, four, etc. glass tubes 330) or to all of the glass tubes 330 in the preform 300. The heat-treating unit 610 may apply heating / cooling functions at specific locations along the preform 300 (e.g., specific locations within the draw furnace 620).

[0060] 4, 6A, and 6B, the thermal treatment unit 610 applies a heating / cooling function to the first section 350 of the glass tube 330. In some embodiments, the thermal treatment unit 610 heats / cools the entire length of the first section 350. The heating / cooling function may be uniform along the first section 350 or may be gradient. For example, an upper portion of the first section 350 may be heated to a higher temperature or at a faster rate than a lower portion of the first section 350. In other embodiments, the thermal treatment unit 610 heats / cools less than the entire length of the first section 350. The thermal treatment unit 610 may heat / cool a length of the first section 350 that is, for example, about three-quarters, two-thirds, half, one-third, or one-quarter of the length of the first section 350.

[0061] Also, note that during the drawing of preform 300, second section 360 of preform 300 is heated to a higher temperature than first section 350 of preform 300 in order to create a root-forming portion in second section 360. Thus, in embodiments, the formation of the root-forming portion requires a higher glass temperature than the temperature to increase the pressure within glass tube 330.

[0062] Heating of the preform 300 by the heat treatment unit 610 and the heating elements 622 can create at least two hot zones on the preform 300, with a first hot zone in the first section 350 and a second hot zone in the second section 360. The first hot zone can be equal to or less than the entire length of the first section 350. Furthermore, the first hot zone can be used to control the size of the structural tube 30, while the second hot zone can be used to form a root forming portion for drawing the fiber 10. As discussed above, the second hot zone can have a higher temperature than the first hot zone. With the first and second hot zones, the temperature profile along the length of the preform (during the drawing process) can be referred to as bimodal Gaussian.

[0063] Due to the heating / cooling function of the thermal processing unit 610, the preform 300 may undergo a temperature change in the range of about 50°C to about 250°C. Thus, in one exemplary embodiment, the preform 300 may have a first temperature before entering the thermal processing unit 610 and a second temperature upon exiting the thermal processing unit 610, such that the second temperature is about 50°C to about 250°C higher than the first temperature. In other embodiments, the second temperature is about 50°C to about 250°C lower than the first temperature. It is also contemplated that the change in temperature due to the heating / cooling function of the thermal processing unit 610 may be about 50°C to about 200°C, or about 100°C to about 175°C, or about 50°C to about 150°C.

[0064] The thermal processing unit 610 may operate at a heating rate of about 1° C. per second or greater, or about 5° C. per second or greater, or about 10° C. per second or greater, or about 20° C. per second or greater, or about 25° C. per second or greater, or about 50° C. per second or greater, or about 75° C. per second or greater, or about 100° C. per second or greater, or about 150° C. per second or greater, or about 200° C. per second or greater, or about 250° C. per second or greater, or about 300° C. per second or greater, or about 350° C. per second or greater. In some embodiments, the heating rate is in the range of about 100° C. per second to about 350° C. per second, or about 150° C. per second to about 300° C. per second, or about 200° C. per second to about 250° C. per second. Additionally, the thermal processing unit 610 may operate at a cooling rate of about −1° C. per second or less, or about −5° C. per second or less, or about −10° C. per second or less, or about −15° C. per second or less, or about −20° C. per second or less, or about −25° C. per second or less, or about −30° C. per second or less, or about −40° C. per second or less, or about −50° C. per second or less, or about −60° C. per second or less, or about −70° C. per second or less, or about −80° C. per second or less, or about −90° C. per second or less, or about −100° C. per second or less. In some embodiments, the thermal processing unit 610 operates at a cooling rate of about −5° C. per second to about −50° C. per second, or about −10° C. per second to about −40° C. per second, or about −20° C. per second to about −35° C. per second, or about −25° C. per second to about −35° C. per second.

[0065] It is contemplated that in some embodiments, the heat treatment unit 610 and the heating element 622 within the furnace 620 are one component. Thus, in these embodiments, the heating element 622 may be used to both (i) heat the second section 360 of the preform 300 to draw a fiber from the root portion of the preform 300, and (ii) heat / cool the first section 350 of the preform 300 to increase / decrease the gas pressure within the capillary 335.

[0066] In some exemplary embodiments, the pressure in the furnace 620 may be used to control the pressure in the glass tube 330. As such, it may be used in addition to the heat treatment of step 150b of process 100. For example, during the heat treatment of step 150b, the pressure in the furnace 620 may be controlled to facilitate changes in pressure in the glass tube 330. A relatively high pressure in the furnace 620 may require a smaller temperature change (e.g., by the heat treatment unit 610) to increase the gas pressure in the glass tube 330. Conversely, a relatively low pressure in the furnace 620 may require a larger temperature change (e.g., by the heat treatment unit 610) to increase the gas pressure in the glass tube 330. In embodiments, the relationship between the pressure in the furnace, the pressure in the glass tube, and the temperature of the glass tube may be optimized to provide a desired capillary size and structural tube wall thickness in the drawn optical fiber.

[0067] Illustrative Examples In a first exemplary embodiment, hollow-core preform precursors were each formed by positioning six glass tubes within a glass cladding tube. The glass tubes were evenly spaced around the inner diameter of the glass cladding tube such that adjacent glass tubes were separated by gaps. Each glass tube had an inner diameter of 7 mm and an outer diameter of 9 mm. The glass cladding tube had an inner diameter of 40 mm and an outer diameter of 50 mm. The preform precursors were then stretched so that the outer diameter of the glass cladding tube was reduced to 15 mm. Additional soot was then deposited on the precursors to affix them to the preforms. The preforms were then drawn into hollow-core optical fibers. During the drawing process: (i) the first preform glass tube was drawn according to a conventional drawing process without the heat treatment disclosed herein; (ii) the second preform glass tube was subjected to heat treatment by cooling the glass tube at a temperature of -10°C for 2 minutes at a cooling rate of -10°C per minute; (iii) the third preform glass tube was subjected to heat treatment by heating the glass tube at a temperature of 250°C for 2 minutes at a rate of 100°C per minute; and (iv) the fourth preform glass tube was subjected to heat treatment by heating the glass tube at a temperature of 200°C for 2 minutes at a rate of 100°C per minute. Figure 7A shows a drawn fiber produced using the conventional drawing process described above in (i). Figure 7B shows a drawn fiber produced using the exemplary process described above in (ii). Figure 7C shows a drawn fiber produced using the exemplary process described above in (iii). FIG. 7D shows a drawn fiber made using the exemplary process in (iv) above.

[0068] Cooling the glass tube during the drawing process caused the structural tubes in the drawn fiber to have a relatively small size, as shown in Figures 7A and 7B. Heating the glass tube during the drawing process caused the structural tubes in the drawn fiber to have a relatively larger size, as shown in Figures 7A, 7C, and 7D.

[0069] Table 1 below provides an example of a hollow-core fiber made using a conventional process without a heat treatment step (comparative fiber) and a hollow-core fiber made using a heat treatment step as disclosed herein (exemplary fiber). During heat treatment of the exemplary fiber, the preform was heated to a temperature of 250°C throughout the draw of the fiber. [Table 1]

[0070] 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.

[0071] 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 hollow-core optical fiber, 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; drawing the preform into a hollow-core optical fiber; and heat-treating the preform while elongating the preform to manipulate gas pressure within the glass tube by at least one of (i) heating at least a portion of the preform to increase gas pressure within the glass tube, and (ii) cooling at least a portion of the preform to decrease gas pressure within the glass tube.

2. The method of claim 1 , wherein said heat treating comprises heating said portion of said preform to a temperature of about 200° C. or greater.

3. The method of claim 2, wherein said heat treating comprises heating said portion of said preform to a temperature of from about 200°C to about 500°C.

4. The method of claim 1 , wherein a heat treatment is applied to a first section of the preform and a second section of the preform is a root-forming portion of the preform.

5. the heat treating includes heating the first section of the preform to a first temperature; the stretching of the preform includes heating the second section of the preform to a second temperature; The method of claim 4 , wherein the first temperature is less than the second temperature.

6. The method of claim 5 , wherein the first temperature is below the strain point of the glass tube.

7. The method of claim 1 , wherein said heat treating comprises cooling said portion of said preform to a temperature of about 50° C. or less.

8. The method of claim 7, wherein said heat treating comprises cooling said portion of said preform to a temperature of from about -100°C to about 50°C.

9. The method of claim 1 further comprising sealing the first open end of the glass tube.

10. 10. The method of claim 9, wherein the first open end of the glass tube is sealed while the gas within the glass tube is at a temperature of about 25°C.

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

12. 12. The method of claim 11 , wherein the glass tube forms a structural tube within the drawn hollow-core optical fiber, the structural tube comprising an inner capillary, and heating the portion of the preform increases the outer diameter of the inner capillary and decreases the wall thickness of the structural tube.

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

14. 14. The method of claim 13, wherein the glass tube forms a structural tube within the drawn hollow-core optical fiber, the structural tube comprising an inner capillary, and wherein cooling the portion of the preform reduces an outer diameter of the inner capillary and increases a wall thickness of the structural tube.

15. 10. The method of claim 1, wherein the at least one glass tube comprises a second glass tube, the heat treating comprises heating the glass tube to increase gas pressure within the glass tube, and the method further comprises cooling the second glass tube to decrease gas pressure within the second glass tube.

16. 10. The method of claim 1, wherein the at least one glass tube comprises a second glass tube, the heat treating comprises heating the glass tube to a first temperature to increase a gas pressure within the glass tube, and the method further comprises heating the second glass tube to a second temperature to increase a gas pressure within the second glass tube.

17. The method of claim 16 , wherein the first temperature is greater than the second temperature.

18. 10. The method of claim 1, further comprising creating a first hot zone and a second hot zone, wherein the first hot zone controls the size of a structural tube within the drawn hollow core optical fiber and the second hot zone forms a root-forming portion of the preform.

19. The method of claim 1, wherein said heat treating comprises heating said portion of said preform at a heating rate of between about 100° C. per second and about 350° C. per second.

20. 20. The method of claim 19, wherein the heating rate is between about 150° C. per second and about 300° C. per second.

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