HIGH PRESSURE DRAW FURNACE AND METHODS OF PRODUCING OPTICAL FIBERS

A method of forming an optical fiber, the method including heating a forming region of the optical fiber preform within a pressure device while exposing the forming region to a total pressure of about 500 atm or greater, directing the optical fiber preform in a downstream direction along a process pathway to form the optical fiber, and traversing the optical fiber through an aperture of a nozzle to maintain the total pressure of about 500 atm or greater within the pressure device.

FIELD

The present disclosure is directed to a high pressure draw furnace and methods of producing optical fibers with such a draw furnace and, more particularly, to the production of optical fiber having reduced Rayleigh scattering.

BACKGROUND

Optical fiber is increasingly being used for a variety of applications, including but not limited to broadband voice, video, and data transmission. As bandwidth demands increase, optical fiber is migrating deeper into communication networks including fiber-to-the-premises applications, such as FTTx, 5G, and the like.

However, traditional optical fiber inherently induces optical loss in optical signals that propagate within the optical fiber. This optical loss produces signal degradation that can affect network performance. One source of optical loss is the presence of structural voids within the optical fiber that cause Rayleigh scattering and overall signal attenuation.

Consequently, there exists an unresolved need for optical fiber draw production systems and methods of optical fiber production that reduce the presence of structural voids.

SUMMARY

The present disclosure is directed to optical fiber draw production systems, pressure devices, and methods of fabrication of optical fiber that apply pressure to the optical fiber within the draw furnace and at or near the point of optical fiber formation to reduce the presence of structural voids in the formed optical fiber.

In one embodiment, a method of forming an optical fiber comprises heating a forming region of the optical fiber preform within a pressure device while exposing the forming region to a total pressure of about 500 atm or greater, directing the optical fiber preform in a downstream direction along a process pathway to form the optical fiber, and traversing the optical fiber through an aperture of a nozzle to maintain the total pressure of about 500 atm or greater within the pressure device.

In another embodiment, a fiber draw furnace comprises a pressure device, a heater, and a nozzle. The pressure device comprising an inner cavity configured to receive an optical fiber preform. The heater being configured to heat at least a forming region of an optical fiber preform to draw the optical fiber preform into an optical fiber, and the heater being configured to heat the forming region while exposing the optical fiber preform to a total pressure of about 500 atm or greater within the inner cavity. The nozzle being disposed downstream of the inner cavity and configured to maintain the total pressure of about 500 atm or greater within the inner cavity while the optical fiber traverses through an aperture in the nozzle.

It is to be understood that both the foregoing general description and the following detailed description present embodiments that are intended to provide an overview or framework for understanding the nature and character of the claims. The accompanying drawings are included to provide a further understanding of the disclosure and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments and together with the description serve to explain the principles and operation.

DETAILED DESCRIPTION

References will now be made in detail to the embodiments of the disclosure, examples of which are illustrated in the accompanying drawings. Whenever possible, like reference numbers will be used to refer to like components or parts.

The present disclosure is directed to optical fiber draw production systems, pressure devices, and methods of fabrication of optical fiber that apply pressure to an optical fiber preform within a draw furnace and at or near the point of optical fiber formation to reduce the presence of structural voids in the formed optical fiber. Structural voids within the optical fiber cause undesirable Rayleigh scattering and overall signal attenuation. In the optical fiber draw process, an optical fiber is formed from bulk glass (e.g. a preform) by heating the bulk glass to softening and drawing (pulling) an optical fiber from the softened glass through the action of gravity and application of a draw tension. One strategy for reducing the presence of structural voids in glass is to apply pressure to the glass to rearrange or densify the glass structure to remove or minimize structural voids.

Unless otherwise specified, the temperature is expressed herein in units of° C. (degrees Celsius).

The term “process pathway” refers to the pathway traversed by an optical fiber in an optical fiber draw process.

The relative position of one process unit relative to another process unit along the process pathway is described herein as upstream or downstream. The upstream direction of the process pathway is the direction toward the preform and the downstream direction of the process pathway is the direction toward the winding stage. Positions or processing units upstream from a reference position or processing unit are closer, along the process pathway, to the preform than the reference position or processing unit. A process unit located at a position closer to the draw furnace along the process pathway is said to be upstream of a process unit located at a position further away from the draw furnace along the process pathway. The draw furnace is upstream from all other process units and the take-up spool (or winding stage or other terminal unit) is downstream of all other process units.

The term “optical fiber” refers to a glass waveguide. The glass waveguide includes a glass core and a glass cladding. The glass cladding surrounds and is directly adjacent to the glass core. The glass cladding may include two or more concentric glass regions that differ in refractive index. The refractive index of the glass core is greater than the refractive index of the glass cladding (or the average refractive index of the glass cladding when the glass cladding includes multiple concentric regions) at a wavelength of 1550 nm.

Embodiments of the present disclosure apply high pressure (e.g., 500 atm or greater) to a preform in an optical fiber draw process during the draw of the preform to an optical fiber. More particularly, embodiments employ a pressure device that applies pressure to the preform without the preform or drawn optical fiber physically contacting any component of the pressure device or other components of the optical fiber draw furnace.

Pressure is preferably applied to the optical fiber preform by the pressure device while the preform is at an elevated temperature. In some embodiments, pressure is applied to the forming region of the preform. Embodiments further provide centering forces on the optical fiber to ensure no physical contact between it and structures of the optical fiber draw production system.

Various embodiments of optical fiber draw production systems, pressure devices, and methods for forming optical fiber that apply pressure to an optical fiber during fiber formation to reduce or eliminate structural voids are described in detail below. Referring now toFIG.1, an exemplary optical fiber draw furnace100is shown. Draw furnace100comprises a sleeve110, such as a susceptor sleeve, and a pressure device120configured to receive an optical fiber preform10. Sleeve110may be disposed within pressure device120. As discussed further below, draw furnace100draws preform10into an optical fiber20. In the embodiments disclosed herein, draw furnace100draws preform10into optical fiber20under a high pressure to reduce any Rayleigh scattering and attenuation in the drawn fiber.

Preform10may be composed of any well-known glass or other material and may be doped suitable for the manufacture of optical fibers. In some embodiments, preform10includes a core and a cladding. As discussed further below, preform10is heated and consumed during the draw process. A downfeed handle (not shown) may be attached to preform10to lower the preform within pressure device120as the preform is consumed.

Sleeve110forms a tubular member comprising an inner cavity115through which preform10may be moveably disposed. An upper heater130is disposed adjacent to sleeve110to create a hot zone within draw furnace100. In embodiments, upper heater130is an induction coil. The heat of the hot zone decreases the viscosity of preform10to draw preform50into optical fiber20. Upper heater130and sleeve110may be encased by insulation140comprising a refractory material such as, for example, silicon, aluminum, magnesium, calcium, boron, chromium, zirconium, or mixtures thereof. As shown inFIG.1, inner cavity115extends through both sleeve110and insulation140from a first end117of the cavity to a second end119of the cavity. In some embodiments, a liner (not shown) may be disposed between inner cavity115and insulation140.

As also shown inFIG.1, pressure device120comprises a first end122and a second end124such that first end122is proximate to preform10and second end124is proximate to the drawn optical fiber20. It is noted that first end122of pressure device120is proximate to first end117of inner cavity115and that second end124of pressure device120is proximate to second end119of inner cavity115. Furthermore, inner cavity115extends for the length of pressure device120from first end122to second end124. A nozzle160is disposed at second end124of pressure device120. With the exception of a very thin aperture in nozzle160, as discussed further below, pressure device120is sealed at first end122and at second end124so that inner cavity115of sleeve110is also sealed. It is noted that optical fiber20is positioned within the very thin aperture of nozzle160, which allows pressure device120to be sealed (or effectively sealed). Furthermore, a gas inlet150is configured to inject process gas into inner cavity115during the draw process. The process gas may comprise helium, argon, nitrogen, air, oxygen, krypton, xenon, or a combination thereof. The combination of the sealing of pressure device120and the injection of the process gas into inner cavity115allows inner cavity115to be pressurized to a relatively high pressure. In embodiments disclosed herein, inner cavity115is pressurized to a relatively high pressure before optical fiber20is drawn from preform10.

The flow rate of the process gas into inner cavity115is about 100 g/min or less, or about 75 g/min or less, or about 50 g/min or less. In some embodiments, the temperature of the process gas injected into inner cavity115is between about 10° C. and about 1100° C., or between about 15° C. and about 500° C., or between about 20° C. and about 200° C., or at room temperature (i.e., about 25°° C.). It is also contemplated that inner walls of inner cavity115are at an elevated temperature, such as between about 300° C. and about 600° C. The elevated temperature of the inner walls may assist with minimizing any cooling of preform10and/or optical fiber20within pressure device120.

In embodiments, inner cavity115is pressurized to a relatively high total gas pressure and preform10is heated by upper heater130and consumed into optical fiber20while exposed to this relatively high pressure within pressure device120. Therefore, the glass of preform10is compressed to a higher density under the relatively high pressure before it is drawn into optical fiber20. More specifically, the relatively high pressure within inner cavity115, as disclosed herein, compresses the glass of preform10before it is drawn into an optical fiber. This compression increases the density of preform10before it is drawn into an optical fiber, which reduces the size of any voids in preform10. Without being bound by theory, it is believed that the hydrostatic pressure applied on the voids from the relatively high pressure overcomes any pressure forces within the voids, thereby causing the voids to collapse. The collapse of voids in preform10translates to a reduction of void size in the drawn optical fiber20, which reduces Rayleigh scattering and attenuation in the drawn optical fiber20.

Inner cavity115is pressurized to a relatively high total gas pressure of about 500 atm or greater, or about 750 atm or greater, or about 1,000 atm or greater, or about 1,250 atm or greater, or about 1,500 atm or greater, or about 1,750 atm or greater, or about 2,000 atm or greater, or about 5,000 atm or greater, or about 10,000 atm or greater, or about 20,000 atm or greater, or about 30,000 atm or greater, or about 40,000 atm or greater, or about 50,000 or greater, or about 60,000 atm or greater, or about 70,000 atm or greater, or about 80,000 atm or greater. Additionally or alternatively, inner cavity115is pressurized to a total gas pressure of about 80,000 atm or less, or about 70,000 atm or less, or about 60,000 atm or less, or about 50,000 atm or less, or about 40,000 atm or less, or about 30,000 atm or less, or about 20,000 atm or less, or about 10,000 atm or less, or about 5,000 atm or less, or about 2,000 atm or less, or about 1,750 atm or less, or about 1,500 atm or less, or about 1,250 atm or less, or about 1,000 atm or less, or about 750 atm or less, or about 500 atm or less. In embodiments, inner cavity115is pressurized to a total gas pressure from about 500 atm to about 80,000 atm, or about 500 atm to about 50,000 atm, or about 500 atm to about 10,000atm, or about 500 atm to about 5,000 atm, or about 500 atm to about 2,000 atm, or about 750atm to about 1,750 atm, or about 1,000 atm to about 1,500 atm, or about 1,250 atm to about 2,000 atm, or about 1,500 atm to about 2,000 atm, or about 1,750 atm to about 2,000 atm. As noted above, this total gas pressure within inner cavity115compresses preform10within inner cavity115. The total gas pressure within inner cavity115may be uniform and constant throughout the length of the cavity. Therefore, the pressure within inner cavity115at a first end117of inner cavity115is the same (or approximately the same) as the pressure at a second end119of inner cavity115.

FIG.2shows an enlarged image of a portion of draw furnace100of the embodiment ofFIG.1. As discussed above, upper heater130creates a hot zone within draw furnace100to lower the viscosity of the preform to draw the preform into optical fiber20. At region A (above the neckdown region of the preform), preform10is heated to a temperature below the softening point of the glass. At region B, preform10is heated to a temperature at or above the softening point of the glass to draw the preform into an optical fiber. In embodiments, region B of preform10is heated to a temperature from about 1570° C. to about 2100°° C., or about 1585° C. to about 2075° C., or about 1600° C. to about 2050° C., or about 1625° C. to about 2000° C., or about 1650°° C. to about 1975° C., or about 1670° C. to about 1975° C., or about 1675° C. to about 1950° C., or about 1700° C. to about 1925° C., or about 1725° C. to about 1900°° C., or about 1750° C. to about 1875° C., or about 1670° C. to about 2100° C. Therefore, the temperature of preform10at region A is below the temperature of perform10at region B.

Region B of preform10comprises a forming region of preform10, which includes the portion of preform10that extends from the end of region A to optical fiber20. As shown inFIG.2, Region B comprises the neckdown region of preform10. The transition point between region B and optical fiber20is the point at which the diameter of the preform reaches the diameter of optical fiber20(e.g., 125 microns). Once the diameter of preform10is the diameter of optical fiber20, region B of preform10has terminated and transitioned into optical fiber20. Therefore, throughout the entirety of region B, preform10has a larger diameter than the diameter of optical fiber20. In some embodiments, the diameter of optical fiber20is 125 microns, in which case region B terminates at the point at which the diameter of the preform has reached 125 microns. In other embodiments, the diameter of optical fiber20is 150 microns, in which case region B terminates at the point at which the diameter of the preform has reached 150 microns. It is noted that region B may terminate closer to nozzle160than depicted inFIG.3A(or within nozzle160or downstream of nozzle160). In some embodiments, region B terminates upstream of nozzle160. Furthermore, as shown inFIGS.2and3A, region B is downstream of region A. It is also noted that the temperature of the drawn optical fiber20within pressure device120is at the same temperature (or approximately the same temperature) as the temperature of preform10at region B.

As discussed above, inner cavity115is pressurized to a relatively high gas pressure of, for example, about 500 atm or greater. Therefore, the glass of preform10is subject to the compression forces caused by the relatively high gas pressure within inner cavity115. As is known in the art, when particles are compressed, the density of that component increases. Therefore, as the glass particles within preform10are compressed by the relatively high gas pressure, the glass particles of preform10densify. The densification of preform10is further amplified by the heating of preform10within region B, which lowers the viscosity of the glass so that it is more susceptible to the compression forces. As noted above, region B of preform10is heated to a higher temperature than region A so that the glass obtains a higher density in region B than in region A. The increased density in region B of preform10is maintained in the optical fiber20drawn directly from region B. In particular, the increased density is maintained in the drawn optical fiber20because the fiber is also subject to the same compression forces within inner cavity115. Therefore, both region B of preform10and the drawn optical fiber20have a relatively higher density.

In some embodiments, preform10may be pre-densified before the draw process (for example, preform10may be densified before being disposed within pressure device120). In these embodiments, when preform10is then subject to the high gas pressure within pressure device120, the increased density of preform10is maintained due to the relatively high gas pressure within inner cavity115. More specifically, the relatively high gas pressure within inner cavity115prevents the densified preform10from reverting back to its state before such densification. In these embodiments, both region A and region B of preform10have the relatively higher density. Therefore, in these embodiments, the drawn optical fiber20also has the relatively higher density.

As discussed above, the relatively higher density causes voids to collapse in the glass. It is noted that the voids remain collapsed and do not reopen as long as the glass is subject to the relatively high gas pressure until the glass is cooled. Therefore, in the embodiments disclosed herein, the length of inner cavity115is at the relatively high gas pressure so that the drawn optical fiber is exposed to such relatively high gas pressure until the optical fiber is cooled. This allows the voids in the drawn optical fiber to remain collapsed.

As discussed further below, optical fiber20exits pressure device120through nozzle160. As discussed further below, nozzle160advantageously helps to maintain the relatively high gas pressure within inner cavity115of pressure device120. In particular, nozzle160comprises an aperture through which optical fiber20extends. The aperture has a small diameter to maintain the desired pressure within inner cavity115. Furthermore, the aperture is structured so that optical fiber20is centered within the narrow opening, to prevent optical fiber20from contacting an inner wall of nozzle60.

During the draw process, the draw speed of optical fiber20is about 5 m/s or greater, or about 20 m/s or greater, or about 40 m/s or greater. However, embodiments of the present disclosure are not limited by any particular draw speed.

FIG.3Ashows pressure device120ofFIG.1A(with nozzle160attached thereto) separate from the remaining components of draw furnace100.FIG.3Bshows another embodiment of pressure device120in which a molten metal30is disposed within nozzle160. In particular,FIG.3Bshows a partial cross-sectional view of pressure device120and of nozzle160with optical fiber20disposed therein. Molten metal30may fill up an entire interior space of nozzle160or just a portion thereof. In some embodiments, molten metal30is at a bottom portion of nozzle160. Molten metal30helps to seal pressure device120and inner cavity115while allowing optical fiber20to exit through the aperture in nozzle160. In embodiments, molten metal30comprises a metal material such that the melting point of the metal material is less than the softening point of preform10. Therefore, molten metal30is in a liquified state within nozzle160. Exemplary examples of molten metal30include, for example, tin (Sn), gold (Au), copper (Cu), aluminum (Al), and combinations thereof.

FIG.3Cshows another embodiment of pressure device120in which molten metal30is disposed within nozzle160and within inner cavity115. In particular,FIG.3Cshows a cross-sectional view of pressure device120and of nozzle160with optical fiber20disposed therein. In the embodiment ofFIG.3C, molten metal30extends all the way from nozzle160to the neckdown region of preform10. However, molten metal30should, preferably, not extend within region A of preform10so that the glass injected into inner cavity115can reach the desired relatively high pressure, as discussed above.FIG.3Dshows pressure device120of the embodiment ofFIG.3Cin draw furnace100. In this embodiment, draw furnace100further comprises a lower heater132to heat and melt molten metal30. Lower heat132may be configured to maintain molten metal30in the liquefied state. In embodiments, lower heater132is an induction coil.

The relatively high gas pressure within inner cavity115applies a downward pressure on molten metal30, so that molten metal30is also pressurized to a relatively high pressure state. Therefore, the glass of region B of preform and the glass of optical fiber20are subject to the compression forces of molten metal30as the fiber is drawn and passes through the molten metal. The pressurized molten metal30may be at the same relatively high pressures as disclosed above (e.g., about 500 atm or greater). However, the pressure applied by molten metal30may be a liquid pressure (as opposed to a gas pressure). For example, molten metal30may comprise a liquid pressure of about 500 atm or greater (or any of the pressures disclosed above with reference to the total gas pressure of inner cavity115). And the pressurized molten metal30helps to maintain the voids in optical fiber20in the collapsed state so that the voids do not reopen as the fiber is drawn.

In embodiments in which molten metal30extends to the neckdown region of preform10(such that molten metal30surrounds the neckdown region (e.g., region B)), molten metal30advantageously isolates this region of preform10from any turbulent gas within inner cavity115. As in known in the art, process gas is subject to flow instabilities as it flows within a draw furnace during a drawing process. These flow instabilities result in an uneven and irregular diameter in the drawn optical fiber. Unsteady convection, due to density stratification of the gas within the draw furnace and due to irregular flow of the process gas, causes such flow instabilities. For example, the flow of the process gas may form recirculations of gas in the draw furnace. These flow instabilities are manifested as temperature variations, pressure variations, and mass flow variations that are translated to the neckdown region of the preform, which ultimately cause fluctuations in the diameter of the drawn optical fiber. Molten metal30provides a barrier between any flow instabilities in inner cavity115and the neckdown region of preform10. Therefore, molten metal30advantageously protects the drawn optical fiber from diameter fluctuations.

FIG.4shows the relationship between temperature and dynamic viscosity of molten tin (in units of MPa·s), as an exemplary molten metal30. As shown inFIG.4, as the temperature of the molten tin increases, the viscosity of the molten tin decreases, thus creating a more fluid-like consistency. Conversely, as the temperature of the molten tin decreases, the viscosity of the molten tin increases, thus creating a more tacky-like consistency.

In embodiments, molten metal30at region C has a viscosity from about 0.70 MPa·s to about 0.80 MPa·s, or about 0.72 MPa·s to about 0.78 MPa·s, or about 0.74 MPa·s to about 0.76 MPa·s, or about 0.75 MPa·s to about 0.77 MPa·s. In embodiments, molten metal30at region D has a viscosity from about 0.75 MPa·s to about 1.75 MPa·s, or about 0.80 MPa·s to about 1.70 MPa·s, or about 0.85 MPa·s to about 1.65 MPa·s, or about 0.90 MPa·s to about 1.60 MPa·s, or about 0.95 MPa·s to about 1.55 MPa·s, or about 1.00 MPa·s to about 1.50 MPa·s, or about 1.05 MPa·s to about 1.45 MPa·s, or about 1.10 MPa·s to about 1.40 MPa·s, or about 1.15 MPa·s to about 1.35 MPa·s, or about 1.20 MPa·s to about 1.30 MPa·s, or about 1.25 MPa·s to about 1.30 MPa·s. As discussed above, in embodiments, molten metal30has a higher viscosity at region D than at region C. Furthermore, molten metal30at regions C and D has a density from about 6500 Kg/m3to about 8000 Kg/m3, or about 6700 Kg/m3to about 7800 Kg/m3, or about 6900 Kg/m3to about 7600 Kg/m3, or about 7000 Kg/m3to about 7500 Kg/m3, or about 7100 Kg/m3to about 7400 Kg/m3. In embodiments, the density of molten metal30at region C is the same as the density of molten metal30at region D. In other embodiments, the density of molten metal30at region C is different from the density of molten metal at region D.

In the embodiments disclosed herein that utilize molten metal30, optical fiber20is surrounded by molten metal30as optical fiber20moves downward within inner cavity115and/or nozzle160. Therefore, molten metal30forms an outer coating on optical fiber20once the fiber exits nozzle160. The outer coating is radially outward of any outer cladding glass within optical fiber20.FIG.5shows an exemplary embodiment of optical fiber20, after exiting nozzle160, that comprises a glass core22, a glass cladding24, and a metal coating34. As shown inFIG.5, glass cladding24radially surrounds glass core22, and metal coating34radially surrounds glass cladding24. In embodiments, the thickness of metal coating34is from about 5 microns to about 15 microns, or about 7 microns to about 12microns, or about 8 microns to about 10 microns. Optical fiber20can be further processed and stored with metal coating34thereon. For example, one or more polymeric layers may be disposed on metal coating34in downstream processing of optical fiber20. Alternatively, metal coating34can be removed from optical fiber20by, for example, heat application before any further processing or storing of optical fiber20.

A cross-section of nozzle160with an optical fiber20positioned therein is depicted inFIG.6. In particular,FIG.6shows optical fiber20disposed within aperture162of nozzle160. It is noted that althoughFIG.6does not include molten metal30within nozzle160, as discussed above, molten metal30may fill the entire interior of nozzle160or a portion thereof. It is also noted that the components ofFIG.6are not drawn to scale. Aperture162extends through an entire length of nozzle160, thus forming an opening from a first end165to a second end167of nozzle160. Nozzle160also comprises at least a straight cylindrical member164and a tapered member163, such that straight cylindrical member164is downstream of tapered member163. As shown inFIG.6, straight cylindrical member164extends from a transition region168to second end167. The outer and inner diameters of nozzle160taper radially inward toward second end167within tapered member163. It is also contemplated that just the inner diameter of nozzle160tapers radially inward toward second end167within tapered member163while the outer diameter does not taper inward.

An angle θ is formed between the inner wall of cylindrical member164and the inner wall of tapered member163. More specifically, angle θ is formed between plane X of cylindrical member164and plane Y of tapered member163such that plane X extends along an inner profile of cylindrical member164and that plane Y extends along an inner profile of tapered member163. Angle θ represents the taper angle of tapered member163such that a larger angle θ corresponds to a steeper slope of tapered member163. The slope of tapered member163and the pressure within nozzle160contribute to the centration force required to center optical fiber20within nozzle160.

As used herein, “centration force” is the net force acting on a fiber to center the fiber along a centerline, such as centerline CL of nozzle160. The net force acting on a fiber is a result of the pressure distribution on the fiber. For example, if a fiber is offset from centerline CL when disposed within nozzle160such that the fiber is closer to a top surface of the nozzle than a bottom surface of the nozzle, the pressure distribution at the top of the fiber is not equal to the pressure distribution at the bottom of the fiber. More specifically, the gas or fluid surrounding the fiber (e.g., molten metal30) would have a higher pressure at the top of the fiber than at the bottom of the fiber. The centration force is the force acting on the fiber to counteract any uneven pressure distribution acting on the fiber. A nozzle with a relatively higher centration force is advantageous as it provides a higher force acting on the fiber to center the fiber. Conversely, a nozzle with a relatively lower centration force provides a lower force acting on the fiber, so that the fiber is more prone to move to one side or another.

When optical fiber20is drawn from preform10, upstream forces (for example, turbulent gas flow within pressure device120) can cause the drawn optical fiber to move off-center from centerline CL. The drawn optical fiber20may be biased to one side of the centerline due to such upstream forces, causing an uneven net centration force. Additionally, downstream forces (for example, a downstream winder pulling the fiber to one side or another that is off-center from centerline CL) can cause the drawn optical fiber to move off-center from centerline CL. An even net pressure distribution is desirable so that the optical fiber20does not contact inner walls of nozzle160. Any contact between optical fiber20and the walls of aperture162can cause deformities in the drawn optical fiber, which increase the Rayleigh scattering and attenuation in the drawn optical fiber. As optical fiber20moves through nozzle160, the net pressure distribution acting on optical fiber20should ideally be even so that pressure applied to optical fiber20is even from all sides of the fiber.

The magnitude of the centration force acting on fiber20is measured using equation (1) below:

where Fxis the horizontal centration force acting on the fiber (grams-force), T is the tension force on the fiber (grams-force), and θcis the angle between a centerline CLfof the fiber downstream of pressure device120and centerline CL of nozzle160. As shown inFIG.7A, angle θcis non-zero when the fiber is off-center from centerline CL. In one example, if optical fiber20is subjected to a pulling tension T of 100 grams-force and angle θcis 5 degrees, the centration force Fxrequired to maintain centerline CLfof optical fiber20aligned with centerline CL of nozzle160is about 8.7 grams-force. In this example, a centration force Fxbelow about 8.7 grams-force will allow the fiber to be off-center from centerline CL of nozzle160(which is undesirable). It is desirable to have angle θcbe as low as possible so that the centration force required to center the fiber is also reduced.

In the embodiments disclosed herein, the structure of nozzle160allows gas and/or fluid to flow through the nozzle while providing the necessary centration force so that centerline CFfof optical fiber20remains aligned with centerline CL of nozzle160. For example, the slope of tapered member163and the pressure within nozzle160contribute to the centration force produced by nozzle160.

FIG.6shows an embodiment in which the centerline of optical fiber20is aligned with centerline CL of nozzle160. As shown inFIG.6, an outer diameter Dfof optical fiber20is less than an inner minimum diameter Daof aperture162so that a gap is formed between optical fiber20and nozzle160. At second end167of nozzle160, the gap comprises a first gap G1between the outer diameter of optical fiber20and an inner diameter of aperture20at a top surface23of optical fiber20. Furthermore, at second end167of nozzle160, the gap comprises a second gap G2between the outer diameter of optical fiber20and an inner diameter of aperture162at a bottom surface25of optical fiber20. Preferably, optical fiber20is radially centered about centerline CL of nozzle160so that first gap G1is equal to (or approximately equal to) second gap G2, which occurs when the centration force is sufficiently high to center the optical fiber.

In embodiments, gaps G1and G2are each about 2 microns to about 20 microns, or about 4 microns to about 18 microns, or about 6 microns to about 16 microns, or about 8microns to about 14 microns, or about 10 microns to about 12 microns in length. However, the length of gaps G1and G2depends on the diameter of optical fiber20. As discussed above, the length of gaps G1and G2may be the same or different from each. In some exemplary embodiments, gaps G1and G2are both about 8 microns in length or about 10 microns in length or about 12 microns in length. Furthermore, in embodiments a difference between the length of first gap G1and the length of second gap G2is only about 10% or less, or about 5% or less, or about 2.5% or less, or about 2% or less or about 1.5% or less, or about 1.25% or less, or about 1% or less, or about 0.75% or less, or about 0.5% or less, or about 0.25% or less, or about 0.1% or less of the length of either gap G1or G2.

In the embodiment ofFIG.6, optical fiber20has an outer diameter Dfof about 125 microns. However, optical fiber20may comprise other outer diameters as are well known in the art. The inner minimum diameter of aperture Damay be from about 130 microns to about 165 microns, or about 135 microns to about 160 microns, or about 140 microns to about 155 microns, or about 145 microns to about 150 microns. However, it is noted that the inner minimum diameter Daof aperture162depends on the outer diameter Dfof optical fiber20, such that a larger nozzle160with a relatively larger aperture diameter Damay be required for relatively larger optical fibers. In some exemplary embodiments, optical fiber20has an outer diameter Dfof about 125 microns and aperture162has an inner minimum diameter Daof about 145 microns. The inner minimum diameter Daof aperture162is preferably close to the outer diameter Dfof optical fiber20(but not the same as) to effectively seal inner cavity115and to reduce/prevent flow of molten metal30out of nozzle160while still allowing clearance between optical fiber20and aperture162so that optical fiber20does not contact the inner walls of nozzle160.

Nozzle160has a longitudinal length L (from first end165to second end167and along the centerline CLfof the fiber) of about 0.05 mm or greater, or about 0.10 mm or greater, or about 0.15 mm or greater, or about 0.20 mm or greater, or about 0.22 mm or greater, or about 0.25 mm or greater. Additionally or alternatively, the length L of nozzle160is about 0.25 mm or less, or about 0.22 mm or less, or about 0.20 mm or less, or about 0.15 mm or less or about 0.10 mm or less, or about 0.05 mm or less. In some embodiments, the length L is from about 0.05 mm to about 0.25 mm, or about 0.10 mm to about 0.22 mm, or about 0.15 mm to about 0.20 mm. The length L should be sufficiently long to allow flow speed reduction of the process gas flowing from inner cavity115without inducing turbulence in the gas flow.

In embodiments, tapered member163has a longitudinal length (along the centerline CLfof the fiber) from about 0.05 mm to about 0.25 mm, or about 0.10 mm to about 0.22 mm, or about 0.14 mm to about 0.20 mm, or about 0.14 mm to about 0.18 mm. Furthermore, in embodiments, cylindrical member163has a length from about 0.020 mm to about 0.100 mm, or about 0.040 mm to about 0.080 mm, or about 0.050 mm to about 0.070 mm, or about 0.055 mm to about 0.065 mm. Tapered member163of nozzle160may have the same maximum outer profile as inner cavity115and may directly connect with inner cavity115. Thus, aperture162and inner cavity115may form a continuous opening through the draw furnace.

FIG.7B(also not drawn to scale) depicts an image of when the centerline CLfof optical fiber20is not centered with centerline CL of nozzle160, such as when the centration force is not sufficient to center the fiber. Therefore, optical fiber20is radially offset from centerline CL of nozzle160. In this example, first gap G1is less than second gap G2. More specifically, in one exemplary example, first gap G1is 6 microns, second gap G2is 14 microns, aperture162has an inner minimum diameter Daof 145 microns, and optical fiber20has an outer diameter Dfof 125 microns. Furthermore, in this exemplary example, optical fiber20traverses through nozzle160at a speed of 50 m/s and the length of tapered member163(along the centerline CLfof the fiber) is 0.145 mm. Due to the different lengths of first and second gaps G1and G2, the pressure of the gas flowing through first gap G1is not equal to the pressure of the gas flowing through second gap G2. Therefore, the net centration force surrounding optical fiber20is not even, and optical fiber20is not centered about centerline CL of nozzle160.

FIG.7Cshows the gas pressure at second end167of aperture162through gaps G1and G2of this exemplary example (where G1is 6 microns in length and G2is 14 microns in length).FIG.7Cis a cross-sectional view of gaps G1and G2at second end167of nozzle162. First gap G1is smaller than second gap G2and, therefore, has a higher pressure therethrough. As shown inFIG.7C, the pressure through first gap G1is about 1350 atm while the pressure through second gap G2is only about 1180 atm. Due to this difference in pressure between gaps G1and G2, optical fiber20is not centered within nozzle160about centerline CL. Therefore, the net centration force is not even inFIG.7C.

As noted above, the slope of tapered member163(i.e., the value of taper angle θ) and the total pressure within nozzle160determine the centration force provided by nozzle160to center optical fiber20.FIG.8shows the relationship between centration force of nozzle160and the taper angle θ of tapered member163(using the exemplary nozzle embodiment ofFIG.6). In order to determine the relationship as depicted inFIG.8, the pressure within inner cavity115was at an operating pressure of 2000 atm. As shown inFIG.8, a taper angle θ of 5° produced the highest centration force. However, broader ranges were also shown to produce nozzles with sufficiently high centration force to properly center an optical fiber within the nozzle (so that the fiber does not contact the sides of the nozzle). In the embodiments disclosed herein, a taper angle θ between about 1.5 degrees and about 35 degrees produced a nozzle160with a sufficiently high centration force so that the nozzle was able to maintain optical fiber20centered about centerline CL. In embodiments, the taper angle θ is between about 2 degrees and about 30 degrees, or about 3 degrees and about 25 degrees, or about 4 degrees and about 20 degrees, or about 5 degrees and about 15 degrees, or about 5 degrees and about 10 degrees, or about 3 degrees and about 10 degrees, or about 4 degrees and about 7 degrees.

In the embodiments disclosed herein, the centration force of nozzle160is about 2 grams-force or greater, or about 5 grams-force or greater, or about 18 grams-force or greater, or about 10 grams-force or greater, or about 12 grams-force or greater in order to center optical fiber 20 about centerline CL.

FIG.9shows the relationship between the centration force of nozzle160and the total gas pressure within inner cavity115(using the exemplary nozzle embodiment ofFIG.6). In order to determine this relationship as depicted inFIG.9, the taper angle θ of tapered member163was 8°. As shown inFIG.9, as the total gas pressure within the draw furnace increased, the centration force also increased. Therefore, a higher gas pressure corresponds to a higher centration force.

FIG.10shows the relationship between the flow rate of molten metal30within nozzle160and the total gas pressure within inner cavity115(using the exemplary nozzle embodiment ofFIG.6). In particular,FIG.10shows the relationship between the flow rate of molten tin within nozzle160and the total gas pressure within inner cavity115. As shown inFIG.10, as the gas pressure increases, the flow rate of the molten tin increases. Therefore, higher gas pressures correspond to a higher flow rate of molten metal30. As discussed above with reference toFIG.9, higher gas pressures also correspond to higher centration forces.

With reference again toFIG.1, sleeve110of pressure device120may comprise straight and uniform inner walls or inner walls that vary in diameter.FIG.11shows another embodiment of a sleeve210of pressure device120that may be used in draw furnace100. In the embodiment ofFIG.11, sleeve210comprises inner walls that vary in diameter. More specifically, in embodiments, sleeve210comprises an angled inlet region220with inner walls having a low-angle taper (e.g., about 0.5° to about 2° with reference to a centerline CL′ of sleeve210) to maintain a laminar flow condition for process gas passing in an upstream direction within inner cavity115. Angled inlet region220also has a length L′ that reduces the flow speed of process gas to a degree sufficient to avoid turbulence in the vicinity of preform10. The laminar flow conditions and avoidance of turbulence within sleeve210minimize vibrations of preform10, which helps to prevent physical contact between preform10and inner walls of pressure device120.

Sleeve210further comprises chamber230which comprises a larger inner diameter than that of angled inlet region220. As shown inFIG.11, preform10is heated to a temperature above the softening point of the glass within chamber230to draw preform10into optical fiber20. As discussed above, preform10may be heated to the temperatures disclosed above with reference to regions A and B while preform10is positioned within chamber230. The diameter of chamber230transitions into angled outlet region240, which has a smaller inner diameter than that of chamber230. Angled outlet region240may have the same taper angle and length L′ as disclosed above with reference to angled inlet region220. In embodiments, the length L′ of both angled inlet region220and angled outlet region240is in a range from about 100 mm to about 300 mm, or about 150 mm to about 250 mm, or about 200 mm.

In the embodiments ofFIGS.1-3D and6-7C, nozzle160is depicted as a single, unitary member. However, rather than one member, nozzle160may comprise a series of nozzles.FIG.12shows an embodiment in which nozzle360comprises a series of sub-nozzles360A-360E and a tapered exit370at second end367of nozzle360. Optical fiber20exits pressure device120at tapered exit370, and tapered exit370leads to ambient pressure outside (downstream) of pressure device120. Each sub-nozzle360A-360E provides a centering force and the series of sub-nozzles act collectively to control the reduction in pressure from the relatively high pressure in pressure device120to the atmospheric pressure (downstream of tapered exit370).FIG.12also illustrates in grayscale the static pressure within the sub-nozzles360A-360E of nozzle360in an embodiment in which the pressure within the pressure device120is 1000 atm. As shown inFIG.12, sub-nozzle360A (positioned further from second end367) has a relatively higher pressure than sub-nozzle360E (positioned closest to second end367).

The relationship between centration force vs. total gas pressure within inner cavity115with the series of sub-nozzles embodiment ofFIG.12is shown inFIG.13. In particular, the nozzle comprise ten sub-nozzles arranged in series and the optical fiber disposed within is offset by 4 microns (such that, for example, first gap G1is larger than second gap G2by 4 microns). As shown inFIG.13, at an operating pressure of 2000 atm within inner cavity115, the nozzle produces a centration force of about 20 grams-force. As another example, as shown inFIG.13, at an operating pressure of 1000 atm within inner cavity115, the nozzle produces a centration force of about 10 grams-force. In comparison with the embodiment ofFIG.9, an operating pressure of 2000 atm produced the centration force of about 10 grams-force. Therefore, a lower operating gas pressure is required in the embodiment ofFIG.13to achieve the same centration force as that ofFIG.9. It is further noted that the centration force of the nozzle embodiment ofFIG.13can be increased by adding more sub-nozzles.

FIG.14shows the relationship between the flow rate of molten metal30vs. total gas pressure within inner cavity115with the series of sub-nozzles embodiment ofFIG.12. In particular, the nozzle comprise ten sub-nozzles arranged in series and the optical fiber disposed within is offset by 4 microns (such that, for example, first gap G1is larger than second gap G2by 4 microns). In particular,FIG.14shows the relationship between the flow rate of molten tin within the nozzle and the total gas pressure within inner cavity115. As shown inFIG.14, as the gas pressure increases, the flow rate of the molten tin increases. Therefore, higher gas pressures correspond to higher flow rate of molten metal30. As discussed above with reference toFIG.13, higher gas pressures also correspond to higher centration forces.

Other configurations for nozzle160are contemplated within the scope of the present disclosure.FIG.15shows an embodiment of a nozzle460that comprises a pair of step bearings413A,413B. Step bearing413A forms an inner cavity418A and step bearing143B forms an inner cavity418B, which are separated by step419. Furthermore, groove420is provided between step bearing413A and step bearing413B to accept a seal (e.g., an O-ring). AlthoughFIG.15shows two step bearings, it is contemplated that the nozzles disclosed herein may comprise more or less step bearings.

It should now be understood that the pressure devices and methods described herein enable the ability to make optical fibers with higher glass density due to compression of the optical fiber at or near the neckdown region of the preform as well as lower attenuation due to lower Rayleigh scattering coefficient. While not wishing to be bound by theory, the pressure provided by pressure device120on preform10is thought to decrease structural voids within the optical fiber drawn therefrom. The application of pressure to the optical fiber preform, as disclosed herein, leads to reductions in the Rayleigh scattering and overall attenuation in the drawn optical fiber compared to traditional drawing methods. As non-limiting examples, the pressure devices and methods described herein enable the production of optical fibers wherein the fiber attenuation is less than 0.175 dB/km at 1550 nm, less than 0.165 dB/km at 1550 nm, less than 0.155 dB/km at 1550, or less than 0.145 dB/km at 1550 nm. As further non-limiting examples, the pressure devices and methods described herein enable the production of optical fibers wherein the fiber attenuation is less than 0.31 dB/km at 1310 nm, less than 0.29 dB/km at 1310 nm, less than 0.27 dB/km at 1310, or less than 0.25 dB/km at 1310 nm. As further non-limiting examples, the pressure devices and methods described herein enable the production of optical fibers having a Rayleigh scattering coefficient of less than 0.87 dB/km/micron4, less than 0.82 dB/km/micron4, less than 0.77 dB/km/micron4, or less than 0.72 dB/km/micron4. As further non-limiting examples, the pressure devices and methods described herein enable the production of optical fibers having a Rayleigh scattering coefficient reduction of greater than 4%, greater than 8%, greater than 12%, or greater than 16% compared to an optical fiber drawn under identical conditions but without application of pressure by a pressure device.

For the purposes of describing and defining the embodiments of the present disclosure, it is noted that the terms “about,” “approximately,” and “substantially” are utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. The terms “about,” “approximately,” and “substantially” are also utilized herein to represent the degree to which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue.

According to a first aspect of the present disclosure, a method of forming an optical fiber is disclosed, the method comprising heating a forming region of the optical fiber preform within a pressure device while exposing the forming region to a total pressure of about 500 atm or greater, directing the optical fiber preform in a downstream direction along a process pathway to form the optical fiber, and traversing the optical fiber through an aperture of a nozzle to maintain the total pressure of about 500 atm or greater within the pressure device.

According to a second aspect of the present disclosure, the first aspect wherein the total pressure is about 1000 atm or greater.

According to a third aspect of the present disclosure, the first aspect wherein the total pressure is from about 500 atm to about 2000 atm.

According to a fourth aspect of the present disclosure, the first aspect wherein the total pressure is from about 750 atm to about 1750 atm.

According to a fifth aspect of the present disclosure, the first aspect wherein the

forming region of the optical fiber preform is heated to a temperature at or above a softening temperature of the preform.

According to a sixth aspect of the present disclosure, the first aspect wherein the forming region of the optical fiber preform is heated to a temperature from about 1570° C. to about 2100°° C.

According to a seventh aspect of the present disclosure, the first aspect wherein the nozzle provides a centration force that centers the optical fiber about a centerline of the nozzle.

According to an eighth aspect of the present disclosure, the seventh aspect wherein the centration force is about 2 grams-force or greater.

According to a ninth aspect of the present disclosure, the eight aspect wherein the centration force is about 5 grams-force or greater.

According to a tenth aspect of the present disclosure, the first aspect further comprising heating a molten metal, the molten metal being disposed within the nozzle and radially outward of the optical fiber.

According to an eleventh aspect of the present disclosure, the tenth aspect wherein the molten metal is radially outward of the optical fiber preform.

According to a twelfth aspect of the present disclosure, the tenth aspect wherein a temperature of the molten metal within the nozzle is about 1670° C. or less.

According to a thirteenth aspect of the present disclosure, the twelfth aspect wherein the temperature of the molten metal within the nozzle is about 1000° C. or less.

According to a fourteenth aspect of the present disclosure, the thirteenth aspect wherein the temperature of the molten metal within the nozzle is about 400° C. or less.

According to a fifteenth aspect of the present disclosure, the fourteenth aspect wherein a viscosity of the molten metal within the nozzle is from about 0.70 MPa·s to about 0.80 MPa·s.

According to a sixteenth aspect of the present disclosure, the first aspect wherein the nozzle comprises a cylindrical member and a tapered member, the tapered member having a taper angle θ between about 1.5 degrees and about 35 degrees.

According to a seventeenth aspect of the present disclosure, the sixteenth aspect wherein the taper angle θ is between about 2 degrees and about 30 degrees.

According to an eighteenth aspect of the present disclosure, the seventeenth aspect wherein the taper angle θ is between about 5 degrees and about 15 degrees.

According to a nineteenth aspect of the present disclosure, the first aspect further comprising centering the optical fiber within the nozzle such that a first gap between an outer diameter of the optical fiber and an inner minimum diameter of the aperture at a top surface of the optical fiber is approximately equal to a second gap between the outer diameter of the optical fiber and the inner minimum diameter of the aperture at a bottom surface of the optical fiber.

According to a twentieth aspect of the present disclosure, the nineteenth aspect wherein the first gap and the second gap are each about 2 microns to about 20 microns in length.

According to a twenty-first aspect, the first aspect wherein the aperture of the nozzle comprises a stepped surface.

According to a twenty-second aspect of the present disclosure, a fiber draw furnace is disclosed comprising a pressure device, a heater, and a nozzle. The pressure device comprising an inner cavity configured to receive an optical fiber preform. The heater being configured to heat at least a forming region of an optical fiber preform to draw the optical fiber preform into an optical fiber, and the heater being configured to heat the forming region while exposing the optical fiber preform to a total pressure of about 500 atm or greater within the inner cavity. The nozzle being disposed downstream of the inner cavity and configured to maintain the total pressure of about 500 atm or greater within the inner cavity while the optical fiber traverses through an aperture in the nozzle.

According to a twenty-third aspect of the present disclosure, the twenty-second aspect further comprising a gas inlet configured to inject a process gas into the inner cavity, and wherein the total pressure of about 500 atm or greater is a total gas pressure.

According to a twenty-fourth aspect of the present disclosure, the twenty-second aspect wherein the nozzle comprises a cylindrical member and a tapered member, the tapered member having a taper angle θ between about 1.5 degrees and about 35 degrees.

According to a twenty-fifth aspect of the present disclosure, the twenty-fourth aspect wherein the taper angle θ is between about 2 degrees and about 30 degrees.

According to a twenty-sixth aspect of the present disclosure, the twenty-fifth aspect wherein the taper angle θ is between about 5 degrees and about 15 degrees.

According to a twenty-seventh aspect of the present disclosure, the twenty-second aspect wherein the nozzle comprises a cylindrical member and a tapered member, the cylindrical member being downstream of the tapered member.

According to a twenty-eighth aspect of the present disclosure, the twenty-second aspect wherein an inner minimum diameter of the nozzle is from about 130 microns to about 165 microns.

According to a twenty-ninth aspect of the present disclosure, the twenty-second aspect wherein the nozzle has a longitudinal length from about 0.05 mm to about 0.25 mm.

According to a thirtieth aspect of the present disclosure, the twenty-second aspect wherein the nozzle comprises a tapered member having a longitudinal length from about 0.10 mm to about 0.22 mm.

Although the disclosure has been illustrated and described herein with reference to explanatory embodiments and specific examples thereof, it will be readily apparent to those of ordinary skill in the art that other embodiments and examples can perform similar functions and/or achieve like results. For instance, the connection port insert may be configured as individual sleeves that are inserted into a passageway of a device, thereby allowing the selection of different configurations of connector ports for a device to tailor the device to the desired external connector. All such equivalent embodiments and examples are within the spirit and scope of the disclosure and are intended to be covered by the appended claims. It will also be apparent to those skilled in the art that various modifications and variations can be made to the concepts disclosed without departing from the spirit and scope of the same. Thus, it is intended that the present application cover the modifications and variations provided they come within the scope of the appended claims and their equivalents.