System and method for processing optical fiber - Patents.com

Abstract

The system for processing optical fiber includes a draw furnace (402) holding an optical fiber preform (404) therein, an uncoated optical fiber (406) drawn from the optical fiber preform and extending from the draw furnace along a processing path (408), and an annealer (410) operably connected to the draw furnace downstream in the processing flow for exposing the uncoated optical fiber to an annealer processing temperature in the range of about 1000°C to about 1400°C, the uncoated optical fiber passing through the annealer at least twice.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority under 35 U.S.C. § 119 of U.S. Provisional Application No. 62 / 976,545, filed February 14, 2020, the contents of which are incorporated by reference in their entirety and made a part of this application. [Technical field]

[0002] The present invention relates to systems and methods for producing optical fiber, and more particularly, the present invention relates to systems and methods for producing optical fiber having multiple passes through an annealing apparatus. [Background technology]

[0003] In optical fiber manufacturing, an optical preform is heated above the glass softening point and then drawn at a large drawdown ratio to form an optical fiber with a diameter of 125 μm. The high drawdown temperature, large drawdown ratio, and fast draw speed cause the glass to move far from equilibrium, resulting in a high fictive temperature and therefore high attenuation. To reduce the fictive temperature and attenuation of optical fiber, the glass transition region and temperatures below the glass transition temperature (sub-T g Annealing of the fibers in the θ region has been employed to reduce the attenuation of these fibers, but the dwell time required to induce increased relaxation of the glass is rather short (<0.2 seconds), and only a small reduction in fictive temperature is achieved. Summary of the Invention [Problem to be solved by the invention]

[0004] Accordingly, the present inventors have developed an improved system and method for manufacturing optical fiber. [Means for solving the problem]

[0005] Various embodiments of systems and methods for manufacturing optical fiber are described herein. In some embodiments, a system (100) for processing an uncoated optical fiber includes a draw furnace (402) having an optical fiber preform (404) therein, an uncoated optical fiber (406) drawn from the optical fiber preform and extending from the draw furnace along a process path (408), and an annealer (410) operatively connected to the draw furnace downstream of the process flow for exposing the uncoated optical fiber to an annealer process temperature ranging from about 1000° C. to about 1400° C., wherein the uncoated optical fiber is passed through the annealer at least two times.

[0006] In some embodiments, a method for processing an uncoated optical fiber includes drawing an uncoated optical fiber from an optical fiber preform inside a draw furnace, the uncoated optical fiber extending from the draw furnace along a process path; drawing the uncoated optical fiber through an annealing device operatively connected downstream in the process flow from the draw furnace, the annealing device exposing the uncoated optical fiber to an annealing temperature in the range of about 1000° C. to about 1400° C.; and controlling the orientation of the uncoated optical fiber through the annealing device at least two times.

[0007] Further embodiments of the present disclosure other than those described above will be described later. [Brief description of the drawings]

[0008] Various embodiments of the present disclosure, briefly summarized above and described in more detail below, can be understood by reference to example embodiments of the disclosure that are illustrated in the accompanying drawings, which illustrate only typical embodiments of the disclosure and are therefore to be considered as limiting its scope. It should be noted that: [Figure 1]1 illustrates a system for producing optical fiber according to some embodiments of the present disclosure. [Diagram 2] 1 illustrates a fluid dynamic bearing device for use in an optical fiber manufacturing system according to some embodiments of the present disclosure. [Diagram 3] 1 is a side view illustrating a hydrodynamic bearing device according to some embodiments of the present disclosure. [Figure 4] 1 illustrates an exemplary optical fiber manufacturing system according to some embodiments of the present disclosure. [Diagram 5] 1 illustrates an exemplary optical fiber manufacturing system according to some embodiments of the present disclosure. [Figure 6] 1 illustrates an exemplary optical fiber manufacturing system according to some embodiments of the present disclosure. [Figure 7A] FIG. 7 illustrates a fluid dynamic bearing apparatus for use in the exemplary optical fiber manufacturing system depicted in FIGS. 4-6, according to some embodiments of the present disclosure. [Figure 7B] FIG. 7 illustrates a fluid dynamic bearing apparatus for use in the exemplary optical fiber manufacturing system depicted in FIGS. 4-6, according to some embodiments of the present disclosure. [Figure 8] FIG. 1 illustrates the effect of slow cooling on the temperature of a silica fiber. [Figure 9] Illustrative diagram of the effect of annealing on the fictive temperature of a silica fiber for two different drawing speeds. [Figure 10] An example of the wavelength dependence of Rayleigh scattering loss for a series of silica fibers with different fictive temperatures. [Figure 11] FIG. 1 illustrates the variation of the fictive temperature of a silica fiber over its residence time (up to 2 seconds) when the furnace is operating at a constant temperature ranging from about 900° C. to about 1300° C. in a processing system. [Figure 12] FIG. 1 illustrates the variation of the fictive temperature of a silica fiber over its residence time (up to 10 seconds) when the furnace is operating at a constant temperature ranging from about 900° C. to about 1300° C. in a processing system. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] The embodiments set forth in the drawings are illustrative in nature and are not intended to limit the invention defined by the claims. Moreover, the drawings and individual features of the invention will become more fully apparent and understood in view of the detailed description.

[0010] To facilitate understanding, the same reference numbers have been used, where possible, to designate identical elements common to each of the figures. The figures are not drawn to scale and may be simplified for clarity. It is believed that various elements and features of one embodiment may also form part of other embodiments without further description.

[0011] For purposes of this description, the exemplary embodiments are described with respect to various silica-based optical fibers, including fibers made from pure silica, fibers made from doped silica, or fibers made from combinations of pure and doped silica. The various processing conditions (e.g., temperature, cooling range, cooling rate, draw rate, etc.) and various properties (e.g., fictive temperature, viscosity, attenuation, refractive index, etc.) are described with respect to silica-based optical fibers. However, as one skilled in the art will recognize, the principles of the present disclosure extend to optical fibers based on other material systems, with due consideration given to the properties (e.g., melting temperature, viscosity, fictive temperature, time scale for structural relaxation) of the components of the other material systems.

[0012] This description presents an optical fiber with low attenuation. The optical fiber is a glass filament that is processed under conditions that promote more complete structural relaxation of the glass. The more complete the structural relaxation, the lower the fictive temperature of the fiber, which reduces the attenuation of the fiber.

[0013] In conventional fiber processing, fiber formation is performed by heating a glass preform above its softening point and then drawing the fiber at a large drawdown ratio to form an optical fiber with a desired diameter. For silica glass fiber, the diameter of the preform is about 100 mm to about 120 mm or more, and the glass fiber drawn from the preform typically has a diameter of 125 μm. To manufacture silica glass fiber, the silica glass preform is heated to a temperature above about 2000° C. and the fiber is drawn at a speed of 10 meters per second (m / s) or more. Due to the high drawing temperature, large drawdown ratio, and fast drawing speed, the glass structure of the silica glass fiber is significantly away from equilibrium. Without wishing to be bound by theory, it is believed that the non-equilibrium structure of silica glass fiber is a significant root cause of signal attenuation in silica glass fiber. Therefore, it is believed that the attenuation of optical fiber can be reduced by modifying each processing condition to stabilize the glass structure and bring it closer to the equilibrium structure.

[0014] For the purposes of this discussion, fictive temperature will be used as an indicator of glass structure. Glasses with higher fictive temperatures have structures further away from equilibrium than glasses with lower fictive temperatures. Processing conditions that lower the fictive temperature of the glass more will yield optical fiber with a structure closer to equilibrium. Optical fiber with lower fictive temperatures is expected to have lower attenuation.

[0015] Fictive temperature is the temperature at which the glass structure is in equilibrium. It can be measured by infrared (IR) beam measurement techniques, such as those described in DL Kim and M. Tomozawa, "Fictive Temperature of Silica Glass Fiber - Reexamination," Journal of Non-Crystalline Solids, Vol. 286, pp. 132-138 (2001). As used herein, fictive temperature refers to the average fictive temperature in the radial direction of the optical fiber.

[0016] This section describes various processing conditions that extend the time that the fiber is exposed to temperatures in the glass transition region during cooling, with the purpose of stretching being to facilitate structural relaxation of the fiber and to lower the fictive temperature of the fiber. The glass transition region generally ranges from about 1200°C to about 1700°C for silica glass optical fibers. The temperature region below the glass transition region (sub-T g Another relaxation of the glass may occur in the fictive temperature range (region), which corresponds to a temperature between about 1000° C. and about 1200° C. In one embodiment, the temperature range over which cooling facilitates relaxation of the glass structure and provides the optical fiber with a low fictive temperature range is between about 1000° C. and about 1700° C. In another embodiment, the temperature range is between about 1050° C. and about 1600° C. In yet another embodiment, the temperature range is between about 1100° C. and about 1500° C.

[0017] Optical fibers are typically drawn in air. The cooling rate in air is typically greater than about 12,000° C. per second during the drawing process when the fiber is at a temperature greater than about 1000° C., resulting in a glass fictive temperature greater than about 1550° C. and high attenuation in the optical fiber. Some prior art reports that disclose drawing optical fibers disclose that the optical fiber is exposed to a heated furnace during drawing, where the optical fiber is exposed to temperatures between about 1200° C. and about 1700° C. for less than 0.1 seconds, and in some cases less than 0.2 seconds. This description discloses a method of drawing an optical fiber, where the optical fiber is maintained at temperatures between about 1000° C. and about 1700° C., in some embodiments for a period of time greater than 0.5 seconds, and in some other embodiments for a period of time greater than 1 second, and in still other embodiments for a period of time greater than 2 seconds. The subject methods allow the optical fiber to be maintained at temperatures between about 1000° C. and about 1700° C. for periods of greater than 0.5 seconds, greater than 1 second, or greater than 2 seconds for draw speeds greater than 10 meters per second (m / s), greater than 20 m / s, greater than 30 m / s, greater than 40 m / s, greater than 50 m / s, or greater than 60 m / s.

[0018] FIG. 1 illustrates a typical system for producing optical fiber according to the prior art. The system 108 includes a fiber preform 110 disposed within a furnace 112. The fiber preform 110 is composed of a glass, such as silica glass, and may include regions of different compositions. For example, the preform 110 may include modified and unmodified silica glass regions having compositions corresponding to the desired core and cladding compositions of the fiber to be drawn therefrom. The fiber preform 110 is heated within the furnace 112 from which a fiber 114 is drawn. The fiber 114 enters a treatment zone 130 and begins to cool. The fiber 114 exits the treatment zone 130 and is directed through a cooling region 118 that includes a series of hydrodynamic bearing devices 116. The hydrodynamic bearing devices 116 further cool the fiber and direct the fiber toward a coating device 120 where a coating material is applied to provide a coated fiber 121. After exiting coating apparatus 120, coated optical fiber 121 may pass through a variety of other processing stages (not shown) in systems known in the art. Drawing mechanism 128 is used to provide tension to the optical fiber as it is drawn through system 108.

[0019] Hydrodynamic bearing devices are described in U.S. Patent No. 7,937,971, the disclosure of which is incorporated herein by reference. A general description of the structure and operation of exemplary hydrodynamic bearing devices follows. However, it should be understood that other designs of hydrodynamic bearing devices are possible, and that the advantages achievable by the methods and apparatus disclosed herein are not limited to any particular design of hydrodynamic bearing device.

[0020] A typical design of a hydrodynamic bearing device is shown in Figures 1 and 2. The hydrodynamic bearing device 216 of Figure 2 includes a first plate 230, a second plate 232, an inner member 236, and at least one opening 234 in at least one of the first and second plates. The first plate 230 and the second plate 232 are made of metal and include arcuate circumferential surfaces 238, 239, respectively. The first plate 230 and the second plate 232 are connected by fasteners (e.g., bolts 240) to couple the plates 230, 232 together and allow fluid to pass through the bearing assembly 216. The arcuate circumferential surface 238 of plate 230 and the arcuate circumferential surface 239 of plate 232 lie generally along the circumferential surfaces of the plates 230, 232, respectively. The first plate 230 has an inner surface 242 and an outer surface 243, and the second plate 232 has an inner surface 244 and an outer surface 245, with the inner surface 242 of plate 230 and the inner surface 244 of plate 232 aligned with one another. A recess 247 extends at least partially around the inner circumference of the inner surface 242 or inner surface 244 of either the first plate 230 or the second plate 232 to provide a plenum for fluid flow. In another embodiment, the recess has a variety of shapes to provide uniform flow into the fiber support channel 250, as described herein below.

[0021] In the embodiment illustrated in Figure 2, the arcuate circumferential surface 238 of the first plate 230 and the arcuate circumferential surface 239 of the second plate 232 are preferably generally aligned to define an area between the outer circumferential surfaces 238, 239 of both the first plate 230 and the second plate 232 that is configured to receive the optical fiber and allow the optical fiber to move along the area without rotating the bearing assemblies. This fiber support channel 250 is more clearly shown in the embodiment illustrated in Figure 3 (discussed later in this section). At least one opening 234 extends through at least one of the first plate 230 and the second plate 232. As shown in FIG. 2, an opening 234 in the first plate 230 and the second plate 232 allow a fluid (e.g., air, helium, or other desired gas or liquid) to be pumped through the hydrodynamic bearing device 216 from the opening 234 to a fiber support channel 250 formed between the first plate 230 and the second plate 232. As will be explained more fully below, the fluid fed into the channel 250 creates a high pressure region between the fiber and the surface of the channel 250. The fluid, in combination with the tension driving the drawing, acts to stabilize the fiber in the channel 250 and position the fiber above the surface of the channel 250 to prevent the fiber from making mechanical contact with the hydrodynamic bearing device. The fluid passed through the hydrodynamic bearing device may be referred to herein as a flotation fluid. The flotation fluid may be a gas or a liquid. Exemplary flotation fluids include air, nitrogen gas (N2), various inert gases, and the like.

[0022] Additionally, the fluid dynamic bearing device 216 may include a central member 236 disposed between the first plate 230 and the second plate 232. The central member 236 (e.g., a shim 237) is configured to help guide the buoyant fluid to the area between the outer circumferential surface 238 of the first plate 230 and the outer circumferential surface 239 of the second plate 232, so that the buoyant fluid exits the fiber support channel 250 with a predetermined flow direction. The central member 236 is disposed between the first plate 230 and the second plate 232 to provide a gap between the two members. If necessary, the central member 236 may include a number of finger members (not shown) to further control the fluid flow by restricting non-radial flow. Additionally, the central member 236 functions as a seal to provide sufficient contact between the first plate 230 and the second plate 232. The central member may also include a notch to facilitate the entry and exit of the optical fiber.

[0023] 3, the fiber support channel 250 formed between the outer periphery 238 of the first plate 230 and the outer periphery 239 of the second plate 232 is tapered where the levitation fluid exits between the first plate 230 and the second plate 232. However, in alternative embodiments, the fiber support channel 250 may have a different shape, such as a parallel or reverse tapered shape. Additionally, the opening 260 of the tapered fiber support channel 250 may vary depending on where the optical fiber 214 is located. The configuration of the aperture 260 and the fiber support channel 250 is such that for the particular draw tension and draw rate employed, and the flow rate of the buoyant fluid through the aperture 260, the optical fiber is maintained in a section of the fiber support channel 250, which for a typical fiber with an outer diameter of 125 μm is preferably less than 500 μm wide, more preferably less than 400 μm, more preferably 300 μm, and most preferably 200 μm wide. Thus, the fiber is preferably maintained in a region of the channel 250 between 1 and 2 times its diameter, more preferably between 1 and 1.75 times the diameter of the fiber, and most preferably between 1 and 1.5 times the diameter of the fiber. In placing the fiber within the region of the channel, the distance between the outer fiber and each wall is preferably between 0.05 and 0.5 times the fiber diameter.

[0024] In the embodiment illustrated in FIG. 3, the taper angle is exaggerated for ease of visualization and is indicative of the desired angle of the tapered opening to the fiber support channel 250. In practice, each opposing face of the support channel 250 is preferably angled on both sides, but at least one of the faces is preferably angled at an angle greater than 0 degrees and less than 10 degrees, more preferably between 0.3 and 7 degrees, and most preferably between 0.4 and 3 degrees, such that the width 260 of the top or outer portion of the fiber support channel 250 is greater than the width 260 of the bottom or inner portion 237 of the fiber support channel 250. For example, in such an embodiment, the plates forming the region may be angled at an angle of −0.6 degrees for the first plate 230 and +0.6 degrees for the second plate 232. Alternatively, the fiber support channel 250 may be of any depth, width, or taper angle. By utilizing a tapered fiber support channel 250 to inject fluid into the slot formed by the channel 250 so that the fluid enters the narrower inner portion of the fiber support channel 250 and then exits the wider outer region of the fiber support channel 250, the buoyancy fluid expelled through the channel 250 provides a buffer that causes the fiber to self-settle within the depth of the channel 250. For example, for a given flow rate of the buoyancy fluid, as the fiber draw tension increases, the fiber will move downward within the channel 250 until the gap between the fiber 214 and the channel walls is narrow enough that the pressure in region 237 is high enough to precisely counteract the new increased tension. As the fiber draw tension decreases, the fiber 214 will move upward within the channel 250 until the gap between the fiber 214 and the channel walls is wide enough that the pressure in region 237 is low enough to counteract the new decreased tension. Thus, the channel 250 can be tapered to accommodate a wider range of draw tensions. If the channel 250 were not tapered as shown, and the draw tension were reduced, the fiber would move upward and risk exiting the fiber support channel 250.

[0025] The fiber is preferably located within an area of ​​the channel 250 that is about 1 to about 2 times its diameter, more preferably between about 1 to about 1.75 times the fiber diameter, and most preferably between about 1 to about 1.5 times the fiber diameter. By positioning the fiber within such a relatively narrow area of ​​the channel 250, the fiber will be centered during operation due to the Bernoulli effect. For example, as the fiber approaches one of the opposing sides of the channel 250, the velocity of the buoyancy fluid will increase on the nearest side and decrease on the nearest side. According to the Bernoulli effect, the acceleration of the buoyancy fluid occurs simultaneously with the reduction in pressure. As a result, the increased pressure caused by the reduced flow of buoyancy fluid near one side forces the fiber back toward the center of the channel 250. Thus, in a preferred embodiment, the Bernoulli effect is at least substantially utilized to center the fiber within the fiber support channel 250 during drawing due to the flow of buoyancy fluid passing around the fiber and exiting the fiber support channel 250. Notably, such movement to the center occurs without the need for any buoyancy fluid flow to impinge on the fiber from its sides, e.g., without jets of buoyancy fluid emanating from each sidewall of the channel 250. The velocity of the buoyancy fluid flow moving through the slot is preferably adjusted to maintain the fiber in a position entirely within the tapered region of the channel 250. In the embodiment of FIG. 3, the fiber is positioned within an area of ​​the channel 250 between about one and about two times its diameter, so that the fiber is supported by a pressure differential that exists below the fiber 214 (rather opposite to aerodynamic drag that could also be used to support the fiber, if so selected). Supporting or buoying the fiber within the channel 250 by a fluid pressure differential requires much less fluid flow than would be required if aerodynamic drag were used to buoy the fiber.

[0026] 3, the fluid flow is preferably provided by a single stream of buoyancy fluid that enters fiber support channel 250 through an inner, narrow portion of channel 250 and exits fiber support channel 250 through an outer, wider region 260 of fiber support channel 250. In this manner, the fiber is fully affixed to a location inside the slot formed by fiber support channel 250, allowing the fiber to suspend between the narrowest and widest portions of the slot.

[0027] Employing a tapered fiber support channel 250 and injecting levitation fluid through region 250 in the manner described above allows the fiber to be held in the region of the slot formed by fiber support channel 250, where the slot has a width between 10 μm and 150 μm, more preferably between 15 μm and 100 μm, and most preferably between about 24 μm and about 70 μm, greater than the diameter of the fiber being guided through fiber support channel 250. Also, the fiber is preferably held within the region of the channel such that the distance between the outer fiber and each wall is about 0.05 to about 0.5 times the fiber diameter for the duration of the fiber drawing process.

[0028] In some preferred embodiments, the fiber support channel 250 is provided with a means for applying a vacuum below the fiber as the fiber moves outwardly away from the source of the buoyancy fluid flow. Such a means for releasing pressure can be accomplished in the form of a tapered channel design, as discussed above.

[0029] The hydrodynamic bearing device allows the optical fiber to move along the region of the buoyant fluid buffer while preventing or substantially preventing actual mechanical contact between the optical fiber and the bearing assembly; specifically, the fiber moves within fiber support channel 250 without contacting either plates 230 or 232. Additionally, due to the size and shape of the region, the hydrodynamic bearing device is able to maintain the fiber within the region without mechanical contact through a range of draw tensions without actively controlling the flow of the buoyant fluid.

[0030] The flow of buoyancy fluid is important because it prevents the optical fiber 214 from migrating toward the bottom of the fiber support channel 250 and from contacting the shim 237 or the sides of the fiber support channel 250. This is especially important when the optical fiber is not yet coated, so that the quality of the fiber is not degraded by mechanical contact with the fluid bearing device or the channel 250. Additionally, the closer the optical fiber 214 is relative to the bottom of the fiber support channel 250, the more pressure must be applied within the fiber support channel 250 to maintain the optical fiber 214 in the desired position. Obviously, the tapered sides of the channel create this necessary pressure increase by narrowing the gap between the sides of the channel and the fiber.

[0031] Other factors that affect fiber position within fiber support channel 250 include draw tension. For example, a fiber pulled with a 200 g tension will suspend lower within fiber support channel 250 than a fiber pulled with a 100 g tension for the same fluid flow. Therefore, it is important that the flow of buoyancy fluid exiting the region of the fluid bearing is sufficient to maintain the optical fiber in the desired position for the particular fiber draw speed and draw tension employed.

[0032] For example, in an embodiment utilizing a fiber support channel 250 having an innermost section with a width of about 127 μm and an outermost section with a width of about 380 μm, between plates 230 and 232, the buoyancy fluid flow rate may be about 0.5 liters per second to about 5 liters per second (about 0.5 L / s to 5 L / s). Such a geometry and buoyancy fluid flow may result in local flow velocities around the optical fibers of eight hundred kilometers per hour (800,000 m / h) or greater. Thus, in some embodiments, the maximum buoyancy fluid velocity around the fibers employed in fiber support channel 250 may be greater than one hundred kilometers per hour (100,000 m / h), greater than two hundred kilometers per hour (200,000 m / h), greater than four hundred kilometers per hour (400,000 m / h), or even greater than six hundred kilometers per hour (600,000 m / h). In some embodiments, the maximum levitation fluid velocity around the fiber employed within fiber support channel 250 may be greater than 900 kilometers per hour (900,000 m / h). However, the methods disclosed herein are certainly not limited to these speeds, and in fact may desirably be selected based on the various draw conditions (e.g., draw speed, draw tension, etc.) and fluid bearing designs to result in the desired placement of the fiber within fiber support channel 250. In other embodiments, the levitation fluid flow rate may be from about 3 liters per second to about 4 liters per second. Of course, any levitation fluid flow rate may be utilized as long as it is sufficient to maintain the optical fiber in the desired position at a given draw tension.

[0033] One drawback of the system 108 shown in Figure 1 is that the fiber experiences high cooling rates when the fast draw rates desired for production are employed. Under conditions typically employed in optical fiber production, i.e., for the duration of the draw process when the fiber is at temperatures above 1000°C, cooling rates of greater than 12,000°C per second are encountered. The high cooling rates result in high fiber fictive temperatures (around 1500°C) and increased attenuation.

[0034] One possible approach to reduce the cooling rate is to increase the temperature of the treatment zone 130 as the fiber enters it to more closely match that of the fiber. The smaller the temperature difference between the treatment zone 130 and the furnace 112, the slower the fiber will cool. In principle, a temperature gradient can be created in the treatment zone 130 that allows the fiber to be gradually cooled from its incoming temperature (about 1500° C. or higher) to each lower temperature in a manner that prolongs the fiber's residence time in the range of about 1000° C. to about 1700° C. sufficiently to achieve the structural relaxation necessary to reduce the fictive temperature as described herein. Alternatively, the fiber processing system may include multiple treatment zones that are operated to gradually decrease the temperature to cool the fiber while extending the fiber's residence time in the desired temperature range of about 1000° C. to about 1700° C. Although conceptually feasible, strategies for controlled cooling of the fiber that involve adjusting the conditions in the processing zone 130 are difficult to implement in practice. To best promote the structural relaxation necessary to achieve a low fictive temperature by controlling the cooling of the fiber, it is necessary to maximize the period during which the fiber temperature is at the desired temperature between about 1000° C. and about 1700° C. identified in this section. When operating at production draw speeds, the prior art fiber processing system shown in FIG. 1 limits the period during which the fiber temperature is in the temperature range between about 1200° C. and about 1700° C. to about 0.2 seconds. This period is too short to allow significant structural relaxation of the glass, and the fictive temperature of the fiber produced by the system is correspondingly high (about 1500° C.). In order to increase the residence time between about 1000° C. and about 1700° C., it is necessary to control the temperature in the processing zone 130 to slow the cooling rate. As previously discussed, adequate temperature control requires a slower ramp down of the fiber temperature to ensure adequate residence time for the fiber in the temperature range of about 1000° C. to about 1700° C. However, as the temperature ramp down slows, the length of the treatment zone 130 also increases. To establish the slow controlled anneal necessary to reduce the fictive temperature of the fiber significantly below about 1500° C. at production draw speeds, the required length of the treatment zone 130 exceeds the headspace height (floor to ceiling) available in most manufacturing facilities. Retrofitting existing facilities to accommodate the increased headspace height would increase manufacturing costs prohibitively. Alternative approaches to reduce the draw speed to fit within existing headspace height constraints are also undesirable due to the increased costs associated with reduced manufacturing throughput.

[0035] The incorporation of multiple fluid bearing devices into a fiber processing system is advantageous because the multiple bearing devices allow the fiber to be redirected in a horizontal or other elevational direction during the duration of the process. The incorporation of fluid bearing devices into the system increases the path length available for fiber processing without requiring additional elevational space in the facility. In the system 108 shown in FIG. 1, as the uncoated fiber 114 exits the processing zone 130, the fluid bearing devices 116 redirect the uncoated fiber 114 from elevational to a more horizontal orientation. In the configuration shown in FIG. 1, each fluid bearing device 116 delivers the uncoated fiber 114 to the coating device 120. In an alternative arrangement, the fiber processing system can be modified to include a second processing zone parallel to the processing zone 130, and each fiber bearing device redirects the fiber elevationally upward to deliver the fiber to the second processing zone, further extending the cooling path to slow the cooling rate sufficiently to produce a lower fictive temperature fiber.

[0036] However, in the system configuration of FIG. 1, each of the hydrodynamic bearing devices 116 is counterproductive to the goal of achieving a low fictive temperature fiber. Instead of allowing for slow controlled cooling, each of the hydrodynamic bearing devices 116 deployed in FIG. 1 promotes rapid cooling of the fiber. As the optical fiber 114 is transported across each of the hydrodynamic bearing devices 116, the area of ​​the buoyancy fluid buffer of each of the hydrodynamic bearing devices 116 acts to cool the optical fiber 114. The moving flow of buoyancy fluid utilized by the hydrodynamic bearing devices to support and position the fiber cools the optical fiber at a faster rate than the fiber would cool in still air at room temperature. The greater the temperature difference between the optical fiber and the buoyancy fluid in the hydrodynamic bearing, the greater the ability of the buoyancy fluid bearing to cool the optical fiber 114. In the arrangement of FIG. 1, the buoyancy fluid supplied to each of the hydrodynamic bearing devices 116 is air or an inert gas at room temperature. As mentioned above, the velocity of the buoyancy fluid supplied to the channel 250 is high in order to support and buoy the fiber and prevent mechanical contact of the fiber with the hydrodynamic bearing device. The use of such a high velocity levitation fluid flow significantly increases the rate at which the fiber cools due to convection processes: the greater the difference between the temperature of the fiber and the temperature of the levitation fluid being supplied by the hydrodynamic bearing device, and the greater the flow velocity of the levitation fluid, the faster the fiber will cool.

[0037] In prior art fiber processing systems, the levitation fluid provided by the hydrodynamic bearing device is at room temperature, and the fiber leaving the processing zone and entering the hydrodynamic bearing device assembly is typically at a temperature of about 500° C. or higher, but more commonly about 1000° C. or higher. At typical fiber draw speeds through the hydrodynamic bearing device, and at typical levitation fluid velocities, the fiber temperature can be reduced by several hundred degrees C. to over about 1000° C. over the 1-2 m length of the fiber as it passes through the hydrodynamic bearing device. The rapid fiber cooling provided by multiple hydrodynamic bearing devices as deployed in the prior art has been considered beneficial because it could simplify fiber processing systems by eliminating the need for helium cooling devices.

[0038] While the prior art has recognized the advantages of rapid fiber cooling using multiple hydrodynamic bearing devices, the present disclosure recognizes that the time scale over which fiber cooling occurs is much shorter than the time required to achieve a low fiber fictive temperature and promote the structural relaxation necessary to produce a fiber with low attenuation. The present disclosure presents a new processing system designed to reduce the fiber's fictive temperature. The system includes multiple hydrodynamic bearing devices for rerouting the fiber through the processing system and is designed to extend the fiber's residence time at temperatures ranging from about 1000° C. to about 1700° C., with some embodiments designed to exceed 0.5 seconds, others to exceed 1 second, still others to exceed 2 seconds, still others to exceed 5 seconds, and still others to exceed 10 seconds. To allow longer residence times to be achieved, the drawing process speed may be greater than 10 meters per second (10 m / s), greater than 20 m / s, greater than 30 m / s, greater than 40 m / s, greater than 50 m / s, or even greater than 60 m / s.

[0039] The present system overcomes two aspects of the prior art system shown in Figure 1 that are detrimental to the goal of achieving a fiber with a low fictive temperature. First, unlike the prior art system, the present system extends the period during which the fiber is exposed to temperatures in the glass transition region during cooling, thereby promoting structural relaxation of the fiber and lowering the fiber's fictive temperature. Second, unlike the prior art system, the present system places multiple hydrodynamic bearing devices within a heated enclosure.

[0040] 4 illustrates an exemplary system (400) for processing an uncoated optical fiber according to some embodiments of the present disclosure. As used herein, the phrase "uncoated optical fiber" refers to an optical fiber drawn directly from a preform and prior to the application of a protective coating layer on its outer surface (e.g., before the uncoated optical fiber is coated with a polymer-based material).

[0041] The system 400 includes a drawing furnace (402) that holds an optical fiber preform (404) therein. An uncoated optical fiber (406) is drawn from the optical fiber preform 404. The fiber preform 404 is composed of a glass, such as silica glass, and may include regions of different compositions. For example, the optical fiber preform 404 may include modified and unmodified silica glass regions having compositions corresponding to the desired core and cladding compositions of the fiber drawn from the preform. The fiber preform 404 is heated in the furnace 402, and an uncoated optical fiber 406 is drawn therefrom and extends from the drawing furnace 402 along a process path (408).

[0042] In some embodiments, the speed at which the uncoated optical fiber is drawn from the optical fiber preform exceeds 10 m / s, in some various embodiments exceeds 20 m / s, in some embodiments exceeds 30 m / s, in some various embodiments exceeds 40 m / s, in some embodiments exceeds 50 m / s, or in some embodiments exceeds 60 m / s.

[0043] The annealing device 410 is operatively connected to the draw furnace 402 downstream of the process flow along a first section 412 of the process path 408. The annealing device 410 exposes the uncoated optical fiber to a process temperature ranging from about 1000° C. to about 1400° C. The process temperature is the temperature in the process area of ​​the annealing device. As defined herein, the annealing device provides controlled cooling to the optical fiber at a rate slower than the rate at which the optical fiber cools in unheated air. In some embodiments, the annealing device is temperature controlled in one or more zones. For example, the annealing device may comprise a furnace having one or more temperature control zones. The controlled temperature may be designed to allow the optical fiber to cool more slowly as compared to cooling in unheated air, as well as to increase the residence time of the fiber at the desired process temperature or range of desired process temperatures, thereby facilitating the goal of lowering the fictive temperature of the fiber during cooling. In another embodiment, the annealing device operates under subatmospheric pressure conditions. Compared to cooling by slowing the rate of heat transfer from the fiber to the gaseous atmosphere at atmospheric pressure, reducing the pressure of the gaseous environment surrounding the fiber allows the fiber to cool more slowly, thereby facilitating the goal of lowering the fictive temperature of the fiber during cooling.

[0044] The present invention extends the residence time in the glass transition region by redirecting the optical fiber through the same annealing device multiple times before a protective coating is applied to the optical fiber. In some embodiments, the optical fiber is guided through the same annealing device at least two times. In some embodiments, the optical fiber is guided through the same annealing device at least three times. In some embodiments, as depicted in FIG. 4, the uncoated optical fiber is redirected through an annealing device 410 along a first section 412 of a process path 408 via at least two fluid bearing devices 422a, 422b operably connected to a downstream process flow of the draw furnace 402. The fluid bearing devices 422a, 422b redirect the uncoated optical fiber 406 through the annealing device 410. Within the annealing device 410, the uncoated optical fiber 406 is exposed to a process temperature ranging from about 1000° C. to about 1400° C. In some embodiments, the uncoated optical fiber 406 is exposed to the processing temperature for a residence time of at least 0.4 seconds, with a residence time of 0.5 seconds being preferred, with a residence time of at least 1.0 second being more preferred, and with a residence time of at least 2.0 seconds being even more preferred. "Residence time" refers to the total amount of time that the uncoated optical fiber is within the processing region of the annealing apparatus 410. In some embodiments, the optical fiber is exposed to a temperature of about 1000°C to about 1700°C in the annealing apparatus for at least 0.2 seconds, with a temperature of about 1000°C to about 1700°C being preferred for at least 0.5 seconds, with a temperature of about 1000°C to about 1700°C being more preferred for at least 1 second, and even more preferred for at least 2 seconds.

[0045] Each of the hydrodynamic bearing devices 422a, 422b includes multiple fiber support channels 250. In some embodiments, the hydrodynamic bearing devices 422a, 422b may be multiple hydrodynamic bearing devices connected together, or multiple hydrodynamic bearing devices arranged within a single housing, as already discussed above and illustrated in FIGS. 2 and 3. In some embodiments, the hydrodynamic bearing devices 422a, 422b include an integrally formed body (i.e., a unitary or single-piece body) with multiple fiber support channels 250 as described above. FIG. 7 illustrates an exemplary hydrodynamic bearing device with three fiber support channels 250. The number of fiber support channels illustrated is exemplary only. The hydrodynamic bearing device 422 may include more or fewer fiber support channels than illustrated in the exemplary figures, redirecting the uncoated optical fiber through the same annealing device, for example, at least twice.

[0046] In some embodiments, the uncoated optical fiber is heated after being redirected by the hydrodynamic bearing device and before re-entering the annealing device. In some embodiments, the uncoated optical fiber is heated by at least about 500° C., and in some embodiments, preferably by about 800° C., before being redirected by the hydrodynamic bearing device and before re-entering the annealing device.

[0047] In some embodiments, the temperature of the hydrodynamic bearing device is room temperature (about 25° C.). In some embodiments, the temperature of the hydrodynamic bearing device is between about 5° C. and about 450° C. In some embodiments, the temperature of the fluid in the hydrodynamic bearing device is between about 5° C. and about 200° C. In some embodiments, the temperature of the fluid in the hydrodynamic bearing device is between about 400° C. and about 1200° C. In some embodiments, the hydrodynamic bearing device is disposed within a heated housing. In some embodiments, the optical fiber is exposed to a processing temperature in the hydrodynamic bearing device of greater than about 400° C. In some embodiments, the optical fiber is exposed to a processing temperature in the hydrodynamic bearing device of greater than about 800° C. In some embodiments, the heated housing is a furnace. In some embodiments, the heated housing is an annealing device.

[0048] The temperature at which the fiber is below the glass transition temperature (near-T gWhen the optical fiber is cooled at a process temperature below the fictive temperature range (e.g., below about 1000° C.), the glass structure, fiber state, or both are kinetically quenched and essentially unchanged (based on practical time scales) because the available thermal energy is less than the energy required to relax or modify the glass to bring about structural relaxation or to bring it closer to equilibrium. However, in the exemplary system configuration of FIG. 4, each hydrodynamic bearing device is counterproductive to the goal of achieving a fiber with a low fictive temperature. Instead of allowing for slow, controlled cooling, each hydrodynamic bearing device promotes rapid cooling of the fiber due to the contact of the fluid within each device with the optical fiber. As the optical fiber is transported through the hydrodynamic bearing devices, the regions of levitation fluid cushioning in each hydrodynamic on-axis device act to rapidly cool the optical fiber. Due to the moving flow of levitation fluid employed by the hydrodynamic bearing devices to support and position the fiber, the optical fiber cools at a rate significantly faster than it would cool in still air at room temperature. The greater the temperature difference between the optical fiber and the levitation fluid in the fluid bearing, the greater the ability of the levitation fluid bearing to cool the optical fiber. The levitation fluid supplied to the fluid bearing is air or an inert gas at room temperature. The velocity of the levitation fluid supplied to the channel is high in order to support and levitate the fiber, thereby preventing the fiber from coming into mechanical contact with the fluid bearing device. The use of such a high velocity flow of the levitation fluid significantly increases the cooling rate of the fiber by convection processes. The greater the difference between the temperature of the fiber and the temperature of the levitation fluid supplied by the fluid bearing device, and the higher the flow rate of the levitation fluid, the faster the cooling rate of the fiber. In prior art fiber processing systems, the levitation fluid supplied by the fluid bearing device is at room temperature, and the fiber leaving the annealing device and entering each fluid bearing device assembly is typically at a temperature of about 800° C. or higher, and more commonly at a temperature of about 1000° C. or higher. At typical fiber drawing speeds, and typical velocities of the levitation fluid through the fluid bearing device, the temperature of the fiber can drop by several hundred degrees Celsius to over about 1000 degrees Celsius over the 1 to 2 m length of the fiber as it passes through the fluid bearing device.Typical fiber temperatures while exiting the fluid dynamic bearing device range from about 23° C. to about 600° C., or from about 50° C. to about 800° C. After exiting the fluid dynamic bearing device, the fiber is redirected and directed to a reheating stage for heating to a temperature higher than the temperature at which it exited the fluid dynamic bearing device. The fiber temperature while entering the reheating stage is believed to be in the range of about 23° C. to about 600° C., or from about 50° C. to about 800° C. The subject system includes a reheating stage after the fiber exits the fluid dynamic bearing device and before it re-enters the annealing device a second time (or before it re-enters the annealing device any time thereafter) to return the cooled fiber to a temperature of about 1000° C. or higher for the duration of the process. In one embodiment, the fiber is a silica fiber or a doped silica fiber drawn from a preform, which passes through an annealing device operating at a temperature of about 1000° C. to about 1400° C., cools to a temperature below about 600° C., and is then reheated at some stage to a fiber temperature above about 1000° C. before re-entering the annealing device. Reheating the fiber extends the time that the fiber temperature remains in the temperature range of about 1000° C. to about 1700° C., allowing subsequent cooling to further reduce the fictive temperature of the fiber. The subject system and subject method may include traversing the optical fiber multiple times (e.g., times >2, >3, >5...) through the same annealing device using at least two individual multi-channel hydrodynamic bearing devices. The method involves multiple cycles of cooling the fiber inside the channel of the hydrodynamic bearing device to a fiber temperature below about 600°C and then at some stage reheating the fiber to raise the fiber temperature to above about 1000°C, thereby further extending the time the fiber is at a fiber temperature between about 1000°C and about 1700°C in the annealing device, thereby further reducing the fictive temperature.The subject system also includes a fluid bearing or other fiber redirecting device that redirects the fiber's path through the processing system with multiple channels to allow multiple passes of the fiber through a single annealer, thereby extending the time the fiber is at a fiber temperature between about 1000° C. and about 1700° C. in the annealer, further reducing the fictive temperature while minimizing the need for unnecessary vertical processing headspace and the need for additional anneals along the processing path. The reheating stage applies heat to the optical fiber by convection, radiation, or electromagnetic mechanisms. The reheating stage may be a furnace or may operate with various heating elements or by flowing hot gas or a flame across the fiber. Alternatively, the reheating stage may include a laser, plasma source, or other light source to heat the fiber. The laser or other light source may operate at a wavelength absorbed by the fiber and transfer energy to the fiber by absorption. The plasma source may heat the plasma gas and heat the fiber by conduction. The reheat stage in the meandering process path is advantageous because the fiber cools rapidly when redirected by the fiber redirector. The temperature of the silica fiber when it emerges from the fiber redirector is typically in the range of about 50° C. to about 600° C. Reheating reverses the cooling and raises the temperature of the fiber enough to control the fictive temperature as described herein. In another embodiment, the reheat stage chamber is heated to a temperature and one or more fiber redirectors with multiple channels are arranged within the chamber in a configuration that expands the process path through the reheat stage. In this embodiment, the reheat stage chamber is heated to a temperature higher than the temperature of the fiber, and the fiber is heated as it passes through the chamber. The temperature within the reheat stage may be spatially uniform or may be spatially variable.In one embodiment, the temperature inside the reheating stage increases monotonically along the direction the fiber is transported, with a peak temperature inside that is higher than the temperature of the fiber as it enters the reheating stage, while in another embodiment, the temperature inside the reheating stage decreases monotonically along the direction the fiber is transported, with a peak temperature inside that is higher than the temperature of the fiber as it enters the reheating stage.

[0049] The hydrodynamic bearing device 422 advantageously allows the uncoated optical fiber to be guided through multiple passes of the annealing device, thereby increasing the residence time in the glass transition region without requiring additional cooling devices in the downstream transport section of the fiber drawing system. Increasing the residence time in the glass transition region increases the relaxation of the glass, resulting in a significant decrease in the Rayleigh scattering and attenuation of the optical fiber.

[0050] In some embodiments, the optical fiber exits the hydrodynamic bearing device at a temperature between about 300° C. and about 800° C., preferably between about 500° C. and about 800° C., and more preferably between about 700° C. and about 800° C. After exposure to the processing temperature for a residence time of at least 0.4 seconds, the uncoated optical fiber is directed to a coating device 128 where a coating is applied to provide a coated fiber 130. After exiting the coating device 128, the coated optical fiber 130 may pass through a variety of other processing stages (not shown) in systems known in the art. A drawing mechanism 132, known in the art, is used to provide tension to the optical fiber as it is drawn through the system 100.

[0051] 5 illustrates another exemplary system (500) for processing an uncoated optical fiber in accordance with certain embodiments of the present disclosure. The system 500 includes a draw furnace (502) having an optical fiber preform (504) therein. An uncoated optical fiber (506) is drawn from the optical fiber preform 504. The fiber preform 504 is heated in the furnace 502, and the drawn uncoated optical fiber 506 extends from the draw furnace 502 along a processing path (508).

[0052] A first annealing device (510) is operatively connected to the process flow downstream of the draw furnace 502 along a first section (512) of the process path 508. The first annealing device 510 exposes the uncoated optical fiber to a process temperature in the range of about 1000° C. to about 1400° C. for a residence time of at least 0.1 seconds. Unlike the example system 400 depicted in FIG. 4, the example system 500 does not direct the uncoated optical fiber through the first annealing device.

[0053] A first fluid bearing device 514 is operatively connected in a process flow downstream manner to the first annealing device 510 along a first section 512 of the process path 508. The first fluid bearing device 514 redirects the uncoated optical fiber 504 from the first section 512 of the process path 508 to a second section 516 of the process path.

[0054] A second fluid bearing device 518 is operatively connected to the process flow downstream of the first fluid bearing device 514 along a second section 516 of the process path 508. The second fluid bearing device 518 redirects the uncoated optical fiber 504 from the second section 516 of the process path 508 to a third section of the process path. The uncoated optical fiber 504 is cooled to a temperature in the range of about 400° C. to about 500° C. initially in the third section.

[0055] A second annealing device 526 is operatively connected in a process flow downstream from the second fluid bearing device 518 along a third section 520 of the process path 508. The second annealing device 526 exposes the optical fiber to a process temperature in the range of about 1000° C. to about 1400° C.

[0056] Two fluid bearing devices 522a, 522b with multiple support channels are operatively connected to a second annealing device 526 along a third section 520 of the process path 508. The two fluid bearing devices 522a, 522b redirect the uncoated optical fiber and pass the optical fiber through the second annealing device 526 at least two times.

[0057] Following the second annealing station, the uncoated optical fiber is directed along a fourth section 524 of the process path to a coating station 528 where a coating is applied to the fiber. After exiting the coating station 528, the coated optical fiber 530 may pass through a variety of other processing stages (not shown) in systems known in the art. A drawing mechanism 532, known in the art, is used to provide tension to the optical fiber as it is drawn through the system 500.

[0058] 6 illustrates another exemplary system 600 for processing an uncoated optical fiber in accordance with certain embodiments of the present disclosure. System 600 includes a draw furnace 602 having an optical fiber preform 604 therein. An uncoated optical fiber 606 is drawn from optical fiber preform 604. Fiber preform 604 is heated in furnace 602, and the drawn uncoated optical fiber 606 extends from draw furnace 602 along process path 608.

[0059] A first annealer 610 is operatively connected in the process flow downstream from the draw furnace 602 along a first section 612 of the process path 608. The annealer 610 exposes the uncoated optical fiber 606 to a process temperature in the range of about 1000° C. to about 1400° C.

[0060] Two fluid dynamic bearing devices 622a, 622b, each having multiple support channels, are operably connected along a first section 612 of a process path 608 to a first annealing device 610 downstream in the process flow of the draw furnace 602. The two fluid dynamic bearing devices 622a, 622b redirect the uncoated optical fiber 606 to pass the optical fiber through the first annealing device 610 at least two times, where the uncoated optical fiber 606 is exposed to a process temperature ranging from about 1000°C to about 1400°C.

[0061] A first fluid bearing device 614 is operatively connected in a process flow downstream of the first annealing device 610 along a first section 612 of the process path 608. The fluid bearing device 614 redirects the uncoated optical fiber from the first section 612 of the process path 608 to a second section 616 of the process path.

[0062] The second fluid bearing device 618 is operatively connected to the process flow downstream of the first fluid bearing device 614 along a second section 616 of the process path 608. The second fluid bearing device 618 redirects the uncoated optical fiber 606 from the second section 616 of the process path 608 to a third section 620 of the process path. The uncoated optical fiber 606 is cooled initially in the third section to a temperature in the range of about 400° C. to about 500° C.

[0063] A second annealing device 626 is operatively connected in the process flow downstream from the second fluid bearing device 618 along a third section 620 of the process path 608. The second annealing device 626 exposes the optical fiber to a process temperature in the range of about 1000° C. to about 1400° C.

[0064] Two fluid bearing devices 634a, 634b with multiple support channels are operatively connected to the process flow downstream of the second fluid bearing device 618 along a third section 620 of the process path 608. The two fluid bearing devices 634a, 634b redirect the uncoated optical fiber 606 to pass the optical fiber through a second annealing device 626 at least two times, where the uncoated optical fiber 606 is exposed to a process temperature in the range of about 1000°C to about 1400°C.

[0065] Following completion of the residence time in the second annealing device, the uncoated fiber is directed along a fourth process path segment 624 to a coating device 628 where a coating is applied to result in a coated fiber 630. After exiting the coating device 628, the coated optical fiber 630 may pass through a variety of other processing stages (not shown) in systems known in the art. A drawing mechanism 632, known in the art, is used to apply tension to the optical fiber as it is drawn through the system 600.

[0066] Optical fiber prepared utilizing the systems and methods described herein has an attenuation of less than 0.18 decibels per kilometer (0.18 dB / km) for optical signals at 1550 nm, preferably less than 0.17 dB / km at 1550 nm, and more preferably less than 0.16 dB / km at 1550 nm. EXAMPLES

[0067] FIG. 8 illustrates the effect of an annealing device on the temperature of a silica fiber for a system configuration including a draw furnace and an annealing device operably connected to the draw furnace, but without an intervening reheating step. The fiber is drawn from a preform in the draw furnace and then delivered directly to the annealing device. FIG. 8 illustrates the fiber temperature as a function of axial position of the fiber filament, which increases along the process path away from the draw furnace. Curve 803 is a reference trace showing the temperature profile of the fiber in the absence of an annealing device. The family of curves depicted as 806 are traces illustrating the temperature of the fiber as controlled by an annealing device located downstream of the draw furnace. The annealing device is maintained at a constant temperature of about 1200° C. The different traces correspond to different distances between the annealing device inlet and the draw furnace. The distances range from 0 mm to 1030 mm. Traces 806 show that the inclusion of an annealing device in the processing system results in a wider range of fiber strands being exposed to processing temperatures above about 1000° C. In the absence of an annealing device, trace 803 shows that the fiber temperature drops below about 1000° C. over a short distance of the fiber strand. The use of an annealing device significantly expands the range of fiber strands that maintain a temperature above about 1000° C. As the fiber is transported at a given speed over the duration of processing, the range of fiber strands is a function of the time the fiber is exposed to processing temperatures sufficient to produce a period of time during which the fiber temperature remains above about 1000° C. The inclusion of an annealing device significantly extends the period during which the fiber temperature remains above about 1000° C. during cooling.

[0068] FIG. 9 shows the effect of annealing on the fictive temperature of a silica fiber for draw speeds of 42 m / s and 50 m / s. The system configuration used in FIG. 9 includes an annealing device operatively connected to a draw furnace without any intervening devices. The fiber is drawn from a preform in the draw furnace and is conveyed directly through the annealing device at draw speeds of 42 m / s or 50 m / s. The annealing device is maintained at a uniform temperature of about 1100° C. The inlet of the annealing device (SCD) is positioned downstream in the process flow from the outlet (bottom plate) of the draw furnace along the process path 121. The inlet and outlet locations of the annealing device relative to the bottom plate of the draw furnace are illustrated in FIG. 9. Traces 912 and 914 show the change in fiber temperature as a function of distance from the bottom plate of the draw furnace for draw speeds of 42 m / s and 50 m / s, respectively. The fiber temperature decreases as it passes through the annealer, but the cooling rate is much slower than the cooling rate in air. The fiber cooling rate increases significantly as the fiber exits the annealer, resulting in a corresponding decrease in fiber temperature over the shortened distance along the processing path. Traces 916 and 918 show the change in fiber fictive temperature as a function of distance from the bottom plate of the draw furnace for draw speeds of 42 m / s and 50 m / s, respectively. The fictive temperature of a silica fiber cooled directly in air is about 1550° C. The results shown in FIG. 9 imply that conveying the fiber through the annealer reduces the fictive temperature of the fiber to about 1500° C.

[0069] Figure 10 shows the effect of fictive temperature (T f), which is the primary source of attenuation over the wavelength range shown. The plots show Rayleigh scattering loss as a function of wavelength for silica fibers with fictive temperatures ranging from about 900° C. to about 1550° C. The top trace shows the Rayleigh scattering loss for a fiber made with a standard air-cooled drawing process when no annealing device is installed. The fiber made with the standard air-cooled drawing process has a fictive temperature of about 1550° C. The remaining traces are arranged in descending order of fictive temperature, with decreasing fictive temperature as indicated in the legend. The bottom trace corresponds to a fiber with a fictive temperature of about 900° C. These traces show that the Rayleigh scattering loss decreases as the fictive temperature of the fiber decreases. The lowest attenuation was observed for the fiber with a fictive temperature of about 900° C., and the highest attenuation was observed for the fiber with a fictive temperature of about 1550° C. Moderate attenuation was observed for each fiber with a fictive temperature between about 900° C. and about 1550° C. Attenuation was observed to decrease consistently across the wavelength range shown as the fictive temperature of the fiber decreased.

[0070] FIG. 11 illustrates the modeled variation of fiber fictive temperature as a function of residence time in a furnace set at various temperatures for a silica fiber processed in a fiber drawing system according to the present disclosure. Residence time at a particular temperature or range of temperatures can be controlled by the placement, spacing, and number of hydrodynamic bearing devices as described herein. Many possible system configurations may be employed to achieve the desired residence time. In the present model, the fiber was taken to exit the draw furnace at time t=0.02 seconds and enter the furnace at time t=0.03 seconds (shown as dashed line 10). The fiber had a temperature of about 1650° C. upon entering the furnace. In this model, the heated zone of the furnace was maintained at a constant temperature. Traces 30, 40, 50, 60, and 70 show the change in fiber fictive temperature as a function of time in the furnace for furnaces operated at constant temperatures of about 900° C., about 1000° C., about 1100° C., about 1200° C., and about 1300° C., respectively. These results show that the fictive temperature of the fiber decreases the longer it is in the furnace. The dashed line 20 marks a residence time of about 0.2 seconds in the furnace set at each of the constant operating temperatures. Residence times of about 0.2 seconds are typical of prior art fiber processing systems. The data shown in FIG. 11 show that the fictive temperature of the fiber continues to decrease as residence times in the furnace are extended beyond 0.2 seconds. At a residence time of 2 seconds, the fictive temperature of the fiber is about 100° C. lower, or more, than the fictive temperature of the fiber observed at a residence time of 0.2 seconds. Figure 12 shows an example of extending the residence time to 10 seconds at each furnace temperature for the data presented in Figure 11. The various data curves and labels shown in Figure 12 correspond to those shown in Figure 11.

[0071] It is obvious to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the present invention. Therefore, the present invention is intended to cover the modifications and variations of the present invention provided they come within the scope of the appended claims and their equivalents.

[0072] Preferred embodiments of the present invention will be described below in detail.

[0073] EMBODIMENT 1 The system (400) for processing an optical fiber comprises: a drawing furnace (402) having an optical fiber preform (404) therein; an uncoated optical fiber (406) drawn from an optical fiber preform and extending from a drawing furnace along a processing path (408); an annealing apparatus (410) operatively connected to the draw furnace on a process flow downstream thereof for exposing the uncoated optical fiber to an annealing process temperature in the range of about 1000° C. to about 1400° C.; wherein the uncoated optical fiber passes through the annealing device at least twice.

[0074] EMBODIMENT 2 The system of embodiment 1 further comprises at least two fluid bearing devices (422a, 422b) operably connected to the drawing furnace on the downstream side of the process flow, each of the fluid bearing devices further comprising a body portion having a number of support channels configured to redirect the uncoated optical fiber and pass it through the annealing device at least twice.

[0075] EMBODIMENT 3 3. The system of embodiment 2, wherein the hydrodynamic bearing device comprises at least three fiber support channels.

[0076] EMBODIMENT 4 2. The system of embodiment 1, wherein the optical fiber is drawn from the optical fiber preform at a speed greater than 30 m / s.

[0077] EMBODIMENT 5 2. The system of embodiment 1, wherein the optical fiber is drawn from the optical fiber preform at a speed greater than 40 m / s.

[0078] EMBODIMENT 6 2. The system of embodiment 1, wherein the optical fiber is drawn from the optical fiber preform at a speed greater than 50 m / s.

[0079] EMBODIMENT 7 The system of embodiment 2, wherein the at least two fluid bearing devices are configured to redirect the uncoated optical fiber through the annealing device to expose the uncoated optical fiber to the annealing device processing temperature for a residence time of at least 0.4 seconds.

[0080] EMBODIMENT 8 6. The system of embodiment 5, wherein the residence time in the annealing device is at least 0.5 seconds.

[0081] EMBODIMENT 9 6. The system of embodiment 5, wherein the residence time in the annealing device is at least 1 second.

[0082] EMBODIMENT 10 6. The system of embodiment 5, wherein the residence time in the annealing device is at least 2 seconds.

[0083] EMBODIMENT 11 2. The system of embodiment 1, wherein the optical fiber is at a temperature of about 1000° C. to about 1700° C. inside the annealing device for at least 0.2 seconds.

[0084] EMBODIMENT 12 2. The system of embodiment 1, wherein the optical fiber is at a temperature of about 1000° C. to about 1700° C. inside the annealing device for at least 0.5 seconds.

[0085] EMBODIMENT 13 2. The system of embodiment 1, wherein the optical fiber is at a temperature of about 1000° C. to about 1700° C. inside the annealing device for at least 1 second.

[0086] EMBODIMENT 14 2. The system of embodiment 1, wherein the optical fiber is at a temperature of about 1000° C. to about 1700° C. inside the annealing device for at least 2 seconds.

[0087] EMBODIMENT 15 The system of embodiment 2, wherein the system is configured to increase the temperature of the uncoated optical fiber by at least about 500° C. after the uncoated optical fiber is redirected by the fluid bearing device and before it is re-inserted into the annealing device.

[0088] EMBODIMENT 16 The system of embodiment 2, wherein the system is configured to increase the temperature of the uncoated optical fiber by at least about 800° C. after the uncoated optical fiber is redirected by the fluid bearing device and before it is re-inserted into the annealing device.

[0089] EMBODIMENT 17 3. The system of embodiment 2, wherein the temperature of the hydrodynamic bearing device is between about 5°C and about 450°C.

[0090] EMBODIMENT 18 3. The system of embodiment 2, wherein the temperature of the fluid in the fluid dynamic bearing device is between about 5°C and about 200°C.

[0091] EMBODIMENT 19 3. The system of embodiment 2, wherein the temperature of the optical fiber exiting the hydrodynamic bearing device is between about 300°C and about 800°C.

[0092] EMBODIMENT 20 2. The system of embodiment 1, wherein the optical fiber has an attenuation of less than 0.18 dB / km at 1550 nm.

[0093] EMBODIMENT 21 2. The system of embodiment 1, wherein the optical fiber has an attenuation of less than 0.17 dB / km at 1550 nm.

[0094] EMBODIMENT 22 2. The system of embodiment 1, wherein the optical fiber has an attenuation of less than 0.16 dB / km at 1550 nm.

[0095] EMBODIMENT 23 The system of embodiment 2, wherein the hydrodynamic bearing device is within a heated housing.

[0096] EMBODIMENT 24 3. The system of embodiment 2, wherein the temperature of the fluid in the hydrodynamic bearing device is between about 400°C and about 1200°C.

[0097] EMBODIMENT 25 25. The system of embodiment 24, wherein the optical fiber is exposed to a processing temperature greater than about 400° C. within the fluid dynamic bearing device.

[0098] EMBODIMENT 26 25. The system of embodiment 24, wherein the optical fiber is exposed to a processing temperature greater than about 800° C. within the fluid dynamic bearing device.

[0099] EMBODIMENT 27 The method for treating an uncoated optical fiber comprises: drawing an uncoated optical fiber from an optical fiber preform within a draw furnace such that the optical fiber extends from the draw furnace along a process path; drawing the uncoated optical fiber through an annealing device operatively connected in a process flow downstream from the draw furnace such that the annealing device exposes the uncoated optical fiber to an annealing process temperature in the range of about 1000° C. to about 1400° C.; redirecting the uncoated optical fiber through the annealing device at least two times; The method includes:

[0100] EMBODIMENT 28 The method of embodiment 25, wherein the uncoated optical fiber is redirected through the annealing device at least twice by at least two fluid bearing devices operably connected to the drawing furnace downstream in the process flow, each of the fluid bearing devices having a body portion having a number of support channels formed therein.

[0101] EMBODIMENT 29 26. The method of embodiment 25, wherein the optical fiber is drawn from the optical fiber preform at a speed greater than 30 m / s.

[0102] EMBODIMENT 30 26. The method of embodiment 25, wherein the optical fiber is drawn from the optical fiber preform at a speed greater than 40 m / s.

[0103] EMBODIMENT 31 26. The method of embodiment 25, wherein the optical fiber is drawn from the optical fiber preform at a speed greater than 50 m / s.

[0104] EMBODIMENT 32 The method of embodiment 28, wherein the at least two fluid bearing devices are configured to redirect the uncoated optical fiber through the annealing device, thereby exposing the uncoated optical fiber to the annealing device processing temperature for a residence time of at least 0.5 seconds.

[0105] EMBODIMENT 33 33. The method of embodiment 32, wherein the residence time in the annealing apparatus is at least 1 second.

[0106] EMBODIMENT 34 33. The method of embodiment 32, wherein the residence time in the annealing apparatus is at least 2 seconds.

[0107] EMBODIMENT 35 28. The method of embodiment 27, wherein the optical fiber is at a temperature of about 1000° C. to about 1700° C. (inside the annealing apparatus) for at least 0.2 seconds.

[0108] EMBODIMENT 36 28. The method of embodiment 27, wherein the optical fiber is at a temperature of about 1000° C. to about 1700° C. (inside the annealing apparatus) for at least 0.5 seconds.

[0109] EMBODIMENT 37 28. The method of embodiment 27, wherein the optical fiber is at a temperature of about 1000° C. to about 1700° C. (inside the annealing apparatus) for at least 1 second.

[0110] EMBODIMENT 38 28. The method of embodiment 27, wherein the optical fiber is at a temperature of about 1000° C. to about 1700° C. (inside the annealing apparatus) for at least 2 seconds.

[0111] EMBODIMENT 39 29. The method of embodiment 28, wherein the uncoated optical fiber is heated to an increase in temperature by at least about 500° C. after being redirected by the fluid dynamic bearing device and before being re-entered into the annealing device.

[0112] EMBODIMENT 40 29. The method of embodiment 28, wherein the uncoated optical fiber is heated to an increase in temperature by at least about 800° C. after being redirected by the fluid dynamic bearing device and before being re-entered into the annealing device.

[0113] EMBODIMENT 41 29. The method of embodiment 28, wherein the temperature of the hydrodynamic bearing device is between about 5°C and about 450°C.

[0114] EMBODIMENT 42 29. The method of embodiment 28, wherein the temperature of the fluid in the fluid dynamic bearing device is between about 5°C and about 200°C.

[0115] EMBODIMENT 43 29. The method of embodiment 28, wherein the temperature of the optical fiber exiting the hydrodynamic bearing device is between about 300°C and about 800°C.

[0116] EMBODIMENT 44 2. The system of embodiment 1, wherein the optical fiber has an attenuation of less than 0.18 dB / km at 1550 nm.

[0117] EMBODIMENT 45 28. The method of embodiment 27, wherein the optical fiber has an attenuation of less than 0.17 dB / km at 1550 nm.

[0118] EMBODIMENT 46 28. The method of embodiment 27, wherein the optical fiber has an attenuation of less than 0.16 dB / km at 1550 nm.

[0119] EMBODIMENT 47 29. The method of embodiment 28, wherein the hydrodynamic bearing device is within a heated housing.

[0120] EMBODIMENT 48 29. The method of embodiment 28, wherein the temperature of the fluid in the fluid dynamic bearing device is between about 400°C and about 1200°C.

[0121] EMBODIMENT 49 49. The method of embodiment 48, wherein the optical fiber is exposed to a processing temperature greater than about 400° C. inside the fluid dynamic bearing device.

[0122] EMBODIMENT 50 49. The method of embodiment 48, wherein the optical fiber is exposed to a processing temperature greater than about 800° C. inside the fluid dynamic bearing device. [Explanation of symbols]

[0123] 400 System for processing optical fibers 402 Drawing furnace 404 Optical fiber preform 406 Uncoated Optical Fiber 408 Processing Path 410 Slow cooling device 422a Fluid dynamic bearing device 422b Fluid dynamic bearing device

Claims

1. The system (400) for processing an optical fiber comprises: a drawing furnace (402) having an optical fiber preform (404) therein; an uncoated optical fiber (406) drawn from the optical fiber preform and extending along a processing path (408) from the draw furnace; an annealing apparatus (410) operatively connected to the draw furnace on a process flow downstream side thereof for exposing the uncoated optical fiber to an annealing process temperature in the range of about 1000° C. to about 1400° C.; at least one hydrodynamic bearing device operatively connected downstream in the process flow of the drawing furnace; Equipped with the at least one hydrodynamic bearing device is configured to redirect the uncoated optical fiber exiting the annealing device and re-enter the annealing device. system.

2. The system described in claim 1, wherein the at least one fluid bearing device further comprises a main body portion having a number of support channels configured to redirect the uncoated optical fiber and re-insert it into the slow cooling device.

3. The system of claim 1, further comprising at least one fluid dynamic bearing device configured to redirect the uncoated optical fiber through the annealing device to expose the uncoated optical fiber to an annealing device processing temperature for a residence time of at least 0.4 seconds.

4. The system of claim 1, wherein the uncoated optical fiber is at a temperature of about 1000° C. to about 1700° C. inside the slow cooling device for at least 0.2 seconds.

5. The system of claim 1, wherein the system is configured to increase the temperature of the uncoated optical fiber by at least about 500°C after the uncoated optical fiber has been redirected by the fluid bearing device and before it is reinserted into the slow cooling device.

6. The system described in claim 1, wherein the temperature of the at least one fluid bearing device is between about 5°C and about 450°C.

7. The system described in claim 1, wherein the temperature of the optical fiber exiting the at least one fluid bearing device is between about 300°C and about 800°C.

8. The system of claim 1, wherein the optical fiber has an attenuation of less than 0.18 dB / km at 1550 nm.

9. The system described in claim 1, wherein at least one fluid dynamic bearing device is located inside a heated enclosure.

10. 1. A method for processing an uncoated optical fiber, comprising: drawing an uncoated optical fiber from an optical fiber preform within a draw furnace such that the optical fiber extends from the draw furnace along a process path; drawing the uncoated optical fiber through an annealing device operatively connected downstream in the process flow to the draw furnace such that the annealing device exposes the uncoated optical fiber to an annealing process temperature in the range of about 1000° C. to about 1400° C.; redirecting the uncoated optical fiber so that the uncoated optical fiber exits the annealing apparatus and reenters the annealing apparatus; The method includes:

11. The method described in claim 10, wherein the uncoated optical fiber is redirected and re-inserted into the annealing device by at least one fluid bearing device operably connected to the drawing furnace downstream in the process flow, the at least one fluid bearing device having a body portion provided with a number of support channels.

12. The method of claim 10, further comprising redirecting the uncoated optical fiber through the annealing device, thereby exposing the uncoated optical fiber to an annealing device processing temperature for a residence time of at least 0.5 seconds.

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