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1. WO2010077132 - UVLED APPARATUS FOR CURING GLASS-FIBER COATINGS

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UVLED Apparatus for Curing Glass-Fiber Coatings

Field of the Invention

[0001] The present invention relates to an apparatus and a method for curing coatings on drawn glass fibers. Background

[0002] Glass fibers are typically protected from external forces with one or more coating layers. Typically, two or more layers of coatings are applied during the optical-fiber drawing process {i.e., whereby a glass fiber is drawn from an optical preform in a drawing tower). A softer inner coating layer typically helps to protect the glass fiber from microbending. A harder outer coating layer typically is used to provide additional protection and to facilitate handling of the glass fiber. The coating layers may be cured, for example, using heat or ultraviolet (UV) light.

[0003] UV curing requires that the coated glass fiber be exposed to high intensity UV radiation. Curing time can be reduced by exposing the coating to higher intensity UV radiation. Reducing curing time is particularly desirable to permit an increase in fiber drawing line speeds and thus optical-fiber production rates.

[0004] Mercury lamps (e.g., high pressure mercury lamps or mercury xenon lamps) are commonly used to generate the UV radiation needed for UV curing. One downside of using mercury lamps is that mercury lamps require a significant amount of power to generate sufficiently intense UV radiation. For example, UV lamps used to cure a single coated fiber {i.e., one polymeric coating) may require a collective power consumption of 50 kilowatts. [0005] Another shortcoming of mercury lamps is that much of the energy used for powering mercury lamps is emitted not as UV radiation, but rather as heat. Accordingly, mercury lamps must be cooled (e.g., using a heat exchanger) to prevent overheating. In addition, the undesirable heat generated by the mercury lamps may slow the rate at which the optical fiber coatings cure.

[0006] Furthermore, mercury lamps generate a wide spectrum of electromagnetic radiation, such as having wavelengths of less than 200 nanometers and greater than 700 nanometers {i.e., infrared light). Typically, UV radiation having wavelengths of between about 300 nanometers and 400 nanometers is useful for curing UV coatings. Thus, much of the electromagnetic radiation generated by mercury bulbs is wasted (e.g., 90 percent or more). Additionally, glass fibers possess an exemplary diameter of about 125 microns, which, of course, is much smaller than the mercury bulbs. Consequently, most of the UV radiation emitted by the mercury lamps does not reach the glass fiber's uncured coating (i.e., the energy is wasted).

[0007] It may thus be advantageous to employ UVLEDs, an alternative to conventional mercury lamps, to cure glass-fiber coatings. UVLEDs typically require significantly less energy and correspondingly generate much less heat energy than conventional UV lamps.

[0008] For example, U.S. Patent No. 7,022,382 (Khudyakov et al.) discloses the use of UV lasers (e.g., continuous or pulsed lasers) for curing optical fiber coatings. [0009] U.S. Patent Application Publication No. 2003/0026919 (Kojima et al.) discloses the use of ultraviolet light emitting diodes (UVLEDs) for curing optical fiber coatings. The disclosed optical fiber resin coating apparatus includes a mold assembly in which a UV curable resin is coated onto an optical fiber. Also at the mold assembly, the coated optical fiber is exposed to UV radiation from a number of UVLEDs to cure the UV coating. A control circuit may be used to control the UV radiation output from the UVLED array. For example, the control circuit may reduce the current to one or more UVLEDs to reduce the intensity of emitted UV radiation. The control circuit may also be used to vary the intensity of the UV radiation as the optical fiber progresses through the mold assembly.

[0010] Even so, UVLEDs, though more efficient than mercury lamps, still waste a significant amount of energy in curing glass-fiber coatings. In particular, much of the UV radiation is not used to cure the glass-fiber coatings. [0011] Therefore, a need exists for a UVLED apparatus that, as compared with a conventional mercury-lamp device, not only consumes less power and generates less unwanted heat, but also is capable of curing glass-fiber coatings with improved curing efficiency. Summary [0012] Accordingly, the invention embraces a UVLED apparatus and associated method for curing in situ optical fiber coating. The apparatus and method employ one or more UVLEDs that emit electromagnetic radiation into a curing space. An incompletely cured, coated glass fiber passes through the curing space, thereby absorbing electromagnetic radiation to effect curing of the optical-fiber coating. [0013] The invention hence proposes an apparatus for curing a coating upon a glass fiber, the apparatus comprising one or more UVLED-mirror pairs, each UVLED-mirror pair comprising (/) a UVLED for emitting electromagnetic radiation and (H) a mirror for reflecting electromagnetic radiation; wherein said UVLED and said mirror of said one or more UVLED-mirror pairs define therebetween a curing space for the passage of a coated glass fiber between said UVLED and said mirror; and wherein said mirror is positioned opposite said UVLED of said one or more UVLED-mirror pairs to reflect the electromagnetic radiation emitted from said UVLED into the curing space.

[0014] In embodiments, the apparatus is further characterized in that:

- it comprises a plurality of UVLED-mirror pairs arranged in such a manner that the curing space of each of the plurality of UVLED-mirror pairs is aligned in longitudinal direction to form the curing space or the passage of the coated glass fiber through the plurality of UVLED-mirror pairs;

- the curing space defines a central axis; and each of said UVLED-mirror pairs are configured in two or more distinct planes that are substantially perpendicular to the central axis; - said UVLED-mirror pairs are helically configured around the central axis of the curing space;

- it comprises at least two UVLED-mirror pairs and wherein at least two of said UVLEDs emit electromagnetic radiation at different output intensities;

- it comprises at least two UVLED-mirror pairs and having a dark space between at least two of said UVLEDs;

- substantially all of the electromagnetic radiation emitted by each said UVLED has wavelengths of between about 200 nanometers and 600 nanometers;

- at least 90 percent of the electromagnetic radiation emitted by each said UVLED has wavelengths of between about 250 nanometers and 400 nanometers; - at least 80 percent of the electromagnetic radiation emitted by each said UVLED has wavelengths of between about 300 nanometers and 450 nanometers;

- at least 80 percent of the electromagnetic radiation emitted by each said UVLED has wavelengths of between about 375 nanometers and 425 nanometers;

- each said UVLED emits electromagnetic radiation of wavelengths mostly between about 395 nanometers and 415 nanometers;

-at least 80 percent of the electromagnetic radiation emitted by each said UVLED has wavelengths within a 30-nanometer range;

-at least one said UVLED has a power output of at least about 30 watts;

- each said UVLED has a power output of at least about 30 watts; - at least one of said UVLED-mirror pairs comprises a plurality of UVLEDs;

- at least one of said UVLED-mirror pairs comprises a concave mirror;

- at least one of said UVLED-mirror pairs comprises a parabolic mirror, a spherical mirror, and/or a cylindrical mirror;

- at least one of said UVLED-mirror pairs comprises a plurality of mirrors;

- it comprises a heat sink for dissipating heat from at least one said UVLED;

- said heat sink comprises a heat exchanger that employs a liquid coolant;

- it comprises a heat sink for dissipating heat from each said UVLED;

- it comprises a reflector for focusing UV radiation toward the coated glass fiber, wherein said reflector is attached to at least one said UVLED;

-said reflector has the shape of a rotated teardrop curve;

- it comprises a lens for focusing UV radiation toward the coated glass fiber, said lens attached to at least one said UVLED;

- it comprises a control circuit for controlling the UV radiation output from at least one said UVLED;

- said control circuit adjusts the intensity of electromagnetic radiation output from at least one said UVLED in response to a change in the speed at which the coated glass fiber passes through the apparatus;

- at least one said UVLED-mirror pair comprises a plurality of UVLEDs; - at least one said UVLED-mirror pair comprises a plurality of mirrors positioned opposite said UVLED to reflect the electromagnetic radiation emitted from said UVLED into the curing space.

[0015] An exemplary UVLED apparatus includes one or more UVLED-mirror pairs. Each UVLED-mirror pair includes one or more UVLEDs capable of emitting electromagnetic radiation and one or more mirrors (or other reflective surfaces) that are capable of reflecting electromagnetic radiation. The UVLED(s) and corresponding mirror(s) are positioned apart from one another so as to define a curing space. As noted, this curing space permits the passage of a coated glass fiber between the UVLED(s) and the mirror(s). Moreover, the mirror(s) are typically positioned opposite corresponding UVLED(s) to efficiently reflect the electromagnetic radiation emitted from the UVLED(s) (and not already absorbed by the glass-fiber coating) into the curing space.

[0016] In another aspect, the present invention embraces a method for curing a coating on a glass fiber. UV radiation is emitted from one or more sources of electromagnetic radiation toward a curing space. A portion of the emitted UV radiation is transmitted entirely through the curing space. At least some of the UV radiation transmitted entirely through the curing space is reflected toward the curing space (e.g., with a mirror). A glass fiber having an incompletely cured coating is passed through the curing space to effect the absorption of both emitted and reflected UV radiation, thereby curing the coated glass fiber.

[0017] The foregoing illustrative summary, as well as other exemplary objectives and/or advantages of the invention, and the manner in which the same are accomplished, are further explained within the following detailed description and its accompanying drawings.

Brief Description of the Drawings [0018] Fig. 1 schematically depicts a UVLED configuration for the curing of a coating upon a glass fiber.

[0019] Fig. 2 schematically depicts an alternative staggered UVLED configuration for the curing of a coating upon a glass fiber.

[0020] Fig. 3 schematically depicts a staggered UVLED-mirror configuration for the high-efficiency curing of a coating upon a glass fiber.

[0021] Fig. 4 schematically depicts an alternative staggered UVLED-mirror configuration for the high-efficiency curing of a coating upon a glass fiber.

[0022] Fig. 5 schematically depicts a UVLED-mirror configuration having UVLEDs and mirrors positioned at an angle. [0023] Fig. 6 schematically depicts a UVLED having an attached reflector for focusing emitted UV radiation.

[0024] Fig. 7 depicts a cross-sectional view of an elliptic cylinder having a reflective inner surface.

[0025] Fig. 8 schematically depicts an exemplary apparatus for curing a coated glass fiber. [0026] Fig. 9 schematically depicts a cross-sectional view of an exemplary apparatus for curing a coated glass fiber.

Detailed Description

[0027] In one aspect, the present invention embraces an apparatus for curing glass-fiber coatings. [0028] The apparatus employs one or more UVLEDs that are configured to emit electromagnetic radiation toward a drawn glass fiber to cure its coating(s), typically polymeric coatings. In this regard, a plurality of UV lamps may be positioned in various configurations, such as an apparatus 10 containing opposing UVLEDs 11 schematically depicted in Fig. 1 and an apparatus 20 containing staggered UVLEDs 11 depicted in Fig. 2. The UVLEDs 11 define a curing space 15 having a central axis 14 along which an optical fiber (i.e., a glass fiber having one or more coating layers) may pass during the curing process. A heat sink 12 may be positioned adjacent to each UVLED 11 to dissipate generated heat energy. A mounting plate 13 provides structural support for the UVLEDs 11.

[0029] In one exemplary embodiment, the apparatus for curing glass-fiber coatings includes at least a UVLED-mirror pair, which includes (/) an ultraviolet light emitting diode (UVLED) and (//) a mirror (e.g., a concave mirror) that is positioned to reflect and focus the UV-radiation emitted by the UVLED. [0030] Fig. 3 schematically depicts an apparatus 30 containing a plurality of UVLED-mirror pairs. As depicted in Fig. 3, the UVLEDs 11 and corresponding mirrors 16 define a space through which the coated glass fiber can pass (i.e., a curing space 15). This curing space 15 further defines a central axis 14, typically the axis along which a drawn glass fiber passes during the curing process (i.e., the glass fiber's curing axis). Although the central axis 14 is typically vertical, non-vertical (e.g., horizontal) arrangements of the central axis 14 may also be used. Moreover, although the central axis may be centrally positioned in the curing space, a central axis that is not centrally positioned within the curing space is within the scope of the present invention [0031] Each UVLED 11 may be positioned such that it emits UV radiation toward the central axis 14. In this regard, those of ordinary skill will appreciate that the power per unit area (i.e., the radiant flux) emitted by a UVLED decreases exponentially as the distance of the UVLED from the optical fiber increases. Accordingly, each UVLED may be positioned at a distance of between about 1 millimeter and 100 millimeters (e.g., typically between about 5 millimeters and 20 millimeters) from the optical fiber to be cured (e.g., from the central axis). [0032] Typically, a UVLED and its corresponding mirror are positioned so that a substantial portion of the UV radiation incident to the optical fiber is substantially perpendicular to the optical fiber (i.e., incident at about a 90 degree angle).

[0033] Alternatively, the UVLED and/or its corresponding mirror may be positioned at an angle so that most of the UV radiation incident to the optical fiber is incident at an angle other than 90 degrees. Fig. 5 schematically depicts an apparatus 50 containing UVLEDs 11 and mirrors 16 positioned at an angle (i.e., an angle other than 90 degrees relative to the optical fiber and the central axis 14).

[0034] It may be desirable for the power of the UV radiation incident to the optical fiber to vary as the optical fiber progresses through the apparatus. Varying the power of the UV radiation may aid in the curing of the glass-fiber coating. Depending on the curing properties of a particular coating, it may be desirable to initially expose the optical fiber to high intensity UV radiation. Alternatively, it may be desirable to initially expose the optical fiber to lower intensity UV radiation (e.g., between about 10 percent and 50 percent of the maximum exposure intensity) before exposing the optical fiber to high intensity UV radiation (e.g., the maximum intensity to which the optical fiber is exposed). In this regard, initially exposing the optical fiber to lower intensity UV radiation may be useful in controlling the generation of free radicals in an uncured coating. Those of ordinary skill in the art will appreciate that if too many free radicals are present, many of the free radicals may recombine rather than encourage the polymerization of the glass fiber coating — an undesirable effect.

[0035] Varying the intensity of the UV radiation may, for example, be achieved by positioning the UVLED at an angle. As noted, the intensity of the UV radiation incident to a portion of the optical fiber may vary depending upon the distance from that portion to the UVLED. Alternatively, in an apparatus containing a plurality of UVLEDs the intensity of the UV radiation output from the UVLEDs may vary.

[0036] As noted, each UVLED may be positioned such that it emits UV radiation toward the central axis. That said, it will be appreciated by those of ordinary skill in the art that a UVLED does not emit UV radiation only toward a point or line, but rather emits UV radiation in many directions. Thus, most of the UV radiation emitted by a UVLED will not strike the glass-fiber coating to effect curing. However, in curing an optical fiber coating, it is desirable that as much UV radiation as possible strikes the optical fiber (i.e., a coated glass fiber). In this regard, it will be further appreciated by those of ordinary skill in the art that curing occurs when UV radiation is absorbed by photoinitiators in the glass-fiber coating. [0037] Thus, a UVLED may have one or more associated reflectors that focus emitted UV radiation toward the central axis. For example, Fig. 6 depicts a cross-sectional view of a UVLED 11 having an attached reflector 17. The reflector 17 may, for example and as depicted in Fig. 6, have the shape of a rotated teardrop curve. By having a teardrop shape, the reflector focuses UV radiation having various angles of emittance toward the central axis. That said, the present invention embraces UVLEDs with associated reflectors of various shapes (e.g., a spherical, elliptical, cylindrical or parabolic mirror).

[0038] A UVLED may have one or more lenses attached for focusing UV radiation emitted by the UVLED toward the central axis. Typically, a lens for focusing electromagnetic radiation is convex (e.g., biconvex or plano-convex). In an alternative embodiment, a Fresnel lens may be employed. Moreover, the lens may be selected such that the lens has a focal point at the glass fiber's curing axis (e.g., the central axis).

[0039] One or more mirrors may be positioned opposite a UVLED (i.e., on the opposite side of the central axis) so as to reflect the UV radiation emitted by the UVLED in the general direction of the central axis. In other words, a UVLED emits UV radiation toward the curing space and the central axis, and its corresponding mirror(s) reflect emitted UV radiation not initially absorbed by optical fiber coatings back toward the central axis (e.g., the glass fiber's curing axis). In this respect, Fig. 3 depicts UVLEDs 11 having a mirror 16 positioned opposite the central axis 14. Typically a mirror will be larger than its corresponding UVLED. For example, a mirror may have a height of between about one inch and 1.5 inches; however, other mirror sizes are within the scope of the present invention. The mirror may be formed from a suitable reflective material. For example, the mirror may be formed from polished aluminum, polished stainless steel, or metalized glass (e.g., silvered quartz).

[0040] The mirror may have a concave shape {i.e., the mirror is curved inwards toward the curing space) so that the mirror focuses UV radiation emitted by the UVLED toward the central axis. By way of example, a concave mirror may have, inter alia, a cylindrical, elliptical, spherical, or parabolic shape (e.g., a paraboloid or a parabolic cylinder). A concave mirror can focus otherwise lost UV radiation (e.g., UV radiation not initially incident to an optical fiber to be cured) onto an optical fiber for curing, thus limiting the amount of wasted energy. [0041] A UVLED-mirror pair as used herein is not limited to a single UVLED being paired with a single mirror in a one-to-one relationship. A UVLED-mirror pair may include a plurality of UVLEDs. Alternatively, a UVLED-mirror pair may include a plurality of mirrors. [0042] UVLEDs are capable of emitting wavelengths within a much smaller spectrum than conventional UV lamps. This promotes the use of more of the emitted electromagnetic radiation for curing. [0043] In this regard, a UVLED for use in the present invention may be any suitable LED that emits electromagnetic radiation having wavelengths of between about 200 nanometers and 600 nanometers. By way of example, the UVLED may emit electromagnetic radiation having wavelengths of between about 200 nanometers and 450 nanometers (e.g., between about 250 nanometers and 400 nanometers). In a particular exemplary embodiment, the UVLED may emit electromagnetic radiation having wavelengths of between about 300 nanometers and 400 nanometers. In another particular exemplary embodiment, the UVLED may emit electromagnetic radiation having wavelengths of between about 350 nanometers and 425 nanometers. [0044] As noted, a UVLED typically emits a narrow band of electromagnetic radiation. For example, the UVLED may substantially emit electromagnetic radiation having wavelengths that vary by no more than about 30 nanometers, typically no more than about 20 nanometers (e.g., a UVLED emitting a narrow band of UV radiation mostly between about 375 nanometers and 395 nanometers). It has been observed that a UVLED emitting a narrow band of UV radiation mostly between about 395 nanometers and 415 nanometers is more efficient than other narrow bands of UV radiation. Moreover, it has been observed that UVLEDs emitting UV radiation slightly above the wavelength at which a glass-fiber coating has maximum absorption (e.g., about 360 nanometers) promote more efficient polymerization than do UVLEDs emitting UV radiation at the wavelength at which the glass-fiber coating has maximum absorption. [0045] In this regard, although an exemplary UVLED emits substantially all of its electromagnetic radiation within a defined range (e.g., between 350 nanometers and 450 nanometers, such as between 370 nanometers and 400 nanometers), the UVLED may emit small amounts of electromagnetic radiation outside the defined range. In this regard, 80 percent or more (e.g., at least about 90 percent) of the output {i.e., emitted electromagnetic radiation) of an exemplary UVLED is typically within a defined range (e.g., between about 375 nanometers and 425 nanometers).

[0046] UVLEDs are typically much smaller than conventional UV lamps (e.g., mercury bulbs). By way of example, the UVLED may be a 0.25-inch square UVLED. The UVLED may be affixed to a platform (e.g., a 1-inch square or larger mounting plate). Of course, other UVLED shapes and sizes are within the scope of the present invention. By way of example, a 3-millimeter square UVLED may be employed.

[0047] Each UVLED may have a power output of as much as 32 watts (e.g., a UVLED having a power input of about 160 watts and a power output of about 32 watts). That said, UVLEDs having outputs greater than 32 watts (e.g., 64 watts) may be used as such technology becomes available. Using UVLEDs with higher power output may be useful for increasing the rate at which optical fiber coatings cure, thus promoting increased production line speeds. [0048] Relative to other UV radiation sources, UVLED devices typically generate a smaller amount of heat energy. That said, to dissipate the heat energy created by a UVLED, a heat sink may be located behind the UVLED (e.g., opposite the portion of the UVLED that emits UV radiation). The heat sink may be one-inch square, although other heat sink shapes and sizes are within the scope of the present invention.

[0049] The heat sink may be formed of a material suitable for conducting heat (e.g., brass, aluminum, or copper). The heat sink may include a heat exchanger that employs a liquid coolant (e.g., chilled water), which circulates within the heat exchanger to draw heat from the UVLED.

[0050] Removing heat generated by the UVLED is important for several reasons. First, excess heat may slow the rate at which optical-fiber coatings cure. Furthermore, excess heat can cause the temperature of the UVLED to rise, which can reduce UV-radiation output. Indeed, continuous high-temperature exposure can permanently reduce the UVLED's radiation output. With adequate heat removal, however, the UVLED may have a useful life of 50,000 hours or more.

[0051] In another exemplary embodiment, the apparatus for curing glass-fiber coatings includes two or more UVLEDs. [0052] For instance, the UVLEDs may be arranged in an array (e.g., a planar or non-planar array, such as a three dimensional array). With respect to a three dimensional array, two or more UVLEDs may be configured in two or more distinct planes that are substantially perpendicular to the central axis. As depicted in Figs. 1 and 2, the UVLEDs11 , which emit UV radiation toward the curing space 15, are typically arranged approximately equidistant from the central axis 14.

[0053] The UVLEDs, for instance, may be arranged to define one or more helixes (i.e., a helical array of UVLEDs). In an array containing more than one helix, the helixes may have the same chirality {i.e., asymmetric handedness). Alternatively, at least one helix may have the opposite chirality {e.g., one helix may be right-handed and a second helix may be left-handed). [0054] The three-dimensional array of UVLEDs may define a curing space that is suitable for the passage of a coated glass fiber for curing. As before, the curing space defines a central axis {e.g., the axis along which a drawn glass fiber passes during the curing process). [0055] The apparatus employing a three-dimensional array of UVLEDs for curing glass-fiber coatings may also include one or more mirrors for reflecting UV radiation into the curing space. For example, the apparatus may include a plurality of the foregoing UVLED-mirror pairs. By way of further example and as depicted in Fig. 4, a plurality of UVLEDs 11 may be embedded in a mirror 46 (e.g., a mirror in the shape of a circular, elliptical, or parabolic cylinder), the interior of which defines the curing space 15. [0056] It will be appreciated by those of skill in the art that the position of the UVLEDs in a three-dimensional array may be defined by the cylindrical coordinate system {i.e., r, θ, z). Using the cylindrical coordinate system and as described herein, the central axis of the curing space defines a z-axis. Furthermore, as herein described and as will be understood by those of ordinary skill in the art, the variable r is the perpendicular distance of a point to the z-axis {i.e., the central axis of the curing space). The variable θ describes the angle in a plane that is perpendicular to the z-axis. In other words and by reference to a Cartesian coordinate system {i.e., defining an x axis, a y axis, and a z axis), the variable θ describes the angle between a reference axis (e.g., the x axis) and the orthogonal projection of a point onto the x-y plane. Finally, the z variable describes the height or position of a reference point along the z axis.

[0057] Thus, a point is defined by its cylindrical coordinates (r, θ, z). For UVLEDs employed in exemplary configurations, the variable r is usually constant. Stated otherwise, the UVLEDs may be positioned approximately equidistant from the central axis {i.e., the z axis). Accordingly, to the extent the variable r is largely fixed, the position of the UVLEDs can be described by the z and θ coordinates.

[0058] By way of example, a helical UVLED array may have a first UVLED at the position (1 ,0,0), where r is fixed at a constant distance {i.e., represented here as a unitless 1). Additional UVLEDs may be positioned, for example, every 90° {i.e., ττ/2) with a Δz of 1 {i.e., a positional step change represented here as a unitless 1). Thus, a second UVLED would have the coordinates (1 , π/2, 1), a third UVLED would have the coordinates (1 , π, 2), and a fourth UVLED would have the coordinates (1 , 3π/2, 3), thereby defining a helical configuration. [0059] Those having ordinary skill in the art will appreciate that, as used in the foregoing example, the respective distances r and z need not be equivalent. Moreover, those having ordinary skill in the art will further appreciate that several UVLEDs in an array as herein disclosed need not be offset by 90° (e.g., ττ/2, π, 3π/2, etc.). For example, the UVLEDs may be offset by 60° (e.g., τr/3, 2ττ/3, π, etc.) or by 120° (e.g., 2ττ/3, 4τr/3, 2ττ, etc.). Indeed, the UVLEDs in an array as discussed herein need not follow a regularized helical rotation. [0060] It will be further appreciated by those of ordinary skill in the art that UVLEDs may absorb incident electromagnetic radiation, which might diminish the quantity of reflected UV radiation available for absorption by the glass-fiber coating. See Fig. 1. Therefore, in an apparatus for curing glass-fiber coatings having a plurality of UVLEDs, it may be desirable to position the UVLEDs in a way that reduces UV radiation incident to the UVLEDs. See Figs. 2, 3, and 4. [0061] In an exemplary embodiment described using the cylindrical coordinate system, UVLEDs with a Δθ of π {i.e., UVLEDs positioned on opposite sides of a UVLED array) may be positioned so that they have a Δz that is at least the height of the UVLED. Thus, if each UVLED has a height of 0.5 inch, a UVLED with a Δθ of π should have a Δz of at least 0.5 inch. It is thought that this would reduce the absorption by one UVLED of UV radiation emitted by another UVLED, thereby increasing the availability of UV radiation for reflection by one or more mirrors and absorption by the glass-fiber coating.

[0062] Alternatively (or in accordance with the foregoing discussion), the UVLEDs may employ a reflective surface (e.g., a surface coating) that promotes reflection of incident electromagnetic radiation yet permits the transmission of emitted electromagnetic radiation.

[0063] In view of the foregoing, yet another exemplary embodiment employs a plurality of UVLED-mirror pairs that are arranged in a three dimensional configuration. In particular, the plurality of UVLED-mirror pairs share a common curing space defining a common central axis. In an exemplary configuration, the UVLED-mirror pairs may be helically arranged (e.g., configured in a 60°, 90°, or 120° helical array).

[0064] In yet another exemplary embodiment, the apparatus for curing glass-fiber coatings includes one or more UVLEDs positioned within a cylindrical cavity (or a substantially cylindrical cavity) having a reflective inner surface (e.g., made from stainless steel or silvered quartz, or otherwise including a reflective inner surface). [0065] The interior of the cylindrical cavity defines the curing space. As before, the curing space defines a curing axis (e.g., a central axis) along which a drawn glass fiber passes during the curing process. Moreover, one or more UVLEDs may be positioned within the cylindrical cavity such that they emit UV radiation in the direction of the curing axis. [0066] In a typical embodiment, the cylindrical cavity has a non-circular elliptical cross-section. In other words, the cylindrical cavity typically has the shape of an elliptic cylinder. For an elliptic cylinder the curing axis may correspond with one of the two line foci defined by the elliptic cylinder. In addition, each UVLED may be positioned along the other line focus such that they emit UV radiation in the general direction of the curing axis. This arrangement is useful for improving curing efficiency, because any electromagnetic radiation that is emitted from one line focus (regardless of direction) will be directed toward the other line focus after being reflected at the inner surface of the cylinder. This principle is illustrated in Figure 7, which depicts a cross-section of a reflective elliptic cylinder 55 having a first line focus 51 and a second line focus 52. As depicted in Figure 7, each UV ray 53 emitted from the first line focus 51 will intersect the second line focus 52. [0067] That said, the UV radiation emitted from a UVLED is not emitted from a single point. Therefore and because of the small size of a coated glass fiber, it is desirable to use small UVLEDs (e.g., a 3-millimeter square UVLED or a 1-millimeter square UVLED), because a greater percentage of emitted and reflected light from a small UVLED will be incident to the coated glass fiber. [0068] In accordance with the foregoing, Figures 8-9 depict an exemplary apparatus 60 for curing a coated glass fiber 66. The apparatus 60 includes a substantially cylindrical cavity 65 having an elliptical shape and having a reflective inner surface. The cavity 65 defines a first line focus 61 and a second line focus 62. A plurality of UVLEDs 64 are positioned along the first line focus 61. The second line focus 62 further defines a curing axis along which a coated glass fiber 66 passes so it can be cured. As depicted in Figure 9, UV rays 63 emitted from the UVLEDs 64 may reflect off the inner surface of the cavity 65 such that the reflected UV rays 63 are incident to the coated glass fiber 66. To facilitate uniform curing of the coated glass fiber 66, some of the UVLEDs 64 may be differently oriented. For example, a second apparatus 70 for curing a glass fiber could have a different orientation than the apparatus 60 (e.g., the second apparatus 70 may have UVLEDs positioned along a line focus 71 that differs from the first line focus 61).

[0069] In an alternative exemplary embodiment, rather than placing the UVLEDs along one of the line foci, each UVLED may include a lens for focusing emitted UV radiation. In particular, each lens may have a focus at one of the two line foci (e.g., the line focus not defining a curing axis). By including a lens with each UVLED1 the efficiency of the apparatus can be further improved.

[0070] An apparatus as described herein may include a dark space between one or more UVLEDs. In other words, the apparatus may include a space in which substantially no UV radiation is incident to the optical fiber being cured. A pause in the curing process provided by a dark space can help to ensure even and efficient curing of the optical fiber coatings. In particular, a dark space may be useful in preventing too many free radicals from being present in a glass-fiber coating before it is cured (i.e., dark space helps to control free-radical generation). [0071] For example, it may be desirable to initially expose an optical fiber to low power UV radiation and then pass the optical fiber through a dark space. After the optical fiber passes through a dark space it is exposed to higher power UV radiation to complete the curing process. A curing apparatus employing dark space is disclosed in commonly assigned U.S. Patent No. 7,322,122 for a Method and Apparatus for Curing a Fiber Having at Least Two Fiber Coating Curing Stages.

[0072] An apparatus for curing glass-fiber coatings may include a control circuit for controlling the UV radiation output from the UVLED. The control circuit may be used to vary the intensity of the UV radiation as an optical fiber progresses through the apparatus. [0073] For example, to ensure that the optical fiber receives a consistent dose of ultraviolet radiation, the UV radiation output of the UVLEDs may vary with the speed at which the optical fiber passes through the apparatus. That is to say, at higher speeds (i.e., the speed the optical fiber passes through the apparatus) the output intensity of the UVLEDs may be greater than the output intensity at lower speeds. The output intensity of the UVLEDs may be controlled by reducing (or increasing) the current flowing to the UVLEDs.

[0074] In another aspect, the present invention embraces a method of employing the foregoing apparatus to cure a coating on a glass fiber (i.e., in situ curing). [0075] In an exemplary method, a glass fiber is drawn from an optical preform and coated with a UV curable material. UV radiation is emitted from one or more sources of electromagnetic radiation (e.g., one or more UVLEDs) toward a curing space (e.g., in the general direction of the coated glass fiber). A portion, if not most, of the emitted UV radiation is transmitted entirely through the curing space. Typically, at least some, if not most, of the UV radiation transmitted entirely through the curing space (i.e., at least some UV radiation that has not been absorbed) is reflected (e.g., with a mirror) toward the curing space. A glass fiber having an incompletely cured coating is continuously passed through the curing space to effect the absorption of emitted and reflected UV radiation. The absorption of the UV radiation cures the glass-fiber coating. Moreover, and in accordance with the foregoing, to improve the curing rate, at least a portion of the reflected UV radiation may be focused on the glass fiber (e.g., by using a concave mirror to reflect UV radiation toward the curing space's curing axis). [0076] In accordance with the foregoing, the resulting optical fiber includes one or more coating layers (e.g., a primary coating and a secondary coating). At least one of the coating layers — typically the secondary coating — may be colored and/or possess other markings to help identify individual fibers. Alternatively, a tertiary ink layer may surround the primary and secondary coatings. [0077] For example, the resulting optical fiber may have one or more coatings (e.g., the primary coating) that comprise a UV-curable, urethane acrylate composition. In this regard, the primary coating may include between about 40 and 80 weight percent of polyether-urethane acrylate oligomer as well as photoinitiator, such as LUCERIN TPO, which is commercially available from BASF. In addition, the primary coating typically includes one or more oligomers and one or more monomer diluents (e.g., isobomyl acrylate), which may be included, for instance, to reduce viscosity and thereby promote processing. Exemplary compositions for the primary coating include UV curable urethane acrylate products provided by DSM Desotech (Elgin, Illinois) under various trade names, such as DeSolite® DP 1011 , DeSolite® DP 1014, DeSolite® DP 1014XS, and DeSolite® DP 1016. [0078] Those having ordinary skill in the art will recognize that an optical fiber with a primary coating (and an optional secondary coating and/or ink layer) typically has an outer diameter of between about 235 microns and about 265 microns (μm). The component glass fiber itself (i.e., the glass core and surrounding cladding layers) typically has a diameter of about 125 microns, such that the total coating thickness is typically between about 55 microns and 70 microns.

[0079] With respect to an exemplary optical fiber achieved according to the present curing method, the component glass fiber may have an outer diameter of about 125 microns. With respect to the optical fiber's surrounding coating layers, the primary coating may have an outer diameter of between about 175 microns and about 195 microns (i.e., a primary coating thickness of between about 25 microns and 35 microns) and the secondary coating may have an outer diameter of between about 235 microns and about 265 microns (i.e., a secondary coating thickness of between about 20 microns and 45 microns). Optionally, the optical fiber may include an outermost ink layer, which is typically between two and ten microns thick. [0080] In an alternative embodiment, the resulting optical fiber may possess a reduced diameter (e.g., an outermost diameter between about 150 microns and 230 microns). In this alternative optical fiber configuration, the thickness of the primary coating and/or secondary coating is reduced, while the diameter of the component glass fiber is maintained at about 125 microns. By way of example, in such embodiments the primary coating layer may have an outer diameter of between about 135 microns and about 175 microns (e.g., about 160 microns), and the secondary coating layer may have an outer diameter of between about 150 microns and about 230 microns (e.g., more than about 165 microns, such as 190-210 microns or so). In other words, the total diameter of the optical fiber is reduced to less than about 230 microns (e.g., about 200 microns). [0081] Exemplary coating formulations for use with the apparatus and method described herein are disclosed in the following commonly assigned applications: U.S. Patent Application No. 61/112,595 for a Microbend-Resistant Optical Fiber, filed November 7, 2008, (Overton); International Patent Application Publication No. WO 2009/062131 A1 for a Microbend-Resistant Optical Fiber, (Overton); U.S. Patent Application Publication No. US2009/0175583 A1 for a Microbend-Resistant Optical Fiber, (Overton); and U.S. Patent Application No. 12/614,011 for a Reduced-Diameter Optical Fiber, filed November 6, 2009, (Overton).

[0082] In the specification and/or figures, typical embodiments of the invention have been disclosed. The present invention is not limited to such exemplary embodiments. The figures are schematic representations and so are not necessarily drawn to scale. Unless otherwise noted, specific terms have been used in a generic and descriptive sense and not for purposes of limitation.