FIELD OF THE INVENTION

The present application is directed to polymer-coated optical fibers and fiber optic cables, which find application in a variety of fields.

BACKGROUND OF THE INVENTION

Optical fibers generally are long, thin strands of optically clear glass (silica) or plastic that are used to transmit light. Each fiber generally includes a transparent core (through which the light travels), surrounded by a transparent cladding material with a lower refractive index, which reflects light back into the transparent core. Optical fibers can be provided in bundles, called fiber optic cables, which can include one or more support components (e.g., a central cable or carrier) around which the fibers are bound. Fibers and fiber optic cables are used extensively in an array of industries with exemplary applications, in the oil and gas, aerospace, telecommunications, aviation, nuclear, remote sensing, automotive, and medical industries.

One disadvantage to the use of optical fibers and fiber optic cables is that the glass or plastic itself is very susceptible to damage, e.g., cracking, during handling (having limited ability to flex and bend), rendering the strength of the bare glass or plastic rather low. As such, optical fibers and fiber optic cables are typically provided in coated form, with one or more polymeric coating layers generally coating each fiber. Such coatings make handling of optical fibers possible by providing mechanical protection to the fibers (significantly increasing the strength, e.g., tensile strength) thereof. Primary buffer coatings (also referred to as "primary coatings" or "primary buffer coatings") are typically added immediately following production of the core and cladding (e.g., as an in-line process with production of the core and cladding) and thus are in direct contact with the glass or plastic of the cladding.

For application in certain of the industries referenced herein above, an optical fiber or fiber optic cable must exhibit various other physical properties, e.g., to withstand exposure to harsh conditions of use. For example, for some applications, the optical fiber or fiber optic cable must exhibit one or more of heat resistance, chemical resistance, abrasion resistance, moisture/water resistance, and/or biocompatibility. As such, overlying the primary buffer coating, an outer polymeric coating or "jacket" is applied. The outer polymeric coating can be referred to as a "tight buffer" where it is in contact with the primary buffer coating, having no airspace there between, or a "loose buffer" where an airspace is intentionally present between the primary buffer coating and outer polymeric coating. An outer polymeric coating, e.g., in the form of a tight buffer, can be present around a single optical fiber or a bundle of optical fibers, i.e., a fiber optic cable. Given that optical fibers and fiber optic cables are generally subjected to multiple physical processes before use, the fiber optic properties of the fiber or cable (e.g., signal loss/attenuation) may suffer as a result. Small cracks, microbends, or irregularities in a fiber can cause loss of energy (attenuation) as light travels down the fiber. It would be beneficial to provide optical fibers and fiber optic cables that with good optical properties (signal loss/attenuation) and particularly to provide optical fibers and cables that exhibit such desirable features over a broad range of environmental conditions (e.g., elevated temperature and/or cycled temperature).

SUMMARY OF THE INVENTION

The present invention relates to optical fibers and fiber optic cables with extruded polymeric coatings thereon. In this disclosure, (i) the optical fibers and fiber optic cables with extruded polymeric coatings thereon are often referred to as "thermoplastic polymer-coated optical elements"(sometimes shortened to "polymer-coated optical elements" or "coated optical elements") and (ii) the extruded polymeric coating is often referred to herein as a "thermoplastic polymeric tight buffer coating" (sometimes shortened to "tight buffer coating"). Advantageously, in certain embodiments, the attenuation of the disclosed coated optical element compares well with the attenuation of a corresponding optical element without the extruded polymeric coating thereon. In some embodiments, the attenuation of the disclosed coated optical element does not increase significantly after thermal cycling (e.g., heating to temperatures of at least 170°C and cooling), rendering certain such coated optical elements particularly useful in high-temperature applications.

In one aspect, the disclosure provides a thermoplastic polymer-coated optical element comprising: an optical element having an outer surface, wherein the optical element comprises an optical fiber having a core, cladding, and polymeric primary buffer coating; and a thermoplastic polymeric tight buffer coating on at least a portion of the outer surface of the optical element, wherein the polymer-coated optical element exhibits a first attenuation at room temperature that is no more than 50% greater than that of a comparable optical element with no thermoplastic polymeric tight buffer coating thereon (e.g., a first attenuation that is plus or minus 50% the attenuation of a comparable optical element with no thermoplastic polymeric tight buffer coating thereon), and wherein the polymer-coated optical element exhibits a second attenuation at room temperature after thermal cycling to a temperature of at least 170°C that is about 2 times the first attenuation or less. In another aspect, the disclosure provides a thermoplastic polymer-coated optical element comprising: an optical element having an outer surface, wherein the optical element comprises an optical fiber comprising a core and cladding; and a thermoplastic polymeric tight buffer coating on at least a portion of the outer surface of the optical element, wherein the thermoplastic polymeric tight buffer coating has an average thickness of at least 50 microns, and wherein the polymer-coated optical element exhibits a first attenuation at room temperature that is no more than 50% greater than that of a comparable optical element with no thermoplastic polymeric tight buffer coating thereon (e.g., a first attenuation that is plus or minus 50% the attenuation of a comparable optical element with no thermoplastic polymeric tight buffer coating thereon), and wherein the polymer-coated optical element exhibits a second attenuation at room temperature after thermal cycling to a temperature of at least 170°C that is about 2 times the first attenuation or less.

In a further aspect, the disclosure provides a thermoplastic polymer-coated optical element comprising: an optical element having an outer surface; wherein the optical element comprises an optical fiber having a core, cladding, and primary buffer coating; and a thermoplastic polymeric tight buffer coating on at least a portion of the outer surface of the optical element, wherein the polymer- coated optical element exhibits an attenuation at 1550 nm that is no more than 20% greater than that of a comparable optical element with no thermoplastic polymeric tight buffer coating thereon (e.g., an attenuation that is plus or minus 20% the attenuation at 1550 nm of a comparable optical element with no thermoplastic polymeric tight buffer coating thereon).

The composition of the thermoplastic polymeric tight buffer coating in various embodiments of the coated optical elements disclosed herein can vary. In some embodiments, the thermoplastic polymeric tight buffer coating comprises one or more of a polyaryletherketone (PAEK), a liquid crystal polymer, a polyamide-imide, and a polybenzimidazole or a derivative or copolymer thereof. In some embodiments, the thermoplastic polymeric tight buffer coating is selected from the group consisting of polyetherketone (PEK), polyetheretherketone (PEEK), polyetherketoneketone (PEKK), polyetheretherketoneketone (PEEKK), polyetherketoneetherketoneketone (PEKEKK), and derivatives and copolymers thereof . In one specific embodiment, the thermoplastic polymeric tight buffer coating is PEEK. The thermoplastic polymeric tight buffer coating can, in some embodiments, consist essentially of a thermoplastic polymer and can, in some embodiments, comprise less than about 30% by weight of components other than the thermoplastic polymer or named polymer(s).

The optical element, in certain embodiments, is an optical fiber. Such optical fibers can generally comprise a core and a cladding. In some embodiments, the optical fibers of the disclosed coated optical elements comprise a core and cladding with no primary buffer coating thereon, i.e., the thermoplastic polymeric tight buffer coating is in direct contact with the cladding. Such optical fibers, in some embodiments, consist essentially of silica. In some embodiments, the optical fibers of the disclosed coated optical elements comprise a core and cladding with a primary buffer coating thereon, i.e., the thermoplastic polymeric tight buffer coating is in direct contact with the primary buffer coating. In some such embodiments, the core and cladding can consist essentially of silica and the primary buffer coating can vary and, in certain embodiments, comprises polyimide.

In some embodiments, the optical element is a fiber optic cable. The optical fibers making up the fiber optic cable can include or exclude primary buffer coatings as disclosed herein above.

Furthermore, the entire bundle of optical fibers making up the fiber optic cable can optionally be contained within a cable primary buffer coating encapsulating the bundle of optical fibers. As such, the disclosure provides fiber optic cables with or without a cable primary buffer coating, which further comprise a thermoplastic polymeric tight buffer coating surrounding the outer diameter of a bundle of optical fibers (which as such, can be in direct contact with a portion of the primary buffer- coated and/or uncoated optical fibers) or surrounding a cable primary buffer coating encapsulating the bundle of optical fibers (which as such, can be in direct contact with the cable primary buffer coating).

The first attenuation can, in some embodiments, be no more than 20% greater than the attenuation of a comparable optical element with no thermoplastic polymeric tight buffer coating thereon. In some embodiments, the first attenuation is less than or equal to the attenuation of a comparable optical element with no thermoplastic polymeric tight buffer coating thereon. For example, certain thermoplastic polymer-coated optical elements disclosed herein exhibit attenuations of roughly the same as the attenuation of the corresponding optical element without such a thermoplastic polymeric tight buffer coating thereon. Certain thermoplastic polymer-coated optical elements disclosed herein exhibit attenuations of no more than 15% greater, no more than 10% greater, or no more than 5% greater than the attenuation of the corresponding "uncoated"

(comparative) optical element. Certain thermoplastic polymer-coated optical elements disclosed herein exhibit attenuations of +/- about 20% that of the corresponding "uncoated" (comparative) optical element, +/- about 10% that of the corresponding uncoated optical element, or +/- about 5% that of the corresponding uncoated optical element. In particular embodiments, the attenuation of certain thermoplastic polymer-coated optical elements is less than the attenuation of the

corresponding "uncoated" (comparative) optical element. For example, some such thermoplastic polymer-coated optical elements exhibit first attenuation values that are 50%- 100% the attenuation values of the corresponding "uncoated" (comparative) optical element. In particular embodiments, the first attenuation is less than 1.0 dB/km. The second attenuation, in certain embodiments, is about 1.5 times the first attenuation or less.

These and other features, aspects, and advantages of the disclosure will be apparent from a reading of the following detailed description together with the accompanying drawings, which are briefly described below. The invention includes any combination of two, three, four, or more of the above-noted embodiments as well as combinations of any two, three, four, or more features or elements set forth in this disclosure, regardless of whether such features or elements are expressly combined in a specific embodiment description herein. This disclosure is intended to be read holistically such that any separable features or elements of the disclosed invention, in any of its various aspects and embodiments, should be viewed as intended to be combinable unless the context clearly dictates otherwise. Other aspects and advantages of the present invention will become apparent from the following.

■A- BRIEF DESCRIPTION OF THE DRAWINGS

In order to provide an understanding of embodiments of the invention, reference is made to the appended drawings, which are not necessarily drawn to scale, and in which reference numerals refer to components of exemplary embodiments of the invention. The drawings are exemplary only, and should not be construed as limiting the invention.

FIGs. 1A and IB are schematic representations of certain coated optic fibers provided according to the present application, with 1 A depicting an expanded longitudinal portion of the fiber and IB depicting the cross-section of such a fiber;

FIGs. 2A and 2B are schematic representations of certain coated optical fibers provided according to the present application, with 3A depicting an expanded longitudinal portion of the fiber and 3B depicting the cross-section of such a fiber; and

FIGs. 3 A and 3B are schematic representations of certain coated fiber optic cables provided according to the present application, with 2 A depicting an expanded longitudinal portion of the cable and 2B depicting the cross-section of such a cable.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described more fully hereinafter with reference to the accompanying figures, in which some, but not all embodiments of the inventions are shown. Indeed, these inventions may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout.

The disclosure relates generally to optical elements (e.g., fibers and fiber optic cables) comprising a thermoplastic polymeric tight buffer coating on at least a portion thereof, which coated optical elements exhibit particular optical properties. Optical elements are understood to comprise at least one optical fiber and may include individual optical fibers or bundles thereof, i.e., fiber optic cables. The thermoplastic polymeric tight buffer coating described herein can be applied to a substrate comprising a single optical fiber and/or to a bundle of optical fibers in the context of a fiber optic cable.

A thermoplastic polymeric tight buffer coating is generally a polymeric coating that is in contact with an underlying material (which as outlined herein can be, e.g., a primary buffer-coated or uncoated optical element). A tight buffer coating as associated with the presently disclosed coated optical elements is distinguished from a "loose buffer" coating, which is typically not substantially in contact with the underljang coated or uncoated optical element. The optical element within a loose buffer coating is considered to "float" within the loose buffer coating, having airspace (which may be filled with, e.g., a gel or other material) between the optical element and the loose buffer coating. The composition of the thermoplastic polymeric tight buffer coatings associated with the coated optical elements disclosed herein can vary. In some embodiments, the coating comprises a thermoplastic polymer, e.g., including, but not limited to, polyaryletherketone (PAEK), a liquid crystal polymer, a polyamide-imide (e.g., Torlon® PAI), polybenzimidazole (e.g., Celazole® polybenzimidazole), or any derivative, copolymer, or combination of the foregoing. PAEKs are a family of semi-crystalline thermoplastic polyketones including, for example, polyetherketone (PEK), polyetheretherketone (PEEK), polyetherketoneketone (PEKK), polyetheretherketoneketone (PEEKK), and polyetherketoneetherketoneketone (PEKEKK). Exemplary PEEK resins include, but are not limited to, VICTREX® PEEK polymers (Victrex PLC), Vestakeep® PEEK (Evonik Industries AG), KetaSpire® PEEK (Solvay), and Ketron® PEEK (Quadrant). The thermoplastic polymeric tight buffer coating can comprise more than one thermoplastic polymer, such as a mixture, alloy, or copolymer of two or more, e.g., such that the coating comprises a mixture, alloy, or copolymer of LCP and PEEK or PBI and PEEK, or other combinations from the aforementioned polymeric families.

The thermoplastic polymeric tight buffer coating materials are typically provided in the form of solid resins, which can be melted, applied to the optical element (typically via extrusion, as described more fully hereinafter), and then cooled to provide a coating thereon. "Resin" as used herein refers to a material consisting essentially of a given type of polymer. Resins are typically provided in solid form (e.g., as solid pellets), although they are not limited thereto (with other forms including, but not limited to, powders, granules, dispersions, solutions, gels, and the like). In certain embodiments, polymeric resins are homopolymeric (i.e., comprising a single type of repeating monomer unit). In certain embodiments, polymeric resins are copolymeric resins, comprising, for example, alternating copolymers (having two or more monomer units in a regularly alternating arrangement), periodic copolymers (having two or more monomer units in a regularly repeating sequence), block copolymers (having two or more individual types of monomer segments connected by a covalent bond), or random copolymers (having two or more monomer units randomly arranged with respect to one another). In certain embodiments, polymeric resins can comprise binary copolymers (i.e., comprising two types of repeating monomer units). In certain embodiments, polymeric resins are terpolymeric (i.e., comprising three types of repeating monomer units).

The compositions and molecular weights of the polymers in a particular resin can vary, as generally understood and various grades can be used in the method described herein. It is generally understood that, as will be described in further detail herein below, the grade of a resin is correlated to its viscosity in extrusion and, therefore, selection of a particular resin grade/viscosity may affect the process. Resins of various molecular weights/grades can be relevant to the coated optical elements provided herein, depending on consideration of other processing parameters. In some embodiments, the resins (and thus the coatings) disclosed herein can comprise polymers with various fillers contained therein, many of which are commercially available. In some embodiments, the coatings disclosed herein are prepared from substantially pure resins, with no materials (e.g., fillers) intentionally added. Certain such resins have less than about 30%, less than about 20%, less than about 10%, less than about 5%, less than about 2%, less than about 1%, or less than about 0.5% by weight of added material, including resins with no added material, e.g., resins consisting essentially of the named polymer.

Generally, optical fibers comprise a central core and a cladding material, where both the central core and cladding material comprise silica. The core-to-cladding diameter ratio of the cables is not particularly limiting; however, it is generally understood that the cladding diameter is advantageously and commonly significantly larger than the core diameter. Although the disclosure focuses on optical elements comprising glass/silica-based fibers, it is not intended to be limited thereto and the disclosure in some embodiments is relevant in the context of plastic optical fibers

(POFs) as well. Fibers can be, e.g., single-mode fibers, graded mode fibers, or multi-mode fibers; it is noted that, according to the present disclosure, the specific type of optical fiber is not particularly limited.

"Optical fiber" as used herein includes fibers with a central core and cladding that is covered with a protective, primary buffer coating or that is uncovered (e.g., consisting only of a central core and cladding). The primary buffer coating, where present, can comprise various materials and is generally polymeric. The composition of the primary buffer coating is not particularly limited in the context of the disclosed coated optical elements. Advantageously, in some embodiments, the composition of the primary buffer coating is such that it is capable of withstanding elevated temperatures (e.g., greater than about 170°C) without significant degradation. In certain specific embodiments, the primary buffer coating withstands such temperatures for at least 24 hours without significant degradation as shown, e.g., by thermogravimetric analysis.

In some embodiments, the primary buffer coating is a polyimide or derivative or copolymer thereof. Other exemplary primary buffer coatings include, but are not limited to, silicone and high temperature acrylate (HTA) materials. In some embodiments, the primary buffer coating is relatively thin, e.g., with an average thickness of less than about 50 microns and/or in some embodiments, the primary buffer coating does not comprise a thermoplastic polymer.

FIGs 1A and IB depict one exemplary coated optical fiber 50 provided in accordance with the present disclosure, comprising a core 2, a cladding 4, a primary buffer coating 6 in contact with the cladding, and a thermoplastic polymeric tight buffer coating 8. FIGs. 2A and 2B depict another exemplary coated optical fiber 60, comprising a core 2, a cladding, 4, and a thermoplastic polymeric tight buffer coating 8 in contact with the cladding. The embodiment shown in FIGs. 2A and 2B notably does not include a primary buffer coating in contact with the cladding. FIGs. 3A and 3B depict exemplary coated fiber optic cables 70. Fiber optic cables are bundles of two or more optical fibers (which can comprise primary buffer coatings 6 as shown), which may optionally include various other components, including, but not limited to, support elements. The thermoplastic polymeric tight buffer coatings 8 of the coated optical fibers and fiber optic cables provided herein are considered to be "tight buffers," as they are generally in direct contact with and attached to the underlying layer (e.g., the primary buffer coating 6 or the cladding 4).

Advantageously, with the use of such "tight buffer" coatings little to no gap is present between the thermoplastic polymeric coating 8 and the underlying material (e.g., the primary buffer coating 6 or the cladding 4).

It is noted that the coated optical elements provided herein, in addition to one or more optical fibers, may comprise any number of additional components. For example, coated optical elements can further comprise (e.g., within the outer polymeric coating) various other coating layers (e.g., polymeric coatings), support elements (e.g., wires, cables, rods, yarn), and the like.

The diameters of the final coated optical elements of the present disclosure are not particularly limited. Although not limited thereto, it is noted that the diameter of some common optical fibers can range from about 50 to about 250 microns (based on cladding diameter and/or primary buffer coating diameter), and common fiber optic cables are somewhat larger in diameter, as they generally comprise at least one fiber and at least one other component (e.g., a strengthening component) or at least two fibers. Again, although not limited thereto, the thermoplastic polymeric tight buffer coating (and, thus, the diameter of the coated substrate) disclosed herein can vary. In some embodiments, the average coating thickness is at least about 50 microns (e.g., about 50 microns to about 1000 microns) or at least about 100 microns and in some embodiments, the average coating thickness is about 250 microns to about 900 microns. Interestingly, in various embodiments, attenuation is not significantly affected by the thermoplastic polymeric tight buffer coating thickness although, with traditional coated optical elements, attenuation has been previously noted to decrease with increasing outer polymeric coating thickness. As such, the thickness of the thermoplastic polymeric tight buffer coating on the coated optical elements disclosed herein can vary widely, while maintaining attenuation and other physical properties of the coated optical elements within the ranges generally disclosed herein.

Advantageously, the coated optical elements disclosed herein exhibit good optical properties and in particular, low attenuation over a length of the coated substrate (optical element). Attenuation is generally the loss of optical power along a fiber as a result of absorption, scattering, bending, and other loss mechanisms as light travels through the fiber. Attenuation is commonly reported at a particular wavelength, in units of decibels per kilometer (dB/km). Low attenuation values are desirable because longer lengths of fiber can be used while allowing for the passage of a suitable amount of light therethrough. All attenuation values provided herein, unless otherwise noted, are reported in dB/km at 1550 nm based on LSA (least squares attenuation loss) analysis. As will be described in greater detail herein below, microbending (giving high attenuation) is commonly introduced into optical elements by application of coating(s) thereto, e.g., via application (extrusion) of a thermoplastic polymeric buffer coating (which typically puts stress on the underlying optical fiber). The inventors have demonstrated herein the ability to control various features of the extrusion process to provide coated optical elements comprising a thermoplastic polymeric tight buffer coating wherein the coated optical elements do not exhibit significant microbending, and thus exhibit low attenuations (e.g., falling within the ranges outlined in the present disclosure).

Previously known coated optical elements exhibit substantially higher attenuation (i.e., more light loss over a given length of optical element) than the corresponding uncoated optical element (e.g., at least about 2 times that of the corresponding uncoated optical element) due, e.g., to microbending within the optical fiber (which, as noted above, is understood to arise (at least in part) from stress/compression associated with application of the coating(s) thereto). However, coated optical elements provided in accordance with this disclosure, in some embodiments, exhibit an attenuation that is no more than about 100%, no more than about 75%, no more than about 50%, or no more than about 25% above the attenuation of the corresponding uncoated optical element. In some embodiments, the coated optical element exhibits attenuation that is even lower (i.e., closer to the corresponding uncoated optical element), including an attenuation of no more than about 10% above that of the corresponding uncoated optical element, no more than about 5% above that of the corresponding uncoated optical element, and even an attenuation roughly equal to that of the corresponding uncoated optical element. In fact, in certain embodiments, the coated optical element exhibits attenuation that is less than that of the corresponding uncoated optical element.

Coated optical elements as disclosed herein often exhibit good stability in attenuation at various temperatures. It is generally known that attenuation exhibited by a given optical element is affected by the temperature at which the optical element is used/tested. In particular, many optical elements exhibit significantly higher attenuation at a given temperature after thermal cycling (e.g., to the maximum continuous use temperature for the buffer material).

Advantageously, as demonstrated in Example 1, a PEEK thermoplastic tight buffer-coated optical element according to the present disclosure did not exhibit a significant change in attenuation after thermal cycling. In one embodiment, a coated optical element is provided which exhibits little to no attenuation loss after thermal cycling to about 170°C, about 200 °C, or about 250 °C (or even higher) and cooling to room temperature. For example, such thermoplastic polymer tight-buffer coated optical elements may, in some embodiments, exhibit an attenuation at room temperature after thermal cycling (i.e., raising the temperature of the coated optical element to a given temperature and cooling back to room temperature) that is about 2 times that of the as-produced thermoplastic polymer tight buffer-coated optical fiber at room temperature (i.e., before thermal cycling) or less, including about 1.5 times or less, about 1.2 times or less, and about 1.1 times or less, e.g., an attenuation of about 1 to 2 times that of the corresponding uncoated optical fiber. In preferred embodiments, the attenuation after thermal cycling is at the lower end of that range, e.g., an attenuation of about 1 to about 1.5 times that of the as-produced thermoplastic polymer tight buffer-coated optical fiber, about 1 to about 1.2 times that of the as-produced thermoplastic polymer tight buffer-coated optical fiber, or about 1 to about 1.1 times that of the as-produced thermoplastic polymer tight buffer-coated optical fiber. In certain embodiments, such values are relevant even after multiple heating and cooling cycles (e.g., 2 heat/cool cycles, 3 heat/cool cycles, 4 heat cool cycles, etc.).

In some embodiments, the attenuation after temperature cycling is reported by comparison to the attenuation of the uncoated optical element. The "uncoated optical element" in this context is analogous to the thermoplastic polymer tight buffer-coated optical element, but without the thermoplastic polymer tight buffer coating thereon. For example, when the thermoplastic polymer tight buffer-coated optical element includes a core, cladding, primary buffer coating, and

thermoplastic polymer tight buffer coating, the "uncoated optical element" used for comparison includes a core, cladding, and primary buffer coating. When the thermoplastic polymer tight buffer- coated optical element includes a core, cladding, and thermoplastic polymer tight buffer coating, the "uncoated optical element" used for comparison includes a core and cladding. For example, certain thermoplastic polymer tight buffer-coated optical elements provided herein, after thermal cycling (e.g., over 1 heat/cool cycle) exhibit attenuation values of less than about 150% the attenuation values of the corresponding uncoated optical elements (which have not been subjected to thermal cycling). In various embodiments, such coated optical elements provided herein, after thermal cycling, exhibit attenuation values of less than about 125%, less than about 110%, less than about 105%, or less than about 100% the attenuation value of the corresponding uncoated optical element (which has not been subjected to thermal cycling).

In certain embodiments, other physical properties of the thermoplastic polymer tight buffer- coated optical elements disclosed herein are significant as well. For example, in some embodiments, the thermoplastic polymeric tight buffer coating can exhibit decreased shrinkage (e.g., measured as a change in length and/or a change in diameter using high precision measurement techniques) when heated to a relatively high temperature (e.g., about 300°C) relative to other known coated optical elements. Exemplary shrinkage values (after a single heating and cooling cycle) include, but are not limited to, shrinkages of less than about 50%, less than about 25%, less than about 15%, less than about 10%, less than about 5%, less than about 2%, less than about 1%, or less than about 0.5% in length and/or diameter as compared to the material at room temperature. See Example 2 for data relating to shrinkage for certain exemplary PEEK-coated optical fibers.

High shrinkage is disadvantageous, as it can cause microbending which, in the optical fiber, can dramatically increase attenuation. See, e.g., D. Hardy (W.L. Gore & Associates), "Space Flight Heritage of Optical Fiber Cables, available at https://nepp.nasa.gov/DocUploads/8025759D-3FAA- 483B-BCA4685A1ECDC286/Hardy%20.pdf, which is incorporated herein by reference in its entirety (describing a PEEK over polyimide-coated optical fiber to exhibit significant microbending). As described in the reference, if a thermoplastic polymeric tight buffer coating is heated to a temperature near or above its glass transition temperature, stress relaxation occurs, and for axially stressed layers (e.g., extruded buffer coatings), axial shrinkage will occur when the coating is cooled, which increases the compressive force that the fiber experiences. High modulus primary and secondary buffers (i.e., primary buffer coatings and polymeric tight buffer coatings) are noted therein to be disadvantageous for shock absorption and decreasing this compressive force and, specifically, a PEEK/polyimide combination is noted in the reference to exhibit particularly high compressive force (giving rise to high attenuation values). This understanding has been the standard understanding in the industry, i.e., that a "shock absorbing layer" comprising a polymer with a relatively low modulus is beneficially incorporated within coated optical elements (particularly as a primary buffer coating) to decrease the effect of thermal cycling-induced shrinkage on attenuation of the underlying optical element.

Additionally, high modulus primary and secondary coatings are understood to have relatively high coefficients of linear thermal expansion (with respect to glass, e.g., an order of magnitude difference), which increases attenuation loss. As such, thin, hard, high-modulus primary buffer coatings have been traditionally understood to transmit the thermal expansion of a surrounding thermoplastic polymeric buffer coating to the underlying optical element more readily. Shock absorbing layers, i.e., polymeric layers with relatively low modulus (e.g., acrylates or derivatives thereof, as referenced in the Hardy document above), are generally understood to be beneficial to dampen this effect as well.

Notably, in contrast to these teachings and understanding in the art, the inventors of the presently claimed subject matter have found that coated optical elements (e.g., coated optical elements with high modulus primary buffer coatings, such as a coated element with a polyimide primary buffer coating and a PEEK thermoplastic polymeric tight buffer coating) can exhibit surprisingly low microbending/attenuation even after thermal cycling. Although, based on common understanding in the art, one would expect such compositions to be sensitive to thermal cycling (exhibiting high shrinkage and thus, high attenuation), when prepared in consideration of the features outlined herein, coated optical fibers (including fibers with both PI primary buffer coatings and PEEK thermoplastic polymeric tight buffer coatings) can be provided that exhibit particularly low attenuation values after thermal cycling.

Interestingly, it is also noted that, for typical coated optical elements, concentricity is understood to affect attenuation. Concentricity is a measure of the uniformity of the cross-section of the coated substrate, and describes how well aligned the layers within the coated substrate are with respect to each other. Concentricity relates to the individual circles of the cross-section of the coated substrate and perfectly concentric coated substrates share the same center or axis and thus have uniform layer thicknesses around the core. Each layer can be described by its

concentricity/eccentricity, i.e., how close the center/axis of that layer in cross-section is to the center/axis of the substrate (e.g., optical fiber) in cross-section. It is generally understood that optical elements exhibiting decreased concentricity/increased eccentricity with respect to one or more of the layers contained therein increased attenuation (higher light loss). In the Hardy reference incorporated herein above, it is noted that coating and buffer eccentricity plays an important role in microbending. However, the inventors have found that, with respect to the presently disclosed fibers, concentricity surprisingly does not have a significant effect on microbending/attenuation. In particular, even at values as low as 30% concentricity, coated optical fibers are provided herein which exhibit attenuation values falling within the disclosed ranges (e.g., having attenuation values comparable to those of the uncoated optical element).

Although the disclosed coated optical elements can have concentricities of about 100% (with respect to the primary buffer coating and the thermoplastic polymeric tight buffer coating, based on the center/axis of the optical element), the disclosure uniquely also provides coated optical elements with lower concentricities, having attenuation values falling within the ranges disclosed herein, even after thermal cycling. For example, in some embodiments, coated elements exhibiting satisfactory attenuation values (falling within the disclosed ranges) are provided with concentricities less than about 90%, less than about 80%, less than about 70%, and less than about 50% (e.g., concentricities of about 30% to about 100% about 30% to about 90%, and about 30% to about 80%).

The coated optical elements disclosed herein are generally prepared by extruding a thermoplastic polymer resin onto an optical element to form a thermoplastic polymeric tight buffer coating thereon.

As referenced herein above, the underlying optical element can comprise a primary buffer coating thereon, which is generally capable of withstanding further processing (e.g., the application of the molten thermoplastic polymeric coating thereto to give the thermoplastic polymeric tight buffer coating thereon). As such, selection of an optical element and, in particular, selection of the primary buffer coating associated with an optical element, may require consideration of further processing steps and, particularly, consideration of the melting point of the thermoplastic polymeric material which will be applied thereto (to form the thermoplastic polymeric tight buffer coating). In various embodiments disclosed herein, PI or a derivative or copolymer thereof (or a similar, high modulus polymer) is selected as the primary buffer coating, as such materials are known to be able to withstand the application of high temperature (e.g., as required for the application of a thermoplastic polymeric tight buffer coating and/or as required for the ultimate use of the coated optical element). In some embodiments, the optical element comprises no primary buffer coating thereon (i.e., it comprises only a core and cladding).

Selection of the specific materials and the specific extrusion parameters employed in the process may, in some embodiments, affect the quality of the resulting coated optical elements and, in particular, may affect the extent of microbending/attenuation of the resulting coated optical elements. The materials and extrusion parameters (as described herein below with respect to shear rate) can also affect the onset of melt fracture. Melt fracture is created by extrusion of the thermoplastic buffer material in excess of a condition/material-specific shear rate and manifests itself in the form of micro surface roughness and/or irregularities. Melt fracture can be readily observed visually under a microscope, and one of skill in the art is familiar with determination of whether a material demonstrates melt fracture or not (with melt fractured materials exhibiting a roughened, or

"sharkskin" appearance). It is generally understood that visual evidence of melt fracture is indicative of stress/strain on the underlying optical element, causing irregularities in the optical element and leading to microbending/higher attenuation values. Thus, melt fracture is advantageously minimized/avoided in producing coated optical elements according to the present disclosure.

Melt fracture and negative effects on optical element attenuation (e.g., arising from microbending) are understood to arise in some cases from material variables (resin grade/viscosity) and/or processing variables. Advantageously, the preparation of coated optical elements as disclosed herein is conducted so as to minimize microbending/attenuation effects and/or to minimize melt fracture. As such, melt fracture concerns can generally be avoided/cured by either material means (e.g., modifying the grade/viscosity of the thermoplastic polymer resin used) or mechanical means (e.g., by modifying the processing variables). These means for controlling will be described in further detail herein below.

In certain embodiments, the grade of the thermoplastic polymer resin used in the extrusion to apply the thermoplastic polymeric tight buffer coating may impact the quality of the coated optical element. The viscosity of the resin (which is related to the resin grade), as referenced above, is one consideration in ensuring that extrusion is conducted so as to minimize/avoid melt fracture. One of skill in the art would recognize that material modifications can be made, if desired, to avoid the onset of melt fracture (e.g., selecting a resin with a lower/higher viscosity).

The extrusion process is advantageously operated so as to maintain the shear stress on the molten polymer below the critical shear stress value for melt fracture to obtain coated optical elements of sufficient quality. Values for critical shear stress and thus shear stress maxima in the disclosed extrusion processes can vary and are dependent upon the process as a whole. Critical shear stress can be calculated or can be avoided during process development (e.g., by looking for evidence of melt fracture, as described above and adjusting parameters such that melt fracture is not visible). It is generally known that polymers exhibit non-Newtonian stress behavior and as such, it is understood that in the context of the disclosed process, critical shear stress is difficult to determine. Shear stress can be modified (i.e., reduced), in a variety of ways, including, without limitation, by reducing the extrusion/throughput rate, increasing melt temperature, or reducing melt viscosity, as is generally recognized by one of skill in the art. Accordingly, such parameters can, in some embodiments, be advantageously modified to maintain the shear stress value below the critical shear stress value for melt fracture, to obtain a coated optical element exhibiting the referenced attenuation values referenced herein above.

With respect to the extrusion rate, one means for reducing melt fracture during extrusion is to reduce the line speed. In some embodiments, reducing the line speed (i.e., the rate at which the optical element is drawn through the extrusion apparatus) can reduce the amount of shear imparted into the extruded thermoplastic polymer and can reduce surface defects (and microbending in the underlying optical element, which leads to negative effects on attenuation). See Example 5, providing one example of the effect of dropping the line speed to correct a critical shear concern.

With respect to the melt temperature, another means for reducing the melt fracture during extrusion is to increase the temperature of the molten polymer (by increasing extruder or die apparatus temperatures), reducing the polymer viscosity and thereby lowering the shear stress imparted to the extruded thermoplastic polymer. Again, such control can provide decreased microbending in the underlying optical element, giving coated optical elements with better attenuation.

It can also be beneficial to control fiber tension and preheat in the coating process. Fiber tension can allow for a controlled stretch on the optical element (and the primary buffer coating thereon, if included) so that after the thermoplastic polymeric tight buffer coating is extruded thereon, the cooling of the thermoplastic polymer and elastic restoration of the glass (or other material from which the optical element is made) can be similar to each other. When materials cool, they shrink and, when the underlying optical element (e.g., comprising glass) is stretched, then following the application of a molten thermoplastic polymer buffer coating thereto, both materials can relax together, which can decrease axial compression/microbending in the optical element. Typical means for controlling fiber tension are paying the fiber off into a tension loop (commonly referred to as a "dancer" loop) prior to the extrusion of a thermoplastic polymeric tight buffer coating onto the optical element, wherein weight or stretch is added to the optical element prior to the coating. Tension can be modified/controlled by a clutch and a loop can be used/not used in various embodiments and varying amounts of tension/weight can be applied. Advantageously, throughout a run, the tension is set to a desired level and this level is stably maintained. See Example 4, providing one example of the effect of modifying tension to correct a critical shear concern.

Regarding fiber preheat, it is shown in some embodiments that heating the optical element (e.g., such that the outer surface thereof is heated, i.e., the cladding of an uncoated optical fiber or the primary buffer coating of a coated optical fiber) prior to entering the extrusion apparatus can have a significant effect on the attenuation of the resulting coated optical element. Although not intending to be limited by theory, it is believed that this property is due to slower cooling of the thermoplastic layer where it comes in contact with the coated or uncoated optical element surface (an annealing of sorts) that allows the stress to relieve itself after the thermoplastic polymeric tight buffer coating is applied. Relevant values for fiber preheat in the methods outlined herein are low enough that the primary buffer coating is not degraded, but high enough that substantial axial stress buildup due to differential cooling rates is avoided. See Example 3, providing certain exemplary results of preheating experiments.

Each of the factors referenced herein above with respect to the method of preparation of a coated optical fiber of the present invention is advantageously considered to afford a coated optical fiber exhibiting the attenuation values outlined herein. Each factor, by itself, is not a singular solution to arriving at such a coated optical fiber; rather, it is the sum of each of these considerations that, with an understanding of the principles outlined herein above, can be adjusted to prepare such a coated optical fiber. One of skill in the art is generally aware of how these parameters can be manipulated to avoid melt fracture, and the inventors have recognized that by doing so, they are able to prepare coated optical elements with suitable attenuation values using materials that have been heretofore considered to be largely incompatible with the production of coated optical elements with suitable attenuation values (e.g., including, but not limited to, coated optical elements comprising hard and/or thin primary buffer coatings, as detailed above).

EXAMPLES

Aspects of the present invention are more fully illustrated by the following examples, which are set forth to illustrate certain aspects of the present invention and are not to be construed as limiting thereof.

EXAMPLE 1: Preparation of coated element and attenuation analysis: thermal cycling

A PI/PEEK coated optical element is prepared using a 18mm single screw extruder running

Victrex 381g resin with cylinder temperatures of 690 F, 710 F, and 710 F. The 125 μπι (micron) cladding, 9 μπι (micron) core single mode PI coated optical element, Nufern R1550B-P, is delivered to the extruder crosshead through a tension control system with a setpoint of 150 grams, and is then subjected to a preheat step set at 750 F. The fiber passes to and through an extrusion crosshead where the thermoplastic PEEK is paired with the optical element. The coated fiber (i.e., paired fiber/PEEK) travels through an outer diameter (OD) measurement gauge, and to a dual caterpillar belt haul off. The fiber is then run through a secondary tension control system and wound on a finished package at the end of the line.

The resulting 0.025" outer diameter optical fiber with a 9 micron core diameter and 125 micron cladding diameter, with a 150 micron outer diameter (OD) polyimide coating and an outer polymer coating of PEEK is evaluated for energy loss/attenuation using an optical time domain reflectometer (OTDR). The PEEK-coated fiber with 0.025" diameter exhibited a 0.531 dB/km attenuation based on a least squares attenuation loss (LSA) analysis, which is better than the uncoated (bare) optical fiber. This sample did not exhibit any thermal cycling effects. At room temperature, the attenuation of this PEEK-coated fiber at 1550 nm was 0.53 dB/km. The PEEK-coated sample was heated to 200°C, and the attenuation at 1550 nm at that temperature was 0.58 dB/km. The sample was cooled down to room temperature and the attenuation at 1550 nm after this heating cycle was 0.72 dB/km. Surprisingly (and in contrast to many known coated optical fibers), this coated fiber exhibited low attenuation at room temperature after thermal cycling.

EXAMPLE 2: Shrinkage analysis

Heat-induced shrinkage of a PEEK-coated optical fiber sample in accordance with the present disclosure was evaluated by first using a razor blade to remove a layer of the PEEK material from the coated fiber (to ensure no effect of the underljang optical fiber). The PEEK material was then cut into roughly 3" sample lengths, and the length of each sample was measured. Each sample was then exposed to a temperature of 200°C (see Table 1) or 300°C (see Table 2) for 15 minutes, and after cooling, the length of each sample was again measured. Shrinkage percent was calculated based on these values according to the following formula: (Original Length - New Length) x 100

Shrinkage % =

Original Length

Typically, significant shrinkage is observed under such conditions for PEEK-coated optical fibers (believed to be due, at least in part, to the stress associated with typical extrusion processes, as referenced previously in this application). However, when samples according to the present disclosure were analyzed, low shrinkage was observed (believed to be due, at least in part, to controlling the extrusion parameters, e.g., avoiding onset of melt fracture, as outlined above to minimize stress on the underlying fiber). The following shrinkage data was obtained:

Table 1: Shrinkage at 200°C

Table 2: Shrinkage at 300°C

The average shrinkage at both 200°C and 300°C is minimal, i.e., 0.35% shrinkage at both temperatures.

EXAMPLE 3: Preparation of coated element and attenuation analysis: fiber pre-heat effects

A PI/PEEK coated optical element is prepared using a 18 mm single screw extruder running Victrex 381g resin and cylinder temperatures of 690 F, 710 F, and 710 F. The 125 μπι (micron) cladding, 9 μπι (micron) core single mode PI coated optical element, Nufern R1550B-P, is delivered to the extruder crosshead through a tension control system with a fixed setpoint of 110 grams, and then a variable preheat step is conducted on a single length of fiber set at 400 F, 500 F, 600 F, and 700 F. The fiber then passes to and through an extrusion crosshead where thermoplastic PEEK is paired with the optical element. The coated fiber (i.e., paired fiber/PEEK) travels through an outer diameter (OD) measurement gauge, and to a dual caterpillar belt haul off. The fiber is then run through a secondary tension control system and wound on a finished package at the end of the line. The approximate line speed for the conditions described above was 35 feet per minute (FPM).

The resulting .025" outer diameter optical fiber with a 9 micron core diameter and 125 micron cladding diameter, with a 150 micron outer diameter (OD) polyimide tight buffer coating and a PEEK thermoplastic polymeric tight buffer coating is evaluated for energy loss/attenuation at 1550 nm using an optical time domain reflectometer (OTDR). The attenuation of the product prepared by preheating at 400 F was catastrophic, and no reading was obtainable. The product prepared by preheating at 500 F exhibited an attenuation of 54.8 dB/km, the product prepared by preheating at 600 F exhibited an attenuation of 12.28 dB/km, and the product prepared by preheating at 700 F exhibited an attenuation of 18.2 dB/km. Readings could only be obtained by tracing the fiber from the 700 F end on the ODTR, as light would not pass from the 400 F end through the optical core in measureable amounts. This testing was not optimized, but was a screening test that demonstrated that improper settings on fiber preheat can cause catastrophic failures in attenuation. Although not intending to be limited, in some embodiments, the preheat temperature is advantageously sufficient to slow the thermoplastic polymeric tight buffer coating from cooling at the interface with the underlying prior to extrusion, but below the temperature at which the primary buffer coating degrades.

EXAMPLE 4: Preparation of coated element and attenuation analysis: tension effects

A PI/PEEK coated optical element is prepared using a 18mm single screw extruder running

Victrex 381g resin with cylinder temperatures of 690 F, 710 F, and 710 F. The 125 μπι (micron) cladding, 9 μπι (micron) core single mode PI coated optical element, Nufern R1550B-P, is delivered to the extruder crosshead through a payoff tension control system with a variable setpoint of 400 g and 500 g, and fixed preheat of 700 F. The fiber then passes to and through an extrusion crosshead where the thermoplastic PEEK is paired with the optical element. The coated fiber (paired fiber/PEEK) travels through an outer diameter (OD) measurement gauge, and to a dual caterpillar belt haul off. The fiber is then run through a secondary tension control system and wound on a finished package at the end of the line. The approximate line speed for the conditions described above was 35 FPM.

The resulting .016" outer diameter optical fiber with a 9 micron core diameter and 125 micron cladding diameter, with a 150 micron outer diameter (OD) polyimide tight buffer coating and a PEEK thermoplastic polymeric tight buffer coating is evaluated for energy loss/attenuation using an optical time domain reflectometer (OTDR). The 1550 nM attenuation for the coated fiber prepared using 400 g pretension was 38.96 dB/km and the 1550 nM attenuation for the coated fiber prepared using 500 g pretension was 17.442 dB/km. This shows that adding stress to the glass fiber is one means of further improving attenuation by allowing elastic deformation to reduce the compression effects of the cooling polymer. It is noted that the attenuation values of this Example are higher than those of Example 1. The associated factors (as referenced herein throughout the disclosure and examples) can be controlled/optimized further to give the unexpected result of providing a thermoplastic tight buffer coated optical element exhibiting low attenuation in combination with high thermal stability.

EXAMPLE 5: Preparation of coated element and attenuation analysis: effect of exceeding polymer critical shear rate for conditions and onset of melt fracture

A PI/PEEK coated optical element is prepared using a 18mm single screw extruder running Victrex 150g resin at cylinder temperatures of 690 F, 710 F, and 710 F. The 125μπι (micron) cladding, 9 μπι (micron) core single mode PI coated optical element, Nufern R1550B-P, is delivered to the extruder crosshead through a payoff tension control system with a setpoint of 100 g, and fixed preheat of 750 F. The fiber then passes to and through an extrusion crosshead where thermoplastic PEEK is paired with the optical element. The coated fiber (paired fiber/ PEEK) travels through an outer diameter (OD) measurement gauge, and to a dual caterpillar belt haul off. The fiber is then run through a secondary tension control system and wound on a finished package at the end of the line. The approximate line speed for the conditions of this Example was 20 FPM.

The product had evidence of exceeding critical shear by showing a rough surface texture under magnification (indicative of melt fracture). The product with melt fracture had an attenuation of 14.43dB/km @ 1550 nM. All line parameters were left constant and a very minor adjustment as considered by one skilled in the art was made to the extrusion die apparatus temperature by raising the heat 10 degrees F to better match the extrusion and polymer conditions regarding critical shear rate. In addition to the temperature change the linespeed was lowered to 15 FPM to further lower the polymer shear rate. These two process adjustments were enough to eliminate the melt fracture phenomenon occurring and restoring attenuation to a value of 0.708 dB/km with no other alterations to the process. This shows that, by optimizing extrusion shear conditions for the particular polymer and mass throughput rate, attenuation can be significantly affected (94% reduction).

EXAMPLE 6: Preparation of coated element and attenuation analysis: effect of concentricity

Once the conditions are optimized for a material and optical fiber, many of the factors taught in industry as being key to attenuation do not affect the product attenuation as discovered by the inventors and described herein, one being concentricity of layers. A PI/PEEK coated optical element is prepared using a 18mm single screw extruder running Victrex 381g resin using cylinder temperatures of 690 F, 710 F and 710 F. The 125 μπι (micron) cladding, 9 μπι (micron) core single mode PI coated optical element, Nufern R1550B-P, is delivered to the extruder crosshead through a payoff tension control system with a setpoint of 165 g, and fixed preheat of 750 F. The fiber then passes to and through an extrusion crosshead where the thermoplastic PEEK is paired with the optical element. The resulting coated fiber (paired fiber/PEEK) travels through an outer diameter (OD) measurement gauge, and to a dual caterpillar belt haul off. The fiber is then run through a secondary tension control system and wound on a finished package at the end of the line. The approximate line speed for this example was 28.8 FPM. Two samples were made, with the only changes being the centering of the optical element with PI primary buffer coating within the PEEK thermoplastic polymeric tight buffer coating. To modify the concentricities during production of the coated optical fiber, the extrusion die apparatus is moved relative to the optical element being coated, using an off-center condition of the die opening to introduce eccentricity. A high concentricity sample (having a concentricity of 81.5% using the formula: % Concentricity = (Min tight buffer thickness / Max tight buffer Thickness) x 100 had an attenuation at 1550 nM of 0.158 dB/km. A low concentricity sample (having a concentricity of 39.5% (against the teachings of industry, e.g., Hanson, "Origin of Temperature Dependence of Microbending Attenuation in Fiber Optic Cables," Fiber and Integrated Optics, 3:2-3, 113-148 (1980), which is incorporated herein by reference in its entirety)) had an attenuation at 1550 nM of 0.603 (which is lower than the uncoated optical fiber itself). Both of these coated samples tested thermally stable as claimed in the invention when cycling up to 200 °C.

Incorporation

The present patent application claims the benefit of the filing date of United States provisional patent application no. 62/323,195, filed April 15, 2016, and United States provisional patent application no. 62/374,110, filed August 12, 2016, and United States non-provisional patent application no. 15/291,323, filed October 12, 2016; and all three of the foregoing are incorporated herein by this reference in their entireties.

Many modifications and other embodiments of the invention will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing description. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.