Reduction of axial viscosity asymmetry to reduce geometrical distortions during optical fiber draw

Abstract

An optical fiber or a preform from which the fiber may be drawn may include an outer cladding layer, an inner cladding layer, a first core, and a second core. Each core may be in contact with at least the inner cladding layer and under frozen-in compressive axial stress. The refractive index of the inner cladding layer may be less than the refractive index of the outer cladding layer.

Description

IN THE UNITED STATES PATENT AND TRADEMARK OFFICEAPPLICATION FOR PATENTREDUCTION OF AXIAL VISCOSITY ASYMMETRY TO REDUCE GEOMETRICAL DISTORTIONS DURING OPTICAL FIBER DRAWCROSS-REFERENCE TO RELATED APPLICATION

[0001] The benefit of and priority to U.S. Provisional Patent Application No.63 / 575,565, filed April 5, 2024, entitled “REDUCTION OF AXIAL VISCOSITY ASYMMETRY TO REDUCE GEOMETRICAL DISTORTIONS DURING DRAW,” and U.S. Provisional Patent Application No. 63 / 656,210, filed June 5, 2024, entitled “REDUCTION OF AXIAL VISCOSITY ASYMMETRY TO REDUCE GEOMETRICAL DISTORTIONS DURING DRAW,” is hereby claimed, and the contents thereof incorporated herein by this reference in their entirety as if fully set forth below and for all applicable purposes.BACKGROUND

[0002] A draw tower is an apparatus used in optical fiber manufacturing processes.In a draw tower, glass material is drawn from a large-diameter glass structure known as a preform into a small-diameter fiber. This is achieved by heating up the glass to a temperature (typically on the order of 2000 degrees Celsius) where it behaves as a viscous fluid and thus can be drawn down to the desired dimensions. As the fiber is drawn, it moves (generally in a vertically downward direction) through a sequence of processing stations, which may include cooling, heating, coating, curing, and other processing stations, before the fiber is routed through a turning wheel and taken up on a spool at the end of the draw tower. The preform may comprise multiple layers, such as a core and a cladding, and the resulting fiber that is drawn from the core thus correspondingly comprises the same layers but in smaller diameters. Some types of fiber have an axisymmetric structure, i.e., consisting of concentric layers of generally circular cross-sectional shape, while other types, such as multi-core fiber (MCF) and polarization-maintaining fiber (PMF), have non-axisymmetric structures.

[0003] Fiber drawn from a preform with an axisymmetric structure tends to closely preserve the circular cross-sectional geometry of the preform. However, fiberdrawn from a preform with a non-axisymmetric structure may exhibit geometrical distortion or deformation with respect to the cross-sectional geometry of the preform, such as increased ovality of both the cladding and the fiber-internal elements (e.g., cores, stress areas, etc.)- Such distortion may be caused by non- axisymmetric viscosity of the molten glass layers during fiber draw.

[0004] Such distortions can substantially degrade the optical properties of the fiber. For example, such distortions may increase polarization-mode dispersion (PMD). In addition, the geometrical deformation itself can be a degradation of the fiber quality. At interconnect points, such as among other splices, geometrical deviations from the design values can result in elevated insertion loss, elevated crosstalk or reduced mechanical stability of the interconnect point. It would therefore be desirable to provide fiber having minimal geometrical distortion from a design (e.g., with respect to the cross-sectional geometry of the preform).SUMMARY

[0005] Exemplary’ embodiments of the present invention relate generally to optical fiber having reduced geometrical distortion and to methods for making such optical fiber and preforms from which such optical fiber may be made.

[0006] An exemplary method for making an optical fiber preform may include forming an outer cladding layer, forming an inner cladding layer, forming a first core, and forming a second core. Each of the first core and the second core may be in contact with the inner cladding layer, and each of the first core and the second core may be under frozen-in compressive axial stress. The refractive index of the inner cladding layer may be less than the refractive index of the outer cladding layer.

[0007] An exemplary method for making an optical fiber may include providing a preform and drawing a fiber from the preform using a draw tower. The preform may include an outer cladding layer, an inner cladding layer, a first core, and a second core. Each of the first core and the second core may be in contact with the inner cladding layer, and each of the first core and the second core may be under frozen-in compressive axial stress. The refractive index of the inner cladding layer may be less than the refractive index of the outer cladding layer.

[0008] An exemplary optical fiber may include an outer cladding layer, an inner cladding layer, a first core, and a second core. Each of the first core and the second core may be in contact with the inner cladding layer, and each of the first core and the second core may be under frozen-in compressive axial stress. The refractive index of the inner cladding layer may be less than the refractive index of the outer cladding layer.

[0009] Other systems, methods, features, and advantages will be or become apparent to one of skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the specification, and be protected by the accompanying claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention.

[0011] FIG. 1 is a cross-sectional view of a dual-core optical fiber structure showing an absence of distortion.

[0012] FIG. 2 is a cross-sectional view of a dual-core optical fiber structure showing an example of distortion.

[0013] FIG. 3 is a cross-sectional view of a dual-core optical fiber structure having features for reducing distortion.

[0014] FIG. 4 is a cross-sectional view of a dual-core optical fiber structure having features for reducing distortion and in which each core has multiple layers.

[0015] FIG. 5 illustrates a method for making optical fiber in a draw tower.DETAILED DESCRIPTION

[0016] As illustrated in FIG. 1, an optical fiber 100 may include a cladding layer102 and two cores 104 and 106 and is thus an example of a m ulti-core fiber (MCF) . A multi-core fiber is an example of a non-axisymmetric fiber. A circle 108 that is coaxial with the center axis (not shown) of the fiber 100 and concentric with the circular periphery of the cladding 102 is shown for reference purposes only anddoes not represent a feature of the optical fiber 100. Note that the centers or axes of the cores 104 and 106, which are indicated by cross symbols (“+”), intersect the circle 108 along the diameter of the circle 108. The geometry of the optical fiber 100, which may be characterized in this example by the locations of the circular cores 104 and 106 with respect to each other and the circular cladding 102, may be identical to the geometry of a preform (not shown) from which the fiber 100 was drawn. Such an absence of distortion between a fiber and the preform from which it was drawn may be desirable but may be difficult to achieve due to differences in the viscosity of the molten glass among the various layers or features as the fiber is being drawn.

[0017] A reference to the “preform draw viscosity” of a layer may be used herein to mean the viscosity of the layer around the glass transition temperature while the fiber was being drawn or otherwise processed in the draw tower. Although preform draw viscosity, i.e., the viscosity of molten glass in a draw tower, may be difficult to measure directly, axial stress profiles of the resulting fiber may be used as proxies for preform draw viscosity and may be measured in a fiber sample using conventional techniques well understood by one of ordinary skill in the art. (See, e.g., Aben, H. & Guillemet, C. Photoelasticity of Glass. (Springer, Berlin, Heidelberg, 1993) or Yablon, A. D. Optical and Mechanical Effects of Frozen-in Stresses and Strains in Optical Fibers. IEEE Journal of Selected Topics in Quantum Electronics 10, 300-311 (2004).) The method is based on the photoelastic effect, the effect that stress changes the refractive index. The refractive index profile of the fiber is measured with light of two different polarizations. The difference is then proportional to the axial stress profile.

[0018] As illustrated in FIG. 2, an optical fiber 200 may include a cladding layer202 and two cores 204 and 206 and is thus similar to the above-described optical fiber 100 (FIG. 1) except that in this example the center or axis of the core 204 is further out from the reference circle 208 (i.e., at a greater radial distance from the axis (not indicated in FIG. 2) of the fiber 200. The optical fiber 200 in this example may thus exhibit a type of distortion that may be referred to as ovality. Ovality refers to an atribute of an optical fiber in which the cross-section of a core or cladding deviates from a perfect circle. Although not shown in FIG. 2, the axis ofthe core 206 could similarly be fiirther out horn the reference circle 208, such that both of the cores 204 and 206 are further out. In other words, the whole structure may deform simultaneously during the draw process. With one or both cores 204 and 206 further out from the reference circle 208, the outer periphery of the cladding layer 202 may not be perfectly circular. Rather, the outer periphery of the cladding layer 202 may have a slightly oval shape (not depicted in FIG. 1 for purposes of clarity). In still other examples (not shown) of ovality, one or more layers or features other than, or in addition to, the outermost cladding layer, such as the cores themselves (i.e., their outer peripheries), could exhibit ovality.

[0019] Ovality in optical fiber is generally undesirable. In axisymmetric fiber, such as fiber consisting solely of concentric layers aligned with the fiber axis deformations such as ovality may only occur in cases where this symmetry is broken due to manufacturing imperfections. In contrast, in non-axisymmetric fiber, such as the optical fiber 100 and 200, this symmetry may be broken by design, which may result in non-axisymmetric viscous forces during the draw process and thus the fiber or portions thereof, such as the cladding, the cores, or other features, may deform or assume a less than perfectly circular shape during the process of manufacturing the fiber in a draw tower. The present invention may address undesirable ovality or other distortions and provide solutions for reducing such distortions.

[0020] As illustrated in FIG. 3, an optical fiber 300 may include an outer cladding layer 302, an inner cladding layer 310, a first core 304 and a second core 306. The inner cladding layer 310 is surrounded by the outer cladding layer 302. A reference circle 308 that is coaxial with the center axis (not shown) of the fiber 300 and within the inner cladding layer 310 is also shown. Although in the illustrated example the optical fiber 300 has two cores 304 and 306, in other examples such an optical fiber may have more than two cores.

[0021] The optical fiber 300 may include features that enable it to exhibit less ovality than the above-described fiber 200 (FIG. 2). It should be understood that although the features are described for purposes of clarity with respect to the fiber 300, a preform (not shown) from which the fiber 300 may be drawn may include these same features. Correspondingly, a fiber 300 drawn from a preform havingone or more such features may exhibit less ovality than the above-described fiber 200 (FIG. 2) drawn from a preform not having such features.

[0022] It should be understood in the following descriptions that the boundaries or peripheries of adjacent layers, i.e., layers that are in contact with one another, may be distinguished from each other or detected by measuring differences in dopant content of the glass, measuring differences in refractive index of the glass (e.g., resulting from differences in dopant content), measuring differences in axial stress profiles of the glass (e.g. resulting from the forces during the draw process), or by other methods. Such differences may also correspond to differences in preform draw viscosity. For example, a first layer having a higher fluorine (dopant) content than a second layer may be a characteristic of a fiber in which the first layer preform draw viscosity was (during the draw process) lower than the second layer preform draw viscosity. Also, for example, a first layer having a lower refractive index than a second layer may be a characteristic of a fiber in which the first layer preform draw viscosity was lower than the second layer preform draw viscosity. Conversely, a first layer having a lower fluorine (dopant) content than a second layer may be a characteristic of a fiber in which the first layer preform draw viscosity was higher than the second layer preform draw viscosity. And a first layer having a higher refractive index than a second layer may be a characteristic of a fiber in which the first layer preform draw viscosity was lower than the second layer preform draw viscosity.

[0023] In the fiber 300, the preform draw viscosity of the outer cladding layer 302 may be greater than the preform draw viscosity of the inner cladding layer 310. In addition, the preform draw viscosity of the inner cladding layer 310 may be equal or higher than the average viscosity of either core 304 or core 306. The term “average viscosity” refers to the preform draw viscosity of a core in an example in which the core has only one layer and refers to the average of the preform draw viscosities of all core layers in an example (not shown in FIG. 3) in which the core has more than one layer. The refractive index of the outer cladding layer 302 may be greater than the refractive index of the inner cladding layer 310, the fluorine dopant content of the outer cladding layer 302 may be less than the fluorine dopant content of the inner cladding layer 310, or both. These differences in one or moreof preform draw viscosity, refractive index and fluorine content between the outer cladding layer 302, the inner cladding layer 310, core 304 and core 306 may be an example of one or more characteristics of a fiber 300 that exhibits less ovality than the above-described fiber 200 (FIG. 2). That is, a fiber 300 drawn from a preform (not shown) having this same combination of one or more characteristics may exhibit less ovality than the above-described fiber 200 (FIG. 2). This may come from a reduction of the asymmetry of the viscous forces the glass experiences during the draw process.

[0024] In the fiber 300, each core 304 and 306 is in contact with both the inner cladding layer 310 and the outer cladding layer 302. A first portion of the outer periphery of the first core 304 is in contact with the inner cladding layer 310, while the remaining (second) portion of the outer periphery of the first core 304 is in contact with the outer cladding layer 302. Stated another way, the first portion of the first core 304 is surrounded by the inner cladding layer 310, while the second portion of the first core 304 is surrounded by the outer cladding layer 302.

[0025] The first core 304 and the second core 306 each may be under frozen-in compressive axial stress. The term “frozen-in stress” (e.g., “frozen-in compressive axial stress”) relates to mechanical stress in the fiber which originates from the draw process and stays stable in the fiber subsequently. The term “frozen-in” is therefore used to discriminate this inherent stress from any other stress which could be applied to the fiber subsequently with an external force. Stress from such an external force would, however, vanish again once the force is no longer applied. The term “compressive” means that material under this stress is shortened or compressed (commonly expressed in terms of negative stress values). Areas under compressive axial stress may correspond to a preform draw viscosity below the average preform draw viscosity of the whole fiber. Accordingly, a fiber 300 With an inner cladding layer 310 and outer cladding layer 302, and in which the cores 304 and 306 are under frozen-in compressive axial stress, may have reduced distortion as a result of reduced force asymmetries during the draw process. Such reduced force asymmetries during the draw process may be a result of the preform draw viscosity of the outer cladding layer 302 being greater than the preform draw viscosity of the inner cladding layer 310. In addition, such reduced forceasymmetries during the draw process may be a result of the preform draw viscosity of the inner cladding layer 310 being greater than or equal to the average preform draw viscosity of the core 304 or the core 306. As noted above, the dopant content of the layers may be selected to achieve these differences in preform draw viscosities.

[0026] In an example in which the preform draw viscosity of the inner cladding layer 310 is between the preform draw viscosity of the outer cladding layer 302 and the average preform draw viscosities of the first core 304 and second core 306, the asymmetry of the viscous forces may be reduced during the draw process (which are affected by the viscosity) in comparison to fiber 200 (FIG. 2). In the fiber 300, each of the first core 304 and the second core 306 being under frozen-in compressive axial stress and being in contact with both the inner cladding layer 310 and the outer cladding layer 302 may be an example of one or more characteristics of a fiber 300 that exhibits less ovality than the above-described fiber 200 (FIG. 2). That is, a fiber 300 drawn from a preform (not shown) having this same combination of one or more characteristics may exhibit less ovality than the above-described fiber 200 (FIG. 2). In some examples, the cores being under frozen-in compressive axial stress and being in contact with at least an inner cladding layer may also be an example of characteristics of a fiber that exhibits reduced ovality. That is, the cores contact at least the inner cladding layer but may also contact the outer cladding layer, and such contact with both the inner cladding layer and outer cladding layer may further contribute to the reduced ovality effect.

[0027] As illustrated in FIG. 4, an optical fiber 400 may include an outer cladding layer 402, an inner cladding layer 410, a first core 404 and a second core 406. A reference circle 408 that is coaxial with the center axis (not shown) of the fiber 400 and within the inner cladding layer 410 is also shown. The inner cladding layer 410 m ay be surrounded by the outer cladding layer 402. That is, the outer periphery of the inner cladding layer 410 may be in contact with the inner periphery of the outer cladding layer 402.

[0028] In the fiber 400, the preform draw viscosity of the outer cladding layer 402 may be greater than the preform draw viscosity of the inner cladding layer 410. Correspondingly, the refractive index of the outer cladding layer 402 may begreater than the refractive index of the inner cladding layer 410, the fluorine dopant content of the outer cladding layer 402 may be less than the fluorine dopant content of the inner cladding layer 410, or both. These differences in one or more of preform draw viscosity, refractive index and fluorine content between the outer cladding layer 402 and the inner cladding layer 410 may be an example of one or more characteristics of a fiber 400 that exhibits less ovality than the abovedescribed fiber 200 (FIG. 2). That is, a fiber 400 drawn from a preform (not shown) having this same combination of one or more characteristics may exhibit less ovality than the above-described fiber 200 (FIG. 2).

[0029] In the fiber 400, each core 404 and 406 is in contact with both the inner cladding layer 410 and the outer cladding layer 402. In this example, the first core 404 may have two or more layers, such as an outermost first core layer 412, an innermost first core layer 414, and another first core layer 415 between the outermost first core layer 412 and the innermost first core layer 414. The outermost first core layer 412 is in contact with the inner cladding layer 410. In comparing the fiber 400 (FIG. 4) with the fiber 300 (FIG. 3), it may be noted that the area of contact between the cores 404 and 406 and the inner cladding layer 410 (FIG. 4) is less than the area of contact between the cores 304 and 306 and the inner cladding layer 310 (FIG. 3). In still other examples (not shown), the cores may be entirely within the inner cladding layer.

[0030] In accordance with a feature of the solutions herein, the average refractive index of each core 404 and 406 may be below the refractive index of the outer cladding layer 410. In the illustrated example, the average core refractive index of the first core 404 is the average of the refractive indices of the outermost first core layer 412, the innermost first core layer 414, and the other first core layer 415. In other examples (not shown) having multi-layer cores, the average core refractive index is the average of the refractive indices of all core layers.

[0031] In accordance with another feature of the solutions herein, the outermost first core layer 412, the innermost first core layer 414, and the other first core layer 415 may be under frozen-in compressive axial stress. As described above, the frozen-in axial stress profile in the fiber may be a proxy for the glass viscosity profile during the draw process. Areas under compressive axial stress maycorrespond to a preform draw viscosity below the average preform draw viscosity of the whole fiber. Accordingly, a multi-core fiber with an inner cladding and outer cladding, and in which all layers of a core are under frozen-in compressive axial stress, may have reduced distortion as a result of reduced force asymmetries during the draw process. It may be noted that the axial stress on the cores 404 and 406 may be compressive (i.e., negative), and the axial stress of the outer cladding layer 402 may be tensile (i.e., positive).

[0032] Likewise, the second core 406 may have two or more layers, such as an outermost second core layer 416, an innermost second core layer 418, and another second core layer 417 between the outermost second core layer 416 and the innermost second core layer 418. The outermost second core layer 416 is in contact with the inner cladding layer 410. The average core refractive index of the second core 406 may be below the refractive index of the outer cladding layer 402. The average core refractive index of the second core 406 is the average of the refractive indices of the outermost second core layer 416, the innermost second core layer 418, and the other second core layer 417.

[0033] Also, the outermost second core layer 416, the innermost second core layer418, and the other second core layer 417 each may be under frozen-in compressive axial stress. The frozen-in axial stress of the outer cladding layer 402 may be greater than the frozen-in axial stress of the inner cladding layer 410. Further, the frozen-in axial stress of the inner cladding layer 410 may be greater than the average frozen-in axial stress of the first core 404 and greater than the average frozen-in axial stress of the second core 406.

[0034] The innermost core layers 414 and 418 in this example may consist essentially of pure (i.e., undoped) silica. Core layers that are adjacent to the innermost core layers 414 and 418, such as the other layers 415 and 417, respectively, may have a refractive index greater than the refractive index of the inner cladding layer 410 (or correspondingly, a lower fluorine content than the inner cladding layer). In an example (not shown) in which the layers of each core consist of exactly two layers (i.e., an innermost core layer and an outermost core layer), the outermost core layer may have a refractive index greater than the refractive index of the surrounding inner cladding layer.

[0035] With reference to FIG. 5, examples of the above-described optica! fibers 300(FIG. 3), 400 (FIG. 4), etc., may be made using a draw tower 502. It should be understood that the structure of the draw tower 502 as described below is intended only as an example, and other draw towers may have other structures.

[0036] A glass preform 504 may be provided as a process input to the draw tower502. As the glass preform 504 may have the same structural and material composition as any of the above-described fibers 300 (FIG. 2), 400 (FIG. 4), etc., the glass preform 504 is not shown or described in similar detail. As described above, some of the features or characteristics of the fibers 300, 400, etc., when correspondingly provided in the preform 504, may help reduce any distortion (e.g., ovality) imparted by the process of making the fibers 300, 400, etc., in the draw tower 502. As described above, such distortion is related to differences in viscosity ofthe layers of molten glass as the fiber is drawn. A goal ofthe solutions described herein may be to produce a fiber 300, 400, etc., from the preform 504 using the draw tower 502 that, though much smaller in size than the preform 504, has as precisely as possible the same geometry as the preform 504 (i.e., minimal distortion). Nevertheless, a fiber 300, 400, etc., made from the preform 504 using the draw tower 502 may have a geometry that is distorted by some amount from the geometry of the preform 504. Cladding layers, core layers, or other structural features may distort in different amounts. The solutions described herein may enable fiber to be drawn from a preform with the fiber’s geometry distorting less than a threshold amount.

[0037] A draw furnace 506 may heat a portion of the glass preform 504, and the resulting molten glass may be directed vertically downwardly to form a fiber. A thermal treatment unit 508 (e.g., a heater or a chiller) may apply heat to or remove heat from the fiber. A coating die unit 510 may apply a coating on the fiber. The coating die unit 510 may include curing lamps (not separately shown) that expose the fiber and its coating to ultraviolet (UV) radiation, thereby curing the coating. Although only a single coating die unit 510 is shown in the illustrated example, in other examples there may be more than one such coating die unit. Following coating and curing, the fiber may exit the draw tower 502 and be re-directed by a turning sheave 512 into a take-up unit 514. A capstan 516 may wind the resultingfiber 518 on a take-up spool 520. The fiber 518 that is produced from the preform 504 using the draw tower 502 may be an example of any of the above-described fibers 300, 400, etc.

[0038] One or more illustrative or exemplary embodiments of the invention have been described above. However, it is to be understood that the invention is defined by the appended claims and is not limited to the specific embodiments described.

Claims

CLAIMSWhat is claimed is:

1. An optical fiber, comprising: an outer cladding layer having an outer cladding refractive index; an inner cladding layer within the outer cladding layer and having an inner cladding refractive index less than the outer cladding refractive index: a first core in contact with the inner cladding layer and under frozen-in compressive axial stress; and a second core in contact with the inner cladding layer and under frozen-in compressive axial stress.

2. The optical fiber of claim 1 , wherein: the first core is in contact with the outer cladding layer; and the second core is in contact with the outer cladding layer.

3. The optical fiber of claim 1 , wherein: the outer cladding glass composition includes an outer cladding fluorine content; and the inner cladding glass composition includes an inner cladding fluorine content higher than the outer cladding fluorine content.

4. The optical fiber of claim 1, wherein each of the first and second cores includes a plurality of layers and wherein an average refractive index of each core is less than the outer cladding refractive index.

5. The optical fiber of claim 1, wherein a frozen-in axial stress of the outer cladding layer is greater than a frozen-in axial stress of the inner cladding layer, and wherein the frozen-in axial stress of the inner cladding layer is greater than an average frozen-in axial stress of the first core and greater than an average frozen-in axial stress of the second core.

6. The optical fiber of claim 1 , wherein each of the first and second cores includes a plurality of layers, and each of the plurality of layers of each core is under frozen-in compressive axial stress.

7. A method for making an optical fiber, comprising: providing a preform having an outer cladding layer with an outer cladding refractive index, an inner cladding layer with an inner cladding refractive index, a first core, and a second core, wherein the inner cladding refractive index is less than the outer cladding refractive index, the first core is in contact with the inner cladding layer and under frozen-in axial stress, and the second core is in contact with the inner cladding layer and under frozen-in axial stress; and drawing a fiber from the preform using a draw tower.

8. The method of claim 7, wherein: the first core is in contact with the outer cladding layer; and the second core is in contact with the outer cladding layer.

9. The method of claim 7, wherein: the outer cladding glass composition includes an outer cladding fluorine content; and the inner cladding glass composition includes an inner cladding fluorine content higher than the outer cladding fluorine content.

10. The method of claim 7, wherein each of the first and second cores includes a plurality of layers and wherein an average refractive index of each core is less than the outer cladding refractive index.

11. The method of claim 7, wherein a frozen-in axial stress of the outer cladding layer is higher than the a frozen-in axial stress of the inner cladding layer, and wherein the frozen-in axial stress of the inner cladding layer is greater than an averagefrozen-in axial stress of the first core and greater than an average frozen-in axial stress of the second core.

12. The method of claim 7, wherein each of the first and second cores includes a plurality of layers, and each of the plurality of layers of each core is under frozen-in compressive axial stress.

13. A method for making an optical fiber preform, comprising: forming an outer cladding layer having an outer cladding refractive index; forming an inner cladding layer within the outer cladding layer and having an inner cladding refractive index less than the outer cladding refractive index; forming a first core in contact with the inner cladding layer and under frozen- in compressive axial stress; and forming a second core in contact with the inner cladding layer and under frozen-in compressive axial stress.

14. The method of claim 13, wherein: the first core is in contact with the outer cladding layer; and the second core is in contact with the outer cladding layer.

15. The method of claim 13 , wherein: the outer cladding glass composition includes an outer cladding fluorine content; and the inner cladding glass composition includes an inner cladding fluorine content higher than the outer cladding fluorine content.

16. The method of claim 13, wherein each of the first and second cores includes a plurality of layers and wherein an average refractive index of each core is less than the outer cladding refractive index.

17. The method of claim 13, wherein a frozen-in axial stress of the outer cladding layer is greater than a frozen-in axial stress of the inner cladding layer, and wherein the frozen-in axial stre ss of the inner cladding layer is greater than an average frozen- in axial stress of the first core and greater than an average frozen-in axial stress of the second core.

18. The method of claim 13, wherein each of the first and second cores includes a plurality of layers, and each of the plurality of layers of each core is under frozen-in compressive axial stress.

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