Patent Publication Number: US-2020301065-A1

Title: Multi-core optical fiber

Description:
RELATED APPLICATIONS 
     This patent application claims the benefit of U.S. provisional patent application Ser. No. 62/310,402, filed Mar. 18, 2016, and U.S. provisional patent application Ser. No. 62/310,442, filed Mar. 18, 2016, both of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates generally to optical fibers and, more particularly, to an optical fiber having multiple cores, which may be referred to as a multi-core optical fiber. 
     DESCRIPTION OF RELATED ART 
     Multi-core optical fibers have been developed to increase the signal carrying capacity of traditional single-core optical fibers. Such multi-core optical fibers include a plurality of optical waveguides surrounded and supported by a silica support tube that encircles the waveguides. In some instances, the silica support tube may have optical characteristics matching that of the cladding of each waveguide. A buffer layer surrounds and protects the support tube. Examples of multi-core optical fibers are disclosed in U.S. Pat. No. 6,154,594. 
     In addition to greater signal carrying capacity, multi-core optical fibers also result in space savings because the waveguides are more closely positioned as compared to a plurality of individual optical fibers. This configuration may permit additional space savings when used with lasers and/or detectors that are configured to operate on the reduced spacing of the cores of the multi-core fiber. 
     While multi-core optical fibers increase the density of the waveguides, such structure may increase the crosstalk between adjacent cores. Such a potential increase in crosstalk may require additional physical structure or crosstalk compensation schemes within the optical system to decrease the crosstalk to an acceptable level. In addition, bending of the cores may occur in an inconsistent manner resulting in inconsistent signal carrying characteristics. 
     While multi-core optical fibers increase the density of the waveguides, such structure also increases the complexity of the optical fiber termination process. More specifically, the larger number of optical waveguides carried within the small cross-section of a single optical fiber increases the complexity of optical termination. An active process that sends light through a plurality of the waveguides may be required to determine their positions. This increases the time, complexity, and cost of terminating such multi-core optical fibers. 
     SUMMARY 
     In one aspect, a multi-core optical fiber includes a plurality of optical waveguides. Each optical waveguide has a length, a core and a cladding layer surrounding the core, and each optical waveguide is at least partially fused to an adjacent optical waveguide along the length thereof. At least some of the optical waveguides are aligned to form a linear array and the linear array has a major axis generally parallel to the linear array and a minor axis generally perpendicular to the major axis. A linear support structure is fused to the linear array of optical waveguides along the length of the optical waveguides. The optical waveguides and the linear support structure define an outer perimeter and a buffer engages and surrounds the outer perimeter. The buffer has a buffer modulus of elasticity substantially less than a waveguide modulus of elasticity of each of the waveguides. 
     In another aspect, a multi-core optical fiber includes a plurality of silica rods. Each rod is at least partially fused to an adjacent rod along a length thereof, and at least some of the rods are optical rods having a core and a cladding surrounding the core to define an optical waveguide. At least some of the optical waveguides form a linear array of optical waveguides having a major axis generally parallel to the linear array and a minor axis generally perpendicular to the major axis. The silica rods define an outer cross-sectional perimeter with at least a portion of the outer cross-sectional perimeter being defined by at least some of the optical rods. A buffer engages and surrounds the outer cross-sectional perimeter. The buffer has a buffer modulus of elasticity substantially less than a rod modulus of elasticity of each of the silica rods. 
     In still another aspect, a multi-core glass optical fiber includes a plurality of glass optical waveguides. Each optical waveguide has a length, a core and a cladding layer. The cladding layer has an annular cross section surrounding and co-axial with its core. Each optical waveguide is at least partially fused to an adjacent optical waveguide along the length thereof with at least some of the optical waveguides aligned to form a linear array. The linear array has a major axis generally parallel to the linear array and a minor axis generally perpendicular to the major axis. A glass linear support structure is fused to the linear array of optical waveguides along the length of the optical waveguides and along a side of the linear array and generally parallel to the major axis. The optical waveguides and the linear support structure define an outer perimeter and the optical fiber is devoid of a glass support tube encircling the outer perimeter. A buffer engages and surrounds the outer perimeter. The buffer has a buffer modulus of elasticity substantially less than a waveguide modulus of elasticity of each of the waveguides. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The organization and manner of the structure and operation of the Present Disclosure, together with further objects and advantages thereof, may best be understood by reference to the following Detailed Description, taken in connection with the accompanying Figures, wherein like reference numerals identify like elements, and in which: 
         FIG. 1  is a perspective view of a multi-core optical fiber according to an embodiment of the present disclosure; 
         FIG. 2  is an enlarged end view of the array of the multi-core optical fiber of  FIG. 1 ; 
         FIG. 3  is an end view of a second embodiment of a multi-core optical fiber; 
         FIG. 4  is an end view of a third embodiment of a multi-core optical fiber; 
         FIG. 5  is an end view of a fourth embodiment of a multi-core optical fiber; 
         FIG. 6  is an end view of a fifth embodiment of a multi-core optical fiber; 
         FIG. 7  is an end view of a preform that may be used to form the multi-core optical fiber of  FIG. 1 ; 
         FIG. 8  is an end view of a sixth embodiment of a multi-core optical fiber; 
         FIG. 9  is an end view of a seventh embodiment of a multi-core optical fiber; and 
         FIG. 10  is an end view of an eighth embodiment of a multi-core optical fiber. 
     
    
    
     DETAILED DESCRIPTION 
     While the Present Disclosure may be susceptible to embodiment in different forms, there is shown in the Figures, and will be described herein in detail, specific embodiments, with the understanding that the Present Disclosure is to be considered an exemplification of the principles of the Present Disclosure, and is not intended to limit the Present Disclosure to that as illustrated. 
     As such, references to a feature or aspect are intended to describe a feature or aspect of an example of the Present Disclosure, not to imply that every embodiment thereof must have the described feature or aspect. Furthermore, it should be noted that the description illustrates a number of features. While certain features have been combined together to illustrate potential system designs, those features may also be used in other combinations not expressly disclosed. Thus, the depicted combinations are not intended to be limiting, unless otherwise noted. 
     In the embodiments illustrated in the Figures, representations of directions such as up, down, left, right, front and rear, used for explaining the structure and movement of the various elements of the Present Disclosure, are not absolute, but relative. These representations are appropriate when the elements are in the position shown in the Figures. If the description of the position of the elements changes, however, these representations are to be changed accordingly. 
       FIG. 1  depicts a multi-core optical fiber  10  drawn from a preform as described below and is known in the art. Optical fiber  10  includes an array  11  of rods  12  surrounded or encircled by a buffer  13 . Some of the rods  12  function as optical rods or waveguides  14  and include a core  15  and a cladding or cladding layer  16  that surrounds the core. Others of the rods  12  function as support rods or members  17  that mechanically interact with the optical waveguides  14  to assist in accurately positioning the optical waveguides within the array  11 . 
     As seen in  FIGS. 1-2 , the core  15  of each optical waveguide  14  has a circular cross-section and the cladding layer  16  has an annular cross-section that surrounds and is co-axial with the core. Each of the core  15  and the cladding  16  may be made of glass, a polymer, or any other desired material provided that light will travel through the core  15  of each optical waveguide  14  as desired. To do so, the index of refraction of the core  15  is greater than the index of refraction of the cladding  16 . The core  15  and cladding  16  may be dimensioned or configured so that the optical waveguide  14  functions in any manner such as a single-mode, a multi-mode, or a few- or oligo-mode waveguide. 
     In many instances, both the core  15  and the cladding  16  may be made primarily of silica. The refractive index of the core  15  and/or cladding  16  may be changed by adding elements such as by doping to change the optical characteristics of the silica. For example, the refractive index may be increased by adding elements having a higher atomic mass than silica such as germanium or phosphorous. In other instances, the refractive index may be reduced by adding elements having a lower atomic mass than silica such as fluorine. In still other instances, the core  15  and cladding may be made from other types of glass such as borosilicate and other elements may be used for changing the refractive indices. 
     The support rods  17  are depicted with a circular cross-section in  FIGS. 1-2  and may be made of the same base material as the optical waveguides  14  so as to have the same melting temperature. In other words, if the optical waveguides  14  have a base material (without doping) of silica, the support rods  17  may also be made of silica. The support rods  17  do not include a cladding layer and thus are not capable of or are unsuitable for the efficient transmission of light as required for an optical waveguide. As such, the support rods  17  do not need to be doped during the process of forming a preform as described below. The support rods  17  may be formed of any material that will provide the desired support for the optical waveguides  14  during and after the forming process. 
     The rods  12  are configured so as to form a first row  21  of rods aligned along line  50  ( FIG. 2 ) and form a linear array. A second row  22  of rods  12  are aligned along line  51  to form a second linear array that is offset from line  50  and has one fewer rod  12  as compared to first row  21 . The rods  12  of the second row  22  are positioned adjacent to but offset from the first row  21 , with the center of each rod  12  of the second row being aligned with the intersection of each pair of rods  12  of the first row. Similarly, the center of the interior rods (designated  12   a ) of the first row  21  are aligned with the intersection of each pair of rods  12  of the second row  22 . Such a closely packed array of rods is sometimes referred to as a hexagonal close packed array. 
     As depicted, the rods  12  of the first row  21  are all configured as optical waveguides  14  to create or define a linear array of optical waveguides. The rods  12  of the second row  22  create or define a linear support structure. 
     One of the rods  12  of the second row  22  is configured as polarization waveguide  14   a  and the others are configured as support rods  17 . Other combinations of rods  12  making up the second row  22  are contemplated. The polarization waveguide  14   a  of the second row  22  may function as an orienting or polarization waveguide. More specifically, the polarization waveguide  14   a  is located in a predetermined position along the second row to establish or identify the order of the waveguides  14  within the first row  21 . Determining which of the rods  12  within the second row  22  is the polarizing waveguide  14   a  will assist in positioning the optical fiber  10  relative to another component (not shown) so that each of the waveguides  14  in the first row  21  is aligned as desired with respect to the other component. If desired, the polarization waveguide  14   a  of the second row may be omitted and other techniques or structures for polarization may be used or the optical fiber  10  may not include any polarization. 
     As a result of the drawing process described below, each rod  12  is fused to each of the adjacent rods along the entire length of each rod at each intersection between the rods. Since at least some of the rods  12  have a circular cross-section, the round rods are only partially fused to the adjacent rods as a result of the interstitial air gaps  18  between adjacent rods. By positioning the rods  12  of the first row  21  and the second row  22  in a hexagonal close packed array, a very stable array of rods  12  is formed. In other words, by aligning the center of each rod  12  of the second row  22  with the intersection of each pair of rods  12  of the first row  21  and aligning the center of each interior rod  12   a  of the first row  21  with the intersection of each pair of rods  12  of the second row  22 , the array as drawn is sufficiently stable so that the rods  12  maintain their precise positioning during and after the forming process without the need for an external support member such as a glass or silica tubular support structure used with prior art multi-core optical fibers. 
     As best seen in  FIG. 2 , the drawn array or structure  11  is devoid of a glass tubular support structure surrounding the outer perimeter and thus has an asymmetrical cross-section. This asymmetrical configuration has a major axis  55  generally parallel to the lines  50  and  51  and a minor axis  56  generally perpendicular to the major axis. Such an asymmetrical configuration (i.e., without a silica tubular support structure) is primarily flexible or has greater flexibility along the minor axis  56  and is substantially less flexible along the major axis  55 . 
     Referring back to  FIG. 1 , buffer  13  surrounds and protects the array  11 . Since the array  11  does not have a glass support structure encircling it, the buffer  13  engages the exposed outer perimeter of the array  11  (i.e., the outer arcuate surfaces of the rods  12 ). Buffer  13  may have a circular cross-sectional outer surface  23 . Other outer surface configurations such as an oval cross-sectional outer surface (not shown) are contemplated. Buffer  13  may be formed of resin such as a UV cured acrylate material. Other materials are contemplated. If desired, an additional layer of material (not shown) such as a harder layer of UV cured resin material may also be applied to the buffer  13 . 
     Buffer  13  has a modulus of elasticity that is substantially less than the modulus of elasticity of the rods  12 . For example, if buffer  13  is formed of a UV cured acrylate resin or material, it will have a modulus of elasticity of approximately 40,000 psi. Rods  12  formed of silica (including those doped with various elements) will have a modulus of elasticity of approximately 10 7  psi. Due to the substantially lower modulus of elasticity of the buffer as compared to that of the rods  12 , the flexibility of the optical fiber  10  will not be significantly limited by the buffer. In other words, the addition of buffer  13  will not materially affect the flexibility of the array  11  and thus the optical fiber  10  will have significant flexibility in a direction generally along or parallel to the minor axis  56  of array  11  and be substantially less flexible in a direction generally along or parallel to the major axis  55  of the array. 
       FIGS. 3-6  depict alternate embodiments of multi-core optical fibers. Like elements are identified with like reference numbers and the description thereof may be omitted. Referring to  FIG. 3 , multi-core optical fiber  30  includes a hexagonal close packed array  31  that is similar to the hexagonal close packed array  11  of  FIGS. 1-2  but with the array expanded by adding an additional row  32  of rods  12 . Each of the rods  12  of the third row  32  is a support rod  17  and each is aligned with one of the rods of the second row  22 . Through such a configuration, further stability may be added to the array  31  in order to maintain the precise positioning of the optical waveguides  14 . Buffer  33  surrounds and contacts the array  31 . 
     Multi-core optical fiber  40  depicted in  FIG. 4  includes a hexagonal close packed array  41  that expands upon the array  11  depicted in  FIGS. 1-2  by adding an additional row of optical waveguides  14  on the support structure defined by the second row  22  of rods  12  of  FIGS. 1-2 . The rods  12  of the first row  21  are all configured as optical waveguides  14 . One of the rods  12  of the second row  42  is configured as a polarization waveguide  14   a  and the other rods are configured as support rods  17 . It should be noted that the second row  41  has an additional rod  12  as compared to the number of rods in the first row  21 . 
     A third row  44  of rods  12  is provided with each of the rods of the third row aligned with a rod  12  of the first row and also aligned with the intersection between adjacent rods of the second row  42  to form a hexagonal close packed array. Each of the rods  12  of the third row  44  is configured as an optical waveguide  14  and thus the third row defines a third linear array of rods and a second linear array of waveguides. As such, array  41  includes two linear arrays of optical waveguides  14  that are parallel to each other and positioned on opposite sides of the second row  42  of rods  12 . Array  41  thus has two linear arrays of waveguides  14  with each linear array having four waveguides and may also include an orientation waveguide  14   a , if desired. The orientation waveguide  14   a  is configured as a component of the second row  42  of rods  12  that functions as a linear support structure. Buffer  43  surrounds and contacts array  41 . 
     As may be seen in  FIGS. 1-4 , each of the arrays  11 ,  31 , and  41  include at least one linear array of waveguides  14  and at least one linear array of support rods  17  that define a linear support structure. The linear array of waveguides  14  and the linear support structure are positioned relative to each other to form a hexagonal close packed array. Each of the arrays  31  and  41  include a second linear array of rods  12 . The second linear array of rods in array  31  provides a second linear support structure while the second linear array of rods in array  41  provides a second linear array of waveguides  14 . 
       FIGS. 5-6  depict still further alternate embodiments of multi-core optical fibers. Multi-core optical fiber  50  ( FIG. 5 ) has an array  51  of rods that includes a first row  21  of rods  12  and a support rod  52 . Each rod  12  of the first row  21  has a circular cross-section and is configured as an optical waveguide  14  to define a linear array of optical waveguides. Support rod  52  has a rectangular cross-section and functions as a support rod rather than as an optical waveguide. Support rod  32  is fused to the waveguides  14  along one side of the linear array of waveguides  14 . As such, array  51  is similar to array  11  but includes rectangular support rod  52  rather than the linear array of circular support rods  17  that define the linear support structure in  FIGS. 1-2 . Buffer  53  surrounds and contacts the array  51 . 
     Multi-core optical fiber  60 , depicted in  FIG. 6 , includes an array  61  similar to array  51  of  FIG. 5  but includes a second support rod  62  with a rectangular cross-section that is fused to the first row  21  of optical waveguides  14  but on a side of the waveguides  14  opposite the support rod  52 . As such, the array  61  includes a linear array of waveguides  14  with a linear support structure (i.e., the support rods  52  and  62 ) fused to opposite sides of the waveguides  14 . Buffer  63  surrounds and contacts the array  61 . 
     When forming an optical fiber, a preform having a cross-section substantially identical to the desired cross-section of the optical fiber is initially formed. Referring to  FIG. 7 , a cross-section of preform  70  used to form multi-core optical fiber  10  is depicted. Preform  70  includes preform rods  72  corresponding in location to each of the rods  12 . When forming the preform  70 , the preform rods  72  are formed of the desired materials and precisely positioned with the preform rods corresponding to rods  12  depicted in  FIGS. 1-2 . Some of the preform rods  72  include cores  73  corresponding to cores  15  of waveguides  14 . The preform rods  72  are fused or otherwise secured to each other and sand or other material may be positioned within the interstitial gaps indicated at  74  in  FIG. 7  between the preform rods  72 . If desired, relatively small preform rods indicated in phantom at  75  may be placed within the interstitial gaps  74  to assist in maintaining the positions of the preform rods  72 . After the preform  70  is formed, the optical fiber  10  may be formed by positioning the preform at the top of a draw tower (not shown) and heating the preform within an in-line furnace (not shown). After the array  11  is drawn to the desired size, the buffer  13  is applied and then cured to form the multi-core optical fiber  10 . 
     The multi-core optical fibers  10 ,  30 ,  40 ,  50 , and  60  described above have many advantages over existing multi-core optical fibers in which the cores are surrounded by a cylindrical support tube and thus the glass components have a circular cross-section. Since the arrays  11 ,  31 ,  41 ,  51 , and  61  of the optical fibers  10 ,  30 ,  40 ,  50 , and  60  include a major axis  55  and a minor axis  56 , bending of the optical fiber most easily occurs generally along the minor axis. As a result, distortion within the waveguides  14  caused by bending of the optical fiber will be consistent between adjacent waveguides. Further, since the direction of such bending may be anticipated, compensation for any distortion caused by such bending may be more easily achieved. 
     More specifically, an existing multi-core optical fiber having a circular cross-section may bend in any direction and such bending may affect the waveguide within the fiber and in an inconsistent manner. For example, a multi-core fiber in which the glass components have a circular cross-section (i.e., the fiber includes a structural support tube surrounding the cores) and a linear array of waveguides may bend at any orientation relative to the linear array. As a result, unless the optical fiber is bent in a direction perpendicular to the linear array of waveguides, the bending of the waveguides will be inconsistent and thus the optical characteristics of each waveguide may be affected differently by the bend. 
     In contrast, with the multi-core optical fibers  10 ,  30 ,  40 ,  50 , and  60  disclosed herein, the optical fibers will bend in a consistent manner in a direction generally orthogonal to the major axis  55  or along the minor axis  56  so that the bending of the optical fiber will have an equal or consistent effect on each of the waveguides  14 . This minimizes strain-induced polarization effects that can diminish signal integrity. Still further, since the direction of bending will be known, certain types of distortion may be anticipated and systems in which the multi-core optical fiber is used may be configured to compensate for those types of distortion caused by such bending. 
     The lack of a glass structural support tube around the rods  12  of the multi-core optical fibers  10 ,  30 ,  40 ,  50 , and  60  of the present disclosure also permit the optical fiber to be bent in a smaller radius as compared to a multi-core optical fiber having the same rods plus a glass support tube surrounding the rods. In other words, the cross-sectional structure of the disclosed embodiments in which the optical fiber  10 ,  30 ,  40 ,  50 , and  60  bends orthogonal to the major axis  55  causes a reduction in the mechanical stress caused by bending of the waveguides  14 . Reductions in stress on the optical fibers is desirable as such stress decreases the optical performance of the optical fiber. 
     The multi-core optical fibers  10 ,  30 ,  40 ,  50 , and  60  of the present disclosure also simplify connection and termination of the optical fibers as compared to existing multi-core optical fibers in which the glass components have a circular cross-section. Since the multi-core optical fibers  10 ,  30 ,  40 ,  50 , and  60  of the present disclosure will generally bend orthogonal to the major axis  55 , such bending action may be used to determine the orientation of the waveguides  14 . More specifically, since the waveguides  14  are configured in a linear array generally perpendicular to the minor axis  56 , the orientation of the linear array of waveguides may be determined in a passive manner (i.e., without projecting or sending light through the waveguides) by merely bending the optical fiber. This passive manner of determining the position of the waveguides  14  is substantially less complicated and time consuming than actively determining the position of the waveguides within the multi-core optical fiber of the prior art in which the glass components have a circular cross-section. 
     In some instances, the array of waveguides and support structure may be symmetrical resulting in a major axis  55  that is also a neutral axis of the structure. For example, in  FIG. 3 , the first row  21  of rods  12  is along the major axis and the symmetrical nature of the first row  21 , second row  22 , and third row  32  of array  31  results in the major axis coinciding with the neutral axis. Similarly, in  FIG. 4 , the first row  42  of rods  12  is along the major axis and the symmetrical nature of the first row  21 , second row  22 , and third row  42  of array  41  results in the major axis coinciding with the neutral axis. In  FIG. 6 , the row  21  of rods  12  is along the major axis and configuration of the row of rods, the first support rod  52 , and the second support rod  62  result in the major axis coinciding with the neutral axis. 
     The performance of polarization maintaining optical fibers or waveguides is typically dependent upon minimizing strain on the optical fibers or waveguides. By configuring the rods  12  that are along the major axis and the neutral axis as polarization maintaining optical fibers or waveguides, the strain on the polarization maintaining waveguides may be minimized. Accordingly, it may be desirable to utilize polarization maintaining optical fibers or waveguides along the major axis (which coincides with the neutral axis) of the multi-core optical fibers  30 ,  40  and  60  to isolate such polarization maintaining optical fibers or waveguides from strain-induced signal degradation. 
     In some instances, it may be desirable to configure the multi-core optical fibers to increase the security of the signals being transmitted through the waveguides  14  thereof. For example, it is known that bending certain waveguides will cause leakage of light from the waveguide. Such waveguides that permit leakage are referred to herein as standard waveguides. It is further known to configure certain other waveguides such that they less-readily leak light upon bending of the waveguide. Such waveguides that restrict light leakage are referred to herein as bend-insensitive waveguides. 
     Referring to  FIG. 8 , a multi-core optical fiber  80  with enhanced security is depicted. The structure of multi-core optical fiber  80  is similar to that of multi-core optical fiber  10  of  FIG. 1  and like elements are identified with like reference numbers. In multi-core optical fiber  80 , the waveguides  14  are standard waveguides and thus bending of the waveguide will readily permit light to pass through the cladding adjacent the bent portion so that light leaks from the waveguide. One or more of the waveguides are configured as bend-insensitive waveguides  81  and thus prevent or minimize the amount of light that passes through the cladding or leaks from a bent portion of the waveguide. Through such a configuration, the standard waveguides  14  permit a sufficient amount of light to leak, when bent, to obscure the leakage of light from the bend-insensitive waveguides  81 . 
     A multi-core optical fiber may have any desired combination of standard waveguides  14  and bend-insensitive waveguides  81 . In other words, any of the standard waveguides  14  of any of the multi-core optical fibers  10 ,  30 ,  40 ,  50  and  60 , as well as any other configurations of multi-core optical fibers, may be replaced by bend-insensitive waveguides  81 . The arrangement (i.e., positions and mix) of standard waveguides  14  and bend-insensitive waveguides  81  may be determined based upon any number of factors including the types of signals being sent, the desired degree of security, and the desired fiber interconnection. 
       FIG. 9  depicts another example of a multi-core optical fiber  85 . In one embodiment, an array of waveguides  86  includes a bend-insensitive waveguide  81  positioned in the center of a ring of standard waveguides  14 . Such configuration may be advantageous because any light escaping from the bend-insensitive waveguide  81  is surrounded and obscured by the light escaping from the surrounding standard waveguides  14 . 
     Multi-core optical fiber  85  is capable of being bent along three major axes, each being indicated at  87 , that are 120 degrees apart. By positioning the bend-insensitive waveguide  81  in the center of the array of waveguides, the bend-insensitive waveguide  81  will always be along one of the major axes  87  so that it will be bent less than the standard waveguides  14  that are not positioned along the major axis. As a result, regardless of the orientation of the multi-core optical fiber  85 , any optical signals escaping from the bend-insensitive waveguide  81  will be obscured by the greater signals escaping from the standard waveguides  14  that are not located along the major axis  87  about which the optical fiber is being bent. 
     In some embodiments, standard waveguides  14  and bend-insensitive waveguides  81  may be used in a multi-core optical fiber having a cylindrical support tube such that the glass components have a circular cross-section. For example, as depicted in  FIG. 10 , a multi-core optical fiber  90  includes a waveguide array  91  identical to that of  FIG. 9  with the waveguide array including a plurality of standard waveguides  14  surrounding a single bend-insensitive waveguide  81 . However, the multi-core optical fiber  90  includes a glass cylindrical support tube  92  surrounding and engaging or contacting the waveguide array  91  and a buffer  93  surrounding and engaging or contacting the support tube. With such a configuration, the multi-core optical fiber  90  will be able to bend in any direction as a result of the glass support tube  92  but any light escaping from the bend-insensitive waveguide  81  will be obscured by light escaping from the standard waveguides  14 . 
     In the embodiments depicted in  FIGS. 9 and 10 , if desired, the bend-insensitive waveguide  81  may be replaced by a polarization maintaining optical fiber or waveguide since the symmetrical array results in the major axis being coincident with the neutral axis, thus avoiding or minimizing strain-induced signal degradation. 
     In an embodiment, a mutually fused array of optical fibers or waveguides such as those of a multi-core optical waveguide may be provided for enhanced security. The array has peripheral fibers or waveguides at or near the outside edge of the array and inner fibers or waveguides that are closer to the center of the array than the peripheral fibers or waveguides. The inner fibers or waveguides are constructed in a manner such that they less-readily leak light than the peripheral fibers or waveguides do when the array is bent. 
     In some embodiments, some or all of the inner fibers or waveguides are constructed to minimize the leakage of light when bent. In some embodiments, some or all of the peripheral fibers or waveguides are constructed to sufficiently leak light, when bent, to obscure the leakage of light from some or all of the inner fibers. Some or all of the inner fibers or waveguides may be comprised of bend-insensitive fiber. Some or all of the peripheral fibers or waveguides may be constructed of bend-sensitive fiber. 
     In the embodiments depicted in  FIGS. 9 and 10 , information conveyed by light transmitted via the central fiber or waveguide  81  can be made more secure by also transmitting light via one or more of the surrounding fibers or waveguides  14 . If this array of optical fibers or waveguides is tapped by bending the array, the light leaked from one or more of the surrounding fibers  14  can be used to hide or obscure any light leaked from the central fiber or waveguide  81 . 
     While a preferred embodiment of the Present Disclosure is shown and described, it is envisioned that those skilled in the art may devise various modifications without departing from the spirit and scope of the foregoing Description and the appended Claims.