Patent Publication Number: US-2021181408-A1

Title: Multicore optical fiber with chlorine doped cores

Description:
This application claims the benefit of priority to U.S. Provisional Application Ser. No. 62/946,668 filed on Dec. 11, 2019, the content of which is relied upon and incorporated herein by reference in its entirety. 
    
    
     FIELD 
     The present disclosure generally relates to multicore optical fibers, and in particular to multicore optical fibers having chlorine doped cores. 
     BACKGROUND 
     Optical fibers are utilized in a variety of telecommunication applications. Multicore optical fibers can provide increased fiber density compared to single core optical fibers, which can help to address challenges associated with cable size limitations and duct congestion in passive optical network (“PON”) systems. Multicore optical fibers can also be utilized in high speed optical interconnects where there is a need to increase the fiber density to achieve high fiber count connectors. Multicore optical fibers generally include multiple cores embedded in a single common cladding matrix. The performance of multicore optical fibers can be affected by optical loss, i.e., attenuation, of each core as well as crosstalk between cores. 
     In view of these considerations, there is a need for multicore optical fibers exhibiting low attenuation and low crosstalk. 
     SUMMARY 
     According to an embodiment of the present disclosure, a multicore optical fiber includes a first core, a first inner cladding surrounding the first core, a second core, a second inner cladding surrounding the second core, and a common cladding surrounding the first core and the second core. The first core includes silica and greater than 3 wt % chlorine, wherein the first core comprises a first core centerline, a relative refractive index Δ 1MAX , and an outer radius r 1 . The first inner cladding includes a relative refractive index Δ IC1  and a width δr IC1 , wherein Δ 1MAX &gt;Δ IC1 . The second core includes silica and greater than 3 wt % chlorine, wherein the second core comprises a second core centerline, a relative refractive index Δ 2MAX , and an outer radius r 2 . The second inner cladding includes a relative refractive index Δ IC2  and a width δr IC2 , wherein Δ 2MAX &gt;Δ IC2 . The common cladding includes a relative refractive index Δ CC . A spacing between the first core centerline and the second core centerline is at least 28 micrometers and a crosstalk between the first core and the second core is ≤−30 dB, as measured for a 100 km length of the multicore optical fiber operating at a wavelength of 1550 nm. 
     According to an embodiment of the present disclosure, a multicore optical fiber includes a first core, a second core, and a common cladding formed from silica-based glass surrounding and in direct contact with the first core and the second core. The first core includes silica and greater than 3 wt % chlorine, wherein the first core comprises a first core centerline, a relative refractive index Δ 1MAX , and an outer radius r 1 . The second core includes silica and greater than 3 wt % chlorine, wherein the second core comprises a second core centerline, a relative refractive index Δ 2MAX , and an outer radius r 2 . The common cladding has a relative refractive index Δ CC . A spacing between the first core centerline and the second core centerline is at least 28 micrometers and a crosstalk between the first core and the second core is ≤−30 dB, as measured for a 100 km length of the multicore optical fiber operating at a wavelength of 1550 nm. 
     These and other aspects, objects, and features of the present disclosure will be understood and appreciated by those skilled in the art upon studying the following specification, claims, and appended drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a perspective view of a schematic of a multicore optical fiber, according to aspects of the present disclosure; 
         FIG. 2  is a cross-sectional view of the multicore optical fiber of  FIG. 1  taken along the line II-II, according to aspects of the present disclosure; 
         FIG. 3  is a cross-sectional schematic of a ribbon-type multicore optical fiber, according to aspects of the present disclosure; 
         FIG. 4A  is a cross-sectional schematic of an exemplary core and inner cladding of a multicore optical fiber, according to aspects of the present disclosure; 
         FIG. 4B  is a cross-sectional schematic of an exemplary core, inner cladding, and outer cladding of a multicore optical fiber, according to aspects of the present disclosure; 
         FIG. 5  is a refractive index profile for a multicore optical fiber having a chlorine doped core and an undoped silica common cladding, according to aspects of the present disclosure; 
         FIG. 6  is a refractive index profile for a multicore optical fiber having a chlorine doped core and chlorine doped common cladding, according to aspects of the present disclosure; 
         FIG. 7  is a refractive index profile for a multicore optical fiber having a chlorine doped core and chlorine doped common cladding, according to aspects of the present disclosure; 
         FIG. 8  is a refractive index profile for a multicore optical fiber having a chlorine doped core, a fluorine doped inner cladding, and an undoped silica common cladding, according to aspects of the present disclosure; 
         FIG. 9  is a refractive index profile for a multicore optical fiber having a chlorine doped core, a fluorine doped inner cladding, and a fluorine doped common cladding, according to aspects of the present disclosure; 
         FIG. 10  is a refractive index profile for a multicore optical fiber having a chlorine doped core, a fluorine doped inner cladding, and a chlorine doped common cladding, according to aspects of the present disclosure; 
         FIG. 11  is a plot illustrating crosstalk between adjacent cores as a function of core centerline to core centerline spacing, according to aspects of the present disclosure; 
         FIG. 12  is a refractive index profile for a multicore optical fiber having a chlorine doped core, a fluorine doped inner cladding, and an undoped silica common cladding, according to aspects of the present disclosure; 
         FIG. 13  is a refractive index profile for a multicore optical fiber having a chlorine doped core, a fluorine doped inner cladding, and a chlorine doped common cladding, according to aspects of the present disclosure; 
         FIG. 14  is a refractive index profile for a multicore optical fiber having a chlorine doped core, an undoped silica inner cladding, and a chlorine doped common cladding, according to aspects of the present disclosure; and 
         FIG. 15  is a refractive index profile for a multicore optical fiber having a chlorine doped core, an undoped silica inner cladding, a fluorine doped outer cladding, and an undoped silica common cladding, according to aspects of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, for purposes of explanation and not limitation, example embodiments disclosing specific details are set forth to provide a thorough understanding of various principles of the present disclosure. However, it will be apparent to one having ordinary skill in the art, having had the benefit of the present disclosure, that the present disclosure may be practiced in other embodiments that depart from the specific details disclosed herein. Moreover, descriptions of well-known devices, methods and materials may be omitted so as not to obscure the description of various principles of the present disclosure. Finally, wherever applicable, like reference numerals refer to like elements. 
     As used herein, the term “and/or,” when used in a list of two or more items, means that any one of the listed items can be employed by itself, or any combination of two or more of the listed items can be employed. For example, if a composition is described as containing components A, B, and/or C, the composition can contain A alone; B alone; C alone; A and B in combination; A and C in combination; B and C in combination; or A, B, and C in combination. 
     Modifications of the disclosure will occur to those skilled in the art and to those who make or use the disclosure. Therefore, it is understood that the embodiments shown in the drawings and described above are merely for illustrative purposes and not intended to limit the scope of the disclosure, which is defined by the following claims, as interpreted according to the principles of patent law, including the doctrine of equivalents. 
     As used herein, the term “about” means that amounts, sizes, formulations, parameters, and other quantities and characteristics are not and need not be exact, but may be approximate and/or larger or smaller, as desired, reflecting tolerances, conversion factors, rounding off, measurement error and the like, and other factors known to those of skill in the art. When the term “about” is used in describing a value or an end-point of a range, the disclosure should be understood to include the specific value or end-point referred to. Whether or not a numerical value or end-point of a range in the specification recites “about,” the numerical value or end-point of a range is intended to include two embodiments: one modified by “about,” and one not modified by “about.” It will be further understood that the end-points of each of the ranges are significant both in relation to the other end-point, and independently of the other end-point. 
     The term “formed from” can mean one or more of comprises, consists essentially of, or consists of. For example, a component that is formed from a particular material can comprise the particular material, consist essentially of the particular material, or consist of the particular material. 
     A multicore optical fiber, also referred to as a multicore fiber or “MCF”, is considered for the purposes of the present disclosure to include two or more core fibers disposed within a cladding matrix. Each core fiber can be considered as having a core surrounded by a cladding matrix defining an common cladding. Optionally, each core fiber can include a core surrounded by one or more inner claddings disposed between each core and the cladding matrix of the common cladding. “Radial position,” “radial distance,” when used in reference to the radial coordinate “r” refers to radial position relative to the centerline (r=0) of each individual core in a multicore optical fiber. “Radial position,” “radial distance,” when used in reference to the radial coordinate “R” refers to radial position relative to the centerline (R=0, central fiber axis) of the multicore optical fiber. The length dimension “micrometer” may be referred to herein as micron (or microns) or μm. 
     The “refractive index profile” is the relationship between refractive index or relative refractive index and radial distance r from the core&#39;s centerline for each core fiber of the multicore optical fiber. For relative refractive index profiles depicted herein as having step boundaries between adjacent core and cladding regions, normal variations in processing conditions may result in step boundaries at the interface of adjacent regions that are not sharp. It is to be understood that although boundaries of refractive index profiles may be depicted herein as step changes in refractive index, the boundaries in practice may be rounded or otherwise deviate from perfect step function characteristics. It is further understood that the value of the relative refractive index may vary with radial position within the core region and/or any of the cladding regions. When relative refractive index varies with radial position in a particular region of the fiber (core region and/or any of the cladding regions), it may be expressed in terms of its actual or approximate functional dependence or in terms of an average value applicable to the region. Unless otherwise specified, if the relative refractive index of a region (core region and/or any of the inner and/or common cladding regions) is expressed as a single value, it is understood that the relative refractive index in the region is constant, or approximately constant, and corresponds to the single value or that the single value represents an average value of a non-constant relative refractive index dependence with radial position in the region. Whether by design or a consequence of normal manufacturing variability, the dependence of relative refractive index on radial position may be sloped, curved, or otherwise non-constant. 
     The “relative refractive index” or “relative refractive index percent” as used herein with respect to multicore optical fibers and fiber cores of multicore optical fibers is defined according to equation (1): 
     
       
         
           
             
               
                 
                   
                     Δ 
                      
                     
                         
                     
                      
                     % 
                   
                   = 
                   
                     100 
                      
                     
                       
                         
                           
                             n 
                             2 
                           
                            
                           
                             ( 
                             r 
                             ) 
                           
                         
                         - 
                         
                           n 
                           c 
                           2 
                         
                       
                       
                         2 
                          
                         
                           
                             n 
                             2 
                           
                            
                           
                             ( 
                             r 
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where n(r) is the refractive index at the radial distance r from the core&#39;s centerline at a wavelength of 1550 nm, unless otherwise specified, and n c  is 1.444, which is the refractive index of undoped silica glass at a wavelength of 1550 nm. As used herein, the relative refractive index is represented by Δ (or “delta”) or Δ % (or “delta %) and its values are given in units of “%” or “% A”, unless otherwise specified. Relative refractive index may also be expressed as Δ(r) or Δ(r) %. When the refractive index of a region is less than the reference index n c , the relative refractive index is negative and can be referred to as a trench. When the refractive index of a region is greater than the reference index n c , the relative refractive index is positive and the region can be said to be raised or to have a positive index. 
     The average relative refractive index of a region of the multicore optical fiber can be defined according to equation (2): 
     
       
         
           
             
               
                 
                   
                     Δ 
                      
                     
                         
                     
                      
                     % 
                   
                   = 
                   
                     
                       
                         ∫ 
                         
                           r 
                           inner 
                         
                         
                           r 
                           outer 
                         
                       
                        
                       
                         
                           Δ 
                            
                           
                             ( 
                             r 
                             ) 
                           
                         
                          
                         dr 
                       
                     
                     
                       ( 
                       
                         
                           r 
                           
                             o 
                              
                             u 
                              
                             t 
                              
                             e 
                              
                             r 
                           
                         
                         - 
                         
                           r 
                           
                             i 
                              
                             n 
                              
                             n 
                              
                             e 
                              
                             r 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     where r inner  is the inner radius of the region, r outer  is the outer radius of the region, and Δ(r) is the relative refractive index of the region. 
     The term “α-profile” (also referred to as an “alpha profile”) refers to a relative refractive index profile Δ(r) that has the following functional form (3): 
     
       
         
           
             
               
                 
                   
                     Δ 
                      
                     
                       ( 
                       r 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         Δ 
                          
                         
                           ( 
                           
                             r 
                             0 
                           
                           ) 
                         
                       
                        
                       
                         [ 
                         
                           1 
                           - 
                           
                             
                                
                               
                                 r 
                                 - 
                                 
                                   r 
                                   0 
                                 
                               
                                
                             
                             
                               ( 
                               
                                 
                                   r 
                                   1 
                                 
                                 - 
                                 
                                   r 
                                   0 
                                 
                               
                               ) 
                             
                           
                         
                         ] 
                       
                     
                     α 
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     where r 0  is the point at which Δ(r) is maximum, r 1  is the point at which Δ(r) is zero, and r is in the range r i ≤r≤r f , where r i  is the initial point of the α-profile, r f  is the final point of the α-profile, and α is a real number. In some embodiments, examples shown herein can have a core alpha of 1≤α≤100. In practice, an actual optical fiber, even when the target profile is an alpha profile, some level of deviation from the ideal configuration can occur. Therefore, the alpha parameter for an optical fiber may be obtained from a best fit of the measured index profile, as is known in the art. 
     The term “graded-index profile” refers to an α-profile, where α&lt;10. The term “step-index profile” refers to α-profile, where α≥10. 
     The “effective area” can be defined as (4): 
     
       
         
           
             
               
                 
                   
                     A 
                     eff 
                   
                   = 
                   
                     
                       2 
                        
                       
                         
                           π 
                            
                           
                             [ 
                             
                               
                                 ∫ 
                                 0 
                                 ∞ 
                               
                                
                               
                                 
                                   
                                     ( 
                                     
                                       f 
                                        
                                       
                                         ( 
                                         r 
                                         ) 
                                       
                                     
                                     ) 
                                   
                                   2 
                                 
                                  
                                 rdr 
                               
                             
                             ] 
                           
                         
                         2 
                       
                     
                     
                       
                         ∫ 
                         0 
                         ∞ 
                       
                        
                       
                         
                           
                             ( 
                             
                               f 
                                
                               
                                 ( 
                                 r 
                                 ) 
                               
                             
                             ) 
                           
                           4 
                         
                          
                         rdr 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     where f(r) is the transverse component of the electric field of the guided optical signal and r is radial position in the fiber. “Effective area” or “A eff ” depends on the wavelength of the optical signal. Specific indication of the wavelength will be made when referring to “Effective area” or “A eff ” herein. Effective area is expressed herein in units of “μm 2 ”, “square micrometers”, “square microns” or the like. 
     The “mode field diameter” or “MFD” of an optical fiber is defined as MFD=2w, where w is defined as (5): 
     
       
         
           
             
               
                 
                   
                     w 
                     2 
                   
                   = 
                   
                     
                       
                         ∫ 
                         0 
                         ∞ 
                       
                        
                       
                         
                           
                             ( 
                             
                               f 
                                
                               
                                 ( 
                                 r 
                                 ) 
                               
                             
                             ) 
                           
                           2 
                         
                          
                         rdr 
                       
                     
                     
                       
                         ∫ 
                         0 
                         ∞ 
                       
                        
                       
                         
                           
                             ( 
                             
                               
                                 df 
                                  
                                 
                                   ( 
                                   r 
                                   ) 
                                 
                               
                               dr 
                             
                             ) 
                           
                           4 
                         
                          
                         rdr 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     where fir) is the transverse component of the electric field distribution of the guided optical signal and r is radial position in the fiber. “Mode field diameter” or “MFD” depends on the wavelength of the optical signal. Specific indication of the wavelength will be made when referring to “mode field diameter” or “MFD” herein. 
     As used herein, “trench volume” is the volume of the outer cladding surrounding core i and is defined as (61: 
     
       
         
           
             
               
                 
                   
                     V 
                     trench 
                   
                   = 
                   
                     
                       V 
                       ICi 
                     
                     = 
                     
                        
                       
                         2 
                          
                         
                           
                             ∫ 
                             
                               r 
                               i 
                             
                             
                               
                                 r 
                                 i 
                               
                               + 
                               
                                 δ 
                                  
                                 
                                     
                                 
                                  
                                 
                                   r 
                                   ICi 
                                 
                               
                             
                           
                            
                           
                             
                               ( 
                               
                                 
                                   
                                     Δ 
                                     ICi 
                                   
                                    
                                   
                                     ( 
                                     r 
                                     ) 
                                   
                                 
                                 - 
                                 
                                   Δ 
                                   
                                     C 
                                      
                                     C 
                                   
                                 
                               
                               ) 
                             
                              
                             rdr 
                           
                         
                       
                        
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     where r i  is the inner radius of the trench region surrounding core i, r i +δr ICi , is the outer radius of the trench region surrounding core i, Δ ICi (r) is the relative refractive index of the trench region, Δ CC  is the relative refractive index of the common cladding surrounding the trench region, and r is radial position in the fiber core. Trench volume is an absolute value and a positive quantity and will be expressed herein in units of % Δ-square micrometers, % Δmicron 2 , % Δ-micron 2 , % Δ-μm 2 , or % Δμm 2 , whereby these units can be used interchangeably herein. As used herein, when present, an inner cladding surrounding a core forms a trench, and thus for the purposes of the present disclosure, the terms inner cladding and trench can be used interchangeably to refer to the same region of the multicore optical fiber. 
     “Chromatic dispersion”, herein referred to as “dispersion” unless otherwise noted, of an optical fiber is the sum of the material dispersion, the waveguide dispersion, and the intermodal dispersion. In the case of single mode waveguide fibers, the inter-modal dispersion is zero. Dispersion values in a two-mode regime assume intermodal dispersion is zero. The zero dispersion wavelength (4) is the wavelength at which the dispersion has a value of zero. Dispersion slope is the rate of change of dispersion with respect to wavelength. Dispersion and dispersion slope are reported herein at a wavelength of 1310 nm or 1550 nm, as noted, and are expressed in units of ps/nm/km and ps/nm 2 /km, respectively 
     The cutoff wavelength of an optical fiber is the minimum wavelength at which the optical fiber will support only one propagating mode. For wavelengths below the cutoff wavelength, multimode transmission may occur and an additional source of dispersion may arise to limit the fiber&#39;s information carrying capacity. Cutoff wavelength will be reported herein as a cable cutoff wavelength. The cable cutoff wavelength is based on a 22-meter cabled fiber length as specified in TIA-455-80: FOTP-80 IEC-60793-1-44 Optical Fibres Part 1-44: Measurement Methods and Test Procedures Cut-off Wavelength (21 May 2003), by Telecommunications Industry Association (TIA). 
     The “theoretical cutoff wavelength”, or “theoretical fiber cutoff”, or “theoretical cutoff”, for a given higher-order mode, is the wavelength above which guided light cannot propagate in that higher-order mode. According to an aspect of the present disclosure, the cutoff wavelength refers to the cutoff wavelength of the LP11 mode. A mathematical definition can be found in Single Mode Fiber Optics, Jeunhomme, pp. 39-44, Marcel Dekker, New York, 1990, wherein the theoretical fiber cutoff is described as the wavelength at which the mode propagation constant becomes equal to the plane wave propagation constant in the common cladding. This theoretical wavelength is appropriate for an infinitely long, perfectly straight fiber that has no diameter variations. 
     The bend resistance of an optical fiber may be gauged by bend-induced attenuation under prescribed test conditions. In the present description, bend losses were determined by a mandrel wrap test. In the mandrel wrap test, the fiber is wrapped around a mandrel having a specified diameter and the attenuation of the fiber in the wrapped configuration at 1550 nm is determined. The bend loss is reported as the increase in attenuation of the fiber in the wrapped configuration relative to the attenuation of the fiber in an unwrapped (straight) configuration. Bend loss is reported herein in units of dB/turn, where one turn corresponds to a single winding of the fiber about the circumference of the mandrel. Bend losses for mandrel diameters of 10 mm, 15 mm, 20 mm, 30 mm, 40 mm, 50 mm, and 60 mm were determined. 
     As used herein, the multicore optical fiber can include a plurality of cores, wherein each core can be defined as an i th  core (i.e., 1 st , 2 nd , 3 rd , 4 th , etc. . . . ). Each i th  core can have an outer radius r i  and a relative refractive index Δ iMAX . Each i th  core is disposed within a cladding matrix of the multicore optical fiber which defines a common cladding of the multicore optical fiber. The common cladding includes a relative refractive index Δ CC  and an outer radius R CC . Optionally, each i th  core is surrounded by a corresponding i th  inner cladding having a width δr ICi  and a relative refractive index Δ ICi . Optionally, each i th  core can be surrounded by a corresponding i th  inner cladding having a width δr ICi  and a relative refractive index Δ ICi , which is surrounded by a corresponding i th  outer cladding having a width δroc, and a relative refractive index Δ OCi . Thus, i=1 refers to a first core having an outer radius r 1  and relative refractive index Δ 1  and a maximum relative refractive index Δ 1MAX . When the first core is surrounded by a corresponding it h  inner cladding, where i=1, it is referred to as the first inner cladding and has a width δr IC1  and a relative refractive index Δ IC1 . When the first inner cladding is surrounded by a corresponding i th  outer cladding, it is referred to as the first outer cladding and has a width δr OC1  and a relative refractive index Δ OC1 . When i=2, the core is referred to as a second core having an outer radius r 2  and relative refractive index Δ 2MAX . When the second core is surrounded by a corresponding i th  inner cladding, where i=2, it is referred to as the second inner cladding and includes a width δr IC2  and a relative refractive index Δ IC2 . Each additional i th  core and optional i th  inner cladding and optional i th  outer cladding is referred to as a third core and optional third inner cladding and optional third outer cladding (i=3), a fourth core and optional fourth inner cladding and optional fourth outer cladding (i=4), etc. . . . . The number assigned to each i th  core is used to distinguish one core from another for the purposes of discussion and does not necessarily imply any particular ordering of the cores. 
     According to one aspect of the present disclosure, the core forms the central portion of each core fiber within the multicore optical fiber and is substantially cylindrical in shape. In addition, when present, the surrounding inner cladding region is substantially annular in shape. Annular regions may be characterized in terms of an inner radius and an outer radius. Radial positions r refer herein to the outermost radii of the region (e.g., the core, the inner cladding region, etc. . . . ). When two regions are directly adjacent to each other, the outer radius of the inner of the two regions coincides with the inner radius of the outer of the two regions. For example, in embodiments in which an inner cladding region surrounds and is directly adjacent to a core region, the outer radius of the core region coincides with the inner radius of the inner cladding region and the outer radius of the inner cladding region is separated from the inner radius of the inner cladding region by the width δr IC . 
     The present illustrated embodiments generally relate to multicore optical fibers having at least two core fibers, wherein each core fiber includes a core comprising silica and greater than 3% chlorine, by weight (wt %), and a crosstalk between adjacent cores of ≤−30 dB, as measured for a 100 km length of the multicore optical fiber operating at a wavelength of 1550 nm. Accordingly, elements of the present disclosure have been represented, where appropriate, by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present disclosure so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. Further, like numerals in the description and drawings represent like elements. 
     In this document, relational terms, such as first and second, top and bottom, and the like, are used solely to distinguish one entity or action from another entity or action, without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by “comprises . . . a” does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element. 
     Aspects of the present disclosure generally relate to a multicore optical fiber in which each core includes silica glass and greater than 3 wt % chlorine. The relative refractive index profile of each core, optional inner cladding, and an common cladding can be tailored in combination with the spacing between adjacent cores to provide a multicore optical fiber having a crosstalk between the first core and the second core is ≤−30 dB, as measured for a 100 km length of the multicore optical fiber operating at a wavelength of 1550 nm. In some examples, the relative refractive index profile of each core, optional inner cladding, and the common cladding can be tailored in combination with the spacing between adjacent cores to provide a multicore optical fiber having different mode field diameters and/or attenuation. The multicore optical fibers of the present disclosure can provide higher core density in a single fiber than a conventional fiber including a single core. 
       FIGS. 1 and 2  illustrate an isometric view of a section of a multicore optical fiber  10  and a cross-sectional view of the multicore optical fiber  10 , respectively. The multicore optical fiber  10  includes a central fiber axis  12  (the centerline of the multicore optical fiber  10 ) and a cladding matrix  14  defining a common cladding  20 . The common cladding  20  can have an outer radius R CC , which in the illustrated embodiment of  FIGS. 1 and 2  corresponds to the outer radius of the multicore optical fiber  10 . A plurality of cores C i  (individually denoted C 1  and C 2  in the example of  FIGS. 1 and 2  and collectively referred to as cores “C”) are disposed within the cladding matrix  14 , with each core C i  forming a core fiber CF i  that generally extends through a length of the multicore optical fiber  10  parallel to the central fiber axis  12 . With reference to  FIG. 2 , each core C 1  and C 2  includes a central axis or centerline CL 1  and CL 2  and an outer radius r 1  and r 2 , respectively. A position of each of the centerlines CL 1  and CL 2  within the multicore optical fiber  10  can be defined using Cartesian coordinates with the central fiber axis  12  defining the origin (0, 0) of an x-y coordinate system coincident with the coordinate system defined by the radial coordinate R. The position of centerline CL 1  can be defined as (x 1 , y 1 ) and the position of centerline CL 2  can be defined as (x 2 , y 2 ). A distance D C1-C2  between the centerlines CL 1  and CL 2  can then be defined as √[(x 2 −x 1 ) 2 +(y 2 −y 1 ) 2 ]. Thus, for a given core C i  having a centerline CL i  and an adjacent core C j  having a centerline CL j , a distance D Ci-Cj  is defined as √[(x j −x i ) 2 +(y j −y i ) 2 ]. 
     While  FIGS. 1 and 2  illustrate the multicore optical fiber  10  as having a circular cross-sectional shape, the multicore optical fiber  10  can also be in the form of a ribbon having a rectangular or ribbon cross-sectional shape, as illustrated in  FIG. 3 . In some embodiments, the multicore optical fiber of the present disclosure can have a circular cross-section shape with seven cores, wherein six of the cores are at the vertices of a hexagon and the seventh core is at the center of the circular cross-section. In some embodiments, the multicore optical fiber of the present disclosure can have a circular cross-section and the cores can be arranged in a 2×2 configuration. In still other embodiments, the multicore optical fiber of the present disclosure can have a circular cross-section and the number of cores can be between 10 and 20. When in ribbon form, the multicore optical fiber  10  can have a cross-sectional width  30  and a thickness  32 . The cores C, can be arranged in one or more rows along the thickness  32  and in one or more columns extending along the width  30 . The position of each core C i  and each core centerline CL 1  can be defined using a Cartesian coordinate axis system with respect to the central fiber axis  12 , in a manner similar to that described above with respect to the circular multicore optical fiber  10  of  FIGS. 1 and 2 . 
     The multicore optical fiber  10  can have N number of total cores C i , wherein i=1 . . . N and N is at least 2. According to one aspect of the present disclosure, the total number N of cores Cr in the multicore optical fiber  10  is from 2 to 20, 2 to 18, 2 to 16, 2 to 12, 2 to 10, 2 to 8, 2 to 6, 2 to 4, 2 to 3, 4 to 20, 4 to 18, 4 to 16, 4 to 12, 4 to 10, 4 to 8, 4 to 6, 6 to 20, 6 to 18, 6 to 16, 6 to 12, 6 to 10, 6 to 8, 8 to 20, 8 to 18, 8 to 16, 8 to 12, 8 to 10, 10 to 20, 10 to 18, 10 to 16, 10 to 12, 12 to 20, 12 to 18, or 12 to 16. For example, the total number N of cores C in the multicore optical fiber  10  can be 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, or any total number N of cores C i  between any of these values. The total number N of cores C i  can be even or odd and can be arranged in any pattern within the cladding matrix  14 , non-limiting examples of which include a 2×4 pattern (or multiples thereof), a square pattern, a rectangular pattern, a circular pattern, and a hexagonal lattice pattern. For example, the multicore optical fiber  10  can have N=7 cores C i  arranged in a hexagonal lattice pattern. In another example, the multicore optical fiber  10  can have N=12 cores C i  arranged in a circular pattern. In one example, the multicore optical fiber  10  can have a core C i  positioned such that the core centerline CL i  aligns with the central fiber axis  12 . In another example, the multicore optical fiber  10  can have a core C i  pattern such that the cores C i  are spaced around the central fiber axis  12 . 
     Referring now to  FIG. 4A , according to one aspect of the present disclosure, one or more of the plurality of cores C i  of the multicore optical fiber  10  can optionally be surrounded by an inner cladding IC i . Each inner cladding IC has an outer radius r ICi  and an inner radius that corresponds to the outer radius r i  of the core C i . The inner cladding IC i  has a width δr ICi  defined by the outer radius r i  of the core C and the outer radius r ICi  of the inner cladding IC i . The core C i  can include a diameter d i  corresponding to 2*r i  and the inner cladding IC i  can include a diameter d ICi  corresponding to 2*r ICi . 
     Referring now to  FIG. 4B , according to one aspect of the present disclosure, one or more of the plurality of cores C i  of the multicore optical fiber  10  can optionally be surrounded by an inner cladding IC i  and an outer cladding OC surrounding the inner cladding IC i  between the inner cladding IC i  and the common cladding  20 . Each inner cladding IC i  has an outer radius r ICi  and an inner radius that corresponds to the outer radius r i  of the core C i . The inner cladding IC i  has a width δr IC1  defined by the outer radius r i  of the core C i  and the outer radius r ICi  of the inner cladding IC i . The core C i  can include a diameter d i  corresponding to 2*r i  and the inner cladding IC i  can include a diameter d ICi  corresponding to 2*r ICi . Each outer cladding OC i  has an outer radius r OCi  and an inner radius that corresponds to the outer radius r ICi  of the inner cladding IC i . The outer cladding OC i  has a width δr OCi  defined by the outer radius r ICi  of the inner cladding IC i  and the outer radius δr OCi  of the outer cladding OC i . 
     Referring again to  FIGS. 1-3 , the cladding matrix  14  forming the common cladding  20  can include undoped silica glass or doped silica glass. According to one aspect, the cladding matrix  14  is undoped silica glass. According to another aspect of the present disclosure, the cladding matrix  14  is doped silica glass that includes one or more up-dopants and/or one or more down-dopants. As used herein, the term “up-dopant” is used to refer to a dopant that increases the refractive index relative to pure, undoped silica glass. Non-limiting examples of up-dopants include chlorine (“Cl”), bromine (“Br”), germanium dioxide (“GeO 2 ”), aluminum trioxide (“Al 2 O 3 ”), phosphorus pentoxide (“P 2 O 5 ”), and titanium dioxide (“TiO 2 ”). As used herein, the term “down-dopant” is used to refer to a dopant that decreases the refractive index relative to pure, undoped silica glass. Non-limiting examples of down-dopants include fluorine (“F”) and boron (“B”). In one example, the core can be up-doped with GeO 2 . In another example, the inner and/or common cladding can be down-doped with fluorine. In another example, the inner and/or common cladding can be up-doped with chlorine. 
     For some dopants, the change in refractive index relative to undoped silica glass varies linearly as a function of dopant concentration. For example, up-doping with GeO 2  can result in a relative refractive index due to Ge (“ΔGe %”) that can be estimated as a function of concentration of GeO 2 , in weight percent (“wt % of GeO 2 ”), by the following equation: ΔGe %=0.0601*(wt % of GeO 2 ). In another example, down-doping with fluorine can result in a relative refractive index due to F (“ΔF %”) that can be estimated as a function of concentration of F, in weight percent (“wt % of F”), by the following equation: ΔF %=−0.3053*(wt % of F). In another example, up-doping with chlorine can result in a relative refractive index due to Cl (“ΔCl %”) that can be estimated as a function of concentration of Cl, in weight percent (“wt % of Cl”), by the following equation: ΔCl %=0.10*(wt % of Cl). 
     The amount of dopant in the silica glass can be selected to provide the common cladding  20  with one or more desired characteristics, non-limiting examples of which include a relative refractive index and a viscosity. According to one aspect of the present disclosure, the common cladding  20  includes silica glass doped with chlorine. In one example, an amount of chlorine dopant in the silica glass is from about 0 wt % to about 2 wt %, about 0.01 wt % to about 2 wt %, about 0.1 wt % to about 2 wt %, about 0.5 wt % to about 2 wt %, about 1 wt % to about 2 wt %, about 1.5 wt % to about 2 wt %, 0 wt % to about 1.5 wt %, about 0.01 wt % to about 1.5 wt %, about 0.1 wt % to about 1.5 wt %, about 0.5 wt % to about 1.5 wt %, about 1 wt % to about 1.5 wt %, 0 wt % to about 1 wt %, about 0.01 wt % to about 1 wt %, about 0.1 wt % to about 1 wt %, about 0.5 wt % to about 1 wt %, 0 wt % to about 0.5 wt %, about 0.01 wt % to about 0.5 wt %, or about 0.1 wt % to about 0.5 wt %. According to one aspect of the present disclosure, the common cladding  20  includes silica glass doped with fluorine. In one example, an amount of fluorine dopant in the silica glass is from about 0 wt % to about 2 wt %, about 0.01 wt % to about 2 wt %, about 0.1 wt % to about 2 wt %, about 0.5 wt % to about 2 wt %, about 1 wt % to about 2 wt %, about 1.5 wt % to about 2 wt %, 0 wt % to about 1.5 wt %, about 0.01 wt % to about 1.5 wt %, about 0.1 wt % to about 1.5 wt %, about 0.5 wt % to about 1.5 wt %, about 1 wt % to about 1.5 wt %, 0 wt % to about 1 wt %, about 0.01 wt % to about 1 wt %, about 0.1 wt % to about 1 wt %, about 0.5 wt % to about 1 wt %, 0 wt % to about 0.5 wt %, about 0.01 wt % to about 0.5 wt %, or about 0.1 wt % to about 0.5 wt %. 
     The common cladding  20  can have a relative refractive index Δ CC  of from about −0.25% to about 0.3%. For example, the common cladding  20  can have a relative refractive index Δ CC  of from about −0.25% to about 0.3%, about −0.2% to about 0.3%, about −0.15% to about 0.3%, about −0.1% to about 0.3%, about −0.05% to about 0.3%, about −0.025% to about 0.3%, about 0% to about 0.3%, about 0.025% to about 0.3%, about 0.05% to about 0.3%, about −0.25% to about 0.2%, about −0.2% to about 0.2%, about −0.15% to about 0.2%, about −0.1% to about 0.2%, about −0.05% to about 0.2%, about −0.025% to about 0.2%, about 0% to about 0.2%, about 0.025% to about 0.2%, about 0.05% to about 0.2%, about −0.25% to about 0.1%, about −0.2% to about 0.1%, about −0.15% to about 0.1%, about −0.1% to about 0.1%, about −0.05% to about 0.1%, about −0.025% to about 0.1%, about 0% to about 0.1%, about 0.025% to about 0.1%, about 0.05% to about 0.1%, about −0.25% to about 0.05%, about −0.2% to about 0.05%, about −0.15% to about 0.05%, about −0.1% to about 0.05%, about −0.05% to about 0.05%, about −0.025% to about 0.05%, about 0% to about 0.05%, about 0.025% to about 0.05%, about −0.25% to about 0.025%, about −0.2% to about 0.025%, about −0.15% to about 0.025%, about −0.1% to about 0.025%, about −0.05% to about 0.025%, about −0.025% to about 0.025%, about 0% to about 0.025%, about −0.25% to about 0%, about −0.2% to about 0%, about −0.15% to about 0%, about −0.1% to about 0%, about −0.05% to about 0%, or about −0.025% to about 0%. 
     When the multicore optical fiber  10  has a circular cross-sectional shape, the common cladding  20  can have an outer radius R CC  less than or equal to about 110 μm (≤110 μm). For example, the outer radius R CC  can be ≤110 μm, ≤100 μm, ≤95 μm, or ≤90 μm. In some examples, the outer radius R CC  can be from about 50 μm to about 110 μm. For example, the common cladding  20  can have an outer radius R CC  of from about 50 μm to about 110 μm, about 60 μm to about 110 μm, about 75 μm to about 110 μm, about 100 μm to about 110 μm, about 50 μm to about 100 μm, about 60 μm to about 100 μm, about 75 μm to about 100 μm, about 50 μm to about 95 μm, about 60 μm to about 95 μm, about 75 μm to about 95 μm, about 50 μm to about 90 μm, about 60 μm to about 90 μm, or about 75 μm to about 90 μm. In one example, the outer radius R CC  of the common cladding  20  is about 50 μm, about 60 μm, about 62 μm, about 62.5 μm, about 63 μm, about 70 μm, about 80 μm, about 90 μm, about 100 μm, about 110 μm, or any length between these values. 
     When the multicore optical fiber  10  is in the form of a ribbon having a rectangular or ribbon cross-sectional shape, the outer core  20  can have a width  30  of from about 50 μm to about 400 μm. According to one aspect, the outer core  20  can have a width  30  of from about 50 μm to about 400 μm, about 100 μm to about 400 μm, about 150 μm to about 400 μm, about 200 μm to about 400 μm, about 250 μm to about 400 μm, about 300 μm to about 400 μm, about 350 μm to about 400 μm, 50 μm to about 350 μm, about 100 μm to about 350 μm, about 150 μm to about 350 μm, about 200 μm to about 350 μm, about 250 μm to about 350 μm, about 300 μm to about 350 μm, 50 μm to about 300 μm, about 100 μm to about 300 μm, about 150 μm to about 300 μm, about 200 μm to about 300 μm, about 250 μm to about 300 μm, 50 μm to about 250 μm, about 100 μm to about 250 μm, about 150 μm to about 250 μm, about 200 μm to about 250 μm, 50 μm to about 200 μm, about 100 μm to about 200 μm, about 150 μm to about 200 μm, 50 μm to about 150 μm, about 100 μm to about 150 μm, or about 50 μm to about 125 μm. In another example, the width  30  of the common cladding  20  is less than 200 μm, less than 170 μm, less than 160 μm, less than 140 μm, less than 120 μm, or less than 130 μm. 
     According to an aspect of the present disclosure, each of the cores C i  includes silica glass doped with greater than 3 wt % chlorine. The chlorine doping in each of the cores C i  may be the same or different. In one aspect, the cores C i  can include silica glass doped with chlorine in an amount greater than 3 wt %, greater than 3.25 wt %, greater than 3.5 wt %, greater than 3.75 wt %, greater than 4 wt %, greater than 4.25 wt %, greater than 4.5 wt %, greater than 4.75 wt %, greater than 5 wt %, greater than 5.25 wt %, greater than 5.5 wt %, or greater than 6 wt %. For example, the cores C, can include silica glass doped with chlorine in an amount of from about 3 wt % to about 6 wt %, about 3.25 wt % to about 6 wt %, about 3.5 wt % to about 6 wt %, about 3.75 wt % to about 6 wt %, about 4 wt % to about 6 wt %, about 4.25 wt % to about 6 wt %, about 4.5 wt % to about 6 wt %, about 4.75 wt % to about 6 wt %, about 5 wt % to about 6 wt %, about 5.25 wt % to about 6 wt %, about 5.5 wt % to about 6 wt %, about 5.75 wt % to about 6 wt %, about 3 wt % to about 5 wt %, about 3.25 wt % to about 5 wt %, about 3.5 wt % to about 5 wt %, about 3.75 wt % to about 5 wt %, about 4 wt % to about 5 wt %, about 4.25 wt % to about 5 wt %, about 4.5 wt % to about 5 wt %, about 4.75 wt % to about 5 wt %, about 3 wt % to about 4 wt %, about 3.25 wt % to about 4 wt %, about 3.5 wt % to about 4 wt %, or about 3.75 wt % to about 4 wt %. For example, the cores C i  can include silica glass doped with chlorine in an amount of about 3 wt %, about 3.1 wt %, about 3.17 wt %, about 3.2 wt %, about 3.25 wt %, about 3.3 wt %, about 3.4 wt %, about 3.5 wt %, about 3.75 wt %, about 4 wt %, about 4.25 wt %, about 4.5 wt %, about 4.75 wt %, about 5 wt %, about 5.25 wt %, about 5.3 wt %, about 5.4 wt %, about 5.5 wt %, about 5.75 wt %, about 6 wt %, or any amount of chlorine between these values. 
     According to one aspect of the present disclosure, the cores C i  include silica glass doped with chlorine and are free or substantially free of any other dopants. As used herein, free or substantially free are used interchangeably to mean that no additional dopants are intentionally added to the cores C i , although it is understood that trace amounts of other materials may be present due to impurities and/or contaminants in source materials and/or processing equipment. 
     The cores C i  can have a relative refractive index Δ iMAX  of about 0.15% to about 0.5%. Each of the cores C i  can have the same or different relative refractive index Δ iMAX . In one aspect, the cores C i  can have a relative refractive index Δ iMAX  of about 0.15% to about 0.5%, about 0.2% to about 0.5%, about 0.25% to about 0.5%, about 0.3% to about 0.5%, about 0.325% to about 5%, about 0.35% to about 5%, about 0.15% to about 0.4%, about 0.2% to about 0.4%, about 0.25% to about 0.4%, about 0.3% to about 0.4%, about 0.325% to about 4%, about 0.35% to about 4%, about 0.15% to about 0.35%, about 0.2% to about 0.35%, about 0.25% to about 0.35%, about 0.3% to about 0.35%, about 0.325% to about 0.35%, about 0.15% to about 0.325%, about 0.2% to about 0.325%, about 0.25% to about 0.325%, about 0.3% to about 0.325%, or about 0.15% to about 0.25%. In one example, the cores C i  can have a relative refractive index Δ iMAX  of greater than 0.15%, greater than 0.2%, greater than 0.25%, greater than 0.3%, greater than 0.325%, greater than 0.35%, or greater than 0.4%. In one example, the cores C i  can have a relative refractive index Δ iMAX  of about 0.15%, about 0.2%, about 0.25%, about 0.3%, about 0.325%, about 0.34%, about 0.35%, about 0.4%, about 0.45%, about 0.5%, or any relative refractive index Δ iMAX  between these values. 
     According to an aspect of the present disclosure, the cores C i  can have a core alpha a of about 1≤α≤100. The cores C can each have the same or different core alpha. For example, the cores C i  can have a core alpha a of about 1≤α≤about 100, about 10≤α≤about 100, about 10≤α≤about 50, about 10≤α≤about 20, about 1≤α≤about 8, about 2≤α≤about 6, or about 2≤α≤about 4. In one example, the cores C i  can have a core alpha a of about 20. In another example, the cores C i  can have a core alpha a of about 2.5. 
     According to an aspect of the present disclosure, the cores C i  can have an outer core radius r i  of from about 2.5 μm to about 12.5 μm. Each of the cores C i  can have the same or different outer core radius r i . In one example, the cores C i  can have an outer core radius r i  of from about 2.5 μm to about 12.5 μm, about 5.0 μm to about 12.5 μm, about 10 μm to about 12.5 μm, about 2.5 μm to about 10 μm, about 5 μm to about 10 μm, or about 2.5 μm to about 5 μm. In one example, the cores C i  can have an outer core radius r i  of about 2.5 μm, about 3 μm, about 4 μm, about 4.2 μm, about 4.5 μm, about 4.9 μm, about 5 μm, about 5.5 μm, about 6 μm, about 6.5 μm, about 7 μm, about 7.3 μm, about 7.4 μm, about 8 μm, about 10 μm, about 12.5 μm, or any outer core radius r i  between these values. The cores C i  can be single mode or multimode depending on the operating wavelength of the multicore optical fiber  10 . 
     According to an aspect of the present disclosure, a distance between the centerline CL i  of a given core C i  and the centerline CL j  of an adjacent core C j  is greater than 10 μm, as measured using a Cartesian coordinate system in which the central fiber axis  12  defines the origin (0, 0) of the coordinate system. For a given core C i  having a centerline CL i  and an adjacent core C j  having a centerline CL j , a distance D Ci-Cj  is defined as √[(x j −x i ) 2 +(y j −y i ) 2 ]. For example, a distance D Ci-Cj  between adjacent cores can be greater than 10 μm, greater than 15 μm, greater than 20 μm, greater than 25 μm, greater than 28 μm, or greater than 30 μm. For example, a distance D Ci-Cj  between adjacent cores can be from about 10 μm to about 50 μm, about 10 μm to about 40 μm, about 10 μm to about 30 μm, about 10 μm to about 20 μm, about 20 μm to about 50 μm, about 20 μm to about 40 μm, about 20 μm to about 30 μm, about 28 μm to about 50 μm, about 28 μm to about 40 μm, about 28 μm to about 30 μm, about 30 μm to about 50 μm, about 30 μm to about 40 μm, or about 40 μm to about 50 μm. The distance D Ci-Cj  between adjacent cores can be the same or different for each of the cores of the multicore optical fiber  10 . 
     According to an aspect of the present disclosure, the multicore optical fiber  10  can include an inner cladding ICi, such as illustrated in the exemplary embodiment of  FIG. 4A . When present, the inner cladding IC i  can include silica glass doped with fluorine or undoped silica. In some aspects, the inner cladding IC i  can include silica glass doped with fluorine in an amount of from about 0 wt % to about 0.5 wt %. For example, the inner cladding IC i  can include silica glass doped with fluorine in an amount of from about 0 wt % to about 0.5 wt %, about 0.1 wt % to about 0.5 wt %, about 0.1 wt % to about 0.4 wt %, about 0.1 wt % to about 0.3 wt %, about 0.1 wt % to about 0.2 wt %, about 0.2 wt % to about 0.5 wt %, about 0.2 wt % to about 0.4 wt %, about 0.2 wt % to about 0.3 wt %, about 0.3 wt % to about 0.5 wt %, or about 0.3 wt % to about 0.4 wt %. For example, the inner cladding IC i  can include silica glass doped with fluorine in an amount of about 0 wt %, about 0.1 wt %, about 0.15 wt %, about 0.17 wt %, about 0.2 wt %, about 0.23 wt %, about 0.25 wt %, about 0.3 wt %, about 0.35 wt %, about 0.4 wt %, about 0.45 wt %, about 0.5 wt %, or any amount of fluorine between these values. Each inner cladding IC i  can have the same or different amount of fluorine doping. 
     In some aspects, when the common cladding  20  includes silica glass doped with an up-dopant, such as chlorine, for example, the inner cladding IC can include undoped silica glass. In another example, the common cladding  20  can include silica glass doped with a down-dopant, such as fluorine, for example, and the inner cladding IC i  can include undoped silica glass. In this manner, the inner cladding IC can be defined as a region surrounding the core C i  and between the core C i  and the common cladding  20  in which the relative refractive index Δ ICi  of the inner cladding IC i  is less than the relative refractive index core Δ iMAX  of the core C i  and different than the relative refractive index Δ CC  of the common cladding  20 . For example, the relative refractive index Δ CC  of the common cladding  20  can be less than or greater than the relative refractive index Δ ICi  of each inner cladding IC i . In some examples, the relative refractive index Δ CC  of the common cladding  20  is greater than the relative refractive index Δ ICi  of one or more of the inner claddings IC i . In some examples, the relative refractive index Δ CC  of the common cladding  20  is less than the relative refractive index Δ ICi  of one or more of the inner claddings IC i . 
     Still referring to the exemplary embodiment of  FIG. 4A , according to an aspect of the present disclosure the inner cladding IC i  can be characterized by a relative refractive index Δ ICi ≤0. Each inner cladding IC i  can have the same or different relative refractive index Δ ICi . The relative refractive index Δ ICi  of the inner cladding IC i  is less than the relative refractive index Δ iMAX  of the core C i  which the corresponding inner cladding IC i  surrounds. According to one aspect, a relative refractive index Δ ICi  of the inner cladding IC i  is from about −0.2% to about 0%. For example, the inner cladding IC i  can be characterized by a relative refractive index Δ ICi  of from about −0.7% to about 0%, about −0.6% to about 0%, about −0.5% to about 0%, about −0.4% to about 0%, about −0.3% to about 0%, about −0.2% to about 0%, −0.18% to about 0%, about −0.15% to about 0%, about −0.12% to about 0%, about −0.1% to about 0%, about −0.08% to about 0%, about −0.05% to about 0%, about −0.02% to about 0%, about −0.7% to about −0.1%, about −0.6% to about −0.1%, about −0.5% to about −0.1%, about −0.4% to about −0.1%, about −0.3% to about −0.1%, about −0.7% to about −0.2%, about −0.6% to about −0.2%, about −0.5% to about −0.2%, about −0.4% to about −0.2%, about −0.3% to about −0.2%, about −0.2% to about −0.02%, −0.18% to about −0.02%, about −0.15% to about −0.02%, about −0.12% to about −0.02%, about −0.1% to about −0.02%, about −0.08% to about −0.02%, about −0.05% to about −0.02%, about −0.2% to about −0.05%, −0.18% to about −0.05%, about −0.15% to about −0.05%, about −0.12% to about −0.05%, about −0.1% to about −0.05%, about −0.08% to about −0.05%, about −0.2% to about −0.08%, −0.18% to about −0.08%, about −0.15% to about −0.08%, about −0.12% to about −0.08%, about −0.1% to about −0.08%, about −0.2% to about −0.1%, −0.18% to about −0.1%, about −0.15% to about −0.1%, or about −0.12% to about −0.1%. For example, the inner cladding IC i  can be characterized by a relative refractive index Δ ICi  of about −0.7%, about −0.6%, about −0.5%, about −0.4%, about −0.3%, about −0.2%, about −0.18%, about −0.15%, about −0.12%, about −0.1%, about −0.08%, about −0.07%, about −0.05%, about −0.02%, about −0.01%, or any relative refractive index between these values. 
     Still referring to the exemplary embodiment of  FIG. 4A , according to an aspect of the present disclosure, the inner cladding IC i  can have a width δr ICi  of from about 5 μm to about 30 μm. The width δr ICi  of each inner cladding IC i  can be the same or different. In one aspect, the inner cladding IC i  can have a width δr ICi  of from about 5 μm to about 30 μm, about 10 μm to about 30 μm, about 15 μm to about 30 μm, about 20 μm to about 30 μm, about 25 μm to about 30 μm, about 5 μm to about 25 μm, about 10 μm to about 25 μm, about 15 μm to about 25 μm, about 20 μm to about 25 μm, about 5 μm to about 20 μm, about 10 μm to about 20 μm, about 15 μm to about 20 μm, about 5 μm to about 15 μm, or about 10 μm to about 15 μm. For example, the inner cladding IC i  can have a width δr ICi  of about 5 μm, about 10 μm, about 15 μm, about 15.8 μm, about 16 μm, about 17 μm, about 17.6 μm, about 17.7 μm, about 20 μm, about 22 μm, about 25 μm, about 27 μm, about 30 μm, or any width between these values. As discussed above, the width δr ICi  of the inner cladding IC i  can be defined as a distance between the outer radius r ICi  of the inner cladding IC i  and the inner radius of the inner cladding IC i , which corresponds to the outer radius r i  of the core C i  that is surrounded by the inner cladding IC i . 
     Still referring to the exemplary embodiment of  FIG. 4A , according to an aspect of the present disclosure, the inner cladding IC i  can have a trench volume of greater than about 30%Δ-square micrometers. In one aspect, the inner cladding IC i  can have a trench volume of greater than about 30%Δ-square micrometers, greater than about 40%Δ-square micrometers, greater than about 50%Δ-square micrometers, or greater than about 60%Δ-square micrometers. In some aspects, the inner cladding IC i  can have a trench volume of less than about 75%Δ-square micrometers, less than about 70%Δ-square micrometers, less than about 65%Δ-square micrometers, or less than about 60%Δ-square micrometers. In some aspects, the inner cladding IC i  can have a trench volume of from about 30%Δ-square micrometers to about 120%Δ-square micrometers, about 40%Δ-square micrometers to about 120%Δ-square micrometers, about 50%Δ-square micrometers to about 120%Δ-square micrometers, about 60%Δ-square micrometers to about 120%Δ-square micrometers, about 70%Δ-square micrometers to about 120%Δ-square micrometers, about 80%Δ-square micrometers to about 120%Δ-square micrometers, about 90%Δ-square micrometers to about 120%Δ-square micrometers, about 100%Δ-square micrometers to about 120%Δ-square micrometers, about 110%Δ-square micrometers to about 120%Δ-square micrometers, about 30%Δ-square micrometers to about 110%Δ-square micrometers, about 40%Δ-square micrometers to about 110%Δ-square micrometers, about 50%Δ-square micrometers to about 110%Δ-square micrometers, about 60%Δ-square micrometers to about 110%Δ-square micrometers, about 70%Δ-square micrometers to about 110%Δ-square micrometers, about 80%Δ-square micrometers to about 110%Δ-square micrometers, about 90%Δ-square micrometers to about 110%Δ-square micrometers, about 100%Δ-square micrometers to about 110%Δ-square micrometers, about 30%Δ-square micrometers to about 100%Δ-square micrometers, about 40%Δ-square micrometers to about 100%Δ-square micrometers, about 50%Δ-square micrometers to about 100%Δ-square micrometers, about 60%Δ-square micrometers to about 100%Δ-square micrometers, about 70%Δ-square micrometers to about 100%Δ-square micrometers, or about 80%Δ-square micrometers to about 100%Δ-square micrometers. For example, the inner cladding IC i  can have a trench volume of about 30%Δ-square micrometers, about 40%Δ-square micrometers, about 50%Δ-square micrometers, about 55%Δ-square micrometers, about 57%Δ-square micrometers, about 60% Δ-square micrometers, about 62% Δ-square micrometers, about 70% Δ-square micrometers, about 80% Δ-square micrometers, about 90% Δ-square micrometers, about 100% Δ-square micrometers, about 107% Δ-square micrometers, about 110% Δ-square micrometers, about 120% Δ-square micrometers, or any trench volume between these values. Each inner cladding IC i  can have the same or different trench volume. The trench volume of the inner cladding IC i  can be determined as described above wherein r IC,inner  corresponds to the inner radius of the inner cladding IC i , which corresponds to the outer radius r i  of the core C i  that is surrounded by the inner cladding IC i  and r IC,outer  corresponds to the outer radius r ICi  of the inner cladding IC i . 
     According to an aspect of the present disclosure, the multicore optical fiber  10  can include a core C i  having an inner cladding IC i  and an outer cladding OC i , an exemplary embodiment of which is illustrated in  FIG. 4B . The inner cladding ICi can include undoped silica, silica glass doped with an up-dopant, such as chlorine, or silica glass doped with a down-dopant, such as fluorine. The material of the inner cladding IC i  can be selected such that the relative refractive index Δ ICi  of the inner cladding IC i  is less than the relative refractive index Δ iMAX  of the core C i  and greater than the relative refractive index Δ OCi  of the outer cladding OC i  which surrounds the corresponding inner cladding IC i . The outer cladding OC i  can include undoped silica, silica glass doped with an up-dopant, such as chlorine, or silica glass doped with a down-dopant, such as fluorine. The outer cladding OC i  defines a depressed refractive index region (also referred to as a trench), that is offset from the core C i  (also referred to as an offset trench). The outer cladding OC i  can be made by either down doping the outer cladding OC i  relative to the inner cladding IC i  and the common cladding  20  or by up-doping the inner cladding IC i  and the common cladding  20  relative to the outer cladding OC i . In one exemplary embodiment, the relative refractive index Δ ICi  of the inner cladding IC i ≥0 and is less than the relative refractive index Δ iMAX  of the core C i  and greater than the relative refractive index Δ OCi  of the outer cladding OC i , where Δ CC &gt;Δ OCi . In one example, the relative refractive index Δ OCi  can be ≤0. In another exemplary embodiment, the relative refractive index Δ ICi  of the inner cladding IC i  and the common cladding Δ CC  is 0 and the relative refractive index Δ OCi  of the outer cladding OC i &lt;0. The relative refractive index Δ ICi  of the inner cladding IC i  and the common cladding Δ CC  can be the same or different. 
     Still referring to the exemplary embodiment of  FIG. 4B , according to an aspect of the present disclosure the inner cladding IC i  can be characterized by a relative refractive index Δ ICi ≤0 or a relative refractive index Δ ICi ≥0. Each inner cladding IC i  can have the same or different relative refractive index Δ ICi . The relative refractive index Δ ICi  of the inner cladding IC i  is less than the relative refractive index Δ iMAX  of the core C i  which the corresponding inner cladding IC i  surrounds. According to one aspect, a relative refractive index Δ ICi  of the inner cladding IC i  is from about −0.05% to about 0.05%. For example, the inner cladding IC i  can be characterized by a relative refractive index Δ ICi  of from about −0.05% to about 0.05%, about −0.04% to about 0.04%, about −0.03% to about 0.03%, about −0.02% to about 0.02%, about −0.01% to about 0.01%, or about 0%. For example, the inner cladding IC i  can be characterized by a relative refractive index Δ ICi  of about −0.05%, about −0.04%, about −0.03%, about −0.02%, about −0.01%, about 0%, about 0.01%, about 0.02%, about 0.03%, about 0.04%, about 0.05%, or any relative refractive index between these values. 
     In some embodiments, the inner cladding IC i  can include undoped silica glass. In other embodiments, the inner cladding IC i  can include silica glass doped with fluorine (to decrease the relative refractive index) or silica glass doped with chlorine (to increase the relative refractive index) to provide the desired relative refractive index. For example, the inner cladding IC i  can include silica glass doped with fluorine in an amount of from about 0 wt % to about 0.16 wt %. For example, the inner cladding IC i  can include silica glass doped with fluorine in an amount of from about 0 wt % to about 0.16 wt %, about 0 wt % to about 0.13 wt %, about 0 wt % to about 0.1 wt %, about 0 wt % to about 0.06 wt %, or about 0 wt % to about 0.03 wt %. In another example, the inner cladding IC i  can include silica glass doped with chlorine in an amount of from about 0 wt % to about 0.5 wt %, about 0 wt % to about 0.4 wt %, about 0 wt % to about 3 wt %, about 0 wt % to about 2 wt %, or about 0 wt % to about 1 wt %. Each inner cladding IC i  can have the same or different amount of doping. 
     Still referring to the exemplary embodiment of  FIG. 4B , according to an aspect of the present disclosure, the inner cladding IC i  can have a width δr ICi  of from about 5 μm to about 30 μm. The width δr ICi  of each inner cladding IC i  can be the same or different. In one aspect, the inner cladding IC i  can have a width δr ICi  of from about 5 μm to about 20 μm, about 10 μm to about 20 μm, about 15 μm to about 20 μm, about 5 μm to about 15 μm, about 5 μm to about 10 μm, or about 10 μm to about 15 μm. For example, the inner cladding IC can have a width δr ICi  of about 5 μm, about 6 μm, about 7 μm, about 8 μm, about 9 μm, about 10 μm, about 11 μm, about 12 μm, about 13 μm, about 14 μm, about 15 μm, about 20 μm, or any width between these values. 
     Still referring to the exemplary embodiment of  FIG. 4B , according to an aspect of the present disclosure, the outer cladding OC i  can include undoped silica glass, silica glass doped with fluorine, or silica glass doped with chlorine to provide an outer cladding region having a depressed relative refractive index Δ OCi  relative to the adjacent inner cladding IC and the common cladding  20 . For example, when one or both of the inner cladding IC i  and the common cladding  20  include undoped silica glass or silica glass doped with fluorine, the outer cladding OC i  can include silica glass doped with fluorine in an amount such that Δ OCi &lt;Δ ICi  and Δ OCi &lt;Δ CC . In another example, when both of the inner cladding IC i  and the common cladding  20  include silica glass doped with chlorine, the outer cladding OC can include undoped silica glass, silica glass doped with fluorine, or silica glass doped with chlorine in an amount such that Δ OCi &lt;Δ ICi  and Δ OCi &lt;Δ CC . 
     In one example, the outer cladding OC i  can include silica glass doped with fluorine in an amount of from about 0 wt % to about 3 wt %. For example, the outer cladding OC i  can include from about 0 wt % to about 3 wt %, about 0 wt % to about 2 wt %, about 0 wt % to about 1 wt %, about 0 wt % to about 0.5 wt %, about 0.5 wt % to about 3 wt %, about 0.5 wt % to about 2 wt %, about 0.5 wt % to about 1 wt %, about 1 wt % to about 3 wt %, about 1 wt % to about 2 wt %, or about 2 wt % to about 3 wt %. For example, the outer cladding OC i  can include about 0 wt %, about 0.5 wt %, about 0.8 wt %, about 1 wt %, about 1.1 wt %, about 1.3 wt %, about 2 wt %, about 3 wt %, or any amount of fluorine between these values. Each outer cladding OC i  can have the same or different amount of fluorine doping. 
     Still referring to the exemplary embodiment of  FIG. 4B , the outer cladding OC i  has a relative refractive index that is less than the adjacent inner cladding ICi that the outer cladding OC i  surrounds and less than the common cladding  20 . Each outer cladding OC i  can have the same or different relative refractive index. In one aspect, the outer cladding OC i  can have a relative refractive index Δ OCi ≤0%. For example, the outer cladding OC i  can have a relative refractive index Δ OCi  of from about −1% to about 0%, about −0.75% to about 0%, about −0.5% to about 0%, about −0.25% to about 0%, about −1% to about −0.25%, about −0.75% to about −0.25%, about −0.5% to about −0.25%, about −1% to about −0.5%, or about −0.75% to about −0.5%. For example, the outer cladding OC i  can have a relative refractive index Δ OCi  of about 0%, about −0.25%, about −0.26%, about −0.3%, about −0.33%, about −0.4%, about −0.5%, about −0.75%, about −1%, or any relative refractive index between these values. 
     Still referring to the exemplary embodiment of  FIG. 4B , the outer cladding OC i  can have a width δr OCi  of from about 2 μm to about 20 μm. The width &amp;cc, of each outer cladding OCi can be the same or different. In one aspect, the outer cladding OC i  can have a width δroc, of from about 2 μm to about 20 μm, about 2 μm to about 15 μm, about 2 μm to about 10 μm, about 2 μm to about 8 μm, about 3 μm to about 20 μm, about 3 μm to about 15 μm, about 3 μm to about 10 μm, about 3 μm to about 8 μm, about 4 μm to about 20 μm, about 4 μm to about 15 μm, about 4 μm to about 10 μm, about 4 μm to about 8 μm, about 10 μm to about 20 μm, about 10 μm to about 15 μm, or about 15 μm to about 20 μm. For example, the outer cladding OC i  can have a width δ OCi  of about 2 μm, about 3 μm, about 4 μm, about 5 μm, about 6 μm, about 7 μm, about 8 μm, about 9 μm, 10 μm, about 12 μm, about 15 μm, about 16 μm, about 17 μm, about 18 μm, about 19 μm, about 20 μm, or any width between these values. 
     In one aspect, the outer cladding OC i  can be characterized by a trench volume of greater than about 30% Δ-square micrometers. In one aspect, the outer cladding OC i  can have a trench volume of greater than about 30% Δ-square micrometers, greater than about 40% Δ-square micrometers, greater than about 50% Δ-square micrometers, or greater than about 60% Δ-square micrometers. In some aspects, the outer cladding OC i  can have a trench volume of less than about 75% Δ-square micrometers, less than about 70% Δ-square micrometers, less than about 65% Δ-square micrometers, or less than about 60% Δ-square micrometers. In some aspects, the outer cladding OC i  can have a trench volume of from about 30% Δ-square micrometers to about 75% Δ-square micrometers, about 40% Δ-square micrometers to about 75% Δ-square micrometers, about 50% Δ-square micrometers to about 75% Δ-square micrometers, about 60% Δ-square micrometers to about 75% Δ-square micrometers, about 30% Δ-square micrometers to about 65% Δ-square micrometers, about 40% Δ-square micrometers to about 65% Δ-square micrometers, about 50% Δ-square micrometers to about 65% Δ-square micrometers, about 30% Δ-square micrometers to about 55% Δ-square micrometers, or about 40% Δ-square micrometers to about 55% Δ-square micrometers. For example, the outer cladding OC i  can have a trench volume of about 30% Δ-square micrometers, about 40% Δ-square micrometers, about 45% Δ-square micrometers, about 46% Δ-square micrometers, about 47% Δ-square micrometers, about 48% Δ-square micrometers, about 49% Δ-square micrometers, about 50% Δ-square micrometers, about 55% Δ-square micrometers, about 60% Δ-square micrometers, about 61% Δ-square micrometers, about 62% Δ-square micrometers, about 68% Δ-square micrometers, about 69% Δ-square micrometers, about 70% Δ-square micrometers, about 75% Δ-square micrometers, or any trench volume between these values. Each outer cladding OC i  can have the same or different trench volume. The trench volume of the outer cladding OC i  can be determined as described above. 
     The multicore optical fiber  10  can be characterized by crosstalk between adjacent cores C i  of equal to or less than −20 dB, as measured for a 100 km length of the multicore optical fiber  10  operating at 1550 nm. In some aspects, the multicore optical fiber  10  can be characterized by crosstalk between adjacent cores C i  of equal to or less than −30 dB, as measured for a 100 km length of the multicore optical fiber  10 . In some aspects, crosstalk between adjacent cores C i  is ≤−20 dB, ≤−30 dB, ≤−40 dB, ≤−50 dB, or ≤−60 dB, as measured for a 100 km length of the multicore optical fiber  10  operating at 1550 nm. The crosstalk can be determined based on the coupling coefficient, which depends on the design of the core and a distance between two adjacent cores, and Δβ, which depends on a difference in β values between the two adjacent cores. For two cores placed next to each other, assuming the power launched into the first core is P 1 , using coupled mode theory and considering the perturbations along the fiber, the power coupled to the second core, P 2 , can be determined using the following equation (7): 
     
       
         
           
             
               
                 
                   
                     P 
                     2 
                   
                   = 
                   
                     
                       L 
                       
                         L 
                         c 
                       
                     
                      
                     
                       〈 
                       
                         
                           
                             ( 
                             
                               κ 
                               g 
                             
                             ) 
                           
                           2 
                         
                          
                         
                           
                             sin 
                             2 
                           
                            
                           
                             ( 
                             
                               g 
                                
                               
                                   
                               
                                
                               Δ 
                                
                               
                                   
                               
                                
                               L 
                             
                             ) 
                           
                         
                       
                       〉 
                     
                      
                     
                       P 
                       1 
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     where &lt; &gt; denotes the average, L is fiber length, κ is the coupling coefficient, ΔL is the length of the fiber segment over which the fiber is uniform, L c  is the correlation length, and g is given by the following equation (8): 
     
       
         
           
             
               
                 
                   
                     g 
                     2 
                   
                   = 
                   
                     
                       κ 
                       2 
                     
                     + 
                     
                       
                         ( 
                         
                           
                             Δ 
                              
                             β 
                           
                           2 
                         
                         ) 
                       
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     where Δβ is the mismatch in propagation constant between the modes in two cores when they are isolated. The crosstalk (in dB) can be determined using the following equation (9): 
     
       
         
           
             
               
                 
                   X 
                   = 
                   
                     
                       10 
                        
                       
                         log 
                          
                         
                           ( 
                           
                             
                               P 
                               2 
                             
                             
                               P 
                               1 
                             
                           
                           ) 
                         
                       
                     
                     = 
                     
                       10 
                        
                       
                         log 
                          
                         
                           ( 
                           
                             
                               L 
                               
                                 L 
                                 c 
                               
                             
                              
                             
                               〈 
                               
                                 
                                   
                                     ( 
                                     
                                       κ 
                                       g 
                                     
                                     ) 
                                   
                                   2 
                                 
                                  
                                 
                                   
                                     sin 
                                     2 
                                   
                                    
                                   
                                     ( 
                                     
                                       g 
                                        
                                       
                                           
                                       
                                        
                                       Δ 
                                        
                                       
                                           
                                       
                                        
                                       L 
                                     
                                     ) 
                                   
                                 
                               
                               〉 
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     The crosstalk between the two cores grows linearly in the linear scale, but does not grow linearly in the dB scale. As used herein, crosstalk performance is reported for a 100 km length of optical fiber. However, crosstalk performance can also be represented with respect to alternative optical fiber lengths, with appropriate scaling. For optical fiber lengths other than 100 km, the crosstalk between cores can be determined using the following equation (10): 
     
       
         
           
             
               
                 
                   
                     X 
                      
                     
                       ( 
                       L 
                       ) 
                     
                   
                   = 
                   
                     
                       X 
                        
                       
                         ( 
                         
                           1 
                            
                           0 
                            
                           0 
                         
                         ) 
                       
                     
                     + 
                     
                       10 
                        
                       
                         log 
                          
                         
                           ( 
                           
                             L 
                             
                               1 
                                
                               0 
                                
                               0 
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     For example, for a 10 km length of optical fiber, the crosstalk can be determined by adding “−10 dB” to the crosstalk value for a 100 km length optical fiber. For a 1 km length of optical fiber, the crosstalk can be determined by adding “−20 dB” to the crosstalk value for a 100 km length of optical fiber. 
     Discussions regarding methods for determining crosstalk between cores in a multicore optical fiber can be found in M. Li, et al., “Coupled Mode Analysis of Crosstalk in Multicore Fiber with Random Perturbations,” in Optical Fiber Communication Conference, OSA Technical Digest (online),  Optical Society of America,  2015, paper W2A.35, and Shoichiro Matsuo, et al., “Crosstalk behavior of cores in multi-core fiber under bent condition,”  IEICE Electronics Express , Vol. 8, No. 6, p. 385-390, published Mar. 25, 2011 and Lukasz Szostkiewicz, et al., “Cross talk analysis in multicore optical fibers by supermode theory,”  Optics Letters , Vol. 41, No. 16, p. 3759-3762, published Aug. 15, 2016, the contents of which are both incorporated herein by reference in their entirety. 
     According to one aspect, the cores C i  of the multicore optical fiber  10  can be characterized by an attenuation of less than 0.18 dB/km at an operating wavelength of 1550 nm. For example, the attenuation can be less than 0.18 dB/km, less than 0.175 dB/km, less than 0.17 dB/km, or less than 0.16 dB/km at an operating wavelength of 1550 nm. Each core C i  may have the same or different attenuation. 
     According to one aspect, the cores C i  of the multicore optical fiber  10  can be characterized by an effective area (“A eff ”) of at least 50 μm 2  at 1550 nm. Each core C i  may have the same or different effective area. In one aspect, the effective area is at least 50 μm 2 , at least 75 μm 2 , at least 100 μm 2 , at least 125 μm 2 , at least 150 μm 2 , or at least 200 μm 2  at 1550 nm. For example, the effective area can be from about 50 μm 2  to about 200 μm 2 , about 75 μm 2  to about 200 μm 2 , about 100 μm 2  to about 200 μm 2 , about 125 μm 2  to about 200 μm 2 , about 150 μm 2  to about 200 μm 2 , or about 175 μm 2  to about 200 μm 2  at 1550 nm. For example, the effective area can be about 50 μm 2 , about 75 μm 2 , about 100 μm 2 , about 110 μm 2 , about 112 μm 2 , about 125 μm 2 , about 150 μm 2 , about 152 μm 2 , about 175 μm 2 , about 200 μm 2  or any effective area between these values. 
     According to one aspect, the cores C i  of the multicore optical fiber  10  can be characterized by an effective area of at least 50 μm 2  at 1310 nm. Each core C i  may have the same or different effective area. In one aspect, the effective area is at least 50 μm 2 , at least 55 μm 2 , at least 60 μm 2 , at least 65 μm 2 , at least 70 μm 2 , or at least 75 μm 2  at 1310 nm. For example, the effective area can be from about 50 μm 2  to about 75 μm 2 , about 55 μm 2  to about 75 μm 2 , about 60 μm 2  to about 75 μm 2 , about 65 μm 2  to about 75 μm 2 , or about 60 μm 2  to about 75 μm 2  at 1310 nm. For example, the effective area can be about 50 μm 2 , about 55 μm 2 , about 60 μm 2 , about 65 μm 2 , about 66 μm 2 , about 68 μm 2 , about 69 μm 2 , about 70 μm 2 , about 72 μm 2 , about 75 μm 2  or any effective area between these values. 
     According to one aspect, the mode field diameter of the cores C i  is at least 6 μm, at least 8 μm, at least 10 μm, at least 12 μm, or at least 13 μm at 1550 nm. For example, the mode field diameter can be from about 6 μm to about 15 μm, about 8 μm to about 15 μm, about 10 μm to about 15 μm, about 6 μm to about 12 μm, about 8 μm to about 12 μm, about 10 μm to about 12 μm, about 6 μm to about 10 μm, or about 8 μm to about 10 μm at 1550 nm. For example, the mode field diameter can be about 6 μm, about 8 μm, about 9 μm, about 10 μm, about 11 μm, about 12 μm, about 13 μm, about 14 μm, about 15 μm, or any value between these values at 1550 nm. 
     According to one aspect, the mode field diameter of the cores C i  is at least 5 μm, at least 9 μm, at least 10 μm, at least 12 μm, or at least 13 μm at 1310 nm. For example, the mode field diameter can be from about 5 μm to about 15 μm, about 9 μm to about 15 μm, or about 10 μm to about 15 μm at 1310 nm. For example, the mode field diameter can be about 5 μm, about 8 μm, about 9 μm, about 10 μm, about 11 μm, about 12 μm, about 13 μm, about 14 μm, about 15 μm, or any value between these values at 1310 nm. Each core C i  may have the same or different mode field diameter at each operating wavelength of 1550 nm and 1310 nm. 
     According to an aspect of the present disclosure, the cores C i  may have a theoretical cutoff wavelength of less than about 1500 nm, less than about 1400 nm, less than about 1300 nm, less than about 1260 nm, or less than about 1200 nm. For example, the theoretical cutoff wavelength can be from about 1300 nm to about 1500 nm or about 1300 nm to about 1400 nm. For example, the theoretical cutoff wavelength can be about 1300 nm, about 1310 nm, about 1320 nm, about 1329 nm, about 1330 nm, about 1340 nm, about 1350 nm, about 1360 nm, about 1370 nm, about 1380 nm, about 1400 nm, about 1500 nm, or any theoretical cutoff wavelength between these values. Each of the cores C i  may have the same or different theoretical cutoff wavelength. 
     According to one aspect, a cable cutoff wavelength of the cores C i  is less than about 1500 nm, less than about 1400 nm, less than about 1300 nm, less than about 1260 nm, or less than about 1200 nm. For example, the cable cutoff wavelength can be from about 1200 nm to about 1500 nm, about 1200 nm to about 1400 nm, about 1200 nm to about 1300 nm, about 1300 nm to about 1500 nm, about 1300 nm to about 1400 nm, or about 1400 nm to about 1500 nm. For example, the cable cutoff wavelength can be about 1200 nm, about 1209 nm, about 1210 nm, about 1220 nm, about 1230 nm, about 1240 nm, about 1250 nm, about 1260 nm, about 1300 nm, about 1310 nm, about 1350 nm, about 1400 nm, about 1410 nm, about 1420 nm, about 1430 nm, about 1440 nm, about 1450 nm, about 1460 nm, about 1500 nm, or any cable cutoff wavelength between these values. Each of the cores C i  may have the same or different cable cutoff wavelength. 
     According to one aspect, the cores C i  can have a zero dispersion wavelength of from about 1200 nm to about 1400 nm. For example, the zero dispersion wavelength can be about 1200 nm to about 1400 nm, about 1250 nm to about 1400 nm, about 1300 nm to about 1400 nm, about 1350 nm to about 1400 nm, about 1200 nm to about 1350 nm, about 1200 nm to about 1300 nm, about 1250 nm to about 1350 nm, about 1250 nm to about 1300 nm, or about 1300 nm to about 1324 nm. For example, the zero dispersion wavelength can be about 1200 nm, about 1210 nm, about 1225 nm, about 1250 nm, about 1275 nm, about 1278 nm, about 1280 nm, about 1285 nm, about 1289 nm, about 1290 nm, about 1300 nm, about 1301 nm, about 1305 nm, about 1306 nm, about 1310 nm, about 1325 nm, about 1350 nm, about 1375 nm, about 1400 nm, or any zero dispersion wavelength between these values. Each of the cores C i  may have the same or different zero dispersion wavelength. 
     According to an aspect of the present disclosure, the cores C i  can have a dispersion having an absolute value at 1310 nm of less than 3 ps/nm/km and a dispersion slope at 1310 nm of less than 0.1 ps/nm 2 /km. Each of the cores C i  may have the same or different dispersion and dispersion slope at 1310 nm. For example, the absolute value of the dispersion at 1310 nm can be from about 0.3 ps/nm/km to about 3 ps/nm/km, about 0.3 ps/nm/km to about 2.75 ps/nm/km, about 0.3 ps/nm/km to about 2.5 ps/nm/km, about 0.3 ps/nm/km to about 2.25 ps/nm/km, about 0.3 ps/nm/km to about 2 ps/nm/km, about 0.3 ps/nm/km to about 1.75 ps/nm/km, about 0.3 ps/nm/km to about 1.5 ps/nm/km, or about 0.3 ps/nm/km to about 1 ps/nm/km. For example, the absolute value of the dispersion at 1310 can be about 0.3 ps/nm/km, about 0.35 ps/nm/km, about 0.4 ps/nm/km, about 0.5 ps/nm/km, about 0.75 ps/nm/km, about 1 ps/nm/km, about 1.25 ps/nm/km, about 1.5 ps/nm/km, about 1.75 ps/nm/km, about 2 ps/nm/km, about 2.25 ps/nm/km, about 2.5 ps/nm/km, about 2.75 ps/nm/km, about 3 ps/nm/km, or any value between these values. In one example, the dispersion slope at 1310 nm can be about 0.075 ps/nm 2 /km to about 0.1 ps/nm 2 /km, about 0.08 ps/nm 2 /km to about 0.1 ps/nm 2 /km, about 0.085 ps/nm 2 /km to about 0.1 ps/nm 2 /km, about 0.09 ps/nm 2 /km to about 0.1 ps/nm 2 /km, about 0.075 ps/nm 2 /km to about 0.09 ps/nm 2 /km, about 0.08 ps/nm 2 /km to about 0.09 ps/nm 2 /km, or about 0.085 ps/nm 2 /km to about 0.09 ps/nm 2 /km. For example, the dispersion slope at 1310 nm can be about 0.075 ps/nm 2 /km, about 0.08 ps/nm 2 /km, about 0.085 ps/nm 2 /km, about 0.086 ps/nm 2 /km, about 0.087 ps/nm 2 /km, about 0.088 ps/nm 2 /km, about 0.089 ps/nm 2 /km, about 0.09 ps/nm 2 /km, about 0.01 ps/nm 2 /km, or any value between these values. 
     According to an aspect of the present disclosure, the cores C i  can have a dispersion at 1550 nm of less than 22 ps/nm/km and a dispersion slope at 1550 nm of less than 0.1 ps/nm 2 /km. Each of the cores C i  may have the same or different dispersion and dispersion slope at 1550 nm. For example, the dispersion at 1550 nm can be from about 10 ps/nm/km to about 22 ps/nm/km, about 10 ps/nm/km to about 22 ps/nm/km, about 10 ps/nm/km to about 20 ps/nm/km, about 10 ps/nm/km to about 15 ps/nm/km, about 15 ps/nm/km to about 22 ps/nm/km, or about 15 ps/nm/km to about 20 ps/nm/km. For example, the dispersion at 1550 can be about 10 ps/nm/km, about 15 ps/nm/km, about 16 ps/nm/km, about 17 ps/nm/km, about 17.5 ps/nm/km, about 18 ps/nm/km, about 19 ps/nm/km, about 19.5 ps/nm/km, about 19.6 ps/nm/km, about 20 ps/nm/km, about 20.1 ps/nm/km, about 22 ps/nm/km, or any value between these values. In one example, the dispersion slope at 1550 nm can be about 0.04 ps/nm 2 /km to about 0.1 ps/nm 2 /km, about 0.05 ps/nm 2 /km to about 0.1 ps/nm 2 /km, about 0.055 ps/nm 2 /km to about 0.1 ps/nm 2 /km, about 0.06 ps/nm 2 /km to about 0.1 ps/nm 2 /km, about 0.08 ps/nm 2 /km to about 0.1 ps/nm 2 /km, about 0.04 ps/nm 2 /km to about 0.08 ps/nm 2 /km, about 0.05 ps/nm 2 /km to about 0.08 ps/nm 2 /km, about 0.055 ps/nm 2 /km to about 0.08 ps/nm 2 /km, about 0.06 ps/nm 2 /km to about 0.08 ps/nm 2 /km, about 0.04 ps/nm 2 /km to about 0.06 ps/nm 2 /km, about 0.05 ps/nm 2 /km to about 0.06 ps/nm 2 /km, or about 0.055 ps/nm 2 /km to about 0.06 ps/nm 2 /km. For example, the dispersion slope at 1550 nm can be about 0.04 ps/nm 2 /km, about 0.05 ps/nm 2 /km, about 0.055 ps/nm 2 /km, about 0.057 ps/nm 2 /km, about 0.058 ps/nm 2 /km, about 0.059 ps/nm 2 /km, about 0.06 ps/nm 2 /km, about 0.061 ps/nm 2 /km, about 0.07 ps/nm 2 /km, about 0.08 ps/nm 2 /km, or any value between these values. 
     According to one aspect, the bending loss of the multicore optical fiber  10  at 1550 nm as determined by the mandrel wrap test using a mandrel having a diameter of 10 mm may be less than about 3 dB/turn, less than about 2.5 dB/turn, less than about 2 dB/turn, less than about 1.5 dB/turn, or less than about 1 dB/turn. For example, the bend loss can be from about 0.5 dB/turn to about 3 dB/turn, about 0.5 dB/turn to about 2.5 dB/turn, about 0.5 dB/turn to about 2 dB/turn, about 0.5 dB/turn to about 1.5 dB/turn, about 0.5 dB/turn to about 1 dB/turn, about 1 dB/turn to about 3 dB/turn, about 1 dB/turn to about 2.5 dB/turn, about 1 dB/turn to about 2 dB/turn, about 1 dB/turn to about 1.5 dB/turn, about 1.5 dB/turn to about 3 dB/turn, about 1.5 dB/turn to about 2.5 dB/turn, about 1.5 dB/turn to about 2 dB/turn, about 2 dB/turn to about 3 dB/turn, or about 2 dB/turn to about 2.5 dB/turn using a mandrel having a diameter of 10 mm. For example, the bend loss can be about 0.5 dB/turn, about 0.75 dB/turn, about 0.9 dB/turn, about 1 dB/turn, about 1.5 dB/turn, about 2 dB/turn, about 2.5 dB/turn, about 3 dB/turn, or any value between these values, using a mandrel having a diameter of 10 mm. 
     According to one aspect, the bending loss of the multicore optical fiber  10  at 1550 nm as determined by the mandrel wrap test using a mandrel having a diameter of 15 mm may be less than about 1 dB/turn, less than about 0.5 dB/turn, or less than about 0.3 dB/turn. For example, the bend loss can be from about 0.1 dB/turn to about 1 dB/turn, about 0.1 dB/turn to about 0.75 dB/turn, about 0.1 dB/turn to about 0.5 dB/turn, about 0.2 dB/turn to about 1 dB/turn, about 0.2 dB/turn to about 0.75 dB/turn, about 0.2 dB/turn to about 0.5 dB/turn, about 0.3 dB/turn to about 1 dB/turn, about 0.3 dB/turn to about 0.75 dB/turn, or about 0.3 dB/turn to about 0.5 dB/turn, using a mandrel having a diameter of 15 mm. For example, the bend loss can be about 0.2 dB/turn, about 0.23 dB/turn, about 0.25 dB/turn, about 0.3 dB/turn, about 0.5 dB/turn, about 0.6 dB/turn, about 0.75 dB/turn, about 1 dB/turn, or any value between these values, using a mandrel having a diameter of 15 mm. 
     According to one aspect, the bending loss of the multicore optical fiber  10  at 1550 nm as determined by the mandrel wrap test using a mandrel having a diameter of 20 mm may be less than about 3 dB/turn, less than about 2 dB/turn, less than about 1 dB/turn, less than about 0.5 dB/turn, or less than about 0.3 dB/turn. For example, the bend loss can be from about 0.1 dB/turn to about 3 dB/turn, about 0.1 dB/turn to about 2.5 dB/turn, about 0.1 dB/turn to about 2 dB/turn, about 0.2 dB/turn to about 3 dB/turn, about 0.2 dB/turn to about 2.5 dB/turn, about 0.2 dB/turn to about 2 dB/turn, about 0.3 dB/turn to about 3 dB/turn, about 0.3 dB/turn to about 2.5 dB/turn, or about 0.3 dB/turn to about 2 dB/turn, about 0.1 dB/turn to about 1 dB/turn, about 0.1 dB/turn to about 0.75 dB/turn, about 0.1 dB/turn to about 0.5 dB/turn, 0.5 dB/turn to about 3 dB/turn, about 0.5 dB/turn to about 2.5 dB/turn, about 0.5 dB/turn to about 2 dB/turn, 1 dB/turn to about 3 dB/turn, about 1 dB/turn to about 2.5 dB/turn, or about 1 dB/turn to about 2 dB/turn, using a mandrel having a diameter of 20 mm. For example, the bend loss can be about 0.2 dB/turn, about 0.23 dB/turn, about 0.25 dB/turn, about 0.3 dB/turn, about 0.5 dB/turn, about 0.6 dB/turn, about 0.75 dB/turn, about 0.8 dB/turn, about 0.9 dB/turn, about 1 dB/turn, about 2 dB/turn, about 2.1 dB/turn, about 2.5 dB/turn, about 3 dB/turn, or any value between these values, using a mandrel having a diameter of 20 mm. 
     According to one aspect, the bending loss of the multicore optical fiber  10  at 1550 nm as determined by the mandrel wrap test using a mandrel having a diameter of 30 mm may be less than about 1 dB/turn, less than about 0.5 dB/turn, less than about 0.25 dB/turn, or less than about 0.1 dB/turn. For example, the bend loss can be from about 0.01 dB/turn to about 1 dB/turn, about 0.01 dB/turn to about 0.5 dB/turn, about 0.01 dB/turn to about 0.25 dB/turn, about 0.01 dB/turn to about 0.2 dB/turn, about 0.01 dB/turn to about 0.1 dB/turn, about 0.01 dB/turn to about 0.05 dB/turn, about 0.05 dB/turn to about 1 dB/turn, about 0.05 dB/turn to about 0.5 dB/turn, about 0.05 dB/turn to about 0.25 dB/turn, or about 0.05 dB/turn to about 0.2 dB/turn, about 0.2 dB/turn to about 1 dB/turn, about 0.2 dB/turn to about 0.5 dB/turn, or about 0.5 dB/turn to about 1 dB/turn, using a mandrel having a diameter of 30 mm. For example, the bend loss can be about 0.01 dB/turn, about 0.05 dB/turn, about 0.06 dB/turn, about 0.07 dB/turn, about 0.08 dB/turn, about 0.09 dB/turn, about 0.1 dB/turn, about 0.12 dB/turn, about 0.13 dB/turn, about 0.15 dB/turn, about 0.2 dB/turn, about 0.23 dB/turn, about 0.24 dB/turn, about 0.24 dB/turn, about 0.25 dB/turn, about 0.3 dB/turn, about 0.31 dB/turn, about 0.4 dB/turn, about 0.5 dB/turn, about 0.51 dB/turn, about 1 dB/turn, or any value between these values using a mandrel having a diameter of 30 mm. 
     According to one aspect, the bending loss of the multicore optical fiber  10  at 1550 nm as determined by the mandrel wrap test using a mandrel having a diameter of 40 mm may be less than about 0.3 dB/turn, less than about 0.2 dB/turn, or less than about 0.1 dB/turn. For example, the bend loss can be from about 0.01 dB/turn to about 0.3 dB/turn, about 0.05 dB/turn to about 0.3 dB/turn, about 0.1 dB/turn to about 0.3 dB/turn, about 0.01 dB/turn to about 0.2 dB/turn, about 0.05 dB/turn to about 0.2 dB/turn, about 0.1 dB/turn to about 0.2 dB/turn, about 0.01 dB/turn to about 0.1 dB/turn, or about 0.05 dB/turn to about 0.1 dB/turn, using a mandrel having a diameter of 40 mm. For example, the bend loss can be about 0.01 dB/turn, about 0.02 dB/turn, about 0.03 dB/turn, about 0.04 dB/turn, about 0.05 dB/turn, about 0.06 dB/turn, about 0.07 dB/turn, about 0.08 dB/turn, about 0.09 dB/turn, about 0.1 dB/turn, about 0.11 dB/turn, about 0.12 dB/turn, about 0.13 dB/turn, about 0.2 dB/turn, about 0.3 dB/turn, or any value between these values, using a mandrel having a diameter of 40 mm. 
     According to one aspect, the bending loss of the multicore optical fiber  10  at 1550 nm as determined by the mandrel wrap test using a mandrel having a diameter of 50 mm may be less than about 0.05 dB/turn, less than about 0.04 dB/turn, or less than about 0.03 dB/turn. For example, the bend loss can be from about 0.005 dB/turn to about 0.05 dB/turn, about 0.005 dB/turn to about 0.03 dB/turn, about 0.005 dB/turn to about 0.02 dB/turn, or about 0.01 dB/turn to about 0.05 dB/turn, using a mandrel having a diameter of 50 mm. For example, the bend loss can be about 0.01 dB/turn, about 0.02 dB/turn, about 0.03 dB/turn, about 0.032 dB/turn, about 0.039 dB/turn, about 0.04 dB/turn, about 0.05 dB/turn, or any value between these values, using a mandrel having a diameter of 50 mm. 
     According to one aspect, the bending loss of the multicore optical fiber  10  at 1550 nm as determined by the mandrel wrap test using a mandrel having a diameter of 60 mm may be less than about 0.05 dB/turn, less than about 0.03 dB/turn, or less than about 0.02 dB/turn. For example, the bend loss can be from about 0.001 dB/turn to about 0.05 dB/turn, about 0.001 dB/turn to about 0.03 dB/turn, or about 0.001 dB/turn to about 0.02 dB/turn, using a mandrel having a diameter of 60 mm. For example, the bend loss can be about 0.001 dB/turn, about 0.002 dB/turn, about 0.0023 dB/turn, about 0.005 dB/turn, about 0.006 dB/turn, about 0.007 dB/turn, about 0.008 dB/turn, about 0.009 dB/turn, about 0.01 dB/turn, about 0.016 dB/turn, about 0.02 dB/turn, about 0.03 dB/turn, about 0.05 dB/turn, or any value between these values using a mandrel having a diameter of 60 mm. 
     Exemplary configurations of the multicore optical fiber  10 , Exemplary MCF A-D, according to aspects of the present disclosure are shown in Table 1 below and  FIGS. 5-11 . Table 1 identifies the combination of materials according to the present disclosure. The core C i , inner cladding IC i , and/or common cladding  20  can include additional components according to aspects of the present disclosure discussed herein. While the exemplary configurations Exemplary MCF A-D are discussed and illustrated in  FIGS. 5-11  in the context of a first core C 1  and optional first inner cladding IC 1 , it is understood that the multicore optical fiber  10  can include any combination of any one or more cores and inner claddings corresponding to Exemplary MCF A-D. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Exemplary Multicore Optical Fiber Configurations. 
               
            
           
           
               
               
               
               
            
               
                 Exemplary MCF 
                 Core 
                 Inner Cladding 
                 Common cladding 
               
               
                   
               
               
                 Exemplary MCF  
                 silica glass doped 
                 N/A 
                 undoped silica  
               
               
                 A 
                 with &gt;3 wt % Cl 
                   
                 glass 
               
               
                 Exemplary MCF  
                 silica glass doped 
                 undoped silica  
                 silica glass doped 
               
               
                 B 
                 with &gt;3 wt % Cl 
                 glass 
                 with Cl 
               
               
                 Exemplary MCF  
                 silica glass doped 
                 silica glass  
                 undoped silica  
               
               
                 C 
                 with &gt;3 wt % Cl 
                 doped with F 
                 glass 
               
               
                 Exemplary MCF  
                 silica glass doped 
                 silica glass  
                 silica glass doped 
               
               
                 D 
                 with &gt;3 wt % Cl 
                 doped with F 
                 with F 
               
               
                 Exemplary MCF  
                 silica glass doped 
                 silica glass  
                 silica glass doped 
               
               
                 D 
                 with &gt;3 wt % Cl 
                 doped with F 
                 with Cl 
               
               
                   
               
            
           
         
       
     
       FIG. 5  is a schematic refractive index profile corresponding to an example multicore optical fiber  10  corresponding to Exemplary MCF A. The core C 1  can have an outer radius r 1  and a maximum relative refractive index Δ 1MAX  that is greater than the relative refractive index Δ CC  of the undoped silica glass common cladding. The core C 1  of Exemplary MCF A has a step-index core profile. 
       FIGS. 6-7  illustrate schematic refractive index profiles corresponding to example multicore optical fibers  10  corresponding to Exemplary MCF B.  FIG. 6  illustrates the core C 1  having an outer radius r 1 , a maximum relative refractive index Δ 1MAX , and a step-index core profile. The inner cladding IC 1  is undoped silica glass having an outer radius r IC1  and a relative refractive index Δ IC1 , wherein Δ 1MAX &gt;Δ IC1 . The common cladding  20  is silica glass doped with chlorine and has an outer radius R CC  and a relative refractive index Δ CC , wherein Δ 1MAX &gt;Δ CC  and Δ IC1 &lt;Δ CC .  FIG. 7  illustrates an example that is similar to the example illustrated by  FIG. 6 , except that the core C 1  has a graded-index core profile. It is understood that any of the cores C i  described herein may have a step-index core profile, as illustrated in  FIG. 6 , or a graded-index core profile, as illustrated in  FIG. 7 . It is further understood that some cores of a multicore optical fiber may have a step-index core profile and other cores of the multicore optical fiber may have a graded-index core profile. 
       FIG. 8  illustrates a schematic refractive index profile corresponding to an example multicore optical fiber  10  corresponding to Exemplary MCF C.  FIG. 8  illustrates the core C 1  having an outer radius r 1 , a maximum relative refractive index Δ 1MAX , and a step-index core profile. The inner cladding IC 1  is silica glass doped with fluorine having an outer radius no and a relative refractive index Δ IC1 , wherein Δ 1MAX &gt;Δ IC1  and Δ IC1 &lt;0. The common cladding  20  is undoped silica glass and has an outer radius R CC  and a relative refractive index Δcc, wherein Δ 1MAX &gt;Δ CC  and Δ IC1 &lt;Δ CC . In another embodiment, the core of Exemplary MCF C has a graded-index core profile. 
       FIG. 9  illustrates a schematic refractive index profile corresponding to an example multicore optical fiber  10  corresponding to Exemplary MCF D.  FIG. 9  illustrates the core C 1  having an outer radius r 1 , a maximum relative refractive index Δ 1MAX , and a step-index core profile. The inner cladding IC 1  is silica glass doped with fluorine having an outer radius no and a relative refractive index Δ IC1 , wherein Δ 1MAX &gt;Δ IC1  and Δ IC1 &lt;0. The common cladding  20  is silica glass doped with fluorine and has an outer radius R CC  and a relative refractive index Δ CC , wherein Δ 1MAX &gt;Δ CC  and Δ IC1 &lt;Δ CC  and both Δ IC1  and Δ CC &lt;0. In another embodiment, the core of Exemplary MCF D has a graded-index core profile. 
       FIG. 10  illustrates a schematic refractive index profile corresponding to an example multicore optical fiber  10  corresponding to Exemplary MCF E.  FIG. 10  illustrates the core C 1  having an outer radius r 1 , a maximum relative refractive index Δ 1MAX , and a step core profile. The inner cladding IC 1  is silica glass doped with fluorine having an outer radius no_ and a relative refractive index Δ IC1 , wherein Δ 1MAX &gt;Δ IC1  and Δ IC1 &lt;0. The common cladding  20  is silica glass doped with chlorine and has an outer radius R CC  and a relative refractive index Δ CC , wherein Δ 1MAX &gt;Δ CC , Δ IC1 &lt;Δ CC , and Δ CC &gt;0. In another embodiment, the core of Exemplary MCF E has a graded-index core profile. 
     The multicore optical fibers  10  of the present disclosure can be made using any suitable method for forming a multicore optical fiber. See, for example, U.S. Pat. No. 9,120,693 and U.S. Published Patent Application No. 20150284286, the disclosures of which are incorporated herein by reference in their entirety. For example, the multicore optical fiber  10  can be formed by drawing a multicore preform made using conventional optical-fiber techniques, such as glass drilling or stacking. The glass drilling method can be used to form a multicore preform by drilling holes in a silica glass cylinder (pure, undoped silica or doped silica). The locations and dimensions of the holes are based on the multicore optical fiber design. Core canes having the desired refractive index profile and a diameter that is slightly smaller than the pre-drilled holes are then inserted into the pre-drilled holes to form the multicore preform. The multicore preform is then heated to a temperature sufficient to melt the silica glass forming the pre-drilled holes such that the pre-drilled holes collapse around the core canes. The multicore preform is then drawn into a fiber. The core canes can be made using any suitable conventional preform manufacturing technique, such as outside vapor deposition (OVD), modified chemical vapor deposition (MCVD), or plasma activated chemical vapor deposition (PCVD). 
     Suitable precursors for silica include SiCl 4  and organosilicon compounds. Organosilicon compounds are silicon compounds that include carbon, and optionally oxygen and/or hydrogen. Examples of suitable organosilicon compounds include octamethylcyclotetrasiloxane (OMCTS), silicon alkoxides (Si(OR) 4 ), organosilanes (SiR 4 ), and Si(OR) x R 4-x , where R is a carbon-containing organic group or hydrogen and where R may be the same or different at each occurrence, and wherein at least one R is a carbon-containing organic group. Suitable precursors for chlorine doping include Cl 2 , SiCl 4 , Si 2 Cl 6 , Si 2 OCl 6 , SiCl 3 H, and CCl 4 . Suitable precursors for fluorine doping include F 2 , CF 4 , and SiF 4 . Regions of constant refractive index may be formed by not doping or by doping at a uniform concentration over the thickness of the region. Regions of variable refractive index are formed through non-uniform spatial distributions of dopants over the thickness of a region and/or through incorporation of different dopants in different regions. The OVD, MCVD, PCVD and other techniques for generating silica soot permit fine control of dopant concentration through layer-by-layer deposition with variable flow rate delivery of dopant precursors. 
     One exemplary method that can be used to form the multicore optical fibers of the present disclosure is a method that utilizes a cane-based optical fiber preform and then draws the optical fiber from the cane-based glass preform. An exemplary cane-based glass preform method is disclosed in Applicant&#39;s co-pending U.S. Patent Application Ser. No. 62/811,842, entitled “Vacuum-Based Methods of Forming a Cane-Based Optical Fiber Preform and Methods of Forming an Optical Fiber Using Same,” which was filed on Feb. 28, 2019, the contents of which are incorporated herein by reference in their entirety. 
     Briefly, a cane-based glass preform method for forming the multicore optical fibers  10  can include utilizing one or more glass cladding sections each having one or more precision axial holes formed therein and a top end with a recess defined by a perimeter lip. When using multiple glass cladding sections, the sections can be stacked so that the axial holes are aligned. A core cane can then be added to each axial hole to define a cane-cladding assembly. Top and bottom caps, respectively, can be added to the top and bottom of the cane-cladding assembly to define a preform assembly. The top cap closes off the recess at the top of the glass-cladding section. The bottom cap can have its own raised lip and recess that becomes closed off when the bottom cap is interfaced with the bottom end of the cane-cladding assembly. The closed-off recesses and gaps formed by the canes within the axial holes define a substantially sealed internal chamber. The preform assembly can then be dried and purified by drawing a select cleaning gas (e.g., chlorine) through a small passage in the bottom cap that leads to the internal chamber. A vacuum can be applied through the top cap to create a pressure differential between the internal chamber and the ambient environment. The pressure differential facilitates maintaining the components of the preform assembly together, and can be referred to as a vacuum-held preform assembly. The vacuum-held preform assembly can then be consolidated by heating in a furnace to just above the glass softening temperature so that the glass cladding section(s), the core canes, and the top and bottom caps, which are all made of glass, seal to one another. In addition, the glass flow can remove the internal chamber. The result is a solid glass preform that is ready to be drawn, especially if the furnace used for the consolidation is a draw furnace used for drawing optical fiber. 
     EXAMPLES 
     The following examples describe various features and advantages provided by the disclosure, and are in no way intended to limit the invention and appended claims. 
     Example 1 
     Referring to  FIG. 11 , the crosstalk for three different multicore optical fibers according to aspects of the present disclosure, Examples 1A, 1B, and 1C (“Ex. 1A,” “Ex. 1B,” and “Ex. 1C”), were mathematically modeled. Examples 1A-1C were multicore optical fibers having a circular cross-section, a fiber length of 100 km, and two cores, each core including silica doped with greater than 3.5 wt % chlorine. Table 2 below provides the details and optical properties for Ex. 1A-1C. In Example 1A, both cores had a step-index profile and an effective area of 80 μm 2 . In Example 1B, both cores had a step-index profile, an inner cladding directly adjacent and surrounding the core, and an effective area of 80 μm 2 . The inner cladding for Example 1B included silica doped with fluorine. Ex. 1A is represented schematically in the refractive index profile of  FIG. 5  and Ex. 1B is represented schematically in the refractive index profile of  FIG. 12 . Example 1C was similar to Example 1B, except that each core of Example 1C included an effective area of 100 μm 2 . Ex. 1C is represented schematically in the refractive index profile of  FIG. 8 . The data in  FIG. 11  demonstrates the ability of the cores of the present disclosure to be configured to provide low crosstalk, in most cases below 0 dB for a core spacing of 35 μm or more. The effective area, inner cladding, and/or core spacing can be configured to provide a desired level of crosstalk. 
     The distance between cores for Ex. 1A-1C was measured as described above using a Cartesian coordinate system to determine the (x, y) position of the centerline of each core and then determining the distance between the core centerlines. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Features and Optical Properties of Examples 1A-1C. 
               
            
           
           
               
               
               
               
            
               
                 Parameter 
                 Ex. 1A 
                 Ex. 1B 
                 Ex. 1C 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 Δ 1 , Δ 2  (%) 
                 0.34 
                 0.2 
                 0.34 
               
               
                 Cl in core (wt. %) 
                 5.40 
                 3.17 
                 5.40 
               
               
                 r 1 , r 2  (μm) 
                 4.2 
                 6.2 
                 4.9 
               
               
                 Δ IC1 , Δ IC2  (%) 
                 — 
                 −0.12 
                 −0.07 
               
               
                 inner cladding dopant 
                 — 
                 F 
                 F 
               
               
                 F in inner cladding (wt %) 
                 — 
                 0.4 
                 0.23 
               
               
                 r IC1 , r IC2  (μm) 
                 — 
                 22 
                 14.8 
               
               
                 δr IC1 , δr IC2  (μm) 
                 — 
                 15.8 
                 9.9 
               
               
                 R CC   
                 62.5 
                 62.5 
                 62.5 
               
               
                 Δ CC  (%) 
                 0 
                 0 
                 0 
               
               
                 Common cladding dopant 
                 — 
                 — 
                 — 
               
               
                   
               
            
           
         
       
     
     Example 2 
     Table 3 below provides the details and optical properties for modeled exemplary multicore optical fibers, Examples 2A-2D (“Ex. 2A,” “Ex. 2B,” “Ex. 2C,” and “Ex. 2D”). All four examples, Ex. 2A-2D, had a core consisting of Cl-doped silica with a core alpha value of 100 and an attenuation at 1550 nm of &lt;0.17 dB/km. When core alpha has a value of 100, the relative refractive index Δ 1  of the core is approximately constant and the relative refractive index profile of the core closely approximates a step-index profile. Ex. 2A and Ex. 2B lack an inner cladding and have a common cladding of undoped silica directly adjacent to the core (as shown schematically in  FIG. 5 ). Ex. 2C has an inner cladding of down-doped silica directly adjacent to the core and a common cladding of undoped silica directly adjacent to the inner cladding (as shown schematically in  FIG. 8 ). Ex. 2D has an inner cladding of undoped silica directly adjacent to the core and an common cladding of up-doped silica directly adjacent the inner cladding (as shown schematically in  FIG. 6 ). Each of Ex. 2A-2D included two cores configured identically as indicated in Table 3 with a spacing between adjacent cores (measured between centerlines of adjacent cores) of 55 μm and a core to fiber edge distance of 35 μm. The two cores were disposed symmetrically about the centerline of the multicore optical fiber (as shown schematically in  FIG. 2 ). The centerline-to-centerline spacing of the cores can be based at least in part on the desired crosstalk of the optical fiber and the number of cores accommodated by the optical fiber. In some embodiments, the centerline-to-centerline spacing of the cores is greater than about 28 μm, greater than about 30 μm, or greater than about 40 μm. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Features and Optical Properties of Examples 2A-2D. 
               
            
           
           
               
               
               
               
               
            
               
                 Parameter 
                 Ex. 2A 
                 Ex. 2B 
                 Ex. 2C 
                 Ex. 2D 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Δ 1 , Δ 2  (%) 
                 0.34 
                 0.34 
                 0.34 
                 0.347 
               
               
                 Cl in core (wt. %) 
                 5.40 
                 5.40 
                 5.40 
                 5.51 
               
               
                 r 1 , r 2  (nm) 
                 4.2 
                 4.45 
                 4.9 
                 5.1 
               
               
                 Δ IC1 , Δ IC2  (%) 
                 — 
                 — 
                 −0.07 
                 — 
               
               
                 inner cladding dopant 
                 — 
                 — 
                 F 
                 — 
               
               
                 F in inner cladding (wt %) 
                 — 
                 — 
                 0.23 
                 — 
               
               
                 r IC1 , r IC2  (μm) 
                 — 
                 — 
                 14.8 
                 15.4 
               
               
                 δr IC1 , δr IC2  (μm) 
                 — 
                 — 
                 9.9 
                 10.3 
               
               
                 R CC  (μm) 
                 62.5 
                 62.5 
                 62.5 
                 62.5 
               
               
                 Δ CC  (%) 
                 0 
                 0 
                 0 
                 0.07 
               
               
                 Common cladding dopant 
                 — 
                 — 
                 — 
                 Cl 
               
               
                 Theoretical Cutoff  
                 1330 
                 1370 
                 1329 
                 1329 
               
               
                 wavelength (nm) 
                   
                   
                   
                   
               
               
                 Cable cutoff wavelength  
                 1210 
                 1250 
                 1209 
                 1209 
               
               
                 (nm) 
                   
                   
                   
                   
               
               
                 Zero-dispersion  
                 1306 
                 1301 
                 1289 
                 1278 
               
               
                 wavelength (nm) 
                   
                   
                   
                   
               
               
                 Mode field diameter at  
                 9.1 
                 9.2 
                 9.1 
                 9.3 
               
               
                 1310 nm (μm) 
                   
                   
                   
                   
               
               
                 Effective area at 1310 nm  
                 66.2 
                 68 
                 68.6 
                 72 
               
               
                 (μm 2 ) 
                   
                   
                   
                   
               
               
                 Dispersion at 1310 nm  
                 0.35 
                 0.75 
                 2.55 
                 2.87 
               
               
                 (ps/nm/km) 
                   
                   
                   
                   
               
               
                 Dispersion Slope at 1310  
                 0.086 
                 0.0866 
                 0.0881 
                 0.0888 
               
               
                 nm (ps/nm 2 /km) 
                   
                   
                   
                   
               
               
                 Mode field diameter at  
                 10.3 
                 10.4 
                 10 
                 10.2 
               
               
                 1550 nm (μm) 
                   
                   
                   
                   
               
               
                 Effective area at 1550 nm  
                 80.2 
                 83.6 
                 80.1 
                 83.3 
               
               
                 (μm 2 ) 
                   
                   
                   
                   
               
               
                 Dispersion at 1550 nm  
                 17 
                 17.5 
                 19.6 
                 20.1 
               
               
                 (ps/nm/km) 
                   
                   
                   
                   
               
               
                 Dispersion Slope at 1550  
                 0.0576 
                 0.0579 
                 0.0587 
                 0.0593 
               
               
                 nm (ps/nm 2 /km) 
               
               
                   
               
            
           
         
       
     
     Example 3 
     Table 4 below provides the details and optical properties for modeled exemplary multicore optical fibers, Examples 3A-3C (“Ex. 3A,” “Ex. 3B,” and “Ex. 3C”). All three examples, Ex. 3A-3C, had an attenuation at 1550 nm of &lt;0.17 dB/km. All of the bend loss values listed in Table 4 are for optical fibers operating at 1550 nm. Ex. 3A-3C included two cores configured as indicated in Table 4 with a spacing between adjacent cores (measured between centerlines of adjacent cores) of 55 μm and a core to fiber edge distance of 35 μm. The centerline-to-centerline spacing of the cores can be based at least in part on the desired crosstalk of the optical fiber and the number of cores accommodated by the optical fiber. In some embodiments, the centerline-to-centerline spacing of the cores is greater than about 28 μm, greater than about 30 μm, or greater than about 40 μm. The two cores were disposed symmetrically about the centerline of the multicore optical fiber (as shown schematically in  FIG. 2 ). 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Features and Optical Properties of Examples 3A-3C. 
               
            
           
           
               
               
               
               
            
               
                 Parameter 
                 Ex. 3A 
                 Ex. 3B 
                 Ex. 3C 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 Δ 1 , Δ 2  (%) 
                 0.2 
                 0.2 
                 0.25 
               
               
                 Cl in core (wt%) 
                 3.17 
                 3.17 
                 3.17 
               
               
                 r 1 , r 2  (μm) 
                 6.2 
                 7.3 
                 7.4 
               
               
                 Δ IC1 , Δ IC2  (%) 
                 −0.12 
                 −0.05 
                 0 
               
               
                 inner cladding dopant 
                 F 
                 F 
                 none 
               
               
                 F in inner cladding (wt %) 
                 0.4 
                 0.17 
                 0 
               
               
                 r IC1 , r IC2  (μm) 
                 22 
                 25 
                 25 
               
               
                 δr IC1 , Δr IC2  (μm) 
                 15.8 
                 17.7 
                 17.6 
               
               
                 Trench volume (% Δμm2) 
                 107 
                 57 
                 62 
               
               
                 Δ CC  (%) 
                 0 
                 0.02 
                 0.055 
               
               
                 R CC  (μm) 
                 62.5 
                 62.5 
                 62.5 
               
               
                 Mode field diameter at 1550 nm (μm) 
                 11.785 
                 13.571 
                 13.65 
               
               
                 Effective area at 1550 nm (μm 2 ) 
                 112.338 
                 150.14 
                 152.34 
               
               
                 Dispersion at 1550 nm (ps/nm/km) 
                 20.947 
                 21.3 
                 21.27 
               
               
                 Dispersion Slope at 1550 nm  
                 0.0609 
                 0.061 
                 0.061 
               
               
                 (ps/nm 2 /km) 
                   
                   
                   
               
               
                 22-meter cable cutoff wavelength (nm) 
                 1400 
                 1430 
                 1450 
               
               
                 20 mm bend loss (dB/turn) 
                 0.9332 
                 2.0855 
                 2.0497 
               
               
                 30 mm bend loss (dB/turn) 
                 0.2382 
                 0.5117 
                 0.3109 
               
               
                 40 mm bend loss (dB/turn) 
                 0.0968 
                 0.1287 
                 0.0603 
               
               
                 50 mm bend loss (dB/turn) 
                 0.0394 
                 0.0324 
                 0.0117 
               
               
                 60 mm bend loss (dB/turn) 
                 0.0160 
                 0.0081 
                 0.0023 
               
               
                   
               
            
           
         
       
     
     Each core of each of Ex. 3A, Ex. 3B, and Ex. 3C included an inner cladding directly adjacent to the core and a common cladding directly adjacent to the inner cladding. Ex. 3A is illustrated schematically in the refractive index profile of  FIG. 12 , Ex. 3B is illustrated schematically in the refractive index profile of  FIG. 13 , and Ex. 3C is illustrated schematically in the refractive index profile of  FIG. 14 . As illustrated in  FIGS. 12-14 , the cores of Ex. 3A, 3B, and 3C, respectively, had a graded-index profile. 
     Example 4 
     Table 5 below provides the features and optical properties for modeled exemplary multicore optical fibers, Examples 4A-4D (“Ex. 4A,” Ex. 4B,” “Ex. 4C,” and ‘Ex. 4D”). All of the Examples 4A-4D included a chlorine doped core, an undoped silica inner cladding, a fluorine doped outer cladding, and an undoped silica common cladding. Examples 4A-4D can be referred to as having an offset trench. Ex. 4C is illustrated schematically in the refractive index profile of  FIG. 15 , and includes a graded-index profile. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Features and Optical Properties of Examples 4A-4D. 
               
            
           
           
               
               
               
               
               
            
               
                 Parameter 
                 Ex. 4A 
                 Ex. 4B 
                 Ex. 4C 
                 Ex. 4D 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Δ 1 , Δ 2  (%) 
                 0.31 
                 0.36 
                 0.35 
                 0.33 
               
               
                 Cl in core (wt%) 
                 4.95 
                 5.74 
                 5.63 
                 5.20 
               
               
                 r 1 , r 2  (ina) 
                 4.54 
                 4.70 
                 4.34 
                 4.43 
               
               
                 Core alpha 
                 14.22 
                 5.35 
                 11.45 
                 12.67 
               
               
                 Δ IC1 , Δ IC2  (%) 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
               
               
                 inner cladding dopant 
                 none 
                 none 
                 none 
                 none 
               
               
                 r IC1 , r IC2  (μm) 
                 10.15 
                 10.52 
                 9.74 
                 10.36 
               
               
                 Δr IC1 , Δr IC2  (μm) 
                 5.61 
                 5.82 
                 5.40 
                 5.93 
               
               
                 Δ OC1 , Δ OC2  (%) 
                 −0.40 
                 −0.33 
                 −0.30 
                 −0.26 
               
               
                 r OC1 , r OC2  (μm) 
                 16.01 
                 15.92 
                 17.92 
                 17.11 
               
               
                 Δr OC1 , Δr OC2  (μm) 
                 5.86 
                 5.40 
                 8.18 
                 6.75 
               
               
                 Trench volume (% Δμm 2 ) 
                 61.1 
                 47.7 
                 68.9 
                 47.6 
               
               
                 Outer cladding dopant 
                 F 
                 F 
                 F 
                 F 
               
               
                 F in outer cladding (wt %) 
                 1.31 
                 1.10 
                 1.00 
                 0.84 
               
               
                 R CC  (μm) 
                 62.5 
                 62.5 
                 62.5 
                 62.5 
               
               
                 Δ CC  (%) 
                 0 
                 0 
                 0 
                 0 
               
               
                 Common cladding dopant 
                 none 
                 none 
                 none 
                 none 
               
               
                 Theoretical cutoff wavelength 
                 1193 
                 1209 
                 1199 
                 1191 
               
               
                 (nm) 
                   
                   
                   
                   
               
               
                 Cable cutoff wavelength (nm) 
                 1185 
                 1190 
                 1195 
                 1177 
               
               
                 Zero dispersion wavelength 
                 1301.3 
                 1312.2 
                 1308.9 
                 1308.9 
               
               
                 (nm) 
                   
                   
                   
                   
               
               
                 Mode field diameter at 1310 nm 
                 9.38 
                 8.93 
                 8.85 
                 9.18 
               
               
                 (μm) 
                   
                   
                   
                   
               
               
                 Effective area at 1310 nm (μm 2 ) 
                 68.89 
                 61.67 
                 61.09 
                 65.60 
               
               
                 Dispersion at 1310 nm 
                 0.79 
                 −0.20 
                 0.10 
                 0.10 
               
               
                 (ps/nm/km) 
                   
                   
                   
                   
               
               
                 Dispersion Slope at 1310 nm 
                 0.09 
                 0.09 
                 0.09 
                 0.09 
               
               
                 (ps/nm 2 /km) 
                   
                   
                   
                   
               
               
                 Mode field diameter at 1550 nm 
                 10.51 
                 10.11 
                 9.97 
                 10.38 
               
               
                 (μm) 
                   
                   
                   
                   
               
               
                 Effective area at 1550 nm (μm 2 ) 
                 84.82 
                 77.46 
                 75.89 
                 82.04 
               
               
                 Dispersion at 1550 nm 
                 18.86 
                 17.59 
                 17.91 
                 17.83 
               
               
                 (ps/nm/km) 
                   
                   
                   
                   
               
               
                 Dispersion Slope at 1550 nm 
                 0.06 
                 0.06 
                 0.06 
                 0.06 
               
               
                 (ps/nm 2 /km) 
                   
                   
                   
                   
               
               
                   
               
            
           
         
       
     
     The following non-limiting aspects are encompassed by the present disclosure. To the extent not already described, any one of the features of the first through the twenty-sixth aspect may be combined in part or in whole with features of any one or more of the other aspects of the present disclosure to form additional aspects, even if such a combination is not explicitly described. 
     According to a first aspect of the present disclosure, a multicore optical fiber, includes: a first core comprising silica and greater than 3 wt % chlorine, wherein the first core comprises a first core centerline, a relative refractive index Δ 1MAX , and an outer radius r 1 ; a first inner cladding surrounding the first core and comprising a relative refractive index Δ IC1  and a width δr IC1 , wherein Δ 1MAX &gt;Δ IC1 ; a second core comprising silica and greater than 3 wt % chlorine, wherein the second core comprises a second core centerline, a relative refractive index Δ 2MAX , and an outer radius r 2 ; a second inner cladding surrounding the second core and comprising a relative refractive index Δ IC2  and a width δr IC2 , wherein Δ 2MAX &gt;Δ IC2 , and a common cladding surrounding the first core and the second core and in direct contact with the first inner cladding and the second inner cladding, wherein the common cladding comprises a relative refractive index Δ CC , and wherein a spacing between the first core centerline and the second core centerline is at least 28 micrometers and a crosstalk between the first core and the second core is ≤−30 dB, as measured for a 100 km length of the multicore optical fiber operating at a wavelength of 1550 nm. 
     According to a second aspect of the present disclosure, the multicore optical fiber according to the first aspect, wherein the first inner cladding and the second inner cladding include one of undoped silica and silica doped with fluorine. 
     According to a third aspect of the present disclosure, the multicore optical fiber of the first or the second aspect, wherein the common cladding comprises one of undoped silica, silica doped with fluorine, and silica doped with chlorine. 
     According to a fourth aspect of the present disclosure, the multicore optical fiber of the first aspect, wherein the first inner cladding and the second inner cladding comprise undoped silica and the common cladding comprises silica doped with chlorine. 
     According to a fifth aspect of the present disclosure, the multicore optical fiber of the first aspect, wherein first inner cladding and the second inner cladding comprise silica doped with fluorine and the common cladding comprises one of undoped silica, silica doped with fluorine, and silica doped with chlorine. 
     According to a sixth aspect of the present disclosure, the multicore optical fiber of the fifth aspect, wherein the first inner cladding and the second inner cladding comprise silica doped with from about 0.1 wt % to about 0.5 wt % fluorine. 
     According to a seventh aspect of the present disclosure, the multicore optical fiber of any one of the first aspect to the sixth aspect, wherein: Δ 1MAX ≥0, Δ IC1 ≤0, Δ CC ≥0, and Δ CC &gt;ΔIC1; and Δ 2MAX &gt;0, Δ IC2 ≤0, Δ CC ≥0, and Δ CC &gt;Δ IC2 . 
     According to an eighth aspect of the present disclosure, the multicore optical fiber of any one of the first aspect to the seventh aspect, wherein the first inner cladding and the second inner cladding comprise a trench volume of from about 30% Δ-square micrometers to about 120% Δ-square micrometers. 
     According to a ninth aspect of the present disclosure, the multicore optical fiber of any one of the first aspect to the seventh aspect, further including: a first outer cladding surrounding the first inner cladding between the first inner cladding and the common cladding, wherein the first outer cladding includes a relative refractive index Δ OC1  and a width δr OC1 ; and a second outer cladding surrounding the second inner cladding between the second inner cladding and the common cladding, wherein the second outer cladding includes a relative refractive index Δ OC2  and a width δr OC2 . 
     According to a tenth aspect of the present disclosure, the multicore optical fiber of the ninth aspect, wherein the first inner cladding and the second inner cladding include undoped silica and the first outer cladding and the second outer cladding comprise silica doped with fluorine. 
     According to an eleventh aspect of the present disclosure, the multicore optical fiber of the tenth aspect, wherein the common cladding includes undoped silica. 
     According to a twelfth aspect of the present disclosure, the multicore optical fiber of the ninth aspect, wherein: Δ 1MAX &gt;0, Δ 1MAX &gt;Δ IC1 &gt;Δ OC1 , and Δ CC &gt;Δ OC1 ; and Δ 2MAX &gt;0, Δ 2MAX &gt;Δ IC2 &gt;Δ OC2 , and Δ CC &gt;Δ OC2 . 
     According to a thirteenth aspect of the present disclosure, the multicore optical fiber of the ninth aspect, wherein the first outer cladding and the second outer cladding comprise a trench volume of from about 30% Δ-square micrometers to about 75% Δ-square micrometers. 
     According to a fourteenth aspect of the present disclosure, the multicore optical fiber of any one of the first aspect to the thirteenth aspect, wherein first core and the second core comprise greater than 3 wt % and less than 6 wt % chlorine. 
     According to a fifteenth aspect of the present disclosure, the multicore optical fiber of any one of the first aspect to the fourteenth aspect, wherein the first core outer radius r 1  and the second core outer radius r 2  are from about 2.5 micrometers to about 12.5 micrometers. 
     According to a sixteenth aspect of the present disclosure, the multicore optical fiber of any one of the first aspect to the fifteenth aspect, wherein the first core and the second core are free of fluorine. 
     According to a seventeenth aspect of the present disclosure, the multicore optical fiber of any one of the first aspect to the sixteenth aspect, wherein the first core and the second core comprise one of a step index core profile and a graded index core profile. 
     According to an eighteenth aspect of the present disclosure, the multicore optical fiber of any one of the first aspect to the seventeenth aspect, wherein an attenuation of the first core and the second core is less than 0.175 dB/km at 1550 nm. 
     According to a nineteenth aspect of the present disclosure, the multicore optical fiber of any one of first aspect to the eighteenth aspect, wherein the multicore optical fiber comprises one of: a circular cross-sectional shape having an outer radius of from about 50 micrometers to about 110 micrometers; and a ribbon cross-sectional shape having a width of from about 50 micrometers to about 400 micrometers. 
     According to a twentieth aspect of the present disclosure, the multicore optical fiber of any one of the first aspect to the nineteenth aspect, further comprising: i additional cores comprising silica and greater than 3 wt % chlorine, wherein i is 1 to 18, and wherein each additional core comprises a core centerline, a relative refractive index Δ MAX , and an outer radius r i ; and an inner core surrounding each additional core and comprising a relative refractive index Δ ICi  and a width δr ICi , wherein Δ iMAX &gt;Δ ICi , wherein a spacing between the core centerline of adjacent cores is at least 28 micrometers and a crosstalk between adjacent cores is ≤−30 dB, as measured for a 100 km length of the multicore optical fiber operating at a wavelength of 1550 nm. 
     According to a twenty-first aspect of the present disclosure, the multicore optical fiber of any one of the first aspect to the twentieth aspect, wherein the crosstalk between the first core and the second core is ≤−40 dB, as measured for a 100 km length of the multicore optical fiber operating at a wavelength of 1550 nm. 
     According to an twenty-second aspect of the present disclosure, the multicore optical fiber of any one of the first aspect to the twentieth aspect, wherein the crosstalk is ≤−50 dB, as measured for a 100 km length of the multicore optical fiber operating at a wavelength of 1550 nm. 
     According to a twenty-third aspect of the present disclosure, the multicore optical fiber of any one of the first aspect to the twentieth aspect, wherein the crosstalk is ≤−60 dB, as measured for a 100 km length of the multicore optical fiber operating at a wavelength of 1550 nm. 
     According to a twenty-fourth aspect of the present disclosure, a multicore optical fiber, includes: a first core comprising silica and greater than 3 wt % chlorine, wherein the first core comprises a first core centerline, a relative refractive index Δ 1MAX , and an outer radius r 1 ; a second core comprising silica and greater than 3 wt % chlorine, wherein the second core comprises a second core centerline, a relative refractive index Δ 2MAX , and an outer radius r 2 ; and a common cladding formed from silica-based glass surrounding and in direct contact with the first core and the second core, the common cladding having a relative refractive index Δcc, wherein a spacing between the first core centerline and the second core centerline is at least 28 micrometers and a crosstalk between the first core and the second core is ≤−30 dB, as measured for a 100 km length of the multicore optical fiber operating at a wavelength of 1550 nm. 
     According to a twenty-fifth aspect of the present disclosure, the multicore optical fiber of the twenty-fourth aspect, wherein the common cladding comprises undoped silica. 
     According to a twenty-sixth aspect of the present disclosure, the multicore optical fiber of the twenty-fourth aspect or the twenty-fifth aspect, wherein the first core and the second core comprise greater than 3 wt % and less than 6 wt % chlorine. 
     According to a twenty-seventh aspect of the present disclosure, the multicore optical fiber of any one of the twenty-fourth aspect to the twenty-sixth aspect, wherein the first core outer radius r 1  and the second core outer radius r 2  are from about 2.5 micrometers to about 12.5 micrometers. 
     According to a twenty-eighth aspect of the present disclosure, the multicore optical fiber of any one of the twenty-fourth aspect to the twenty-seventh aspect, wherein the first core and the second core are substantially free of fluorine. 
     According to a twenty-ninth aspect of the present disclosure, the multicore optical fiber of any one of the twenty-fourth aspect to the twenty-eighth aspect, wherein the first core and the second core comprise one of a step index core profile and a graded index core profile. 
     According to a thirtieth aspect of the present disclosure, the multicore optical fiber of any one of the twenty-fourth aspect to the twenty-ninth aspect, wherein an attenuation of the first core and the second core is less than 0.175 dB/km at 1550 nm. 
     According to a thirty-first aspect of the present disclosure, the multicore optical fiber of any one of the twenty-fourth aspect to the thirtieth aspect, wherein the multicore optical fiber comprises one of: a circular cross-sectional shape having an outer radius of from about 50 micrometers to about 110 micrometers; and a ribbon cross-sectional shape having a width of from about 50 micrometers to about 400 micrometers. 
     According to a thirty-second aspect of the present disclosure, the multicore optical fiber of any one of the twenty-fourth aspect to the thirty-first aspect, further comprising: i additional cores comprising silica and greater than 3 wt % chlorine, wherein i is 1 to 18, and wherein each additional core comprises a core centerline, a relative refractive index Δ iMAX , and an outer radius r i , and wherein a spacing between the core centerline of adjacent cores is at least 28 micrometers and a crosstalk between adjacent cores is ≤−30 dB, as measured for a 100 km length of the multicore optical fiber operating at a wavelength of 1550 nm. 
     According to a thirty-third aspect of the present disclosure, the multicore optical fiber of any one of the twenty-fourth aspect to the thirty-second aspect, wherein the crosstalk between the first core and the second core is ≤−40 dB, as measured for a 100 km length of the multicore optical fiber operating at a wavelength of 1550 nm. 
     According to a thirty-fourth aspect of the present disclosure, the multicore optical fiber of any one of the twenty-fourth aspect to the thirty-second aspect, wherein the crosstalk is ≤−50 dB, as measured for a 100 km length of the multicore optical fiber operating at a wavelength of 1550 nm. 
     According to a thirty-fifth aspect of the present disclosure, the multicore optical fiber of any one of the twenty-fourth aspect to the thirty-second aspect, wherein the crosstalk is ≤−60 dB, as measured for a 100 km length of the multicore optical fiber operating at a wavelength of 1550 nm. 
     Many variations and modifications may be made to the above-described embodiments of the disclosure without departing substantially from the spirit and various principles of the disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims. It will be understood that any described processes or steps within described processes may be combined with other disclosed processes or steps to form structures within the scope of the present disclosure. The exemplary structures and processes disclosed herein are for illustrative purposes and are not to be construed as limiting. 
     To the extent not already described, the different features of the various aspects of the present disclosure may be used in combination with each other as desired. That a particular feature is not explicitly illustrated or described with respect to each aspect of the present disclosure is not meant to be construed that it cannot be, but it is done for the sake of brevity and conciseness of the description. Thus, the various features of the different aspects may be mixed and matched as desired to form new aspects, whether or not the new aspects are expressly disclosed.