Patent Publication Number: US-2023138454-A1

Title: Multicore fiber stubs, multicore fan-in, fan-out devices, and methods of fabricating the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority of U.S. Provisional Application Ser. No. 63/273,413 filed on Oct. 29, 2021, the content of which is relied upon and incorporated herein by reference in its entirety. 
    
    
     FIELD 
     The disclosure is directed to fan-in, fan-out devices and, more particularly, to multicore fiber stubs, multicore fan-in, fan-out devices, and methods of fabricating the same. 
     BACKGROUND 
     Optical fiber is increasingly being used for a variety of applications, including but not limited to broadband voice, video, and data transmission. As bandwidth demands increase optical fiber is migrating deeper into communication networks such as in fiber to the premises applications such as FTTx, 5G, and the like. As optical fiber extends deeper into communication networks there exists a need for building more complex and flexible fiber optic networks in a quick and easy manner. 
     Multicore fibers (MCF) have been intensively studied in the last two decades as one of the ways to improve the transmission capacity of optical fibers. Application of MCF for long haul applications was slow in part because of the rapid advances of transmission rate in coherence communications as well as the high cost associated with laying out new long haul cables. However, the rapid growth of hyperscale datacenters opens a new opportunity of MCF. Although the distance within a datacenter campus is typically less than 2 km, a massive number of fibers is used to interconnect the buildings or regional campuses. At the same time, the high-fiber count cables are deployed through existing ducts, which have limited space. As the need for fiber count continues to increase, MCF provides a much needed relieve in duct space. 
     One component in a MCF-based fiber link is the fan-in fan-out (FI/FO) device, which breaks out each optical core in a MCF to separate single-core optical fibers. The device operates by routing the optical cores with a small pitch from the MCF and converting the small pitch into a wider pitch for connectivity to standard single-core optical fibers (e.g., 125 μm optical fibers). 
     Present fan-in fan-out have several disadvantages, including high insertion loss and also expensive and complicated manufacturing requirements. Consequently, there exists an unresolved need for fan-in fan-out devices having low insertion loss that also lends themselves to high volume production and automation. 
     SUMMARY 
     Various embodiments of multicore fiber stubs and multicore fan-in, fan-fan out devices having short lengths and small taper ratios. The multicore fiber stubs and multicore fan-in, fan-fan out devices disclosed herein are also easily manufacturable and provide low insertion loss. 
     In one embodiment, a multicore optical fiber stub includes a plurality of optical cores, each optical core having an inner core and an outer core, and a fiber coupling section having a first diameter. The plurality of optical cores has a first pitch at the fiber coupling section, and each core at the fiber coupling section has a first index profile with three regions. The multicore optical fiber stub also includes a multicore fiber coupling section having a second diameter that is less than the first diameter. The plurality of optical cores has a second pitch at the multicore fiber coupling section that is less than the first pitch, and each core at the multicore fiber coupling section has a second index profile with three regions. The multicore optical fiber stub further includes a taper section between the fiber coupling section and the multicore fiber coupling section. The taper section has a decreasing diameter such that the taper section has a taper ratio of the first pitch to the second pitch being less than 4. 
     In another embodiment, a multicore optical fiber fan-in, fan-out device includes a multicore optical fiber stub, a plurality of single core optical fibers, and a multicore optical fiber. The multicore optical fiber stub includes a plurality of optical cores, each optical core having an inner core and an outer core, and a fiber coupling section having a first diameter. The plurality of optical cores has a first pitch at the fiber coupling section, and each core at the fiber coupling section has a first index profile with three regions. The multicore optical fiber stub also includes a multicore fiber coupling section having a second diameter that is less than the first diameter. The plurality of optical cores has a second pitch at the multicore fiber coupling section that is less than the first pitch, and each core at the multicore fiber coupling section has a second index profile with three regions. The multicore optical fiber stub further includes a taper section between the fiber coupling section and the multicore fiber coupling section. The taper section has a decreasing diameter such that the taper section has a taper ratio of the first pitch to the second pitch being less than 4. The plurality of single-core optical fibers is optically coupled to the plurality of optical cores at a fiber coupling face of the fiber coupling section. The multicore optical fiber includes a plurality of optical cores optically coupled the plurality of optical cores of the multicore optical fiber stub at a multicore coupling face of the multicore fiber coupling section. 
     In yet another embodiment, a method of fabricating an optical interconnect device includes applying heat to a multicore optical fiber to taper the multicore optical fiber from a first diameter to a second diameter less than the first diameter over a taper section having a taper length. The multicore optical fiber includes a plurality of optical cores, each optical core having an inner core and an outer core. The taper section has a taper ratio of a first pitch at the fiber coupling section to the second pitch at the multifiber coupling section being less than or equal to 4. The method further includes cutting the multicore optical fiber to form a multicore optical fiber stub. The multicore optical fiber stub includes a fiber coupling section having the first diameter, a multicore fiber coupling section having the second diameter, and a taper section. The plurality of optical cores has a first pitch at the fiber coupling section, and each core at the fiber coupling section has a first index profile with three regions. The plurality of optical cores has a second pitch at the multicore fiber coupling section that is less than the first pitch, and each core at the multicore fiber coupling section has a second index profile with three regions. The taper section is located between the fiber coupling section and the multicore fiber coupling section. 
     Additional features and advantages will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the same as described herein, including the detailed description that follows, the claims, as well as the appended drawings. 
     It is to be understood that both the foregoing general description and the following detailed description present embodiments that are intended to provide an overview or framework for understanding the nature and character of the claims. The accompanying drawings are included to provide a further understanding of the disclosure and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments and together with the description serve to explain the principles and operation. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG.  1    schematically illustrates an example multicore optical fiber stub, an example multicore optical fiber, and an example bundle of single-core optical fibers according to one or more embodiments described and illustrated herein; 
         FIG.  2    schematically illustrates an example coupling face of a multicore optical fiber and an example multicore fiber coupling face of an example multicore optical fiber stub according to one or more embodiments described and illustrated herein; 
         FIG.  3 A  illustrates an example first index profile of a fiber coupling section of an example multicore optical fiber stub according to one or more embodiments described and illustrated herein; 
         FIG.  3 B  illustrates an example second index profile of a multicore fiber coupling section of an example multicore optical fiber stub according to one or more embodiments described and illustrated herein; 
         FIGS.  4 A- 4 D  are graphs that plot mode field diameter (MFD) changes as a function of outer core radius r 2  after tapering at 1310 nm for fiber designs according to one or more embodiments described and illustrated herein; 
         FIGS.  5 A- 5 D  are graphs that plot mode field diameter (MFD) changes as a function of outer core radius r 2  after tapering at 1550 nm for fiber designs according to one or more embodiments described and illustrated herein; 
         FIG.  6    is a flowchart illustrating an example process to fabricate a multicore optical fiber stub according to one or more embodiments described and illustrated herein; 
         FIG.  7    schematically illustrates a tapering and cutting system for fabricating a multicore optical fiber stub according to one or more embodiments described and illustrated herein; 
         FIG.  8    is a flowchart illustrating an example process to fabricate a multicore fan-in, fan-out device according to one or more embodiments described and illustrated herein; and 
         FIG.  9    schematically illustrates an example multicore fan-in, fan-out device according to one or more embodiments described and illustrated herein. 
     
    
    
     DETAILED DESCRIPTION 
     References will now be made in detail to the embodiments of the disclosure, examples of which are illustrated in the accompanying drawings. Whenever possible, like reference numbers will be used to refer to like components or parts. 
     The concepts disclosed are related to low-cost fan-in fan-out devices for multicore fiber (MCF) applications with low insertion loss as well as their manufacture. Previous fan-in, fan-out devices for solutions have been based on three types of technologies. The first technology is based on a reduced cladding fiber bundle. The reduced cladding fiber bundle is obtained through precisely etching the cladding to a diameter equal to half the core spacing of the MCF and stacking the fibers in a capillary or a ferrule. This is a highly delicate process and flaws may be generated that affects the long term reliability. 
     The second technology for fan-in fan-out devices is waveguide-based devices. With laser inscribed waveguides in glass, pitch conversion and transition from two dimensions to one dimension becomes feasible. The glass waveguide chip is then connected to fibers and a MCF using an active alignment process. However, waveguide-based fan-in fan-out devices are limited by insertion loss performance due to the waveguide propagation loss and the coupling loss. 
     The third technology is fiber-based devices. In this approach, the fan-out fibers are spaced at a proper spacing and placed inside a low index glass preform and tapered to very small diameter, where the fiber claddings become the “cores” and the low index tubing becomes the cladding. The taped end is fusion spliced to a MCF. This method produces lower insertion loss; however, the process is complicated and lacks scalability. Moreover, a more than 10× taper ratio is required to shrink the fiber cladding into the “core.” Although the taper ratio can be reduced by using relay fibers with a second cladding, the need for a precision glass preform remains to hamper automation. 
     The multicore optical fiber fan-in, fan-out devices of the present disclosure are highly manufacturable, have low insertion loss, and a small taper ratio for the multicore optical fiber stub (e.g., less than or equal to 4 for the ratio of a first pitch at the fiber coupling section to a smaller second pitch at the multicore coupling section). Various embodiments of multicore fiber stubs, multicore optical fiber fan-in, fan-out devices, and methods of fabricating an optical interconnect device are described in detail below. 
     Referring now to  FIG.  1   , an example multicore optical fiber stub  100  (MCF stub  100 ) along with a MCF  200  and a bundle of single-core optical fibers  300  are illustrated. The MCF stub  100  is operable to optically couple the optical cores  202  of the MCF  200  to the individual optical cores (not shown) of each optical fiber of the plurality of single-core optical fibers  300 . The phrase “optically coupled” means that an optical signal passes between an interface with less than 1 dB optical loss. 
     Generally, the MCF stub  100  is the component that converts a fiber pitch from that of a bundled single-core optical fiber  300  to a core pitch matched to that of the MCF  200 . The profile of the fiber cores in the MCF stub  100  is such that the mode field diameter of the cores stays consistent at the tapered and un-tapered ends.  FIG.  1    illustrates a 2×2, four core MCF stub  100  as an example. It should be understood that devices for other MCF configurations and core numbers can be designed following the same approach of the embodiments described in the present disclosure. 
     The example MCF stub  100  comprises three sections: a fiber coupling section  130  which is operable to be coupled to the bundled single-core optical fibers  300 ; a taper section  110 , and a multicore fiber coupling section  120  operable to be coupled to the MCF  200 . 
     Within the fiber coupling section  130 , the MCF stub has four cores  131  (i.e., core regions) defined by an inner core  132  and an outer core  134 . The cores  131  are surrounded by a low index cladding  136 . The cores  131  are exposed at a fiber coupling face  139  of the fiber coupling section  130 . 
     The pitch and diameter of the cores  131 ′ are reduced within the multicore fiber coupling section  120 . As shown in  FIG.  1   , the multicore fiber coupling section  120  has an inner core  132 ′, and outer core  134 ′ and the low index cladding  136 ′ that has a reduced diameter as compared to the low index cladding  136  at the fiber coupling section  130 . The reduced-diameter cores  131 ′ are exposed at a multicore fiber coupling face  121 . 
     The lengths of the fiber coupling section  130  and the multicore fiber coupling section  120  are not limited by this disclosure; however, they each may be at least 5 mm long for handling in a subsequent fusion splicing process. The length of taper section  110  may be such to allow loss-less adiabatic transition of the mode in the inner cores into the new composite core  131 ′ formed by the tapered inner and outer cores. As non-limiting examples, the length of the taper section  100  may be at least 500 μm to ensure low transition loss. The length of the taper section may be greater than 600 μm, greater than 800 μm, greater than 1 mm, greater than 2 mm, or greater than 5 mm. The transition loss may be less than 1 dB, less than 0.5 dB, or less than 0.1 dB. 
     In one non-limiting example, the MCF  200  has four cores in a 2×2 array with a core-to-core pitch of 45 μm, and the bundled single-core optical fibers  300  each have a diameter of about 125 μm. In this example, the taper ratio of the taper section  110  is 2.78:1, which is substantially smaller than that of existing vanishing core technology. In embodiments of the present disclosure, the taper ratio of the taper section  110  is less than 5, less than 4, less than 3.5, or less than 3. The outer diameter of the fiber coupling section  130  of the MCF stub  100  is about 347.5 μm. With such a small diameter, the MCF stub  100  can be made continuously in long lengths using a fiber draw process with an applied protective coating. As described in more detail below, the optical fiber from which the MCF stub  100  is fabricated may be stored in reels until more MCF stubs  100  are needed to be made. 
     In another non-limiting example, the outer diameter fiber coupling section  130  of the MCF stub  100  is matched to the peripheral diameter of the bundle of single-core optical fibers  300 . For the 2×2 core MCF  200  and the 125 μm diameter single core fibers example, the outer diameter of the MCF stub  100  is designed to be about 301.8 μm. The diameter of the multicore fiber coupling section  120 ′ is about 101.4 μm, as shown in  FIG.  2   , as compared to 125 μm in the example of  FIG.  1   . A marker (not shown) can be built in the MCF stub  100  for core identification as known in the art. The difference in outer diameter of the multicore fiber coupling section does not affect the insertion loss when spliced to the MCF fiber  200 . 
     The MCF stubs  100  disclosed herein have index profiles with at least three regions comprising different indexes of refraction. Particularly, the fiber coupling section  130  has a first index profile with at least three regions comprising different indexes of refraction, and the multicore fiber coupling section  120  has a second index profile with at least three regions comprising different indexes of refraction. There are two different profiles because of the change in core-to-core pitch due to the taper section  120  and the changing diameter of the MCF stub  100 . 
       FIG.  3 A  illustrates an example first index profile of the fiber coupling section  130  of the MCF stub  100 . The first index profile comprises three segments: an inner core segment provided by the inner core  132 , an outer core segment provided by the outer core  134 , and a cladding segment provided by the cladding  136 . The inner core segment has a relative refractive index of Δ 1  and a core radius of R 1  from the center of the inner core  132 . The outer core segment has a relative refractive index of Δ 2 , and a core radius of R 2  from the center of the inner core  132 . The cores are within the common low-index cladding  136  that has a relative refractive index of Δ 3 , and a radius of R c  from the fiber center. The diameter of the fiber coupling section  130  is D=2R c . The relative index profile satisfies Δ 1 &gt;Δ 2 &gt;Δ 3 . 
       FIG.  3 B  illustrates an example second index profile of the multicore fiber coupling section  120  after tapering provided by the taper section  110 . After tapering, the cladding diameter is reduced from R c  to r c . The relative refractive indices of the inner core Δ 1 , the outer core Δ 2  and the cladding Δ 3  remain the same, but the inner and outer radii are reduced to r 1  (not shown) and r 2 . The diameter is reduced from D to d=2r c  at the multicore fiber coupling section  120 . The ratio of R 1 /r 1 =R 2 /r 2 =R c /r c  is the taper ratio. Although the profiles in  FIG.  3 A  and  FIG.  3 B  are ideal profiles with perfect step changes from each segment, it will be appreciated by those skilled in the art that dopant diffusions can happen during the manufacturing processes that make the profile transition rounded around the step changes without affecting the functionality of the fiber. Also, the profile can be designed with graded refractive index profiles, such as an alpha profile known in the art to have different shape to achieve similar functionally of the fiber. 
     Sixteen experimental MCF stubs were designed having different taper ratios and different profile designs. The design parameters are shown in Tables 1A and 1B below. In these examples, the outer core was chosen to be pure silica glass with a relative refractive index Δ 2 =0. The inner core has a positive relative refractive index Δ 1 &gt;0, which can be made with an up-dopant in silica glass, for example with Germanium (Ge), Titanium (Ti), Aluminum (Al), Phosphorus (P) or Chlorine (Cl). The cladding has a negative relative refractive index Δ 3 &lt;0, which can be made with a down-dopant, for example, Fluorine (F), or Boron (B). The fiber can also be designed by choosing the cladding to be the pure silica with Δ 2 =0. In this case, both the relative refractive index of both the inner and outer cores are shifted higher. The relative A differences between the inner core and the outer core, and between the outer core and the cladding remain about the same. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 1A 
               
               
                   
               
               
                 Design 
                 1a 
                 1b 
                 1c 
                 1d 
                 2a 
                 2b 
                 2c 
                 2d 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Δ 1   
                 0.34 
                 0.34 
                 0.34 
                 0.34 
                 0.34 
                 0.34 
                 0.34 
                 0.34 
               
               
                 Δ 2   
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 Δ 3   
                 −0.15 
                 −0.15 
                 −0.15 
                 −0.15 
                 −0.20 
                 −0.20 
                 −0.20 
                 −0.2 
               
               
                 R 2 /R 1  = r 2 /r 1   
                 0.413 
                 0.330 
                 0.275 
                 0.236 
                 0.413 
                 0.330 
                 0.275 
                 0.236 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 1B 
               
               
                   
               
               
                 Design 
                 3a 
                 3b 
                 3c 
                 3d 
                 4a 
                 4b 
                 4c 
                 4d 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Δ 1   
                 0.34 
                 0.34 
                 0.34 
                 0.34 
                 0.34 
                 0.34 
                 0.34 
                 0.34 
               
               
                 Δ 2   
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                 Δ 3   
                 −0.25 
                 −0.25 
                 −0.25 
                 −0.25 
                 −0.30 
                 −0.30 
                 −0.30 
                 −0.30 
               
               
                 R 2 /R 1  = r 2 /r 1   
                 0.413 
                 0.330 
                 0.275 
                 0.236 
                 0.413 
                 0.330 
                 0.275 
                 0.236 
               
               
                   
               
            
           
         
       
     
       FIGS.  4 A- 4 D  plot the mode field diameter (MFD) changes as a function of outer core radius r 2  after tapering at 1310 nm for each of the sixteen experimental designs. Similarly,  FIGS.  5 A- 5 D  plot the mode field diameter (MFD) changes as a function of outer core radius r 2  after tapering at 1550 nm for each of the sixteen experimental designs. From the plots of  FIGS.  4 A- 4 D and  5 A- 5 D , one can choose the larger core radius before tapering and the small radius after tapering to have an MCF stub  100  with a MFD that matches the MFD of the multicore optical fiber  200  and the MFD of the single-core optical fibers  300 . To obtain low insertion losses between the multicore optical fiber  200  and the single-core optical fibers  300 , preferably the MFD mismatch at both ends of the MCF stub  100  is less than 1 μm, less than 0.5 μm, or less than 0.25 μm. A typical MFD range is between 8 μm and 11 μm at 1310 nm, and between 9 μm and 12 μm at 1550. A taper ratio between 2 and 4 may be utilized to match the mode field diameters at both ends of the MCF stub  100 . 
     In addition to the sixteen designs of MCF stubs identified by Tables 1A and 1B, five multicore optical fiber fan-in, fan-out devices  10  (e.g., see  FIG.  9   ) comprising a MCF stub  100  optically coupled to a MCF  200  and a bundle of single-core optical fibers  300  disposed within a housing  150  were designed. The parameters of the five multicore optical fiber fan-in, fan-out devices are provided in Table 2 below. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Ex. 1 
                 Ex. 2 
                 Ex. 3 
                 Ex. 4 
                 Ex. 5 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Core design 
                 2 × 2 
                 2 × 2 
                 2 × 2 
                 1 × 4 
                 7 cores 
               
               
                   
                 Profile design 
                 1b 
                 3c 
                 4d 
                 1a 
                 4b 
               
               
                 Large 
                 Core spacing S 
                 125 
                 80 
                 160 
                 80 
                 125 
               
               
                 end 
                 (μm) 
               
               
                   
                 Inner core radius 
                 4.62 
                 2.06 
                 4.27 
                 5.82 
                 3.96 
               
               
                   
                 R 1  (μm) 
               
               
                   
                 Outer core radius 
                 14 
                 7.5 
                 18.1 
                 14.1 
                 12 
               
               
                   
                 R 2  (μm) 
               
               
                   
                 Fiber diameter D 
                 347.5 
                 142.4 
                 443.8 
                 228.8 
                 375 
               
               
                   
                 (μm) 
               
               
                   
                 MFD at 1310 nm 
                 9.2 
                 9.1 
                 9.0 
                 9.3 
                 8.8 
               
               
                   
                 (μm) 
               
               
                   
                 MFD at 1550 nm 
                 10.4 
                 10.8 
                 10.3 
                 10.4 
                 10.1 
               
               
                   
                 (μm) 
               
               
                 Small 
                 Core spacing s 
                 45 
                 45 
                 45 
                 28 
                 41.7 
               
               
                 end 
                 (μm) 
               
               
                   
                 Inner core radius 
                 1.65 
                 1.16 
                 1.2 
                 2.1 
                 1.32 
               
               
                   
                 r 1  (μm) 
               
               
                   
                 Outer core radius 
                 5.03 
                 4.2 
                 5.1 
                 5.1 
                 4 
               
               
                   
                 r 2  (μm) 
               
               
                   
                 Fiber diameter d 
                 125 
                 80 
                 125 
                 80 
                 125 
               
               
                   
                 (μm) 
               
               
                   
                 MFD at 1310 nm 
                 8.7 
                 8.8 
                 9.0 
                 8.6 
                 7.9 
               
               
                   
                 (μm) 
               
               
                   
                 MFD at 1550 nm 
                 10.4 
                 10.4 
                 10.3 
                 10.9 
                 9.3 
               
               
                   
                 (μm) 
               
               
                   
                 Taper ratio 
                 2.78 
                 1.78 
                 3.55 
                 2.86 
                 3 
               
               
                   
               
            
           
         
       
     
     As shown in Table 2, in these examples, core designs of 2×2 cores are arranged in a square around the core center, 1×4 cores arranged in a line across the center, and 7 cores with one core in the center and 6 cores arranged in a hexagonal shape. At the large end of the taper section  110 , a core spacing of 125 and 80 μm can be used to adapt fiber arrays of with fiber diameter of the 125 and 80 μm. In Example 3, the core spacing is 160 μm, which can be used to attach individual single-core optical fibers without them touching one another. In all five examples, the diameter of the fiber coupling section  130  is less than 400 μm. As described in more detail below, the MCF fabrication process may be used to make such multicore fibers and wind them onto a fiber reel. Therefore, the proposed large diameter MCF can be made with long length and low costs. Because the taper ratio is less than 4, the multicore optical fiber fan-in, fan-out devices described herein are easy to make with low manufacturing costs. 
       FIG.  6    is a flowchart  400  illustrating an example method of fabricating a MCF stub  100 . At block  402 , a length of a MCF is drawn by any known or yet-to-be developed draw process. In one embodiment, a silica-based glass substrate blank and core canes are prepared first using a known fiber preform manufacturing process such outside vapor deposition (OVD), modified chemical vapor deposition (MCVD) or plasma chemical vapor deposition (PCVD). The substrate blank and core canes are made according to a refractive index profile design with appropriate dopants that described in previous sections. In some embodiments, the substrate blank has a diameter of in the range of 1 cm to 20 cm. In some embodiments, the core canes have a diameter of 1 mm to 20 mm. The substrate blank is drilled with multiple holes according to a multicore fiber structure design and the hole diameter is slightly larger than the cane diameter to enable the insertion of core canes into the holes. Then the core canes are inserted into the holes of the substrate blank and sealed by applying heat and vacuum for form a multicore preform. The multicore preform is then drawn into MCF using a fiber draw tower known in the art. The MCF may have any designed diameter and any number of optical cores in any desired core arrangement. In some embodiments, a polymeric coating layer or a dual polymeric coating including a primary coating layer and a secondary coating layer is applied to the drawn multicore optical fiber. The drawn multicore optical fiber may be wound in a reel. The fiber length may be greater than 1 m, greater than 10 m, greater than 100 m, greater than 1 km, or greater than 10 km. 
     At block  404 , a section of the MCF has its coating removed, and is heated to reduce the diameter of the MCF over a taper length.  FIG.  7    illustrates an example heating process to reduce the diameter of a MCF  100 ′. A section of the MCF  100 ′ is disposed within a heating device  150 , which may be configured to completely enclose the portion of the MCF  100 ′ or only partially enclose the portion of the MCF  100 ′ (e.g., from one or more sides of the MCF  100 ′). The heating device  150  is operable to apply heat  152  to the MCF  100 ′ to raise its temperature to above its softening point. As non-limiting example, the heating device  150  may be an electric arc, a resistive heater, or CO 2  lasers. In some embodiments, a pulling force is applied to the MCF  100 ′ to form the taper section and reduce the diameter of the MCF  100 ′. In other embodiments, gravity causes the reduction in diameter of the MCF  100 ′. A diameter monitoring device may be used to control the diameter of the MCF  120  to get an accurate and uniform diameter. 
     The taper shape follows the taper profile described above. Because of the smaller diameter of the MCF  100 ′ compared with a glass preform, the taper process is better controlled than tapering a multicore glass cane, typically around 1 to a few mm in diameter into a fiber of about 125 μm in diameter. At block  406 , the tapered MCF  100 ′ is then cleaved off based on a pre-designed length.  FIG.  7    illustrates a cleaving component  154  operable to cleave the MCF  100 ′ to form the MCF stub  100 . The cleaving may be done by a CO 2  laser, or an ultra-fast laser in the visible or near infrared wavelength range, or a mechanical device. Thus, the tapered MCF stubs described herein are automatically produced continuously without having to load the preforms and feeding the optical fibers through the preforms, which provides for high throughput. 
     Referring now to  FIG.  8   , a flowchart  500  of an example process for fabricating a multicore fan-in, fan-out device  10  is provided. At block  502 , a multicore fiber coupling section  120  of a MCF stub  100  is coupled to a MCF  200 . The other end of the MCF  200  may be terminated with a MCF connector. At block  504 , a fiber coupling section  130  of the MCF stub  100  is coupled to a plurality of single-core optical fibers  300 . The other end of the plurality of single-core optical fibers  300  may be terminated by individual single-mode fiber connectors. In some embodiments, the coupling between the MCF stub  100  and the MCF  200  and the plurality of single-core optical fibers  300  is effectuated by fusing splicing. 
     The multicore fiber coupling section matches the MCF in both core pitch and mode field diameter. Very low insertion loss is obtained through fusion splicing. At the fiber coupling section, single-core optical fibers, (e.g., single-core optical fibers or single-core optical fibers with titanium doped strength layer) are stripped of coating and bundled into a 2×2 array matching the core pattern of the MCF stub  100 . When a titanium doped stress layer is present, the bare optical fiber can be handled without mechanical defects. In some embodiments, the bundled single-core optical fiber  300  has a pitch of 125 μm. Alternatively, smaller cladding diameter fiber with or without titanium cladding layer can be used to further reduce the taper ratio. With a 125 μm cladding diameter, the single-core optical fibers  300  can be terminated using standard connector ferrules. Once again, because of the relatively smaller diameter of the MCF stub  100  compared with glass preforms, fusion splicing of the un-tapered end to the bundle of single-core optical fibers  300  is feasible using a similar heat source as the that used in the tapering process. In another embodiment, the single-core optical fibers  300  may be laser-fused to the fiber coupling face  139  sequentially but at a larger pitch than 125 μm. In this case, a larger core pitch in the MCF stub  100  and a higher taper ratio may be needed. 
     At block  506  of  FIG.  8   , the MCF stub  100  and a portion of the MCF  200  and a portion of the bundle of single-core optical fibers  300  into a housing  350 , an example of which is shown in  FIG.  9   . The housing  350  may be rigid, such as fabricated from a hard polymer. In other embodiments, the housing  350  may be flexible (e.g., fabricated from a heat shrink material, such as a soft polymer). The exposed fibers are encapsulated in polymeric material. 
     It should now be understood that embodiments of the present disclosure are directed to MCF stubs, multicore fan-in, fan-out devices, and methods of fabricating the same. The multicore fan-in, fan-out devices described herein have smaller footprints compared with previous fan-in, fan-out devices using tapered preforms. As a non-limiting example, the cross section of the fan-in, fan-out device may be only 0.5 mm at its largest diameter. The smaller taper ratio of the devices disclosed herein translates to shorter overall length. The taper and splices may be kept straight in the package without tension. 
     The MCF stub  100  may be terminated with two connector interfaces. The connector interface at the coupling section  120  is to connect with the MCF  200  terminated with a multicore connector. The connector interface at the coupling section  130  is to connect with fiber bundle terminated with a multi-fiber connector. In this configuration, a compact fan-in or fan-out connector can be realized with the MCF stub  100 . 
     It is noted that recitations herein of a component of the embodiments being “configured” in a particular way, “configured” to embody a particular property, or function in a particular manner, are structural recitations as opposed to recitations of intended use. More specifically, the references herein to the manner in which a component is “configured” denotes an existing physical condition of the component and, as such, is to be taken as a definite recitation of the structural characteristics of the component. 
     It is noted that one or more of the following claims utilize the term “wherein” as a transitional phrase. For the purposes of defining the embodiments of the present disclosure, it is noted that this term is introduced in the claims as an open-ended transitional phrase that is used to introduce a recitation of a series of characteristics of the structure and should be interpreted in like manner as the more commonly used open-ended preamble term “comprising.” 
     Although the disclosure has been illustrated and described herein with reference to explanatory embodiments and specific examples thereof, it will be readily apparent to those of ordinary skill in the art that other embodiments and examples can perform similar functions and/or achieve like results. For instance, the connection port insert may be configured as individual sleeves that are inserted into a passageway of a device, thereby allowing the selection of different configurations of connector ports for a device to tailor the device to the desired external connector. All such equivalent embodiments and examples are within the spirit and scope of the disclosure and are intended to be covered by the appended claims. It will also be apparent to those skilled in the art that various modifications and variations can be made to the concepts disclosed without departing from the spirit and scope of the same. Thus, it is intended that the present application cover the modifications and variations provided they come within the scope of the appended claims and their equivalents.