Patent Publication Number: US-11644630-B2

Title: High-density optical fiber ribbon interconnect and method of making

Description:
PRIORITY APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 16/689,144, filed on Nov. 20, 2019, now U.S. Pat. No. 11,287,588, which claims the benefit of priority of U.S. Provisional Application No. 62/778,593, filed on Dec. 12, 2018, both applications being incorporated herein by reference. 
    
    
     FIELD 
     The present disclosure relates to optical fiber ribbon interconnects, and in particular to a high-density optical fiber ribbon interconnect with a fiber ribbon having cladding-strengthened glass optical fibers in a common protective coating. 
     BACKGROUND 
     The push for higher data rates in digital communications has driven the integration of optics with electronics. In particular, the use of silicon photonics for electro-optical transceivers has resulted in very dense optical circuitry concentrating many separate optical signal lines in the form of optical waveguides (e.g., channel waveguides) into one silicon photonics chip. For many optical signal transmission applications, the optical signals generated on the silicon photonics chips need to be coupled from the optical waveguides into optical fibers. Likewise, optical signals generated at a remote location (e.g., a telecommunications device) need to be coupled from optical fibers to the optical waveguides to be detected by the silicon photonics chip. 
     Optical fiber ribbons and multicore optical fibers are two approaches to increase the fiber density to achieve the parallel connectivity required for the optical circuitry of silicon photonics chips. Unfortunately, using conventional optical fibers in a fiber ribbon does not result in a sufficiently high fiber density due to their relatively large size (as defined by the core, cladding and protective coating) as compared to the size of the optical waveguides of the silicon photonics chips. Likewise, the use of multicore optical fibers is problematic due to manufacturing shortcomings (e.g., maintaining concentricity of the hard protective coating), the high connectivity costs, and the lack of component ecosystems. 
     SUMMARY 
     A high-density fiber ribbon interconnect includes an optical fiber ribbon and at least one connector. The optical fiber ribbon includes two or more cladding-strengthened glass optical fibers each having an outer surface and each not individually including a protective polymer coating. A protective polymer coating substantially surrounds the outer surfaces of the two or more cladding-strengthened glass optical fibers so that the protective polymer coating is common to the two or more cladding-strengthened glass optical fibers. A fiber ribbon cable is formed by adding a cover assembly to the fiber ribbon. A fiber ribbon interconnect is formed by adding one or more optical connectors to the fiber ribbon or fiber ribbon cable. Optical data transmission systems that employ the fiber ribbon to optically connect to a photonic device are also disclosed. 
     Present day optical transceivers used on silicon photonics chips of photonic devices operate at a speed of 100 Gb/s based on 4 lanes at 25 Gb/s per lane. The roadmap for electrical lane speed has been defined for the next generation, and the existing 25 Gb/s lane speed will increase to 56 GBaut/s in the PAM4 signaling protocol which amounts to 112 Gb/s per lane (PAM stands for “pulse amplitude modulation”). The 400 Gb/s Ethernet speed will therefore continue to follow the 4-lane architecture. The optics to support 4 electrical lanes are currently based on either PSM4 (parallel single mode 4 fibers) or the CWDM4 signaling protocol (i.e., 4 wavelength coarse wavelength division multiplexing). The transceiver optical interface is typically an 8 fiber MPO for PSM-4, and a duplex LC for CWDM4. 
     The pro and cons of the PSM4 versus CWDM4 signaling protocols has been an ongoing debate. The PSM4 transceiver that employs a standard ribbon fiber is currently the lower cost solution, even though the connectivity cost is considerably higher than CWDM4 due to the use of a manual push-on pull-off (MPO) connector. The PSM4 protocol consumes more chip space for coupling to fibers. On the other hand, the CWDM4 transceivers suffer from the excess insertion loss of the WDM multiplexer and de-multiplexer, which typically exceeds 4 decibels (dB). As the transceiver speed increases, the link budget will be challenged to accommodate the high insertion loss of WDM components. Moreover, CWDM transceivers require multiple laser sources and consume more power than PSM4 transceivers. As mega data centers increasingly focus on energy efficiency, parallel single mode remains an appealing solution if the connectivity density can be improved. 
     The fiber ribbons, cables and assemblies disclosed herein substantially enhance the density of parallel fiber connectivity with photonic devices that include silicon photonics chips (e.g., transceiver chips that support optical waveguides) without resorting to the use of multicore fibers. The conventional approach to improving the fiber density in an optical fiber-based connection has been to reduce the thickness of the protective coating(s) of the optical fibers. A 200 μm diameter fiber, for instance, is designed to reduce the protective coating thickness from 250 μm while using the same glass cladding diameter of 125 μm. The improvement in fiber density has been appreciable for high-fiber-count cables when protective coating thickness is reduced. For transceiver chip coupling, however, the density improvement is incremental at best. 
     In an example, the high-density fiber ribbons disclosed herein comprise a closely packed array of single-mode fibers in one or more rows, with each fiber having a strengthened cladding and a single common protective coating that directly encapsulates all the fibers, with the exception of the fiber ends as well as the fiber end sections in some examples. Each fiber is made entirely of glass and does not have an individual non-glass protective coating, other than perhaps a thin hermetic seal coating. The lack of individual protective coatings allows for maximizing the fiber density in a fiber ribbon configuration without compromising the optical transmission properties of the fibers. 
     The fiber cladding is made of silica and includes an inner cladding and an outer cladding. The outer cladding is compositionally distinct from the inner cladding and has higher mechanical strength, greater abrasion resistance, and/or greater fatigue resistance than the inner cladding. The outer cladding is referred to herein as a “strengthened cladding” or a “strengthened outer cladding” and a fiber having the outer cladding is referred to herein as a “cladding-strengthened fiber” or a “cladding-strengthened optical fiber” or a “cladding-strengthened glass optical fiber”. The outer cladding is strengthened by doping silica glass. In one aspect, a strengthened outer cladding is made by doping silica glass with titanium dioxide (TiO 2 ). Doping of silica glass to form a strengthened outer cladding improves the scratch and fatigue resistance of the fiber and permits handling and installation of the fiber (e.g. in a ribbon) without damage. The strengthened outer cladding is sufficiently robust to obviate the need for an individual protective coating for each fiber in a ribbon. The overall fiber diameter is accordingly reduced and a higher packing density of fibers in a ribbon is achieved. 
     The common protective coating can be based on an ultraviolet (UV) curable acrylate, a thermoplastic, or other adhesives. The collectively coated fiber array can include indicia (i.e., features, shapes, markings, etc.) to identify the polarity of the fiber ribbon. The pitch of the fiber array of the fiber ribbon can be transitioned from a relatively high fiber density (e.g., substantially equal to the fiber diameter) at one end to up to 250 μm at the other end. In an example, this can be accomplished using a fan-out structure configured to minimize bending. The different fiber densities can be exploited for mass fusion splicing or termination by MPO ferrules for subsequent termination by an MPO connector. The fiber ribbon can be made compatible to standard 127 μm pitch grooves of a grooved substrate, enabling a simple assembly process for high-density connector assemblies, such as fiber array units (FAUs). 
     The fiber array can be collectively coated into a relatively small form factor. With a standard outer cladding diameter of 125 μm, the high-density fiber array can utilize existing fiber termination equipment and connectivity components. Higher fiber densities can be achieved by reducing the outer cladding diameter to 80 μm or even lower. Without protective coatings on the individual fibers, the tightly packed fiber array has inherent geometric precision due to the high manufacturing tolerance and consistency of the fiber outer cladding diameter associated with the fiber drawing process. 
     The present disclosure extends to a fiber ribbon interconnect that includes a fiber ribbon, a first optical connector at a first end of the fiber ribbon and a second optical connector at a second end of the fiber ribbon. The fiber ribbon includes two or more cladding-strengthened glass optical fibers each having an outer surface and a common protective coating that substantially surrounds the outer surfaces of the two or more cladding-strengthened glass optical fibers. 
     The present disclosure also extends to a fiber ribbon interconnect that includes a fiber ribbon and a first optical connector at a first end of the fiber ribbon. The fiber ribbon includes two or more glass optical fibers each having an outer surface, wherein the two or more glass optical fibers do not include individual protective coatings. The fiber ribbon also includes a common protective coating that surrounds the outer surfaces of the two or more glass optical fibers. 
     The present disclosure also extends to a fiber ribbon interconnect that includes a high-density optical fiber ribbon having a first end section with a first end, and at least one second end section with at least one second end. The fiber ribbon also includes two or more glass optical fibers arranged in at least one row, with each of the two or more glass optical fibers having an outer surface and not having an individual protective coating. The fiber ribbon also includes a common protective coating that substantially surrounds the outer surfaces of the two or more glass optical fibers. The fiber ribbon interconnect also includes a high-density optical fiber connector connected to the first end of the high-density optical fiber ribbon. 
     The present disclosure also extends to a method for making a fiber ribbon interconnect. The method includes forming a protective polymer coating surrounding outer surfaces of two or more glass optical fibers, wherein the two or more glass optical fibers do not include individual protective coatings, and wherein the protective polymer coating is common to the two or more glass optical fibers to define an optical fiber ribbon having first and second ends. The method also includes coupling a first optical connector to the first end of the optical fiber ribbon. 
     Additional features and advantages are set forth in the Detailed Description that follows, and in part will be apparent to those skilled in the art from the description or recognized by practicing the embodiments as described in the written description and claims hereof, as well as the appended drawings. It is to be understood that both the foregoing general description and the following Detailed Description are merely exemplary, and are intended to provide an overview or framework to understand the nature and character of the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s), and together with the Detailed Description explain the principles and operation of the various embodiments. As such, the disclosure will become more fully understood from the following Detailed Description, taken in conjunction with the accompanying Figures, in which: 
         FIG.  1    is a front elevated view of an example cladding-strengthened glass optical fiber as disclosed herein. 
         FIG.  2 A  is a cross-sectional view of the cladding-strengthened glass optical fiber of  FIG.  1   . 
         FIG.  2 B  is similar to  FIG.  2 A , but shows an embodiment of the cladding-strengthened glass optical fiber that includes a hermetic sealing layer. 
         FIG.  3 A  is a schematic diagram of an optical fiber drawing system used to form the cladding-strengthened glass optical fibers disclosed herein. 
         FIGS.  3 B and  3 C  are close-up views of the glass outer surface of the cladding-strengthened glass optical fiber and illustrate how the outer surface can be readily functionalized using a fluourinated silane. 
         FIG.  3 D  is a plot of the contact angle θ (°) versus measurement position P (mm) for a bare fiber ( 10 B) and for a fiber ( 10 ) having a silane-based hermetic sealing layer. 
         FIG.  4 A  is a top-down view of an example fiber ribbon that employs the cladding-strengthened glass optical fibers embedded in a common protective coating. 
         FIG.  4 B  is a cross-sectional view of the fiber ribbon of  FIG.  4 A . 
         FIG.  4 C  is similar to  FIG.  4 B  and shows two rows of the cladding-strengthened glass optical fibers within the common protective coating. 
         FIG.  4 D  is a close-up cross-sectional view of a portion of the fiber ribbon of  FIGS.  4 A and  4 B , illustrating an example where the common protective coating comprises two different materials that define a primary (inner) layer and a secondary (outer) layer. 
         FIG.  4 E  is similar to  FIG.  4 B  and shows an example where the common protective coating is thicker on one side of the cladding-strengthened glass optical fibers than the other as an indication of the polarity of the fiber ribbon. 
         FIG.  5 A  is a top down view of an example fiber ribbon cable formed using the fiber ribbon. 
         FIGS.  5 B through  5 D  are cross-sectional views of the fiber ribbon cable of  FIG.  5 A , wherein the fiber ribbon cable comprises the fiber ribbon and a cover assembly that surrounds the outside of the fiber ribbon. 
         FIG.  6    is an elevated view of an example fiber ribbon interconnect that comprises the fiber ribbon cable connectorized at its opposite ends with optical fiber connectors. 
         FIG.  7 A  is an exploded front elevated view of an example method of forming a high-density connector assembly that can be used directly as a high-density optical fiber connector or that can be used to form a high-density optical fiber connector. 
         FIG.  7 B  is a front elevated view of the assembled high-density connector assembly. 
         FIG.  7 C  is a cross-sectional view of the high-density connector assembly of  FIG.  7 B , illustrating an example of the cladding-strengthened glass optical fibers extracted from the protective coating of the fiber ribbon, and the fiber ribbon extracted from the cover assembly. 
         FIG.  7 D  is an elevated view illustrating how the high-density connector assembly of  FIG.  7 B  can be used to form a high-density optical fiber connector by adding additional components in the form of a connector housing and alignment features. 
         FIG.  8 A  is a top-down view of an example fan-out fiber ribbon. 
         FIGS.  8 B and  8 C  are x-y cross-sectional views of the fan-out fiber ribbon of  FIG.  8 A  taken at the lines B-B and C-C respectively, and illustrating how the fiber pitch (density) can be made different at the opposite ends of the fan-out fiber ribbon using a fan-out region. 
         FIG.  9    is a top-down view of an example fiber ribbon interconnect that comprises the fan-out fiber ribbon of  FIG.  8 A  with a high-density optical fiber connector operably attached to the high-density (narrow) end and a conventional (e.g., MPO) connector attached to the standard-density (wide) end. 
         FIG.  10 A  is an elevated view of an example furcated fiber ribbon. 
         FIG.  10 B  is a cross-sectional view of each of the furcations of the furcated fiber ribbon of  FIG.  10 A . 
         FIG.  10 C  is similar to  FIG.  10 A  and shows an example interconnect that employs the furcated fiber ribbon. 
         FIG.  11 A  is as schematic diagram of an optical data transmission system that employs the fiber ribbon as disclosed herein. 
         FIG.  11 B  is a close-up view of one end of the fiber ribbon connectorized with a high-density optical fiber connector, which is in position to operably engage a photonics chip of a photonic device. 
         FIG.  11 C  is similar to  FIG.  11 B  and shows the high-density optical fiber connector operably engaged with the photonics chip of the photonic device so that the cladding-strengthened glass optical fibers are in optical communication with optical waveguides supported by the photonics chip. 
         FIG.  11 D  is a side view of a connectorized end of an example fiber ribbon interconnect, wherein the connectorized end includes a standard connector, and illustrating an example of how the fiber ribbon interconnect can be optically connected to a standard optical fiber cable using an adapter. 
         FIG.  11 E  is a top-down view of an example of an optical data transmission system wherein a fiber ribbon interconnect terminated by two high-density optical connectors is used to optically connect two different photonic devices. 
         FIG.  11 F  is similar to  FIG.  11 E  and shows an example of an optical data transmission system wherein the fiber ribbon interconnect utilizes a bifurcated fiber ribbon to provide high-density optical interconnections between one photonic device at one end of the fiber ribbon interconnect and two photonic devices at the other end of the fiber ribbon interconnect. 
     
    
    
     DETAILED DESCRIPTION 
     Reference is now made in detail to various embodiments of the disclosure, examples of which are illustrated in the accompanying drawings. Whenever possible, the same or like reference numbers and symbols are used throughout the drawings to refer to the same or like parts. The drawings are not necessarily to scale, and one skilled in the art will recognize where the drawings have been simplified to illustrate the key aspects of the disclosure. 
     The claims as set forth below are incorporated into and constitute part of this Detailed Description. 
     Cartesian coordinates are shown in some of the Figures for the sake of reference and are not intended to be limiting as to direction or orientation. 
     The expression “comprises” as used herein includes the term “consists of” as a special case, so that for example the expression “A comprises B and C” is understood to include the case of “A consists of B and C.” 
     Relative terms like top, bottom, side, horizontal, vertical, etc. are used for convenience and ease of explanation and are not intended to be limiting as to direction or orientation. 
     The acronym MPO as used herein stands for multifiber push on and is used to describe a type of optical fiber connector known in the art and that is standard in the art. 
     The term “elastic modulus” as used herein refers to Young&#39;s modulus. 
     The terms “optical fiber” and “glass optical fiber” as used herein refer to a glass fiber configured to operate as a waveguide. 
     Fiber with Strengthened Cladding 
       FIG.  1    is a front elevated view of an example cladding-strengthened glass optical fiber (“fiber”)  10  as disclosed herein.  FIG.  2 A  is a cross-sectional view of the fiber  10  of  FIG.  1   . The fiber  10  has a centerline AC, a core  16  centered on the centerline and having a diameter D 1 , and a cladding  20  surrounding the core and having a diameter D 2  and a thickness TH 2 . The core  16  and cladding  20  are both made of glass and can have the configuration (i.e., refractive index profile, dimensions, etc.) of a conventional optical fiber. The refractive index profile of the fiber  10  can be that for a standard single mode fiber, a bend-insensitive single mode fiber, a few-mode fiber, or a multimode fiber. 
     An example diameter D 1  of the core  16  for a single mode fiber is in the range from about 5 microns to about 10 microns, with about 8 microns being typical value for telecommunications wavelengths of 1310 nm and 1550 nm. An example diameter D 2  of the cladding  20  is in the range from about 30 μm to about 250 μm, depending on the desired overall fiber diameter DF, which in an example can be in the range from 50 μm to 200 μm. 
     The fiber  10  also includes an additional cladding  30  that surrounds the cladding  20 , so that the cladding  20  can be considered an inner cladding and the cladding  30  considered an outer cladding of a two-part cladding region  40 . The outer cladding  30  has a diameter D 3  and a thickness TH 3 . An example range on the thickness TH 3  is between 1 μm and 20 μm. 
     The outer cladding  30  comprises silica (SiO 2 ) doped with titanium dioxide (TiO 2 , which is also called “titania”). An example of such a fiber  10  is the Corning® Titan® single-mode optical fiber, available from Corning, Inc., Corning, N.Y. The outer cladding  30  strengthens the fiber  10  and in particular provides abrasion and/or fatigue resistance to the fiber  10 . That is, outer cladding  30  has a higher strength parameter S and/or a higher dynamic fatigue parameter nd than inner cladding  20 . A fiber with a strengthened outer cladding  30  is referred to herein as a “cladding-strengthened fiber”, or a “cladding-strengthened optical fiber”, or a “cladding-strengthened glass fiber”, or a “cladding-strengthened glass optical fiber”. The outer cladding  30  need not be stripped off during splicing or termination. The outer cladding  30  has an outer surface  32 , which defines the outer surface of the cladding region  40 . The outer surface  32  is a glass surface. 
     The outer cladding  30  has an amount of compressive stress SC that strengthens the outer surface  32  of the outer cladding layer in a manner similar to chemically strengthened glasses. In an example, the amount of compressive stress SC is in the range from 30 MPa to 100 MPa. In addition, the formation of microcrystals due to doping of the SiO 2  with titania can stop defects such as scratches from propagating through the fiber  10 , resulting in further fatigue resistance. In an example, the doping concentration of titania in the SiO 2  of the outer cladding  30  is in the range from 5 wt % to 25 wt %. The outer cladding  30  contributes to the overall refractive index profile of the fiber  10 , but is also designed to avoid excess loss of the guided light from the core  16 . Because the titania dopant in silica glass increases the refractive index, it can cause tunneling loss of guided light traveling in the core  16  to the titania doped outer cladding  30 . To avoid this excess loss, the starting position (inner radius) of outer cladding  30  needs to be sufficiently away from the core. Preferably, the spacing (radial distance) between the core  16  and the outer cladding  30  (i.e., the thickness TH 2  of the inner cladding  20  as shown in  FIG.  2 A ) is greater than 25 μm and more preferably greater than 30 μm. 
     The fiber  10  also has an outer surface  70 . In the example of  FIG.  2 A , fiber  10  is bare fiber  10 B and outer surface  70  is the same as the outer surface  32  of the outer cladding  30 .  FIG.  2 B  is similar to  FIG.  2 A  and illustrates an example sealed fiber  10 H that includes a hermetic sealing layer  50  on the outer surface  32  of the outer cladding  30 . In an example, the hermetic sealing layer  50  comprises or consists essentially of carbon. In one example, the hermetic sealing layer  50  has a thickness TH 5 &lt;100 nm and so does not contribute substantially to the overall size (diameter DF) of the sealed fiber  10 H. In an example, the hermetic sealing layer is made of an inorganic material, e.g., is not made of an organic polymer such as acrylate. The hermetic sealing layer  50  is designed to prevent moisture and other adverse materials in the environment from entering and possibly damaging the fiber. The hermetic sealing layer  50  is thus substantially different in chemical composition from a protective coating used on conventional individual optical fibers or the common protective coating for fibers in an array or bundle. Protective coatings for individual optical fibers and fiber arrays or bundles are organic polymers. The organic polymers are formed by polymerizing organic monomers, usually acrylate or methacrylate monomers. Thermoplastic organic polymers are also used as protective coatings for individual optical fibers and fiber arrays or bundles. The hermetic sealing layer  50  is not an organic polymer and has a thickness well below the thickness used for the protective coatings of individual optical fibers or fiber arrays or bundles. In an example discussed below, the hermetic sealing layer comprises a self-assembled monolayer (SAM), such as formed using a silane compound. Example hermetic sealing layers  50  are discussed in greater detail below. In the discussions below, the fiber  10  can be a fiber such as sealed fiber  10 H in  FIG.  2 B  or a bare fiber  10 B such as in  FIG.  2 A , unless the particular type of fiber is specified. 
     A given length of fiber  10  has opposite end sections  60  each with an end face  62 , as shown in  FIG.  1   . The fiber  10  has the aforementioned outer surface  70 , which can be defined by the cladding region  40  as in bare fiber  10 B of  FIG.  2 A  or the hermetic sealing layer  50  as in sealed fiber  10 H of  FIG.  2 B , depending on the configuration of the fiber. 
     In the discussion and in the drawings, reference to fiber  10  refers to either bare fiber  10 B or the sealed fiber  10 H, unless otherwise noted. 
     Fabricating the Fiber 
     The fiber  10  can be made by drawing a fiber from a preform using standard optical fiber fabrication drawing techniques.  FIG.  3 A  is a schematic diagram of an example optical fiber drawing system (“drawing system”)  100 . The drawing system  100  may comprise a draw furnace  102  for heating the preform to the glass melt temperature, non-contact measurement sensors  104 A and  104 B for measuring the size of the drawn fiber as it exits the draw furnace for size (diameter) control, a cooling station  106  to cool the drawn fiber, a tensioner  120  with a surface  122  to pull (draw) the fiber, guide wheels  130  with respective surfaces  132  to guide the drawn fiber, and a fiber take-up spool (“spool”)  150  to store the drawn fiber. 
     The drawing system  100  also includes a preform holder  160  located adjacent the top side of the draw furnace  102  and that holds a glass preform  10 P used to form the fiber  10 . With reference to the close-up inset of  FIG.  3 A  that shows a cross-sectional view of the glass preform  10 P, the preform has a preform core  16 P, an inner preform cladding  20 P and a outer preform cladding  30 P. The glass preform  10 P has generally the same relative configuration and dimensional proportions in the radial direction as bare fiber  10 B but is much larger, e.g.,  25 X to  100 X larger. 
     The preform core  16 P can be made by doping silica with an index-increasing dopant such germanium oxide. The inner preform cladding  20 P and outer preform cladding  30 P start out as pure silica (SiO 2 ). The preform outer cladding  30 P is then doped with titania to strengthen it. The glass preform  10 P and in particular preform core  16 P and the inner preform and outer preform cladding layers  20 P and  30 P may be produced in a single-step process or multi-step process. Suitable methods or processes include: the double crucible method, rod-in-tube procedures, and doped deposited silica processes, also commonly referred to as chemical vapor deposition (CVD). A variety of CVD processes are known and are suitable for producing the core and cladding layers used in the optical fibers of the present invention. They include outside vapor deposition process (OVD) process, vapor axial deposition (VAD) process, modified CVD (MCVD), and plasma-enhanced CVD (PECVD). 
     After the glass preform  10 P is formed, it is operably supported in the preform holder  160  relative to the draw furnace as shown in  FIG.  3 A . The glass preform  10 P is then heated by the draw furnace  102  and drawn into fiber  10  using the drawing system  100 . The drawing process is similar to a conventional fiber draw process, except that no polymer coatings are added to the fiber  10 , i.e. the fiber is a bare glass fiber  10 B with a strengthened outer cladding  30  or a sealed glass fiber  10 H with a strengthened outer cladding  30  and a hermetic sealing layer  50  as discussed above. Note in particular that the doped outer preform cladding  30 P defines the chemically strengthened (doped) outer cladding  30  when the preform  10 P is drawn to form the fiber  10 . 
     In the fabrication process, the fiber drawn from glass preform  10 P exits the draw furnace  102 , with tension applied by the tensioner  120 . The dimensions (e.g., the diameter) of the fiber are measured by the non-contact sensors  104 A and  104 B and the measured dimensions are used to control the draw process. The fiber can then pass through the cooling mechanism  106 , which can be filled with a gas that facilitates cooling at a rate slower than air at ambient temperatures. At this point, the fiber  10  is a bare fiber  10 B. 
     The fiber  10  passes from the tensioner  120  to the guide wheels  130 , then through the guide wheels to the spool  150 , where the fiber  10  is taken up and stored. It is noted that a bare glass fiber without a protective coating that lacks outer cladding  30  cannot be collected on a take-up spool as a practical matter due to the high break rate due to surface damage to the fiber. The strengthened outer cladding  30  of fiber  10  makes possible collecting this glass fiber on the spool  150  without breaks. Also, in an example, the tensioner surface  122  and the guide wheel surfaces  132  preferably comprise either a polymer material such as a fluoropolymer (e.g., polytetrafluoroethylene or PTFE), or a plastic material or a rubber material, to protect the fiber  10  from surface damage. 
     The configuration of the glass preform  10 P and the various drawing parameters (draw speed, temperature, tension, cooling rate, etc.) dictate the final form of the fiber  10 . 
     Embodiments of the Hermetic Sealing Layer 
     In the example sealed fiber  10 H of  FIG.  2 B  that includes the hermetic sealing layer  50  (i.e., the coated fiber), the drawing system  100  can include an applicator device  170  that applies a hermetic sealing layer material  50 M to the drawn bare fiber  10 B as the bare fiber  10 B passes by the applicator device  170 . The applicator device  170  can also be one that is off-line, i.e., in another location besides in the drawing system  100  and employed after the bare fiber  10 B is collected on the spool  150 , with the fiber distributed from the spool for application of the hermetic sealing layer material  50 M to bare fiber  10 B by the applicator device  170  for form hermetic sealing layer  50 . 
     In an example, the hermetic sealing layer material  50 M comprises an inorganic material, such as an inorganic hydrophobic material. Hermetic sealing layer materials that include silicon are regarded herein as inorganic materials even if carbon or an organic fragment is bonded to silicon. Organosilanes, for example, are regarded as inorganic materials for purposes of the present disclosure. 
     In another example, the hermetic sealing layer material  50 M comprises a self-assembled monolayer (SAM). In an example, the SAM is formed using a silane, preferably an organosilane, which can be applied in liquid form onto the bare fiber  10 B using the applicator device  170 . The SAM hermetic sealing layer material  50 M that defines an example hermetic sealing layer  50  produces a hydrophobic outer surface  70  for the sealed fiber  10 H that shields the outer cladding  30  from moisture, thereby slowing down the development of glass fatigue and reduce fiber breaks. Because the SAM layer is thin (e.g., &lt;10 nm), it does not need to be removed when making the fiber ribbon (introduced and discussed below), and does not affect fiber positioning (e.g., the fiber ribbon pitch). In some embodiments, the SAM layer covers or is uniformly distributed over the entirety of the outer surface  32  of the outer cladding  30 . In other embodiments, the SAM layer does not cover or is not uniformly distributed over the entirety of the outer surface  32  of the outer cladding  30 . For example, gaps may exist in the SAM layer and portions of the outer surface  32  of the outer cladding  30  may be exposed. 
     One example of a silane in liquid form comprises octadecyldimethyl trimethoxysilylpropyl ammonium chloride (60 wt % in MeOH), acetic acid (0.05 wt %) and deionized water (18 Mohm, 0.2 micron filtered). In an example, the proportions by weight of the three ingredients can be 16.7:1:19823.4. The deposited layer of the silane liquid as the hermetic sealing layer material  50 M produces on the outer surface  32  of the outer cladding  30  a hermetic sealing layer  50  with a hydrophobic fiber outer surface  70  that can inhibit moisture from getting into the glass material of the fiber. 
       FIGS.  3 B and  3 C  are close-up views of the outer surface  32  of the sealed fiber  10 H and illustrate how the outer surface can be functionalized using a fluourinated silane  80  as hermetic sealing material  50 M. The fluorinated silane  80  includes a silane core  82  and fluorinated chain  84  attached thereto. The fluorinated silane  80  can be can be introduced to the outer surface  32  ( FIG.  3 B ) so that the silane core  82  bonds to silanol groups  86  on the outer surface  32 , thereby forming the thin silane-based SAM hermetic sealing layer  50  ( FIG.  3 C ). 
     In an example, a solution of perfluoropolyether-functionalized silane  80  was used as the hermetic sealing layer material  50 M to create a hydrophobic hermetic sealing layer  50 . The silane-based hermetic sealing layer material  50 M was prepared as a solution by adding 0.12 vol % perfluoropolyether-functionalized silane to a fluorinated solvent. A silane-based hermetic sealing layer material  50 M can be applied to the bare fiber  10 B by using an applicator device  170  that includes a die containing the liquid hermetic sealing layer material  50 M. After passing through the die, a thin layer of the hermetic sealing layer material  50 M is coated on the outer surface  32 . Some hermetic sealing layer materials  50 M can be dried in air at the ambient temperature. Some hermetic sealing layer materials  50 M require curing. The curing can be done by heating for heat-curable materials or by UV light for UV-curable materials. 
     In another example, the hermetic sealing layer material  50 M comprises carbon to define a carbon-based hermetic sealing layer  50 . The carbon can be deposited on the outer surface  32  of the outer cladding  30  by the applicator device  170  in the form of an atmospheric chemical vapor deposition chamber in which a hydrocarbon gas, such as methane, acetylene, ethylene, propane, etc. undergoes pyrolysis and a heterogeneous reaction on the outer surface  32 . Carbon is strongly bonded to silica with Si—C bond. The carbon layer has typically a randomly oriented graphite platelet structure or amorphous cross-linked graphite structure. Forms of carbon with graphite structures are regarded herein as inorganic materials. 
     The wettability of example sealed fibers  10 H with the hermetic sealing layer  50  was evaluated by using dynamic contact angle measurement using a tensiometer (K100C-MK2, Kruss GmbH, Germany). The contact angle θ is a quantitative measure of wettability of the outer surface  70  of the fiber  10 H by a liquid. Generally, if the contact angle θ is less than 90° the surface is said to be hydrophilic. On the other hand, if the contact angle θ is greater than 90°, the surface is said to be hydrophobic. 
       FIG.  3 D  is a plot of the contact angle θ (°) versus measurement position P (mm) for a bare fiber  10 B (i.e., no hermetic sealing layer  50 ; see  FIG.  2 A ) and for a sealed fiber  10 H (see  FIG.  2 B ) having a silane-based hermetic sealing layer  50  as described above. The plot includes the advancing contact angle measured as the sample fiber  10  is immersed in the liquid (+P direction) and the receding contact angle measured as the sample fiber emerges from liquid (−P direction). Table 1 below lists the advancing, receding and the contact angle hysteresis (difference between the advancing and receding contact angle). The contact angle hysteresis is a measure of surface heterogeneity and surface roughness. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                   
                 Water contact angle θ (°) 
               
            
           
           
               
               
               
               
            
               
                 Sample 
                 Advancing 
                 Receding 
                 Hysteresis 
               
               
                   
               
               
                 Bare 
                  29.3 ± 18.2 
                 63.0 ± 7.0 
                 33.7 ± 25.2 
               
               
                 fiber 
                   
                   
                   
               
               
                 Coated 
                 131.4 ± 5.0 
                 114.9 ± 5.6  
                 16.6 ± 10.6 
               
               
                 fiber 
                   
                   
                   
               
               
                 Control 
                 118.4 ± 0.3 
                 95.8 ± 3.1 
                 22.6 ± 3.3  
               
               
                 (flat glass) 
               
               
                   
               
            
           
         
       
     
     Contact angles θ of the bare fibers  10 B were in the range of 29-63° indicating the hydrophilic nature of the bare fibers  10 B. In contrast, contact angles θ measured on the sealed fiber  10 H were in the range of 115-131°, indicating that the hermetic sealing layer  50  is hydrophobic. In addition, the contact angle hysteresis of the sealed fibers  10 H is lower than that for the bare fibers  10 B, suggesting good uniformity of the hermetic sealing layer  50  on the outer surface  70 . 
     In the case of a silane-based hermetic sealing layer  50 , the thickness TH 5  of this layer need not exceed 10 nm. The thickness of a hermetic sealing layer  50  made in the form of a SAM from a silane-based hermetic sealing material  50   m  is less than or equal to 10 nm, or less than or equal to 8 nm, or less than or equal to 6 nm, or in the range from 3 nm-10 nm, or in the range from 4 nm-8 nm. These ranges for the thickness TH 5  are suitable if the main purpose of the hermetic sealing layer  50  is to provide hydrophobicity. In other embodiments, the hermetic sealing layer  50  is also used to improve the mechanical properties of the fiber and the thickness TH 5  of the hermetic sealing layer  50  is preferably greater than 10 nm. In an example, the hermetic sealing layer material  50  can comprise a fluorosilicone and/or a perfluoroelastomer and the layer thickness TH 5  can be in the range from 10 nm-10 microns, or in the range from 50 nm-5 microns, or in the range from 100 nm-3 microns. One example of the fluorosilicone comprises a polysiloxane backbone with a fluorinated pendant groups (e.g. trifluoropropyl groups). Such fluorosilicones can be moisture cured or catalyst-cured (e.g. platinum-cured) with a hydride crosslinker at room temperature and the cure can be accelerated by applying heat. The fluorosilicones, in addition to providing hydrophobicity, can also provide shock and vibration absorption and increased durability, thereby increasing the mechanical properties of the fibers. 
     In another example, the hermetic sealing layer material  50 M comprises a perfluoroelastomer (FKM). A type of FKM suitable for use as the hermetic sealing layer material  50 M is known as VITON®, which is a registered trademark of The Chemours Company of Wilmington, Del. In an example, the FKM can be cured by peroxide and can also be grafted with silicones using amino-functionalized polydimethyl siloxane. These silicone-grafted FKMs can provide the flexibility of the silicones along with the durability and tensile strength of the FKMs. 
     Fiber Ribbon 
       FIG.  4 A  is a top-down view of an example optical fiber ribbon (“fiber ribbon”)  200  that employs an array  8  of the fibers  10  disclosed herein.  FIG.  4 B  is an x-y cross-sectional view of the example fiber ribbon  200  as taken along the line A-A in  FIG.  4 A . The example fiber ribbon  200  of  FIG.  4 A  has a substantially constant x-y cross-sectional shape in the z-direction. The array  8  of fibers  10  are arranged in a row i.e., with their centerlines AC substantially residing on reference line R 1  that runs in the x-direction (see the close-up inset of  FIG.  4 B ). The fiber ribbon  200  has a first end section  201  with a first end  202  and a second end section  203  with a second end  204 . 
     While the fiber ribbon  200  can generally comprise two or more fibers  10  in the fiber array  8 , in some preferred embodiments, the fiber ribbon  200  comprises multiples of 8 fibers, e.g., 8, 16, 24, etc. In addition, while the fiber ribbon  200  described in detail below includes cladding-strengthened optical fibers  10 , in other embodiments the fiber ribbon  200  may include other types of glass or plastic optical fibers. 
     The cladding-strengthened optical fibers  10  reside within a common protective coating  210 , i.e., the common protective coating  210  generally surrounds the outer surfaces  70  of the fibers  10 . The cross-sectional view of  FIG.  4 B  shows the fibers  10  encapsulated within the common protective coating  210 . In some examples, one or both end sections  60  of one or more of the fibers  10  are exposed, as explained below. The common protective coating  210  has an outer surface  221  with a top side  222 , a bottom side  224  and edges  226 . In an example, the common protective coating  210  has an oval, rectangular (with sharp or rounded corners), or similar elongate shape, with a long dimension LX in the x-direction and a short dimension LY in the y-direction. Note that in an embodiment where the outer surfaces  70  of the fibers  10  are in contact with each other to maximize the fiber density, the common protective coating  210  may not be contact the entire outer surface  70  of each fiber  10  at the location where adjacent fibers are in contact. In other embodiments where the fibers  10  are spaced apart from each other, the common protective coating  210  surrounds the entire outer surface  70  of each fiber over at least a portion of the length of the fiber ribbon  200 . 
     As discussed above, typical optical fibers, in an array or individually, each have an individual protective coating. In contrast, the fibers  10  in array  8  do not each include individual protective coatings. Instead, the common protective coating  210  is common to fibers  10  in array  8  of fiber ribbon  200 . As used herein, a coating is said to be “common” if it is applied to a plurality of fibers  10  of an array  8  and if it makes direct contact with an outer surface  70  of at least some of the plurality of fibers  10  of the array  8 . In some embodiments, for example, the common protective coating directly contacts the outer surface of two or more of the cladding-strengthened glass optical fibers. The common protective coating  210  may be applied simultaneously to the plurality of fibers  10  of the array  8 . If the plurality of fibers  10  includes two or more bare fibers  10 B, a coating is common if it makes direct contact with outer surface  32  of at least two of the fibers in the plurality. If the plurality of fibers includes two or more hermetic sealed fibers  10 H, a coating is common if it makes direct contact with outer surface  70  of at least two of the fibers in the plurality. 
     In an example, the common protective coating  210  is made of a non-glass material, and further in the example is made of a polymer such as those used as protective primary or secondary coatings for individual optical fibers. The polymer is preferably an organic polymer. Such polymers include acrylates, methacrylates, and polyamides. Other non-glass materials known in the art for coating glass optical fibers can also be used for the common protective coating  210 , including thermoplastics and adhesives. In an example, the material used for the common protective coating  210  is curable by exposure to ultraviolet (UV) light (i.e., is UV curable). An example thermoplastic has a melt temperature in the range of 160° C. to 260° C., a melt viscosity in the range of 100 centipoise (cP) to 10,000 cP, and an operating temperature from −40° C. to 100° C., noting that telecommunications standards involve testing telecommunications components (including fibers) over a temperature range from −40° C. to 85° C. 
     In an experiment, an example fiber  10  having an outer cladding  30  with a thickness of 2 μm was formed using the above-described drawing process using the drawing system  100  and wound around the take-up spool  150 . Then eight fibers  10  were bundled together to form a fiber array  8 . The fiber array  8  was then coated using a thermoplastic split die coating drawing process to form a common protective coating  210  around the eight fibers  10 . The polymer used to form the common protective coating  210  was polyamide (Henkel PA652, also known as MACROMELT OM 652, available from Henkel Corporation, Rocky Hill, Conn.), which was heated to 190° C. as the fiber array  8  passed through. The polyamide coating solidified when cooled down to less than 120° C. to form the (solid) common protective coating  210 . With reference to  FIG.  4 B , the resulting fiber ribbon  200  had a dimension LY in the y-direction 0.21 mm and a dimension LX in the x-direction of 1.1 mm. The fiber ribbon  200  was measured for bending loss and exhibiting the preferential bending properties as expected. 
       FIG.  4 C  is similar to  FIG.  4 B  and shows an example embodiment wherein the fibers  10  are arranged in two arrays  8  in the form of rows defined by respective reference lines R 1  and R 2  that run in the x-direction. More than two rows of fibers  10  can also be implemented, and the discussion below focuses on a single-row embodiment of the fiber ribbon  200  for ease of illustration and explanation. 
     The fibers  10  define a fiber pitch PR for the fiber ribbon  200 , which defines the fiber density for the fiber ribbon i.e., the number of fibers per unit length along the given row of fibers. As discussed below, the fiber pitch PR for the fiber ribbon  200  can be constant with length along the fiber ribbon, or can change, depending on the particular fiber ribbon configuration. The fiber pitch PR can be in the range from the fiber diameter DF to 250 microns, wherein the greatest fiber density is about 2× of the greatest fiber density of a conventional fiber ribbon. This factor increases to 4× for two-dimensional arrays  8  (i.e., two rows) of fibers  10 . 
     In one embodiment, fiber density is expressed as the separation between adjacent glass optical fibers in a fiber ribbon. Each of the fibers in a ribbon has a centerline and fiber density is expressed in terms of the separation between centerlines of adjacent glass optical fibers in the fiber ribbon. The separation of centerlines of adjacent glass optical fibers is less than 150 microns, or less than 125 microns, or less than 100 microns, or less than 80 microns, or less than 60 microns, or in the range from 40 microns-150 microns, or in the range from 60 microns-125 microns, or in the range from 75 microns-110 microns. 
     The fiber density of the fiber ribbon  200  is greater than that of a conventional fiber ribbon mainly because conventional optical fibers include individual protective coatings applied during the fiber drawing process. Such protective coatings typically have an outer diameter of about 240 microns, which approximately doubles the diameter of the fiber relative to the cladding-strengthened fibers disclosed herein. Without the protective coating, a conventional optical fiber has very high chance of breaking when wound onto the take-up spool because a conventional optical fiber lacks a strengthened outer cladding  32  as described herein, which makes the ribbon fabrication process with high fiber density as disclosed herein impractical to implement. The local stress imparted to a bare glass conventional fiber also present long-term reliability risks that are mitigated through inclusion of the strengthened outer cladding  32 . 
     In one example, the common protective coating  210  is made of a single polymer material. In another example illustrated in the close-up cross-sectional view of  FIG.  4 D , the common protective coating  210  includes multiple polymer materials, which in one aspect are layered to define a primary (inner) layer  212  and a secondary (outer) layer  214  in the case of two different polymer materials. In an example, the secondary layer  214  has a higher elastic modulus than the primary layer  212 , with the elastic modulus of the entire common protective coating  210  being an effective elastic modulus that is substantially an average of the respective elastic moduli of the primary and secondary layers. This configuration provides a relatively soft, cushioning layer closest to the fibers  10  that protects the fibers from mechanical loads while also providing a harder, abrasion-resistant layer on the outermost surface  221  of the fiber ribbon. An example of a dual-layer common protective coating  210  that can be effectively utilized to form the fiber ribbon  200  is the Corning® CPC® protective coating, available from Corning, Inc., Corning, N.Y. In an example, the effective elastic modulus of a protective coating with one or a plurality of layers is in the range from 10 MPa-1000 MPa, or in the range from 20 MPa-800 MPa, or in the range from 50 MPa-600 MPa. 
     The common protective coating  210  may be deposited over the fibers  10  using techniques known in the art such as by disposing a curable coating composition on the fibers  10  and then curing the curable coating composition using, for example, ultraviolet (UV) light, heat, or by other means known in the art. In this embodiment, the common protective coating  210  is a cured product of the curable coating composition. 
     The fiber ribbon  200  may optionally include indicia  230 , such as geometrical features, markings, colorings, etc. to identify the polarity of the fiber ribbon. For instance, with reference to  FIG.  4 E , the common protective coating  210  may be made asymmetric relative to the reference line R 1 , e.g., the thickness of the common protective coating  210  may be different between the reference line R 1  and the top surface  222  as compared to the thickness between the reference line R 1  and the bottom surface  224 . In other example, the top/bottom sides or left/right sides of the common protective coating  210  can be formed by polymers having different colors. 
     Fiber Ribbon Cable 
       FIG.  5 A  is a top-down view similar to  FIG.  4 A  and shows an example of a fiber ribbon cable  300  formed using the fiber ribbon  200 . The fiber ribbon cable  300  has first and second end sections  301  and  303  that respectively include first and second ends  302  and  304 . The first and second ends  202  and  204  of the fiber ribbon are shown as coinciding with the first and second ends  302  and  304  of the fiber ribbon cable, but this need not be the case. 
       FIGS.  5 B through  5 D  are cross-sectional views of examples of the fiber ribbon cable  300  of  FIG.  5 A . The fiber ribbon cable  300  includes the fiber ribbon  200  and a cover assembly  310  that surrounds the outer surface  221  of the common protective coating  210  of the fiber ribbon. In an example, one or more fiber ribbons  200  can be loosely arranged within the cover assembly  310 , i.e., the fiber ribbon cable  300  can be a loose-buffered cable. In another embodiment, the fiber ribbon cable  300  can be a tight-buffered cable. 
     In the example cover assembly  310  of the fiber ribbon cable of  FIG.  5 B , the fiber ribbon  200  is surrounded by a strength layer  314  (e.g., aramid yarn), and an outer jacket  320  that surrounds the strength layer. The example configuration of  FIG.  5 C  comprises a binder layer  312  that surrounds the fiber ribbon  200 , with the strength layer  314  surrounding the binder layer  312  and the outer jacket  320  surrounding the strength layer. 
       FIG.  5 D  shows an example where the outer jacket  320  is provided directly to the outer surface  221  of the common protective coating  210  of the fiber ribbon  200 . 
     Various other configurations for the cover assembly  310  as known in the art can also be effectively employed. For example, multiple fiber ribbons  200  can supported within the cover assembly  310 . 
     Fiber Ribbon Interconnect 
     The fiber ribbon  200  is compatible with existing fiber processing tools and termination components. In one example, a fiber ribbon interconnect is formed by terminating the first and second ends of the fiber ribbon  200  with respective optical fiber connectors (“connectors”). In another example, the fiber ribbon interconnect is formed by connectorizing the fiber ribbon at only one end. In another example, a fiber ribbon interconnect includes the cover assembly that forms a fiber ribbon cable  300 . Most of the example fiber ribbon interconnects discussed below are formed from a fiber ribbon cable  300 , but the fiber ribbon interconnect as disclosed herein need not include the cover assembly  310 . Furthermore, as noted above, the fiber ribbon interconnect disclosed herein can include only one connectorized end. 
     Connectorizing the fiber ribbon  200  with connectors to form a fiber ribbon interconnect typically requires extracting the fibers  10  from the common protective coating  210 . This process can be done mechanically, though it may be relatively difficult as compared to conventional fiber ribbons. Thus, other stripping approaches, such as chemical stripping and thermal stripping, can be used. An example chemical stripping approach includes using hot sulfuric acid. An example thermal stripping process includes the use of a hot nitrogen jet. 
       FIG.  6    is an elevated view of an example fiber ribbon interconnect  400  that includes the fiber ribbon cable  300  terminated at the first and second ends  402  and  404  by respective optical fiber connectors (“connectors”)  450 . In an example, the connectors  450  can be the same or substantially the same connector. In an example, the one or both of the connectors  450  can be high-density connectors, denoted  450 H. The term “high density” generally means a fiber density that is greater than a conventional optical fiber connector, such as an MPO connector, which typically has a fiber density (pitch) of 250 microns. In the discussion below, some connectors  450  can be standard-density connectors, and these are denoted  450 S. 
     High-Density Connector Assembly 
       FIG.  7 A  is an exploded front elevated view of an example method of forming a high-density connector assembly (“connector assembly”)  452  that can be used directly as the high-density connector  450 H or that can be used to form the high-density connector  450 H (e.g., by adding further connector components, as discussed below). 
     The connector assembly  452  includes a grooved substrate  460  having a front-end section  461  with a front end  462 , a back-end section  463  with a back end  464 , and a central axis A 2  that runs in the z-direction. The front-end section  461  has a planar top surface  472  while the back-end section  463  has a planar top surface  474  that is lower than the top surface  472  of the front section. The front-end section  461  includes an array  480  of grooves  482  formed in the planar top surface  472  and that run parallel to the central axis A 2 . In an example, the grooved substrate  460  comprises a glass or glass-based material. In an example, the grooves  482  are V-grooves, but other cross-sectional shapes for the grooves can also be effectively employed. 
       FIG.  7 A  shows the end sections  60  of the array  8  of fibers  10  of the fiber ribbon  200  extracted from the common protective coating  210 . The grooves  480  of the front-end section  461  of the V-groove support substrate  460  are sized to accommodate the end sections  60  of the fibers  10  while the back-end section  463  accommodates the fiber ribbon  200 , which in an example has been removed from the cover assembly  310  of a fiber ribbon cable  300  (e.g., a portion of the cover assembly has been stripped away). In an example, the fiber pitch P 1  at the first end  202  of the fiber ribbon  200  is 125 μm and is closely matched to the pitch PV of standard 127 μm grooves  482 . 
     Once the end sections  60  of the fibers  10  are supported in the grooves  482 , then with reference to  FIGS.  7 B and  7 C , an adhesive  490  is applied to the fiber ribbon  200  at the back-end section  463 . A cover  500  having a front end  502 , a back end  504 , a top surface  512  and a bottom surface  514  is then placed over the top of the array  480  of grooves  482  to secure the end sections  60  of the fibers  10  in the front-end section  461  of the grooved support substrate  460 . The cover  500  is held in place by the adhesive  490  contacting the back end  504  of the cover. The adhesive  490  can also be added to the grooves  482 . The cover  500  can be used to press the end sections  60  of the fibers  10  into their respective grooves  482 . In an example, the cover  500  is a thin planar sheet made of a glass or a glass-based material. 
       FIG.  7 B  shows the resulting connector assembly  452 , with the end faces  62  of the fibers  10  residing substantially at the front end  462  of the grooved substrate  460 . The connector assembly  452  supports the fibers at a pitch P 1  (wherein P 1 =PV), which can be the same as or different than the fiber pitch PR of the fiber ribbon  200 . The fiber end faces  62  are typically polished once assembly of the connector assembly  452  is completed. 
       FIG.  7 C  is y-z cross-sectional view of the connector assembly  452  as attached to the end section  201  of the fiber ribbon  200 , which is shown as incorporated into a cover assembly to form a fiber ribbon cable  300 . In this case, a portion of the cover assembly  310  is stripped away to expose the first-end section  201  of the fiber ribbon  200 . In an example, the front end  462  of the grooved substrate, the front end  502  of the cover  500  and the end face  62  of the fiber  10  define a tilt angle θ relative to a vertical plane VP to reduce reflection losses. In an example, the tilt angle θ can be up to about 8 degrees. 
       FIG.  7 D  is similar to  FIG.  7 B  and illustrates an example high-density connector  450 H formed by at least partially enclosing the connector assembly  452  within a connector housing  454  having a front end  456  and a back end  457 . In an example, the high-density connector  450  includes at least one alignment feature  458 , which is shown by way of example as alignment pins that extend from the front end  456  of the connector housing to define a plug type connector. The at least one alignment feature  458  can also comprise alignment holes sized to receive alignment pins, thereby defining a receptacle type connector. 
     The front end  462  of the grooved substrate  460  is shown residing at or proximate to the front end  456  of the connector housing  450  while the fiber ribbon  200  extends from the back end  457  of the connector housing. In another example, the connector assembly  452  can extend from the front end  456  of the connector housing  454 . 
     Fan-Out Fiber Ribbon 
       FIG.  8 A  is a top-down view of an example fiber ribbon  200  similar to that shown in  FIG.  4 A , except that the fiber pitch PR is not constant along the length of the fiber ribbon.  FIGS.  8 B and  8 C  are cross-sectional views of the fiber ribbon  200  as taken along the lines B-B and C-C respectively, and show two different fiber pitches PR 1  and PR 2  as a function of the z-position, and in particular at the front-end section  201  and the back-end section  203 . In an example, the change in the fiber pitch PR occurs over a fan-out region  250  of length LF, with the fiber pitch PR 1  in the front-end section  201  being substantially constant and the fiber pitch PR 2  in the back-end section  203  being substantially constant. The length LF is called the transition length, and the fiber ribbon  200  with the fan-out region  250  is called a fan-out fiber ribbon. In an example, the fiber pitch PR 1  at the first end  202  of the fiber ribbon  200  is in the range from 80 microns to 125 microns (which can also be the range of the diameter DF of the fiber  10 ) while the fiber pitch PR 2  at the second end  204  is greater, such as that for a standard connector, e.g., 250 microns. 
     The transition length LF of the fan-out region  250  can be designed to minimize the bending stress on the fibers  10 . As can be seen in  FIG.  8 A , the outermost fibers  10  experience the greatest amount of bending, i.e., the tightest bend radius. In an example, the shape of the fiber transition in the fan-out region  250  is designed to maximize the bend radius along the fiber&#39;s path. In one example, the fiber path is a half period of a cosine function, which is also known in the art of waveguide design as an “S-bend.” For example, for an 8-fiber fan out, the outermost fiber  10  has an x position as a function of axial position z as given by x={3.5·(PR 2 −PR 1 )/2}·cos{πz/LF}. In an example the transition length LF is about 10 mm, so that the minimum bend radius, which occurs at both ends of the transition, is at least 46 mm. The corresponding maximum tensile stress in the fiber  10  at this bend radius is about 14 kpsi, which is well within the capability of the fiber. 
     Reliability research of example fibers  10  showed a significantly increased fatigue resistance factor as compared to conventional fibers, even though the tensile strength of the fiber was on par or even slightly less than conventional fiber. As a result, while conventional fiber can one operate in the 20%˜30% of proof tested stress level, examples of the fiber  10  are fatigue resistant and can operate at about 80% of proof test stress level. 
       FIG.  9    is a top-down view of an example fiber ribbon assembly  400  that includes the fan-out fiber ribbon  200  of  FIG.  8 A  with a high-density connector  450 H operably attached to the first (narrow) end  202  of the fan-out fiber ribbon and a standard-density connector  450 S attached to the second (wide) end of the fan-out fiber ribbon. In an example, the standard-density connector  450 S comprises an MPO connector that supports the fibers  10  at a standard pitch P 2  of 250 microns. In an example, the fiber ribbon assembly  400  can include the cover assembly  310 , only a portion of which is shown in dashed-line outline in  FIG.  9    for ease of illustration. 
     Furcated Fiber Ribbon 
       FIG.  10 A  is similar to  FIG.  8 A  and illustrates an example embodiment of a furcated fiber ribbon  200  wherein the fiber ribbon is bifurcated at a furcation location  209  into two sub-sections (bifurcations)  200 A and  200 B having respective second ends  204 A and  204 B and each containing a sub-set of the total set (array) of fibers  10 . This furcated configuration for the fiber ribbon  200  is useful for example when making optical connections to transmitters and receivers that reside on separate silicon-photonics chips. In other embodiments, there can be two or more furcations ( 200 A,  200 B,  200 C, etc.), with the bifurcation embodiment shown by way of example. Also in an example the furcations need not have the same number of fibers  10  and need not have the same length. The furcation location  209  can also be selected for convenience based on the given connection application. 
       FIG.  10 B  shows cross-sectional views of the example sub-sections  200 A and  200 B of the furcated fiber ribbon  200 , wherein each sub-section includes four of the eight total fibers  10 . In the example furcated fiber ribbon  200  of  FIG.  10 B , the first end section  201  of the furcated fiber ribbon  200  is unfurcated (see also  FIG.  8 B ) and is terminated by a single connector  450  (e.g., high-density connector  450 H) while the second end section  203  now comprises two end sections  203 A and  203 B with respective ends  204 A and  204 B each terminated by a connector  450  (e.g., high-density connectors  450 H), as shown in  FIG.  10 C , to define an example fiber ribbon interconnect  400 . In an example, the furcated fiber ribbon  200  can be incorporated into a furcated cover assembly  310  to form a furcated fiber ribbon cable  300 . 
     Optical Data Transmission System 
       FIG.  11 A  is a schematic diagram of an example optical data transmission system  700 . The optical data transmission system  700  comprises a photonic device  710  having a circuit board  720  having a top surface  722  and that operably supports a photonics chip  730 , e.g., a silicon-photonics chip. The photonics chip  730  has a front end  732 . The example optical data transmission system  700  also includes a telecommunications device  800  having a connector receptacle  810 . The optical data transmission system  700  also includes an example fiber ribbon interconnect  400  with a high-density connector  450 H and a standard-density connector  450 S. 
       FIG.  11 B  is a top-down view of an example photonic device  710 .  FIG.  11 B  also shows the high-density connector  450 H at the first end  402  of the fiber ribbon interconnect  400  in position to operably engage the photonic device. The photonics chip  730  operably supports an array of optical waveguides  740 . Eight optical waveguides  740  that terminate at the front end  732  of the photonics chip  730  are shown by way of example. In an example, the optical waveguides  740  comprise channel waveguides. 
     The photonics chip  730  may be fabricated from any material capable of having optical waveguides  740  disposed thereon or therein. As non-limiting examples, the photonics chip  730  may be fabricated from a glass-based material (e.g., glass, glass-ceramic, and fused silica) or a semiconductor material (e.g., silicon). The optical waveguides  740  may be configured as any known or yet-to-be-developed optical waveguides. Non-limiting example optical waveguides  740  include thin-film deposition, photolithographic masking and etching processes, laser written waveguides, ion-exchanged waveguides, optical waveguides, among others. It will be understood that the optical waveguides  740  may be suitably configured for the operations of the photonics chip  730  and are merely schematically depicted in a straight-line configuration. 
     The optical waveguides  740  are operably connected to respective active photonic elements  750 , which in an example can comprise an optical transceiver, an optical light source (e.g., a vertical-cavity surface-emitting laser or VCSEL) or an optical detector. In an example, the photonics chip  730  can comprise a first sub-chip that includes the optical waveguides  740  and a second sub-chip that includes the active photonic elements  750 . 
     In an example, the photonics chip  730  is configured to generate and/or receive optical data signals using the active photonic elements  750  and the optical waveguides  740 . The optical waveguides  740  terminate the front end  732  of the photonics chip  730 . The front end  732  of the photonics chip  730  can include one or more alignment features  734 , which are shown by way of example as alignment holes. In an example, the front  732  and the one or more alignment features  734  define an optical connector  760 , which is shown by way of example as receptacle type of optical connector that complements the plug type of high-density connector  450 H of the fiber ribbon interconnect  400  and that allows for mating and de-mating of the photonic device  710  with the fiber ribbon interconnect to establish optical communication between the optical waveguides of the photonics chip  730  and the fibers  10  of the fiber ribbon interconnect. 
     Also shown in  FIG.  11 B  is the connector high-density connector  450  having one or more alignment features  458  that are complementary to the one or more alignment features  734  of the photonic device  710 . The example alignment features  458  are shown in the form of alignment pins sized and configured to closely engage the alignment holes  734  when the high-density connector  450  is operably engaged with the photonic device  730 , as shown in  FIG.  11 C . The photonic device  710  is shown as having additional alignment features  734  that help guide the second connector into position relative to the photonics chip  730 . The optical waveguides  740  have a pitch that matches that of the fibers  10  supported by the connector  450  so that when the high-density connector  450  is operably engaged with the photonic device  730 , the fibers  10  are in optical communication with respective optical waveguides  740 . The optical waveguides  740  have a high waveguide density, i.e., greater than that associated with standard connectors used in standard optical fiber cables. 
     At the other end of the ribbon interconnect  200 , the standard-density connector  450 S is operably engaged with the connector receptacle  610  of the telecommunications device  600 . The telecommunications device  600  can be a wide variety of standard telecommunication devices known in the art, such as a server, a fiber optic cable, an electronics panel in a data center, etc. The standard-density connector  450 S has the aforementioned standard fiber density associated with industry standard telecommunication systems and devices. 
     With reference to  FIG.  11 D , in the example where the telecommunications device  800  is a fiber optic cable  840  terminated by a standard-density connector  450 S, the connector receptacle  810  can be defined by a connector adapter  880  having two connector receptacles and used to operably connect optical fiber cables as is known in the art. Thus, in an example, the ribbon interconnect  400  disclosed herein can be used to optically connect a photonic device  710  having a high waveguide density to a remote telecommunications device  800  having a standard fiber density. 
     In another example of the optical data transmission system  700  illustrated in  FIG.  11 E , the ribbon interconnect  400  can be used to establish optical data communication between one photonic device  710  and another photonic device. 
       FIG.  11 F  is similar to  FIG.  11 E  and shows an example of an optical data transmission system  700  wherein the fiber ribbon interconnect  500  utilizes a bifurcated fiber ribbon  200  to provide high-density optical interconnections between one photonic device  710  at the first end  402  of the fiber ribbon interconnect  400  and two photonic devices  710  at the second ends  404  of the fiber ribbon interconnect. 
     It will be apparent to those skilled in the art that various modifications to the preferred embodiments of the disclosure as described herein can be made without departing from the spirit or scope of the disclosure as defined in the appended claims. Thus, the disclosure covers the modifications and variations provided they come within the scope of the appended claims and the equivalents thereto.