Abstract:
Described are optical fiber distribution cables that simplify the installation process and significantly reduce the number of field splices. The distribution cables contain optical splitters within the cable structure itself, and the drop cables are also housed within the distribution cable. The optical splitters are preferably bi-directional to facilitate placement of the optical splitters inside the distribution cable.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of provisional application 61/537,745 filed Aug. 22, 2011, which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to optical fiber cables for local distribution of optical signals in optical networks. They are specially adapted for drop line installations in a Passive Optical Network (PON). 
       BACKGROUND OF THE INVENTION 
       [0003]    (Parts of this background may or may not constitute prior art.) Fiber-to-the-premises (FTTP) from local telephone and cable service providers is rapidly being implemented. This service requires a broadband optical fiber distribution network comprising local optical fiber distribution cables that are installed in neighborhoods and city streets. These are commonly referred to as Passive Optical Networks (PONs). The basic architecture is point-to-multipoint. The local distribution cable is a large fiber count (multi-fiber) cable. Single fiber or few fiber cables are used for the “drop” line from the street to the premises. In many cases, aerial drop lines are used, and these may have special requirements. In other cases, buried drop lines are used, and these may have different requirements. 
         [0004]    A key to a PON is some form of effective optical splitter. The optical signal from the cable service provider and/or telephone service provider is routed into a local neighborhood over a distribution cable. At a point in the PON, typically in or near the neighborhood to be serviced, the optical signal in the fiber cable is split using a 1×N optical splitter. The output of the optical splitter is a group of N optical fibers, each with the identical optical signal as the main feeder fiber. Each of the N optical fibers is intended to be optically connected to a given subscriber. The optical splitters are typically housed in a splitter box located in the neighborhood being served. 
         [0005]    As will be described in more detail below, conventional PONs contain multiple feeder and distribution fibers, and serve many subscribers in multiple neighborhoods. The fibers and cables that serve as the input to the splitter are typically known as feeder fibers and cables. The output fibers and cables from the splitter to a final drop closure are known as distribution fibers and cables. Dozens of fibers for either the feeder or distribution function are housed in one cable. Finally, the fibers and cables that connect the final drop closure to the home are known as drop fibers and cables. Since only one fiber is most often needed to provide service to the home, drop cables are typically lower fiber count cables. 
         [0006]    A typical installation procedure for a PON is to route the feeder cable to the neighborhood to be served. The cable is opened and one or more feeder fibers are removed from the cable and suitably connected to the neighborhood splitter box. If for example the splitter is a 1×32 optical splitter, thirty three splices are made at the splitter location. If more than one splitter is housed at the splitter location, 33 splices are made for each additional splitter. The thirty two distribution optical fibers at the output of the splitter are routed along the network right-of-way and ultimately field spliced or connected to drop cables in drop closures or drop terminals, which are then routed to thirty two individual subscriber locations. Each typical drop closure feeds an average roughly 5-6 subscribers and sometimes as few as 1-2 subscribers per closure. The procedure is repeated for each neighborhood served. 
         [0007]    Existing methods for making the drop to the home include field splicing and using pre-connectorized hardened or non-hardened connectors. Field splicing, while very reliable and relatively inexpensive, is more time consuming and requires a drop closure. Using non-hardened pre-connectorized cables also requires a drop closure to house them, which adds material cost, labor cost, light loss (attenuation) due to the mechanical nature of the connection, and complexity to the network. Hardened connectors also require a closure and a terminal that houses connector adapters. This scenario adds significantly more cost to the network, adds more light loss to the network, and potentially reduces reliability. 
         [0008]    More efficient PON distribution systems, in terms of both improved design and simpler installation, would be an important advance in the technology. 
       STATEMENT OF THE INVENTION 
       [0009]    We have designed an optical fiber distribution cable that simplifies the installation process, eliminates the last drop splice or connector closure, and significantly reduces the number of field splices. This is enabled by a cable structure wherein the optical splitters are contained within the cable itself, and the feeder and distribution fibers, and drop cables are also housed within the distribution cable. The optical splitters are preferably bi-directional to facilitate placement of the optical splitters inside the distribution cable. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0010]    The invention may be more clearly described with the aid of the drawing, in which: 
           [0011]      FIG. 1  is a schematic view of a typical PON from the main optical signal source and distribution and feeder cable to the subscriber locations; 
           [0012]      FIG. 2  is schematic representation of multiple main distribution fibers in a distribution cable showing the locations of optical fiber splitters according to one aspect of the invention; 
           [0013]      FIG. 3  is a more detailed representation of the distribution cable of  FIG. 1  showing the organization of the distribution fibers, the optical splitters, and the drop cables; 
           [0014]      FIGS. 4 and 5  are two embodiments of a distribution cable in cross-section; 
           [0015]      FIG. 6  shows a bi-directional optical splitter useful in the implementation of the invention; 
           [0016]      FIG. 7  shows details of a tandem optical splitter; and 
           [0017]      FIG. 8  shows details of one embodiment of a bi-directional optical splitter that uses serially arranged conventional unidirectional optical splitters. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    It should be pointed out at the outset that any number of subscribers may be served by a given PON and its associated fiber optic cable. The distribution cable may have any number of main feeder fibers, with each feeder fiber connected as the input of the optical splitter. Conventional splitters come in split ratios from 1×2 to 1×16, 1×32, 1×64 or beyond, or any number of multiple signals to serve a local area. A distribution cable with 16 feeder fibers, each connected to a 1×32 optical splitter, may comprise a PON serving 512 subscribers. However, for simplicity, the following describes a PON with 4 feeder fibers connected to a 1×4 splitter and the OLT, and four 1×8 optical splitters. This PON will serve up to 32 subscribers. Larger networks are easily designed by extension from the network described. 
         [0019]      FIG. 1  is a schematic diagram of a portion of this PON where  11  represents the remote signal source, referred to below as the Optical Line Terminal (OLT) of the PON, and  12  is one of the four main feeder fibers. The other three feeder fibers are shown at  13 ,  14 , and  15 . For simplicity, the associated optical splitters for these distribution fibers are not shown. The feeder fiber  12  is connected to a 1×8 optical splitter  17 . The eight outputs from the optical splitter are routed to the subscribers  16  by distribution cables  18  and drop cables  21 . A drop closure or terminal  20  is used to either splice or connect the drop cables to the distribution cables. 
         [0020]    The combination of optical splitters  17  with the input feeder fiber  12  and the distribution fibers  18 , serve as an access/distribution point for the PON, and, in conventional PONs, are housed in a street cabinet, or other suitable closure. The contents of the street cabinet are indicated by dashed box  19 . This facility often serves both as a signal splitter and as a patch panel, where the split optical signals are patched to distribution cables  18  which ultimately lead to the individual drop cables  21 . 
         [0021]    The portion  19  of the PON is the focus of an important aspect of this invention. 
         [0022]    Street cabinets are metal or plastic enclosures placed above ground near the subscribers  15 . They intrude on the landscape and are constantly being accessed by installation/repair crews. The small PON just described requires up to seventeen field splices or connections. Cabinets for larger PONs may contain hundreds of field connections. As mentioned earlier, field connections are known to be weak links in a PON. 
         [0023]    According to a feature of this invention, street cabinets, along with many of the associated field boxes and closures, are essentially eliminated. The contents of the access/distribution cabinet  19  are contained within a newly designed distribution cable. The optical splitter  17  is housed permanently within the cable structure and the distribution and drop cables  18 / 21  are housed initially within the cable structure and become the same item, eliminating the final drop closure or terminal  20 . This will be recognized as a major advance in PON technology. A portion of each of the drop cables will be removed from the distribution cable during installation. This is described in more detail below. 
         [0024]      FIG. 2  shows schematically the contents of the distribution cable according to a main feature of the invention. The distribution fibers, four in this embodiment, are shown at  12 - 15 . The optical splitters  17  and the drop cables  18  are shown spaced along the distribution cable length and are housed within the distribution cable. The space “d” represents a distance between clusters of eight (or fewer) subscribers, and a corresponding distance between access/distribution points for the PON. As should be evident, distance d may vary considerably and may be dictated by spacing between customer houses. 
         [0025]    An important aspect of some embodiments of the invention is the inclusion of drop cables, as contrasted with drop fibers, within the cable structure. 
         [0026]    Optical fiber drop cables may be made in several designs. Many of these designs mimic earlier copper cable versions. For example, “A-drop” optical fiber cable is an optical fiber version of A-drop copper cable, and is made in the same flat or ribbon-like configuration. More recently, round drop cables have become widely used, and, for reasons to become apparent, these are preferred for implementing the invention. However, the invention may be implemented with flat or ribbon cables. 
         [0027]    A drop cable is defined as a cable suitable for transmitting an optical signal from the distribution fiber to a subscriber&#39;s premises. It comprises one or more optical fibers within a cable jacket. Optical fibers comprise a core and a cladding with at least one polymer coating. The core and cladding may be plastic, but are more typically glass. Optical fibers are normally too fragile to be used alone as a drop between the distribution cable and the subscriber&#39;s terminal. Accordingly, at least one protective coating for the fiber(s) is used. This is referred to here as a drop cable encasement or jacket. Thus a drop cable is defined as at least one optical fiber covered with a drop cable jacket. The drop cable may have additional protective layers including armor, and may have one or more strength layers or strength members. It may or may not be gel-filled. It may include metallic or other components to facilitate underground traceability. For indoor installations it may comprise fire-resistant materials. A wide variety of drop cable designs may be used in the practice of the invention. 
         [0028]    A particularly suitable drop cable comprises an optical fiber encased in a tight-buffered polymer encasement. This optical fiber cable is typically 900 microns in diameter to meet standard coupling and splicing equipment and techniques. Other sizes may be used, e.g. 600 microns. The tight-buffer material is preferably a stiff, robust dual-layer nylon/ethylene-acrylic acid copolymer. Details of this encasement layer are given in U.S. Pat. No. 5,684,910, incorporated herein by reference. The encasement material can be any suitable plastic material, including PVC, thermoplastic elastomers such as DuPont&#39;s “Hytrel” materials, fluoropolymers, nylon, poly(butylene terephtalate), or UV-cured acrylate resins. The encasement is tightly fitting to the optical fiber polymer coating. 
         [0029]    The term “encasement” as used above is defined as the primary medium that surrounds the optical fibers and may be considered equivalent to the drop cable “jacket”. While drop cables with encased designs are preferred, the invention may also be implemented with loose fiber cable designs. 
         [0030]    The tight-buffered optical fiber may be wrapped with a strength layer of aramid yarns. Teijin Twaron BV&#39;s Twaron Type 1055 waterswellable high modulus material is suitable. The yarn may be coated with a waterswellable coating. 
         [0031]      FIG. 3  shows in more detail how the drop cables are used for implementing the invention. For clarity, the contents of the distribution optical fiber cable are shown without the cable sheath. The bold arrow in  FIG. 3  indicates the direction of the PON from the head end to the subscribers. The bold arrow indicates a downstream direction “d” and an upstream direction “u”. 
         [0032]    The four feeder fibers are shown as  12 ,  13 ,  14 , and  15 , as in  FIGS. 1 and 2 . This figure shows a portion of the PON interconnecting the feeder fibers  12  and  13  with  16  subscriber locations (not shown). It will be understood that feeder fibers  14  and  15  are likewise interconnected downsteam with groups of 8 subscriber locations respectively.  FIG. 3  shows optical splitters  17  associated with main feeder fibers  12  and  13  as the input to these optical splitters, and eight optical fiber drop cables as the output of each of the optical splitters. The system is designed such that the splitter is placed in the middle of its service area. Four of the optical fiber drop cables,  32   d,  extend downstream of the optical splitter  17 , and four optical fiber drop cables,  32   u,  extend upstream from the optical splitter  17 . 
         [0033]    Each of the drop cables being used in the PON will be removed from the distribution cable by snaking the drop cable from the cable sheath through a cable access hole  34 . The sections of the drop cables that are removed from the distribution cable are indicated as  31   d  and  31   u.  The sections of the cable that remain with the cable after installation are indicated as  32   d  and  32   u.  The original section of each drop cable, prior to installation, is indicated as  33   d  and  33   u.  Sections  33   d  and  33   u  represent the positions of the drop cable sections after the sections  31 d and  32   d  are snaked from the cable sheath through the access hole  34 . Thus cable lengths  33   d,    33   u  and  31   d,    31   u  show sections of the drop cables  32   d  and  32   u  before ( 33 ) and after ( 31 ) installation. To allow the drop cable sections to be removed from the distribution cable in the manner just described each drop cable must be cut at a suitable location along the distribution cable. These locations are shown in  FIG. 3  as severance points  35 . These cuts could be made either as the cable is being made, or as the cable is being deployed in the field during construction of the network, but before installation of the service to the customer. 
         [0034]    It should be evident that the figures are not to scale. The optical splitters  17  may be a few centimeters in length, while the feeder and drop cables  32   u  and  32   d  may be tens, hundreds, or thousands of meters along the interior of the cable. 
         [0035]    On inspection of  FIG. 3 , two conclusions may be drawn. One, the optimum location for the access/distribution points (i.e., the splitter/splice location) would be at or near the center of the cluster of eight subscriber locations. Two, the placement of the severance points  35  may advantageously take into account the length of the drop cable needed for a given subscriber connection. 
         [0036]    From  FIG. 3  it is evident that, after installation, a continuous length of each drop cable preferably extends from the optical splitter  17  to the Optical Node Terminal (ONT) location at each subscriber&#39;s premises. That eliminates the need for problematical field connections or splices at the cable access holes  34  between the distribution cable and the conventional drop to the subscriber location. 
         [0037]    It is also important to recognize that most of the fiber splices between the head end of the PON and the subscriber location are physically located within the distribution cable. That means that, not only are the splice locations protected from potentially hostile environments, but the splices may be factory installed. However, another embodiment of the invention could entail the cable trunk and drop cable structure without the splitter spliced into it, to enable the customer to splice in the splitter at an appropriate location. 
         [0038]    An advantage to distribution cable designed according to the invention is that the main part of the PON can be custom engineered for given clusters of subscribers. The installation of the PON is thereby greatly simplified, resulting in very substantial savings in installation cost. 
         [0039]    The feeder fibers in  FIG. 3  are shown as extending downstream of the associated access/distribution point. However, the fiber  12  is shown as a dashed line in  FIG. 3  indicating it is not connected past the access/distribution point. It may be retained in the cable structure as a dummy fiber, either to fill the cable or for use as a spare fiber downstream. If the cable is factory engineered and manufactured, it may be omitted from the cable structure downstream of where it is connected to its associated optical splitter. These feeder fibers may be contained in a conventional buffer tube, and in most embodiments of the invention will not be encased in a cable structure similar to the drop cables, in order to minimize the overall size of the composite cable. 
         [0040]    It will be recognized that the distribution cable is preferably designed so that the drop cables can be snaked easily from the overall cable structure. A variety of expedients for facilitating this will occur to those skilled in the art. For example, the drop cable structure may mimic cables designed for duct installations by using friction-reducing materials, where friction between cables is minimized to allow cables to be pulled through ducts. In addition, duct cable installation techniques may be used in connection with the installation of PONs according to this invention. 
         [0041]    An expedient that may be useful in the installation phase is to install shorter drops before longer drops. This can be used when the drop cable lengths are factory designed and custom manufactured. Shorter lengths of drop cable will normally be easier to pull than longer drops. When a short drop cable is removed from the distribution cable it leaves added space to facilitate removal of the longer drops. 
         [0042]    Pre-engineering distribution cables also allows cable access openings ( 34  in  FIG. 3 ) to be installed in the factory. In some applications, a pre-engineered distribution cable may contain a combination of drop cables, jacketed as just described, along with drop fibers that are connected in the usual way, i.e., spliced to conventional drop cables on exit from the distribution fiber cable. 
         [0043]    Similarly, it is within the scope of this invention to provided drop cable stubs contained within the distribution cable. In this case one or more stubs may be shorter than the overall required drop cable length. With reference to  FIG. 3 , one or more of the drop cables  31  may not complete the entire drop length to the subscriber, or even a substantial part of the drop length, before being spliced to another drop cable length. 
         [0044]    A section  4 - 4  of the distribution cable of  FIG. 3  is shown in  FIG. 4  with outer cable sheath indicated by  41 . An optional buffer tube  42  contains the feeder fibers (corresponding to fibers  12 - 15  in  FIG. 3 ). The drop cables  32  are shown randomly bundled within the cable. 
         [0045]      FIG. 5  shows the section  5 - 5  in  FIG. 3 . Here the buffer tube is omitted and the distribution fibers  13 ,  14 , and  15 , as well as the drop cables  32 , are bundled within cable  51 . (Distribution fiber  12  has been dead-ended at this point along the cable length). It should be noted that in both  FIG. 4  and  FIG. 5  the drop cables are represented as optical fibers within a cable jacket as described earlier. The distribution fibers may or may not be jacketed, but are shown as unjacketed. 
         [0046]    The optical splitters  17  ( FIG. 1-3 ) may have a variety of constructions. Conventional PONs use fused bi-conic splitters or PLC (Planar Lightwave Circuit) splitters. However, for placement within the cable structure, according to a main feature of this invention, it is necessary that the splitters be small enough to fit within the cable, preferably resulting in a minimum or no bulge to the outside cable sheath that may otherwise hinder duct or similar constricted space installations. The splitters may or may not be housed in an appropriate housing to facilitate appropriate fiber routing. Optical splitters with 1×8 functionality, and even 1×32 functionality, are available with a width of 10 mm or less. The length of the PLC splitter is of less importance than the width, since the cable diameter is the limiting parameter. 
         [0047]    According to a preferred embodiment of the invention a bi-directional splitter is used. Since the drop cables in the distribution cable design of  FIG. 3  extend both downstream from the optical splitter, as well as upstream, it should be evident from inspection that a bi-directional optical splitter will implement this design without the need for severe bends in the optical fibers. An example of a bi-directional optical splitter is shown in  FIG. 6 , where bi-directional optical splitter  61  is shown contained within distribution fiber cable  62 . The bi-directional optical splitter  61  has a dual design. Part of the optical splitter is a PLC, and part is a MEMS. The dual design is used for convenience to illustrate two forms of optical splitters that may be combined to produce a bi-directional splitter. In the optical splitter  61  a distribution fiber is input to the device as shown. The downstream direction is indicated by the bold arrow below cable  62 . The PLC splitter section splits the signal in the distribution fiber into eight outputs. Four of these,  63 ,  64 ,  65 , and  66 , extend in the downstream direction. The signal is split as shown so that a fifth output is input into the MEMS splitter section. As is well known, a MEMS splitter is capable of re-directing an optical beam through 180 degrees as shown, producing four outputs  71 ,  72 ,  73  and  74  on the upstream direction. 
         [0048]    As just described, the bi-directional optical splitter  61  is shown with two sections, a PLC splitter section and a MEMS section. It will be apparent to those skilled in the art that a similar bi-directional splitter may be implemented in an all MEMS device. It should also be evident that, while the optical splitter  61  is shown with both the PLC section and the MEMS section on a common device substrate or support, the bi-directional splitter may be just as easily produced in two separate devices with e.g., one downstream of the other. In that manner the splitter may have a smaller overall width dimension. For example, a small inexpensive 1×2 splitter may be connected to a given distribution fiber, with the two outputs made inputs, respectively, to a splitter providing downstream outputs and a splitter providing upstream outputs. Alternatively, a PLC downstream splitter may be designed with an extra output waveguide routed through a 180 degree turn to be connected to the upstream splitter. Or, the PLC splitter will have outputs in 2 directions, upstream and downstream, simplifying the installation and eliminating the need for fiber bends within the splitter structure. 
         [0049]    Likewise, for a very large PON the optical splitter may be separated into multiple devices at the same access/distribution point to accommodate a large number of splits. In this manner, the issue of the size (width) of the splitter fitting into a given cable diameter may be surmounted by recognizing that the limiting dimension is the width not the length. So one may use, for example, a 1×9 splitter and route the 9th output to another 1×8 splitter just downstream to produce a 1×16 splitter with half the width of a conventional 1×16 splitter. This device is referred to here as a tandem splitter and is illustrated in  FIG. 7 , where the distribution cable  76  is shown with distribution fiber  77 . For clarity, the other distribution fibers and the drop cables are not shown in the figure. The tandem splitter has two sections, arranged in tandem as shown, with 1×9 splitter  78  producing 9 outputs as shown, and one output  79  connected as the input to the second tandem splitter  81 . The device produces the 16 outputs shown to the right of the figure, and has a width of half of a conventional 1×16 splitter. It will be evident that any number of splitter combinations may be used to implement the tandem splitter concept. Also, more than two splitters may comprise the tandem sequence. Nominally, a 1×N tandem splitter with x splitters comprises: (x−1) tandem splitter sections each described by 1×(N/x)+1, connected to a 1×N/x section as the last section in the tandem. However, it should be evident that N need not be the same in each section. In that case, where N=N′+N″, and x=2, the 1×N tandem splitter comprises a 1×N′+1 section, connected to a 1×N″ section. A variety of equivalent arrangements may occur to those skilled in the art. 
         [0050]    A simpler, and possibly more cost-effective, bi-directional splitter may be produced by folding one of the distribution optical fibers through a 180 degree arc to attach to a PLC optical splitter oriented with the outputs facing upstream in the optical fiber distribution cable. This is illustrated in  FIG. 8  where a distribution optical fiber  84  in optical fiber distribution cable  83  is wound around an optional element  85  that serves as a mandrel to smoothly redirect the distribution fiber through a 180 degree arc. The redirected distribution fiber is and connected to a 1×4 PLC splitter  87  upstream from the mandrel. The element  85  that serves a mandrel function is preferably small, e.g., a disk or ring. In a preferred embodiment it comprises a rigid overlay sleeve. The mandrel element ensures a smooth bend in the optical fiber and, if the element is approximately the diameter of the cable, ensures the maximum allowed bend diameter. The output fibers  88  may correspond to output fibers  71 - 74  in  FIG. 6 . 
         [0051]      FIG. 8  shows an arrangement for redirecting an upstream distribution fiber to four downstream optical fibers, and represents a general technique for producing upstream outputs. The downstream outputs are not shown in this figure. Corresponding downstream outputs may be produced by splitting the distribution fiber into a downstream fiber and an upstream fiber. For example, a small inexpensive 1×2 splitter may be connected to a given distribution fiber, with the two outputs made inputs, respectively, to a conventional PLC splitter providing downstream outputs and a splitter, like splitter  87  in  FIG. 8 , providing upstream outputs. 
         [0052]    The drop cables in  FIGS. 4 and 5  are shown as containing two optical fibers. However, the drop cables may contain a single optical fiber, or more than two optical fibers. For FTTH applications, and small business installations, drop cables with 1-3 optical fibers will normally be used. 
         [0053]    Various other modifications of this invention will occur to those skilled in the art. All deviations from the specific teachings of this specification that basically rely on the principles and their equivalents through which the art has been advanced are properly considered within the scope of the invention as described and claimed.