Abstract:
An optical star assembly having a ribbon fiber optic mixing element with the first and second ends. A first and second bundle of fiber optic cables are provided each with an engagement end for connection with a respective end of the ribbon fiber optic mixing element. The ribbon fiber optic mixing element and a portion of the fiber optic cables are carried in the housing so that the housing biases each end of the ribbon fiber optic mixing element towards an engagement end of the first and second bundles of fiber optic cables respectively. The fiber optic cables may be connected to a plurality of optical receivers so that the ribbon fiber optic mixing element distributes optical information from each of the fibers to all of the optical receivers in the assembly.

Description:
The benefit of a provisional application U.S. Ser. No. 60/066,127, filed Nov. 12, 1997, entitled “Passive Optical Star” is hereby claimed. 
    
    
     TECHNICAL FIELD 
     This invention relates to fiber optic couplers, and more particularly to a passive fiber optic star assembly. 
     BACKGROUND OF THE INVENTION 
     A passive fiber optic star is a device used to distribute the optical information from one fiber optic source to several fiber optic receivers simultaneously, without an external source of power. The heart of the fiber optic star is its mixing element, a device by which the optical signal coming in from any of several input fibers is distributed more or less evenly among the output fibers. Characteristic features of mixing elements include a number of input and output ports, connection method, uniformity, insertion loss and excess loss. Insertion loss of the amount of attenuation experience between an input and output port. Excess loss of the amount of attenuation of the input signal before reaching the output ports. Such a system as described in U.S. Pat. No. 5,367,595 issued to Jennings et al on Nov. 22, 1994. 
     U.S. Pat. No. 5,402,512 issued to Jennings et al on Mar. 28, 1995 describes a seven fiber optic line star connection with a retainer that forms a mixing element into a predefined shape. The retainer constraints the mixing element on all sides. The mixing element in one embodiment is made from polymethylmethaacyalate, a material which shrinks approximate two percent when heated and cooled repeatedly. Because the mixing element is constrained on all sides, shrinkage occurs at the ends of the mixing element causing a gap to appear between the linear arrayed fibers and the mixing element. This gap greatly increases the optical insertion loss of the star. 
     This fiber optic coupling system includes an individual spring and terminal for each fiber plug into the star. As each fiber is plugged into the star, it is retained by a plastic lock. Because the space required for the “push, click, tug” locking mechanism of the system, the fibers must be spaced on 5 mm center lines. The incoming fibers are transitioned in each of three dimensions or directions down into a linear array. In order to minimize light loss, the fiber should not be bent on a radius smaller than 25 mm. Because of the large spacing between the fibers, each fiber must be transitioned many mm in each direction in order to lineup in the linear array without substantially reducing light loss. Accordingly, under this type of system configuration, the transition occurs over a length of 50 mm. 
     Further, under this fiber optical coupling system a complex assembly of parts is utilized to create channels which guide individual fibers into their appropriate positions in the linear array. The channel was created by assembling a convergent piece with a stop. A combination of two parts creates the channel which guides each fiber into position. The channels are not significantly tight however. The fibers when heated, lose some of their column strength, and tend to relax resulting in extra space in the channels. As a result, the ends of the fibers in the array tend to back away from the mixing element creating a gap and increasing the optical loss. Thus, a solution to the drawbacks of this type of system is needed. 
     SUMMARY OF THE INVENTION 
     The invention generally includes an optical star assembly having a ribbon fiber optic mixing element with first and second ends. A first and second bundle of fiber optic cables are provided each with an engagement end for connection with a respective end of the ribbon fiber optic mixing element. The ribbon fiber optic mixing element and a portion of the fiber optic cables are carried in the housing so that the housing biases the ribbon fiber optic mixing element towards an engagement end of the first and second bundles of fiber optic cables respectively. The fiber optic cables may be connected to a plurality of optical receivers so that the ribbon fiber optic mixing element distributes optical information from each of the fibers to all of the optical receivers in the assembly. 
     In one embodiment of the invention, first and second ribbon holders are secured to respective ends of the ribbon fiber optic mixing element and a bridge extends between the ribbon holders. A stop with a sloped surface is provided on one the of the upper or lower housing halves. The stop is positioned so that when the upper and lower halves are connected together, the sloped surface of the stop engages the bridge to bias the ribbon holder ends, and thus the ends of the ribbon fiber optic mixing element, towards the first and second bundles of fiber optic cables respectively. 
     In another embodiment of the invention, a bundle of fiber optic cables is provided including a first row of cables aligned in a first plane and overlying a second row of cables aligned in a second plane. A star ferrule is provided including a plurality of channels formed therein each for receiving a fiber optic cable and constructed and arranged so that the fiber optic cable is transitioned in the X, Y and Z directions so that two fibers from the first row are transitioned into a linear array and are spaced apart a distance sufficient to receive a third fiber therebetween from the second row. Accordingly, all the fibers in the first and second row are transitioned into a single linear array engaging the engagement end of the ribbon fiber optic mixing element. More particularly, two fibers from one row are transitioned a distance just sufficient to receive another fiber from the other row therebetween and preferably so that the cables are positioned on 2.2 mm center lines. 
     These and other objects, features and advantages of the present invention will become apparent from the following brief description of the drawings, detailed description, and appended claims and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a passive optical star assembly including a passive star connected to a plurality of modular connections received in headers according to the present invention; 
     FIG. 2 is an exploded view of the optical star subassembly shown in FIG. 1; 
     FIG. 3 is a perspective view of a lower housing subassembly of the optical star assembly according to the present invention; 
     FIG. 4 is a perspective, partially exploded view of the ribbon fiber optic mixing element, ribbon holder, fiber optic bundles and star ferrule combination according to the present invention; 
     FIG. 5 illustrates a fiber optic bundle and star ferrule combination with a cable retention clip according to the present invention; 
     FIG. 6 illustrates a fiber optic bundle and first and second halves of the star ferrule according to the present invention; 
     FIG. 7 illustrates the assembly of two fiber optic bundles and the ribbon fiber optic mixing element in the lower housing subassembly according to the present invention; 
     FIG. 8 illustrates the engagement of a stop on the upper housing with the bridge connecting the ribbon holders of the optical star assembly according to the present invention; and 
     FIG. 9 illustrates a ribbon holder having a V-shaped grooved end face according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates a passive optical star system including a passive star subassembly  10  connected to a plurality of modular connections  12  received in headers  14 . Each modular connection  12  includes an input optical fiber line  16  and output optical fiber line  18  connected to the passive star so that each header  14  receive signals from all the other headers. 
     According to the present invention, a passive optical star as shown in FIG. 2 includes a housing lid  20  and a lower housing  22  (i.e., upper and lower half portions) for receiving a flexible ribbon fiber optic mixing element  24 . The ribbon mixing element  24  has an element ribbon holder  26  and of V-grooved end face (described in greater detail hereafter) attached at each end of the ribbon mixing element  24 . A plurality of fiber optic lines  30  are each received in a star ferrule  28  and positioned for engagement and communication with the ribbon fiber optic mixing element  24 . According to the present invention, two star ferrules  28  are used, one for each of the fiber optic cable bundles. One of the fiber optic cable bundles is for input to the ribbon mixing element  24  and one bundle is for output from the ribbon mixing element  24 . Preferably, the lower housing  22  includes upwardly extending walls  61  that define a first set of connection bays  52  each for receiving a portion of the star ferrule. Preferably guide rails  54  extend upwardly from the floor  56  of a lower housing  22  inside the construction bay walls  61  and are constructed and arranged to receive wings  58  that extend outwardly from the star ferrule  28 . Preferably the lower housing  22  also a second group of upwardly extending walls  63  that define a second set of connection bays  60  each for receiving a portion of a ribbon holder  26  and a front portion of a star ferrule  28  between alignment walls  62  defining the bays  60 . Portions (described in greater detail hereafter) of the ribbon holder  26  and the star ferrule  28  together form a guide arm  60  for that is received between the guide walls  62  defining a portion of the second set of connection bays  60 . 
     FIG. 3 illustrates a ribbon mixing element  24  received in the lower housing  22 . The ribbon mixing element  24  is positioned in the lower housing  22  in a generally U-shaped configuration. A bridge  34  extends between the ribbon holder elements  26 . Preferably, the bridge  34  and the ribbon holder elements  26  are a single piece and is made from a material such as polycarbonate or the like that includes a sufficient spring character to bias the ribbon holder elements  26 . Thus, the ends of the ribbon fiber optic mixing element  24 , are biased against the ends of the fiber optic lines held in the star ferrule  28  so that no signal is lost. 
     FIG. 4 illustrates a star ferrule  28  which includes a first half  28  A and a second half  28  B and may also include an insulation displacement clip  40  for holding the fiber optic cable  30  in position in the star ferrule  28 . The insulation displacement clip  40  is held in position by ribs  70  extending from walls of the first half  28  A of the star ferrule, as also shown in FIG.  5 . 
     As illustrated in FIG. 6, the fiber optic cables are arranged in lower  30  A and upper  30  B groupings, that is, in two overlying planar rows. The upper and lower star ferrule halves  28  A,  28  B have cable channels  42  defined therein that are constructed and arranged to transition each fiber optic cable of the respective groupings  30  A and  30  B into a single linear array for communication with the flat ribbon fiber optic mixing element  24 . The channels  42  each are constructed and arranged to transition one of the cables from the plane of the fiber optic bundles  30  A or  30  B in the X, Y and Z direction so that two of the fiber optic cables from one of the groupings are spaced apart a distance sufficient to receive a fiber optic cable from the other groupings. That is, two fibers from the first row are transitioned into a linear array and are spaced apart just enough to receive a third fiber from the second row therebetween. As such, fiber optic cables from the upper and lower planar groupings  30  A and  30  B are transitioned into a single plane and exit the star ferrule  28  through an opening  44  in an end face  45  of the star ferrule. 
     As shown in FIG. 7, one of the star ferrules  28  A includes alignment projections  46  extending from the end face  45  and are positioned so that each is received in a respective slot  50  defined in the front face of the ribbon holder  26 . As indicated earlier, the end face  36  of the ribbon holder  26  with the slots  50  is connected to the end face  45  of the star ferrule  28  so that the projections  46  are received in the slot and together these portions of the ribbon holder  26  and star ferrule  28  defined an outwardly extending alignment arm  64  (see FIGS. of  2 ,  3 ,  4 , and  9 ) that is received in a connection bay  60  and held in position by alignment walls  62  of a lower housing  22 . 
     FIG. 8 is another view of the present invention showing a stop  32  on the housing cover  20 . The stop  32  includes a sloped surface  33  that coverages on a ledge  35 . As the cover  20  is connected to the lower housing  22 , the bridge  34  (extending between the ribbon holder elements  26 ) engages the sloped surface  33  and rides along the sloped surface  33  until the bridge lands on the ledge  35 . As the bridge  34  rides down the sloped surface  33 , the ribbon holder elements  26  and the ends of the ribbon fiber optic mixing element  24  are biased towards ends of the fiber optic cables  30  in each of the star ferrules  28 . As such, the stop is responsible for maintaining z-axis control of the ribbon mixing element  24 . 
     FIG. 9 shows the ribbon holder  26  with a ribbon element passage  39  therethrough and a V-groove  37  formed in the end face  36  immediately adjacent the ribbon element passage  39 . The ribbon fiber optic mixing element  24  is extended through the passage in the end face and the end of the ribbon element  24  is melted so that the melted material flows into the V-shaped groove in the end face  36 . After the material cools, a mechanical connection is formed to hold the ribbon mixing element  24  firmly to the ribbon holder  26 . The melting process also provides a smooth uniform surface, at the end of the ribbon, to interface with a fiber optic cables of the star ferrule thereby reducing light loss. 
     As will be appreciated from the foregoing, the present invention overcomes many of the disadvantages of the prior art in that the present invention stacks fiber optic lines immediately next to one another in the same plane and so that the fibers sit on approximately 2.2 mm center lines. Because the fiber optic lines are tightly grouped into overlying planar rows by the star ferrule, the fibers do not have to be transitioned as far in the X, Y and Z direction in order to form a linear array for communication with the flat ribbon fiber optic mixing element  24 . The reduced transition distance means that all the fibers can be stripped to the same length, and no spring is necessary to take up the differences in path length between different fiber optic cables. A 12 way star ferrule assembly according to the present provides distinct channels to guide fibers into their appropriate positions in the linear array with sufficiently fewer and/or smaller pieces than that required by prior art structures. This also reduces the packaging sides of the star ferrule assembly. 
     According to the present invention a 12 way star ferrule is designed so that fibers will protrude from the end face about 0.5 mm. When mated with the ribbon fiber optic mixing element  24 , the fibers  30  are forced back flush with a star ferrule end face surface  45 . Instead of individually spring loading each fiber optic cable as in the prior art, the present invention uses a single spring element which in this case is be ribbon retainer to assure flush contact with the fiber optic mixing element  24  and the ends of the cable in the star ferrules  28 . When the star housing cover  20  is attached, the stop  32  pushes against the bridge  34  of the ribbon retainer subassembly forcing it forward. This design relies upon the spring characteristic of the plastic ribbon retainer and bridge combination to provide the normal force. The alignment projections  46 /slot  50  feature of the ribbon holder and star ferrule reduce the tolerance stack to provide improved alignment over prior art designs.