Abstract:
A fiber optic connector having improved alignment and manufacturing characteristics over traditional connectors. A fiber optic connector of the present invention has a first connector ferrule and alignment means for aligning the first connector ferrule with a matching second connector ferrule. The fiber optic connector includes a first connector ferrule including a first mating surface. The mating surface defines at least one receiving cavity, and at least one protruding alignment rod is seated in the cavity and extends from the mating surface. The alignment rod has a diameter D 1  and protruded from the mating surface a protrusion distance p 1 , wherein p 1 ≦2D 1 . In an exemplary embodiment the rod has a spherical tip and 0.5D 1 ≦p 1 ≦2D 1 .

Description:
RELATED APPLICATIONS 
     The present application is a continuation-in-part of U.S. application Ser. No. 08/953,950 filed Oct. 10, 1997 now U.S. Pat. No. 5,845,028, entitled “Alignment Assembly for Multifiber or Single Fiber Optical Cable Connector”, which is a continuation of U.S. application Ser. No. 08/614,412 filed Mar. 12, 1996, issued as U.S. Pat. No. 5,778,123. The present application also is a continuation-in-part of U.S. patent application Ser. No. 08/819,247, entitled “Stubless Optoelectronic Device Receptacle”, filed Mar. 17, 1997. All of the related applications are commonly assigned to the Assignee of the present invention and the disclosures of the above which are herein incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to connectors for optical fibers and devices. In particular, the present invention relates to an optical connector including a rounded rod alignment feature. 
     BACKGROUND OF THE INVENTION 
     Optical fibers are increasingly being used for the transmission of optical signals. Optical fibers offer greatly increased transmission capability and transmission characteristics over traditional copper wires. 
     The use of optical fibers, however, does present some difficulties. Optical fibers are, in fact, conductors of light signals. To avoid losing or degrading the light signals being transmitted, there is a need for precise alignment and coupling any time optical fibers are connected to each other or to optical devices. Optic transfer efficiency is the term used to measure the ability of a connector to accurately couple the transmitted light signals. 
     Use of optical cables has generally been limited to large scale long haul trunking installations, such as those of the telecommunications industry, where the improved transmission characteristics of the optical fibers justify the greater expense and typical difficulty associated with their manufacturing and installation. Nevertheless, as demands on communication media and data volume continue to increase, the advantages of using optical cable for transmission of signals across shorter distances, or for interconnecting local devices, continues to grow. With this growth has come a need to connect fiber optic cables accurately and economically to each other and to a multiplicity of devices. 
     Of considerable relevance to the problem of developing practical fiber optic connectors is the question of the optic transfer efficiency at the connector. Various factors affect the optic transfer efficiency at a connector including (a) gap separation at the point of abutment, (b) lateral separation due to axial misalignment, and (c) thermal expansion characteristics of connectors. 
     Numerous optical cable connectors have been developed to aid in the connection of fiber optic cables. As data requirements grow, single fiber cables have given way to multiple fiber cables, such as parallel ribbon cables including a plurality of optical fibers aligned in parallel. As the number of fibers grow, such do the difficulties in maintaining the transfer efficiency of the connector. 
     Examples of known multi-fiber connectors include the MAC™ connector by Berg Electronics and the MT Connector by U.S. Conec. Further examples of optical connectors are illustrated in U.S. Pat. No. 5,420,952 to Katsura, et al.; U.S. Pat. No. 5,276,755 to Longhurst; U.S. Pat. No. 5,500,915 to Foley et al.; U.S. Pat. No. 4,784,457 to Finzell; U.S. Pat. No. 5,430,819 to Sizer, II, et al.; and U.S. Pat. No. 5,287,426 to Shahid. 
     Many of the known connectors have disadvantages associated with them. A MT-type connector, illustrated in FIG. 1, is one of the most common connectors currently used. Connector  10  includes a ferrule  12  having two protruding long pins  20  and  22 . The proposed TIA/EIA-604-5 MT connector intermateability standard specifies that the alignment pins must protrude at least 2.285 pin diameters (1.6 mm protrusion for a 0.7 mm diameter pin) from the face of the ferrule. 
     Long thin pins, such as those of the MT connector, attempt to control movement of the connector in the x, y and z axis. Long pins may help achieve suitable optical connections for some applications and the coupling of pins and holes may be intuitive to users. However, the use of such long pins does present significant coupling, alignment, durability and manufacturing disadvantages. 
     As illustrated in FIGS. 2 and 3, during coupling of a MT-type connector, the ferrule  12  is interference fit upon a receptacle  14 . The receptacle  14  defines a receiving orifice or hole  30 . The pin  20  is inserted into the corresponding receiving hole  30 . Significant insertion force is needed to seat each small diameter (˜0.7 mm) pin fully into the respective hole. It has been calculated that the interference fit of a nominal MT connector pin inserted into a matching receptacle hole could require approximately six Newtons of force to fully seat. If the pins are not fully seated, an air gap between the two ferrules results that can cause severe light loss. 
     Correct alignment of the pins is very important before coupling. FIG. 2 illustrates a 0.5 mm lateral misalignment of the 0.7 mm MT connector pin  20 . The small diameter of the pin  20  and of the matching receiving hole  30  results in complete failure to couple even under very small (e.g., half a millimeter) lateral misalignment. 
     FIG. 3 illustrates the effects of angular misalignment of pin  20 . As the effects of even a small angular misalignment are magnified by the length of the pin, even a small angular misalignment (5 degrees) may again result in complete failure to couple. 
     If the pin  20  is not perfectly aligned before engagement into the mating hole  30 , the pin  20  may miss the hole  30  and crack the mating ferrule  14  causing a catastrophic failure. The long and thin metal pins  20  and  22  also are liable to bend during insertion and withdrawal and damage the mating ferrule  14  on subsequent insertions. The high interference fit of the long pin to the mating hole can cause the hole to be “skived” and deposit unwanted debris onto the connector mating face which can cause signal failure. Because the pins protrude so far from the mating face of the MT, the mating face is difficult to clean. 
     Manufacture of a MT connector further requires tight control of the tolerances of at least nine critical dimensions: (1) pin diameter, (2) pin straightness, (3) pin taper, (4) hole diameter, (5) hole straightness, (6) hole angle, (7) hole taper, (8) hole placement relative to matching hole, (9) hole placement relative to fibers. Accordingly, the use of traditional alignment pins further drives up manufacturing difficulty and costs. 
     A further consideration is that the long protruding metal MT alignment pins have a tendency to act as “antennas” and may cause electro-magnetic interference when placed near high frequency components. This interference may in turn cause signal interference to other equipment and components. 
     An alternative optical connector design is disclosed in U.S. Pat. No. 5,778,123, entitled “Alignment Assembly for Multifiber or Single Fiber Optical Cable Connector”, commonly assigned with the present invention to Minnesota Mining and Manufacturing and which is hereby incorporated by reference. The patent discloses a “ball and socket” alignment structure, illustrated in FIG. 4, where an opening or socket  130  in a ferrule  100  seats a ball  120 , rather than a long pin. The opening  130  has a depth d 1 . The ball  120  has a radius R, where R&gt;d 1 . The ball and socket structure offers significant advantages as the design does not overconstrain the z-axis alignment and requires control of only two manufacturing tolerances: the size of the alignment ball, which is easily controllable, and the spacing between the two openings. 
     However, the ball  120  offers only a limited bonding surface to the associated alignment hole  130 . A limited bonding surface may result in inadequate bonding of the ball  120  to the ferrule  100 . Also, the ball and socket design may be susceptible to damage from overpolishing of the ferrule and fiber ends. As illustrated in FIG. 5, overpolishing a ball-in-socket ferrule face may damage or obliterate the ball alignment opening or chamfer, thus inhibiting accurate attachment of the ball. 
     The opportunity remains for an improved optical connector and alignment feature. 
     SUMMARY OF THE INVENTION 
     The present invention is a fiber optic connector including a novel alignment feature having improved alignment and manufacturing characteristics over traditional connectors, while offering the advantages of both the traditional pin connectors and the ball-in-socket connector. The connector includes large diameter alignment rods tightly fit into appropriately sized holes to align optical fiber cores and produce a low loss optical interconnection. Large diameter rods are defined as rods wherein the rods have a diameter such that the rods protrude from the containing holes less than two rod diameters. 
     A fiber optic connector of the present invention has a first connector ferrule and alignment means for aligning the first connector ferrule with a matching second connector ferrule. In an exemplary embodiment, the fiber optic connector includes a first connector ferrule including a first mating surface. The mating surface defines at least one receiving cavity, and at least one protruding alignment rod is seated in the cavity and extends from the mating surface. The alignment rod has a diameter D 1  and protruded from the mating surface a protrusion distance p 1 , wherein p 1 ≦2D 1 . In an exemplary embodiment the rod has a hemispherical tip and 0.5D 1 ≦p 1 ≦2D 1 . 
     The fiber optic connector may further include a second connector ferrule having a second mating surface configured to be aligned opposite the first connector ferrule. The second mating surface defines a second receiving cavity having a depth P 2  wherein and p 1 &lt;P 2 . 
     The second connector ferrule may further includes a second cavity and at least one second protruding alignment rod seated in the cavity, the alignment rod having a diameter d 1  and protruding from the mating surface a protrusion distance p 1 , wherein p 1 &lt;2d 1 . In turn, the first mating surface of the first connector ferrule may define a second receiving cavity having a depth p 2 , wherein p 1 &lt;p 2 , and wherein the first receiving cavity is configured to be aligned to receive the first alignment rod and the second receiving cavity is configured to be aligned to receive the second alignment rod. 
     The fiber optic connector may further include longitudinal grooves designed to receive at least one optical fiber. In an exemplary embodiment, the mating surfaces are positioned at a longitudinal end (defined by the direction of the optical fibers) of the connector ferrule. 
     In an alternative embodiment of the connector of the present invention, the connector ferrule defines a device-receiving cavity. An optoelectronic component is placed within the device-receiving cavity and a flexible circuit is electrically coupled to the optoelectronic component. In an exemplary embodiment, the flexible circuit encapsulates and protect the optoelectronic component within the receiving cavity. The flexible circuit may include at least one light-transmissive window optically aligned with the optoelectronic component. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a MT-type connector. 
     FIG. 2 is a cross-sectional view of a laterally misaligned pin of the connector illustrated in FIG.  1 . 
     FIG. 3 is a cross-sectional view of an angularly misaligned pin of the connector illustrated in FIG.  1 . 
     FIG. 4 is a cross-sectional view of a connector end of a ball and socket connector. 
     FIG. 5 is a cross-sectional view of a portion of an overpolished ball-and-socket connector. 
     FIG. 6 is an isometric view of a connector ferrule in accordance with the present invention. 
     FIG. 7 is a cross-sectional view of a portion of a connector end of the connector ferrule illustrated in FIG.  6 . 
     FIG. 8 is an isometric view of the connector ferrule illustrated in FIG.  6  and supporting hardware. 
     FIG. 9 is an exploded isometric view of a connector assembly in accordance with the present invention. 
     FIG. 10 is a cross-sectional view of a laterally misaligned rod in a connector in accordance with the present invention. 
     FIG. 11 is a cross-sectional view of an angularly misaligned rod in a connector in accordance with the present invention. 
     FIG. 12 is a cross-sectional view sequence illustrating two connector ferrules in accordance with the present invention registering to eliminate misalignment. 
     FIG. 13 is a cross-sectional view of an overpolished connector end for a connector in accordance with the present invention. 
     FIG. 14 is an isometric view of an optoelectronic device receptacle in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 6 illustrates an exemplary embodiment of a connector ferrule  200  in accordance with the present invention. The term connector in the present invention is meant to include devices for coupling and aligning two or more optical fiber cable ends, an optical fiber cable end with an optical device, or two or more optical devices. Accordingly, those skilled in the art will readily appreciate that the present invention may be utilized to align two optical fiber connector ferrules to one another, or alternately, to align one optical fiber ferrule to an optical source, detector, or other optical device. 
     The ferrule  200  includes an alignment block or body  210 . For descriptive purposes, the longitudinal z-axis of the ferrule  200  is defined by the direction along which the ferrule  200  receives and aligns optical fibers. The body  210  has a mating surface  212  located at a longitudinal end of the body  210  and a fiber-receiving surface  214  located on a top middle portion of the body  210 . The fiber-receiving surface  214  allows for top-loading of one or more optical fibers and includes a plurality of v-grooves  216  that retain and align the optical fibers in parallel. The body  210  further includes a first and a second receiving sockets  230  and  232  located longitudinally along the sides of the body  210 . In the present exemplary embodiment, the body is composed of a thermoplastic, such as Fortron 6165A6 from the Ticona Company. The ferrule could also be made of any of many other materials including metal, ceramic, thermoset plastic, or other suitable materials. 
     The sockets  230  and  232  define first and second receiving cavities  234  and  236  respectively. A longitudinal cross-section of socket  230  is illustrated in FIG.  7 . An alignment rod  220  is interference fit into the receiving cavity  234  of socket  230 . A second alignment rod  240  (illustrated in FIGS. 8 and 9) may be seated in the second receiving cavity  236 . 
     In the present embodiment, the rod  220  is cylindrical and has a diameter D of 2.0 mm, a length of 6 mm and has spherical ends  222  and  224 . The alignment rod  220  may be made of hard, durable materials, including plastic, metal, ceramic, or metal alloys, such as stainless steel alloy  303  SS. The receiving cavity  234  similarly is cylindrical and has a diameter of 2.0 mm and a depth of 4.5 mm. When inserted into the receiving socket  230 , the alignment rod  220  protrudes a distance p of about 1.5 mm from the mating surface  212 . The desired relationship between the protrusion distance p for a spherical tipped rod is between 0.5 to 2 times the diameter D of the rod  220  (i.e., 0.5D≦p≦2.0D), providing a sphere-ended post. For a spherically or hemispherically tipped rod a distance p of 0.5 D equals the radius of the sphere, thus allowing the full diameter of the rod to be available for alignment with an opposing receiving cavity. 
     In alternative embodiments, the shape of the rod and of the cavity may be polyhedral and the diameter of the cavity, depending on the material used for the body and the desired fit, may be slightly larger, equal or even smaller than D. In such embodiments, the diameter D is meant to be measured as the cross-sectional thickness of the rod. Also, the end geometry of the alignment rod may be a variety of shapes, such as spherically radiused, chamfered, or sloped. If the rod tip is not hemispherical, the rod could protrude less than 0.5 diameters, while preferably allowing the full diameter of the rod to be exposed for aligning the mating connector. 
     FIGS. 8 and 9 illustrate a connector ferrule assembly  900  including the ferrule  200 . FIG. 8 illustrates an assembled connector showing the intermating of a fiber optic cable  800 , a strain relief boot  804 , a latching body  250 , the ferrule  200 , a ferrule cover  816 , and alignment rods  220  and  240 . 
     FIG. 9 illustrates an exploded view of the connector assembly  900 . The fiber optic cable  800  includes an outer jacket  801 , containing a fibrous kevlar strength member  803 , which surrounds a horizontal array of optical fibers  802 . The optic cable  800  is threaded into the strain relief boot  804 , which restricts the bending of the cable  800 . A cable jacket retention sleeve  805  is slid over cable  800  to prevent cable jacket  801  from slipping rearward within strain relief boot  804 . 
     The optical fibers  802  are inserted into a connector body  810 . The connector body  810  consists of a central body  811  connected to a crimp region  812 , as well as ferrule alignment posts  813  and ferrule retention tongue  815 . The cable strength member  803  is sandwiched between crimp region  812  and crimp ring  806 . Crimp ring  806  is subsequently compressed, thus trapping strength member  803  between crimp region  812  and crimp ring  806 . 
     Springs  817  are placed over ferrule alignment posts  813  and ferrule  200  is inserted over alignment posts  813  and retention tongue  815 , thus compressing springs  817 . The array of optical fibers  802  passes through connector body  810  and rests in ferrule  200 , aligned to the v-grooves  216 . The array  802  is then held in place by ferrule cover  816 . The end of array  802  is polished flush with the ferrule mating surface  212 . One or two alignment rods  220  are the inserted into ferrule sockets  230  and  232 . Finally, latching body  250  is installed over the ferrule  200  and attached to connector body  810 . 
     FIGS. 10-12 illustrated the coupling of ferrule  200  with a matching ferrule  300 . The ferrule  300  is generally symmetrical to the ferrule  200  and includes an opposite mating surface  312  and matching opposite sockets  330 . As illustrated in FIGS. 10-12, during coupling, the matching ferrule  300  may be placed opposite the ferrule  200 . The rod  220  is aligned opposite a matching receiving cavity  336 , while a rod  320  of the matching ferrule  320  is aligned opposite receiving cavity  236 . The matching receiving cavity has a depth P 2 , where P 2 &gt;P. When the corresponding rods and cavities are matched, the v-grooves, and accordingly the optical fibers, become aligned. 
     The larger sphere ended post  220  of the present invention allows for much greater alignment mismatch and more reliable mating. Ease of mating is a significant improvement because many of the applications for the connector, such as connection between a daughtercard to a backplane, require “blind mate”, that is, visual orientation of the connector prior to engaging is not possible. FIG. 10 illustrates how the large diameter rod  220  compensates for a 0.5 mm lateral misalignment, such as that previously illustrated in FIG.  2 . The advantage is directly proportional to the diameter of the alignment feature; the larger the feature, the more misalignment the rod may accommodate. 
     FIG. 11 illustrates compensation on an angular misalignment of the same magnitude as that previously illustrated in FIG.  3 . An alignment rod with limited protrusion (from 0.5 to 2 rod diameters) is substantially less susceptible to misalignment based on angular inaccuracies in the alignment holes. In addition, a large diameter alignment rod with limited protrusion is less susceptible to bending and breakage than a thinner, more exposed alignment pin. FIG. 12 illustrates a mating sequence of ferrules  200  and  300 , from initial alignment (FIG. 12A) to compensation for lateral misalignment (FIG.  12 B), to coupling (FIG.  12 C). 
     One further advantage of the connector of the present invention is a more robust bonding between the rod and the ferule. As illustrated in FIGS. 7 and 13, the larger sphere ended post offers a much larger bonding and seating area, which results in better adhesion between the rod and the ferrule. Also, as illustrated in FIG. 13, the present invention is less susceptible to overpolish. Overpolishing the face  212  of ferrule  200  merely results in slightly greater rod protrusion which has little effect on the alignment of the connector. 
     The larger diameter, smaller protruding rod arrangement of the present invention has other significant benefits. The rod has larger surface area to align the two ferrules and is not inserted as deeply into the socket as a long pin connector, thus dramatically decreasing the required insertion force. Furthermore, since the sphere ended post is not sharp and does not penetrate the mating ferrule as deeply, unwanted debris is minimized. The sphere ended post connector is easy to clean, since the rod does not protrude far from the face. The present invention increases the reliability and quality of the interface. The sphere-ended post is much more robust and resistant to damage due to its larger cross-sectional area, and minimal protrusion from the connector face. 
     Finally, the present invention minimizes the unwanted “antenna effect” since the rod is significantly shorter than long pins and does not protrude as far out from the connector face as traditional pins. 
     Those skilled in the art will appreciate that the present invention may be used when coupling a variety of optical devices and even non-optical devices that require precise alignment. FIG. 14 illustrates an optoelectronic device receptacle in accordance with the present invention. Receptacle  400  includes a body or alignment block  410 , having an engagement surface  412 . The surface  410  defines a first and a second receiving cavity  430  and  432 . An alignment rod  420  may be placed in either or both of sockets  430  and  432  to compliment to corresponding placement of rods within a matching ferrule, such as ferrule  200 . The rod  420  has a diameter D and protrudes a distance between 0.5 to 2 times D from the engagement surface  412 . The surface  410  further defines a device-receiving cavity  452 . An optoelectronic component  450  is placed within the cavity. A flexible circuit  454  is placed over and is electrically coupled to the optoelectronic component. The flexible circuit  454  may have a plurality of windows  456  comprising of a light-transmitting polymer aligned with optical receptors or emitters of the optoelectronic component. The flexible circuit  454  encapsulates and protects the optoelectronic component within the receiving cavity  452 . 
     Those skilled in the art will appreciate that the present invention may be used when coupling a variety of optical devices and even non-optical devices that require precise alignment. While the present invention has been described with a reference to exemplary preferred embodiments, the invention may be embodied in other specific forms without departing from the spirit of the invention. Accordingly, it should be understood that the embodiments described and illustrated herein are only exemplary and should not be considered as limiting the scope of the present invention. Other variations and modifications may be made in accordance with the spirit and scope of the present invention.