Abstract:
A ferrule structure for an optical connector includes a central member disposed in the ferrule. The central member is configured so that an exterior dimension of the central member can change between a smaller size and a larger size. A plurality of optical fibers are disposed in the ferrule externally to the central member. A method for assembling an optical connector ferrule includes providing a central member, wherein the central member is configured so that an exterior dimension of the central member can change between a smaller size and a larger size. The method further includes placing an axial load on the central member to cause the exterior dimension to assume the smaller size. The central member is disposed in the ferrule. A plurality of optical fibers are disposed in the ferrule, external to the central member. The axial load is removed from the central member.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/219,481, filed on Jun. 23, 2009, which is herein incorporated by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     The invention relates generally to the field of optical fiber signal communication. More specifically, the invention relates to structures for terminals in a connector used to join segments of an optical fiber cable and methods of utilizing same. 
     Optical fibers are used in cables to transmit signals in the form of modulated light along the cable. Often, a single cable may carry a dozen or more optical fibers. The cable may have a preselected length with optical connectors on one or both ends. The optical connectors are used to join one cable to another while making optical connection between corresponding optical fibers in each cable. Consequently, the optical connectors should provide secure optical connections (i.e., providing a high transmission rate) between corresponding optical fibers. Ideally, an optical connector would provide such connections for each pair of corresponding fibers in a single step. 
     U.S. Pat. No. 6,827,597 issued to Metzbower et al. shows one type of optical cable connector. In connectors such as the one shown in the &#39;597 patent, each optical fiber that is to be optically coupled to a corresponding fiber is held in place in the connector body by an individual ferrule. Each ferrule may be spring loaded from the connector body end so that when the corresponding connectors are mated, the corresponding fiber ends are urged into contact with each other with a preselected biasing force. 
     As optical cables are made with increasing numbers of fibers, suitable optical connectors for such cables becomes increasingly large and cumbersome because one ferrule is used for each optical fiber, and thus the size of the connector body increases accordingly. 
     Equipment for geophysical surveying of subterranean formations may utilize optical fibers for signal transmission. Optical signals may advantageously avoid electromagnetic interference problems which would accompany electrical signals. This may be particularly advantageous given the cable lengths required in typical surveys. Equipment for geophysical surveys could benefit from smaller, less complex, more reliable, and more cost efficient optical connectors. 
     There exists a need for an optical connector that can couple larger numbers of optical fibers between cables without the need for corresponding increase of the size of the connector body. 
     SUMMARY OF THE INVENTION 
     A ferrule structure according to one aspect of the invention includes a central member disposed in the ferrule, wherein the central member is configured so that an exterior dimension of the central member can change between a smaller size and a larger size. A plurality of optical fibers is disposed in the ferrule externally to the central member. A method for assembling an optical connector ferrule according to another aspect of the invention includes providing a central member, wherein the central member is configured so that an exterior dimension of the central member can change between a smaller size and a larger size. The method further includes placing an axial load on the central member to cause the exterior dimension to assume the smaller size. The central member is disposed in the ferrule. A plurality of optical fibers is disposed in the ferrule, wherein the optical fibers are disposed external to the central member. The axial load is removed from the central member. 
     An optical connector for marine geophysical surveys according to yet another aspect of the invention comprises at least one ferrule. Each ferrule comprises a central member disposed in the ferrule, wherein the central member is configured so that an exterior dimension of the central member can change between a smaller size and a larger size, and an axial load on the central member causes the exterior dimension to assume the smaller size. Each ferrule also comprises a plurality of optical fibers disposed in the ferrule external to the central member. 
     Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an example optical connector ferrule wherein twelve optical fibers are frictionally held in place by an expandable central member. 
         FIG. 2  shows the example ferrule of  FIG. 1  wherein the central member is stressed to reduce its diameter, thus enabling insertion of the fibers into the ferrule. 
         FIG. 3  shows an example ferrule with a twelve fiber ribbon wrapped around an expandable central member. 
         FIGS. 4A and 4B  show example ferrules with another form of central member in stressed and relaxed states, respectively. 
         FIGS. 5A and 5B  show example ferrules with another form of central member in stressed and relaxed states, respectively. 
         FIG. 6A  shows an example ferrule using shape memory alloy as the central member in a stressed state. 
         FIG. 6C  shows a side view of the central member of  FIG. 6A   
         FIGS. 6B and 6D  show the same elements of  FIGS. 6A and 6C , wherein the central member is relaxed, respectively. 
         FIGS. 7A through 7D  show similar structures to those shown in  FIGS. 6A through 6D , with the addition of a coating on the central member. 
         FIG. 8  illustrates parameters useful in calculating the dimensions of the central member and the ferrule given a selected number of optical fibers and dimensions of the fibers. 
         FIG. 9  shows an example optical connector having multiple optical fiber cavities. 
         FIG. 10  shows an example optical fiber having a core, cladding, buffer, and an outer jacket. 
     
    
    
     DETAILED DESCRIPTION 
     As described in U.S. Pat. No. 6,827,597 issued to Metzbower et al., which is herein incorporated by reference, optical connectors typically require a mechanism for securing the orientation and location of the optical fibers. The present invention works with optical connectors to retain the optical fibers in position to make optical contact with corresponding optical fibers in the mating part of the connector. Ferrules according to embodiments of the invention may be made from a ceramic material, a metal, or any other suitable material. For example, steel ferrules generally known in the telecommunications industry as “LC” or “MU” may be used, although the type of ferrule is not a limitation on the scope of the present invention. The ferrule may also be derived from connector types known in the art having, for example, outside diameters of about 1.25 mm, 2.0 mm, or 2.5 mm. In other embodiments, the ferrule may be of a user-specified shape and diameter. 
       FIG. 1  shows an example ferrule  10  having therein twelve optical fibers  12  disposed around a central member  14 . As will be further discussed, the specific number and dimensions of optical fibers  12  may vary, and the dimensions of the ferrule  10  and central member  14  may vary to accommodate the required optical fiber configuration. The assembly in  FIG. 1  shows the central member  14  in the absence of an axial load. The central member  14  may be, for example, a solid wire, a tube, a straight wire, a helically wound wire, or a shape memory alloy structure (such as a helically wound wire, explained further below). In many embodiments, the material from which the central member  14  is made enables the central member  14  to reduce in size under an axial load to a sufficient degree to enable insertion of the fibers  12  into the ferrule  10  as shown in  FIG. 2 . In some embodiments, central member  14  is made from an ultra elastic nickel-titanium memory metal alloy known as Nitinol. In other embodiments, central member  14  is made from a material having an elastic nanostructure. In still other embodiments, the central member is made from a rigid epoxy. In some embodiments, the central member is created by an injection molding process. Returning to  FIG. 1 , after the axial load is removed from the central member  14 , the central member  14  expands in size, thereby applying a radial force to the optical fibers  12  against the interior of the ferrule  10 . The optical fibers  12  are, thereby, retained in place between the central member  14  and the ferrule  10 . The ends of the fibers so retained may be inspected, for example, microscopically to determine their rotational alignment. After the alignment is determined, an orientation key  13  may be affixed to or formed in the exterior of the ferrule  10 . Examples of implementations of an orientation key include crimping, adhesive bonding, and swaging. A corresponding receptacle (not shown) in the connector body (not shown) may engage the orientation key  13  so that the multiple optical fibers  12  in the ferrule are disposed in a known rotary orientation. In some embodiments, orientation key  13  will actually be a notch or negative space created in ferrule  10  into which a tooth from the connector body (not shown) is inserted. 
       FIG. 3  shows another example wherein a fiber ribbon  16  includes twelve optical fibers disposed in an embedding material. The fiber ribbon  16  may be wrapped around the central member  14 , and both fiber ribbon  16  and central member  14  may be inserted into the ferrule  10 . The central member  14  may then have the axial load relieved. The fiber ribbon  16  will then be retained inside the ferrule  10  by friction. 
       FIGS. 4A and 4B  show, respectively, an alternative example of the central member in stressed and relaxed conditions. As can be observed in the figures, the central member  14 A need not be circular in cross section as in the previous examples, but may have a cross section suited to retain the optical fibers  12  in a particular position about the exterior of the central member  14 A.  FIGS. 5A and 5B  show stressed and relaxed states of a differently shaped central member  14 B, respectively. Is should be understood that the shape of a cross-section of central member  14  may vary, for example a circle, a triangle, a square, a pentagon, a hexagon, a regular polygon, or an irregular polygon. 
       FIG. 6A  shows an end view of a ferrule  10  having a plurality of optical fibers therein in which the central member  14 C may be a helically wound spring, made for example from shape memory alloy, spring steel, or other suitable spring material. A side view of the central member  14 C of  FIG. 6A  is shown in the stressed state in  FIG. 6C .  FIGS. 6B and 6D  show the ferrule  10  and the central member  14 C of  FIGS. 6A and 6C  in the relaxed state, respectively. 
       FIGS. 7A through 7D  show, respectively, similar structures to those shown in  FIGS. 6A through 6D , with the addition of a coating  15  on the exterior of the central member  14 C. The coating  15  may be a rose metal, a low melting point metal, a thermosetting adhesive compound, or a hot melt adhesive. As used herein, “low melting point” meaning temperatures below the transformation temperature of the central member  14 C in cases wherein the central member  14 C comprises a memory alloy, or temperatures which could affect the optical properties of the optical fibers. In some embodiments, the coating  15  may provide a locking mechanism for the central member  14 C and optical fibers  12 . For example, the temperature of the coating  15  may be raised above its melting point during assemblage of the optical fibers  12  within the ferrule  10 . The coating  15  may thereby fill interstitial spaces or voids between and around central member  14 C and optical fibers  12 . After central member  14 C returns to a relaxed state, the temperature of coating  15  may be lowered below its melting point, thereby solidifying the arrangement of central member  14 C and optical fibers  12 . 
     As previously mentioned, the dimensions of the ferrule  10  and central member  14  may be determined by the number and dimensions of required optical fibers  12 .  FIG. 8  provides an illustration helpful in determining appropriate dimensions in embodiments wherein the radius of the optical fiber is small compared to the interior radius of the ferrule. In such embodiments, for a given number of optical fibers x to be arranged in a ring, the arc angle θ of the ring occupied by each of the optical fibers is approximated by: 
                   θ   ≈       360   ⁢   °     x             (     Eq   .           ⁢   1     )               
In many embodiments, the number of optical fibers will range between 3 and 100. In some embodiments, the number of optical fibers will range between 7 and 24. This approximation holds well for embodiments with at least 6 optical fibers.
 
     For a known radius r of each optical fiber, the minimum interior radius of the ferrule R interior  would then be: 
                     R   interior     ≈       r     tan   ⁢     θ   2         +   r             (     Eq   .           ⁢   2     )               
The radius of optical fibers will typically range from about 10 μm to about 100 μm (about 0.01 mm to about 0.10 mm), resulting in a typical range for R interior  of between about 27 μm and about 3,282 μm (about 0.03 mm to about 3.28 mm). Similarly, the maximum exterior radius of the central member R CM  (when in a relaxed state) would be:
 
                     R   CM     =       r     tan   ⁢     θ   2         -   r             (     Eq   .           ⁢   3     )               
resulting in a typical range for R CM  of between about 70 μm and about 30,900 μm (about 0.07 mm to about 31 mm).
 
     When the central member  14  is a wire or rod, the change in diameter in one dimensional axial stress may be expressed in terms of the Poisson ratio of the central member material. For example, the diameter change of the central member under stress Δd may be given by the following expression: 
                     Δ   ⁢           ⁢   d     =     -       d   ⁡     (     1   -     (     1   +       Δ   ⁢           ⁢   L     L       )       )         -   v                 (     Eq   .           ⁢   4     )               
wherein d is relaxed diameter of the central member, ν is Poisson&#39;s ratio for the rod material (example Poisson ratio of 0.33), L is the original length of the rod in the relaxed state, and ΔL is the change of length (e.g., 8% elongation).
 
     The typical end face on an optical ferrule of the nature described is flat and normal to the optical fibers exiting the ferrule. A well know method of reducing optical back reflection in a ferrule to ferrule optical connection is to provide an angled end face, usually about 8 degrees from normal to the axis of the exiting optical fibers This method is also known as “angle polishing”. The two ferrules are then aligned axially with the two angled end face in parallel plains. This configuration may be employed in certain embodiments of the invention. 
     Ferrules for optical connectors and connectors made according to the various aspects of the invention may enable inclusion of more optical fibers in a particular optical connector without having to increase the size of the optical connector. For example, as illustrated in  FIG. 9 , optical connector  20  may have multiple optical fiber cavities  25 . As would be understood by one of ordinary skill in the art with the benefit of this disclosure, an optical fiber may be either single mode or multi mode, and may comprise several concentric layers.  FIG. 10  illustrates a typical optical fiber  30 , having a core  31 , a cladding  32 , a buffer  33 , and an outer jacket  34 . When the optical fiber  30  is single mode, the core  31  will typically be about 8 μm in diameter, while a multi mode optical fiber will typically have a core  31  of about 62.5 μm in diameter. The cladding  32  will have a diameter of about 125 μm for both single mode and multi mode optical fibers. Depending on the application, the diameter of the buffer  33  may range anywhere from about 250 μm to about 500 μm. If used, the diameter of the outer jacket  34  may range anywhere from about 400 μm to about 2.0 mm. Consequently, in many standard optical connectors  20 , the cavity  25  may be designed accept an optical terminus (an optical fiber enclosed in a ferrule) of any one of the three standard sizes: 1.5 mm, 2.0 mm, and 2.5 mm. For a grouping of 12 optical fibers, each of about 125 μm diameter, a ferrule according to embodiments of the invention could have an interior diameter of about 590 μm (0.59 mm). Consequently, utilizing ferrules according to embodiments of the invention would convert optical connector  20  from a 6-fiber connector to a 72-fiber connector using the existing optical termini cavities. 
     It should be appreciated that ferrules for optical connectors may have applicability in many fields which utilize optical data transmission. For example, ferrules for optical connectors may be utilized in geophysical surveys of subterranean formations. In a specific embodiment, ferrules for optical connectors may facilitate data transmission in electromagnetic survey cables for marine geophysical surveys. In such embodiments, the geophysical survey could utilize ferrules which are smaller, less complex, more reliable, and more cost efficient than conventional ferrules. 
     While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.