Abstract:
An optical fiber terminus includes an elongated body with a passage along a central axis for receiving a portion of an optical fiber cable therethrough and an indexing section. A ferrule is secured to the body and has an end portion of said optical fiber cable therein. A collar is positioned on the elongated body and has an engagement section for engaging the indexing section. The collar is movable along the axis between first and second operative positions. In the first operative position relative rotational movement between the collar and the body is prevented and in the second operative position the collar may rotate relative to the body. A biasing member is provided to bias the collar towards the first operative position.

Description:
REFERENCE TO RELATED APPLICATIONS  
   This application claims priority from prior U.S. provisional patent application No. 60/636,879 filed Dec. 20, 2004. 

   BACKGROUND OF THE INVENTION  
   The present invention relates to an apparatus for inter-connecting optical devices and, more particularly, to a connector for terminating an optical fiber. 
   Optical fiber connectors are an essential part of substantially any optical fiber based communication system. For instance, such connectors may be used to join segments of fiber into longer lengths, to connect fiber to active devices such as transceivers, detectors and repeaters, or to connect fiber to passive devices such as switches and attenuators. The central function of an optical fiber connector is to maintain or position two optical fiber ends such that the core of one fiber is axially aligned with the core of the other fiber. Consequently, the light from one fiber is coupled to the other fiber or transferred between the fibers as efficiently as possible. This is a particularly challenging task because the light-carrying region or core of an optical fiber is quite small. In single mode optical fibers, the core diameter is about 9 microns. In multi-mode fibers, the core can be as large as 62.5 to 100 microns, and hence alignment is less critical. However, precision alignment is still a necessary feature to effectively interconnect the optical fibers. 
   Another function of the optical fiber connector is to provide mechanical stability to and protection for the optical junction in its working environment. Achieving low insertion loss in coupling two fibers is generally a function of the alignment of the fiber ends, the width of the gap between the ends, and the optical surface condition of either or both ends. Stability and junction protection is generally a function of connector design (e.g., minimization of the different thermal expansion and mechanical movement effects). Precision alignment of the optical fiber is typically accomplished within the design of the optical terminus assembly. The typical optical terminus assembly utilizes a method of retention of the terminus within the connector(s) integrated within it and a method of holding and aligning the optical fiber. To align the optical fiber, a terminus typically includes a small cylinder of metal or ceramic at one end commonly referred to as a “ferrule.” The ferrule has a high precision hole passing along it centerline and glass or plastic optical fiber can be installed into the hole within the ferrule using mechanical, adhesive or other retention methods. The primary operational sections of an optical terminus are the support structure around the ferrule and the mechanism (typically a spring) used to create a force to push the ferrule into an opposing ferrule of a mating optical connector. 
   In a connection between a pair of optical fibers, a pair of ferrules is butted together in an end to end manner and light travels from one to the other along their common central axis. In this conventional optical connection, it is highly desirable for the cores of the glass fibers to be precisely aligned in order to minimize the loss of light (such loss being referred to as insertion loss) caused by the connection. As one might expect, it is presently impossible to make a perfect connection. Manufacturing tolerances may approach “zero” but practical considerations such as cost, and the fact that slight misalignment is tolerable, suggest that perfection is unnecessary although stability across the operating environment of the fiber joint is critical. 
   Historically, due to manufacturing costs and design features, optical termini have tended to be manufactured as an assembly of loose components. In high performance connectors intended for single mode application, there exists a specific need to tune out the eccentricity of assemblies and such tuning has been achieved by the interaction between the terminus or ferrule support structure and the connector housing. This is an undesirable effect as the housing becomes an integral element in tuning and if the terminus is removed from the housing (such as for cleaning or replacement), the tuning is in effect lost. 
   Optical terminus assembly tuning is used to reduce the random position of the optical fiber within an optical connector. The randomness of this positioning may be in the order of fractions of microns to several microns. However, when consideration is taken of single mode optical fiber with an optical waveguide of only 8-9 microns in diameter, it can be seen how optical insertion loss can be dramatically impacted if control of the placement of the optical core is not maintained. Fiber eccentricity compensation is currently most commonly found on single channel “LC” style connectors. Compensation is attained using a faceted structure (such as a square or hexagon) to register on the front end of the ferrule support structure. The support structure engages an appropriate complementary pattern within the LC connector body and retains positioning by engaging the LC body. Thus tuning or fiber eccentricity compensation is only retained as the ferrule and its support is retained within the connector body. Once removed it is not possible to determine the exact positional relationship between the fiber holding structure and the connector body. 
   Recognizing the engineering challenge posed by the alignment of two very small optical fiber cores, it is desirable to provide termini that are smaller, less expensive, and yet more convenient for customers to manipulate. One of the key features associated with the design of termini is the system for retaining the termini in a connector. The retention feature affects the ability of the terminus to be engaged into a connector system and retained within the connector system during mating of the two connector halves. The retention system must enable users of the optical terminus system and its associated connector system the ability to remove the optical termini individually for service, repair, inspection or other reasons. Existing optical termini systems are typically utilized in military connector systems and some designs incorporate anti-rotation features but none include an operative retention system and tuning capability as an integral part of the terminus. 
   SUMMARY OF THE INVENTION  
   It is an object of the present invention to provide a terminus retention system that removes complexity from the connector system and enables users to quickly service connectors, yet retain the tuning of a terminus. As such, a connector is disclosed for terminating an optical fiber including a fiber holding structure for maintaining eccentricity compensation and having an end face in which an associated fiber is terminated within the holding structure and including an axial passageway which terminates in the end face and which is adapted to receive an end portion of the associated optical fiber. A connector housing has internal surfaces that define a cavity to accept the fiber-holding structure and includes first and second openings extending into the cavity and being positioned at opposite ends of the housing. The first opening is configured to receive an optical fiber and the second opening is configured to enable the end face of the holding structure to protrude through the opening. A latch is provided integral to the fiber holding structure to secure the fiber holding structure within an associated cavity. To preclude unintended decoupling therebetween, the latch includes a protrusion positioned on one or more surfaces of a sliding collar integral to the fiber holding structure. The latch is configured to engage the cavity structure by having the protrusion sweep an arc beneath an upper surface of the cavity. When the latch protrusion is swept through the arc, it is held beneath the rear face of the cavity by spring pressure created by compression of a primarily helical spring coaxially located along the fiber holding structure longitudinal axis. 
   In the preferred embodiment, the spring member interacts between two surfaces within the fiber-holding structure. The fiber holding structure also provides a keying structure to engage the housing and likewise urge an end face or ferrule through the second opening in the housing. 
   The terminus is a cylindrical fiber-holding structure with a ferrule that includes the end face in which the associated fiber is terminated and an axial passageway which terminates in the end face. This passageway is adapted to receive an uncoated end portion of the associated fiber. A base member holds an end portion of the ferrule within the terminus assembly and includes an axial passageway which is collinear with the axial passageway of the ferrule. A shoulder may also be provided to engage a spring of the terminus assembly. A rear portion of the base member provides a multi-positional eccentricity index feature, such as a hexagonal section. A sliding collar which has a shoulder to engage a spring, an axial pass way in which the base member assembly is positioned and an external index “key” formed by one or more protrusions. A spring member is provided to push the sliding collar towards the rear of the base member. In one embodiment, the cylindrical ferrule has a diameter of about 1.25 millimeters. 
   The cylindrical plug of the present invention includes a tube whose outer cylinder surface has a circular cross section and whose axial passageway is substantially concentric with the outer cylinder surface and wherein the tube is made from ceramic or metallic materials. The fiber-holding structure is adapted to be held within the housing in a singular stable angular position such that the angular position of the fiber-holding structure with respect to the housing is constant. In addition, the fiber-holding structure sliding collar index key allows the entire fiber-holding structure to be removed from the connector housing yet maintain its singular stable angular position when returned to the connector housing. The connector housing includes first and second interconnecting housing members which each include an internal cavity for receiving the fiber-carrying structure. The second interconnecting member is generally cylindrical in shape so as to mate with the first interconnecting member. The first and second interconnecting members combine to form a structure that substantially encloses the fiber-holding structure. The first and second interconnecting members are made from a metallic, plastic or ceramic material and are secured together using a positive locking device such as a threaded collar, a coupling screw or external physical clamp. 
   An optical cable and a connector are also disclosed in which the optical cable includes a glass fiber enclosed within a plastic buffer material and the connector includes a fiber-holding structure with an axial passageway which receives the optical fiber and which terminates in a planar end face that is perpendicular to the passageway. A housing has internal surfaces that define a cavity and surround the fiber-holding structure as well as a first opening at the back end of the housing which receives the optical cable and a second opening at the front end of the housing through which the end face of the fiber-holding structure protrudes. The openings extend into the cavity and are positioned at opposite ends of the housing. The housing captures the fiber holding structure in a manner such that eccentricity is confined to a unique, known position. A manually operated latch for securing the fiber holding structure to the associated receptacle is also provided to preclude unintended decoupling therebetween. The latch is positioned on a one or more side surfaces of the sliding collar section integrated within the fiber holding structure. The latch includes a spring element contained within the fiber holding structure. The fiber-holding structure includes an annular spring that interacts with two flanges or shoulders within the fiber-holding structure. One of the shoulders is free to move relative to the other along the primary axis of the fiber-holding structure and engages the housing thus urging the end face of the fiber-holding structure through the second opening in the housing. 
   A connector for terminating an optical fiber includes a fiber-holding structure that terminates in an end face and is adapted to receive an end portion of the optical fiber. A housing includes a plurality of internal surfaces that define a cavity and surround the fiber-holding structure, a first opening for receiving an optical fiber holding structure/optical fiber and a second opening for enabling the end face of the fiber-holding structure to protrude therethrough. The openings extend into the cavity and are positioned at opposite ends of an axial passageway through the housing. The fiber-holding structure includes a compression spring which presses two shoulders or flanges on the fiber holding structure. The flanges are free to move axially relative to one another to urge the end face of the fiber-holding structure through the second opening in the housing. 
   An optical fiber connector is disclosed for effecting optical end-to-end coupling between two optical fibers, each of which terminates in a ferrule having a precision cylindrical outside surface. One end of each ferrule is held within an opening in a base member. The base member is generally cylindrical and has a flange which is disposed around the circumference of the base member and interacts with one end of an annular spring which is also disposed around the base member. The ferrule, base member and spring are joined to a secondary member that includes a hexagonal or other even sided geometric shaped indexing feature. A sliding member including a latch protrusion feature that engages the secondary member to permit indexing of the hexagonal or other even sided geometric shaped indexing feature and further engages a connector body housing. This engagement is accomplished with one or more unique indexing keys that extend approximately perpendicular to the longitudinal axis of the siding member and engage an appropriate slot in the connector body housing. 
   An optical fiber terminus body has a helical spring trapped between front shoulder on a main inner body and a rear shoulder created by a thin flange on a sliding collar. The sliding collar is likewise trapped between the rear of the spring and a rear shoulder on the main inner body. Typically, the inner body is formed using two components that are pressed, bonded, welded or otherwise assembled. The collar has an alignment ring on it to retain precise alignment of the terminus within a stepped cylindrical bore. The collar also has a protrusion that enables keying and positive positioning of the terminus assembly within a stepped cylindrical bore when the bore has an appropriate slot formed in it or a slot that is created with a secondary piece. The slot is configured with a cut that extends along an arc around the axis of the bore so that the protrusion can act as a retainer mechanism for the terminus assembly in the cylindrical bore. This is accomplished by inserting the terminus into the bore until the front edge of the section having the front spring retention shoulder engages a step in the bore. This presents further penetration of the terminus assembly through the stepped bore. At that time, the sliding collar begins to move forward along the main inner body. The protrusion on the collar moves through the slot along the side of the bore and the spring is compressed. As the protrusion on the collar reaches the end of the slot in the bore, it can be rotated in an undercut arc in the bore. When rotated to the end of the arc, the protrusion cannot pass back upward along the axis of the bore. Hence, there remains compression of the spring and the entire assembly is captured within the bore by the spring pressure between the front shoulder on the main inner body and the sliding collar that has engaged the cylindrical bore. To facilitate tuning of the terminus, a hexagonal or other faceted shaped section integral to the main terminus body is provided at the rear of the main terminus body and engages the sliding collar. The hexagonal or other faceted shaped section is included to allow tuning or minimization of eccentricity of the internal bore relative to a mating terminus of the same type. Tuning is accomplished by determining a desired position for the offset in the bore centerline in the main inner body relative to the sliding collar. If a hexagonal tuning section is used, one of six positions is available. The sliding collar engages one of the available tuning sections on the main inner bodies. These and other objects, features and advantages of the present invention will be clearly understood through a consideration of the following detailed description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS  
     Other objects and advantages of the present invention will be understood from the following description according to one preferred embodiment of the present invention which is shown in accompanying drawings in which: 
       FIG. 1  is a perspective view of one embodiment of an optical fiber terminus in accordance with the principles of the present invention; 
       FIG. 2  is an exploded perspective view of the optical fiber terminus of  FIG. 1 ; and 
       FIG. 3  is a side elevational view of the optical fiber terminus of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
   In accordance with one embodiment of the present invention and referring first to  FIG. 1 , an optical fiber support assembly or terminus  10  and an optical fiber connector that includes such terminus, as well as a method of assembly, are disclosed. The terminus includes three main components, inner main body or member  12 , a sliding collar or outer member  14  with protrusion boss or tab  16  extending radially therefrom and substantially helical spring or biasing member  18 . The inner main body is typically an assembly of three components ( FIG. 2 ) a ferrule  20  (typically made of ceramic or metal), a forward section or body  22  that is joined to the ferrule with an adhesive or by a press-fit and a rear section or body  24  that is assembled with forward section  22  and captures the sliding collar and helical spring  18  therebetween. As described in more detail below, sliding collar  16  is indexable through the interaction between registration structure integral to the collar and indexing structure integral to the rear of inner main body. The sliding collar is further indexed relative to the connector assembly by the interaction of features included in the connector body and the protrusion on the sliding collar. 
   As described above, the terminus  10  has a ferrule  20  attached to the inner main body to position an optical fiber along the longitudinal centerline or axis “A” of the terminus assembly. The terminus has an opening or bore  26  therein for receiving an end of an optical fiber. The inner main body  12  has a shaft portion  28  ( FIGS. 1 and 3 ) formed by the combination of forward section  22  and rear section  24  about which the spring  18  can be positioned and aligned. A forward shoulder  30  on main body  12  forms a front abutment that abuts a front end  32  of the spring, and a shaft recess forming a shoulder. Sliding collar  14  is also installed onto the shaft portion  28  of the main inner body  12  adjacent spring  18 . 
   Sliding collar forms the rear abutment  34  that abuts the rear end  36  of the spring. An engagement section  38  is formed at the rear of collar  14  with opposing arms  42  having inwardly facing flat surfaces  44 . The flat surfaces  44  of the arms engage a multi-faceted (typically hexagon) indexing section  40  on the rearward end of the inner main body. Opposing arms  42  engage opposite sides of the hexagonal indexing section  40  to prevent rotation of the collar  14  relative to the inner main body  12  and further enable selection of multiple orientations of the inner main body relative to the collar  14  and the protrusion boss  16  projecting therefrom. The main terminus body  12  has a rear shoulder  46  that prevents the collar  16  and spring  18  from sliding off the shaft  28  and provides a pre-load compression of the spring when assembled. The main terminus body  12  is typically a two piece component that is either press fit together, bonded together, welded together or affixed together into a single piece using another method of securement. The assembly of ferrule  20 , main terminus body  12 , spring  18  and sliding collar  16  is commonly referred to as a terminus assembly. 
   The terminus assembly  10  must be retained within a connector body in order to form a single or multiple optical pathways interconnect system. An interconnect system is typically formed with a plug connector and a mating receptacle connector (not shown). During mating, opposing optical termini are brought into direct end face contact with one another and the optical fiber (shown in phantom lines in  FIG. 1 ) positioned within each terminus are optically coupled together. When mating of the optical termini is properly implemented, a very low optical loss interconnection is formed. When utilizing termini of the present invention, arrays of very dense, very high performance optical interconnect solutions can be formed. 
   The terminus assembly  10  is retained within a connector housing (not shown) through the interaction between the protrusion boss  16  on sliding collar  14  and structure of the connector housing. Retention is achieved when the terminus assembly  12  is installed into a principally cylindrical bore or terminus cavity within a connector housing or body. The terminus cavity has two or more primary diameters. A smaller, forward diameter generally approximates the diameter of the ferrule  20  and is smaller than the diameter of the leading section  52  of forward section  22  into which the ferrule is pressed. The largest diameter in the terminus cavity is adjacent the rear of the connector and this diameter is slightly larger than the diameter of the main body  48  of the sliding collar. In the embodiment shown, the sliding collar has a full periphery precision shoulder  50  that interacts with the rear bore diameter to provide very precise alignment of the sliding collar with respect to the rear bore diameter of the terminus cavity. This is desirable to maintain axial alignment of the entire optical termini assembly  10  relative to the axis of the terminus cavity. Other methods of precision alignment may be feasible such as multiple raised sections or a precision machined main body for the sliding collar. 
   In the preferred embodiment, the rear opening of the bore in the terminus cavity has a slot extending from a rear face of the housing along an edge of the bore a relatively short distance into the terminus cavity. An arcuate recess extends along an arc from the slot with the arc being formed about the central axis of the cavity and principally perpendicular to the slot. This arcuate recess forms a turning section adjacent the slot that extends generally at a right angle to axis A. A small recess is added at the end of the arc in a direction parallel to the central axis of the cavity for receiving the protrusion boss  16  of sliding collar  14  to secure the terminus assembly  10  in the housing as described below. 
   During assembly, the terminus assembly is retained within the housing by positioning the terminus assembly at the rear of the terminus cavity with protrusion boss  16  and the slot aligned and moving terminus assembly  10  along the central axis of the cavity by gripping or engaging the sliding collar with an appropriate tool (not shown). This forward movement continues until the front or forward edge or shoulder  52  of the inner terminus body engages the forward wall of the smaller diameter bore in the terminus cavity. The ferrule  20  will be extending through front face of the terminus cavity bore and positions the terminus assembly  10  to substantially a central location along the terminus cavity so that the central axis of the cavity and the central axis A of the terminus assembly coincide. When front edge  52  of the terminus inner body  12  engages the front face in the terminus cavity, forward movement of inner body  12  is stopped. By continuing to apply force to sliding collar  14 , collar  14  continues to move forward relative to terminus inner body  12  and, thus, also compressing spring  18  that is an integral part of terminus assembly  10 . The protrusion boss  16  on the terminus collar  14  is aligned with the slot in the wall of the terminus cavity and passes along it until it reaches the end of the slot. Preferably, the opposing arms  42  of collar  14  and hexagonal indexing section  40  are dimensioned so that arms  42  still engage indexing section  40  when protrusion boss  16  reaches the end of the terminus cavity slot. Through such structure, the tuning of terminus assembly  10  is not affected or changed during insertion of the assembly into the terminus cavity. 
   Once protrusion boss  16  abuts the end of the slot, the collar  14  and entire terminus assembly  10  are rotated together about the axis of the terminus cavity with protrusion boss  16  traveling through the arcuate slot until the protrusion boss  16  engages the end wall of the arcuate slot. As force is released from the collar  14  by a technician, spring  18  provides a force that pushes collar  14  axially rearward so that protrusion boss  16  enters the recess at the end of the retention arc to retain the protrusion boss. This spring force maintains the terminus assembly  10  both radially and axially in the terminus cavity bore and hence the connector assembly. In other words, the orientation of the terminus assembly is retained in a predetermined position since the position of collar  14  is determined by the location of the retention arc, and the terminus inner body  12  is fixed relative to collar  14  by the indexing features, as described above. In industrial vernacular, the terminus retention system described above is known as a “quarter turn” fastener, although in the present embodiment, the quarter turn fastener is modified in that only a single protrusion boss  16  is used. In addition, the single protrusion boss  16  is what enables tuning of the optical connector system. 
   The present invention incorporates an optical ferrule holding structure  10 , termed the optical terminus assembly and a support structure, termed the connector. The connector has an optical terminus cavity for each channel in a single or multiple channel connector system. The cavity has a “key” feature that identifies positional location for proper tuning by aligning the protrusion boss  16  feature on the sliding collar  14  of opposing termini to be in-line. In this manner, by establishing eccentricity compensation relative to the protrusion boss, the relative eccentricity of two mating ferrules will be minimized and the resulting optical loss likewise minimized. Further, according to the present invention, by properly positioning the boss and retaining it within the connector body, the entire assembly can retain its eccentricity compensation even when the fiber support structure or terminus  10  is removed from the connector body. 
   Since retaining eccentricity compensation is a key feature of the disclosed invention, it is important to understand the eccentricity issues. Alignment variations between a pair of interconnected ferrules  20  are principally attributable to the parameter known as “eccentricity” of the optical fiber core with respect to the ferrule. Eccentricity is defined as the distance between the longitudinal centroidal axis of the ferrule at an end face of the ferrule and the centroidal axis of the optical fiber core held within the passageway of the ferrule. Generally, the passageway is not exactly concentric with the outer cylindrical surface that is the reference surface. Also, the optical fiber may not be exactly centered within the ferrule passageway and the fiber core may not be exactly concentric with the outer surface of the fiber. Hence, the eccentricity is comprised of the eccentricity of the optical fiber within the ferrule passageway and the eccentricity of the passageway within the ferrule. 
   If one could view the end portion of a “lit” optical fiber, what would be seen is a circle with a dot of light somewhat displaced from the exact center of the circle. Eccentricity can be understood as a two-dimensional vector having magnitude and direction components. The “magnitude component” of the eccentricity vector is the straight line distance between the center of the circle and the dot of light, while the “direction component” of the eccentricity vector is the angle made by that straight line with respect to the X-axis of a 2-dimensional Cartesian coordinate system whose origin is at the center of the circle. It is noted that ferrules used in conventional optical connectors (i.e., ST, SC and FC) have a 2.5 mm diameter while the ferrule disclosed in a preferred embodiment of the present invention has a diameter of 1.25 mm as utilized by the LC connection system. With the use of the smaller ferrule, the magnitude component of the eccentricity vector is proportionally reduced and thus precision is improved. 
   Rotating one of two interconnected ferrules typically changes the relative position of the fibers held within their passageways because of the eccentricity of the optical fiber cores with respect to the ferrules. Because it is very difficult to control the eccentricity of the optical fiber core in the ferrule in which it is terminated, it is difficult to achieve desired losses of 0.1 dB or less in single mode fibers without maintaining close tolerances so that the opposed cores are aligned to within about 0.7 microns. This scale of precision increases the manufacturing cost. If the total eccentricities of the two optical fiber ends to be joined are identical, or at least very nearly so, then a low-loss connection can be achieved by merely rotating, within the collar  14 , one ferrule  20  with respect to the other, until maximum coupling is observed (minimum insertion loss). 
   The present invention enables fiber eccentricity to be compensated through the use of an indexing slot (between arms  42 ) in the terminus assembly. The terminus assembly is designed such that it can be configured with one of six (hex) rotational positions relative to a master indexing key (protrusion boss  16  on the sliding collar  14 ). More or fewer registration features may be used. The key is an integral part of the sliding collar and although the preferred embodiment uses only one key, one or more keys may be used so long as unique orientation identification is retained. 
   Such a design enables the terminus assembly  10  to be installed in a connector body in one of six rotational positions (0 degrees, 60 degrees, 120 degrees, 180 degrees, 240 degrees, 300 degrees). The particular position selected is determined during fabrication of the connector by measuring fiber eccentricity, rotating the main body  12  relative to collar  14  by an amount based on optical power loss minimization measurement. The final requirement for a high optical performance connector is to align the terminus assembly to a specific location when installed into the connector body. As has been described above, this is accomplished by using a slot in the terminus cavity. When mated connectors are brought together, their structures both provide for the retention of orientation relative to the opposing optical terminus assemblies. 
   Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art, and consequently, it is intended that the claims be interpreted to cover such modifications and equivalents. The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.