Abstract:
An apparatus for tuning PM optical fiber connections has a first assembly including a light source which is connected to a first coupling stage having a rotatable polarizer, and a second assembly having a second coupling stage and a rotatable polarization analyzer for directing light transmitted through a terminated PM jumper cable coupled between the first and second coupling stages to a power meter. The output of the power meter is applied to a processing unit which, in turn, controls a rotation arrangement for the polarizing and analyzer. Crosstalk of this jumper is determined by ascertaining the angular positions of the maximum and minima outputs of the rotating analyzer, and the terminating connectors of the jumper are turned to the maximum value by aligning with a connector key. The process similarly applies to the connections between two jumper cables, with the alignment of the slow wave vectors thereof having achieved by determining the difference in the angular positions of the maximum output, and determining therefrom the positions of the reference keys of both connectors.

Description:
RELATED APPLICATIONS  
       [0001]    This application is related to U.S. patent application Ser. No. 09/811,074 filed Mar. 16, 2001 the disclosure of which is incorporated herein by reference, and to U.S. patent application Ser. Nos. ______ (Lampert et al. 56-2-13-25) and ______ (Lampert et al. 57-3-14) filed concurrently herewith. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to connectorizing and tuning polarization maintaining (PM) optical fibers.  
         BACKGROUND OF THE INVENTION  
         [0003]    In optical fiber communications, connectors for joining fiber segments at their ends, or for connecting optical fiber cables to active or passive devices, are an essential component of virtually any optical fiber system. The connector or connectors, in joining fiber ends, for example, has, as its primary function, the maintenance of the ends in a butting relationship such that the core of one of the fibers is axially aligned with the core of the other fiber so as to maximize light transmissions from one fiber to the other, or, put another way, to reduce insertion loss. Another goal is to minimize back reflections. Alignment of these small diameter fibers is extremely difficult to achieve, which is understandable when it is recognized that the mode field diameter MFR of, for example, a singlemode fiber is approximately nine (9) microns (0.009 mm). The MFR is slightly larger than the core diameter. Good alignment (low insertion loss) of the fiber ends is a function of the transverse offset, angular alignment, the width of the gap (if any) between the fiber ends, and the surface condition of the fiber ends, all of which, in turn, are inherent in the particular connector design. The connector must also provide stability and junction protection and thus it must minimize thermal and mechanical movement effects.  
           [0004]    In the present day state of the art, there are numerous, different, connector designs in use for achieving low insertion loss and stability. In most of these designs, a pair of ferrules (one in each connector), each containing an optical fiber end, are butted together end to end and light travels across the junction. Zero insertion loss requires that the fibers in the ferrules be exactly aligned, a condition that, given the necessity of manufacturing tolerances and cost considerations, is virtually impossible to achieve, except by fortuitous accident. As a consequence, most connectors are designed to achieve a useful, preferably predictable, degree of alignment, some misalignment being acceptable.  
           [0005]    However, in connecting or terminating polarization maintaining (PM) fibers, such is not the case. Many optical fiber components, such as, for example, interferometers and sensors, lasers, and electro-optic modulators, are extremely sensitive to and dependent upon, for proper operation, the polarization of the light. Even very slight alterations or changes in the light polarization orientation can result in wide swings in the accuracy of response of such devices. PM fiber has polarization-dependent refractive indices, and the speed of light in an optical fiber is inversely proportional to the magnitude of the refractive index. A birefringent optical fiber is one having two polarizations having different velocities of propagation, thus giving rise to a “fast” wave and a “slow” wave. In a PM fiber, the polarization of a linearly polarized light wave input to the fiber, with the direction of polarization parallel to that of the one of the two principal polarizations, will remain or be maintained in that polarization as it propagates along the fiber, hence the term “polarization maintaining.” If the polarization of the light wave is to be maintained at a splice or other connection, the principal axes of birefringence of the two joined fibers must be aligned in parallel, otherwise there will be polarization crosscoupling, i.e., crosstalk, which is highly undesirable. Thus, where two PM fibers, for example, are to be connected together, they should be terminated carefully to reduce the crosstalk during the connectorization process. Also, the connectors must be capable of aligning then maintaining the fiber orientation to the connector key position. Connectors with tolerances adequate for connecting non-PM fibers usually are inadequate for maintaining polarization alignment at the connector junction.  
           [0006]    Typical PM connector requirements are an insertion loss of less than 0.3 dB, and the prior art PM connector arrangements comprise numerous, different connector configurations aimed at meeting these requirements for different connectors, such as an SC type connector as shown in U.S. Pat. No. 5,216,733 of Ryo Nagase et al. The connector of that patent comprises a ferrule body and a ring shaped flange having a keyway mounted on the periphery of the ferrule body. Alignment is achieved by rotating the ferrule body with respect to the flange keyway. The combination of ferrule and flange comprises a plug which is inserted into a push-pull SC connector having a key therein for mating with the flange keyway and springs bias the flange in the longitudinal direction to maintain the alignment.  
           [0007]    In U.S. Pat. No. 4,784,458 of Horowitz, a splice joint for PM fibers is shown wherein aligned fibers are joined with UV curing epoxy, and the joint is overlaid with epoxy cement for rigidity. Such a joint is permanent, and does not function as a connect-disconnect optical fiber connector.  
           [0008]    U.S. Pat. No. 5,561,726 of Yao discloses an apparatus for controlling the polarization state of the light within a fiber by squeezing a portion of the fiber to produce a birefringent fiber, and the squeezer is then rotated to change the polarization of the light within the fiber. The device is not a connector, but is intended for use with polarization sensitive devices such as interferometers and electro-optic modulators, however, it may also be used with connectors for connecting two PM fibers.  
           [0009]    It is common practice in the prior art for creating PM fibers to include a pair of rods in the fiber cladding which extend parallel to the core as shown in U.S. Pat. No. 4,515,436 of Howard et al. Such rods, which are preferably of glass, are, in manufacture of the fiber, included in the fiber preform from which the fiber is drawn. As the fiber is drawn, the rods are accordingly diminished in diameter and are located within the cladding, preferably on either side of the core. The rods have different thermal expansion characteristics than the surrounding glass, and the stress they exert on the core causes the index of refraction to change along that axis. The axes then have different indices of refraction value and thus propagate light at different speeds. Variations on the two rod arrangement are also known, such as the elliptical stress member disclosed in U.S. Pat. No. 5,488,683 of Michal et al. Also, squeezing the fiber to create birefringence, as shown in the aforementioned Yao patent is feasible. The two rod PM fiber, so called “Panda” type PM fiber, however, has proven quite satisfactory in use, and it is toward the connectorization of such a fiber that the present invention is directed, although other types of PM fibers may be used with the present invention.  
         SUMMARY OF THE INVENTION  
         [0010]    In the copending U.S. patent application Ser. Nos. ______ (Lampert et al. 56-2-13-25) and ______ (Lampert et al. 57-3-14) ______ are shown, respectively, a PM connector plug and an adapter therefor the principles of which are applicable to any of a large number of optical fiber connectors, but are embodied in a modified LC connector in those applications. For optimum performance, i.e., maximum transmission of a polarized beam, it is highly desirable to provide accurate rotational positioning of better than ±1° or even as accurate a &lt;¼° between connectors equipped with polarization maintaining fibers.  
           [0011]    The present invention is an apparatus and method for tuning the PM connectors of those applications to achieve this desideratum.  
           [0012]    When a PM jumper cable, for example, is terminated by connectors, it is most desirable that the cable/connector combination be tuned to align the fiber slow axis with the connector key which serves as a reference point. In accordance with the present invention, there is provided a tuning apparatus for performing the tuning method of the invention which yields extremely accurate rotational positioning of the connectors.  
           [0013]    The apparatus, which is similar to that shown in TIA/EIA Standard FOTP-193, comprises a first assembly including a coupling stage comprising a light source, and a polarizer interposed between first and second connector adapters and connector plugs. A second assembly having a second coupling stage, spaced from the first coupling stage comprises a connector adapter (the second coupling stage), a power meter, the output of which is connected to a PC, and another polarizer (or analyzer). Both polarizer and analyzer can be rotated to any angle and controlled by a rotation controller connected to the PC. In use, a jumper cable, fir example, terminated by connector plugs, is inserted into the adapters in the first and second coupling stages, and the polarizer in the first stage is rotated to match the slow polarization axis of the connector, determined by the increased power reading. Linear polarized light is then launched into the slow axis of the PM fiber. The analyzer in the second stage is rotated and the output power varies between maximum and minimum, as indicated by the power meter. As will be discussed hereinafter, the crosstalk in dB is calculated as the difference between maximum and minimum power.  
           [0014]    The tuning of the PM connector is to set the PM fiber slow axis to correspond to the key position of the connector, which preferably is the connector latch or latching arm. Therefore, two joined PM jumpers can have the same slow axis alignment according to key position to minimize crosstalk due to misalignment. Once the polarization direction of the analyzer is aligned to the connector key which can be regarded as a master position, the tuning process can easily be done by matching the fiber slow axis to the analyzer direction as indicated by output light power. In order to align the analyzer to the key or master position, a pair of PM jumper cables are connected to each other and to the first and second stages, and the crosstalk of the connection is then measured, and one connector is tuned for the lowest crosstalk in the connection of two jumpers.  
           [0015]    One of the jumpers is then moved and the other is connected between the first and second stages. The angle of the maximum output is defined as zero degrees. By measuring the crosstalk, the analyzer can be aligned to the slow axis of the fiber and the analyzer position is recorded. The one jumper is then removed and the second is connected between the stages and the analyzer is also aligned to the second jumper&#39;s slow axis which generally will occur at a different angle than that of the one jumper.  
           [0016]    The position of the key will be midway between the two angles and the analyzer is rotated to the master position and thus the ferrules of the connectors are rotated to this value, which is designated as zero. At this point the slow wave orientation of the connectors is parallel to the connector key, and the connector of the PM jumpers are optimally lined. With the master analyzer position thus determined, subsequent alignment of connectors becomes a single rotation of the ferrules to conform.  
           [0017]    With the connector plug and adapter of the aforementioned Lampert, et al applications, the ferrules of the connector plug are maintained, with very slight possibility of variation, in the optimum tuned position. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is an exploded perspective view of a prior art LC type connector plug;  
         [0019]    [0019]FIG. 2 is a perspective view of the plug of FIG. 1 as assembled;  
         [0020]    [0020]FIG. 3 a  is a view of the flanged barrel member of the plug of FIG. 1;  
         [0021]    [0021]FIG. 3 b  is an end view of the barrel member of FIG. 3;  
         [0022]    [0022]FIG. 4 a  is an end view of the plug of FIGS. 1 and 2;  
         [0023]    [0023]FIG. 4 b  is a cross-sectional view of a portion of the housing of the plug of FIG. 4A;  
         [0024]    [0024]FIG. 5 is a cross-sectional view of the plug of the previous figures as assembled and terminating an optical fiber;  
         [0025]    [0025]FIG. 6 is an exploded view of an alternative form of tunable barrel member for a connector plug;  
         [0026]    [0026]FIG. 7 is a side elevation view of the assembled barrel member of FIG. 6;  
         [0027]    FIGS.  8 , and  9  are two views of the plug housing for use with PM optical fiber;  
         [0028]    [0028]FIG. 10 is a section along the line A-A of FIG. 8;  
         [0029]    [0029]FIG. 11 is a cross-sectional elevation view of one embodiment of the PM plug terminating an optical fiber;  
         [0030]    [0030]FIG. 12 is a perspective view of another embodiment of the PM plug;  
         [0031]    [0031]FIG. 13 is a perspective view of a connector adapter for use with the plug of FIG. 5, for example;  
         [0032]    [0032]FIG. 14 is a top plan view of the adapter of FIG. 13;  
         [0033]    [0033]FIG. 15 is a side elevation view of the adapter of FIG. 13;  
         [0034]    [0034]FIG. 16 is a front elevation view of the adapter of FIG. 13;  
         [0035]    [0035]FIG. 17 is a diagrammatic representation of the relationship between two PM ferrules to be connected together;  
         [0036]    [0036]FIG. 18 is a diagram of the apparatus for tuning an optical fiber jumper;  
         [0037]    [0037]FIG. 19 is a diagram of one of the steps in tuning the optical fiber jumper;  
         [0038]    [0038]FIG. 20 is a diagram (or graph) of the variation in output power as the polarizer and analyzer are rotated;  
         [0039]    [0039]FIG. 21 is a graph showing the effect of misalignment on crosstalk;  
         [0040]    [0040]FIG. 22 is the apparatus of FIG. 18 as set up to determine the master reference position;  
         [0041]    [0041]FIG. 23 is a graph resulting from the operation of the apparatus of FIG. 23; and  
         [0042]    [0042]FIGS. 24 through 27 are diagrams illustrating the several steps in establishing the master reference position. 
     
    
     DETAILED DESCRIPTION  
       [0043]    [0043]FIG. 1 is an exploded perspective view of the principal components of an LC type connector  11  as disclosed in the aforementioned US patent applications and U.S. Pat. No. 6,155,146. It is to be understood that the principles of the present invention are also applicable to other types of connectors, such as an ST, SC, or other amenable to modification to incorporate these principles. Connector  11  comprise a plug housing formed of a front section  12  and a rear section  13  having an extended portion  14  which fits into section  12  and latches thereto by means of slots  16 - 16  in front section  12  and latching members  17 - 17 . Members  12  and  13  are preferably made of a suitable plastic material. Front section  12  has a resilient latching arm  18 , having latching shoulders  20 , extending therefrom for latching the connector  11  in place in a receptacle or adapter. The arm  18  and shoulders  20  together define a latch. Rear or section  13  has extending therefrom a resilient arm or trigger  19 , the distal end of which, when the two sections  12  and  13  are assembled, overlies the distal end of arm  18  to protect it from snagging and to prevent nearby cables from becoming entangled. Usually latch arm  18  and guard  19  are molded with their respective housing sections  12  and  13 , respectively, and form “living hinges” therewith, which enable them to be moved up and down between latching and unlatching positions. Front section  12  has a bore  21  extending therethrough which, when the parts are assembled, is axially aligned with a bore  22  extending through rear sections  13 . The bores  21  and  22  accommodate a barrel assembly  23  which comprises a hollow tubular member  24  having a bore  25  extending therethrough and having a ferrule holding apparatus shown here as an enlarged flange or barrel member  26  from which extends a ferrule  27  which may be made of a suitably hard material such as, preferably, ceramic, glass, filled-plastic, or metal. Ferrule  27  has a bore  28  extending therethrough for receiving and holding an optical fiber therein. When the connector  11  is assembled, a coil spring  29  surrounds the tubular portion  24  of the assembly  23 , with one end bearing against the rear surface of flange  26  and the other end bearing against an interior shoulder in rear section  13 , as will best be seen in subsequent figures.  
         [0044]    In practice, the uncoated portion of the optical fiber is inserted into bore  28  of ferrule  27  and adhesively attached thereto. Spring  29  is compressed as the sections  12  and  13  are connected and supplies a forward bias against the rear of flange  26  and, hence, to ferrule  27 . This arrangement of ferrule  27  and spring  29  is considered to be a “floating” design. Prior to connection, the spring  29  causes ferrule  27  to overtravel its ultimate connected position. When connector  11  is connected within a suitable adapter and the distal end of ferrule  27  butts against the corresponding ferrule end of another connector or of other apparatus, spring  29  will be compressed, thereby allowing backward movement of ferrule  27  to where its end, and the end of the abutting ferrule, lie in the optical plane (transverse centerline) between the two connectors.  
         [0045]    The rear end of rear section  13  has a ridged member  31  extending therefrom for attachment of optical fiber cable and a strain relief boot, not shown. For protection of the distal end of ferrule  27  during handling and shipping, a protective plug  32 , sized to fit within bore  21 , is provided. FIG. 2 depicts the assembled connector  11  in its shipping or handling configuration.  
         [0046]    As best seen in FIGS. 3 a  and  3   b , flange  26  has a hexagonally shaped portion  33  and a front tapered portion  34  with tuning notches  35 , as shown in U.S. Pat. No. 6,155,146, which can be a tapered extension of the hexagonal portion. While the following discussion relates to a multi-faceted ferrule holding member, it is to be understood that the term “faceted” is intended to include other locating arrangements such as, for example, slots or splines, such as are shown in, for example, the U.S. Pat. No. 6,155,146 patent. As shown in FIGS. 4 a  and  4   b , front section  12  has a flange seating cavity  36  formed in a transverse wall  37  thereof which has a hexagonally shaped portion  38  and a tapered portion  39  dimensioned to receive and seat surface  34  of flange  26 . That portion  41  of bore  21  immediately to the rear of portion  38  has a diameter sufficient to allow rotation of flange  26  when it is pushed to the rear against spring  29  and disengaged from the cavity  36 . Thus, as will be discussed more fully hereinafter, when flange  26  is pushed to the rear it may be rotated and, when released, re-seated by spring  29  with tapered portion  34  acting as a guide and centering arrangement. The hexagonal configuration makes it possible to seat the flange  26  in any of six angular rotational positions, each sixty degrees (60°) apart. It has been found that a flange having fewer than six sides cannot be rotated in the assembled connector unless the diameter of bore portion  41  is increased because the diagonal of a four sided flange is too great for rotation of the flange. However, increasing the diameter of portion  41  seriously weakens the walls of the housing section  12 . Further, in the tuning of the connector it has been found that six sides gives a more accurate tuning for reduction in insertion loss. The use of a flange with more than six sides is possible, and gives an even greater tuning accuracy by creating smaller increments of rotation. However, the increased accuracy is not sufficiently great to justify the increased difficulty in achieving a stable and firm seating of the flange. As the number of flange sides is increased, the periphery thereof approaches a circular configuration, which would possibly be undesirably rotatable even when seated. As a consequence, it has been found that a six sided flange is optimum for tuning non-PM type connector plugs. For the PM type fiber connections, greater precision, including incremental control of angular orientation of the polarized fiber in the ferrule is required if optimum light transmission with polarization unimpaired or altered is to be realized.  
         [0047]    The present disclosure comprises three separate apparatuses and a tuning method for achieving optimum or near optimum polarization maintenance and transmission through the connector assembly, which comprises a connector plug and an adapter therefor.  
         [0048]    PM Connector Plug  
         [0049]    In the foregoing, the tuning process for non-PM connections is shown and discussed. The PM connector plug, which is basically a modified LC type connector plug, is shown in cross-section in FIG. 5 and comprises a plug housing  51  which includes a front section  52  and a rear section extender cap  53 , within which is contained a barrel assembly  54  having a fiber bearing ferrule  56  mounted thereto. The barrel assembly  54  has an enlarged nut, such as a hexagonal nut  57  which is a light press fit on a tubular member  58  through which the coated fiber  59  passes. Nut  57  has a sloping front surface  61  and is held in a matching seat  62 , which has a sloped surface  60  for receiving surface  61 , pressed into engagement by means of a coil spring  63 , as shown. Seat  62  is also hexagonal so that barrel assembly  54  is prevented from rotating when seated in front portion  52 . A boot  64  extends from the rear extender cap  53  in accordance with common practice. Front section  52  has a resilient latch comprising latching arm  66  and latching shoulders  20  mounted in cantilever fashion thereon, which is a feature of an LC connector plug.  
         [0050]    [0050]FIG. 6 is an exploded perspective view and FIG. 7 a  side elevation view of the barrel assembly  54  comprising the tubular member  58  in which the ferrule  56  is fixedly mounted. Member  58  has a bore  67  extending therethrough for receiving the coated fiber  59 , and the front end  68  has first and second tuning notches  69  and  71 . Nut  57  is mounted in a light press fit on the front portion  68  of tubular member  58  and butts against a stop ridge  72 . By light press fit is meant a fit such that with application of a substantial rotational torque on tubular member  58  it can be rotated with respect to nut  57 , yet the fit is tight enough that relative rotation between the member  58  and nut  57  will not occur under the forces, if any, likely to be encountered in use. Thus incremental rotation of the fiber containing ferrule  56 , which is fixed in tubular member  58 , relative to the nut  57 , may be performed. As thus far described, the connector plug  51  is substantially similar to the aforementioned Lampert et al. application. In keeping with the necessity of eliminating as much play or float as possible so that subsequent polarization tuning may be maintained, connector plug  51 , more particularly the plug housing, comprising front and rear sections  52  and  53 , is made to be a firm fit within the connector adapter, described hereinafter, which is the subject of the co-pending application Ser. No. ______ (Lampert et al. 57-3-14) filed concurrently herewith. In addition, as best seen in FIGS. 8, 9, and  10 , the latching arm  66  has a cross-section that is in the form of a truncated wedge, with the sides  73  and  74  thereof being at an angle θ of approximately four to eight degrees (4°-8°) to the vertical, as shown in FIG. 10, although other angles or angle ranges might be used. Latching shoulders  20  may be tapered, as shown in FIG. 10.  
         [0051]    In FIG. 11 there is shown a second embodiment  76  of the connector plug. For simplicity, like parts to those in FIGS.  5 - 7  bear the same reference numerals. FIG. 12 is an exploded perspective view of a portion of the plug  76 , illustrating the unique details thereof.  
         [0052]    As can be seen in FIG. 11, tubular member  54  is made with the nut  57  integral therewith, although the arrangement of FIGS. 6 and 7 may also be used. The sloped surface  61  bears against the sloped surface  60  within front section  52 , as is the case with the embodiment of FIG. 5. However, front section  52  does not have, within the bore thereof, the hexagonal seating or locating surface  62  of the embodiment of FIG. 5. Instead of the surface  62 , member  53  has extending longitudinally therefrom toward the connector end of the plug  75  three separate resilient clamping arms  77 ,  78 , and  79 , the distal end of each of which ends in a clamping pad  81 , as best seen in FIG. 12. Arms  77 ,  78 , and  79  are radially positioned 120° from each other and pads  81  have flat faces for bearing against corresponding flat surfaces on nut  57  thus forming a three-jaw collet. The diametric spacing of the pads  81  is slightly less than the corresponding faces of nut  57  against which they bear, thus insuring a positive clamping action on nut  57 . However, the resilience of the arms, which are preferably made of a suitable plastic, is such that nut  57  may be rotated with respect thereto upon application of sufficient torque. Thus nut  57 , and consequently ferrule  56 , may be rotated with respect to latching arm  66 , and clamped firmly in place after such rotation. Further, as pointed out hereinbefore, where, as in the embodiment of FIGS. 6 and 7, the nut  57  is not integral with tubular member  54 , incremental rotations of ferrule  56  with respect to latching arm  66  are possible. A circular ridge  82  surrounds the member  53  and rides in a corresponding groove  83  in front section  52  to permit relative rotation of the barrel assembly and three-jaw collet of FIG. 9 with respect to latching arm  66 , which likewise permits incremental angular position changes of ferrule  56  with respect to latching arm  66 . After the turning process, which will be discussed more fully hereinafter, ridge  82  may be cemented within groove  83  to maintain the tuned position of ferrule  56  with respect to arm  66 . While the collet chuck formed by the arms  77 ,  78 , and  79  is shown with three arms, it is to be understood that fewer arms, or more arms, may be used so long as the barrel assembly is located and firmly held in place within front portion  52 .  
         [0053]    The tapered cross-section of arm  66  in both embodiments is intended to fit within as tapered slot within the adapter, to be discussed more fully hereinafter, but can fit within a straight side slots as will inasmuch as the tapered sides of the arm  66  will engage the straight sided walls at some point, thus limiting lateral float. Thus the plug may be used in a typical LC connection as well as a PM connection.  
         [0054]    The PM connector plug as shown and described herein forms the basis of U.S. patent application Ser. No. ______ (1220-Lampert et al. 56-2-13-25).  
         [0055]    PM Adapter  
         [0056]    [0056]FIG. 13 is a perspective view of the PM adapter  86  for receiving the connector plug. FIGS. 14 and 15 are, respectively, a plan view and an elevation view of the adapter, and FIG. 16 is a front elevation view with connector inserted. The adapter  86  is depicted in the drawings as a duplex adapter, which is a common LC adapter form, but it is to be understood that the principles herein set forth may readily be used in a simplex or multiplex adapter.  
         [0057]    PM adapter  86  is basically similar to the conventional LC adapter and comprises a housing  87  made up of first and second plug receiving members  88  and  89 , each of which has a pair of openings  91  and  92  for receiving the connector plugs. Each opening has a transverse wall  93  and  94  from which project tubular ferrule receiving members  96  and  97  into which alignment sleeves  98  and  99  fit. Member  89  is constructed in the same way so that the alignment sleeves  98  and  99  are situated in the ferrule receiving members  96  and  97  in both members  88  and  89  so that the members  96  and  97  are aligned. As thus far described, housing  87  is the same as a conventional LC adapter housing. In order that PM connections may be realized, each member  88  and  89 , which are preferably made of molded plastic, has spring biasing members  101  and  102  molded in the outer walls of openings  91  and  92 , each has a pad  103  (only one of which is shown) which, when a plug is inserted into either opening  91  or  92  bears against the body thereof to produce a positive, repeatable, transverse seating of the plug. Where a simplex adapter is used, the biasing members  101  and  102  will preferably be located in the side walls opposition each other.  
         [0058]    Further repeatable location of the PM plug is produced by first and second slots  104  and  106  for receiving the latching arm  66  of the connector plug housing  51  which has a truncated wedge shape as discussed in the foregoing. To receive and seat the latching arms, slots  104  and  106  have tapered side walls, as best seen in FIGS. 13 and 16, so that, as shown in FIG. 16, latching arms  66  fit snugly therein so that virtually any and all transverse float is eliminated.  
         [0059]    As shown in FIGS. 13 and 14, PM adapter  86  is configured to be panel mounted. To this end, each of members  88  and  89  has a flange  107  thereon which, when the members are assembled together, forms a panel mounting flange. A metallic member  108  straddles member  88  as shown in FIG. 13 and is preferably affixed thereto. First and second spring locking members  109  and  111  in the form of cantilevered leaf springs extend from member  108  as shown in FIGS. 13 and 14 and bear against the back side (or front side) of the panel, shown in dashed lines in FIG. 14, thereby locking adapter  86  in place on the panel.  
         [0060]    The adapter as described herein can be used as a conventional LC adapter as well as a PM adapter, and shown and as described herein forms the basis for U.S. patent application Ser. No. ______ (1230-Lampert et al. 57-3-14) filed concurrently herewith.  
         [0061]    Tuning Apparatus and Method  
         [0062]    The following discussion is directed to measuring crosstalk in a jumper cable terminated at each end by the connector plug and adapter described in the foregoing, and to establishing a reference position of the apparatus for tuning the connectors terminating a jumper cable.  
         [0063]    As discussed at length in the foregoing, in order to maintain polarization and optimum light transmission at a connection, it is necessary to match the slow wave polarization of the jumper connector to the slow wave polarization of the receiving connector as closely as possible. FIG. 17 is a diagrammatic representation of the relationship between two ferrules  112  and  113  to be connected together is abutting relationship. Contained within ferrule  112  is a PM fiber  114  having first and second stress rods  116  and  117  and which propagates light in a slow axis X and a fast axis Y. Contained within ferrule  113  is a PM fiber  118  having first and second stress rods  119  and  121  and propagating light in a slow wave X 1  and a wave axis Y 1 . In order that there be optimum light transmission (lowest crosstalk) between fibers  114  and  118 , slow waves sectors X and X 1  should be parallel or (coincident) and aligned with reference points  122  and  123 , which may be, for example, the latching arms of the connectors in which the ferrules  112  and  113  are contained. Unfortunately, it is seldom that such an ideal alignment is obtained. It therefore becomes necessary, for optimum performance, that the connectors terminating the jumper, for example, be tuned to at least approach the optimum in performance.  
         [0064]    In FIG. 18 there is shown an apparatus  125  for tuning a terminated jumper cable  126  having PM connectors  127  and  128  as shown and described in the foregoing. A first assembly  129 , to which connector  127  is connected comprises a light source  131  connected by a connector  130  to a coupling stage  132  having a rotatable polarizer  133  interposed between connectors  130  and  127  through which the light passes to the jumper cable  126 . A second assembly  134  spaced from the first assembly  129  comprises a coupling stage  136  to which the connector  128  is connected, and a rotatable analyzer  137  through which light is directed to a power meter  138 , the output of which is directed to a processor or computer  139 . The processor  139  controls a rotation controller  141  for rotating polarizer  133  and analyzer  137 . With the apparatus set-up  125  as shown in FIG. 18, the crosstalk of jumper cable  126  can be ascertained and the connectors  127  and  128  can be tuned by the following steps.  
         [0065]    Step Ia) Light is launched on fiber  126  from light source  131  through the stage  132 .  
         [0066]    Step IIa) Linear polarized light is then launched into the slow axis of the fiber  126 .  
         [0067]    Step IIIa) Analyzer  137  is rotated as in FIG. 19 to create a transmitted power graph by means of power meter  138 . Such a graph is shown in FIG. 20 and gives an indication of transmitted power variation through a full 360° of rotation of analyzer  137 . It can be seen that the output minimum is more sensitive to angular changes than is the output maximum, which is 90° of rotation therefrom, thus it is easier to pinpoint the angle at which the minimum occurs and simply add 90° to that to determine the angle at which the maximum occurs.  
         [0068]    Step IVa) Ascertaining the crosstalk, which is a negative value, by subtracting the maximum power from the minimum power. In FIG. 20 it can be seen that the minimum power is −34:2 db at 90° and the maximum power is −3.4 db at 0°, which yields a crosstalk figure of −30.8 db.  
         [0069]    Step Va) It is possible that the output maximum may occur at a rotational angle of the analyzer  137  that differs from cable to cable. The PM connector  128  may be tuned by rotating the ferrule, as discussed in the foregoing, and the measurements of steps I-III repeated. The crosstalk will not be changed thereby, but the angular position of the maxima and the minima, as indicated by the analyzer  137 , will be. The connector can, therefore, be incrementally tuned to a setting such as 0° for the maximum, which aligns the slow wave with the connector key, the analyzer and the software of processor  139  having previously been set for a zero indication to correspond to connector key alignment with the slow wave.  
         [0070]    [0070]FIG. 21 is a graph showing variations in crosstalk with misalignment, expressed in degrees of angle. The typical crosstalk of the fiber is approximately −40 dB per meter, so the characterization of a one meter jumper with no added crosstalk from the connectors would fall on the −40 dB curve. On the other hand, where the crosstalk of the connector terminated jump is −30 dB at approximately zero degrees, the crosstalk variation with angle would fall on the −30 dB curve. With a crosstalk of −20 dB at approximately zero degrees, the variation with angle follows the −20 dB curve. Misalignment of one degree (1°) can be dramatically different, depending upon how good the jumper is. The graph thus illustrates the extreme sensitivity to turning in a connectorized jumper.  
         [0071]    In production milieu, it is desirable to establish a master position for the analyzer so that jumpers may be tuned thereto at a production rate, with the slow axis of each being aligned with the connector key. The joined PM jumpers can have the same slow axis alignment according to key position to minimize crosstalk resulting from misalignment. Once the polarization direction of the analyzer is aligned to the connector key, the tuning process can easily be performed by matching the fiber slow axis to the analyzer direction as indicated by output light power.  
         [0072]    The second stage has a keyed receptacle for receiving the keyed connector plug thus the keys are aligned with each other. The master reference position will be that position where the analyzer zero coincides with the key of the receptacle, and hence the connector key.  
         [0073]    In FIG. 22 the apparatus  125  is shown by being set up to tune the connector plugs  147  and  148  in connector adapter  142  of a pair of jumper cables  143  and  144 . Cable  143  is terminated by PM connector plugs  146  and  147  of the type disclosed hereinbefore, and cable  144  is terminated by similar PM connector plugs  148  and  149 . The steps in tuning the connectors are similar to preceding crosstalk measurement and tuning, and are as follows, (refer also to FIGS. 24 through 27);  
         [0074]    Step Ib) Measure the crosstalk of the connector as in Steps Ia, IIa, and IIIa;  
         [0075]    Step IIb) Adjust all of connectors  147  and  148  to the other for lowest crostalk;  
         [0076]    Step IIIb) Remove adapter  142  and jumper  144  and reconnect connector plug  147  second coupling stage  136 ;  
         [0077]    Step IVb) Measure the crosstalk of jumper  143  to determine the angular position of output maximum and minimum, for example, +6° for the maximum; as shown in FIG. 23;  
         [0078]    Step Vb) Replace jumper  143  with jumper  144  and reconnect connector plug  148  to second stage  136 ;  
         [0079]    Step VIb) Measure the crosstalk of jumper  144  to determine the angular position of the output maximum and minimum, for example 340° or −20°, as shown in FIG. 23;  
         [0080]    Step VIIb) The key position will be at one-half the difference between the two angles, or −7°;  
         [0081]    Step VIIIb) Adjust the analyzer, as discussed in the foregoing to −7° as indicated by the analyzer, which aligns the zero angle of the analyzer with the receptacle key. This is the master reference position. The sequence or order of jumpers  143  and  144  may be reversed, if desired or necessary. The connectors can now be tuned for optimum performance. In the subsequent production of connectorized fibers, particularly jumpers, it becomes a simple matter to tune the connectors by rotation of the ferrule to the key position, with the master reference position set as in the foregoing, thus eliminating the many steps involved in tuning PM connectors (or jumpers).  
         [0082]    In practice, the foregoing method has yielded crosstalk negative values of better than −38 db. Because of the unique configuration of the PM connector plug and the PM connector adapter, when the connector plugs are tuned in accordance with the foregoing steps, the tuning is maintained in normal usage, due to the reduced float within the connectors described in the foregoing and resistance to any accidental or unintentional change of the setting of the ferrule in the connector plug.  
         [0083]    It is to be understood that the various features of the present invention lend themselves to other types of PM optical fiber connectors, and that other modifications or adaptations might occur to workers in the art. All such variations and modifications are intended to be included herein as being within the scope of the present invention as set forth. Further, in the claims hereinafter, the corresponding structures, materials, acts, and equivalents of all means or step-plus-function elements are intended to include any structure, material or acts for performing the functions in combination with other elements as specifically claimed.