Abstract:
A fiber optic cable suitable for blowing into and through ducts containing pressurized gas, such as natural gas, has the fibers therein arranged in arrays that form longitudinally extending vents. The vents are blocked from transmission of any gas by each being filled with a compliant member which assumes the cross-sectional shape of the vents thereby blocking it. 
     In a second embodiment of the invention, the first and third ferrules are fixed within the housing with their offset bores in alignment. A second ferrule is interposed between the first and third ferrules and is mounted in a rotatable member for misaligning the bore in the second ferrule with the bores in the first and third ferrule to produce attenuation of a signal passing through the ferrules by misaligning the fibers contained in the bores.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of copending U.S. utility application entitled, “Variable Attenuator for Optical Fiber Applications and Method of Making,” having Ser. No. 10/232,078, filed Aug. 30, 2002 of Jones, et al, which is entirely incorporated herein by reference. 
     RELATED APPLICATIONS 
     This application deals with subject matter similar to that in U.S. patent application Ser. No. 10/061,601 of Robert Holman, et al, filed Jan. 31, 2002, the disclosure of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to end-to-end connection of optical fibers, and more particularly, to a variable attenuator for use in such connections. 
     BACKGROUND OF THE INVENTION 
     Fiber optics has become, in most cases, the preferred mode of signal transmission, especially where the signals are in the higher frequencies. Optical fibers produce greatly increased bandwidth over conventional electrical conductors, and are relatively immune to ambient conditions that can disrupt electrical signal transmission. The numerous advantages of optical fibers come with a price, however. Whereas with electrical conductors, splicing may be had by simply butt welding, soldering, or otherwise joining the ends of the conductors being spliced, such is not presently possible with optical fibers, which have a diameter of, for example 125 microns and a core diameter of from 6 to 30 microns, and, in a satisfactory splice, must permit light transmission with a minimum of insertion loss while providing a stable junction. As a consequence, there has been a constant and ongoing effort to achieve, in a connector splice arrangement, an alignment of the butting ends of the fibers to minimize insertion loss. In addition to alignment, the width of the gap between the fiber ends and the surface condition of the ends are factors which must be considered in reducing or minimizing loss. The prior art arrangements are, for the most part, dependent upon or directed toward the centering of the fiber cores, and in most instances, the fibers are contained in ferrules which have centered fiber containing bores therein. On the other hand, in U.S. Pat. No. 4,544,234 of DeVeau et al., the fibers are contained in a slotted tube and centered prior to their being cemented in place within the slot. Such an arrangement does not require centering of the bores of the ferrules, but it does not permit relative movement between the fibers for optimum transmission after the fibers are cemented in place. 
     In U.S. Pat. No. 4,691,986 of Aberson et al., the disclosure of which is incorporated herein by reference, there is shown an arrangement wherein alignment of the fiber containing bores of plugs or ferrules is achieved, thereby obviating at least to a large extent alignment of the fiber cores. The invention of that patent involves the use of “contiguous” plugs derived from contiguous segments of tubular stock, with the contiguous ends being the mating ends, with the rotational relationship between the plugs being that that existed prior to cutting or sectioning of the tubular stock. With such an arrangement, the fiber containing bores are automatically aligned. It doesn&#39;t matter if the bores are slightly eccentric relative to the outer diameter of the plugs, they will still be aligned, thereby enabling alignment of the fiber cores. The invention of that patent is usable in almost any of the large variety of prior art connectors wherein the fibers are contained within ferrules or plugs. 
     The foregoing is directed to the general problems inherent in most fiber connectors of achieving proper fiber alignment in connectors or splices. These problems also exist in various optical fiber components, such as, for example, variable attentuators. In U.S. Pat. No. 4,986,627 of Boscher et al., there is shown a variable attenuator arrangement that has abutting ferrules having substantially identical outer diameters, but with fiber containing bores which are eccentric with respect to the outer diameters. The ferrules are contained in a V-block, with one of the ferrules being affixed thereto. The bores, each of which has a diameter substantially the same as the outer diameter of the fiber contained therein, are rotatable relative to each other to vary the alignment of the fibers relative to each other and thereby to vary the attenuation by increasing or decreasing the offset between the fiber cores. Such an arrangement requires, for optimum performance, that, at least one rotational position, the ferrule bores are substantially aligned, thereby aligning the fiber cores at least to the extent possible with whatever eccentricity may exist for the fiber cores themselves. The Boscher et al. arrangement produces an attenuation coefficient which varies between 3 and 60 dB for fibers which are off center by 7 to 10 microns (μm) for single mode fibers. For multimode fibers, the offset from center is from 30 μm to 50 μm. 
     In addition to arrangements such as shown in Boscher et al, other prior art variable attenuators make use of air gaps and/or angled ferrule (and fiber) ends. Ideally, a variable attenuator should provide the needed or desired attenuation, should have low return loss, and should have stable performance in high power systems and should comprise a single unit that can be inserted within a transmission system between two fiber ends. 
     SUMMARY OF THE INVENTION 
     The present invention, hereinafter shown as embodied in a variable attenuator, overcomes or obviates many of the shortcomings of the prior art, as enumerated hereinbefore, while providing reliable desired variations in the attenuation of signals passing therethrough. It should be recognized that in many instances some level of attenuation may be desirable in order to achieve a power balance among several related transmission lines rather than simply a maximum signal power throughput. 
     The variable attenuator embodying the principles of the invention comprises a fiber connector having a ferrule or glass capillary mounted in a stationary holder and a butting ferrule mounted in a rotatable holder. Glass or similar material ferrules have distinct advantages over ceramic or other material ferrules. A glass ferrule spreads the energy over a large area, thereby preventing energy absorption at the ferrule end face. For high power applications this directing energy away from the end face prevents long term reductions in performance due to heat concentration. The centerlines of the two, substantially identical, ferrules are aligned, and each ferrule has a fiber containing bore extending therethrough, the bores being offset from the ferrule centerline by equal amounts, approximately 35 μm to 40 μm. The ferrules are contained in barrel members and their distal ends are spaced from each other by a gap of approximately 5 μm to 15 μm, which is filled with an index matching material. The ferrules are contained in a sleeve of suitable material such as ceramic or metal whose ends butt against the two ferrule containing barrels and which aligns the centerlines of the two ferrules. The sleeve length is the determinant for the width of the gap when the ferrules are completely inserted therein. 
     Prior to final assembly, the ferrule end faces are polished, either normal to or at an angle to the centerline thereof and the barrels function as stops for the polishing mechanism, thereby insuring that the front face of each ferrule is a desired distance from the front face of the barrel. Thus, the barrel and sleeve together provide a consistent gap which may be normal to or at an angle to the centerline between the distal ends of the ferrule in the production of the attenuators, insuring consistent results among the several attenuators thus produced. 
     In assembly of the barrels, ferrules, and sleeve, virtual identity of the ferrules is assured by mounting a glass rod on proper diameter and having an offset bore within and extending between the barrels. The rod is then cleaved at the middle thereof, creating two, substantially identical ferrules. The barrels are keyed in a housing against accidental rotation, so that when each barrel/ferrule assembly, when removed from the housing and ferrule distal ends are polished, is replaced in the housing, and the offset bores are in approximately perfect alignment. 
     One of the ferrules/barrel assemblies is made rotatable with respect to the other by suitable rotating means. Initially, with both barrels keyed in the housing, the offset bores are aligned, as are the ends of the fibers contained therein, and there is substantially no attenuation (0 dB) present in the transmission line into which the attenuator is inserted. Rotation of the rotatable barrel moves the fiber ends out of registry, thereby attenuating the signal within the transmission line. In practice, it has been found that approximately 30° of rotation away from alignment produces approximately 60 dB of attenuation. Thus, the attenuator is capable of producing a range of attenuation from 0 to −60 dB. 
     In a second embodiment of the invention, first and second ferrules and barrel assemblies containing aligned glass fibers extend from either end of a two part housing member toward the center thereof and are keyed therein to prevent any rotation relative to each other. Between the end faces of the ferrules is a third ferrule having two polished ends mounted to a rotatable member. Each of the two parts of the housing, which are latched together by suitable means, has a wall portion from which extends toward the stationary ferrules a sleeve housing member which contains a sleeve which abuts a planar surface of the rotatable member. 
     Each sleeve contains one end of the ferrule affixed to the rotatable member and a butting end of the corresponding stationary ferrule. The rotatable member, when rotated, rotates the ferrule affixed thereto which can thereby be positioned to align the fibers in all of the ferrules to yield maximum signal transmission. When the rotatable member is rotated, the fiber in its ferrule becomes misaligned to produce attenuation of the signal in the fibers in the same manner as in the first embodiment, but only a very slight rotation is necessary to produce any equivalent amount of rotation. Moreover, the rotatable member can be rotated as a thumb wheel and does not require any external mechanism for producing the rotation. The ferrules fit within the sleeves including the rotatable ferrule with sufficient tightness to maintain the position to which the rotatable ferrule is moved. 
     Such an arrangement, in addition to not requiring an external rotating mechanism produces accurate results with only slight rotations of the “thumb wheel” and is particularly amenable to use in the field. 
     These and other features of the present invention will be readily apparent from the following detailed description, read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic elevation view of the variable attenuator of the invention; 
     FIG. 2 a  is a diagrammatic end view of a ferrule showing the offset of the fiber containing bore from the ferrule centerline; 
     FIG. 2 b  is a side elevation view of a ferrule blank prior to cleaving to make two substantially identical ferrules; 
     FIG. 3 is a partial diagrammatic view of the ferrule/sleeve/barrel assembly; and 
     FIG. 4 is a graph illustrating the performance of the attenuator of the present invention; 
     FIG. 5 a  is a side view and FIG. 5 b  an end view of a ferrule blank for forming three aligned ferrules, with an offset bore; 
     FIG. 6 is a cross-sectional view of the second embodiment of the invention; and 
     FIG. 7 is an end view of the embodiment of FIG.  6 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a diagrammatic elevation view of the invention as embodied in a variable attenuator  11  which is a stand alone unit for insertion into a transmission line between connector terminated buffered optical fibers  12  and  13 . Fibers  12  and  13  are coupled into unit  11  by connector and adapter combinations  14  and  16  respectively, which are shown diagrammatically as blocks, intended here to be representative of any of a number of such combinations known in the art, and which, preferably, are mounted in the ends  10  and  15  of an outer housing  17 , which may be of any suitable material such as metal or plastic. 
     The attenuator assembly comprises a fixed rotary splice housing or barrel  18  which terminates in a capillary or ferrule  19  which contains the end of fiber  12  and which is ground and polished to a flat surface  21  approximately normal to the centerline of the attenuator  11 , as shown, but which may be at an angle thereto. A movable rotary splice housing  22  is axially aligned with housing  18  and terminates in a second capillary or ferrule  23  which contains the end of fiber  13  and which is ground and polished to a flat surface  24  which abuts surface  21  preferably over the entire area thereof. Ferrules  19  and  23  are preferably made of glass. The ferrules or capillaries  19  and  23  are preferably covered and held in alignment, especially in the junction region of end faces  21  and  24 , by a sleeve  26  of metal or other suitable material which, preferably, is a tight slip fit thereon. The significance and function of the sleeve will be discussed more fully with reference to FIG.  3 . The attenuator assembly as thus far described is contained in a housing  27  having a first, stationary part or member  28 , mounted to outer housing  17  by any suitable means such as brackets  29 ,  31 , and a movable part or member  32  which is rotatable with respect to member  28 . Rotary splice housing  18  is prevented from rotating within member  28  by a key  33  thereon, which fits in a keyway  34  in member  28 . It is to be understood that other means for preventing rotation of member  18  might readily be used, that shown here being one alternative. Movable member  32  likewise has a similar keyway  36  into which a key  37  on rotary splice housing  22  fits. Member  32  is rotatably attached to member  28 , and, when rotated with respect thereto, causes splice housing  22  to rotate also, and as a consequence, capillary or ferrule  23  to rotate relative to capillary or ferrule  19 . In order that the faces  21  and  24  of the ferrules  19  and  23  be protected from damage during rotation they are separated by a gap  20  filled with index matching materials  25 . The index matching material  25  may be in the form of a gel, a film, or a plastic wafer, for example. A coil spring  41  is housed in an enlarged portion  42  of member  32  and bears against an enlarged portion of housing  22  and the rear wall  43  of member  32  as shown. The dimensions of the spring  41  and the enlarged portion  42  of member  32  are such that the spring  41  is under compression when the apparatus is assembled, thereby holding the front face of housing  22  firmly against the end of sleeve  26 . 
     Rotation of portion or member  32  is by means of an enlarged control wheel  46  which forms part of member  32  and which penetrates to the exterior through a slot  47  in outer housing  17 . Rotation of the wheel produces rotation of member  32  and therefore, of housing  22  and ferrule  23 . Wheel  46  may be a knurled wheel or, for example, a spur gear or worm driven gear and can be rotated manually or by a driving wheel, gear, or worm  48  driven by a control member  49 , preferably a motor. Inasmuch as very small displacement of the wheel  46  will produce substantial variation in attenuation as shown in FIG. 4 it is preferable, although not necessary, that a gearing arrangement for moving wheel  46  be used. 
     FIGS. 2 a  and  2   b  are diagrammatic views of a glass rod  56 , having a diameter D of approximately 2.49±0.005 mm from which the ferrules or capillaries  19  and  23  are made. Rod  56  has a bore  57  extending therethrough of a diameter A of approximately 126-129 microns which is displaced from the centerline of rod  56  a distance B of approximately 35-40 microns, and which is parallel to the centerline of rod  56 . It is to be understood that the dimensions given herein are preferred for an attenuator for use with single mode fibers having a diameter of 125 microns, but it is to be understood that the values may be called for with different fiber sizes. 
     In fabricating the attenuator  11 , the glass rod  56  is mounted between and affixed to splice housings or barrels  18  and  22 , which, in turn, are keyed to their housings  28  and  32 . A notch  58  is formed in rod  56  at the midpoint of the length thereof and the glass rod  56  is broken or cleaved at this point. The splice housings or barrels  18  and  22  at this point hold rough ferrules that are substantially identical, with fully mating bores  57  which also defines the zero attenuator position of the keys. It has been found that ceramic or other material ferrules are not amenable to the breaking and polishing for producing the desired results. 
     Each splice housing or barrel is then removed and the end face (distal end) of its capillary or ferrule is polished. The splice housing or barrel serves as a stop for the polishing mechanism, so that each ferrule has its distal end face spaced from the front face of the distance which is the same for both ferrules. While the polished end faces shown as being normal to the centerline, they can be at an angle thereto so long as the angles are complementary, i.e., the same, to produce a gap of uniform width. After the polishing step, the fiber is inserted into the bore  57  and affixed thereto, with the fiber end being flush with the end face of its ferrule. Some polishing of the fiber end to make it flush is generally necessary. 
     The ferrules  19  and  23  are then inserted into sleeve  26 , with the front faces of barrels or splice housings  18  and  22  butting against the ends of sleeve  26 . The length of sleeve  26  is so chosen that there is a gap  20  between their end faces, which is filled with index matching material  25  such as a matching gel. With the foregoing assembly, with keys  33  and  37  in their respective keyways  34  and  36 , the offset bores  57  in the ferrules are aligned, as are the fiber therein. In operation, rotation of member  46  and hence, housing  22 , produces rotation of end face  24  of ferrule  23  relative to end face  21  of ferrule  19 . The gap  20  and matching gel  25  prevent such rotation from causing damage to the end faces. In FIG. 3 the barrel  18 ,  22  and sleeve  26  arrangement is shown whereby sleeve  26  governs the width of the gap, which, desirably, is from 5 μm to 15 μm. The ends of sleeve  26  butt against the front faces of the barrels  18  and  22 , thereby preventing the end faces  21  and  24  from contacting each other. If, in the initial assembly, it is found that the gap  20  thus formed is too wide, it is reduced by grinding one of the ends of sleeve  26  until the desired gap width is achieved. The spring  41  bears against barrel  22  to force it against the sleeve  26 , which is, as a result, forced against the front face of barrel or housing  18 . 
     With the width of gap  20  thus set within the range of 5 μm to 15 μm, and it is filled with an index matching material  25 , such as silica gel, barrel  22  can then be rotated without damaging end faces  21  and  24  to vary the attenuation in the transmission system, comprising, in the present embodiment, fibers  12  and  13 . The results of the rotation are shown in FIG. 4, curve β, as contrasted with the rotation of the ferrules in the aforementioned U.S. patent application Ser. No. 10/061,601 as shown in curve α. It can be seen from these curves that the present invention, as represented by curve β produces the same amount of attenuation, starting at 0 dB, in 45° of rotation as does the prior apparatus in approximately 160° of rotation. Furthermore, because of its unique structure, the attenuation of the invention has a range of from 0 dB to −65 dB, which is reached in approximately 75° of rotation, as compared to the prior attenuation which has a range of from approximately 0 dB to −38 dB. 
     A second embodiment  60  of the present invention is shown in FIG. 6, which utilizes three aligned glass ferrules  61 ,  62 , and  63 . The ferrules are formed in the same manner as shown in FIGS. 2 a  and  2   b , except, as shown in FIGS. 5 a  and  5   b  the glass rod  64  is notched at  66  and  67  to form three sections  61 ,  62 , and  63 , which, as described previously, become the ferrules of the same member. 
     The attenuator  60  comprises a housing  68  having two sections  69  and  71  which, after assembly, are latched together by suitable means, shown in block form in FIG. 7 as members  72  and  73 . At the distal ends  74  and  76  of the sections are mounted adapters and/or connectors  77  and  78 , to which incoming and outgoing fibers  79  and  81  are connected. It is to be understood that the connectors and/or adapters  77  and  78  may take any of a number of forms known in the art to accommodate the terminated ends of the fibers. Extending into housing section  69  from connector/adapter  77  is a barrel/ferrule assembly  82  having at its proximal end an enlarged portion  83  within which ferrule  61  is affixed. Portion  83  has a key  84  formed thereon which fits into a keyway  86  mounted within section  69 , as shown, and ferrule  61  extends from portion  83 . In the proximal end of section  69  is mounted a wall member  87  from which extends toward the ferrule  61  a sleeve housing member  88 . Wall member  87  may be integral with section  69  or it may be a separate member mounted within and affixed to section  69 , in which case shoulders  89  and  91  serve to locate wall  87  longitudinally. 
     Section  71  of housing  68  is substantially identical to section  69 , having a barrel/ferrule member  92  having an enlarged portion  93  having a key  94 ; a keyway  96 ; a wall  97 ; and a sleeve housing member  98 . 
     As was the case in the first embodiment, enlarged portion  83  and  93  which are bored to receive the glass sections  61  and  63  are mounted thereto with their keys  84  and  94  aligned. The rod  64  is then cleaved at the notches  66  and  67  and the end faces of sections  61  and  63  are then polished. As a consequence of the foregoing, the offset fiber holding bores  99  and  101  are, when the keys  84  and  94  are fitted in their respective keyways, substantially exactly aligned. 
     Section  62  has mounted thereon a rotatable member  102  and is located between the ends of ferrules  61  and  63  as shown, with the rotatable member  102  protruding from slots  103  and  104 , as best seen in FIG.  7 . Thus, as assembled, ferrule  62  is rotatable while ferrules  61  and  63  are stationary, being held in place by the keyways and members  77  and  78 . The end faces of the ferrules are separated slightly and the gaps  106  and  107  then formed are filled with an index matching material  108  which protects the end faces from damage when ferrule  62  is rotated. The material  108  may be, for example, a gel, a film, or a plastic wafer. The ferrules are inserted in sleeves  105  which, as in the embodiment of FIG. 1, determine the width of the gaps  106  and  107 . 
     Rotation of member  102 , and hence, ferrule  62  functions in much the same manner as the rotation of the ferrule  23  by wheel  46  in the embodiment of FIG.  1 . However, there results a two stage attenuation, i.e., the attenuation resulting from misalignment of bores  99  in ferrule  61  and bore  100  in ferrule  62 , and the misalignment of bore  100  with bore  101  in ferrule  63 . As a consequence, only small rotations of member  102  produce larger amounts of attenuation than the arrangement of FIG.  1 . It is not necessary, therefore, to have any rotation drive means, as shown in FIG. 1, inasmuch as the desired rotation of member  102 , and hence, the desired amount of attenuation can be accurately achieved by hand in which case member  102  may be regarded as a thumb wheel. Inasmuch as the end ferrules  61  and  63  are not rotatable, the device of FIGS. 6 and 7 can be used in a splicing setup wherein the cables do not rotate, or as a connection to an adapter apparatus such as, for example, a circuit board. 
     From the foregoing, it can be seen that the variable attenuators of the invention produces a wide range of attenuation, extending from zero attenuation, in a relatively small amount of rotation and comprises a relatively simple, reproducible unit adaptable to a wide range of transmission arrangements. 
     It is to be understood that the various features of the present invention might be incorporated into other variable attenuator mechanism, 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 in the claims. 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.