Abstract:
Apparatus and methodology for mechanically splicing two optical fibers of equal or different diameters. Instead of flat V-groove structure for holding the optical fibers to be spliced, embodiments of the present invention use a concave-walled channel to better align two optical fibers if they have different diameters. The cover holding the fibers in place in the concave channel is similarly curved. The improved alignment results in more area overlap between end surfaces of the two optical fibers to be spliced. This reduces insertion loss by 0.1 dB or better, at the splice junction and, therefore, improves light signal transmission. The radius of curvature of the concave structure can be approximately two to three times the radius of the optical fibers being spliced.

Description:
BACKGROUND 
     A fiber optic cable contains multiple, mutually-isolated, coated glass fibers. Sometimes the fibers in one cable are not identical in each of their diameters to the fibers in a second cable. Different optical fibers that meet different performance standards may not be identically manufactured which may result in slightly different optical fiber diameters. Different standard specifications for optical fibers are published by the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T). These specifications vary from the ITU-T G.652 specification to the ITU-T G.657 specification, with some eighteen or more specifications or sub-specifications in between which may result in optical fiber diameter variations. 
     A mismatch in diameter between two optical fibers, for any reason, can result in significant insertion loss (signal loss) at a splice junction between the two optical fibers if their cross-sections at the splice junction do not optimally overlap. Even small variations in diameters on the order of 10% can be problematic. Consequently, when splicing optical fibers with different diameters, for example, by technicians working on a fiber optic cable installation at a construction site of a multi-dwelling unit (MDU), the technicians try to align the fibers optimally to mitigate insertion loss at the splice junction. 
     Different splicing techniques offer different alignment capabilities. For example, a fusion splicer can make use of photonics for alignment purposes, and thereby achieve a mode-field diameter alignment, a fiber-core alignment or a fiber cladding alignment, each of which probably provides a better overlap between the spliced optical fibers&#39; cross sections as compared with the overlap achievable by a mechanical splicer. The mechanical splicer generally can not align two fibers as well as a fusion splicer because it is limited to geometrical/mechanical alignment constraints only. 
     But, a mechanical splicing technique has advantages; it is much less costly and easier to use than a fusion splicing technique. The latter requires a relatively expensive splicing instrument, access to electrical power which is sometimes not readily available during initial phases of building construction, more highly trained technicians, and more money for repairs if the fusion splicer is dropped or otherwise damaged during use. In a cable installation for a multi-dwelling unit (MDU) such as a large apartment building, the large number of required splices makes fusion splicing cost prohibitive. For that reason, and because of the other factors noted above, it would be preferable to use mechanical splicing, provided that misalignments resulting from mechanical splicing of optical fibers with unequal diameters could be mitigated. 
     There are different kinds of mechanical splicers, but a current widespread design uses a “V” groove as a channel to hold two optical fibers to be spliced together. The walls of the V groove are flat, and a cover pressing down on top of the open V channel presses against at least the larger of two unequally-diametered fibers. Because of geometry and gravitational force, the smaller diameter optical fiber is displaced downward in the direction of the bottom of the V channel, relative to the supported location of the larger diameter fiber. Thus, there is non-concentric overlap between the cross-sections of these two fibers at their splice junction, as a function of diameter difference. A portion of the cross-section of the smaller fiber hangs below the bottom of the cross-section of the larger fiber, or if the portable splicer is momentarily rotated by the technician on the job site for whatever reason, where either of the normally-up corners of the V-channel is momentarily located in a down position, a like portion of the cross-section of the smaller fiber could then protrude beyond the periphery of the larger fiber in the direction of that momentarily down corner. 
     Applicant provides an improvement to this V groove mechanical splice design by moving the cross-sections towards concentricity and thereby achieving increased cross sectional overlap and reduced insertion loss. Insertion loss has been reduced by as much as 0.1 dB-0.2 dB from use of Applicant&#39;s improvement, which shall be appreciated as being significant by those of skill in the art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exemplary end-view schematic diagram of a prior art V-groove mechanical splicer; 
         FIG. 2  is an exemplary end-view schematic diagram of apparatus configured in accordance with principles of the present invention; 
         FIG. 3  is an enlarged end view of a portion of  FIG. 1  showing two optical fibers having dramatically different diameters for illustrative purposes; 
         FIG. 4  is an enlarged end view of a portion of  FIG. 2  showing two optical fibers with diameters equal to those in  FIG. 3 , for comparison purposes; 
         FIG. 5  is an exemplary schematic diagram depicting a longitudinal view of two optical fibers with unequal diameters as they might be supported by apparatus of  FIG. 1 ; and, 
         FIG. 6  is an exemplary schematic diagram depicting a longitudinal view of the two optical fibers of  FIG. 5  as they might be supported by apparatus of  FIG. 2 ; and 
         FIG. 7  is an exemplary end view schematic diagram of an alternative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In this description, the same reference numeral in different Figs. refers to the same entity. Otherwise, reference numerals of each Fig. start with the same number as the number of that Fig. For example,  FIG. 3  has numerals in the “300” category and  FIG. 4  has numerals in the “400” category, etc. 
     In overview, a V-groove mechanical splicer for splicing optical fibers relies entirely on mechanical constraints to hold the optical fibers in place during the splicing operation. Before the actual splicing takes place, the coated optical fibers need to be properly stripped, cleaned and cleaved which is standard procedure. Cleaving is performed in an optical fiber cleaver, which cuts the ends of the optical fibers in a manner that provides flat and smooth end glass surfaces. These surfaces are either orthogonal to their respective longitudinal axes or are angled at other than 90 degrees to their respective longitudinal axes such as, e.g., 8 degrees, and perfectly mated to each other via alignment and/or keying techniques. An optical gel material that matches the optical properties of the glass of the fibers is used at the splice junction to reduce optical signal reflection and enhance optical transmission. 
     After preparation of the fibers as described above, embodiments of the present invention can be used. These embodiments include apparatus utilized by qualified technicians for splicing together two optical fibers of equal or unequal diameters. The apparatus has a closed splice-junction support channel. The closed channel is configured with three curved, concave surfaces which, if the optical fiber diameters are unequal, together hold or grip at least the larger of the two optical fibers therein. The three concave surfaces are utilized to increase overlap between cross-sections of the two optical fibers at their splice junction with concomitant reduction of insertion loss generated by the splice junction. This improved alignment between the two optical fibers at their splice junction results from usage of embodiments of the present invention to cause relative radial displacement of the two optical fibers in a direction towards a state of optical fiber concentricity or coaxiality. (If two optical fibers have equal diameters they would be likely to achieve actual concentricity or coaxiality when spliced together.) 
     This increase in overlap is measured with respect to results obtained from similar apparatus also having a closed splice junction support channel, but configured with three standard flat surfaces. The improved curvilinear, or arcuate, surfaces in embodiments of the present invention inherently and mechanically increase cross-sectional overlap and reduce signal loss at the splice interface as compared with results obtained from apparatus having optical fiber holding channels configured from flat surfaces. 
     The closed channel of a preferred embodiment of the present invention is configured from two curved restraining walls formed in the body of the mechanical splicer, and a curved cover (i.e., a curved ceiling). Instead of a curved cover, an arc formed within a portion of the cover (a “cover arc”) can be used. The cover is hinged from the body of the splicer and closes upon, or towards, the body of the splicer from which the two curved restraining walls are formed. The two curved restraining walls have substantially equal radii of curvature and meet together at one of their ends to form a linear channel. The channel&#39;s curvature, as viewed from inside the channel, is concave—the channel has concave walls for supporting two separate optical fibers therein. The channel is closed when either the curved cover or the cover arc, both also concave as viewed from inside the channel, and hinged at one end of the cover from the splicer body or chassis, is locked into its closed position. The cover or the cover arc has a radius of curvature that is equal to the radii of curvature of the walls. The cover or the cover arc is configured to firmly hold both inserted fibers having identical diameters, or the one of the two inserted fibers having the larger of the two unequal diameters, within concave constraints of both the channel and the cover or the cover arc when the cover is locked closed. 
     The radii of curvature of the walls and ceiling cover are equal to each other and are approximately two to three times the radius of curvature of the enveloped optical fibers, but these are not absolute limits and other radii of curvature of the walls and ceiling, larger or smaller, can be used. The body or chassis of the mechanical splicer of the preferred embodiments, including the walls and cover, is made from hard and inflexible material but not as hard as the glass which it envelopes. That material should be softer than the glass to avoid damaging the glass in the event that the glass happens to be larger than the space in the closed channel. For example, the body or chassis of embodiments of the present invention can be configured from metal such as aluminum and/or hard plastic. 
       FIG. 1  is an end view of prior art V-groove mechanical splicing apparatus  100 , showing various detail in schematic format. Body or chassis  101  has a V-groove or channel formed in it by flat side walls  102  and  103  which are shown on edge and which meet together at a line shown on end and representing channel bottom  109 . Cylindrically-shaped optical fiber  104  is shown on end as a circle resting in the V-groove. Another optical fiber to which optical fiber  104  shall be spliced also lies in the groove, hidden behind optical fiber  104  and is not shown in this Fig. The other optical fiber is also circular in cross-section and is equal in diameter to the diameter of optical fiber  104 . If the other optical fiber, not shown, were smaller in diameter than optical fiber  104 , a portion of it would have been visible below the bottom of optical fiber  104 . This shall be explained in detail in connection with  FIGS. 3 and 4 . 
     Cover  105  is shown connected to body  101  by way of hinging mechanism  106  at the left hand side of the apparatus. Cover  105  has a flat inner surface  107  shown on edge in  FIG. 1 . When cover  105  is rotated clockwise into a closed position via hinge  106 , flat surface  107  closes down upon optical glass fiber  104 . Cover  105  is held in place by way of hinge  106  at its left in cooperation with resilient clamping or locking means  108  shown at its right which can be made from metal such as spring steel. 
     In clamp  108 , control ends  112  and  113  are separated from each other by support brace  116  and can be squeezed together in directions  114  and  115  to open the mouth of the clamp. Depressions  111   a  and  111   b , formed in the clamp, interlock with lip  110   a  on the end of the top of cover  105  and lip  110   b  on the end of the bottom of body  101 , respectively, when the control ends are released. The reverse procedure is performed to remove the clamp. 
     When cover  105  is locked closed, optical fiber  104  and the other optical fiber, not shown, are held in a closed, longitudinally-linear channel comprised of three flat walls  102 ,  103  and  107 . Physical contact between the cylindrical surfaces of the two optical glass fibers and the three flat surfaces of the closed linear channel is made along three straight lines (not shown), each being parallel to the axes of rotation of the fibers, the lines being substantially equidistant from each other. The end view of that closed linear channel would appear as an equilateral triangle, or substantially close thereto. Apparatus designed in accordance with this principle of operation is commercially available. For example, 3M Company, Senko Co., Ltd and Corning Incorporated are three sources of commercially-available flat V groove alignment splicer models. 
       FIG. 2  is an exemplary end-view schematic diagram of apparatus  200  configured in accordance with principles of the present invention. Body or chassis  201  has a concave channel formed in it by curved side walls  202  and  203  having identical radii of curvature. The side walls are shown on edge and meet together at a line shown on end and representing channel bottom  208 . Cylindrically-shaped optical fiber  204  is shown on end as a circle resting in the concave channel. Another optical fiber to which optical fiber  204  shall be spliced also lies in the groove, hidden behind optical fiber  204  and is not shown in this Fig. The other optical fiber is also circular in cross-section and is equal in diameter to the diameter of optical fiber  204 . If the other optical fiber, not shown, were smaller in diameter than optical fiber  204 , a portion of it would have been visible below the bottom of optical fiber  204 . This shall be explained in detail in connection with  FIGS. 3 and 4 . 
     Cover  205  is shown connected to body  201  by way of hinging mechanism  206  at the left hand side of the apparatus. Cover  205  has a curved inner surface or cover arc  207  having the same radius of curvature as those of side walls  202  and  203 , and is shown on edge in  FIG. 2 . When cover  205  is rotated clockwise into a closed position via hinge  206 , curved surface  207  closes down upon optical glass fiber  204 . Cover  205  is held in place by way of hinge  206  at its left in cooperation with a clamping or locking means not shown in this Fig. to enhance clarity of presentation, but which is similar to clamping mechanism  108  shown in  FIG. 1 . When cover  205  is locked closed, optical fiber  204  and the other optical fiber, not shown, are held in a closed, longitudinally-linear channel comprised of three concave walls  202 ,  203  and  207 . Physical contact between the outer surfaces of the two optical glass fibers and the three curved surfaces of the closed linear channel is made along three straight lines (not shown), each being parallel to the axes of rotation of the Fibers. Because the radii of curvature of the two side walls and the radii of curvature of the cover arc are substantially the same, the lines of contact between the glass fibers and the curved walls are substantially equidistant from each other. The gap between the lower surface of cover  205  associated with cover arc  207  and the upper surface of body  201  can be larger or smaller than the gap shown; the actual gap distance is a function of diameter, or radius, of optical fiber  204  relative to radius of curvature of  202 ,  203  and  207 . 
     The end view of that closed linear channel appears as three equal arc lengths of circular geometry. That end view would approach that of an equilateral triangle, having sixty degrees per angle, as the radius of curvature of each of those arcs was simultaneously increased, in a mathematical limit sense, to a distance of infinity. In a preferred embodiment, the radius of curvature of the three circular arcs is the same and fixed at approximately two to three times the radius of curvature of the encapsulated optical fiber, although larger and smaller radii of curvature can be used. 
       FIG. 3  is an enlarged end view  300  of a portion of  FIG. 1 . However, instead of depicting two optical fibers having the same radius or diameter with one optical fiber hidden behind the other as presented in  FIG. 1 ,  FIG. 3  shows two optical fibers  104  and  302  having dramatically different diameters. This diameter difference is greater than that which is expected to occur in actual practice, but is presented herein, along with  FIG. 4 , to clearly illustrate the principle of operation of the present invention as well as the advantages of the present embodiment over the prior art embodiment. 
       FIG. 3  shows end views of flat surfaces  102  and  103  as depicted in  FIG. 1  and also shows an end view of optical fiber  104  as it rests in the V groove upon side walls  102  and  103 . Cover  105  ( FIG. 1 ) is assumed to be in a closed position wherefore the end view of flat surface  107  is a straight and horizontal line, as shown, tangent to the top-most location of optical fiber  104 . Significantly, smaller-diameter optical fiber  301 , shown on end, is located in the V-groove and is partially visible. The cross-hatched area  302  represents the effective splice junction overlap between the two optical fibers. Although this may not be a realistic fiberoptic match-up, it can be seen that with the dramatically different diameters depicted, cross-hatched area  302  is less than half of the cross-sectional area of optical fiber  301 . In the mis-matched optical fiber circumstance shown, there would be substantial insertion loss at the splice junction of these two optical fibers. 
       FIG. 4  is an enlarged end view  400  of a portion of  FIG. 2  showing two optical fibers with diameters equal to those of the optical fibers in  FIG. 3 , for comparison purposes.  FIG. 4  shows end views of curved surfaces  202  and  203  as depicted in  FIG. 2  and also shows an end view of optical fiber  204  as it rests in the concave channel upon side walls  202  and  203 . Cover  205  ( FIG. 2 ) is assumed to be in a closed position wherefore the end view of curved surface  207  is a curved line, as shown, tangent to the top-most location of optical fiber  204 . The radii of curvature of surfaces  202 ,  203  and  207  are all equal to each other and, in  FIG. 4 , each is depicted as being three times the radius of curvature of optical fiber  204 . Although the radii of curvature are the same in a particular embodiment of the present invention, they need not be limited to a thrice constraint relative to the optical fiber being spliced, and the same larger, or smaller, radii of curvature for each of surfaces  202 ,  203  and  207  can be used and are intended to be covered by the appended claims. 
     Smaller-diameter optical fiber  401 , shown on end and equal in diameter to optical fiber  301  of  FIG. 3 , is resting in the concave channel formed by walls  202  and  203  and is, again, partially visible but, significantly, is less partially-visible than in  FIG. 3 . The cross-hatched area  402  represents the effective splice junction overlap between the two optical fibers and this means that there is more overlap depicted in  FIG. 4  than in  FIG. 3 . It further appears that cross-hatched area  402  is more than half of the cross-sectional area of optical fiber  401  as compared with that of  FIG. 3  which was less than half of the cross-sectional area of same-sized optical fiber  301 . It further appears that the cross-hatched area in  FIG. 4  is approximately twice as large as that of  FIG. 3 . Although there would still be some insertion loss at the splice junction of these two optical fibers in  FIG. 4 , such loss would be much less than that depicted in  FIG. 3 . 
     Thus, when any two optical fibers of different diameter are mechanically spliced in an embodiment of the present invention, there would always be less insertion loss at that splice junction as compared with the loss at a splice junction of those same two fibers as created by a prior art mechanical V-groove splicer. 
       FIG. 5  is an exemplary schematic diagram depicting a longitudinal view  500  of two optical fibers  104  and  501  with unequal diameters as they might be supported by apparatus of prior art  FIG. 1 . These diameters are closer in size to each other than those shown in  FIGS. 3 and 4 . The splice junction overlap is shown by dimension L 1 , and gel  502  is shown between the two optical fibers. The dimensions of gel thickness and optical fiber diameter are not necessarily in realistic proportions, but are depicted as such to enhance clarity of presentation. 
       FIG. 6  is an exemplary schematic diagram depicting a longitudinal view of the two optical fibers having the same size as those of  FIG. 5  as they might be supported by apparatus of  FIG. 2 . Optical fiber  204  has the same diameter as optical fiber  104 ; optical fiber  601  has the same diameter as optical fiber  501 ; gel  602  is used. The overlap is shown by dimension L 2 . L 2  is larger than L 1 . The dimensions of gel thickness and optical fiber diameter are again not necessarily in realistic proportions, but are depicted as such to enhance clarity of presentation. 
     In the preceding specification, various preferred embodiments have been described with reference to the accompanying drawings. It will, however, be evident that various modifications and changes may be made thereto, and additional embodiments may be implemented, without departing from the broader scope of the invention as set forth in the claims that follow. 
     For example,  FIG. 7  depicts an alternative embodiment  700  of the present invention.  FIG. 7  is similar to  FIG. 2  with the exception of the design of cover  702 . Splicer body  701  has equally curved walls  703  and  704  formed therein, similar to side walls  202  and  203 , respectively, in  FIG. 2 . But, rather than having an inner surface  207  formed in cover  205 , that inner surface having the same radius of curvature (and virtually the same arc length) as those of side walls  202  and  203  as shown in  FIG. 2 ,  FIG. 7  shows an extended curved ceiling  705  or a continuously curved inner cover ceiling  705 . That continuous curve runs from near the hinge at the left hand side of cover  702  to near the end of the cover at the right hand side of cover  702 . The curvature of the curved inner cover ceiling  705  is the same as the curvature of the arcs  703  and  704 . 
     The purpose of the alternative embodiment is to accommodate small variations in dimensions resulting from variations in the manufacturing process, when fabricating the embodiments of the present invention. The splicer body and cover can be stamped from aluminum, where the location and configuration of walls  202 / 203  or  703 / 704  in the body is precisely repeatable, but there could be some variation in cover/body alignment when the cover closes upon the splicer body. That is, the embodiment configured in accordance with  FIG. 2  requires manufacturing techniques offering virtually perfect repeatability from manufactured unit to unit, in terms of all three of the splicer&#39;s arc and linear dimensions and in terms of hinge action. Without that level of repeatability, an end of arc  207  in cover  205  of  FIG. 2  making unwanted contact with glass fiber  204  might cause an unwanted displacement of the glass fiber. The cover  205  must mate virtually perfectly and, if linearly offset relative to body  201 , the otherwise achievable optimum alignment cannot be achieved. 
     But, in the alternative embodiment, with a continuous arcuate inner cover ceiling  705 , similar slight displacements or similar slight variations in tolerance will be mitigated because the arc is continuous and without an end point in the vicinity of optical fiber  706 . If the cover is displaced slightly from optimum setting, the cover still presents virtually the same arc to the optical fiber that is encapsulated. Clamp  108  in  FIG. 7  operates with respect to this embodiment as described above with respect to  FIG. 1 . As with  FIG. 2 , the gap between the lower surface  705  of cover  702  and the upper surface of body  701  can be larger or smaller than the gap shown; the actual gap distance is a function of diameter, or radius, of optical fiber  706  relative to radius of curvature of  703 ,  704  and  705 . 
     The present invention is thus not to be interpreted as being limited to particular embodiments and the specification and drawings are to be regarded in an illustrative rather than restrictive sense.