Abstract:
An optical fiber coupler according to the present invention comprises two optical fibers which each include a core portion serving to propagate light and a clad portion surrounding said core portion, the two optical fibers extending substantially in parallel in a same flat plane, and a melting portion in which the clad portions of the two optical fibers are fused together substantially in a line contact. Two optical fibers are arranged in parallel to permit at least parts of the clad portions to be brought into contact with each other. In this state, the two optical fibers are heated and fused mutually substantially in a line contact and are further drawn for fabrication thereof. The optical fibers are heated preferably employing an electric ceramic microheater. The two optical fibers may be same or different in structural parameters.

Description:
[0001]    This application is based on Patent Application No. 2001-58828 filed Mar. 2, 2001 in Japan, the content of which is incorporated hereinto by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a wide-bandwidth optical fiber coupler with less polarization dependency loss and less excessive loss, and a method and an apparatus for easily and stably fabricating the optical fiber coupler.  
           [0004]    2. Description of the Related Art  
           [0005]    An optical coupler with less wavelength dependency, e.g. a wide-bandwidth optical fiber coupler for branching and coupling a wide-bandwidth optical signal may enjoy wide utilization as various optical parts for use in general optical fields and micro optical fields and in optical communication and optical data processing.  
           [0006]    Upon fabrication of such a wide-bandwidth optical fiber coupler, there is known a prior art method wherein one of two optical fibers possessing the same structural parameters of the optical fibers is previously heated and drawn to force the structural parameters of these two optical fibers to be different or two optical fibers possessing mutually different structural parameters of the optical fibers are used, melted and drawn in a twisted state of the fibers.  
           [0007]    Referring to FIGS.  29  to  31 , fabrication procedures of such a prior art wide-bandwidth optical fiber coupler is illustrated in schematically. Opposite ends of one optical fiber  1  is held on a pair of carrying blocks  2 . A coating is removed from a central portion of the optical fibers  1  located between these carrying blocks  2  as illustrated in FIG. 29. A distance between the pair of the carrying blocks  2  is increased while heating an exposed optical fiber strand  3 . The portion of the optical fiber strand  3  is preliminarily drawn, as illustrated in FIG. 30. There are twisted a preliminarily drawn optical fiber  5  and the optical fiber  1  which optical fiber  1  is not preliminarily drawn and in which a coating thereof is only removed from a central portion thereof are twisted along that portions of the optical fiber strands. In this state, both ends of these optical fibers  1  and  5  are again held on the pair of the carrying blocks  2 . The distance between the pair of the carrying blocks  2  are increased while again heating the portions of the twisted optical fiber strands  3  with the burner  4 , as illustrated in FIG. 31. The two optical fiber strands  3  are hereby melted and drawn at those twisted portions to obtain such an optical fiber coupler  6  as illustrated in FIG. 32.  
           [0008]    When a wide-bandwidth optical fiber coupler  6  with reduced wavelength dependency is fabricated, the foregoing prior art method where there are used two optical fibers with mutually different structural parameters requires that two kinds of optical fibers previously should be fabricated. This is not general in view of the production cost.  
           [0009]    To solve this difficulty, the following method is exclusively employed: two optical fibers of the same kind are prepared, one of which is preliminarily drawn to change the parameters concerning the structure.  
           [0010]    The prior art method illustrated in FIGS.  29  to  31 , however, needs to preliminarily draw the optical fiber strand  3  by about 10 mm using the burner  4 . In the method, it is essentially impossible to finely control heating temperature for the optical fiber strand  3  in the unit of several tens of ° C. for example so as to prevent a wire diameter from being varied. In the prior art method, the wire diameter of a portion of the preliminarily drawn optical fiber strand  3  is narrowed, so that it is necessary to mutually twist the preliminarily drawn optical fiber strand  3  and the optical fiber strand  3  of the optical fiber  1  from which sheath is only simply removed for securely bringing them into close contact. As illustrated in FIG. 23, when the two optical fibers  1  and  5  are mutually twisted, it is necessary to apply tension uniformly to those two optical fibers  1  and  5 , so that those fibers should be twisted with subtle discretion. The prior art method therefore suffers from difficulty that work is complicated with deteriorated reproducibility. The prior art method further suffers from a difficulty that the optical fibers  1  and  5  are subjected to twisting, so that the optical fiber strand  3  suffers from larger polarization dependency owing to internal stress produced in itself, and allows polarization dependency loss (hereinafter, may be simply referred to PDL.) to be increased together with increased excessive loss.  
           [0011]    Referring to FIG. 33, there is illustrated wavelength dependency of a coupling ratio, i.e. a branch ratio between a symmetrical optical fiber coupler A where an optical coupling portion is configured symmetrically and asymmetrical wide band optical fiber couplers B and C where an optical coupling portion is not symmetrical). For such a branch ratio of a general symmetrical optical fiber coupler in which parameters concerning the structures of two optical fibers constituting an optical fiber coupler A are same for each other, it varies periodically from 0 to 100% owing to the wavelength of an optical signal. In contrast, for wide band optical fiber couplers B and C possessing different parameters concerning the structures of two optical fibers constituting an optical coupling portion, a branch ratio is reduced to a specific value, 100% or less by combining optical fibers with outer diameters of clad portions thereof for example being respectively 115 μm and 125 μm. There is thereupon utilized a flat portion located in the vicinity of the maximum of the just-mentioned branch ratio. As illustrated in FIG. 33, the wide-bandwidth optical fiber coupler B has the branch ratio of 50% at wavelength of 1.4 μm and the wide-bandwidth optical fiber coupler C has the optical branch ratio of 20% at wavelength of 1.4 μm. It can be understood therefrom that a change in the branch ratio in the vicinity of the maximum branch ratio is more flattened than the wavelength characteristic of the symmetrical optical fiber coupler A would be.  
           [0012]    For fabricating the aforementioned wide-bandwidth optical fiber coupler there is known a method proposed by International Application PCT/GB 86/00445. More specifically, one single mode optical fiber is preliminarily drawn, and the preliminarily drawn optical fiber and another optical fiber not preliminarily drawn are combined, melted and drawn to successfully obtain a wide-bandwidth optical fiber coupler  6  as illustrated in FIG. 34 and FIG. 35 which illustrates a cross sectional structure viewed along XXXV-XXXV shown in FIG. 34. In the resulting optical fiber coupler  6 , a propagation constant of the one single mode optical fiber  1  is altered with the aid of preliminary drawing to obtain different parameters from those concerning the structure of the optical fiber  5  not preliminarily drawn with the maximum branch ratio brought into a specific value less than 100%, and a flat wavelength characteristic in the vicinity of the maximum value of the branch ratio is utilized. In FIG. 34, designated at  7   a  is a core portion,  7   b  is a clad portion,  8  is preliminarily drawn portion, and  9  is a melted drawn portion.  
           [0013]    The aforementioned prior art fabrication method for the wide-bandwidth optical fiber coupler  6  using the preliminary drawing, however, has a difficulty that uniform preliminary drawing is difficult. More specifically, uniform heating control for the preliminary drawn portion  8  is difficult to make impossible precision configuration control and hence to make it difficult to obtain a uniform outer diameter of the melted drawn portion  9 . The optical fibers  1  and  5  might thereupon been sometimes bent upon processing the resulting wide-bandwidth optical fiber coupler  6 . This might cause set value control for the branch ratio to be difficult and therefore accuracy of flatness of the branch ratio of the drawn optical fiber to be unsatisfactory with the very bad yield of the coupler.  
           [0014]    To solve the aforementioned difficulty with the preliminary drawing described above, Japanese Patent Application Laid-Open No. 2-171705 discloses a method wherein two optical fibers possessing mutually different propagation constants are drawn with the mutually equal drawing, and are then melted and further drawn. Further, Japanese Patent Application Laid-Open No. 2-259704 discloses another method wherein between two single mode optical fibers including clad portions thereof having mutually different diameters thereof the one optical fiber having the same outer diameter as that of the same optical fiber at the other end is previously melted and connected with both ends of the other optical fiber, and then the two optical fibers are melted and drawn.  
           [0015]    Also in these two methods, however, structural parameters of the two optical fibers are made asymmetrical and the maximum value of the branch ratio is brought to a specific value less than 100%, and wavelength flatness in the vicinity of the maximum value is utilized. The degree of melting is still high so as to provide close coupling.  
           [0016]    When two optical fibers having the same structural parameters are mutually melted and drawn to fabricate a symmetrical optical fiber coupler, the maximum value of the branch ratio thereof becomes 100% as illustrated in FIG. 33. Thereupon, it is contemplated from the viewpoint of the neighborhood of the maximum value of the branch ratio being used that a wide-bandwidth optical fiber coupler having an arbitrary branch ratio can not been fabricated. Accordingly, a wide-bandwidth optical fiber coupler is conventionally fabricated by preparing two optical fibers having mutually different structural parameters such as a diameter of a clad portion or combining a preliminary drawn optical fiber and a not preliminarily drawn optical fiber.  
           [0017]    In optical fiber communication, there are required wide-bandwidth optical fiber couplers possessing various branch ratios not only of 50% but also of 20, 10, 5, 2%, etc. Further, each time a branch ratio is altered, there must be prepared optical fibers having different structural parameters such as a core diameter, a specific refractive index, and a clad outer diameter, resulting in the high fabrication cost. When a wide-bandwidth optical fiber coupler fabricated with a different clad diameter optical fiber is assembled in an optical communication network, it is necessary to connect a different clad diameter optical fiber with opposite ends of the assembled optical fiber. It is generally not easy technically to connect optical fibers possessing different parameters concerning such a structure mutually in series, resulting in the costing-up of fiber fabrication.  
         SUMMARY OF THE INVENTION  
         [0018]    In view of the above description it is an object of the present invention to provide a method and an apparatus capable of easily fabricating with excellent reproduction a wide-bandwidth optical fiber coupler that possesses less polarization dependency loss and less excessive loss.  
           [0019]    It is another object of the present invention to provide an optical fiber coupler that eliminates a preliminary drawing process, and a method capable of inexpensively fabricating the foregoing optical fiber coupler.  
           [0020]    A first aspect of the present invention is an optical fiber coupler which comprises two optical fibers each including a core section for serving to transmit light and a clad section surrounding the former core section, the two optical fibers extending substantially parallel in a same flat plane, and a melting section where the clad sections of the two optical fibers are mutually melted substantially in a line contact state.  
           [0021]    In the optical fiber coupler according to the first aspect of the present invention, outer diameters of clad portions of two optical fiber couplers may be substantially equal to each other. In this case, even if a coupling rate of the optical fiber coupler is 100%, the optical fiber coupler with arbitrary branch ratio can be provided by making use of a portion with a mild modulation of the branch ratio.  
           [0022]    It is advantageous that the branch ratio of the optical fiber coupler increases substantially monotonically in response to the wavelength of light that propagates in an optical fiber, and in the wavelength range of the light of from 1.3 μm to 1.55 μm the amount of a change in the branch ratio effectively lies within 20%.  
           [0023]    In accordance with the first aspect of the present invention, in the wavelength range of light of from 1.3 μm to 1.55 μm the branch ratio of the optical fiber coupler may be 1 to 20%.  
           [0024]    Outer diameters of clad portions of two optical fibers may be mutually different.  
           [0025]    It is preferable that when the size of the maximum width size of the melting portion is assumed to be W, and outer diameters of the clad portions of the two optical fibers in the melting portion are assumed d 1  and d 2  respectively, a melting rate C represented by  
             C=[ 1−{ W /( d   1   +d   2 )}]×100  
           [0026]    lies in the range of from 0.5 to 10%, preferably 1 to 7% which range corresponds to “substantially line contact state” in the present invention.  
           [0027]    A second aspect of the present invention is a method for fabricating an optical fiber coupler which comprises the steps of arranging mutually in parallel two optical fibers each including a core portion serving to propagate light therethrough and a clad portion surrounding the core portion to bring at least parts of the clad portions into close contact, melting mutually parts of the clad portions of the two optical fibers substantially in a line contact state by heating these clad portions in a mutual close contact state of at least parts of the clad portions, and heating and drawing the mutually melted two optical fibers.  
           [0028]    In the method for fabricating the optical fiber coupler according to the present invention, the step of mutually melting the clad portions of the two optical fibers may include a step of heating the clad portions to 1500° C. or higher.  
           [0029]    The step of heating and drawing of the two optical fibers mutually melted may further comprise the steps of lowering heating temperature with respect to the two optical fibers after those optical fibers are melted, drawing melted portions of the two optical fibers in the state where the heating temperature is lowered, forcing monitor light to impinge from a one end side of any of the two optical fibers and detecting the monitor light from the other end side of at least one of the two optical fibers to measure a branch ratio thereof, and interrupting the drawing of the melted portions of the two optical fibers at the time when the branch ratio reaches a predetermined value. Therefore, an optical fiber coupler having a desired branch ratio can be obtained.  
           [0030]    It is preferable that when the size of the maximum width size of the melting portion is assumed to be W, and outer diameters of the clad portions of the two optical fibers in the melting portion are assumed d 1  and d 2  respectively, a melting rate C represented by  
             C=[ 1−{ W/ ( d   1   +d   2 )}]×100  
           [0031]    lies in the range of from 0.5 to 10%, preferably 1 to 7% which range corresponds to “substantially line contact state” in the present invention. Therefore, the optical fiber coupler possessing excellent characteristic can be obtained.  
           [0032]    For the heating for the optical fiber an electric ceramic microheater may be preferably employed in view of the ease of temperature control. It is then ensured that heating temperature for the optical fiber becomes accurately controllable to smoothly change the diameter of the optical fiber strand as well as successfully obtain a long drawn region possessing a uniform wire diameter. This assures fabrication of an optical fiber coupler possessing excellent characteristics. It is additionally possible to preliminarily draw, in the state where two optical fibers are previously arranged in parallel very closely, only the one optical fiber.  
           [0033]    There may be substantially equal outer diameters of the clad portions of the two optical fibers where at least parts of the clad portions are brought into contact with each other.  
           [0034]    The two optical fibers where at least parts of the clad portions are brought into contact with each other may be different from each other in their structural parameters. Therefore, it becomes possible to melt such two optical fibers without twisting them, whereby a high quality wide-bandwidth optical fiber coupler with less PDL and less excessive loss is ensured stably.  
           [0035]    In this case, it is possible to further provide a step in which only one of the two optical fibers are preliminarily drawn to bring structural parameters thereof to different ones. Therefore, melting of two optical fibers is ensured without twisting them, and a high quality wide-bandwidth optical fiber coupler with less PDL and less excessive loss is ensured stably at the low cost.  
           [0036]    The method for fabricating an optical fiber coupler may further comprise the steps of arranging mutually in parallel two optical fibers having the same structural parameter, and preliminarily drawing only one of the two optical fibers to provide mutually different structural parameters thereof. Therefore, melting of two optical fibers is ensured without twisting them, and a high quality wide-bandwidth optical fiber coupler with less PDL and less excessive loss is ensured stably at the low cost.  
           [0037]    In this case, the step of preliminarily drawing one optical fiber may include the steps of heating both the two optical fibers to distortion eliminating temperature or higher, and drawing only one of the heated two optical fibers. Otherwise, the same step may be performed at a heating temperature where one optical fiber does not melt with the other optical fiber. Therefore, it becomes possible to previously hold the two optical fibers closely and hence improve workability whereby an optical fiber coupler effectively ensured.  
           [0038]    A third aspect of the present invention is an apparatus for fabricating an optical fiber coupler which comprises a pair of fiber carrying blocks for holding longitudinal opposite sides of two optical fibers possessing mutually different structural parameters, fiber fixing means provided on the fiber carrying blocks for fixing the two optical fibers to the fiber carrying blocks, a base for carrying the pair of the fiber carrying blocks movably longitudinally of the optical fibers, carrying block drive means for moving the pair of the fiber carrying blocks mutually oppositely in the opposite directions of the fiber carrying blocks, fiber forcing means provided on the fiber carrying blocks for forcing the two optical fibers such that parts of portions of the optical fibers from which coatings of the two optical fibers are removed and brought into contact with each other, and a heater mounted on the base movably in the direction intersecting the longitudinal direction of the two fibers along a flat plane containing the two optical fibers for heating the two optical fibers.  
           [0039]    In accordance with the third aspect of the present invention, melting of two optical fibers is ensured without twisting them, and a high quality wide-bandwidth optical fiber coupler with less PDL and less excessive loss is ensured stably at the low cost.  
           [0040]    A fourth aspect of the present invention is an apparatus for fabricating an optical fiber coupler which comprises a pair of first fiber carrying blocks for holding longitudinal opposite sides of a first optical fiber, first fiber fixing means provided on the pair of the first fiber carrying blocks to fix the first optical fiber to the fiber carrying blocks, a pair of second fiber carrying blocks for holding a second optical fiber at longitudinal opposite sides thereof in parallel with the first optical fiber, second fiber fixing means provided on the pair of the second fiber carrying blocks for fixing the second optical fiber to the second fiber carrying blocks, a base for movably carrying the first and second fiber carrying blocks longitudinally of the optical fiber, first carrying block drive means for mutually oppositely moving the pair of the first fiber carrying blocks in opposite direction thereof, second carrying block drive means for mutually oppositely moving the pair of the first fiber carrying blocks in the opposite direction thereof, fiber biasing means provided on the first and second fiber carrying blocks for biasing the first and second optical fibers such that parts of portions of the first and second optical fibers where coatings thereof are removed make contact with each other, a heater mounted movably in a direction intersecting a longitudinal direction of the optical fibers along a flat plane containing the first and second optical fibers for heating the first and second optical fibers, and heater movement means for driving the heating means in the direction intersecting the longitudinal direction of these optical fibers along a flat plane containing the first and second optical fibers.  
           [0041]    In accordance with the fourth aspect of the present invention, melting of two optical fibers is ensured without twisting them, and a high quality wide-bandwidth optical fiber coupler with less PDL and less excessive loss is ensured stably at the low cost.  
           [0042]    In the apparatus for fabricating an optical fiber coupler according to the third or fourth aspect of the present invention, there can be further provided a pair of coupling means for respectively integrally coupling first and second fiber carrying blocks located mutually closely. Therefore, when two optical fibers are heated, melted, and drawn, it becomes possible to securely integrally move them.  
           [0043]    The fiber forcing means may include a fixing pin fixed to any one of the first and second fiber carrying blocks, a plunger disposed oppositely to the fixing pin between the first and second optical fibers and being movable oppositely to the fixing pin, and plunger fixing means for fixing the plunger at a predetermined position oppositely to the fixing pin. Therefore, melting of two optical fibers is ensured without twisting them, and a high quality wide-bandwidth optical fiber coupler with less PDL and less excessive loss is ensured stably at the low cost.  
           [0044]    The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0045]    [0045]FIG. 1 is a front view illustrating an external appearance of an embodiment of an apparatus for fabricating an optical fiber coupler according to the present invention;  
         [0046]    [0046]FIG. 2 is a plan view of the embodiment of FIG. 1;  
         [0047]    [0047]FIG. 3 is a cross sectional view viewed along a line III-III in FIG. 1;  
         [0048]    [0048]FIG. 4 is an enlarged view obtained by extracting part of a strand-fixing portion in FIG. 1;  
         [0049]    [0049]FIG. 5 is an enlarged view corresponding to FIG. 4 in the state where forcing means is operated to bring the optical fiber strands to contact with each other;  
         [0050]    [0050]FIG. 6 is a graph illustrating a heating temperature characteristic of a heater in the present embodiment;  
         [0051]    [0051]FIG. 7 is a view of a working concept illustrating procedures of fabrication of the optical fiber coupler according to the present embodiment together with FIGS. 8, 9,  11 ,  12 ,  14 ,  16 , and  17 , in which there are arranged two optical fibers mutually in parallel;  
         [0052]    [0052]FIG. 8 is a view of a working concept illustrating procedures of fabrication of the optical fiber coupler according to the present embodiment together with FIGS. 7, 9,  11 ,  12 ,  14 ,  15 , and  17 , in which heating is started to preliminarily draw one optical fiber strand;  
         [0053]    [0053]FIG. 9 is a view of a working concept illustrating procedures of fabrication of the optical fiber coupler according to the present embodiment together with FIGS. 7, 8,  11 ,  12 ,  14 ,  15 , and  17 , in which one optical fiber strand is preliminarily drawn;  
         [0054]    [0054]FIG. 10 is a graphical representation illustrating a relationship between time and heating temperature in a preliminary drawing process according to the present embodiment;  
         [0055]    [0055]FIG. 11 is a view of a working concept illustrating procedures of fabrication of the optical fiber coupler according to the present embodiment together with FIGS.  7  to  9 ,  12 ,  14 ,  15 , and  17 , in which preliminary drawing of the one optical fiber strand completes;  
         [0056]    [0056]FIG. 12 is a view of a working concept illustrating procedures o fabrication of the optical fiber coupler according to the present embodiment together with FIGS.  7  to  9 ,  11 ,  14 ,  15 , and  17 , in which heating for the two optical fiber strands is started;  
         [0057]    [0057]FIG. 13 is a cross sectional view viewed along a line XIII-XIII in FIG. 11;  
         [0058]    [0058]FIG. 14 is a view of a working concept illustrating procedures of fabrication of the optical fiber coupler according to the present embodiment together with FIGS.  7  to  9 ,  11 ,  12 ,  15 , and  17 , in which forcing means is operated to bring the two optical fiber strands to contact with each other;  
         [0059]    [0059]FIG. 15 is a graphical representation illustrating a relationship between time and heating temperature in a melting drawing process in the present embodiment;  
         [0060]    [0060]FIG. 16 is a view of a working concept illustrating procedures of fabrication of the optical fiber coupler according to the present embodiment together with FIGS.  7  to  9 ,  11 ,  12 ,  14 , and  17 , in which the two optical fiber strands is in the state of heating;  
         [0061]    [0061]FIG. 17 is a cross sectional view viewed along a line XVII-XVII in FIG. 15;  
         [0062]    [0062]FIG. 18 is a view of a working concept illustrating procedures of fabrication of an optical fiber coupler according to the present embodiment together with FIGS.  7  to  9 ,  11 ,  12 ,  14 , and  15 , in which heating and drawing for the two optical fiber strands are completed;  
         [0063]    [0063]FIG. 19 is a graphical representation illustrating a relationship between drawing time and the branch ratio in the embodiment of the optical fiber coupler fabricated by the present invention;  
         [0064]    [0064]FIG. 20 is a graphical representation illustrating wavelength dependency in the present embodiment of the optical fiber coupler fabricated by the present invention;  
         [0065]    [0065]FIG. 21 is a graphical representation illustrating a relationship between the amount of preliminary drawing of the one optical fiber strand and a branch ratio of the optical fiber coupler;  
         [0066]    [0066]FIG. 22 is a perspective view illustrating an external appearance of a melting portion in another embodiment of the optical fiber coupler according to the present invention;  
         [0067]    [0067]FIG. 23 is a cross sectional view illustrating a line XXIII-XXIII in FIG. 22;  
         [0068]    [0068]FIG. 24 is a view of a concept illustrating a schematic construction of another embodiment of an apparatus for fabricating an optical fiber coupler according to the present invention;  
         [0069]    [0069]FIG. 25 is a graphical representation illustrating a relationship between time and heating temperature in a melting and drawing process for an optical fiber coupler using the apparatus illustrated in FIG. 24;  
         [0070]    [0070]FIG. 26 is a graphical representation illustrating wavelength dependency in another embodiment of the optical fiber coupler according to the present invention;  
         [0071]    [0071]FIG. 27 is a graphical view illustrating wavelength dependency in another embodiment of the optical fiber coupler of the present invention;  
         [0072]    [0072]FIG. 28 is a graphical view illustrating wavelength dependency in further another embodiment of the optical fiber coupler of the present invention;  
         [0073]    [0073]FIG. 29 is a view of a working concept illustrating an example of a method for fabricating a prior art optical fiber coupler together with FIGS. 30 and 31, in which an optical fiber before preliminary drawing is fixed;  
         [0074]    [0074]FIG. 30 is a view of a working concept illustrating an example of a method for fabricating a prior art optical fiber coupler together with FIGS. 29 and 31, in which an optical fiber strand is preliminarily drawn;  
         [0075]    [0075]FIG. 31 is a view of a working concept illustrating an example of a method for fabricating a prior art optical fiber coupler together with FIGS. 29 and 30, ion which two twisted optical fibers are fixed;  
         [0076]    [0076]FIG. 32 is a perspective view illustrating an external appearance of an optical fiber coupler obtained by the method for fabricating a prior art optical fiber coupler illustrated in FIGS.  29  to  31 ;  
         [0077]    [0077]FIG. 33 is a graphical representation illustrating wavelength dependency of an optical branch ratio of a prior art symmetrical optical fiber coupler and an asymmetrical optical fiber coupler;  
         [0078]    [0078]FIG. 34 is a view of a concept illustrating schematic construction of a prior art optical fiber coupler;  
         [0079]    [0079]FIG. 35 is a cross sectional view viewed along a line XXXV-XXXV in FIG. 34; and  
         [0080]    [0080]FIG. 36 is a graphical representation illustrating a relationship between a melting rate and a branch ratio of a symmetrical optical fiber coupler. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0081]    Although in the following embodiments of the present invention will be described with reference to FIGS.  1  to  28 , the present invention is not limited to such embodiments, and there are possible further partial combinations and proper partial alteration at need.  
         [0082]    [0082]FIG. 1 illustrates an apparatus for fabricating an optical fiber coupler according to the present invention in a flat plane configuration thereof, FIG. 2 illustrates a front surface configuration thereof, FIG. 3 is a cross sectional configuration partly enlarged and viewed along a line III-III in FIG. 1, and FIG. 4 illustrates an extracted and enlarged configuration of a portion of an strand fixing section in FIG. 1. Two rails  12  and  13  extending mutually in parallel are laid on a base  11 . Two sets of fiber carrying blocks  14 L,  14 R,  15 L and  15 R are respectively slidably engaged with these two rails  12  and  13 . A pair of first fiber carrying blocks  14 L and  14 R (upper side in FIG. 1) that slide on the one rail  12  and a pair of first fiber carrying blocks  15 L and  15 R (lower side in FIG. 1) that slide on the other rail  13  have fundamentally the same structure. Opposite sides of the portion of each of the optical fibers  16  and  17  are hereby held. In the present embodiment, base ends of flexible fiber clamping plates  18 L and  18 R for holding the optical fibers  16  and  17  at tip ends thereof are screwed to the second fiber carrying blocks  15 L and  15 R (hereinafter sometimes simply referred to as  15 ). The one optical fiber  16  placed on the first fiber carrying blocks  14 L and  14 R (hereinafter sometimes simply referred to as  14 ) and the other optical fiber  17  placed on the second fiber carrying block  15  can be held mutually substantially in parallel in the state where these coatings of the fibers are brought into close contact by making use of spring forces of these fiber clamping plates  18 L and  18 R. The force for holding the optical fibers  16  and  17  by making use of the fiber clamping plates  18 L and  18 R is not needed to be so large. The force may be enough simply prevented the fibers  16  and  17  from falling from the fiber carrying blocks  14  and  15 . As illustrated in FIG. 1, the left side first fiber carrying block  14 L and the second fiber carrying block  15 L, and the right side first fiber carrying block  14 R and the second fiber carrying block  15 R can be mutually integrally coupled in an arbitrary position relation using demountable coupling metal fittings  19 . For such a coupling metal fittings  19  use may be suitably made of those utilizing a leaf spring and magnetic force.  
         [0083]    Two feed screws  20  and  21  extending in parallel with the rails  12  and  13  are disposed above the rails  12  and  13  so as to penetrate the aforementioned fiber carrying blocks  14  and  15 . One end sides of these feed screws  20  and  21  are born rotatably with respect to two bearing brackets  22  and  23  disposed on the base  11 , while the other end sides are born rotatably with respect to two gearboxes  24  and  25  disposed likely on the base  11 . On the respective feed screw shafts  20  and  21 , there are formed male screws  26 , directed oppositely on the one end side and on the other end side thereof with the central portion thereof taken as a boundary, which male screws are in a threaded state with respect to feed nuts (not shown) fixed to the fiber carrying blocks  14  and  15 . Stepping motors  27  and  28  are mounted on the respective gearboxes  24  and  25  for rotating the feed screw shafts  20  and  21 . The respective stepping motors  27  and  28  and the feed screw shafts  20  and  21  are coupled with each other through reduction gear mechanisms (not shown) assembled in the gearboxes  24  and  25 .  
         [0084]    Thus, as the stepping motors  27  and  28  are rotated in one direction or the reverse direction, the feed screw shafts  20  and  21  are correspondingly positively or negatively rotated, whereby a pair of the opposing fiber carrying blocks  14  and  15  move along the rails  12  and  13  such that they close or leave each other. The amount of movement of these fiber carrying blocks  14  and  15  is specified by the number of drive pulses to the stepping motors  27  and  28 .  
         [0085]    In the preferred embodiment, four locating blocks  29  and  30  are protruded on the base  11  for specifying the home position of the pair of the fiber carrying blocks  14  and  15 . Stoppers  31  and  32  capable of being contact with these locating blocks  29  and  30  are protruded on the side surfaces of the respective fiber carrying blocks  14  and  15 . More specifically, the fiber carrying blocks  14 L,  15 L or  14 R,  15 R are obstructed in their movement from the state where the stoppers  31  and  32  for the fiber carrying blocks are at a home position as illustrated in FIG. 1 where they simultaneously make contact with the corresponding locating blocks  29  and  30  to the side of the other fiber carrying blocks  14 R,  15 R or  14 L,  15 L. The left side fiber carrying blocks  14 L,  15 L and the right side fiber carrying blocks  14 R,  15 R may be moved such that they are mutually separated taking the home position as a fiducial position.  
         [0086]    Referring to FIG. 1, a heater  35  is disposed between the left side fiber carrying blocks  14 L,  15 L and the right side fiber carrying blocks  14 R,  15 R for heating the optical fiber strands  33  and  34  trained between these carrying blocks. The heater  35  in the present embodiment is an electric ceramic microheater capable of easy and precise temperature control, which has a channel-like configuration and provides thereinside such temperature distribution as illustrated in FIG. 6. It is therefore possible to set an about 10 mm wide region to be heated to 1350° C. or higher centered at the neighborhood of the center of such a recessed portion of the configuration of the ceramic microheater as that of the channel member by keeping the temperature of the neighborhood of the center of the recessed portion at about 1550° C. Electric power is supplied to the heater  35  from a power supply (not shown) such that upon preliminary drawing the one optical fiber  16  is located in the neighborhood of a channel part in the heater  35  and is heated while upon melting and drawing center portions of the two optical fiber strands  33  and  34  kept in a close contact are located in the neighborhood of the center of the channel in the heater  35  and are heated. A controller (not shown) controls the amount of supply of electric power from a power supply and the movement of the pair of the fiber carrying blocks  14  and  15 . In a melting and drawing process, a laser diode having excitation wavelength of 1.55 μm for example is connected with one end of any one of the optical fibers  16  and  17  as a monitor light source, to the other ends of which optical fibers  16  and  17  an optical detection sensors are connected. The two optical detector sensors detect the monitor light emitted from the monitor light source to obtain a branch ratio changing during drawing in real time and hence to interrupt the heating and drawing when a desired branch ratio is attained.  
         [0087]    The heater  35  is supported on a heater movement apparatus  36  mounted on the base  11  through a bracket (not shown). The heater movement apparatus  36  includes a plunger  37  capable of reciprocation traversing the two optical fiber strands  33  and  34 . The heater  35  is coupled with the tip end of the plunger  37 . The heater  35  mounted on the plunger  37  ensures heating a longitudinal central portion of the optical fiber strand  33  exposed by removing the coating on the one optical fiber  16  supported on the first optical fiber  16  for preliminary drawing, and heating and melting the longitudinal central portions of the two optical fiber strands  33  and  34  simultaneously for further drawing. Subtle temperature control in the unit of several tens of ° C. is ensured by controlling the amount of supply of electric power by the controller.  
         [0088]    Elementary wire fixing sections  38  and  39  for mounting forcing means according to the present invention are integrally formed, protruded on the opposite sides of a pair of fiber carrying blocks  14  and  15 . A flexible strand fixing plate  40  is screwed removably through a screw  41  to the tip end of the strand fixing section  38  of the first fiber carrying block  14  for integrally fix the optical fiber strand  33  from which the coating is removed integrally with respect to the strand fixing section  38 . Spring force of the strand fixing plate  40  is employed to integrally fix the one optical fiber strand  33  to the strand fixing portion  38  of the first fiber carrying block  14  and move the aforementioned heater  35  to a predetermined position for its preliminary drawing.  
         [0089]    A receiving pin  42  is provided on the strand fixing section  38  of the first fiber carrying block  14 , with which pin there can be made contact with the side end of the optical fiber strand  33  which is an exposed state of the optical fiber  16  placed on the first fiber carrying block  14  by removing the coating on the optical fiber  16  from the center of the same. A pressure rod  43  is fitted slidably to a rod operation member  44  mounted on the strand fixing section  39  of the second fiber carrying block  15 , which pressure rod  43  has its tip end opposing to the aforementioned receiving pin  42  putting therein the optical fiber strand  34  becoming an exposed state by removing the coating from the center of the optical fiber  17  placed on the second fiber carrying block  15  and the optical fiber strand  33  of the first fiber carrying block  14 . The pressure rod  43  can be changed over between a retreat position where the tip end is retreated to the bracket side described later as illustrated in FIG. 4 and an advance position where the two optical fiber strands  33  and  34  are put in a contact with the tip end portion and the receiving pin  42  as illustrated in FIG. 5. It is noticed that in the present embodiment the receiving pin  42 , pressure rod  43 , and rod-operating member  44  serve as the forcing means.  
         [0090]    The rod operating member  44  is mounted on the bracket  46  fixed to the strand fixing section  39  of the second fiber carrying block  15  through the adjusting screw  45 . The bracket  46  is finely adjustable in its position in a parallel direction to that of the sliding of the pressure rod  43  for the strand fixing section  39  of the second fiber carrying block  15  in response to the diameters of the optical fiber strands  33  and  34 . Further, to the base end of the strand fixing section  39  of the second fiber carrying block  15  there is screwed removably through the screw  48  the flexible strand fixing plate  47  that serves to integrally fix the optical fiber strand  34  from which the coating has been removed to the strand fixing section  39 . The spring force of the strand fixing plate  47  is used to integrally fix the other optical fiber strand  34  to the strand fixing section  39  of the second fiber carrying block  15  and move the aforementioned heater  35  to a predetermined position whereby the two optical fiber strands  33  and  34  are simultaneously heated and melted and are integrally successfully drawn.  
         [0091]    Although in the aforementioned embodiment the two fiber carrying blocks  14  and  15  can be assumed to be independently driven, provided the first fiber carrying block  14 L and the second fiber carrying block  15 L both located on the left side and the first fiber carrying block  14 R and the second fiber carrying block  15 R both located on the right side can be securely integrated using a coupling fittings  19  as illustrated in FIG. 1, there may be eliminated the feed screw shaft  21 , gear box  25 , and stepping motor  28  arranged ion the side of the second fiber carrying block  15 , and the driving mechanism may simplified.  
         [0092]    Referring here to FIGS.  7  to  18 , there are illustrated fabrication procedures for an optical fiber coupler according to the present invention using the optical fiber coupler fabrication apparatus illustrated in FIGS.  1  to  5 . As illustrated in FIG. 7, two optical fibers  16  and  17  from which a central coating was previously removed are mounted on the fiber carrying blocks  14  and  15  located at a home position through fiber clamping plates  18 L and  18 R. The one optical fiber strand  33  is integrally fixed to the strand fixing section  38  using the strand fixing plate  40 . Then, as illustrated in FIGS. 8 and 9, the stepping motor  27  is driven while heating the one optical fiber strand  33  with the heater  35  to separate the first fiber carrying block  14  mutually and hereby preliminarily draw the optical fiber  16 . Since in the present embodiment, heating for the one optical fiber strand  33  is achieved by the heater  35  using an electric ceramic microheater, the diameter of the optical fiber strand  33  can smoothly be changed by accurately controlling heating temperature.  
         [0093]    Referring to FIG. 10, there is illustrated a relationship between the heating time and a change in heating temperature in such a present embodiment. Firstly, the heater  35  is heated to 1200° C. to eliminate distortion in the optical fiber strand  33 , and hen heating for the preliminary drawing. The heating temperature may be arbitrarily set within a range of from 1300 to 1550° C. In the present embodiment, the heating temperature is set to 1400° C., and the optical fiber is preliminarily drawn at a speed of a few to several tens of micrometers per second.  
         [0094]    As illustrated in FIG. 11, there is formed a gap at portions of the optical fiber strands  33  and  34  that corresponds to the thickness of the coating between the optical fiber  16  preliminarily drawing as such and the not drawing optical fiber  17 . A gap of about 125 μm is produced in the present embodiment. The restriction of the one optical fiber strand  33  due to the strand fixing plate  40  is therefore previously released, and the two optical fiber strands  33  and  34  are brought into close contact at not yet drawing portions thereof using the receiving pin  42  and the pressure rod  43 , as illustrated in FIGS. 12 and 13. Keeping this state, the two optical fiber strands  33  and  34  are integrally fixed to the strand fixing sections  38  and  39  with the aid of the strand fixing plates  40  and  47 , as illustrated in FIG. 5. Further, the first fiber carrying block  14 L and the second fiber carrying block  15 L both located on the left side and the second fiber carrying block  15 L and the second fiber carrying block  15 R both on the right side are integrally coupled using the coupling fittings  19 . Designated at a symbol  50  in FIG. 13 is core portions of the optical fiber strands  33  and  34  associated with the propagation of the optical signal, and  51  is a clad portion surrounding the core section.  
         [0095]    Thereafter, the position of the heater  35  is set for melting and drawing process such that the centers of the two optical fiber strands  33  and  34  are located at the neighborhood of the center of the channel of the heater  35 , as illustrated in FIG. 14. FIG. 15 illustrates a relationship between the heating time and the heating temperature in the melting and drawing process in the present embodiment. More specifically, the heater  35  is firstly heated to 1200° C. to eliminate the distortion of the optical fiber strands  33  and  34 . Thereafter, maximum heating temperature is set to 1550° C. for heating for melting, as illustrated in FIGS. 16 and 17. Further, the stepping motors  27  and  28  are driven in synchronism to separate the left side fiber carrying blocks  14 L,  15 L and the right side fiber carrying blocks  14 R,  15 R from each other for drawing for the optical fibers  16  and  17  for a predetermined time, as illustrated in FIG. 1. The heating temperature in the drawing process in the present embodiment is lowered stepwise from 1550° C. that is the heating temperature in the melting process. The two optical fibers  16  and  17  are drawn at the speed of a few to several tens of micrometers per second. Only the melting is performed at the heating temperature of 1550° C., and upon the drawing process the heating temperature is lowered to about 1520° C., and the optical fibers  16  and  17  may be drawn while lowering the temperature stepwise.  
         [0096]    There is ensured an optical fiber coupler  49  possessing a drawing melting section  52  drawn as illustrated in FIG. 18. The reason where the heating temperature is changed in multiple steps for drawing is that any defect is prevented from happening with an improvement of the yield and that an optical fiber coupler t 49  possessing a desired branch ratio is securely obtained. In the present embodiment, there is ensured an optical fiber coupler  49  in which a melting rate C of the drawing melting section  52  is 4% and which is in a substantially line contact state.  
         [0097]    There is prepared optical fibers  16  and  17  where the diameter of the core section  50  is 8 μm, the outer diameter of the clad section  51  is 125 μm, and a difference between refractive indexes of the core section and the clad section is about 0.3%. An optical fiber coupler  49  according to the one embodiment of the present invention is thus obtained following the aforementioned procedures by preliminarily drawing the one optical fiber strand  33  by about 0.3 mm, and heating and melting these two optical fibers  16  and  17  and further drawing them. Referring to FIG. 19, there is illustrated a relationship between the heating drawing time in the final process and the branch ratio of the resulting optical fiber coupler  49 . Referring further to FIG. 20, there is illustrated wavelength dependency with respect to the branch ratio of the resulting optical fiber coupler  49 . These characteristics correspond to the optical fiber coupler  49  according to the present embodiment which coupler is melted in a substantially line contact state with the melting rate C of the melting section  52  being 4%. It is confirmed as clearly demonstrated in FIGS. 19 and 20 that deflection with respect to the branch ratio of 50% is substantially ±5% over the wavelength range of from about 1.2 μm to 1.6 μm, ensuring an optical fiber coupler  49  with less wavelength dependency. It is confirmed that the resulting coupler is a high performance optical fiber coupler  49  with less PDL and less excess loss, e.g. its PDL of 0.03 dB excess loss of 0.07 dB.  
         [0098]    In the aforementioned embodiment, workability can be improved by previously arranging the two optical fibers  16  and  17  in parallel to each other and preliminarily drawing the one optical fiber  16  while keeping the aligned state. For securely avoiding a thermal influence to the other optical fiber upon preliminarily drawing the fiber, however, it may be allowed to previously removing the other optical fiber  17  from the fiber carrying blocks  14  and  15 , and preliminarily drawing only the one optical fiber  16 , and thereafter mounting the other optical fiber  17  not preliminarily drawing on the fiber carrying blocks  14  and  15 , and aligning them in parallel to each other to simultaneously heating and drawing the two optical fibers  16  and  17 . It is herein noticed that there can be eliminated the use of the ones of the two sets of the fiber carrying blocks  14  and  15  and the driving mechanisms for the formers, and hence the installation cost can be sharply reduced.  
         [0099]    Even with use such a method there can be fabricated a wide-bandwidth optical fiber coupler where wavelength dependency is only ±5% with respect to the branch ratio of 50%. The excess loss of the optical fiber coupler is 0.05 dB with PDL 0.02 dB, which can be confirmed to be very high quality optical fiber coupler.  
         [0100]    Referring to FIG. 21, there is provided a relationship in an initial heating process between the amount of preliminary drawing of the preliminarily drawing optical fiber  16  and a branch ratio of the optical fiber coupler obtained by the present invention. As clarified from FIG. 21, it is understood that it is possible to fabricate an optical fiber coupler possessing an arbitrary branch ratio by varying the amount of preliminary drawing of the one optical fiber  16 .  
         [0101]    In the aforementioned embodiment, although there was described the optical fiber coupler  49  using the two optical fibers  16  and  17  including the clad section  51  possessing different outer diameters upon melting, the present invention may be applicable to an optical fiber coupler using two optical fibers including the clad section possessing the same outer diameter upon melting.  
         [0102]    Referring to FIG. 22, there is illustrated the structure of another embodiment of such an optical fiber coupler according to the present invention, and further referring to FIG. 23, there is illustrated a view of a cross sectional structure, viewed along a line XXIII-XXIII, in which the same symbol will be simply applied to the same function element as the aforementioned embodiment, and overlapped description will be omitted. More specifically, as illustrated in FIG. 22, structural parameters of the optical fiber strands  33  and  34 , there are the same as those of the previous embodiments the diameter of the core section  50 , the diameter of the clad section  51 , a specific refractive index, and a cutoff wavelength, for example. The diameter d of the clad section  51  of the optical fiber strands  33  and  34  is 125 μm. A refractive index difference between the clad section  51  and the core section  50  possessing the diameter of 6.5 μm is 0.3%. The degree C of melting of the drawing melting section  52  is expressed by  
           C ={1−( W/ 2 d )}×100  
         [0103]    with the assumption of the maximum width size of the melting section  52  to be W, which is 5% or less in the present embodiment and is a state substantially equal to linear melting. The degree C of the melting is 10% or less to the utmost, especially 7% or less.  
         [0104]    Since upon fabrication of such an optical fiber coupler  49 , there is eliminated the need of the preliminary drawing process as in the aforementioned embodiments, it is also possible to employ the optical fiber coupler fabrication apparatus illustrated in FIGS.  1  to  5 . It is however possible to employ the more simple structure optical fiber coupler fabrication apparatus as illustrated in FIG. 24 having the construction demonstrated by a substantially upper half of that illustrated in FIG. 1. More specifically, the two optical fibers  16  and  17  possessing the same structural parameter with the coatings of their central portions previously removed are mounted on the fiber carrying block  14  located at the home position through the fiber clamping plates  18 L and  18 R, and non-drawn regions of the two optical fiber strands  33  and  34  are brought into contact with each other using the receiving pin  42  (not shown) and the pressure rod  43 , as illustrated in FIG. 5. Keeping this state, the strand fixing plate  40  is employed to integrally fix the two optical fiber strands  33  and  34  to the strand fixing plate  38 , as illustrated in FIG. 5.  
         [0105]    Thereafter, the position of the heater  35  is set for the melting and drawing process such that the centers of the two optical fiber strands  33  and  34  are located in the vicinity of the center of the channel in the heater  35 . The portions of the optical fiber strands  33  and  34  that form the melting section  52  of the same are heated and melted by supplying a current from the power supply  53  to the heater  35 . The controller  54  controls supplied electric power from the power supply  53  and the movement Of the fiber carrying block  14 . Upon the work being first done the heater  35  is withdrawn to its standby position before the process enters the drawing process. The melting state of the melting section  52  is observed with a microscope to determine the optimum melting temperature and the melting time. This is preferably stored in the controller  54 . Provided that the optimum melting temperature and the melting time have previously been determined as described above, the melting process and the drawing process can be continuously performed from the next time. In the drawing process the drawing process is achieved while measuring the branch ratio at a predetermined wavelength 1.55 μm from the monitor light source  55 . Laser diode of 1.55 μm is herein connected with the one end of the one optical fiber  16  as the monitor light source  55 . The optical detection sensor  56  is connected with the other end of the one optical fiber  16 , and the optical detection sensor  57  is also connected with the other end of the other optical fiber  17 . Lights from the monitor light source  55  of 1.55 μm wavelength are employed to measure the branch ratio thereof. The branch ratio is fed-back to the controller  54 , and when the branch ratio becomes 10% for example, the drawing process is finished.  
         [0106]    Referring to FIG. 25, there is illustrated a relationship between the heating time and the heating temperature in the melting and drawing process in the present embodiment. The heater  35  is first heated to 1150° C. to remove distortion of the optical fiber strands  33  and  34 , and the highest heating temperature is set to 1650° C. for example for heating for melting. The stepping motor  27  is driven as illustrated in FIG. 1 to mutually separate the left side fiber carrying block  14 L and the right side fiber carrying block  14 R and hence draw the optical fiber strands  33  and  34  in the melted state for a predetermined time. The heating temperature in the drawing process in the present embodiment ranges from 1650° C. that is the heating temperature in the melting process to 1400° C. and is lowered stepwise from 1400° C. The two optical fiber strands  33  and  34  are drawn, while lowering the temperature at the speed of a few to several tens micrometers per second. There is thus obtained the optical fiber coupler  49  including such a melting portion  52  as illustrated in FIG. 22 in which the melting rate C is 10% and it is in substantially in the linear contact state.  
         [0107]    Referring to FIG. 26, there is illustrated the wavelength characteristic of an optical fiber coupler thus fabricated. As illustrated in the same Figure, the branch ratio substantially monotonically increases within the wavelength range of from 1.1 μm to 1.7 μm within the range of from 2 to 12%. The branch ratio ranges within 2 to 12% in the using wavelength range 1.3 μm to 1.55 μm (i.e. the amount of a change of the branch ratio ranges within 5%), that is satisfactorily flat. It will be understood that this is practical as the optical fiber coupler  49 . Even taking into consideration of variations of fabrication rots, most of the variations of the branch ratio falls within 20%.  
         [0108]    Although in the foregoing embodiment the drawing is interrupted at the time when the branch ratio becomes 10%, when the optical fiber coupler  49  possessing the branch ratio of 5% is fabricated, it can be fabricated in the same manner as in the foregoing embodiment excepting that the drawing is finished when the branch ratio in measurement becomes 5%. Referring to FIG. 27, there is illustrated the wavelength characteristic of the optical fiber coupler possessing the branch ratio becoming 5%. The branch ratio increases substantially monotonically within the wavelength range of from 1.1 to 1.7 μm, and the branch ratio falls within 2 to 7%. The branch ratio ranges within 2.5 to 5% in the using wavelength range 1.3 μm to 1.55 μm (i.e. variations thereof falls within 3.5%). It will be understood that the branch ratio is satisfactorily flat and a resulting optical fiber coupler is practical as the optical fiber coupler.  
         [0109]    It is therefore possible to fabricate a wide-bandwidth optical fiber coupler possessing a desired branch ratio only by changing the setting of the branch ratio when the drawing is finished without the use of two optical fibers having different structural parameters.  
         [0110]    It is also possible to obtain an optical fiber coupler possessing different structural parameters such as the diameter of the core section, specific refractive index or cut-off frequency as in the previous embodiments even when the outer diameter of the clad section is same. For example, there is adopted an optical fiber coupler in which only the core diameter is altered concretely as the structural parameter, and the diameter of the core section of the one optical fiber is 6 μm and the diameter of the core section of the other optical fiber is 10 μm. Referring to FIG. 28, there is illustrated wavelength characteristic of an optical fiber coupler fabricated in such an embodiment. As illustrated in the FIG. 28, the branch ratio increases substantially monotonically in the wavelength range of from 1.1 μm to 1.7 μm, and the branch ratio falls within 4 to 9% in the wavelength range 1.3 to 1.55 μm (i.e. variations fall within 5%). It is therefore found that an optical fiber coupler can be fabricated without any trouble even when optical fibers possessing different structural parameters are combined.  
         [0111]    The present invention has been described in detail with respect to preferred embodiments, and it will now be apparent from the foregoing to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and it is the intention, therefore, in the appended claims to cover all such changes and modifications as fall within the true spirit of the invention.