Abstract:
A guide block assembly for aligning and retaining fiber bore forming pins and guide pin bore fanning pins in precise relation to each other during the molding of a multi-fiber ferrule includes a unitary member defining at least one fiber bore and at least one guide pin bore. Each fiber bore, and optionally each guide pin bore, is formed by creating a starter hole using a first electric discharge machining (EDM) wire and enlarging the starter hale using a second EDM wire. Each fiber bore has a length to diameter ratio of between approximately 3::1 to 10::1, more preferably between approximately 4::1 to 8::1, and most preferably approximately 6::1. The guide block assembly may further include a cavity behind the fiber bore and a front face that forms a non-rectilinear surface on the face of the female. The unitary block assembly contains fewer parts and is less expensive to manufacture.

Description:
This application is a continuation-in-part of co-pending application Ser. No. 09/621,226, filed on Jul. 21, 2000. Priority to this application is hereby claimed under 35 U.S.C. § 120, and the co-pending application is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to an apparatus and a method for molding multi-fiber optical connector ferrules. 
     BACKGROUND OF THE INVENTION 
     A fiber optic cable may include one or more optical fibers capable of transmitting audio, video or other information. Examples of optical fibers are disclosed in U.S. Pat. Nos. 5,561,730 and 5,457,762. Fiber optic cables are laid over long distances and require optical connectors or ferrules to link discrete segments of the optical fibers. As used herein, the term “ferrule” refers to a plug assembly or a structure that receives a terminal end of an optical fiber or optical fiber ribbon and then abuts against an opposing ferrule to align corresponding optical fiber or ribbon for transmission of an optical signal or signals. 
     An example of an optical ferrule is disclosed in U.S. Pat. No. 5,214,730 to Nagasawa et al. FIG. 1 illustrates an optical ferrule similar to that depicted in Nagasawa, and shows multi-fiber ferrules  3  and  3 ′ connected to optical fiber ribbons  1  and  1 ′, respectively. Ribbon  1  comprises multiple optical fibers  2  to be aligned with corresponding optical fibers  2 ′ (not shown) from ribbon  1 ′. Ferrule  3  defines a plurality of optical fiber bores adapted to receive fibers  2  and two guide pin bores  4  adapted to receive guide pins  6 . Guide pins  6  align ferrule  3  with ferrule  3 ′, when the two ferrules are connected to each other to align optical fibers  2  and  2 ′ to optimize optical transmission. 
     During a typical molding process to produce ferrules  3 , bore forming pins are inserted through the mold cavity to create the guide pin bores and the optical fiber bores in the ferrules. Molten plastic is then injected into the mold cavity, and after the plastic solidifies sufficiently the pins are withdrawn to form the bores in the ferrules to receive the optical fibers and guide pins. Prior to connecting to ferrule  3 , optical ribbon  1  is stripped of its outer matrix coating and its buffer layer to expose fibers  2 . The individual fibers  2  are inserted into the fiber bores on ferrule  3 . Various well-known techniques are used to permanently affix fibers  2  to ferrule  3 . End faces  5  and  5 ′ of ferrules  3  and  3 ′ are then polished along with the exposed ends of fibers  2 . A pair of guide pins  6  is then inserted into guide holes  4  to connect and align the ferrules. A spring clip (not shown) may be used to clamp the two ferrules together. 
     There is a premium placed on the precise alignment of opposing optical fibers at a connection to minimize signal losses, which diminishes the quality of the optical transmission through the connection. The precision of aligning opposing optical fibers is more sensitive with multi-fiber ferrules due to the presence of multiple optical fibers and to each fiber&#39;s location relative to each other and relative to the guide pins within the ferrules. Additionally, when an optical fiber is a single-mode fiber, i.e., the optical signal is transmitted through only a small portion of the fiber, the alignment needs to be even more precise. 
     A conventional ferrule molding method uses a series of V-shaped open grooves machined into a block of the mold cavity to retain the bore forming pins inserted into the mold cavity. FIG. 2 shows a cross-sectional view of this conventional molding method, where fiber bore forming pins  7  and guide pin bore forming pins  8  are shown disposed in V-shaped grooves  9 . The disadvantages of this or similar open groove constructions include a tendency of the pins  7  and  8  to float within the V-shaped grooves in the direction of arrow A during the molding process. This float contributes to imprecise alignment of the bores formed in the molded ferrule. Additionally, after repeated uses of a mold cavity with this groove construction, flash begins to build up in areas indicated by B. This flash build up requires frequent cleaning of the grooves. Also, as can be seen, pins  7  contact the V-shaped grooves only along two lines of contact and thus all the friction forces of the repeated insertion and removal of the pins are imparted along these two lines of contact, thereby causing uneven wear along the sides of the V-shaped groove. This causes the alignment of the pins to become progressively more imprecise. 
     The drawbacks of the molding process with the V-shaped grooves have been addressed by the “small hole technology” disclosed by U.S. Pat. No. 5,786,002 to Dean et al. As shown in FIG. 3, Dean et al. discloses a guide block assembly comprising a plurality of fiber bore blocks  12 , at least two guide pin bore blocks  14  and a plurality of spacer blocks  16  arranged in any desirable configuration in a mold cavity. Each fiber bore block  12  defines a small hole or bore  18  adapted to receive during the molding process a pin having the diameter of an optical fiber, and each guide pin bore blocks  14  defines a bore  19  adapted to receive a pin having a diameter of a guide pin. Molten plastic is injected into the mold cavity and the pins are thereafter withdrawn from the holes and the mold cavity to form receptacles in the ferrules to receive optical fibers  2  or guide pins  6 . The use of bores more precisely retains the pins during the molding process than the use of V-shaped open grooves. Dean et al. resolves the known drawbacks from the V-shaped open groove molding technique, and provides the additional benefits of establishing precise spatial relationship among the modular blocks, by machining the surfaces of the adjoining blocks. 
     Dean et al., however, requires the fabrication of multiple blocks, which increases the costs and may become less economical when used to fabricate ferrules for a small number of optical fibers. Hence, there remains a need in the art for a molding apparatus that has the advantages realized in the Dean et al. &#39;002 patent, but requires fewer components and is more economical to produce. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a device for molding multi-fiber ferrules using small hole technology. 
     Another object of the invention is to minimize the costs of fabricating a device for molding multi-fiber ferrules. 
     Yet, another object of the invention is to provide a device capable of precisely aligning and retaining the bore forming pins during the molding of multi-fiber ferrules. 
     These and other objects of the present invention are accomplished by a guide block assembly for aligning and retaining at least one fiber bore forming pin and at least one guide pin bore forming pin during the molding of a ferrule. The guide block assembly comprises a unitary member defining at least one fiber bore and at least one guide pin bore. The fiber bore is created by an electric discharge machining (EDM) wire. One starter hole is created for each bore with the EDM wire attached at one end to an EDM machine. The starter hole is then enlarged by a second EDM wire connected to an EDM machine at both ends. 
     The ratio between the length of the fiber bore to its diameter is preferably from approximately 3::1 to 10::1, more preferably from approximately 4::1 to 8::1, and most preferably approximately 6::1. In accordance with another aspect of the invention, the unitary guide block assembly has a front face, wherein the front face is altered to form a non-rectilinear surface. The non-rectilinear surface can be a curve surface, a stepped surface, an angled surface, or a pedestal surface, among others. 
     In accordance with other aspects of the invention, an open cavity behind the fiber bore is provided to reduce the flash build-up, and longitudinal slots are formed around the fiber bore to reduce the wear and tear on the bore forming pins. 
     In accordance with another aspect of the invention, a method for fabricating a guide block assembly defining at least one fiber bore for aligning and retaining at least one fiber bore forming pin during a molding of a ferrule is provided. This method comprises the steps of securing a blank to a wire electric discharge machining (EDM) machine, forming a starter hole in said blank with a wire attached at one end to the EDM machine, and enlarging said starter hole to a predetermined size and dimension of the fiber bore. The enlarging step may comprise the steps of threading a second wire through the starter hole, and connecting both ends of the second wire to an EDM machine (which may be a different EDM machine) to enlarge the starter hole. This method may also comprise the step of forming a non-rectilinear surface on a front surface of the guide block assembly before or after the fiber bore is formed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is illustrated by way of example in the accompanying drawings, in which: 
     FIG. 1 is a perspective view of a pair of conventional multi-fiber optical connector ferrules; 
     FIG. 2 is a cross-sectional view of a conventional V-shaped open groove guide block assembly; 
     FIG. 3 is front view of another prior art guide block assembly; 
     FIG. 4 is a perspective view of a representative arrangement of a mold cavity environment illustrating a preferred embodiment of a guide block assembly of the present invention; 
     FIG. 5 is an exploded view showing the top and bottom portions of the guide block assembly of the present invention; 
     FIG. 6 is an enlarged front view showing an array of fiber bores and guide pin bores of the guide block assembly of the present invention; 
     FIG. 7 is a cross-sectional view of the bottom portion of the guide box assembly along line  7 — 7  shown in FIG. 5; 
     FIG. 8 is a top view of the bottom portion of the guide block assembly showing another aspect of the invention; 
     FIG. 9 is a longitudinal cross-sectional view of a fiber bore with a fiber bore forming pin inserted therein; 
     FIG. 10 is a front view of an alternative embodiment of the present invention; 
     FIG. 11 is a front view of another alternative embodiment of the present invention; 
     FIG. 12 is a perspective front view of another embodiment of the present invention; 
     FIG. 13 is a perspective back view of the embodiment show n in FIG. 12; 
     FIG. 14 is a top view of the embodiment shown in FIGS. 12 and 13; 
     FIG. 15A is a front view of the embodiment shown in FIGS. 12,  13  and  14 , and 
     FIG. 15B is an enlarged view of a portion of FIG. 15A; 
     FIG. 16A is a front view of the alternative embodiment of the present invention, and 
     FIG. 16B is an enlarged view of a portion of FIG. 16A; 
     FIGS. 17A-E illustrate variations of the end face of the ferrule produced by embodiments of the guide block assembly of the present invention; and 
     FIGS. 18A-B are enlarged views of a portion of the guide block assembly showing the milled front face. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings, wherein reference numbers are used to designate like parts, FIG. 4 shows one preferred embodiment of the guide block assembly  20 , disposed in a mold cavity  22  to illustrate the environment for the guide block assembly  20 . Assembly  20  has a mold face  24 , which can serve as one of the walls  26  defining mold cavity  22 . 
     Referring to FIG. 5, assembly  20  comprises a top portion  28  and a bottom portion  30 . Top portion  28  defines on its lower surface a number of semi-circular guide pin bore grooves  32   a  and a number of semi-circular fiber bore grooves  34   a . As illustrated by FIG. 5, fiber bore grooves  34   a  are positioned on the inside of the guide pin bore grooves  32   a . Grooves  32   a  and  34   a  are sized and configured to match with semi-circular guide pin bore grooves  32   b  and semicircular fiber grooves  34   b  located on the top surface of bottom portion  30 , such that when the top and bottom portions are assembled together, the semi-circular fiber bore grooves  34   a  and  34   b  are joined to form fiber bores  34  and semi-circular guide pin bore grooves  32   a  and  32   b  are joined to form guide pin bores  32 . The top portion  28  also defines two vertical channels  36   a  corresponding to vertical channels  36   b  defined on the bottom portion  30 , such that conventional fasteners such as nuts and bolts may clamp the top portion  28  to the bottom portion  30 . 
     Additionally, the top and bottom portions  28  and  30  may have a pair of corresponding key pin grooves  35   a  and  35   b , respectively, as illustrated in FIG.  5 . Key pin grooves  35   a  and  35   b  together form key pin bore  35  adapted to receive a key pin when the top and bottom portions  28  and  30  are assembled. The key pin  37  is inserted into bore  35  to align the top portion to the bottom portion. The bottom portion may also have receiving channels  40  disposed on its bottom surface. Receiving channels  40  are sized and dimensioned to receive corresponding bosses on the mold cavity (not shown), such that the guide block assembly  20  can be securely affixed onto the mold cavity. Also, top and bottom portions  28  and  30  may also have through holes  42  and  44 , whose function is described below. 
     FIG. 6 is an enlarged exemplary view of the assembly  20  showing the relative dimension and location of guide pin bore grooves  32   a  and  32   b  in relation to fiber grooves  34   a  and  34   b . Although, only two sets of guide pin bore grooves and fiber grooves are shown, any number of grooves can be defined by assembly  20 . FIG. 7 shows a cross-sectional view of bottom portion  30  illustrating by example the location of the guide pin bore grooves  32   b , channels  36   b  and key pin groove  35   b  in relation to each other. 
     In accordance with another aspect of the invention illustrated in FIG. 8, the length of the fiber bores  34  is kept relatively short relative to its diameter, and an open cavity or space  39  is provided behind the bores  34 , such that the molding residue can be pushed through the bores on repeated molding cycles and collect in the open cavity or space  39  instead of clogging the fiber bores. While the open cavity or space  39  is illustrated on bottom portion  30 , it may also be on the top portion  28  or both. The preferred ratio between the length and diameter of the fiber bore is approximately between 3::1 and 10::1; the more preferred ratio is approximately between 4::1 to 8::1; and the most preferred ratio is approximately 6::1. 
     Guide block assembly  20  is configured to retain a plurality of fiber bore forming pins  50  receivable in fiber bores  34  and retain guide pin bore forming pins  51  receivable in guide pin bores  32 , as shown in FIG.  4 . Any suitable jig, not shown, can be used to hold and to move pins  50  and  51  into and out of bores  34  and  32 , respectively. During the molding of a multi-fiber ferrule, the pins are inserted into the bores and the molding material is injected into the mold cavity formed in part by walls  24  and  26  around the pins. For example, as shown in FIG. 9, the distal end portion  52  of a representative fiber bore forming pin  50  is partially inserted into fiber bore  34  and molding material is injected into mold cavity  22  and covers mold zone  54  of fiber bore forming pin  50  outside of bore  34 . After the mold material sets, the pins are retracted to leave behind a plurality of molded bores in the ferrules. Fiber bore forming pins  50  will create a number of fiber bores sized and dimensioned to receive optical fibers in close tolerance. Since the location of the bores  32  and  34  can be precisely machined as described below, and the pins  50  and  51  are held in these precisely positioned bores during the molding process, the molded bores in the ferrules created by the withdrawal of the pins are also precisely positioned to receive the optical fibers and guide pins, especially at the front face  24  of assembly  20 . It should be noted that the front face of the ferrule would be formed at the front face  24 . 
     Fiber bore forming pins  50  may be the actual fibers when the ferrules are molded directly around the fibers. As shown in FIGS. 6-8, the guide pin bores  32  and guide pin bore forming pins  51  typically create larger diameter molded guide pin bores than the molded fiber bores to receive the guide pins to align two opposing multi-fiber ferrules. The shape of the guide bore forming pins  51  and guide pin bores  32  is shown to be circular. This shape, however, can be any shape, such as oval, triangular or polygonal. 
     The present invention is directed to an apparatus and method to precisely arrange the fiber bore forming pins and the guide pin bore forming pins relative to each other in such a way that the precision is repeatable over a large number of molding cycles. As discussed in the background of the invention, the method of arranging the bores with V-shaped grooves as shown in FIG. 2 suffers from floating of pins, flash build up, and premature and uneven wear of the guide block assembly. By using pre-arranged bores in the guide block assembly and insertable pins, float and uneven wear are reduced and flash build up is substantially eliminated. Specifically, bores  32  and  34  provide less room than the V-shaped open grooves for the floating of the pins  50  and  51  during the molding process. Furthermore, by providing bores the contact between the pins and the bores is spread out over the circumferential contact surface between the bores and the pins, thereby decreasing wear on the bores. Additionally, by adopting the preferred range of ratios between the length and diameter of the fiber bore  34  and by providing an open cavity  39  behind the fiber bores  34  as shown in FIG. 8, the clogging problem is substantially reduced. 
     Also by having only a limited number of components, e.g., two portions  28  and  30  in the above-described preferred embodiment, the present invention reduces the costs of fabricating the guide block assembly over the guide block assembly discussed in Dean et al., which comprises a relatively high number of blocks. 
     In another aspect of the present invention, the bottom surface of the top portion  28  of the guide block assembly  20  and the top surface of the bottom portion  30  are mirror images of each other. When the two portions are clamped or bolted face-to-face together, any remaining misalignment after the key pin  37  in inserted into the key pin bore  35  can be readily detected. Such misalignment would make the diameter of the bores in the guide block assembly smaller in the direction from the mold face  24  toward the back of the assembly, when the grooves  34   a  and  34   b  are aligned at the mold face but misaligned elsewhere. A simple lapping process performed on the bore can readily remove any such misalignment. The lapping process comprises covering a precision gauge wire having a diameter smaller than the bore with a lapping compound, e.g., an abrasive compound such as one-quarter micron diamond grit, and then using the precision gauge wire with the lapping compound into the bore to remove any misalignments. 
     The guide block assembly  20  of the present invention can be manufactured by machining the semi-circular grooves into a metal or ceramic block using known precision grinding techniques. Preferably, the top and bottom portions  28  and  30  can be manufactured by an electric discharge machining (EDM) process. A precision wire EDM machine, or more preferably a submersible wire EDM machine, removes metals from metal blocks by creating thousands of electrical discharges per second that flow between a wire and the metal blocks, vaporizing metal in the controlled area. In the preferred submersible wire EDM machine, a zinc-coated brass, molybdenum or tungsten wire of approximately 0.0005 to 0.003 inch in diameter is submerged in a tank of dielectric fluid, such as deionized water, along with the metal blocks. As the wire is moved relative to the metal blocks, semi-circular grooves are formed on the blocks. Typically, eight to twelve passes from the EDM wire can create the preferred fiber pin groove. The motion of the wire may be controlled by any commercially available computer numerical control (CNC) software. A detailed discussion the EDM processes is provided in the Machinery&#39;s Handbook, by E. Oberg et al, (Industrial Press, 1996)(25 th  edition) at page 1266. This discussion is hereby incorporated by reference. 
     At least one manufacturing advantage is realized by the fact that opposing surfaces on the top/bottom portions of the assembly  20  are mirror-images of each other. Hence, regardless of the actual manufacturing technique used, e.g., grinding, machining, or EDM processes, the two corresponding opposing surfaces can be manufactured at the same time using the same equipment. For example, the bottom surface of top portion  28  and the top surface of the bottom portion  30  illustrated in FIGS. 5 and 6 can be manufactured at the same time by securing two metal blanks side by side, and corresponding pairs of semi-circular grooves  34   a  and  34   b  or  32   a  and  32   b  are created by the EDM wire or by the blade of a cutting tool across the two metal blanks. This ensures that any one pair of grooves is properly cut and positioned on the metal blanks. As discussed above and illustrated in FIGS. 5,  7 , and  8 , the through holes  42  and  44  provided on the metal blanks are dimensioned and configured to receive fasteners, such as screws or bolts and nuts, to secure the metal blanks together. The holes  42 , 44  may have countersinks (not shown) for the fasteners that hold the top and bottom portions  28 , 30  together during the EDM or machining process. 
     By utilizing only a small number of components to construct the guide block assembly while still employing the “small hole technology,” the present invention is able to avoid the drawbacks of the conventional V-shaped open groove method, and accomplishes the same objectives as Dean et al. at lower costs. 
     It will also be noted that although only two semi-circular fiber grooves  34   a,b  on the top and bottom portions are illustrated in FIG. 5, any number of fiber grooves can be machined on the top and bottom portions. Furthermore, although only one row of fiber bores is shown on guide block assembly  20 , the present invention may have any number of rows, as shown in FIG.  11 . The guide block assembly  70  may have a plurality of rows of fiber bores, for example two rows of fiber bores. Assembly  70  comprises three portions: a top portion  72 , a middle portion  74  and a bottom portion  76 . In this example, top portion  72  defines five semi-circular fiber grooves on its lower surface to correspond with the five semi-circular fiber grooves on the top surface of the middle portion  74 . Middle portion  74  in turn has three semi-circular fiber grooves and two semicircular guide pin bore grooves defined on its lower surface to correspond with the three semicircular fiber grooves and two semi-circular guide pin bore grooves defined on the top surface of the bottom portion  76 . Hence when the three portions of assembly  70  are assembled, a first row of five fiber bores and a second row of three fiber bores disposed between two guide pin bores are formed, as shown. In accordance with the present invention, any number of rows of any number of bores can be formed and the guide pin bores can be located on any row using the manufacturing processes described above. For example, the bottom surface of top portion  72  and top surface of middle portion  74  can be fabricated at the same time, and the bottom surface of middle portion  74  and top surface of bottom portion  76  can be fabricated at the same time. 
     Alternatively, the guide block assembly can be fabricated from a single block as shown in FIG. 10 to further reduce the costs of fabricating the guide block assembly. Using the wire EDM process, after a starter bore  62  is first created by conventional techniques such as drilling, the EDM wire may be inserted in the starter hole and the cut a path  64  to form fiber bores  34 . Path  64  may then be filled with a high temperature epoxy. Guide pin bores  32  may be drilled as shown, or path  64  may extend from fiber bores  34  to create guide pin bores  32 . 
     In accordance with another aspect of the invention, another unitary guide block assembly  80  is shown in FIGS. 12-15. Guide block assembly  80  is made from a single block of material. Unlike the guide block assembly  60  discussed above, guide block assembly  80  does not require the EDM wire to form path  64  to connect the fiber bores and/or guide pin bores together, and therefore obviates the needs to back fill path  64  with epoxy. As shown, unitary guide block assembly  80  has relief cavity  39  disposed behind fiber bores  34 , as described above. Similar to guide block assembly  20 , assembly  80  also has guide pin bores  32  disposed to the outside of fiber bores  34 , and receiving channels  40  sized and dimensioned to received corresponding bosses on the mold cavity (not shown), such that the guide block assembly can be securely affixed onto the mold cavity, as discussed above and as illustrated in FIG.  4 . Assembly  80  also has front face  24 , which serves as one of the walls of mold cavity  22 . 
     The advantages of unitary guide block assemblies  80  and  60  over guide block assembly  20  include the elimination of a number of components, such as key pin bore  35  and key pin  37  to align the two halves of the guide block assembly, through holes  42  and  44  to clamp the halves together during the manufacturing process, and the vertical channels  36   a  and  36   b  to clamp the halves together during the ferrule molding process. Additionally, the lapping process to ensure proper alignment of the fiber bore grooves is also not necessary. 
     Guide block assembly  80  is preferably manufactured by a novel EDM manufacturing process. First, the relief cavity  39  is cut by conventional method in the blank block. An additional relief channel  82  may be provided behind relief cavity  39 . Next, a series of starter holes is fashioned into the blank. One starter hole is prepared for each guide pin bore  34  and for each fiber bore  32 . Due to the relative sizes of these bores, the starter holes for the guide pin bores can be larger than the starter holes for the fiber bores. The starter holes are formed by a single EDM wire, which is connected to the EDM machine only at one end. The free end of the EDM wire is positioned at the desired location and electrical discharges are emitted therefrom to create the starter holes. Advantageously, the starter holes for the fiber bores  34  are located opposite from relief cavity  39 , where the width of the blank is thinnest, which, in addition to preventing flash build-up, also facilitates the creation of the starter holes. After the starter holes are made, a longer continuous feed EDM wire is threaded through each starter hole. This longer wire is then connected to the EDM machine at both end and the electrical discharges from this longer wire are emitted to enlarge the starter hole until the hole reaches the desired size of the fiber bore  32 . Advantageously, the blank block remains clamped to the EDM machine during the entire manufacturing process, thereby eliminating possible location and sizing errors due to handling and repositioning of the blank. The starter holes for the guide pin bores  34  may also be created the same way. Due to the relative larger size of the guide pin bores, their starter hole may also be created by conventional methods, such as drilling. 
     The starter-hole EDM wire typically is 0.0020-0.0025 inch in diameter and 3 mm (0.12 inch) in length. As described above, EDM wires are typically made from zinc coated brass, molybdenum, or tungsten. Due to the relative shortness of the wire, it can be formed rigid and straight, which increase the accuracy of the position and orientation of the starter holes. As a result, the fiber holes  32  made in accordance with this EDM wire method extend in a precise straight line and running perpendicular to the front surface  24  of the unitary guide block assembly  80 . 
     Several advantages directly flow from this EDM wire method. First, due to the ability to pin point the starter hole and then create straight fiber bores, the radial offset between corresponding optical fibers from two adjoining ferrules made in accordance with this method has been reduced to ¼ μm for single mode fibers and ½ μm for multi mode fibers. As discussed above, single mode fiber use only a relatively small portion of the fiber&#39;s cross-section for signal transmission, while multi-mode fibers use more of the fiber&#39;s cross-section. For example, a single mode fiber uses approximately 9 μm section of a 125 μm optical fiber, while a multi-mode fiber uses approximately 50-60 μm of the 125 μm optical fiber. Hence, tight control of the radial offset between connecting optical fibers, particularly a single mode fiber, is desirable and can be achieved by the ferrules made in accordance with the present invention. Furthermore, the radial offset between the fiber bores  32  and the fiber bore forming pins  50  during the molding operation has also been reduced to about ½ μm. 
     Another advantage realized from the EDM wire method is that due to the more precise perpendicular orientation of the fiber bores  32  relative to the front surface  24  of guide block assembly  80 , front surface  24  may have an arcuate surface or other non-rectilinear surfaces milled or otherwise formed thereon, after the fiber bores have been formed while maintaining the precise location of the fiber bores on the milled front surface  24 . In other words, if the fiber bores  32  are not precisely oriented perpendicular to front surface  24  when front surface  24  is milled, the location of the fiber bores  32  on the milled surface will shift relative to the location of the fiber bores  32  on the pre-milled surface. This shifting produces inaccurate guide block assemblies, which in turn produces inaccurate placement of the fibers at the front face of the ferrules. The fiber bores  32  may also be formed after the front surface  24  is milled. 
     It is known in the art that providing a protruding curve or other non-rectilinear surfaces on the end face  5  of ferrule  3  positions the terminal ends of the optical fibers  2  forward of the ferrule to reduce back reflectance and improve signal transmission between ferrules. Examples of milled front surface  24  are illustrated in FIGS. 18A and 18B. Heretofore, each ferrule is typically ground or polished after molding to achieve the curve surface. A unitary milled guide block assembly in accordance with the present invention obviates the need to grind each ferrule separately, thereby reducing manufacturing costs. In addition to a milled curved front face  24 , which produces the curved end face  5  in ferrule  3  shown in FIG. 17A, front face  24  may have other shapes milled thereon to produce other shapes for end face  5 . For example, front face  24  may have a step milled therein, shown in FIG. 18A, to produce the stepped face  5  shown in FIG.  17 B. Front face  24  may also have individual holes milled around each fiber bore  32  to produce the end face  5  shown in FIG.  17 C. Front face  24  may also have an elongated channel milled around all the fiber bores  32 , shown in FIG. 18B, to produce the pedestal end face  5  shown in FIG. 17D, and front face  24  may have a slant formed thereon to produce the angled end face  5  shown in FIG.  17 E. 
     Additionally, while FIGS. 12-15 show the unitary guide block assembly  80  with two fiber bores  32  defined thereon, it may have any number of fiber bores. For example, unitary guide block assembly  84  shown in FIGS. 16A-16B has 12 fiber bores and the ferrules shown in FIGS. 17A-E were made with a guide block assembly defining  4  fiber bores. Hence, this present invention is not limited to any specific number of fiber bores. 
     In accordance with another aspect of the present invention, a plurality of longitudinal slots may be cut along the periphery of the fiber bores  32  or the guide pin bores  34  to reduce the wear and tear on the fiber bore forming pins  50  and the guide pin bore forming pins  51 . While it remains desirable to evenly distribute the contact between the pins and the bores during the molding process as discussed above, it is also advantageous to reduce the contact areas between these two components. Preferably, four longitudinal slots disposed along the bores reduce such contact areas. 
     While various descriptions of the present invention are described above, it is understood that the various features of the present invention can be used singly or in combination thereof. Therefore, this invention is not to be limited to the specifically preferred embodiments depicted therein.