Patent Publication Number: US-6669378-B2

Title: Ferrule assembly and optical module

Description:
This application is a divisional of application Ser. No. 10/114,031, filed Apr. 3, 2002, now U.S. Pat. No. 6,488,415, which is divisional of application Ser. No. 10/059,023 filed Jan. 30, 2002, now pending, which is a divisional of application Ser. No. 09/810,399 filed Mar. 19, 2001, U.S. Pat. No. 6,394,663, which is a divisional of Ser. No. 09/810,540, filed Mar. 19, 2001, U.S. Pat. No. 6,390,686, which is a divisional of application Ser. No. 09/349,706, filed Jul. 8, 1999, U.S. Pat. No. 6,241,399. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to an optical transmission module for use in the optical communication field, and more particularly to a receptacle type optical transmission module. 
     2. Description of the Related Art 
     In the recent information communications field, high-speed large-capacity processing and high-speed data transmission are required in response to the advancement of information. To meet this requirement, optical transmission is indispensable and preparation is now proceeding toward the expansion and diffusion of an optical communications network. 
     Known as a device used at many sites in an optical transmission system is an optical transmission module having an optical circuit and an electrical circuit in combination for performing opto-electrical conversion or electro-optical conversion. At present, the production scale of the optical transmission module per communications maker is about 10 5  products per year. However, it is said that the production scale required in the future will become 10 6  or more products per year in response to the diffusion of an optical communications network and that the production cost must be reduced to about {fraction (1/10)} or less of the present level. Accordingly, it is strongly desired to establish any form of the optical transmission module which can realize mass production and low cost by minimizing the number of components to simplify the assembly process and can also ensure high reliability and long service life. 
     The components mounted on a printed wiring board built in a communications device are generally classified into a surface mount type and a through hole mount type. A typical example of the surface mount type components is an LSI, which has a form called a flat package. Such a component is soldered to the printed wiring board by a reflow soldering process. This process is performed by printing a solder paste on the printed wiring board, making the surface mount type component stick to the printed solder paste, and heating the whole in a conveyor oven to a solder surface temperature of 220° C. or higher. 
     A typical example of the through hole mount type components is a large-capacity capacitor or a multi-terminal (200 or more terminals) LSI. The multi-terminal LSI has a terminals form called a PGA (Pin Grid Array). Such a through hole mount type component is soldered to the printed wiring board by a flow soldering process. This process is performed by inserting the terminals of the through hole mount type component into through holes of the printed wiring board, and putting the printed wiring board into a solder bath heated at about 260° C. from the side opposite to its component mounting surface. 
     In mounting an optical module on the printed wiring board by soldering like the surface mount type component or the through hole mount type component, a so-called pigtail type of optical module with an optical fiber cord is not suitable as the optical module. That is, the optical fiber cord usually has a nylon coating, and the nylon coating has a low resistance to heat at about 80° C., so that it is melted in the soldering step. Furthermore, the optical fiber cord itself invites inconveniences in accommodation and handling at a manufacturing location, causing a remarkable reduction in mounting efficiency to the printed wiring board. 
     Accordingly, to allow a soldering process for the optical module and reduce a manufacturing cost, the application of a so-called receptacle type of optical module is indispensable. An example of the receptacle type optical module allowing a soldering process is known from 1996 IEICE, General Meeting Proc., C-207 (Ref. 1). In Ref. 1, there is described a receptacle type optical module manufactured by retaining a photoelectric converter and a ferrule with a bare optical fiber on a silicon substrate, next covering the whole with a silicon cap to hermetically seal an optical coupling region, and finally molding the whole with an epoxy resin. 
     The silicon substrate is formed with a V groove for positioning the optical fiber and the ferrule, both of which are simultaneously fixed by the silicon cap. A lead frame is fixed by an adhesive directly to the silicon substrate, so that the lead frame forms electrical input and output terminals. A commercially available MU type connector housing is mounted on an optical fiber connecting portion to realize connection and disconnection of another optical fiber. By flow soldering of the lead frame extending from the molded package, the optical module is mounted on a printed wiring board. 
     Another example is known from 1997 IEICE, General Meeting Proc., C-361 (Ref. 2). In Ref. 2, a V groove for positioning a bare optical fiber and a ferrule is formed on a silicon substrate as in Ref. 1. The bare optical fiber is fixed to the silicon substrate by a glass plate through a UV curable adhesive, thereby realizing optical coupling between the optical fiber and a photoelectric converter. 
     An optical coupling region between the photoelectric converter and the optical fiber is sealed by a transparent epoxy resin. The silicon substrate is fixed to a lead frame forming an electrical input terminal, and the lead frame is connected through a gold wire to the photoelectric converter. The whole except an end portion of the ferrule is molded with a resin to form a molded package. An optical connector adapter is mounted onto the molded package to complete an optical module. The optical connector adapter is used to detachably connect another optical fiber to the optical module. By flow soldering of the lead frame extending from the molded package, the optical module is mounted on a printed wiring board. 
     In an optical subscriber transmission system, economization of the optical transmission system as a whole is also necessary. To this end, there has been proposed and standardized a wavelength division multiplexing bidirectional communication system having a single office terminal to be used commonly by many subscribers. To realize this configuration, an optical module having wavelength multiplexing/demultiplexing functions is required both in each of the subscriber terminals and in the office terminal. In particular, an optical module incorporating a PLC (planar lightwave circuit) formed by integrating the wavelength multiplexing/demultiplexing functions in one chip is expected from the viewpoints of mass production and cost reduction. 
     In reducing an assembly cost for such a subscriber optical transmission module, it is important to ensure a cost reducing technique for a receptacle structure of an optical fiber interface, especially, an interface between a PLC having wavelength multiplexing/demultiplexing functions and an optical fiber. Conventionally known is a self-alignment technique for the connection between a PLC and an optical fiber. In this conventional technique, a fiber guide is formed on a silicon substrate so as to make alignment of the core of an optical waveguide in the PLC and the core of the optical fiber, thereby determining optimum positions of the PLC and the optical fiber in a self-aligned fashion. 
     According to such a self-alignment mounting method, it is not necessary to supply a current to an optical semiconductor element, and it is also not necessary to provide a complicated aligning device for aligning the core of the optical waveguide and the core of the optical fiber. Further, no time for the alignment is needed. Accordingly, this method is suitable for mass production and cost reduction. 
     Known as another example of the receptacle type optical module in the prior art is a technique of optically connecting an optical element and a receptacle ferrule through a V-grooved silicon substrate in a self-aligned fashion. By replacing-the optical element with an optical waveguide to follow this prior art technique, it is possible to obtain a structure such that the optical waveguide and the receptacle ferrule are to be optically connected through a V-grooved PLC substrate in a self-aligned fashion. 
     Also known as another prior art technique is a receptacle type optical module for providing an interface between a PLC having a plurality of optical waveguide cores and multiple optical fibers. In this prior art technique, V grooves for two guide pins are formed on a substrate, and optical coupling between a plurality of optical elements mounted on the substrate or the plurality of optical waveguide cores and the multiple optical fibers is attained through the two guide pins. 
     The above-mentioned conventional receptacle type optical module has the following problems. First, a deep V groove must be formed on the substrate, so as to mount the ferrule on the substrate. Accordingly, the silicon substrate on which the optical element is mounted or the PLC substrate on which the optical waveguide is formed must be made thick, resulting in an increase in material cost. Further, the substrate must be left under the ferrule, causing a disadvantage in reducing the thickness of the optical module. 
     Secondly, in the conventional receptacle type optical module, the ferrule mounted in the V groove and the optical element or the optical waveguide core are aligned with each other. Accordingly, there is a possibility of large misalignment between the optical waveguide core (or an active layer in the optical element) and the core of the optical fiber fixed in the ferrule, causing a large optical coupling loss. As a result, characteristics of the optical module are degraded. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a receptacle type optical module suitable for cost reduction and size reduction. 
     It is another object of the present invention to provide a ferrule assembly required for assembling of the receptacle type optical module. 
     In accordance with an aspect of the present invention, there is provided a ferrule assembly comprising a ferrule having a through hole; and an optical fiber inserted and fixed in the through hole; the ferrule having a flat cut portion for semicylindrically exposing a part of the optical fiber inserted and fixed in the through hole. 
     In accordance with another aspect of the present invention, there is provided an optical module comprising a substrate having a groove; an optical waveguide layer formed on the substrate, the optical waveguide layer comprising an optical waveguide core having first and second ends, the first end being aligned with the groove, and an optical waveguide cladding covering the optical waveguide core; a ferrule having a through hole; and an optical fiber inserted and fixed in the through hole; the ferrule having a flat cut portion for semicylindrically exposing a part of the optical fiber inserted and fixed in the through hole; the ferrule being fixed at the flat cut portion to the substrate so that the part of the optical fiber exposed to the flat cut portion is inserted into the groove of the substrate until one end of the optical fiber abuts against the first end of the optical waveguide core. 
     Preferably, an optical element such as a laser diode or a photodiode is mounted on the substrate at its one end portion opposite to the other end portion on which the ferrule is mounted so that the optical element is optically coupled to the second end of the optical waveguide core. 
     In accordance with still another aspect of the present invention, there is provided an optical module comprising a substrate having first and second grooves at opposite end portions thereof; an optical waveguide layer formed on an intermediate portion of the substrate, the optical waveguide layer comprising an optical waveguide core having first and second ends respectively aligned with the first and second grooves, and an optical waveguide cladding covering the optical waveguide core; first and second ferrules each having a through hole; and first and second optical fibers inserted and fixed in the through holes of the first and second ferrules, respectively; the first and second ferrules respectively having first and second flat cut portions for semicylindrically exposing a part of the first optical fiber inserted and fixed in the through hole of the first ferrule and a part of the second optical fiber inserted and fixed in the through hole of the second ferrule, respectively; the first ferrule being fixed at the first flat cut portion to the substrate so that the part of the first optical fiber exposed to the first flat cut portion is inserted into the first groove of the substrate until one end of the first optical fiber abuts against the first end of the optical waveguide core; the second ferrule being fixed at the second flat cut portion to the substrate so that the part of the second optical fiber exposed to the second flat cut portion is inserted into the second groove of the substrate until one end of the second optical fiber abuts against the second end of the optical waveguide core. 
     In accordance with a further aspect of the present invention, there is provided an optical module comprising a substrate having a groove; an optical waveguide layer formed on the substrate, the optical waveguide layer comprising a first optical waveguide core having first and second ends, a second optical waveguide core having third and fourth ends, the third end being connected to an intermediate portion of the first optical waveguide core, and an optical waveguide cladding covering the first and second optical cores; an optical wavelength filter mounted on the substrate so as to intersect a junction between the first and second optical waveguide cores; a semicut ferrule assembly comprising a ferrule having a through hole, and an optical fiber inserted and fixed in the through hole, the ferrule having a flat cut portion for semicylindrically exposing a part of the optical fiber inserted and fixed in the through hole, the ferrule being fixed at the flat cut portion to the substrate so that the part of the optical fiber exposed to the flat cut portion is inserted into the groove of the substrate until one end of the optical fiber abuts against the first end of the first optical waveguide core; a first optical element mounted on the substrate so as to be optically coupled to the second end of the first optical waveguide core; and a second optical element mounted on the substrate so as to be optically coupled to the fourth end of the second optical waveguide core. 
     For example, the first optical element is a photodiode for detecting a laser beam having wavelengths in a 1.55 μm band, and the second optical element is a laser diode for emitting a laser beam having wavelengths in a 1.3 μm band. 
     In accordance with a further aspect of the present invention, there is provided an optical module comprising a substrate having a first maker at one end portion thereof; an optical waveguide layer formed on the substrate, the optical waveguide layer comprising an optical waveguide core and an optical waveguide cladding covering the optical waveguide core, the optical waveguide cladding having a narrow first portion and a wide second portion; a glass plate having a groove and a second marker, the glass plate being fixed to the substrate so that the second marker is aligned with the first marker, and that the groove accommodates the first portion of the optical waveguide cladding; and a semicut ferrule assembly comprising a ferrule having a through hole, and an optical fiber inserted and fixed in the through hole, the ferrule having a flat cut portion for semicylindrically exposing a part of the optical fiber inserted and fixed in the through hole, the ferrule being fixed at the flat cut portion to the glass plate so that the part of the optical fiber exposed to the flat cut portion is inserted in the groove of the glass plate to optically couple the optical fiber to the optical waveguide core. 
     In accordance with a further aspect of the present invention, there is provided an optical module comprising a substrate having a plurality of grooves; an optical waveguide layer formed on the substrate, the optical waveguide layer comprising a plurality of optical waveguide cores having a plurality of first ends respectively aligned with the grooves, and an optical waveguide cladding covering the optical waveguide cores; and a connector assembly comprising a block having a plurality of through holes, a plurality of optical fibers inserted and fixed in the through holes, respectively, and a plurality of guide pins fixed to the block, the block having a flat cut portion for semicylindrically exposing a part of each of the optical fibers inserted and fixed in the through holes; the block being fixed at the flat cut portion to the substrate so that the parts of the optical fibers exposed to the flat cut portion are inserted into the grooves of the substrate until front ends of the optical fibers abut against the first ends of the optical waveguide cores, respectively. 
     In accordance with a further aspect of the present invention, there is provided an optical module comprising a substrate having an end portion formed with a first groove and another end portion formed with a plurality of second grooves; an optical waveguide layer formed on an intermediate portion of the substrate, the optical waveguide layer comprising an optical waveguide core having a first end aligned with the first groove and a plurality of second ends respectively aligned with the second grooves, and an optical waveguide cladding covering the optical waveguide core; a first connector assembly comprising a first block having a first through hole, a first optical fiber inserted and fixed in the first through hole, and a plurality of first guide pins fixed to the first block, the first block having a first flat cut portion for semicylindrically exposing a part of the first optical fiber inserted and fixed in the first through hole; and a second connector assembly comprising a second block having a plurality of second through holes, a plurality of second optical fibers inserted and fixed in the second through holes, respectively, and a plurality of second guide pins fixed to the second block, the second block having a second flat cut portion for semicylindrically exposing a part of each of the second optical fibers inserted and fixed in the second through holes; the first connector assembly being fixed at the first flat cut portion to the substrate so that the part of the first optical fiber exposed to the first flat cut portion is inserted into the first groove of the substrate until a front end of the first optical fiber abuts against the first end of the optical waveguide core; the second connector assembly being fixed at the second flat cut portion to the substrate so that the parts of the second optical fibers exposed to the second flat cut portion are inserted into the second grooves of the substrate until front ends of the second optical fibers abut against the second ends of the optical waveguide cores, respectively. 
     The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view of a first preferred embodiment of the present invention; 
     FIG. 2 is a partially sectional, side view of the first preferred embodiment in its assembled condition; 
     FIG. 3 is a cross section taken along the line  3 — 3  in FIG. 2; 
     FIG. 4 is an exploded perspective view of a second preferred embodiment of the present invention; 
     FIG. 5 is a partially sectional, side view of the second preferred embodiment in its assembled condition; 
     FIG. 6 is a cross section of the second preferred embodiment as similar to FIG. 3; 
     FIG. 7 is a perspective view showing another preferred embodiment of a ferrule assembly; 
     FIGS. 8A to  8 C are perspective views showing other preferred embodiments of the ferrule assembly; 
     FIGS. 9A and 9B are perspective views showing still other preferred embodiments of the ferrule assembly; 
     FIG. 10 is a perspective view showing a coupling structure of different types of ferrules; 
     FIG. 11 is a perspective view of a third preferred embodiment of the present invention; 
     FIG. 12 is a perspective view of a fourth preferred embodiment of the present invention; 
     FIG. 13 is a perspective view of a fifth preferred embodiment of the present invention; 
     FIG. 14 is a perspective view of a sixth preferred embodiment of the present invention; 
     FIG. 15 is a perspective view of a seventh preferred embodiment of the present invention; 
     FIG. 16 is a perspective view of an eighth preferred embodiment of the present invention; 
     FIG. 17 is a perspective view of a ninth preferred embodiment of the present invention; 
     FIG. 18 is a perspective view of a tenth preferred embodiment of the present invention; 
     FIG. 19 is a perspective view of an eleventh preferred embodiment of the present invention; 
     FIG. 20 is a perspective view of a twelfth preferred embodiment of the present invention; 
     FIG. 21 is a perspective view of a thirteenth preferred embodiment of the present invention; 
     FIG. 22A is a perspective view of a fourteenth preferred embodiment of the present invention; 
     FIG. 22B is a perspective view showing a modification of the fourteenth preferred embodiment; 
     FIG. 23 is a perspective view of a fifteenth preferred embodiment of the present invention; 
     FIG. 24 is a plan view of a sixteenth preferred embodiment of the present invention; 
     FIG. 25 is a perspective view of a seventeenth preferred embodiment of the present invention; 
     FIG. 26 is a perspective view of an eighteenth preferred embodiment of the present invention; 
     FIG. 27 is a perspective view of a nineteenth preferred embodiment of the present invention; 
     FIG. 28 is a perspective view showing a PLC used in the nineteenth preferred embodiment; 
     FIG. 29A is a perspective view of a glass plate used in the nineteenth preferred embodiment; 
     FIG. 29B is a perspective view showing a modification of the glass plate; 
     FIG. 30A is an exploded perspective view of a twentieth preferred embodiment of the present invention; 
     FIG. 30B is a perspective view of the twentieth preferred embodiment in its assembled condition; 
     FIG. 31 is an exploded perspective view of a twenty-first preferred embodiment of the present invention; 
     FIG. 32 is a perspective view showing another preferred embodiment of a multfiber semicut connector; 
     FIG. 33 is an exploded perspective view of a twenty-second preferred embodiment of the present invention; 
     FIG. 34 is an exploded perspective view of a twenty-third preferred embodiment of the present invention; 
     FIG. 35A is an exploded perspective view of a twenty-fourth preferred embodiment of the present invention; 
     FIG. 35B is a perspective view of the twenty-fourth preferred embodiment in its assembled condition; 
     FIG. 36A is an exploded perspective view of a twenty-fifth preferred embodiment of the present invention; 
     FIG. 36B is a perspective view of the twenty-fifth preferred embodiment in its assembled condition; 
     FIG. 37A is an exploded perspective view of a twenty-sixth preferred embodiment of the present invention; 
     FIG. 37B is a perspective view of the twenty-sixth preferred embodiment in its assembled condition; 
     FIG. 38 is an exploded perspective view of a twenty-seventh preferred embodiment of the present invention; 
     FIG. 39A is an exploded perspective view of a twenty-eighth preferred embodiment of the present invention; and 
     FIG. 39B is a perspective view of the twenty-eighth preferred embodiment in its assembled condition. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Various preferred embodiments of the present invention will now be described in detail with reference to the drawings. In the following description of the preferred embodiments, substantially the same or similar parts will be denoted by the same reference numerals and the description thereof will be partially omitted to avoid repetition. 
     Referring to FIG. 1, there is shown an exploded perspective view of an optical module  2  according to a first preferred embodiment of the present invention. FIG. 2 is a partially sectional, side view of the optical module  2 , and FIG. 3 is a cross section taken along the line  3 — 3  in FIG.  2 . The optical module  2  includes a PLC (planar lightwave circuit)  4  and a semicut ferrule assembly  16  connected to the PLC  4 . The PLC  4  includes a silicon substrate  6  and an optical waveguide layer  8  formed on the silicon substrate  6 . The optical waveguide layer  8  includes an optical waveguide core  10  and an optical waveguide cladding  12  covering the optical waveguide core  10 . The optical waveguide core  10  has a square cross section whose side is about 8 μm. An optical signal propagates in the optical waveguide core  10  having a refractive index higher than that of the optical waveguide cladding  12 . 
     The upper surface of the silicon substrate  6  is exposed at its one end portion  6   a , and a V groove  14  is formed on the exposed upper surface  6   a  of the substrate  6  by anisotropic etching of silicon. The position and size of the V groove  14  are set so that when a bare optical fiber having a circular cross section whose diameter is 125 μm is mounted in the V groove  14 , the core (diameter: 9.5 μm) of the bare optical fiber is aligned with the optical waveguide core  10 . The semicut ferrule assembly  16  includes a cylindrical ferrule  18  having a through hole  20  and a bare optical fiber  22  inserted and fixed in the through hole  20 . The ferrule  18  is formed of zirconia, for example. The ferrule  18  further includes a flat cut portion  24  for semicylindrically exposing a part of the optical fiber  22  inserted and fixed in the through hole  20 . 
     The semicut ferrule assembly  16  is fabricated by semicylindrically cutting a part of a completely cylindrical ferrule to thereby form the ferrule  18  having the flat cut portion  24 , and next inserting the bare optical fiber  22  into the through hole  20  so that the opposite end faces of the bare optical fiber  22  become substantially flush with the opposite end faces of the ferrule  18 . The bare optical fiber  22  is fixed by an adhesive in the through hole  20  defined in a cylindrical portion of the ferrule  18  except the flat cut portion  24 . The semicut ferrule assembly  16  is mounted to the PLC  4  by inserting the bare optical fiber  22  exposed to the flat cut portion  24  into the V groove  14  exposed to the upper surface  6   a  of the silicon substrate  6  until one end  22   a  of the bare optical fiber  22  abuts against one end  10   a  of the optical waveguide core  10 , and fixing the flat cut portion  24  of the ferrule  18  to the upper surface  6   a  of the silicon substrate  6 , by using an adhesive. 
     As shown in FIG. 3, a gap  26  is defined between the upper surface  6   a  of the silicon substrate  6  and the flat cut portion  24  of the ferrule  18 . The adhesive is applied to this gap  26  to thereby fix the ferrule  18  to the silicon substrate  6 . Reference numeral  28  in FIG. 3 denotes the core of the bare optical fiber  22 . It is shown that the core  28  of the bare optical fiber  22  is aligned with the optical waveguide core  10 . 
     The optical module  2  according to the first preferred embodiment has the following advantages. 
     (1) A pressure plate for fixing the ferrule  18  or the bare optical fiber  22  is not required, but a minimum number of components (only the PLC  4  and the ferrule assembly  16 ) and a minimum assembly cost are required. 
     (2) It is not necessary to provide adhesive bonding areas on the opposite sides of the diametrical portion of the ferrule  18  on the PLC substrate (the silicon substrate)  6 , so that the width of the PLC substrate  6  can be reduced. 
     (3) It is not necessary to form a groove for mounting the ferrule  18  on the PLC substrate  6 , but the V groove  14  for mounting the bare optical fiber  22  is only formed on the PLC substrate  6 , so that the thickness of the PLC substrate  6  can be reduced. 
     (4) The bare optical fiber  22  does not project from the ferrule  18 , so that there is no possibility of breaking of the optical fiber  22  in assembling the optical module  2 , thereby improving the worker safety and yield rate. 
     (5) The bare optical fiber  22  is fixed by adhesion in the ferrule  18  so as to prevent generation of fiber bends, so that the stability against temperature variations or the like can be ensured to thereby attain high reliability with less characteristics variations. 
     Referring to FIG. 4, there is shown an exploded perspective view of an optical module  30  according to a second preferred embodiment of the present invention. FIG. 5 is a partially sectional, side view of the optical module  30 , and FIG. 6 is a cross section of the optical module  30  as similar to FIG.  3 . The optical module  30  includes a PLC  4 A, a semicut ferrule assembly  16 , and a glass plate  34  having a V groove  36 . The PLC  4 A includes a silicon substrate  6  and an optical waveguide layer  8 . The optical waveguide layer  8  of the PLC  4 A includes an optical waveguide core  10  and an optical waveguide cladding  12 . The optical waveguide cladding  12  is partially removed to form a narrow portion  8   a  in which the optical waveguide core  10  extends. The optical waveguide core  10  extends over the length of the silicon substrate  6 . 
     The upper surface of the silicon substrate  6  is exposed on the opposite sides of the narrow portion  8   a  of the optical waveguide layer  8 , and a pair of marker grooves  32  for positioning to the glass plate  34  are formed by etching on this exposed upper surface of the silicon substrate  6 . The semicut ferrule assembly  16  is similar in structure to that of the optical module  2  according to the first preferred embodiment. The V groove  36  is formed on the lower surface of the glass plate  34  so as to extend over the length thereof. A pair of marker grooves  38  for positioning to the PLC  4 A are also formed on the lower surface of the glass plate  34  so as to extend over the length thereof. The V groove  36  and the marker grooves  38  are formed by cutting or glass molding, for example. 
     As best shown in FIG. 6, the glass plate  34  and the PLC  4 A are fixed together by an adhesive in such a manner that the narrow portion  8   a  of the optical waveguide layer  8  is accommodated in the V groove  36  of the glass plate  34 , and that the marker grooves  32  of the PLC  4 A are vertically aligned with the marker grooves  38  of the glass plate  34 . On the other hand, the glass plate  34  and the semicut ferrule assembly  16  are fixed together by an adhesive in such a manner that the bare optical fiber  22  exposed to the flat cut portion  24  of the ferrule  18  is fitted into the V groove  36  of the glass plate  34  to effect self-aligned positioning. 
     A gap of about 10 μm is defined between the flat cut portion  24  of the ferrule assembly  16  and the lower surface of the glass plate  34 , and the adhesive is charged into the gap to thereby fix the glass plate  34  and the ferrule assembly  16 . The shape and size of the V groove  36  and the position of the V groove  36  relative to the marker grooves  32  and  38  are set so that the optical waveguide core  10  is aligned with the core  28  of the bare optical fiber  22 , shown in FIG.  6 . 
     The optical module  30  according to the second preferred embodiment has the following advantages. 
     (1) It is not necessary to form a V groove on the PLC substrate  6 , but the semicut ferrule assembly  16  is connected through the V-grooved glass plate  34  to the PLC  4 A, thereby realizing low-loss optical connection at a low cost. 
     (2) It is not necessary to provide adhesive bonding areas on the opposite sides of the diametrical portion of the ferrule  18  on the PLC substrate (the silicon substrate)  6 , so that the width of the PLC substrate  6  can be reduced. 
     (3) The bare optical fiber  22  does not project from the ferrule  18 , so that there is no possibility of breaking of the optical fiber  22  in assembling the optical module  30 , thereby improving the worker safety and yield rate. 
     (4) The bare optical fiber  22  is fixed by adhesion in the ferrule  18  so as to prevent generation of fiber bends, so that the stability against temperature variations or the like can be ensured to thereby attain high reliability with less characteristics variations. 
     Referring to FIG. 7, there is shown a perspective view of a semicut ferrule assembly  16 A according to another preferred embodiment of the present invention. The ferrule assembly  16 A includes a cylindrical ferrule  18  having a through hole  20  and a bare optical fiber  22  inserted and fixed in the through hole  20 . The ferrule  18  has a cylindrical intermediate portion and a pair of flat cut portions  24  formed at the opposite end portions for semicylindrically exposing the opposite end portions of the optical fiber  22 . 
     FIGS. 8A,  8 B, and  8 C show semicut ferrule assemblies  16 B,  16 C, and  16 D, respectively, according to other preferred embodiments of the present invention. The ferrule assembly  16 B shown in FIG. 8A is different from the ferrule assembly  16  shown in FIG. 1 in only the point that the width of the flat cut portion  24  is reduced. The ferrule assembly  16 C shown in FIG. 8B is different from the ferrule assembly  16 B shown in FIG. 8A in only the point that the width of the flat cut portion  24  is reduced. The ferrule assembly  16 D shown in FIG. 8C is different from the ferrule assembly  16  shown in FIG. 1 in only the point that the cylindrical portion of the ferrule  18  is formed at one end thereof with a taper  40 . The ferrule assembly  16 D is effective in configuring a compact wavelength filter module to be hereinafter described. 
     FIGS. 9A and 9B show semicut ferrule assemblies  16 E and  16 F, respectively, according to still other preferred embodiments of the present invention. The ferrule assembly  16 E shown in FIG. 9A is similar to the ferrule assembly  16 A shown in FIG. 7 except that a rectangular prismatic ferrule  42  is adopted. That is, the ferrule assembly  16 E includes the ferrule  42  having a through hole  44  and a bare optical fiber  22  inserted in the through hole  44 . The ferrule  42  has a rectangular prismatic intermediate portion and a pair of flat cut portions  46  formed at the opposite end portions for semicylindrically exposing the opposite end portions of the optical fiber  22 . 
     The ferrule assembly  16 F shown in FIG. 9B is similar to the ferrule assembly  16 E shown in FIG. 9A except that a plurality of bare optical fibers  22  are inserted and fixed in a plurality of through holes  44  extending through a rectangular prismatic ferrule  42 ′. Thus, the outside shape of the ferrule used in the present invention is not necessarily cylindrical for the connection of a PLC and an optical element or the connection of a PLC and another PLC. The preferred embodiments shown in FIGS. 9A and 9B intended for size reduction are effective in providing high-density optical connection. 
     Referring to FIG. 10, there is shown a coupling structure of different types of ferrules. By combining two semicut ferrules  18  and  18 ′ different in diameter, a ferrule structure for converting an external size can be simply obtained. This structure is excellent in reliability because no bending of the bare optical fiber  22  occurs. 
     Referring to FIG. 11, there is shown a perspective view of an optical module  50  according to a third preferred embodiment of the present invention. The optical module  50  includes a V-grooved PLC  4 B, a semicut ferrule assembly  16 , and an optical element  52 . The PLC  4 B includes a silicon substrate  6  and an optical waveguide layer  8  formed on an intermediate portion of the silicon substrate  6 . The ferrule assembly  16  is mounted on one end portion of the silicon substrate  6 , and the optical element  52  is mounted on the other end portion of the silicon substrate  6 . 
     The optical element  52  is a laser diode or a photodiode, for example, and it is mounted on the substrate  6  so as to be optically coupled to the optical waveguide core  10  of the optical waveguide layer  8 . Electrodes  54  for the optical element  52  are also formed on the other end portion of the substrate  6 . The one end portion of the substrate  6  of the PLC  4 B is formed with a V groove  14  aligned with the optical waveguide core  10  in the layer  8 . The flat cut portion  24  of the ferrule assembly  16  is bonded to the one end portion of the substrate  6  so that the bare optical fiber  22  is engaged with the V groove  14  of the substrate  6 . Accordingly, the core of the optical fiber  22  is substantially aligned with the optical waveguide core  10 , thereby realizing low-loss optical coupling. 
     The one end portion of the silicon substrate  6  to which the flat cut portion  24  of the ferrule assembly  16  is bonded is further formed with a plurality of grooves  56  for receiving an adhesive. The grooves  56  extend over the width of the substrate  6  in perpendicular relationship to the V groove  14 . An optical functional circuit such as a wavelength filter, optical branching circuit, optical modulator, and optical switch is incorporated in the optical waveguide layer  8 . A plurality of optical elements or an optical element array rather than the single optical element  52  may be mounted on the substrate  6 . Further, a plurality of semicut ferrule assemblies rather than the single semicut ferrule assembly  16  may be mounted on the substrate  6 . 
     Referring to FIG. 12, there is shown a perspective view of an optical module  58  according to a fourth preferred embodiment of the present invention. The optical module  58  includes a V-grooved PLC  4 C and a pair of semicut ferrule assemblies  16 . The PLC  4 C includes a silicon substrate  6  and an optical waveguide layer  8  formed on an intermediate portion of the silicon substrate  6 . The pair of ferrule assemblies  16  are mounted on the opposite end portions of the silicon substrate  6 . The opposite end portions of the substrate  6  of the PLC  4 C are formed with a pair of V grooves  14  each aligned with the optical waveguide core  10  in the layer  8 . The flat cut portions  24  of the ferrule assemblies  16  are bonded to the opposite end portions of the substrate  6  so that the bare optical fibers  22  of the ferrule assemblies  16  are engaged with the V grooves  14  formed on the opposite end portions of the substrate  6 . Accordingly, the core of the optical fiber  22  of each ferrule assembly  16  is substantially aligned with the optical waveguide core  10 , thereby realizing low-loss optical coupling. 
     Each end portion of the silicon substrate  6  to which the flat cut portion  24  of each ferrule assembly  16  is bonded is further formed with a plurality of grooves  56  for receiving an adhesive. The grooves  56  formed on each end portion of the substrate  6  extend over the width of the substrate  6  in perpendicular relationship to the V groove  14  formed on the same end portion of the substrate  6 . An optical functional circuit such as a wavelength filter, optical branching circuit, optical modulator, and optical switch is incorporated in the optical waveguide layer  8 . 
     Referring to FIG. 13, there is shown a perspective view of an optical module  60  according to a fifth preferred embodiment of the present invention. The optical module  60  includes a V-grooved silicon substrate  6 , a semicut ferrule assembly  16 , and an optical element  52 . The ferrule assembly  16  is mounted on one end portion of the silicon substrate  6 , and the optical element  52  is mounted on the other end portion of the silicon substrate  6  so as to be optically coupled to the bare optical fiber  22  of the ferrule assembly  16 . Electrodes  54  for the optical element  52  are formed on the other end portion of the substrate  6 . 
     The one end portion of the substrate  6  is formed with a V groove  14  aligned with an active layer of the optical element  52 . The flat cut portion  24  of the ferrule assembly  16  is bonded to the one end portion of the substrate  6  so that the bare optical fiber  22  is engaged with the V groove  14  of the substrate  6 . Accordingly, the core of the optical fiber  22  is substantially aligned with the active layer of the optical element  52 , thereby realizing low-loss optical coupling. The one end portion of the silicon substrate  6  to which the flat cut portion  24  of the ferrule assembly  16  is bonded is further formed with a plurality of grooves  56  for receiving an adhesive. The grooves  56  extend over the width of the substrate  6  in perpendicular relationship to the V groove  14 . A plurality of optical elements or an optical element array rather than the single optical element  52  may be mounted on the substrate  6 . 
     Referring to FIG. 14, there is shown a perspective view of an optical module  62  according to a sixth preferred embodiment of the present invention. The optical module  62  includes a V-grooved silicon substrate  6 , a pair of semicut ferrule assemblies  16 , and an optical element  52 . The pair of ferrule assemblies  16  are mounted on the opposite end portions of the silicon substrate  6 . The optical element  52  is mounted on an intermediate portion of the silicon substrate  6  so as to be sandwiched between the pair of ferrule assemblies  16 . Electrodes  54  for the optical element  52  are formed on the intermediate portion of the silicon substrate  6 . 
     The opposite end portions of the substrate  6  are formed with a pair of V grooves  14  each aligned with an active layer of the optical element  52 . The flat cut portions  24  of the ferrule assemblies  16  are bonded to the opposite end portions of the substrate  6  so that the bare optical fibers  22  of the ferrule assemblies  16  are engaged with the V grooves  14  formed on the opposite end portions of the substrate  6 . Accordingly, the core of the optical fiber  22  of each ferrule assembly  16  is substantially aligned with the active layer of the optical element  52 , thereby realizing low-loss optical coupling. Each end portion of the silicon substrate  6  to which the flat cut portion  24  of each ferrule assembly  16  is bonded is further formed with a plurality of grooves (not shown) for receiving an adhesive. 
     Referring to FIG. 15, there is shown a perspective view of an optical module  64  according to a seventh preferred embodiment of the present invention. The optical module  64  includes a V-grooved silicon substrate  6 , a pair of semicut ferrule assemblies  16 , and a thin-film or thin-sheet passive optical component  68 . The pair of ferrule assemblies  16  are mounted on the opposite end portions of the silicon substrate  6 . The passive optical component  68  is vertically inserted and fixed in a rectangular groove  66  formed at an intermediate portion of the silicon substrate  6  so as to be sandwiched between the pair of ferrule assemblies  16 . The rectangular groove  66  extends over the width of the silicon substrate  6 . The opposite end portions of the substrate  6  are formed with a pair of V grooves  14  aligned with each other. The flat cut portions  24  of the ferrule assemblies  16  are bonded to the opposite end portions of the substrate  6  so that the bare optical fibers  22  of the ferrule assemblies  16  are engaged with the V grooves  14  formed on the opposite end portions of the substrate  6 . Accordingly, the cores of the optical fibers  22  positioned in the V grooves  14  are substantially aligned with each other, thereby realizing low-loss optical coupling. 
     The passive optical component  68  in the form of thin film or thin sheet fixed in the rectangular groove  66  projects from the upper surface of the silicon substrate  6 , and the opposite side surfaces of the passive optical component  68  at its projecting portion are sandwiched between the opposite end faces of the ferrule assemblies  16  and bonded thereto. Accordingly, a large bonding area of the passive optical component  68  is ensured, and it is supported from the opposite sides by the ferrule assemblies  16 , thereby obtaining a high fixing strength to stabilize the passive optical component  68 . Each end portion of the silicon substrate  6  to which the flat cut portion  24  of each ferrule assembly  16  is bonded is further formed with a plurality of grooves (not shown) for receiving an adhesive. 
     Referring to FIG. 16, there is shown a perspective view of an optical module  70  according to an eighth preferred embodiment of the present invention. The optical module  70  includes a V-grooved PLC  4 D, a pair of semicut ferrule assemblies  16   a  and  16   b , an optical element  52 , and a thin-film optical wavelength filter  84 . The PLC  4 D includes a V-grooved silicon substrate  6  and an optical waveguide layer  72  formed on an intermediate portion of the silicon substrate  6 . The optical waveguide layer  72  is a Y-branch type optical waveguide including a first optical waveguide core  74 , a second optical waveguide core  76  connected to an intermediate portion of the first optical waveguide core  74 , and an optical waveguide cladding  78  covering the first and second optical waveguide cores  74  and  76 . The pair of ferrule assemblies  16   a  and  16   b  are mounted on one end portion of the silicon substrate  6 , and the optical element  52  is mounted on the other end portion of the silicon substrate  6 . Electrodes  54  for the optical element  52  are formed on the other end portion of the silicon substrate  6 . 
     The thin-film optical wavelength filter  84  is vertically inserted and fixed in a rectangular groove  82  cut through the optical waveguide layer  72  into the substrate  6  so as to intersect a Y branch  80  of the Y-branch type optical waveguide, i.e., a junction between the first optical waveguide core  74  and the second optical core  76 . The rectangular groove  82  extends over the width of the substrate  6 . The one end portion of the silicon substrate  6  is formed with a pair of V grooves  14  aligned with the first and second optical waveguide cores  74  and  76 . The flat cut portions  24  of the ferrule assemblies  16   a  and  16   b  are bonded to the one end portion of the substrate  6  so that the bare optical fibers  22  of the ferrule assemblies  16   a  and  16   b  are engaged with the V grooves  14  formed on the one end portion of the substrate  6 . Accordingly, the core of the optical fiber  22  of the ferrule assembly  16   a  is substantially aligned with the first optical waveguide core  74 , and the core of the optical fiber  22  of the ferrule assembly  16   b  is substantially aligned with the second optical waveguide core  76 , thereby realizing low-loss optical coupling. 
     The one end portion of the silicon substrate  6  to which the flat cut portions  24  of the ferrule assemblies  16   a  and  16   b  are bonded is further formed with a plurality of grooves (not shown) for receiving an adhesive. For example, a certain component of light entered the first optical waveguide core  74  from the ferrule assembly  16   a  is transmitted by the wavelength filter  84  to enter the optical element  52 , and the remaining component of the light is reflected by the wavelength filter  84  to enter the second optical waveguide core  76  and to emerge from the ferrule assembly  16   b.    
     Referring to FIG. 17, there is shown a perspective view of an optical module  86  according to a ninth preferred embodiment of the present invention. Like the eighth preferred embodiment mentioned above, the optical module  86  includes a V-grooved PLC  4 E, a pair of semicut ferrule assemblies  16   a  and  16   b , an optical element  52 , and a thin-film optical wavelength filter  84 . The PLC  4 E includes a V-grooved silicon substrate  6  and an optical waveguide layer  72  formed on an intermediate portion of the silicon substrate  6 . The optical waveguide layer  72  has the same structure as that in the eighth preferred embodiment shown in FIG.  16 . The pair of ferrule assemblies  16   a  and  16   b  are mounted on the opposite end portions of the silicon substrate  6 , and the optical element  52  is mounted on one end portion of the silicon substrate  6  where the ferrule assembly  16   a  is mounted. Electrodes  54  for the optical element  52  are formed on the one end portion of the silicon substrate  6 . The thin-film optical wavelength filter  84  is vertically inserted and fixed in a rectangular groove  82  as similar to the structure in the eighth preferred embodiment shown in FIG.  16 . 
     The opposite end portions of the silicon substrate  6  are formed with a pair of V grooves  14  aligned with the opposite ends of the first optical waveguide core  74 . The flat cut portions  24  of the ferrule assemblies  16   a  and  16   b  are bonded to the opposite end portions of the substrate  6  so that the bare optical fibers  22  of the ferrule assemblies  16   a  and  16   b  are engaged with the V grooves  14  formed on the opposite end portions of the substrate  6 . Accordingly, the core of the optical fiber  22  of the ferrule assembly  16   a  is substantially aligned with the first end of the first optical waveguide core  74 , and the core of the optical fiber  22  of the ferrule assembly  16   b  is substantially aligned with the second end of the first optical waveguide core  74 , thereby realizing low-loss optical coupling. Each of the opposite end portions of the silicon substrate  6  to which the flat cut portions  24  of the ferrule assemblies  16   a  and  16   b  are bonded is further formed with a plurality of grooves (not shown) for receiving an adhesive. 
     For example, a certain component of light entered the first optical waveguide core  74  from the ferrule assembly  16   a  is reflected by the wavelength filter  84  to enter the optical element  52  through the second waveguide core  76 , and the remaining component of the light is transmitted by the wavelength filter  84  to enter the ferrule assembly  16   b  and to emerge therefrom. In the case that the optical element  52  is a light emitting element such as a laser diode, a certain component of light emitted from the optical element  52  is reflected by the wavelength filter  84  to enter the ferrule assembly  16   a  and to emerge therefrom, and the remaining component of the light is transmitted by the wavelength filter  84  to enter the ferrule assembly  16   b  and to emerge therefrom. 
     Referring to FIG. 18, there is shown a perspective view of an optical module  88  according to a tenth preferred embodiment of the present invention. Like the eighth and ninth preferred embodiments mentioned above, the optical module  88  includes a V-grooved PLC  4 F, a semicut ferrule assembly  16 , a pair of optical elements  52   a  and  52   b , and a thin-film optical wavelength filter  84 . The PLC  4 F includes a V-grooved silicon substrate  6  and an optical waveguide layer  72  formed on an intermediate portion of the silicon substrate  6 . The optical waveguide layer  72  has the same structure as that in the eighth preferred embodiment shown in FIG.  16 . The ferrule assembly  16  and the optical element  52   a  are mounted on one end portion of the silicon substrate  6 , and the optical element  52   b  is mounted on the other end portion of the silicon substrate  6 . Electrodes  54  for the optical element  52   a  are formed on the one end portion of the silicon substrate  6 , and electrodes  54  for the optical element  52   b  are formed on the other end portion of the silicon substrate  6 . The thin-film optical wavelength filter  84  is vertically inserted and fixed in a rectangular groove  82  as similar to the structure in the eighth preferred embodiment shown in FIG.  16 . For example, the wavelength filter  84  transmits light having wavelengths in a 1.55 μm band, and reflects light having wavelengths in a 1.3 μm band. 
     The one end portion of the silicon substrate  6  is formed with a V groove  14  aligned with the first end of the first optical waveguide core  74 . The flat cut portion  24  of the ferrule assembly  16  is bonded to the one end portion of the substrate  6  so that the bare optical fiber  22  of the ferrule assembly  16  is engaged with the V groove  14  formed on the one end portion of the substrate  6 . Accordingly, the core of the optical fiber  22  of the ferrule assembly  16  is substantially aligned with the first end of the first optical waveguide core  74 , thereby realizing low-loss optical coupling. The one end portion of the silicon substrate  6  to which the flat cut portion  24  of the ferrule assembly  16  is bonded is further formed with a plurality of grooves (not shown) for receiving an adhesive. 
     For example, a certain component of light entered the first optical waveguide core  74  from the ferrule assembly  16  is reflected by the wavelength filter  84  to enter the optical element  52   a , and the remaining component of the light is transmitted by the wavelength filter  84  to enter the optical element  52   b . In the case that the optical element  52   a  is a light emitting element such as a laser diode, a certain component of light emitted from the optical element  52   a  is reflected by the wavelength filter  84  to enter the ferrule assembly  16  and to emerge therefrom, and the remaining component of the light is transmitted by the wavelength filter  84  to enter the optical element  52   b.    
     Referring to FIG. 19, there is shown a perspective view of an optical module  90  according to an eleventh preferred embodiment of the present invention. The optical module  90  includes a V-grooved silicon substrate  6 , a semicut ferrule assembly  16 A similar to that shown in FIG. 7, and an optical element  52 . The silicon substrate  6  is formed at its one end portion with a V groove  14 . The ferrule assembly  16 A is mounted on the silicon substrate  6  in such a manner that one of the two cut flat portions  24  of the ferrule assembly  16 A is bonded to the one end portion of the substrate  6  in the condition where the bare optical fiber  22  is engaged with the V groove  14 . By connecting the optical module  90  at the other flat cut portion  24  to a PLC (not shown), an optical functional system can be simply configured. 
     Referring to FIG. 20, there is shown a perspective view of an optical module  92  according to a twelfth preferred embodiment of the present invention. The optical module  92  is configured by connecting two optical modules each similar to the optical module  90  shown in FIG. 19 to a PLC  4 G at one end portion thereof. A semicut ferrule assembly  16  is mounted on the other end portion of the PLC  4 G. In modification, a wavelength filter or an optical switch circuit may be mounted in the optical waveguide layer  8  of the PLC  4 G. 
     Referring to FIG. 21, there is shown a perspective view of an optical module  94  according to a thirteenth preferred embodiment of the present invention. The optical module  94  includes a V-grooved silicon substrate  6 , a bare optical fiber  22 , and an optical element  52 . A V groove  14  is formed on the upper surface of the silicon substrate  6  at one end portion thereof by anisotropic etching of silicon. The bare optical fiber  22  is fitted in the V groove  14  of the substrate  6 , and the optical element  52  is mounted on the upper surface of the substrate  6  so as to be substantially aligned with the core of the optical fiber  22 . The size of the V groove  14  is set so that the core of the optical fiber  22  fitted in the V groove  14  is optically coupled to the optical element  52 . A rectangular groove  96  perpendicular to the V groove  14  is formed by cutting on the upper surface of the substrate  6 , so as to eliminate a slant portion formed at one end of the V groove  14  opposed to the optical element  52 . 
     Referring to FIG. 22A, there is shown a perspective view of an optical module  98  according to a fourteenth preferred embodiment of the present invention. The optical module  98  includes a V-grooved silicon substrate  6 , a pair of bare optical fibers  22 , and an optical element  52 ′. A pair of V grooves  14  aligned with each other are formed on the upper surface of the silicon substrate  6  at its opposite end portions. The bare optical fibers  22  are fitted in the V grooves  14  of the substrate  6 , and the optical element  52 ′ is mounted on the upper surface of the substrate  6  at its intermediate portion so as to be substantially aligned with the cores of the optical fibers  22 . The size of each V groove  14  is set so that the cores of the optical fibers  22  are optically coupled to the optical element  52 ′. A pair of rectangular grooves  96  for the V grooves  14  are formed on the upper surface of the substrate  6  as similarly to the preferred embodiment shown in FIG.  21 . For example, the optical element  52 ′ is an LD amplifier which amplifies an optical signal. 
     FIG. 22B shows an optical module  100  as a modification of the preferred embodiment shown in FIG.  22 A. The optical module  100  differs from the optical module  98  shown in FIG. 22A in only the point that two pairs of bare optical fibers  22  are fitted in two pairs of V grooves  14 , and that a pair of optical elements  52 ′ are mounted on the silicon substrate  6 . 
     Referring to FIG. 23, there is shown a perspective view of an optical module  102  according to a fifteenth preferred embodiment of the present invention. The optical module  102  includes a silicon substrate  6  having a V groove  14 , a semicut ferrule assembly  16 D having a taper  40  similar to that shown in FIG. 8C, and an optical element  52 . The semicut ferrule assembly  16 D is mounted on the silicon substrate  6  so that the optical fiber  22  of the ferrule assembly  16 D is fitted in the V groove  14  of the substrate  6 , and the optical element  52  is mounted on the substrate  6  so as to be optically coupled to the core of the optical fiber  22  fitted in the V groove  14 . 
     Referring to FIG. 24, there is shown a plan view of an optical module  104  according to a sixteenth preferred embodiment of the present invention. The optical module  104  includes a substrate  105 , two cylindrical ferrule assemblies  112   a  and  112   b , two semicut ferrule assemblies  16 D 1  and  16 D 2 , two optical elements  52   a  and  52   b , and a wavelength filter  110 . 
     Two grooves  106  and  108  orthogonal to each other are formed on the upper surface of the substrate  105 . The cylindrical ferrule assemblies  112   a  and  112   b  each having a tapered front end-are inserted in the grooves  106  and  108 , respectively. The semicut ferrule assemblies  16 D 1  and  16 D 2  each having a tapered front end are inserted in the grooves  108  and  106 , respectively. Thus, the ferrule assemblies  112   a  and  16 D 2  are inserted in the groove  106  in such a manner that their respective tapered front ends are opposed to each other, and the ferrule assemblies  112   b  and  16 D 1  are inserted in the groove  108  in such a manner that their respective tapered front ends are opposed to each other. Although not shown, optical fibers are inserted and fixed in the through holes of the ferrule assemblies  112   a ,  112   b ,  16 D 1 , and  16 D 2 . The wavelength filter  110  is inserted and fixed in a groove formed on the upper surface of the substrate  105  so as to be inclined 45° with respect to the grooves  106  and  108 . The optical elements  52   a  and  52   b  are mounted on silicon substrates  6  on which the ferrule assemblies  16 D 1  and  16 D 2  are mounted. 
     Incident light from the cylindrical ferrule assembly  112   a  is transmitted and reflected by the wavelength filter  110 , wherein a transmitted component of the light enters the optical element  52   b  through the ferrule assembly  16 D 2  and a reflected component of the light enters the optical element  52   a  through the ferrule assembly  16 D 1 . On the other hand, incident light from the cylindrical ferrule assembly  112   b  is also transmitted and reflected by the wavelength filter  110 , wherein a transmitted component of the light enters the optical element  52   a  through the ferrule assembly  16 D 1  and a reflected component of the light enters the optical element  52   b  through the ferrule assembly  16 D 2 . In the case that the optical element  52   a  is a light emitting element and the optical element  52   b  is a photodetecting element, a bidirectional wavelength division multiplexing optical transmission module can be simply fabricated, and the module can be reduced in size. 
     Referring to FIG. 25, there is shown a perspective view of an optical module  114  according to a seventeenth preferred embodiment of the present invention. The optical module  114  includes a substrate  116 , an LD amplifier array  118 , a pair of semicut ferrule assemblies  16 F′, a pair of PLCs  4 H and  4 H′, and a plurality of semicut ferrule assemblies  16 . The LD amplifier array  118  is mounted on the substrate  116 . The semicut ferrule assemblies  16 F′ are mounted on the substrate  116  with the LD amplifier array  118  interposed therebetween. Each ferrule assembly  16 F′ is similar to the ferrule assembly  16 F shown in FIG.  9 B. The ferrule assemblies  16 F′ are mounted also on the substrates  6  of the PLCs  4 H and  4 H′. Thus, the LD amplifier array  118  is optically connected through the ferrule assemblies  16 F′ to the PLCs  4 H and  4 H′. 
     The semicut ferrule assemblies  16  are mounted on the substrate  6  of each of the PLCs  4 H and  4 H′ so as to be optically connected to the optical waveguide layer  8  of each of the PLCs  4 H and  4 H′. For example, optical signals input through the left ferrule assemblies  16  into the PLC  4 H are amplified by the LD amplifier array  118 , and amplified optical signals from the LD amplifier array  118  are input into the PLC  4 H′ and output from the right ferrule assemblies  16 . 
     Referring to FIG. 26, there is shown a perspective view of an optical module  120  according to an eighteenth preferred embodiment of the present invention. The optical module  120  includes a V-grooved PLC  4 I, a plurality of semicut ferrule assemblies  16  each having a larger diameter, and a plurality of semicut ferrule assemblies  16 ′ each having a smaller diameter. The ferrule assemblies  16  are mounted on one end portion of the substrate  6  of the PLC  4 I, and the ferrule assemblies  16 ′ are mounted on the other end portion of the substrate  6 . The ferrule assemblies  16  are arranged with a pitch larger than that of the ferrule assemblies  16 ′. Thus, the external size and pitch of plural ferrule assemblies can be freely changed. 
     Referring to FIG. 27, there is shown a perspective view of an optical module  122  according to a nineteenth preferred embodiment of the present invention. FIG. 28 shows a PLC  4 J used in the nineteenth preferred embodiment, and FIG. 29A is a glass plate  34  used in the nineteenth preferred embodiment. As shown in FIG. 28, the PLC  4 J includes a silicon substrate  6  and an optical waveguide layer  8  formed on the silicon substrate  6 . The optical waveguide layer  8  has a pair of narrow portions  8   a  and  8   b  at the opposite end portions formed by partially cutting the cladding region. 
     A pair of marker grooves  32  for positioning to the glass plate  34  are formed on the opposite sides of the narrow waveguide portion  8   a  on the upper surface of the substrate  6 , and a pair of marker grooves  33  for positioning to another member (not shown) are formed on the opposite sides of the narrow waveguide portion  8   b  on the upper surface of the substrate  6 . The substrate  6  has no V groove in this preferred embodiment. 
     As shown in FIG. 29A, the glass plate  34  has a V groove  36  formed by cutting or the like and a pair of marker grooves  38  formed by cutting or the like on the opposite sides of the V groove  36 . The PLC  4 J and the glass plate  34  are bonded together with a high dimensional accuracy by a passive alignment technique in such a manner that the marker grooves  32  of the PLC  4 J are vertically aligned with the marker grooves  38  of the glass plate  34 , and that the narrow waveguide portion  8   a  of the PLC  4 J is accommodated in the V groove  36  of the glass plate  34 . 
     The glass plate  34  and the semicut ferrule assembly  16  are bonded together in such a manner that the bare optical fiber  22  of the ferrule assembly  16  is fitted in the V groove  36  of the glass plate  34  to effect positioning by a self alignment technique, and that the flat cut portion  24  of the ferrule assembly  16  is bonded to the glass plate  34 . Although the PLC  4 J has no V groove, high-precision optical coupling between the bare optical fiber  22  of the ferrule assembly  16  and the optical waveguide core of the PLC  4 J can be relatively simply obtained by using the V-grooved glass plate  34 . In the case that the width of the narrow waveguide portion  8   a  is relatively small, the glass plate  34  shown in FIG. 29A is used, whereas in the case that the width of the narrow waveguide portion  8   a  is relatively large, a glass plate  34 ′ having a wide groove  124  shown in FIG. 29B is used. In the latter case, the narrow waveguide portion  8   a  is accommodated in the wide groove  124 . 
     Referring to FIG. 30A, there is shown an exploded perspective view of an optical module  126  according to a twentieth preferred embodiment of the present invention. FIG. 30B is a perspective view of the optical module  126  in its assembled condition. The optical module  126  includes a V-grooved PLC  4 K and a multifiber semicut connector  128 . The PLC  4 K includes a silicon substrate  6  and an optical waveguide layer  8  formed on the silicon substrate  6 . The optical waveguide layer  8  includes a Y-branched optical waveguide core  10  and an optical waveguide cladding  12  covering the core  10 . The Y-branched optical waveguide core  10  consists of a first core portion  10   a  and a second core portion  10   b  connected to an intermediate portion of the first core portion  10   a . An optical signal propagates in the optical waveguide core  10  having a refractive index higher than that of the optical waveguide cladding  12 . 
     The silicon substrate  6  has an exposed surface  6   a  formed with two V grooves  14  respectively aligned with the first and second core portions  10   a  and  10   b . The multifiber semicut connector  128  has two bare optical fibers  22  optically coupled to the first and second core portions  10   a  and  10   b . The position and size of the V grooves  14  are set so that when the bare optical fibers  22  are fitted in the V grooves  14 , the cores of the optical fibers  22  are substantially aligned with the first and second core portions  10   a  and  10   b  of the Y-branched optical waveguide core  10 . 
     The multifiber semicut connector  128  includes a block  130  having two through holes in which the two optical fibers  22  are inserted and fixed. The block  130  has a flat cut portion  132  for semicylindrically exposing the optical fibers  22 . The block  130  having the flat cut portion  132  is formed by transfer molding of plastic using a mold. The bare optical fibers  22  are inserted in the through holes of the block  130  and bonded thereto at its portion except the flat cut portion  132 . A pair of guide pins  134  are inserted and fixed in other through holes formed in the block  130 . The insertion and fixing of the guide pins  134  may be carried out after connecting the PLC  4 K to the multifiber semicut connector  128 . 
     The PLC  4 K is bonded at the exposed surface  6   a  to the flat cut portion  132  of the multifiber semicut connector  128  so that the optical fibers  22  of the connector  128  are fitted in the V grooves  14  of the PLC  4 K. Accordingly, the cores of the optical fibers  22  are substantially aligned with the first and second core portions  10   a  and  10   b  of the Y-branched optical waveguide core  10 , thereby realizing low-loss optical coupling. The exposed surface  6   a  of the silicon substrate  6  of the PLC  4 K to which the flat cut portion  132  of the connector  128  is bonded is further formed with a plurality of grooves  56  for receiving an adhesive. 
     The optical module  126  according to this preferred embodiment can solve the problems in the conventional receptacle structure, and has the following advantages. 
     (1) It is not necessary to form deep V grooves for mounting the guide pins  134  on the PLC substrate  6 , so that the PLC substrate  6  can be reduced in thickness and width to thereby reduce material cost. 
     (2) It is not necessary to provide a pressure plate for fixing the bare optical fibers  22  to the block  130 , so that the number of parts and assembly cost can be minimized. 
     (3) Dimensional errors of each V groove  14  and each bare optical fiber  22  are small, so that a misalignment between each of the first and second core portions  10   a  and  10   b  and the core of the corresponding bare optical fiber  22  can be reduced, thereby minimizing an optical coupling loss. 
     Referring to FIG. 31, there is shown an exploded perspective view of an optical module  136  according to a twenty-first preferred embodiment of the present invention. The optical module  136  differs from the optical module  126  shown in FIG. 30A in only the structure of a PLC  4 L. The PLC  4 L includes a V-grooved silicon substrate  6  having exposed surfaces  6   a  and  6   b  at the opposite end portions, and an optical waveguide layer  8  formed on an intermediate portion of the silicon substrate  6 . Two V grooves  14  are formed on the exposed surface  6   a  of the substrate  6 , and three optical elements  52   a ,  52   b , and  52   c  are mounted on the exposed surface  6   b  of the substrate  6 . The optical waveguide layer  8  includes a Y-branched optical waveguide core  10  and an optical waveguide cladding  12  covering the core  10 . The optical waveguide core  10  includes first and second core portions  10   a  and  10   b  respectively aligned with the two V grooves  14 , and third and fourth core portions  10   c  and  10   d  respectively aligned with the optical elements  52   a  and  52   b . For example, the optical element  52   a  is a laser diode, the optical element  52   b  is a photodiode for detection of an optical signal, and the optical element  52   c  is a photodiode for monitoring of light. Reference numerals  54  are electrodes for the optical elements  52   a ,  52   b , and  52   c.    
     Referring to FIG. 32, there is shown a perspective view of a multifiber semicut connector  138  according to another preferred embodiment. The multifiber semicut connector  138  differs from the connector  128  shown in FIG. 30A in only the point that a groove  140  is additionally formed. The groove  140  is formed on the flat cut portion  132  near the boundary between the exposed part of the optical fibers  22  and the unexposed part of the optical fibers  22  so as to extend in a direction perpendicular to the optical fibers  22 . The groove  140  has a width of 0.1 to 1 mm, for example, and is slightly lowered from the horizontal upper surface of the flat cut portion  132 . In fixing the optical fibers  22  inserted in the through holes of the block  130  by means of an adhesive, the groove  140  functions to receive the adhesive leaked from the through holes, thereby preventing the adhesive from sticking to the horizontal surface of the flat cut portion  132  where the exposed part of the optical fibers  22  is placed. 
     Referring to FIG. 33, there is shown an exploded perspective view of an optical module  142  according to a twenty-second preferred embodiment of the present invention. The optical module  142  includes a multifiber semicut connector  128 ′ and a PLC  4 K′. The multifiber semicut connector  128 ′ is similar to the connector  128  shown in FIG. 30A except that the bare optical fibers  22  of the connector  128 ′ project from the block  130  by about 0.1 to 2 mm. The PLC  4 K′ is similar to the PLC  4 K shown in FIG. 30A except that the exposed surface  6   a  of the PLC  4 K′ is longer than that of the PLC  4 K. By projecting the optical fibers  22  from the block  130 , a space between the front ends of the optical fibers  22  and the first and second core portions  10   a  and  10   b  of the optical waveguide core  10  or optical elements (not shown) can be easily controlled. 
     Referring to FIG. 34, there is shown an exploded perspective view of an optical module  144  according to a twenty-third preferred embodiment of the present invention. The optical module  144  includes a semicut ferrule assembly  16 F similar to that shown in FIG. 9B, a PLC  4 K similar to that shown in FIG. 30A, a substrate  146  having two V grooves  148 , and two optical elements  52  mounted on the substrate  146 . The two optical elements  52  are aligned with the two V grooves  148 , respectively. A plurality of grooves  150  for receiving an adhesive are also formed on the substrate  146 . The grooves  150  extend over the width of the substrate  146  in perpendicular relationship to the V grooves  148 . 
     The PLC  4 K is bonded to the flat cut portion  46  formed at one end portion of the ferrule assembly  16 F so that an exposed part of the optical fibers  22  of the ferrule assembly  16 F is fitted in the V grooves  14  of the PLC  4 K. On the other hand, the substrate  146  is bonded to the other flat cut portion  46  of the ferrule assembly  16 F so that the other exposed part of the optical fibers  22  of the ferrule assembly  16 F is fitted in the V grooves  148  of the substrate  146 . According to this preferred embodiment, the optical elements  52  mounted on the substrate  146  are optically coupled through the ferrule assembly  16 F to the PLC  4 K. 
     Referring to FIG. 35A, there is shown an exploded perspective view of an optical module  152  according to a twenty-fourth preferred embodiment of the present invention. FIG. 35B is a perspective view of the optical module  152  in its assembled condition. The optical module  152  includes a V-grooved PLC  4 M and two multifiber semicut connectors  126 ′ and  154  optically coupled to each other through the PLC  4 M. 
     The PLC  4 M includes a silicon substrate  6  and an optical waveguide layer  8  formed on an intermediate portion of the substrate  6 . The substrate  6  has an exposed surface  6   a  at one end portion and another exposed surface  6   b  at the other end portion. The exposed surface  6   a  is formed with two V grooves  14 , and the exposed surface  6   b  is formed with at least four V grooves  14 . The optical waveguide layer  8  includes two Y-branch type (1×N branch type where N is an integer greater than 1) optical waveguide cores  10  and an optical waveguide cladding  12  covering the cores  10 . Each optical waveguide core  10  has one end aligned to one of the V grooves  14  formed on the exposed surface  6   a  and has N ends aligned to N of the V grooves  14  formed on the exposed surface  6   b . The flat cut portion  132  of the connector  126 ′ is bonded to the exposed surface  6   a  of the PLC  4 M so that the optical fibers  22  of the connector  126 ′ are fitted in the V grooves  14  formed on the exposed surface  6   a . On the other hand, the flat cut portion  132  of the connector  154  is bonded to the exposed surface  6   b  of the PLC  4 M so that the optical fibers  22  of the connector  154  are fitted in the v grooves  14  formed on the exposed surface  6   b . Accordingly, an optical signal input from the connector  126 ′ can be branched into a plurality of optical signals in the PLC  4 M, and the resultant optical signals can be output from the connector  154 . Conversely, a plurality of optical signals input from the connector  154  can be combined to an optical signal in the PLC  4 M, and the resultant optical signal can be output from the connector  126 ′. 
     Referring to FIG. 36A, there is shown an exploded perspective view of an optical module  156  according to a twenty-fifth preferred embodiment of the present invention. FIG. 36B is a perspective view of the optical module  156  in its assembled condition. The optical module  156  is similar to the optical module  144  shown in FIG. 34 with the exception that a multifiber semicut connector  128  is added. That is, the optical module  156  includes a PLC  4 N, a semicut ferrule assembly  16 F, a substrate  146 , and the multifiber semicut connector  128 . The PLC  4 N includes a V-grooved silicon substrate  6  and an optical waveguide layer  8  formed on an intermediate portion of the silicon substrate  6 . The silicon substrate  6  has exposed surfaces  6   a  and  6   b  at the opposite end portions. Each of the exposed surfaces  6   a  and  6   b  is formed with two V grooves  14 . The optical waveguide layer  8  includes a Y-branch type optical waveguide core  10  and an optical waveguide cladding  12  covering the core  10  as similar to the structure of the PLC  4 L shown in FIG. 31. A wavelength filter  84  is mounted on the optical waveguide layer  8 . 
     The PLC  4 N is bonded at the exposed surface  6   a  to the flat cut portion  46  of the ferrule assembly  16 F at its one end portion so that the optical fibers  22  exposed to this portion of the ferrule assembly  16 F are fitted in the V grooves  14  formed on the exposed surface  6   a  of the PLC  4 N. The substrate  146  is bonded to the flat cut portion  46  of the ferrule assembly  16 F at its other end portion so that the optical fibers  22  exposed to this portion of the ferrule assembly  16 F are fitted in the V grooves  148  of the substrate  146 . The multifiber semicut connector  128  is similar to that shown in FIG.  31 . The connector  128  is bonded at the flat cut portion  132  to the exposed surface  6   b  of the PLC  4 N so that the optical fibers  22  exposed to the flat cut portion  132  are fitted in the V grooves  14  formed on the exposed surface  6   b . According to this preferred embodiment, the optical elements  52  mounted on the substrate  146  can be optically coupled through the ferrule assembly  16 F and the PLC  4 N to the connector  128 . 
     Referring to FIG. 37A, there is shown an exploded perspective view of an optical module  158  according to a twenty-sixth preferred embodiment of the present invention. FIG. 37B is a perspective view of the optical module  158  in its assembled condition. The optical module  158  includes a V-grooved substrate  160 , an optical element array  162  mounted on the substrate  160 , and a multifiber semicut connector  154  similar to that shown in FIG.  35 A. The optical element array  162  is an LD array or a PD array, for example. A plurality of electrodes  164  for the optical element array  162  are formed on the substrate  160 . The substrate  160  has a mount surface  160   a  formed with a plurality of V grooves  166  respectively corresponding to a plurality of individual optical elements constituting the optical element array  162 . The substrate  160  is bonded at the mount surface  160   a  to the flat cut portion  132  of the connector  154  so that the optical fibers  22  of the connector  154  are fitted in the V grooves  166  of the substrate  160 . With this configuration, the individual optical elements of the optical element array  162  are optically coupled to the optical fibers  22 , respectively. 
     Referring to FIG. 38, there is shown an exploded perspective view of an optical module  168  according to a twenty-seventh preferred embodiment of the present invention. The optical module  168  is similar to the optical module  126  shown in FIG. 30A except that a multifiber semicut connector  128 ′ is used in place of the connector  128 . The multifiber semicut connector  128 ′ includes a silicon substrate  170  having two V grooves  172  and two V grooves  174 , two optical fibers  22  fitted in the two V grooves  172 , two guide pins  134  fitted in the two V grooves  174 , and a cover  176  fixed to the silicon substrate  170  for partially covering the optical fibers  22  and the guide pins  134 . The silicon substrate  170  has an exposed surface  170   a  for semicylindrically exposing the optical fibers  22 . The cover  176  also has two V grooves respectively opposed to the two V grooves  172  for the optical fibers  22  and two V grooves respectively opposed to the two V grooves  174  for the guide pins  134 . The PLC  4 K is bonded at the exposed surface  6   a  to the exposed surface  170   a  of the substrate  170  so that the optical fibers  22  are fitted in the V grooves  14  of the PLC  4 K. 
     Referring to FIG. 39A, there is shown an exploded perspective view of an optical module  178  according to a twenty-eighth preferred embodiment of the present invention. FIG. 39B is a perspective view of the optical module  178  in its assembled condition. The optical module  178  includes a PLC  4 P, a multifiber semicut connector  128  similar-to that shown in FIG. 30A, and a V-grooved glass plate  182 . The PLC  4 P includes a silicon substrate  6  and an optical waveguide layer  8  formed on the silicon substrate  6 . The optical waveguide layer  8  includes a Y-branched optical waveguide core  10  and an optical waveguide cladding  12  covering the core  10 . The Y-branched optical waveguide core  10  consists of a first core portion  10   a  and a second core portion  10   b  connected to an intermediate portion of the first core portion  10   a . The cladding  12  is partially removed at one end portion of the layer  8  to form a narrow waveguide portion  8   a . The narrow waveguide portion  8   a  includes the first and second core portions  10   a  and  10   b . Accordingly, the silicon substrate  6  has two exposed surfaces  6   a  on the opposite sides of the narrow waveguide portion  8   a . A pair of marker grooves  180  are formed on the exposed surfaces  6   a  of the substrate  6 . 
     The glass plate  182  is formed with two V grooves  184  for receiving the optical fibers  22  of the connector  128 , a relatively wide groove  186  for receiving the narrow waveguide portion  8   a  of the optical waveguide layer  8  of the PLC  4 P, and a pair of marker grooves  188  to be vertically aligned with the pair of marker grooves  180  of the PLC  4 P. The PLC  4 P is bonded at the exposed surfaces  6   a  to the glass plate  182  so that the narrow waveguide portion  8   a  of the PLC  4 P is accommodated in the groove  186  of the glass plate  182  and that the marker grooves  180  of the PLC  4 P are vertically aligned with the marker grooves  188  of the glass plate  182 , thereby positioning and fixing the PLC  4 P and the glass plate  182  with a high dimensional accuracy by a passive alignment technique. 
     The glass plate  182  is bonded to the flat cut portion  132  of the connector  128  so that the optical fibers  22  of the connector  128  are fitted in the V grooves  184  of the glass plate  182  to thereby position the glass plate  182  to the connector  128 . Although the PLC  4 P has no V grooves, high-precision optical coupling between the bare optical fibers  22  of the connector  128  and the first and second core portions  10   a  and  10   b  of the optical waveguide core  10  of the PLC  4 P can be realized relatively simply by using the V-grooved glass plate  182 . 
     According to the present invention, it is possible to provide a receptacle type optical module suitable for cost reduction and size reduction by using a semicut ferrule assembly at an interface to an optical fiber.