Patent Publication Number: US-RE46633-E

Title: Optical module

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a reissue of U.S. Pat. No. 7,454,104, which issued from U.S. patent application Ser. No. 11/456,730, filed Jul. 11, 2006. The present inventionapplication contains subject matter related to Japanese Patent Application JP 2005-209022 filed in the Japanese Patent Office on Jul. 19, 2005, to which priority is claimed and the entire contents of which beingare incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical module on which at least two optical elements are mounted. 
     2. Description of Related Art 
     For an optical transmitter module for transmitting an a optical signal or an optical transmitter/receiver module for transmitting/receiving an optical signal, a technology has been proposed for mounting plural optical elements on the same substrate. As a processing speed of a transmission signal by the optical module increases, a crosstalk such that electromagnetic radiation generated from one of optical elements spreads to the other elements to interfere them may occur at the optical module. 
     A technology has been proposed to increase a distance between these elements to electrically reduce the crosstalk between the optical elements. For an optical transmitter module, a technology has been proposed to increase a distance between light-emitting elements (see article, “10 Gbps×4ch Parallel LD Module” by ANAGURA Masato, conference of Electronics Society in Institute of Electronics, Information and Communication Engineers, C-3-50, p. 160, 2001). 
       FIG. 1  shows a configuration of an optical transmitter module as related art, which has been disclosed in the article. The optical transmitter module  101 A has an optical waveguide  102 A in which cores  104 A as curved waveguides and light-emitting elements  103  that are apart from each other. Increasing a distance between the adjacent light-emitting elements enables to be reduced a crosstalk between the elements. It is to be noted that a pitch between the light-emitting elements  103  is set to 1 mm. 
     Further, for an optical transmitter/receiver module, a technology has been proposed to increase a distance between a light-emitting element and a light-receiving element (see Japanese Patent Application Publication No. Hei 10-307238). 
       FIG. 2  shows a configuration of an optical a transmitter/receiver module as related art, which has been disclosed in the patent publication. The optical transmitter/receiver module  101 B has an optical waveguide  102 B, a light-emitting element  103 , and a light-receiving element  105 . The optical waveguide  102 B includes a branching core  104 B and also has, on its end face, a reflecting mirror  106  to fold back its optical path. 
     In an optical transmitter/receiver module, electromagnetic radiation generated on the side of a light-emitting element spreads to a light-receiving element to interfere it, which is then subject to any significant crosstalk on an electrical signal due to signal light. Thus, the optical transmitter/receiver module is more sensitive to the crosstalk than the optical transmitter module. 
     If the optical transmitter/receiver module has such a configuration that a light-emitting element and a light-receiving element that are parallel with each other are separated to an extent as to eliminate an influence of crosstalk, the curved waveguide becomes very long to increase its curvature, so that a module becomes very large. Therefore, in the optical transmitter/receiver module  101 B as shown in  FIG. 2 , the light-emitting element  103  and the light-receiving element  105  are separated from each other so that the reflecting mirror  106  is provided on an end face of the waveguide  102 B to fold back its optical path, thereby arranging these elements at the opposite ends of the optical waveguide  102 B. 
     SUMMARY OF THE INVENTION 
     In a configuration as shown in  FIG. 1  such that the optical elements are parallel arranged to separate from each other by an increased distance between them in order to reduce the crosstalk, this increased distance reduces a curvature of a curved waveguide, thus increasing a loss. Therefore, in order to increase the distance between the elements while reducing the loss, it is necessary for a waveguide to elongate, thereby resulting in a large size of the module. 
     Thus, crosstalk may be insufficiently reduced only by increasing a distance between the optical elements, so that additional measures may be taken against crosstalk. 
     Moreover, in a configuration as shown in  FIG. 2  such that an optical path is folded back to reduce a crosstalk, the reflecting mirror is arranged on the optical path, thereby decreasing an intensity of an optical signal. Further, to reduce the crosstalk sufficiently, it is again necessary to increase a distance between the optical elements, thus resulting in an increased size of a module. Moreover, there is only a small degree of freedom in positions where the optical elements are to be mounted, thus bringing about limitations on design of a circuit substrate for driving the optical elements. 
     It is desirable to provide an optical module that can reduce the crosstalk without resulting in a large size of the module. 
     According to an embodiment of the invention, there is provided an optical module. The optical module has at least two optical elements mounted in parallel with each other, a first electrode pad which is formed between the paralleled optical elements and grounded to a ground potential and a second electrode pad which is arranged along a line that is intersected with a direction in which the optical elements are arranged, which faces the first electrode pad and grounded to the ground potential; and a conductive shield member which is connected to the first electrode pad and the second electrode pad and placed between electrical signal transmission paths each connected to the optical elements. 
     According to an optical module of an embodiment of the present invention, when optical elements are driven, electromagnetic radiation occurs along an electrical signal transmission path connected to the optical elements. Electromagnetic radiation occurred at one of the optical elements does not spread to the electrical signal transmission path of any other optical elements since this electromagnetic radiation is coupled to the shield member that is arranged between the parallel optical elements and connected to the ground potential. 
     According to an optical module of the embodiment of the present invention, electromagnetic radiation that occurred on any one of optical elements is coupled to a grounded shield member and so does not spread to an electrical signal transmission path of any other optical elements, thereby enabling crosstalk between the elements to be reduced. 
     Further, even if the parallel optical elements are brought closer to each other, crosstalk between the elements can be reduced, thereby decrease a size of an optical module and increasing a degree of freedom in arrangement of the optical elements. This mitigates restrictions on an electric circuit configuration to realize a simple module structure with less crosstalk. 
     The concluding portion of this specification a particularly points out and directly claims the subject matter of the present invention. However, those skilled in the art will best understand both the organization and method of operation of the invention, together with further advantages and objects thereof, by reading the remaining portions of the specification in view of the accompanying drawing(s) wherein like reference characters refer to like elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram for showing a configuration of an optical transmitter module as related art; 
         FIG. 2  is a diagram for showing a configuration of an optical transmitter/receiver module as related art; 
         FIG. 3A  is a plane view of an optical module as a first embodiment of the invention, for showing a configuration thereof, and  FIGS. 3B and 3C  are cross-sectional views thereof taken along lines IIIB-IIIB, IIIC-IIIC, respectively, shown in  FIG. 3A ; 
         FIGS. 4A and 4B  are graphs for showing results of measuring a light-receiving sensitivity owing to whether or not crosstalk prevention measures according to embodiments of the present invention are taken; 
         FIG. 5A  is a plane view of an optical module as a second embodiment of the invention, for showing a configuration thereof, and  FIG. 5B  is a cross-sectional view thereof taken along a line VB-VB shown in  FIG. 5A ; and 
         FIG. 6A  is a plane view of an optical module as a third embodiment of the invention, for showing a configuration thereof, and  FIG. 6B  is a cross-sectional view thereof taken along a line VIB-VIB shown in  FIG. 6A . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following will describe embodiments of an optical module of preferred embodiments of the present invention with reference to drawings. 
     Configuration of Optical Module according to a First Embodiment 
       FIGS. 3A-3B  are diagrams of a configuration of an optical module according to the first embodiment.  FIG. 3A  is a plane view of the optical module as a first embodiment of the invention,  FIG. 3B  is a cross-sectional view thereof taken along a line IIIB-IIIB shown in  FIG. 3A , and  FIG. 3C  is a sectional view thereof taken along a line IIIC-IIIC shown in  FIG. 3A . 
     The optical module  1 A of the first embodiment has an optical waveguide sheet  2 A having a core/clad structure, a mounting substrate  3 A on which the optical waveguide sheet  2 A is mounted, and a surface-type light-emitting element  4  such as Vertical Cavity-Surface Emitting Laser (VCSEL) and a surface-type light-receiving element  5  such as Photo Diode (PD) that are mounted on the mounting substrate  3 A. 
     The optical waveguide sheet  2 A is one example of optical signal transmission device and made of, for example, a polymer material. The optical waveguide sheet  2 A has two straight cores  6 A 1  and  6 A 2  extending roughly parallel with each other and a clad  7  that covers the cores  6 A 1  and  6 A 2 . The light sheet  2 A is configured in such a manner that a refractive index of each of the cores  6 A 1  and  6 A 2  may be slightly larger than that of the clad  7 . This causes light coupled to the cores  6 A 1  and  6 A 2  to propagate therethrough with it being confined therein. 
     The optical waveguide sheet  2 A has an inclined end face  8  formed on one side thereof that intersects the cores  6 A 1  and  6 A 2 . The inclined end face  8  is an oblique plane having an inclination of about 45 degrees with respect to a plane of the optical waveguide sheet  2 A, wherein end faces of the cores  6 A 1  and  6 A 2  are exposed to form reflecting faces  6 a. The reflecting face  6 a is formed by exposing the end face of any one of the cores  6 A 1  and  6 A 2  to the same plane as the inclined end face  8  and has an inclination of about 45 degrees with respect to an extending direction of the cores  6 A 1  and  6 A 2 . 
     Accordingly, light made incident upon a surface of the optical waveguide sheet  2 A is reflected by the reflecting face  6 a and coupled with the core  6 A 1  to propagate therethrough and light propagating through the core  6 A 2  is reflected by the reflecting face  6 a and is then emitted from the optical waveguide sheet  2 A roughly perpendicularly with respect to a surface of the optical waveguide sheet  2 A. 
     The optical waveguide sheet  2 A is directly adhered and fixed to a surface of the mounting substrate  3 A. The mounting substrate  3 A is made of, for example, silicon (Si) and has an element-mounting concave portion  9 A where the surface-type light-emitting element  4  and the surface-type light-receiving element  5  are mounted. The element-mounting concave portion  9 A is formed by concaving a part of a surface of the mounting substrate  3 A by utilizing anisotropic etching. 
     The two element-mounting concave portions  9 A,  9 A are formed at positions that face the reflecting faces  6 a,  6 a of the cores  6 A 1 ,  6 A 2  in the optical waveguide sheet  2 A mounted on the surface of the mounting substrate  3 A. In this embodiment, two element-mounting concave portions  9 A,  9 A are parallel with each other at places pulled in from a rear end of the mounting substrate  3 A. 
     One of the element-mounting concave portions  9 A is configured to have an opening large enough to contain the surface-type light-emitting element  4  and a depth a little deeper than its height in order to mount the surface-type light-emitting element  4  thereon. The other element-mounting concave portion  9 A is configured to have an opening large enough to contain the surface-type light-receiving element  5  and a depth a little deeper than its height in order to mount the surface-type light-receiving element  5  thereon. 
     The mounting substrate  3 A has insulation films  10 ,  10 , which is made of silicon oxide (SiO2), formed on its right side and back side and also has an element-mounting bonding pad  11 A formed on each of the element-mounting concave portions  9 A,  9 A. On the mounting substrate  3 A, the surface-type light-emitting element  4  is mounted in one of the element-mounting concave portions  9 A,  9 A on which the bonding pad  11 A is formed and the surface-type light-receiving element  5  is mounted in the other element-mounting concave portion  9 A on which the bonding pad  11 A is formed. 
     The surface-type light-emitting element  4  and the surface-type light-receiving element  5  are each one example of an optical element. The surface-type light-emitting element  4  has a light-emitting portion  4 a that emits light. The surface-type light-emitting element  4  is mounted on the element-mounting concave portion  9 A at a position that the light-emitting portion  4 a faces the reflecting face  6 a of the core  6 A 1  of the optical waveguide sheet  2 A. Accordingly, the surface-type light-emitting element  4  is optically coupled with the core  6 A 1  of the light guide  2 A via the reflecting face  6 a. 
     The surface-type light-receiving element  5  has a light-receiving portion  5 a to which light is made incident upon. The surface-type light-receiving element  5  is mounted on the other element-mounting concave portion  9 A at a position that the light-receiving portion  5 a faces the reflecting face  6 a of the core  6 A 2  of the optical waveguide sheet  2 A. Accordingly, the surface-type light-receiving element  5  is optically coupled with the core  6 A 2  of the light guide  2 A via the reflecting face  6 a. 
     Then, the surface-type light-emitting element  4  and the surface-type light-receiving element  5  are fixed to the bonding pad  11 A by using a conductive adhesive agent or through soldering so that a back-surface electrode, not shown, of each of the surface-type light-emitting element  4  and the surface-type light-receiving element  5  may be electrically connected to the bonding pad  11 A. 
     On a surface of the mounting substrate  3 A, a first ground potential electrode pad  12  is formed between the element-mounting concave portions  9 A,  9 A. The first ground potential electrode pad  12  is one example of a first electrode pad and has such a shape as to extend from a position between the element-mounting concave portions  9 A,  9 A to the read end of the mounting substrate  3 A. It is to be noted that the first ground potential electrode pad  12  and each of the bonding pads  11 A can be manufactured on the surface of the mounting substrate  3 A by the same step. 
     The mounting substrate  3 A has a grounding electrode  13  formed all over the back surface, so that the first ground potential electrode pad  12  formed on the surface of the mounting substrate  3 A and the grounding electrode  13  formed on the back surface of the mounting substrate  3 A are electrically connected to each other through a conducting electrode  14 A formed at the rear end of the mounting substrate  3 A. The conducting electrode  14 A is constituted of an electrode pattern formed on the end surface on a side of the rear end of the mounting substrate  3 A and has its upper side connected to the first ground potential electrode pad  12  and its lower end connected to the grounding electrode  13 . 
     The mounting substrate  3 A mounting the optical waveguide sheet  2 A, the surface-type light-emitting element  4  and the surface-type light-receiving element  5  is then installed on an electric circuit substrate  15 . 
     The electric circuit substrate  15  has a circuit substrate ground potential electrode pad  16  formed on its surface. The circuit substrate ground potential electrode pad  16  is one example of a ground electrode and has at least the same size as the grounding electrode  13  formed all over the back surface of the mounting substrate  3 A and is grounded (GND) through a bonding wire etc., which are not shown. 
     The mounting substrate  3 A is placed on the circuit substrate ground potential electrode pad  16  on the electric circuit substrate  15  and fixed thereto by using a conductive adhesive or through soldering, so that the grounding electrode  13  of the mounting substrate  3 A and the circuit substrate ground potential electrode pad  16  of the electric circuit substrate  15  are electrically connected to each other. 
     As described above, the first ground potential electrode pad  12  on the mounting substrate  3 A is electrically connected to the grounding electrode  13 . The grounding electrode  13  is formed all over the back surface of the mounting substrate  3 A so as to be in contact with the circuit substrate ground potential electrode pad  16  all over a back surface thereof. 
     According to such a configuration, the first ground potential electrode pad  12  is connected to the circuit substrate ground potential electrode pad  16  on the electric circuit substrate  15  through a large area. Accordingly, the first ground potential electrode pad  12  has no frequency dependency and functions as a good ground pad at a high frequency too. 
     On the electric circuit substrate  15 , adjacent to the mounting substrate  3 A, a driver integrated circuit (IC)  17  is mounted behind the surface-type light-emitting element  4  and a receiver IC  18  is mounted behind the surface-type light-receiving element  5 . 
     The surface-type light-emitting element  4  and the driver IC  17  are connected to each other in such a manner that an electrode pad  4 b on the surface of the surface-type light-emitting element  4  and the bonding pad  11 A connected with a back surface electrode, not shown, of the surface-type light-receiving element  4  are connected to an electrode pad  17 a on a surface of the driver IC  17  through a bonding wire  19 . The bonding wire  19  is one example of an electrical signal transmission path and made of, for example, gold (Au) and connected to the surface-type light-emitting element  4  and the driver IC  17  by wire bonding. 
     Similarly, the surface-type light-receiving element  5  and the receiver IC  18  are connected to each other in such a manner that an electrode pad  5 b on the surface of the surface-type light-receiving element  5  and the bonding pad  11 A connected with a back surface electrode, not shown, of the surface-type light-receiving element  5  are connected to an electrode pad  18 a on the surface of the receiver IC  18  through the bonding wire  19 . 
     The electric circuit substrate  15  has a second ground potential electrode pad  20  formed on it independently of the circuit substrate ground potential electrode pad  16 . The second ground potential electrode pad  20  is one example of a second electrode pad and formed at a position that faces the first ground potential electrode pad  12  in such a direction as to intersect with a direction in which the surface-type light-emitting element  4  and the surface-type light-receiving element  5  are arranged. In the present embodiment, the second ground potential electrode pad  20  is formed on a position between the driver IC  17  and the receiver IC  18  on the surface of the electric circuit substrate  15  and grounded through a bonding wire etc. 
     The first ground potential electrode pad  12  on the mounting substrate  3 A and the second ground potential electrode  20  on the electric circuit substrate  15  are connected to each other by a bonding wire  21 . The bonding wire  21  is one example of a shield member and is made, for example, gold (Au) and has its one end connected to the first ground potential electrode pad  12  and the other end thereof connected to the second ground potential electrode pad  20 . The first ground potential electrode pad  12  is grounded through the grounding electrode  13  and the circuit substrate ground potential electrode pad  16  and the second ground potential electrode pad  20  is also grounded, so that the bonding wire  21  is connected to the ground potential. 
     The bonding wire  21  is stretched at almost the same height as the bonding wire  19  that connects the surface-type light-emitting element  4  and the driver IC  17  and the surface-type light-receiving element  5  and the receiver IC  18 . Further, although the number of the bonding wires  21  may be singular, the number thereof may be plural; in the present embodiment, the three bonding wires  21  are stretched roughly parallel with each other. 
     Example of Operations of Optical Module of First Embodiment 
     The following will describe an example of operations of the optical module of the first embodiment. An electrical signal output from the driver IC  17  passes through the bonding wire  19  and enters the surface-type light-emitting element  4  where an electrical signal is converted into an optical signal and issued therefrom. 
     The optical signal is emitted from the surface-type light-emitting element  4  roughly perpendicularly to the mounting substrate  3 A and enters the optical waveguide sheet  2 A through its lower surface. The optical signal made incident upon the lower surface of the optical waveguide sheet  2 A roughly perpendicularly is reflected by the reflecting face  6 a and coupled with one of the cores  6 A 1  to propagate through it. 
     In contrast, another optical signal propagating through the other core  6 A 2  is reflected by the reflecting face  6 a and issued from the lower surface of the optical waveguide sheet  2 A roughly perpendicularly. The optical signal issued roughly perpendicularly from the lower surface of the optical waveguide sheet  2 A is made incident upon the surface-type light-receiving element  5  to be converted into an electrical signal. The electrical signal output from the surface-type light-receiving element  5  passes through the bonding wire  19  to enter the receiver IC  18 . 
     Accordingly, the optical module  1 A constitutes a parallel transmitter/receiver module that has a function to transmit an optical signal emitted from the surface-type light-emitting element  4  through the core  6 A 1  of the optical waveguide sheet  2 A and has a function to receive an optical signal from the other core  6 A 2  by using the surface-type light-receiving element  5 . 
     The optical module  1 A thus having a light-emitting element and a light-receiving element, for example, the surface-type light-emitting element  4  and the surface-type light-receiving element  5  encounters electromagnetic radiation generated from the bonding wire  19  when an electrical signal that drives the surface-type light-emitting element  4  is sent from the driver IC  17  to the surface-type light-emitting element  4  via the bonding wire  19 . In a related optical module, electromagnetic radiation generated from the transmission-side bonding wire  19  connecting the surface-type light-emitting element  4  and the driver IC  17  to each other has been coupled with the reception-side bonding wire connecting the light-receiving element and the receiver IC to each other, thus resulting in a crosstalk. 
     In contrast, in the optical module  1 A of the present embodiment, by stretching the bonding wire  21  connected to the ground potential between the transmission-side bonding wire  19  and the reception-side bonding wire  19 , electromagnetic radiation generated from the transmission-side bonding wire  19  is coupled with the bonding wire  21  stretched between the transmissions-side bonding wire  19  and the reception-side bonding wire  19 . 
     Accordingly, electromagnetic radiation generated by the transmission-side bonding wire  19  is not propagated to the reception-side bonding wire  19  to reduce the crosstalk. Further, the bonding wire  21  is connected to the ground potential, so that the coupled electromagnetic radiation has no influence on the surface-type light-receiving element  5  or the receiver IC  18 . 
     To efficiently couple the electromagnetic radiation generated from the transmission-side bonding wire  19  and propagated to the reception-side bonding wire  19  with the bonding wire  21 , this wire  21  is stretched at almost the same height as the transmission-side and reception-side binding wires  19  by aligning it in height with respect to the transmission-side and reception-side binding wires  19  and the cross-talk-reducing bonding wire  21 . 
     The larger the number of the bonding wires  21  is, the higher the effects of reducing crosstalk become; in fact, in the present embodiment, the three bonding wires  21  are stretched in consideration of sizes of the first ground potential electrode pad  12  and the second ground potential electrode pad  20 , workability of wire bonding, etc. 
       FIGS. 4A and 4B  are graphs showing a result of measuring a light-receiving sensitivity owing to whether crosstalk prevention measures of the embodiment of the present invention are taken, which result was obtained by checking effects of the embodiment of the present invention by using an optical transmitter/receiver module in which a surface-type light-emitting element and a surface-type light-receiving element were arranged in parallel with each other. 
       FIG. 4A  shows a result of measuring a bit error ratio (BER) of a reception system when the surface-type light-emitting element was driven (VCSEL ON) and when it was not driven (VCSEL OFF) in a case where crosstalk prevention measures according to the embodiment of the present invention were taken.  FIG. 4B  shows, for comparison, a result of measurement in a case where the crosstalk prevention measures of the embodiment of the present invention were not taken. 
     In the case where the crosstalk prevention measures were not taken, a light reception sensitivity of BER&lt;10 −12  deteriorated by about 3 dB when the surface-type light-emitting element was driven. In contrast, by taking the crosstalk prevention measures of the embodiment of the present invention, the light reception sensitivity deteriorated by 0.5 dB when the surface-type light-emitting element was driven, confirming an effect of reducing the crosstalk by about 2.5 dB. 
     As described above, in the optical module  1 A of the first embodiment, by stretching the bonding wire  21  connected to the ground potential between the surface-type light-emitting element  4  and the surface-type light-receiving element  5 , crosstalk between the elements can be reduced. Accordingly, even if the surface-type light-emitting element  4  and the surface-type light-receiving element  5  arranged in parallel with each other are brought closer to each other, crosstalk between these elements can be reduced, thereby miniaturizing the optical module. For example, even if a distance between the surface-type light-emitting element  4  and the surface-type light-receiving element  5  is decreased to about 600 μm, crosstalk between them can be reduced. 
     Further, a degree of freedom of arrangement of the surface-type light-emitting element  4  and the surface-type light-receiving element  5  is increased, so that limitations on an electric circuit configuration etc. are also mitigated. 
     Moreover, the first ground potential electrode pad  12  which is formed on the surface of the mounting substrate  3 A and to which the crosstalk-reducing bonding wire  21  is connected is connected to the grounding electrode  13  formed on the back surface of the mounting substrate  3 A by forming the conducting electrode  14 A at the rear end of the mounting substrate  3 A, so that the first ground potential electrode pad  12  is connected with the circuit substrate ground potential electrode pad  16  on the electric circuit substrate  15  in a large area. 
     Thus, the first ground potential electrode pad  12  on the surface of the mounting substrate  3 A can function as a good ground pad having no frequency dependency at a high frequency too, so that electromagnetic radiation is coupled with the bonding wire  21  to improve an effect of reducing the crosstalk. 
     Configuration Example of Optical Module of Second Embodiment 
       FIGS. 5A and 5B  show a configuration of an optical module of the second embodiment of the invention.  FIG. 5A  is a plan view of the optical module  1 B and  FIG. 5B  is a cross-sectional view taken along a line VB-VB of  FIG. 5A . 
     An optical module  1 B of the second embodiment has an optical waveguide sheet  2 B with a core/clad structure, a mounting substrate  3 B on which the optical waveguide sheet  2 B is mounted, and a surface-type light-emitting element  4  and a surface-type light-receiving element  5  that are mounted on the mounting substrate  3 B. 
     In the optical module  1 B of the second embodiment, the optical waveguide sheet  2 B has a Y-branched core in which one core  6 B spreads into two cores  6 B 1  and  6 B 2 . In the optical waveguide sheet  2 B, on one side that the branched cores  6 B 1  and  6 B 2  intersect, an inclined end face  8  is formed. The cores  6 B 1  and  6 B 2  are exposed on the inclined end face  8 , to form a reflecting face  6 a. 
     To the branched core  6 B 1 , the surface-type light-emitting element  4  is coupled via the reflecting face  6 a, and to the branched core  6 B 2 , the surface-type light-receiving element  5  is coupled via the reflecting face  6 a, so that an optical signal output from the surface-type light-emitting element  5  is combined with an optical signal input to the surface-type light-receiving element  5  into the one core  6 B. Accordingly, a one-core-double type optical module is configured in which optical signals are transmitted/received by one optical fiber. 
     Further, in the optical module  1 B, on the mounting substrate  3 B, each of the element-mounting concave portions  9 B,  9 B on which the surface-type light-emitting element  4  and the surface-type light-receiving element  5  are mounted is formed close to a rear end of the mounting substrate  3 B. In the present embodiment, the rear end of the mounting substrate  3 B intersects each of the element-mounting concave portions  9 B,  9 B. 
     Accordingly, positions on which the surface-type light-emitting element  4  and the surface-type light-receiving element  5  are mounted are brought close to the rear end of the mounting substrate  3 B. This enables a distance between the surface-type light-emitting element  4  and a driver IC  17  and a distance between the surface-type light-receiving element  5  and a receiver IC  18  to be shortened. 
     By thus shortening the distance between the surface-type light-emitting element  4  and the driver IC  17  and the distance between the surface-type light-receiving element  5  and the receiver IC  18 , a bonding wire  19  connecting the surface-type light-emitting element  4  and the driver IC  17  and another bonding wire  19  connecting the surface-type light-receiving element  5  and the receiver IC  18  can be shortened. 
     This is because the bonding wire  19 , if it is long, deteriorates greatly when a high-frequency signal is transmitted through it so that the bonding wire  19  may be preferably configured as short as possible. 
     Further, this is because if the bonding wire  19  is long when a signal is transmitted at a high frequency, electromagnetic radiation generated on the transmission-side bonding wire  19  is increased, which bring about any increase in crosstalk due to the electromagnetic radiation so that the bonding wire  19  may be preferably configured as short as possible. 
     Moreover, in the optical module  1 B, on the mounting substrate  3 B, bonding pads  11 B formed in the element-mounting concave portions  9 B,  9 B are respectively formed so as to extend to a surface of the mounting substrate  3 B. Then, the bonding wires  19  are connected to the bonding pads  11 B on the side of the surface of the mounting substrate  3 B. With this, a height of a position where each of the bonding wires  19  is connected to the electrode pad is roughly equalized to each other to align wire bonding heights, thereby improving workability of wire bonding. 
     As described above, in the optical module  1 B, the bonding pad  11 B formed in each of the element-mounting concave portions  9 B,  9 B is extended up to the surface of the mounting substrate  3 B in such a shape that the rear end of the mounting substrate  3 B may intersect each of the element-mounting concave portions  9 B,  9 B, so that each of the bonding pads  11 B connected with the surface-type light-emitting element  4  and the surface-type light-receiving element  5  reaches the read end of the mounting substrate  3 B. 
     Therefore, if a first ground potential electrode pad  12  formed on the surface of the mounting substrate  3 B and a grounding electrode  13  formed all over the back surface of the mounting substrate  3 B are connected to each other by forming a conducting electrode  14  on a rear end of the mounting substrate as in the case of the optical module  1 A of the first embodiment, the bonding pad  11 B also is connected to a ground potential. 
     If such a configuration is employed that the conducting electrode is formed at the rear end of the mounting substrate to make the first ground potential electrode pad  12  and the grounding electrode  13  conductive to each other, in order to prevent the bonding pad  11 B from connecting to the conducting electrode, it is necessary to mask the conducting electrode so that it disconnects the bonding pad  11 B and then form the conducting electrode on the rear end of the mounting substrate, thus resulting in an increase in numbers of steps to be performed. 
     Therefore, in the optical module  1 B of the second embodiment, a conducting electrode  14 B that connects the first ground potential electrode pad  12  and extends to one side end of the mounting substrate  3 B is formed on the surface of the mounting substrate  3 B. 
     Then, by forming a conducting electrode  14 C by sputtering or evaporation on the end face of one of the side ends of the mounting substrate  3 B, the first ground potential electrode pad  12  formed on the surface of the mounting substrate  3 B is connected with the grounding electrode  13  formed all over the back surface of the mounting substrate  3 B. It is to be noted that the first ground potential electrode pad  12  formed on the surface of the mounting substrate  3 B, the conducting electrode  14 B, and the bonding pads  11 B can be manufactured by the same step. 
     Such a configuration makes it possible to easily connect the first ground potential electrode pad  12  on the surface side of the mounting substrate  3 B to the ground potential of an electric circuit substrate  15 . 
     Example of Operations of Optical Module of Second Embodiment 
     The following will describe an example of operations of the optical module of the second embodiment. An electrical signal output from the driver IC  17  passes through the bonding wire  19  and enters the surface-type light-emitting element  4 . The surface-type light-emitting element  4  in turn converts the electrical signal into an optical signal and emits it. 
     The optical signal is emitted from the surface-type light-emitting element  4  roughly perpendicularly to the mounting substrate  3 B and enters into the optical waveguide sheet  2 B through its lower surface. The optical signal made incident upon the lower surface of the optical waveguide sheet  2 B roughly perpendicularly is reflected by the reflecting face  6 a and coupled to the one branched core  6 B 1 , thereby propagating from the core  6 B 1  to the core  6 B. 
     In contrast, another optical signal propagating through the other core  6 B 2  branching from the core  6 B is reflecting face  6 a and issued from the lower surface of the optical waveguide sheet  2 B roughly perpendicularly. This optical signal issued from the lower surface of the optical waveguide sheet  2 B roughly perpendicularly impinges on the surface-type light-receiving element  5  where it is converted into an electrical signal. The electrical signal output from the surface-type light-receiving element  5  is transferred through the bonding wire  19  to the receiver IC  18 . 
     Accordingly, the optical module  1 B constitutes a one-core double type transmitter/receiver module that has a function to transmit an optical signal from the surface-type light-emitting element  4  through the core  6 B of the optical waveguide sheet  2 B and has a function to receive an optical signal entered from the same core  6 B by using the surface-type light-receiving element  5 . 
     Like the optical module  1 A of the first embodiment, the optical module  1 B having the surface-type light-emitting element  4  and the surface-type light-receiving element  5  generate electromagnetic radiation from the bonding wire  19  when an electrical signal that drives the surface-type light-emitting element  4  is sent from the driver IC  17  to the surface-type light-emitting element  4  through the bonding wire  519 . 
     Therefore, in the optical module  1 B also, by stretching the bonding wire  21  connected to the ground potential between the transmission-side bonding wire  19  and the reception-side bonding wire  19 , electromagnetic radiation generated by the transmission-side bonding wire  19  is coupled with the bonding wire  21  stretched between the transmission-side and the reception-side bonding wires  19 . 
     Accordingly, electromagnetic radiation generated by the transmission-side bonding wire  19  does not propagate to the reception-side bonding wire  19  to reduce crosstalk. Further, the bonding wire  21  is connected to the ground potential, so that the coupled electromagnetic radiation has no influence on the surface-type light-receiving element  5  or the receiver IC  18 . 
     As described above, in the optical module  1 B of the second embodiment also, by stretching the bonding wire  21  connected to the ground potential between the surface-type light-emitting element  4  and the surface-type light-receiving element  5 , crosstalk between these elements can be reduced to obtain the same effects as the optical module  1 A of the first embodiment. 
     Further, even if the paralleled surface-type light-emitting element  4  and the surface-type light-receiving element  5  are brought close to each other, crosstalk between the elements can be reduced and a curvature can be increased even if using a curved waveguide, thereby reducing a loss. 
     Moreover, the waveguide is not lengthened, so that the optical module  1 B can be miniaturized. 
     Configuration of Optical Module of Third Embodiment 
       FIGS. 6A and 6B  show a configuration of an optical module of the third embodiment of the invention.  FIG. 6A  is a plan view of the optical module  1 C and  FIG. 6B  is a cross-sectional view thereof taken along a line VIB-VIB of  FIG. 6A . 
     In an optical module  1 C of the third embodiment, a first ground potential electrode pad  12  and a circuit substrate ground potential electrode pad  16  are connected to each other by a bonding wire  22  instead of forming electrode patterns on an end face of a mounting substrate. 
     Supposing that the optical module  1 C of the third embodiment has the same configuration as the optical module  1 B of the second embodiment except that the first ground potential electrode pad  12  and the circuit substrate ground potential electrode pad  16  are connected to each other, like reference characters refer to like elements in the second embodiment. 
     In the optical module  1 C of the third embodiment, the first ground potential electrode pad  12  is formed on a surface of a mounting substrate  3 C and a conducting electrode  14 D connecting to the first ground potential electrode pad  12  is formed on the surface of the mounting substrate  3 C. The conducting electrode  14 D extends to a direction of one of the side ends of the mounting substrate  3 C to form a ground pad  23  at a position where the electrode  14 D is exposed from a optical waveguide sheet  2 B, thereby making the first ground potential electrode pad  12  and the ground pad  23  conductive to each other via the conducting electrode  14 D. 
     The first ground potential electrode pad  12  on the mounting substrate  3 C and the circuit substrate ground potential electrode pad  16  on an electric circuit substrate  15  are connected to each other by a bonding wire  22 . The bonding wire  22  is made of, for example, gold (Au) and has its one end connected to the ground pad  23  by wire bonding and the other end thereof connected to the circuit substrate ground potential electrode pad  16 . 
     Accordingly, the first ground potential electrode pad  12  is connected to the ground via the bonding wire  22  and the circuit substrate ground potential electrode pad  16 . The number of the bonding wires  23  is, for example, at least two; in the present embodiment, the four bonding wires  23  are stretched substantially in parallel with each other. Although in  FIG. 4 , the ground pad  23  has been formed approximately at a middle of the mounting substrate  3 C to arrange the plural bonding wires  22  close to each other, the ground pad  23  may be formed in a length direction of the mounting substrate  3 C to arrange the plural bonding wires  22  with a spacing therebetween in the length direction of the mounting substrate  3 C. 
     The optical module  1 C of the third embodiment has such a configuration to thereby enable to be performed in the same process a step of binding a wire between each optical element and the driver IC or the receiver IC, a step of connecting the first ground potential electrode pad  12  and the second ground potential electrode pad  20  to each other, and a step of connecting the ground pad  23  connected with the first ground potential electrode pad  12  and the circuit substrate ground potential electrode pad  16  to each other. This enables the first ground potential electrode pad  12  on the surface side of the mounting substrate  3 C to be simply connected to the ground potential of the electric circuit substrate  15 . 
     Further, to the first ground potential electrode pad  12 , a crosstalk-reducing bonding wire  21  is connected, so that by forming the ground pad  23  connected via the conducting electrode  14 D at a position different from a place where the first ground potential electrode pad  12  is formed, the grounding bonding wire  22  is connected to the ground pad  23 . Accordingly, the bonding wire  22  can be connected at a position different from a position where the bonding wire  21  is connected to the first ground potential electrode pad  12 , thereby eliminating complicated control etc. in the wire bonding step. 
     Example of Operations of Optical Module of Third Embodiment 
     The optical module  1 C of the third embodiment performs the same operations as those of the optical module  1 B of the second embodiment when transmitting/receiving an optical signal. In the optical module  1 C of the third embodiment, electromagnetic radiation is generated from a bonding wire  19  when an electrical signal that drives a surface-type light-emitting element  4  is sent from a driver IC  17  to the surface-type light-emitting element  4  through the bonding wire  19 . 
     Therefore, in the optical module  1 C, by stretching a bonding wire  21  connected to the ground potential between the transmission-side bonding wire  19  and the reception-side bonding wire  19 , electromagnetic radiation generated by the transmission-side bonding wire  19  is coupled with the bonding wire  21  stretched between the transmission-side and the reception-side bonding wires  19 . 
     Accordingly, electromagnetic radiation generated by the transmission-side bonding wire  19  does not propagate to the a reception-side bonding wire  19  to reduce crosstalk. Further, as the bonding wire  21  is connected to the ground potential, the coupled electromagnetic radiation has no influence on the surface-type light-receiving element  5  or a receiver IC  18 . 
     As described above, in the optical module  1 C of the third embodiment also, by stretching the bonding wire  21  grounded to the ground potential between the surface-type light-emitting element  4  and the surface-type light-receiving element  5 , crosstalk between these elements can be reduced to obtain the same effects as those of the optical module  1 A of the first embodiment and the optical module  1 B of the second embodiment. 
     Further, the first ground potential electrode pad  12  formed on the mounting substrate  3 C and the circuit substrate ground potential electrode pad  16  formed on the electric circuit substrate  15  can be connected to each other in the same process as a step of bonding a wire between the optical element and the driver IC, so that the manufacturing process can be simplified. 
     Modification of Optical Module of Each Embodiment 
     Although the optical module  1 A of the above first embodiment, the optical module  1 B of the above second embodiment, and the optical module  1 C of the above third embodiment have been described with reference to an example of an optical transmitter/receiver module having one light-emitting element and one light-receiving element as optical elements, an optical transmitter module may be employed which has no light-receiving element but plural light-emitting elements. Further, a light-receiving module may be employed which has no light-emitting element but plural light-receiving elements. Furthermore, an optical transmitter/receiver module may be employed which has plural light-emitting elements and plural light-receiving elements. In such a manner, it is of course possible to reduce crosstalk by the same configuration in a variety of embodiments of optical modules having plural optical elements. 
     A core structure configuring a straight waveguide in the optical waveguide sheet  2 A of the optical module  1 A of the first embodiment may be applied to the optical waveguide sheet  2 B of the optical modules  1 B and  1 C of the respective second and third embodiments. Similarly, a core structure configuring branching waveguides in the optical waveguide sheet  2 B of the optical modules  1 B and  1 C of the respective second and third embodiments may be applied to the optical waveguide sheet  2 A of the optical module  1 A of the first embodiment. 
     Although the optical modules  1 A,  1 B, and  1 C of the embodiments have employed an optical waveguide sheet made of a polymer material as optical signal propagation device, it is clear that an optical signal can be propagated by using a light guide made of a quartz-based material, an optical fiber made of a quartz-based material, an optical fiber made of a plastic, or a combination of these. 
     Although the optical modules  1 A,  1 B, and  1 C of the embodiments have provided roughly the same height of the plural crosstalk-reducing bonding wires  21 , the bonding wires  21  may have different heights in consideration of a spread of electromagnetic radiation generated by the transmission-side bonding wire  19 . Further, positions where the bonding wires  21  are respectively connected with the electrode pad may be shifted in a back-and-forth direction. 
     Although the optical modules  1 A,  1 B, and  1 C of the embodiments have utilized a bonding wire as a crosstalk-reducing shield member, it may be constituted of a thin sheet material having conductivity. However, by utilizing a bonding wire as the shield member, the crosstalk-reducing bonding wire can also be connected in the step of binding wires to electrically connect the surface-type light-emitting element  4  and the surface-type light-receiving element  5 , to facilitate a mounting step and enable utilizing of the existing mounting equipment, thereby reducing manufacturing costs. 
     The present invention is applied to an optical communication module between boards or chips of an electronic device, a connector of a communication cable utilizing an optical fiber, etc. It should be understood by those skilled in the art that various modifications, combinations, sub-combination and alternations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.