Abstract:
A connection structure is applied to mutually connect a first and a second transmission line that is planar in shape and that has a ground conductor on a second main surface, such as a microstrip line or a coplanar line with a ground. The first and the second transmission lines are superposed one upon the other, and their signal wiring patterns are electrically connected, as are their ground conductors. An end surface of the first transmission line is substantially covered by a conductor layer connected to the ground conductor. The connection structure can achieve good signal transmission characteristics up to high-frequency bands on the order of several tens of gigahertz.

Description:
[0001]     This is a continuation application of U.S. Ser. No. 10/628,234, filed Jul. 29, 2003. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a technique for connection between transmission lines for transmitting signals at high speeds. Particularly, it relates to a technique for connection between transmission lines suitable for network apparatuses which transmit data at rates on the order of several tens of Gbps.  
         [0004]     2. Background Art  
         [0005]     Network apparatuses for high-speed data transmission are equipped with many components for signal processing, and many transmission lines are used for connection between these components. Types of transmission lines are different for individual components, such as coaxial cables, strip lines, and coplanar lines.  
         [0006]     Referring to  FIGS. 1A  to  2 D, an example of a structure for connecting two transmission lines according to the prior art will be described. One of the transmission lines is a coplanar line with a ground and will be referred to as a component  1 . The other of the transmission lines is a microstrip line and will be referred to as a component  2 . By connecting the signal wiring patterns of the two components  1  and  2  and by connecting the ground conductors of the two components  1  and  2 , the two transmission lines are connected.  
         [0007]     As shown in  FIG. 1A , the lower component  1  includes a dielectric  103 , a signal wiring pattern  101  and a ground conductor  104  both disposed on an upper surface of the dielectric  103 , and a ground conductor  102  disposed on an lower surface of the dielectric  103 . The upper component  2  includes a dielectric  203 , a signal wiring pattern  201  disposed on an upper surface of the dielectric  203 , and a ground conductor  202  disposed on a lower surface of the dielectric  203 .  
         [0008]     As shown in  FIG. 1B , on the lower surface of the component  2 , a conductor pattern  207  is disposed. As shown in  FIG. 1C , the conductor pattern  207  is connected to the signal wiring pattern  201  on the upper surface via a conductor  205  disposed in a through-hole formed in the dielectric  203 .  
         [0009]     Solders  121  and  122  are disposed on the conductor pattern  207  and the ground conductor  202  on the lower surface of component  2 , respectively. These solders function to electrically and mechanically connect the conductors of the components  1  and  2 .  
         [0010]     Referring to  FIG. 2A , the components  1  and  2  are arranged such that an end of the component  2  is superposed on an end of the component  1 . Now referring to  FIG. 2B , the signal wiring pattern  101  on the upper surface of the component  1  is electrically connected to the conductor pattern  207  on the lower surface of the component  2  via the solder  121 . The ground conductor  104  on the upper surface of the component  1  is electrically connected to the ground conductor  202  on the lower surface of the component  2  via the solder  122 , as shown in  FIG. 2C . The ground conductors  104  and  102  of the component  1  are connected to each other via a conductor  106  in a through-hole formed in the dielectric  103 .  
         [0011]     Referring to  FIG. 2D , the signal wiring pattern  101  on the upper surface of the component  1  is electrically connected to the signal wiring pattern  201  on the upper surface of the component  2  via the solder  121 , conductor pattern  207 , and conductor  205  in through-hole. Thus, an electric signal can be transmitted from the signal wiring pattern  101  on the upper surface of the component  1  to the signal wiring pattern  201  on the upper surface of the component  2 .  
         [0012]     This or other similar structures for connecting transmission lines are disclosed in U.S. Pat. No. 6,501,352, JP Patent Publication (Kokai) Nos. 2001-358246 A, 2000-286614 A, 2000-77902 A, and 9-283574 A (1997).  
         [0013]     The conventional structure for connecting transmission lines as shown in  FIGS. 1A  to  2 D has such a problem that when an electric signal is transmitted using this structure, the signal transmission characteristics deteriorate particularly in frequency bands of several tens of GHz. The inventors&#39; analysis indicated that due to the discontinuous structure of the connecting portion, part of high-frequency signals are emitted to the air from the transmission lines.  
       SUMMARY OF THE INVENTION  
       [0014]     It is an object of the invention to provide a structure for connecting transmission lines that can prevent the emission of radio wave of a high-frequency signal at the connecting portion.  
         [0015]     It is another object of the invention to provide a transmission line structure having good signal transmission characteristics up to high-frequency bands.  
         [0016]     According to a representative embodiment of the invention, the present invention is characterized in that in a connection structure for transmitting an electrical signal from a signal wiring pattern of a first transmission line to that of a second transmission line, an end surface of the first transmission line is substantially covered with a conductor that is connected to a ground conductor.  
         [0017]     More specifically, the first and the second transmission lines each include a signal wiring pattern on a first main plane of a dielectric plate, and a ground conductor on a second main plane thereof. A lower surface of the second transmission line is superposed on an upper surface of the first transmission line at the connecting portion so that the signal wiring pattern and the ground conductor of the first transmission line can be connected to the signal wiring pattern and the ground conductor of the second transmission lines, respectively. In this superposed connection structure, the dielectric plate is not exposed at the end surface of the first transmission line, but the end surface thereof is substantially covered with a conductor layer that is connected to the ground conductor.  
         [0018]     The structure reduces the reflection of a signal at the connecting portion, so that good signal transmission characteristics can be obtained up to high-frequency regions on the order of several tens of gigahertz. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]      FIGS. 1A  to  1 C show an example of the connection structure for transmission lines according to the prior art.  FIG. 1A  is a perspective view illustrating how two transmission lines are connected.  FIG. 1B  is a bottom view of a component  2 .  FIG. 1C  is a cross sectional view along line A 1 -A 2  of the component  2 .  
         [0020]      FIGS. 2A  to  2 D similarly show the connection structure according to the prior art.  FIG. 2A  is a top view of the structure when the components  1  and  2  are connected.  FIG. 2B  is a cross sectional view along line B 1 -B 2  of the components  1  and  2 .  FIG. 2C  is a cross sectional view along line C 1 -C 2  of the components  1  and  2 .  FIG. 2D  is a cross sectional view along line D 1 -D 2  of the components  1  and  2 .  
         [0021]      FIGS. 3A  to  3 D show the connection structure for transmission lines according to a first embodiment of the invention.  FIG. 3A  is a perspective view illustrating how components  3  and  4  are connected.  FIG. 3B  is a bottom view of the component  4 .  FIG. 3C  is a cross sectional view along line E 1 -E 2  of the component  4 .  
         [0022]      FIGS. 4A  to  4 E similarly show the first embodiment of the invention.  FIG. 4A  is a top view of components  3  and  4  when they are connected.  FIG. 4B  is a cross sectional view along line F 1 -F 2  of the components  3  and  4 .  FIG. 4C  is a cross sectional view along line G 1 -G 2  of the components  3  and  4 .  FIG. 4D  is a cross sectional view along line H 1 -H 2  of the components  3  and  4 .  FIG. 4E  is a cross sectional view along line i 1 -i 2  of the components  3  and  4 .  
         [0023]      FIGS. 5A and 5B  show the signal transmission characteristics of the transmission lines of the first embodiment in comparison to those of the conventional example.  FIG. 5A  show the frequency characteristics of signal reflectance.  FIG. 5B  show the frequency characteristics of signal transmittance.  
         [0024]      FIGS. 6A  to  6 C show the connection structure for transmission lines according to a second embodiment of the invention.  FIG. 6A  is a perspective view illustrating how components  5  and  6  are connected.  FIG. 6B  is a bottom view of the component  6 .  FIG. 6C  is a cross sectional view along line J 1 -J 2  of the component  6 .  
         [0025]      FIGS. 7A  to  7 E similarly show the second embodiment of the invention.  FIG. 7A  is a top view of the components  5  and  6  when they are connected.  FIG. 7B  is a cross sectional view along line K 1 -K 2  of the components  5  and  6 .  FIG. 7C  is a cross sectional view along line L 1 -L 2  of the components  5  and  6 .  FIG. 7D  is a cross sectional view along line M 1 -M 2  of the components  5  and  6 .  FIG. 7E  is a cross sectional view along line N 1 -N 2  of the components  5  and  6 .  
         [0026]      FIGS. 8A  to  8 C show the connection structure for transmission lines according to a third embodiment of the invention.  FIG. 8A  is a perspective view illustrating how components  7  and  8  are connected.  FIG. 8B  is a bottom view of the component  8 .  FIG. 8C  is a cross sectional view along line O 1 -O 2  of the component  8 .  
         [0027]      FIGS. 9A  to  9 E similarly show the third embodiment of the invention.  FIG. 9A  is a top view of the components  7  and  8  when they are connected.  FIG. 9B  is a cross sectional view along line P 1 -P 2  of the components  7  and  8 .  FIG. 9C  is a cross sectional view along line Q 1 -Q 2  of the components  7  and  8 .  FIG. 9D  is a cross sectional view along line R 1 -R 2  of the components  7  and  8 .  FIG. 9E  is a cross sectional view along line S 1 -S 2  of the components  7  and  8 .  
         [0028]      FIG. 10  is a functional block diagram of an optical transmission module to which the connection structure for transmission lines according to the invention is applied.  
         [0029]      FIG. 11  is an enlarged cross sectional view of the connection structure between devices in the optical transmission module of  FIG. 10 . 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0030]     Referring to  FIGS. 3A  to  4 E, a first embodiment of the structure and method for connecting two transmission lines according to the invention will be described.  
         [0031]     One of transmission lines is a coplanar line with a ground and will be referred to as a component  3 . The other of transmission lines is a microstrip line and will be referred to as a component  4 . By connecting the signal wiring patterns of the two components  3  and  4  and by connecting the ground conductors of the two components  3  and  4 , the two transmission lines are connected.  
         [0032]     As shown in  FIG. 3A , the lower component  3  includes a dielectric  303 , a signal wiring pattern  301  and a ground conductor  304  both disposed on an upper surface of the dielectric  303 , and a ground conductor  302  disposed on a lower surface of the dielectric  303 .  
         [0033]     In the first embodiment, the lower component  3  further includes a conductor  3001  disposed at an end surface thereof. The conductor  3001  is disposed perpendicular to the signal wiring pattern  301 , such that it covers an end surface of the dielectric  303 . The conductor  3001  is electrically connected to the ground conductor  302 .  
         [0034]     The upper component  4  includes a dielectric  403 , a signal wiring pattern  401  disposed on an upper surface of the dielectric  403 , and a ground conductor  402  disposed on a lower surface of the dielectric  403 .  
         [0035]     As shown in  FIG. 3B , on a lower surface of the component  4 , a conductor pattern  407  is disposed. The conductor pattern  407  is connected to the signal wiring pattern  401  via a conductor  405  in a through-hole formed in the dielectric  403 , as shown in  FIG. 3C .  
         [0036]     On the conductor  407  on the lower surface of the component  4 , a solder  141  is disposed, and on the ground conductor  402 , solders  142  and  143  are disposed. These solders function to electrically and mechanically connect the conductors of the components  3  and  4 .  
         [0037]     Referring to  FIGS. 4A  to  4 E, the structure and method for connecting the components  3  and  4  will be described in more detail. As shown in  FIG. 4A , the two components are arranged such that an end of the component  4  is superposed on an end of the component  3 . As shown in  FIG. 4B , the signal wiring pattern  301  on the upper surface of the component  3  is electrically connected to the conductor pattern  407  on the lower surface of the component  4  via the solder  141 . As shown in  FIG. 4C , the ground conductor  304  on the upper surface of the component  3  is electrically connected to the ground conductor  402  on the lower surface of the component  4  via the solder  142 . The ground conductors  304  and  302  of the component  3  are electrically connected to each other via a conductor  306  in a through-hole formed in the dielectric  303 .  
         [0038]     Further, in the first embodiment, the upper surface of the conductor  3001  of the component  3  is electrically connected to the ground conductor  402  on the lower surface of the component  4  via the solder  143 , as shown in  FIGS. 4B, 4C , and  4 D.  
         [0039]     Referring to  FIG. 4E , the signal wiring pattern  301  on the upper surface of the component  3  is electrically connected to the signal wiring pattern  401  on the upper surface of the component  4  via the solder  141 , conductor pattern  407 , and a conductor  405  in a through-hole. Thus, an electric signal can be transmitted from the signal wiring pattern  301  of the component  3  to the signal wiring pattern  401  of the component  4 .  
         [0040]     In comparison to the conventional example shown in  FIGS. 1 and 2 , it will be seen that the first embodiment of the invention shown in  FIGS. 3A  to  4 E differs in that the conductor  3001  is added. The conductor  3001  is disposed on an end surface of the component  3  that corresponds to a surface  3000  of the component  1  of the conventional example. It is electrically connected to the ground conductor  302 , and is also electrically connected the ground conductor  304  via a conductor  306  in a through-hole.  
         [0041]     By disposing the conductor  3001  as the first embodiment of the invention, it can prevent the emission of radio waves from the surface  3000 , which causes the deterioration in the signal transmission characteristics in high-frequency bands in the conventional example.  
         [0042]     The distance S between the edge of the signal wiring pattern  301  and the conductor  3001  in  FIG. 4B  should preferably be set to be smaller than ¼ of the wavelength of the electric signal passing through the signal wiring pattern  301 . For example, when the relative permittivity of the dielectric  303  is 10 and the frequency band of the electric signal passing through the signal wiring pattern  301  is 40 GHz, ¼ of the wavelength of the electric signal is about 750 μm and therefore the distance S should be set to be smaller than 750 μm.  
         [0043]     Referring to  FIGS. 5A and 5B , the signal transmission characteristics of the first embodiment of the invention will be compared with those of the conventional example.  FIG. 5A  shows the frequency characteristics of signal reflectance. A curve  1001  indicates the characteristics of the transmission line connection structure according to the conventional example. A curve  1002  indicates the characteristics of the connection structure according to the first embodiment of the invention.  FIG. 5B  shows frequency characteristics of signal transmittance. A curve  2001  indicates the characteristics of the connection structure of the conventional example. A curve  2002  indicates those of the connection structure of the first embodiment. It will be seen that the connection structure of the invention can achieve lower signal reflectance and higher signal transmittance in frequency bands of over 30 GHz in particular.  
         [0044]     The signal transmission characteristics shown in  FIGS. 5A and 5B  were obtained by three-dimensional electromagnetic field simulations, which employed the following values and materials:  
         [0045]     Thickness of dielectrics  103 ,  303 : 200 μm  
         [0046]     Relative permittivity of dielectrics  103 ,  303 : 10  
         [0047]     Width of signal wiring patterns  101 ,  301 : 150 μm  
         [0048]     Distance between signal wiring patterns  101 ,  301  and ground conductors  104 ,  304 : 225 μm  
         [0049]     Thickness of dielectrics  203 ,  403 : 50 μm  
         [0050]     Relative permittivities of dielectrics  203 ,  403 : 2, 9 respectively  
         [0051]     Width of signal wiring patterns  201 ,  401 : 100 μm  
         [0052]     Distance (S in  FIG. 4B ) between signal wiring pattern  301  and conductor  3001 : 93 μm  
         [0053]     Material of all of the conductors: copper  
         [0054]     Now referring to  FIGS. 6A  to  7 E, a second embodiment of the structure and method for connecting two transmission lines according to the invention will be described. One of transmission lines is a coplanar line with a ground and will be referred to as a component  5 . The other of transmission lines is a microstrip line and will be referred to as a component  6 . By connecting the signal wiring patterns of the two components  5  and  6  and by connecting the ground conductors of the two components  5  and  6 , the two transmission lines are connected.  
         [0055]     As shown in  FIG. 6A , the lower component  5  includes a dielectric  503 , a signal wiring pattern  501  and a ground conductor  504  both disposed at an upper surface of the dielectric  503 , and a ground conductor  502  disposed on a lower surface of the dielectric  503 .  
         [0056]     In the second embodiment, the component  5  further includes a conductor  5001  disposed at an end surface thereof. The conductor  5001  is disposed perpendicular to the signal wiring pattern  501 , such that it covers an end surface of the dielectric  503 . The conductor  5001  is electrically connected to the ground conductors  502  and  504 .  
         [0057]     The upper component  6  includes a dielectric  603 , a signal wiring pattern  601  disposed on an upper surface of the dielectric  603 , and a ground conductor  602  disposed on a lower surface of the dielectric  603 . The upper component  6  has a structure similar to that of the component  2  shown in  FIG. 1 .  
         [0058]     As shown in  FIG. 6B , a conductor pattern  607  is disposed on a lower surface of the component  6 . As shown in  FIG. 6C , the conductor pattern  607  is connected to a signal wiring pattern  601  via a conductor  605  in a through-hole formed in the dielectric  603 .  
         [0059]     Solders  161  and  162  are disposed on the conductor  607  and the ground conductor  602 , respectively, on the lower surface of the component  6 . These solders function to electrically and mechanically connect the conductors of the components  5  and  6 .  
         [0060]     Referring to  FIGS. 7A  to  7 E, the structure and method for connecting the components  5  and  6  will be described in more detail. As shown in  FIG. 7A , an end of the component  6  is superposed on an end of the component  5 . As shown in  FIG. 7B , the signal wiring pattern  501  on the upper surface of the component  5  is electrically connected to the conductor pattern  607  on the lower surface of the component  6  via the solder  161 .  
         [0061]     As shown in  FIG. 7C , the ground conductor  504  on the upper surface of the component  5  is electrically connected to the ground conductor  602  on the lower surface of the component  6  via the solder  162 . The ground conductors  504  and  502  of the component  5  are electrically connected to each other via a conductor  506  in a through-hole formed in the dielectric  503 .  
         [0062]     As shown in  FIG. 7E , the signal wiring pattern  501  on the upper surface of the component  5  is electrically connected to the signal wiring pattern  601  on the upper surface of the component  6  via the solder  161 , conductor pattern  607 , and a conductor  605  in a through-hole. Thus, an electric signal can be transmitted from the signal wiring pattern  501  of the component  5  to the signal wiring pattern  601  of the component  6 .  
         [0063]     In comparison to the first embodiment of the invention shown in  FIGS. 3A  to  4 E, the second embodiment shown in  FIGS. 6A  to  7 E differs in that the conductor  5001  of the component  5  is electrically connected directly to the ground conductors  502  and  504 . Further, the conductor  5001  is not connected directly to the ground conductor  602  of the component  6  as shown in  FIG. 7D , but the conductor  5001  is electrically connected to the ground conductor  602  via the ground conductor  504  and the solder  162 , as shown in  FIG. 7E .  
         [0064]     Thus, the conductor  5001  of the component  5  prevents the emission of radio waves of the electric signal passing through the component  5 , thus preventing the deterioration of the signal transmission characteristics in the high-frequency bands.  
         [0065]     Now referring to  FIGS. 8A through 9E , a third embodiment of the structure and method for connecting two transmission lines according to the invention will be described. One of transmission lines is a microstrip line and will be referred to as a component  7 . The other of transmission lines is a coplanar line with a ground and will be referred to as a component  8 . By connecting the signal wiring patterns of the two components  7  and  8  and by connecting the ground conductors of the two components  7  and  8 , the two transmission lines are connected.  
         [0066]     As shown in  FIG. 8A , the lower component  7  includes a dielectric  703 , a signal wiring pattern  701  disposed on an upper surface of the dielectric  703 , and a ground conductor  702  disposed on a lower surface of the dielectric  703 .  
         [0067]     In the third embodiment, the lower component  7  further includes a conductor  7001  disposed on an end surface thereof. The conductor  7001  is disposed perpendicular to the signal wiring pattern  701 , such that it covers an end surface of the dielectric  703 . The conductor  7001  is electrically connected to the ground conductor  702 .  
         [0068]     The upper component  8  includes a dielectric  803 , a signal wiring pattern  801  and a ground conductor  804  both disposed on an upper surface of the dielectric  803 , and a ground conductor  802  disposed on a lower surface of the dielectric  803 .  
         [0069]     As shown in  FIG. 8B , a conductor pattern  807  is disposed on the lower surface of the upper component  8 . As shown in  FIG. 8C , the conductor pattern  807  is connected to the signal wiring pattern  801  via a conductor  805  in a through-hole formed in the dielectric  803 .  
         [0070]     Solders  181  and  183  are disposed on the conductor pattern  807  and the ground conductor  802 , respectively, on the lower surface of the component  8 . These solders function to electrically and mechanically connect the conductors of the components  7  and  8 .  
         [0071]     Referring to  FIGS. 9A  to  9 E, the structure and method for connecting the components  7  and  8  will be described in more detail. As shown in  FIG. 9A , the components are arranged such that an end of the component  8  is superposed on an end of the component  7 . As shown in  FIG. 9B , the signal wiring pattern  701  on the upper surface of the component  7  is electrically connected to the conductor pattern  807  on the lower surface of the component  8  via the solder  181 . As shown in  FIG. 9C , the ground conductors  804  and  802  of the component  8  are electrically connected to each other via a conductor  806  in a through-hole formed in the dielectric  803 .  
         [0072]     Further, in the third embodiment, an upper surface of the conductor  7001  of the component  7  is electrically connected to the ground conductor  802  on the lower surface of the component  8  via the solder  183 , as shown in  FIGS. 9B, 9C , and  9 D.  
         [0073]     As shown in  FIG. 9E , the signal wiring pattern  701  on the upper surface of the component  7  is electrically connected to the signal wiring pattern  801  on the upper surface of the component  8  via the solder  181 , conductor pattern  807 , and a conductor  805  in a through-hole. Thus, an electric signal can be transmitted from the signal wiring pattern  701  to the signal wiring pattern  801  of the component  8 .  
         [0074]     In comparison to the first and second embodiments, the third embodiment shown in  FIGS. 8A  to  9 E differs in that the lower component  7  is a microstrip line having no ground conductor on the upper surface of the dielectric  703 , and in that the upper component  8  is a coplanar line with a ground that has further a ground conductor on the upper surface of the dielectric  803 . Further, the conductor  7001  is added to the end surface of the lower component  7  that corresponds to the end surface  3000  of the component  1  of the conventional example, and is electrically connected to the ground conductor  702 .  
         [0075]     In this structure too, the conductor  7001  prevents the emission of radio waves of the electric signal passing through the component  7 , thus preventing the deterioration of the electric characteristics in high-frequency bands.  
         [0076]      FIG. 10  shows an example of an optical transmission module to which the connection structure according to the invention can be applied. A plurality of parallel electric signals enter an optical transmission module  22  via a signal wiring pattern  23 . The mutual phases of the signals are adjusted by a phase adjuster  12 , and the signals are then converted into a single high-frequency signal by a multiplexer  11  before being transmitted to a light-emitting device  10 . An optical signal emitted by the light-emitting device  10  is transmitted to the outside via an optical fiber cable  16 . The optical signal introduced into the optical transmission module  22  via an optical fiber cable  17  is converted into a high-frequency signal by a photodetector  13 . The signal is then converted into a plurality of parallel electric signals by a demultiplexer  14  and a phase adjuster  15 , and the signals are transmitted to an external apparatus via a signal wiring pattern  24 .  
         [0077]     Of all the transmission lines connecting the devices making up the optical transmission module, a transmission line  18  between the multiplexer  11  and the light-emitting device  10  and a transmission line  20  between the light-emitting device  13  and the demultiplexer  14  carry high-frequency electric signals. Thus, it is preferable to apply the connection structure according to the invention to these inter-device transmission lines. Preferably, the connection structure of the invention may be applied to other portions, such as a transmission line  19  between the phase adjuster  12  and the multiplexer  11  and a transmission line  21  between the demultiplexer  14  and the phase adjuster  15 .  
         [0078]      FIG. 11  is a cross-sectional view of an example in which the connection structure described with reference to  FIGS. 8A  to  9 E is applied to the inter-device transmission line  18  of the optical transmission module shown in  FIG. 10 .  
         [0079]     The multiplexer  11  of  FIG. 10  is formed in a semiconductor chip  25 , of which a partial cross-section is shown in  FIG. 11 . A component  7  functions as a substrate for supporting the semiconductor chip  25  and also as a wiring lead. Specifically, the semiconductor chip  25  is mounted on a dielectric plate  703 , and a pad of the semiconductor chip is electrically connected to a signal wiring pattern  701  via a bonding wire  26 . On the other hand, the photodetector of  FIG. 10  is formed in a semiconductor chip  27 . A component  8  functions as a substrate for supporting the semiconductor chip  27  and also as a wiring lead. A pad on the chip  27  is electrically connected to a signal wiring pattern  801  via a bonding wire  28 . The components  7  and  8  are similar in structure to the components  7  and  8 , respectively, described with reference to  FIGS. 8A  to  9 E. The structure for connecting them is also similar to that described by referring to  FIGS. 8A  to  9 E. Thus, the end of the dielectric plate  703  of the component  7  is substantially covered by a conductor  7001  that is electrically connected to a ground conductor  702 .  
         [0080]     Thus, the technique according to the invention can be applied to the optical transmission module, which is one of network apparatuses. When the invention is applied to the optical transmission module, the signal transmission characteristics of the transmission lines can be satisfactorily maintained up to high-frequency bands, so that the performance of the relevant apparatus can be enhanced.  
         [0081]     In the first, second and third embodiments, the transmission lines are either coplanar lines with grounds or microstrip lines. However, those skilled in the art will readily appreciate that the connection structure for transmission lines according to the present invention can be also applied to cases where the transmission lines are formed by strip lines.  
         [0082]     It will be readily appreciated by those skilled in the art that the embodiments described above are merely exemplary and that various modifications or variations may be made within the scope and spirit of the invention as defined in the appended claims.  
         [0083]     In accordance with the invention, the emission of radio waves of a high-frequency signal at a connection of transmission lines can be prevented.  
         [0084]     In accordance with the invention, a transmission line structure can be realized that has good signal transmission characteristics up to high-frequency bands.