Patent Publication Number: US-2016246156-A1

Title: Optical module

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2015-033388, filed on Feb. 23, 2015, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiment discussed herein is related to an optical module. 
     BACKGROUND 
     In recent years, along with the increase in the capacity of optical transmission systems, for optical modules of optical modulators and the like, scale of their configurations is increasing, together with the increase in their modulation speed. Therefore, an optical transmitter equipped with an optical module is desirably downsized by integration of a plurality of Mach-Zehnders forming optical waveguides into a single chip. In the optical module, the optical waveguides are parallelly formed of, for example, four Mach-Zehnders, and on each waveguide, two strips of a signal electrode and a ground electrode are patterned. The optical module generates a signal of multi-level modulation by input of different electric signals to the two signal electrodes. In such an optical module, in order to simplify mounting of input units and reduce the mounting area, the input units for all electric signals are arranged on one side of the package. 
     In an optical module having input units arranged on one side thereof, electric signals, such as radio frequency (RF) signals, are input via a coaxial connector provided on a side surface of the package. Further, a coaxial adapter for inputting the electric signals from outside is connected to the coaxial connector. However, in the optical module, since a pitch between the signal electrodes, to which the electric signals are input, needs to be widened according to the width of the coaxial adapter, when the number of channels is increased, the mounting area is increased along therewith. 
     In order to suppress the above described increase in the mounting area, a surface mounted type optical module, to which electric signals are input from a printed circuit board (PCB) side via a flexible printed circuit (FPC) provided in the package, has been developed. In such an optical module, in order to input electric signals, an electrode pattern on the PCB is connected to an electrode pad on the FPC via solder. Thereby, the coaxial adapter is able to be removed from the optical module, and thus the pitch between the signal electrodes, to which the electric signals are input, is able to be narrowed and the mounting area is able to be decreased. As a result, the optical transmitter is able to be downsized. 
     However, since the electrode pattern on the PCB is connected to the electrode pad on the FPC by soldering work in a state where the FPC has been bent along the PCB, correspondingly with the bending of the FPC, the mounting area is increased. In order to prevent this, a structure for electrically connecting the PCB to the FPC without bending of the FPC has been developed. In a surface mounted type optical module adopting this structure, in a state where an electrode terminal extended from a signal pattern on an FPC has been inserted into a through hole provided in an electrode pattern on a PCB, the electrode terminal is connected to the electrode pattern on the PCB via solder. Thereby, since the FPC is not bent in this optical module, increase in the mounting area accompanying bending of the FPC is able to be suppressed (see, for example, Japanese Laid-open Patent Publication No. 04-286808). 
     In an FPC, two ground terminals are parallelly arranged to interpose a single electrode terminal extended from a signal pattern on the FPC. A width, a length, and an area of the signal pattern on the FPC are set, such that characteristic impedance at a connecting portion between the PCB and the FPC becomes an ideal value, 50Ω. Widths, lengths, and areas of the electrode terminal and the ground terminals are set, similarly to the signal pattern, such that characteristic impedance at the above mentioned connecting portion becomes 50Ω, and the widths, lengths, and areas are set to design values that are the same between the electrode terminal and the ground terminals. Further, the ground terminals are connected to the ground pattern on the PCB via solder with the ground terminals being inserted into through holes provided in the ground pattern on the PCB. 
     Therefore, the width, length, and area of the ground terminals are desirably large to a certain extent, in order to prevent the ground terminals from being detached from the ground pattern on the PCB. However, since the widths, lengths, and areas are set to the same design values between the electrode terminal and the ground terminals, the larger the width, length, and area of the ground terminals are, the larger the width, length, and area of the electrode terminal become. If the width, length, and area of the electrode terminal are too large, the width, length, and area will steeply change at a boundary between the electrode terminal and the signal pattern, and at this changing point, mismatch of impedances will occur. This mismatch is a factor that causes the characteristic impendence at the connecting portion between the PCB and FPC to be deviated from the ideal value, 50Ω. In particular, in an optical module, such as an optical modulator, which handles high frequency signals, the above described mismatch of impedances increases reflection of the high frequency signals, and as a result, the high frequency characteristics are deteriorated. 
     In contrast, if the width, length, and area of the electrode terminal are too small, the electrode terminal will not be strongly connected to the electrode pattern on the PCB via the solder, and thus strength of the connecting portion between the PCB and FPC will not be ensured. 
     SUMMARY 
     According to an aspect of an embodiment, an optical module includes a first circuit board having: an electrode pattern; a ground pattern arranged on both sides of the electrode pattern; a first through hole penetrating through the electrode pattern; and a second through hole penetrating through the ground pattern; and a second circuit board having: a signal line; an electrode terminal that is extended from the signal line and is electrically connected to the electrode pattern with the electrode terminal being inserted into the first through hole; and a ground terminal that is arranged on both sides of the electrode terminal, is electrically connected to the ground pattern with the ground terminal being inserted into the second through hole, and has a width, length, or an area or any combination thereof larger than a width, length, or an area or any combination thereof of the electrode terminal. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a top view illustrating a configuration of an optical module according to an embodiment; 
         FIG. 2  is a partial cross sectional view illustrating an example of a connecting portion between a PCB and an FPC according to the embodiment; 
         FIG. 3  is an enlarged cross sectional view illustrating the example of the connecting portion between the PCB and the FPC according to the embodiment; 
         FIG. 4A  is a side cross sectional view illustrating the example of the connecting portion between the PCB and the FPC; 
         FIG. 4B  is a side cross sectional view illustrating another example of the connecting portion between the PCB and the FPC; 
         FIG. 5  is an enlarged cross sectional view illustrating an example of the connecting portion between the PCB and the FPC according to a first modification; 
         FIG. 6  is an enlarged cross sectional view illustrating an example of the connecting portion between the PCB and the FPC according to a second modification; 
         FIG. 7  is an enlarged cross sectional view illustrating an example of the connecting portion between the PCB and the FPC according to a third modification; 
         FIG. 8  is an enlarged cross sectional view illustrating an example of the connecting portion between the PCB and the FPC according to a fourth modification; and 
         FIG. 9  is a diagram illustrating a configuration of a transmitter mounted with the optical module according to any of the embodiment and modifications. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     Preferred embodiment of the present invention will be explained with reference to accompanying drawings. The disclosed techniques are not limited by this embodiment. 
     First, a configuration of an optical module according to an embodiment disclosed by this application will be described.  FIG. 1  is a top view illustrating a configuration of an optical module  1  according to this embodiment. As illustrated in  FIG. 1 , the optical module  1  is formed by: a crystal circuit board  11  being provided on a printed circuit board (PCB)  10 ; and electrodes  13  being provided near optical waveguides  12  formed on the crystal circuit board  11 . The PCB  10  is, for example, a glass epoxy circuit board, or the like, and is mounted with various parts forming the optical module  1 . The crystal circuit board  11  is formed of an electro-optic crystal, such as LiNbO 3 (LN), LiTaO 2 , or the like. Further, the optical waveguides  12  are formed by forming a metal film of Ti or the like and causing the metal film to be thermally diffused, or by proton exchange in benzoic acid after patterning. The optical waveguides  12  form a Mach-Zehnder interference system, and the electrodes  13  are provided on Mach-Zehnder parallel waveguides. 
     Further, the electrodes  13  are arranged right on top of the optical waveguides  12  in order to use the refractive index change due to an electric field in a z-axis direction. The electrodes  13  are coplanar electrodes formed by signal electrodes and ground electrodes being patterned on the optical waveguides  12 . The optical module  1  has a buffer layer between the crystal circuit board  11  and the electrodes  13 , in order to prevent light propagated through the optical waveguides  12  from being absorbed by the signal electrodes and the ground electrodes. The buffer layer is formed of SiO 2  or the like, which has a thickness of about 0.2 to 2 μm. 
     When the optical module  1  is driven at high speed, terminals of the signal electrodes and ground electrodes are connected via a resistance to form a traveling-wave electrode, and microwave signals are applied from an input side thereof. When this is done, due to the electric field, refractive indices of the two optical waveguides  12  forming the Mach-Zehnder respectively change by +Ana and −Anb, and accompanying this, a phase difference between the optical waveguides  12  changes. As a result, signal light phase-modulated due to the Mach-Zehnder interference is output from the optical waveguides  12 . By controlling an effective refractive index of microwaves by changing a cross sectional shape of the electrodes  13  and matching speeds of the light and microwaves, high speed optical response characteristics of the optical module  1  are able to be obtained. 
     In the optical module  1 , as illustrated in  FIG. 1 , in a package  14  accommodating the crystal circuit board  11 , optical waveguides  12 , and electrodes  13 , an FPC  16  is provided via a relay circuit board  15 . If a high frequency propagation loss in electrodes on the FPC  16  is large, the modulation bandwidth is narrowed and the driving voltage is increased. Therefore, in order to reduce the high frequency loss in the optical module  1  handling high frequency signals, the FPC  16  is desirably made short as much as possible. 
     The PCB  10  is connected to the FPC  16 . At a connecting portion between the PCB  10  and the FPC  16 , when the FPC  16  and the PCB  10  are connected to each other via solder in a state where the FPC  16  has been bent along the PCB  10 , correspondingly with the bending of the FPC  16 , the mounting area is increased. In order to avoid this, in a state where an electrode terminal  16   a  extended from a signal pattern of the FPC  16  has been inserted into a through hole provided in an electrode pattern  10   a  of the PCB  10 , the electrode terminal  16   a  is connected to the electrode pattern  10   a  of the PCB  10  via solder. Thereby, since the FPC  16  is not bent in this optical module  1 , increase in the mounting area accompanying bending of the FPC  16  is able to be suppressed. 
     Further, in the optical module  1 , if mismatch of impedances at the connecting portion between the PCB  10  and the FPC  16  occurs, reflection of high frequency signals is increased and the transmission frequency bandwidth is narrowed. In order to prevent this, it is important to make the impedance at the connecting portion between the electrode terminal  16   a  from the FPC  16  and the electrode pattern  10   a  of the PCB  10  to 50Ω as much as possible. 
     An electric signal, such as an RF signal or the like output from the electrode pattern  10   a  of the PCB  10  is input to the electrode  13 , via the electrode terminal  16   a  of the FPC  16  installed in the package  14 . Since the PCB  10  (electrode pattern) and the FPC  16  (electrode terminal) are connected to each other via solder, as compared to a case where a coaxial adapter is used, a pitch between the electrode terminals  16   a  is able to be narrowed and high density mounting is enabled. 
       FIG. 2  is a partial cross sectional view illustrating an example of the connecting portion between the PCB  10  and the FPC  16  according to the embodiment. As illustrated in  FIG. 2 , the electrode pattern  10   a  of the PCB  10  and one end (electrode terminal  16   a  side) of the FPC  16  are connected to each other via a solder S 1 . The FPC  16  extends upward, contacts the package  14  at the other end thereof, and is fixed to a glass terminal  18  on the package  14  via a lead pin  18   a  and solders S 2  and S 3 . Further, the FPC  16  is electrically connected to the relay circuit board  15  and the electrode  13  via the lead pin  18   a.  Thereby, after an electric signal, such as an RF signal or the like, which has been input to the electrode terminal  16   a  from the electrode pattern  10   a,  reaches the lead pin  18   a  via the FPC  16 , the electric signal flows through the electrode  13  via the relay circuit board  15 . 
       FIG. 3  is an enlarged cross sectional view illustrating the example of the connecting portion between the PCB  10  and the FPC  16  according to the embodiment. As illustrated in  FIG. 3 , the electrode pattern  10   a  is formed only on a front surface (an FPC  16  side surface) of the PCB  10  of the optical module  1  according to this embodiment. Further, on the front surface of the PCB  10 , on both sides of the electrode pattern  10   a,  two ground patterns  10   b  and  10   c  are formed in parallel with the electrode pattern  10   a . On a back surface of the PCB  10 , two ground patterns  10   b  and  10   c  are formed at positions opposite to the ground patterns  10   b  and  10   c  on the front surface. That is, the electrode pattern  10   a  is formed only on the front surface of the PCB  10 , and the ground patterns  10   b  and  10   c  are formed on the front surface and back surface of the PCB  10 . Further, in the PCB  10 , a through hole  10   a - 1  penetrating through the electrode pattern  10   a,  and through holes  10   b - 1  and  10   c - 1  penetrating through the ground patterns  10   b  and  10   c  are formed. The through hole  10   b - 1  is a through hole for electrically connecting the ground patterns  10   b  formed on the front surface and back surface of the PCB  10  to each other. The through hole  10   c - 1  is a through hole for electrically connecting the ground patterns  10   c  formed on the front surface and back surface of the PCB  10  to each other. 
     On a back surface (a package  14  side surface) of the FPC  16 , a microstrip line M serving as a signal pattern is formed. Further, on a back surface of the FPC  16 , the electrode terminal  16   a  extended from the microstrip line M is formed. Further, on the front surface of the FPC  16 , on both sides of the electrode terminal  16   a,  two ground terminals  16   b  and  16   c  are formed in parallel with the electrode terminal  16   a.  Further, on the back surface of the FPC  16 , an electrode terminal  16   a  and two ground terminals  16   b  and  16   c  are formed at positions opposite to the terminals on the front surface. That is, the electrode terminals  16   a  and the two ground terminals  16   b  and  16   c  are formed on the front surface and back surface of the FPC  16 . The electrode terminals  16   a  are connected to the electrode pattern  10   a  formed only on the front surface of the PCB  10 , via the solder S 1 , in a state where the electrode terminals  16   a  have been inserted into the through hole  10   a - 1  of the PCB  10 . The ground terminals  16   b  and  16   c  are electrically connected to the ground patterns  10   b  and  10   c  formed on the front surface and back surface of the PCB  10  via solders S 4  and S 5  in a state where the ground terminals  16   b  and  16   c  have been respectively inserted into through holes, which are the through holes  10   b - 1  and  10   c - 1  of the PCB  10 . 
     As illustrated in  FIG. 3 , a width, a length, and an area of the ground terminals  16   b  and  16   c  are larger than those of the electrode terminals  16   a.  That is, conventionally, the ground terminals  16   b  and  16   c  would have the same width, length, and area as those of the electrode terminals  16   a,  such that the characteristic impedance at the connecting portion between the PCB  10  and the FPC  16  would become the ideal value, 50Ω. Therefore, the greater the width, length, and area of the ground terminals  16   b  and  16   c  were, the greater the width, length, and area of the electrode terminals  16   a  became, and at a boundary between the electrode terminals  16   a  and the microstrip line M, mismatch of impedances sometimes occurred. As a result, there was a risk that reflection of high frequency signals at the connecting portion between the PCB  10  and FPC  16  might be increased and the high frequency characteristics might be deteriorated. On the contrary, if the width, length, and area of the electrode terminals  16   a  were too small, the electrode terminals  16   a  and the electrode pattern  10   a  of the PCB  10  would not be strongly connected to each other via the solder and thus there was a risk that strength of the connecting portion between the PCB  10  and FPC  16  would not be ensured. Therefore, the ground terminals  16   b  and  16   c  in the optical module  1  according to this embodiment have a width, a length, and an area, which are larger than those of the electrode terminals  16   a . Accordingly, increase in the width, length, and area of the electrode terminals  16   a  is able to be suppressed, and mismatch of impedances at the connecting portion between the PCB  10  and FPC  16  is able to be suppressed. Thereby, reflection of high frequency signals at the connecting portion between the PCB  10  and FPC  16  is suppressed, and the high frequency characteristics are improved. Further, since the ground terminals  16   b  and  16   c  are connected strongly to the ground patterns  10   b  and  10   c  of the PCB  10  via solder, the strength of the connecting portion between the PCB  10  and FPC  16  is ensured. Although the width, length, and area of the ground terminals  16   b  and  16   c  are larger than those of the electrode terminals  16   a  in the optical module  1  illustrated in  FIG. 3 , at least any one of the width, length, and area of the ground terminals  16   b  and  16   c  just needs to be larger than that or those of the electrode terminals  16   a.    
     Further, as illustrated in  FIG. 3 , in the electrode terminal  16   a,  a through hole T 1 , through which the solder S 1  for electrically connecting the electrode terminal  16   a  to the electrode pattern  10   a  of the PCB  10  is flown, is formed. In the ground terminals  16   b  and  16   c , through holes T 2  and T 3 , through which the solders S 4  and S 5  for electrically connecting the ground terminals  16   b  and  16   c  to the ground patterns  10   b  and  10   c  of the PCB  10  are flown, are respectively formed. Thereby, the connection between the electrode terminal  16   a  and the electrode pattern  10   a  of the PCB  10 , and the connection between the ground terminals  16   b  and  16   c  and the ground patterns  10   b  and  10   c  of the PCB  10  are strengthened. 
       FIG. 4A  is a side cross sectional view illustrating the example of the connecting portion between the PCB  10  and FPC  16 .  FIG. 4A  corresponds to a cross sectional view along an A-A line in  FIG. 3 . As illustrated in  FIG. 4A , the electrode terminals  16   a  formed on the front surface and back surface of the FPC  16  protrude over an end face  16   a - 1  of the FPC  16  and are inserted into the through hole  10   a - 1  of the PCB  10 . A length of a portion of the electrode terminal  16   a,  the portion protruding from the end face  16   a - 1  of the FPC  16 , is desirably less than a thickness of the PCB  10 , and is desirably, for example, equal to or less than 500 μm. Further, between the electrode terminals  16   a  formed on the front surface and back surface of the FPC  16 , a reinforcing portion  16   a - 2  extended from the end face  16   a - 1  of the FPC  16  is inserted. Thereby, bending of the electrode terminals  16   a  formed on the front surface and back surface of the FPC  16  is suppressed, and the connection between the electrode terminal  16   a  and the electrode pattern  10   a  of the PCB  10  is more stabilized. Further, the microstrip line M and the electrode terminals  16   a  of the FPC  16  are coated with a plating C in order to prevent them from being detached from the FPC  16 . Furthermore, the microstrip line M and the electrode terminals  16   a  of the FPC  16  are desirably formed of the same material (for example, copper foil) in order to facilitate the formation thereof. 
     Although illustration thereof is omitted in  FIG. 4A , the ground terminals  16   b  and  16   c  formed on the front surface and back surface of the FPC  16  protrude over the end face  16   a - 1  of the FPC  16  and are inserted into the through holes  10   b - 1  and  10   c - 1  of the PCB  10 , respectively. Further, between the ground terminals  16   b  formed on the front surface and back surface of the FPC  16 , a reinforcing portion extended from the end face  16   a - 1  of the FPC  16  is inserted. Furthermore, between the ground terminals  16   c  formed on the front surface and back surface of the FPC  16 , a reinforcing portion extended from the end face  16   a - 1  of the FPC  16  is inserted. Thereby, bending of the ground terminals  16   b  and  16   c  formed on the front surface and back surface of the FPC  16  is suppressed, and the connection of the ground terminals  16   b  and  16   c  to the ground patterns  10   b  and  10   c  of the PCB  10  is more stabilized. 
       FIG. 4B  is a side cross sectional view illustrating another example of the connecting portion between the PCB  10  and the FPC  16  according to the embodiment.  FIG. 4B  corresponds to a cross sectional view along the A-A line in  FIG. 3 . As illustrated in  FIG. 4B , the electrode terminals  16   a  formed on the front surface and back surface of the FPC  16  protrude over the end face  16   a - 1  of the FPC  16  and are inserted into the through hole  10   a - 1  of the PCB  10 . A length of a portion of the electrode terminal  16   a  is desirably less than the thickness of the PCB  10 , the portion protruding from the end face  16   a - 1  of the FPC  16 , and for example, the length is desirably equal to or less than 500 μm. Further, between the electrode terminals  16   a  formed on the front surface and back surface of the FPC  16 , the reinforcing portion illustrated in  FIG. 4A  is not inserted. Thereby, the structure of the FPC  16  is simplified. 
     As described above, the optical module  1  has the PCB  10  and the FPC  16 . The PCB  10  has the electrode pattern  10   a,  the ground patterns  10   b  and  10   c,  the through hole  10   a - 1 , and the through-holes  10   b - 1  and  10   c - 1 . The ground patterns  10   b  and  10   c  are arranged on both sides of the electrode pattern  10   a.  The through hole  10   a - 1  penetrates through the electrode pattern  10   a.  The through holes  10   b - 1  and  10   c - 1  penetrate through the ground patterns  10   b  and  10   c.  The FPC  16  has the microstrip line M, the electrode terminals  16   a,  and the ground terminals  16   b  and  16   c.  The electrode terminals  16   a  are extended from the microstrip line M, and are electrically connected to the electrode pattern  10   a  in a state where the electrode terminals  16   a  have been inserted into the through hole  10   a - 1 . The ground terminals  16   b  and  16   c  are arranged on both sides of the electrode terminals  16   a,  and are electrically connected to the ground patterns  10   b  and  10   c  in a state where the ground terminals  16   b  and  16   c  have been inserted into the through holes  10   b - 1  and  10   c - 1 . At least any one of the width, length, and area of the ground terminals  16   b  and  16   c  is larger than that of the electrode terminals  16   a . Therefore, reflection of high frequency signals caused by mismatch of impedances at the connecting portion between the PCB  10  and FPC  16  is suppressed, and the ground terminals  16   b  and  16   c  are strongly connected to the ground patterns  10   b  and  10   c  of the PCB  10  via solder. As a result, the strength of the connecting portion between the PCB  10  and FPC  16  is able to be ensured and the high frequency characteristics are able to be improved. 
     First Modification 
     Next, a first modification will be explained. An optical module according to the first modification has the same configuration as that of the optical module  1  according to the above described embodiment, except for that plural through holes are formed in the FPC  16 . Therefore, in the first modification, the same reference signs will be used for components common to those of the above described embodiment, and detailed description thereof will be omitted. 
       FIG. 5  is an enlarged cross sectional view illustrating an example of a connecting portion between the PCB  10  and the FPC  16  according to the first modification. As illustrated in  FIG. 5 , in the electrode terminal  16   a  of the FPC  16 , plural through holes T 1 , through which the solder S 1  for electrically connecting the electrode terminal  16   a  to the electrode pattern  10   a  of the PCB  10  is flown, are formed. In the ground terminals  16   b  and  16   c  of the FPC  16 , plural through holes T 2  and T 3 , through which the solders S 4  and S 5  for electrically connecting the ground terminals  16   b  and  16   c  to the ground patterns  10   b  and  10   c  of the PCB  10  are flown, are respectively formed. Thereby, the connection of the electrode terminals  16   a  to the electrode pattern  10   a  of the PCB  10 , and the connection of the ground terminals  16   b  and  16   c  to the ground patterns  10   b  and  10   c  of the PCB  10  are strengthened further. As a result, strength of the connecting portion between the PCB  10  and FPC  16  is able to be increased. 
     Second Modification 
     Next, a second modification will be explained. An optical module according to the second modification has a configuration similar to that of the optical module  1  according to the first modification, except for a shape of ground terminals of the FPC  16 . Therefore, in the second modification, the same reference signs will be used for components common to those of the above described first modification, and detailed description thereof will be omitted. 
     In the optical module  1  according to the above described first modification, since the solder goes into the through holes T 2  and T 3  of the ground terminals  16   b  and  16   c,  there is concern that the characteristic impedance may be deviated from the ideal value, 50Ω. That is, when solder goes into the through holes T 2  and T 3  of the ground terminals  16   b  and  16   c,  correspondingly with the amount of the solder that has gone into the through holes T 2  and T 3 , a portion of the conductive substances is increased. Therefore, there is a risk that the impedance of the portion of the through holes T 2  and T 3  does not match the impedance of the other portion (a portion of the ground terminals  16   b  and  16   c  not connected by the solder), and that as a result, the characteristic impedance at the connecting portion may deviate from 50Ω. Such mismatch of the impedances is a factor that increases reflection of high frequency signals and deteriorates the high frequency characteristics. 
     For the optical module  1  according to the second modification, adjustment of the above described impedance is aimed.  FIG. 6  is an enlarged cross sectional view illustrating an example of a connecting portion between the PCB  10  and the FPC  16  according to the second modification. As illustrated in  FIG. 6 , the ground terminal  16   b  has an extended portion  16   d  extended towards the electrode terminal  16   a  from a proximal end portion of the ground terminal  16   b,  the proximal end portion not being inserted into the through hole  10   b - 1  of the PCB  10 . Similarly, the ground terminal  16   c  has an extended portion  16   e  extended towards the electrode terminal  16   a  from a proximal end portion of the ground terminal  16   c,  the proximal end portion not being inserted into the through hole  10   c - 1  of the PCB  10 . Further, a gap g 2  smaller than a gap g 1  in  FIG. 3  is present between a distal end of the extended portion  16   d  on the left and the electrode terminal  16   a.  Similarly, a gap g 2  smaller than the gap g 1  in  FIG. 3  is present between a distal end of the extended portion  16   e  on the right and the electrode terminal  16   a.    
     Parameters for adjusting the above described characteristic impedance include, for example, an interval between a signal pattern (electrode terminal  16   a ) and a ground pattern (ground terminals  16   b  and  16   c ), or the like. Therefore, a manufacturer of the optical module  1  may approximate the characteristic impedance to 50Ω by adjusting the gap g 2  corresponding to the interval between the signal pattern and ground pattern to an appropriate value. 
     As described above, in the optical module  1  according to the second modification, the ground terminals  16   b  and  16   c  have the extended portions  16   d  and  16   e  extended toward the electrode terminals  16   a  from the proximal end portions of the ground terminals  16   b  and  16   c,  the proximal end portions not being inserted into the through holes  10   b - 1  and  10   c - 1 . Thereby, the above described mismatch of the impedances is suppressed. Therefore, reflection of high frequency signals is reduced. As a result, the high frequency characteristics are improved. 
     Third Modification 
     Next, a third modification will be explained. An optical module according to the third modification has a configuration similar to that of the optical module  1  according to the first modification, except for a shape of electrode terminals of the FPC  16 . Therefore, in the third modification, the same reference signs will be used for components common to those of the above described first modification, and detailed description thereof will be omitted. 
     In the optical module  1  according to the above described first modification, in order to strengthen the connection between the electrode terminal  16   a  of the FPC  16  and the electrode pattern  10   a  of the PCB  10 , plural through holes, through which the solder S 1  is flown, are formed in the electrode terminal  16   a.  However, the electrode terminals  16   a  are fixed only to the front surface of the PCB  10  by being electrically connected to the electrode pattern  10   a  formed only on the front surface of the PCB  10  via the solder S 1 . Therefore, if the amount of the solder S 1  is small, the connection between the electrode terminals  16   a  of the FPC  16  and the electrode pattern  10   a  of the PCB  10  may be weakened. The weakening of the connection between the electrode terminals  16   a  of the FPC  16  and the electrode pattern  10   a  of the PCB  10  is a factor that reduces the strength of a connecting portion between the PCB  10  and the FPC  16 . 
     Accordingly, in the optical module  1  according to the third modification, the electrode terminals  16   a  are fixed, not only to the front surface of the PCB  10 , but also to the back surface thereof.  FIG. 7  is an enlarged cross sectional view illustrating an example of the connecting portion between the PCB  10  and the FPC  16  according to the third modification. As illustrated in  FIG. 7 , the electrode terminal  16   a  has an insulating portion  16   f  protruded towards an end edge of the through hole  10   a - 1  from a distal end portion of the electrode terminal  16   a , the distal end portion being inserted into the through hole  10   a - 1  of the PCB  10 . Further, the electrode terminal  16   a  has an electrode portion  16   g  insulated from the electrode terminal  16   a  by the insulating portion  16   f.  The electrode portion  16   g  is electrically connected to the ground patterns  10   b  and  10   c  formed on the back surface of the PCB  10  via a solder S 6 . Thereby, the connection between the electrode terminal  16   a  of the FPC  16  and the electrode pattern  10   a  of the PCB  10  is strengthened by the connection of the electrode portion  16   g  to the ground patterns  10   b  and  10   c  formed on the back surface of the PCB  10 . As a result, strength of the connecting portion between the PCB  10  and FPC  16  is increased even further. 
     Fourth Modification 
     Next, a fourth modification will be explained. An optical module according to the fourth modification has a configuration similar to that of the optical module  1  according to the third modification, except for a shape of electrode terminals of the FPC  16 . Therefore, in the fourth modification, the same reference signs will be used for components common to those of the above described third modification, and detailed description thereof will be omitted. 
       FIG. 8  is an enlarged cross sectional view illustrating an example of a connecting portion between the PCB  10  and the FPC  16  according to the fourth modification. As illustrated in  FIG. 8 , the insulating portion  16   f  and the electrode portion  16   g  have a width wider than that of the distal end portion of the electrode terminal  16   a,  the distal end portion being inserted into the through hole  10   a - 1  of the PCB  10 . A through hole T 4 , through which the solder S 6  for electrically connecting the electrode portion  16   g  to the ground patterns  10   b  and  10   c  formed on the back surface of the PCB  10  is flown, is formed in the electrode portion  16   g.  Thereby, the connection of the electrode portion  16   g  to the ground patterns  10   b  and  10   c  formed on the back surface of the PCB  10  is strengthened even further. As a result, strength of the connecting portion between the PCB  10  and FPC  16  is increased even further. 
     Application Example 
     In an optical modulator using the above described optical module  1 , while the strength of the connecting portion between the two circuit boards is ensured, the high frequency characteristics are able to be improved, and thus, the optical modulator is effectively applied to a transmitter, for example.  FIG. 9  is a diagram illustrating a configuration of a transmitter  100  mounted with the optical module  1  according to any of the above described embodiment and modifications. As illustrated in  FIG. 9 , the transmitter  100  has a data generation circuit  101 , an optical modulator  102 , and an optical fiber  103 . Further, the data generation circuit  101  has a driver  101   a,  and the optical modulator  102  has a laser diode (LD)  102   a.  These components are respectively connected to one another to be able to input and output various signals and data, unidirectionally or bidirectionally. Data generated by the data generation circuit  101  are transmitted to outside the apparatus after being converted from electric signals to optical signals by the optical modulator  102 , with the optical fiber  103  being a transmission medium. 
     In particular, the optical module  1  is effectively applied to an optical modulator, to which an electric signal from a PCB  10  side is input by use of the FPC  16 . Such optical modulators include, for example, an in-phase/quadrature (I/Q) optical modulator, a polarization multiplexed optical modulator, an ITXA, an ICR, an optical transmission and reception integrated device, and the like. Not being limited to the transmitter, the optical module  1  may be applied to a receiver. 
     Further, in the above description, the individual configuration and operation have been described for each of the embodiment and modifications. However, the optical module  1  according to each of the above described embodiment and modifications may also have a component specific to any of the other modifications. Further, not being limited to a combination of two of the embodiment and modifications, any mode including a combination of three or more thereof may be adopted. For example, the optical module  1  according to the second modification may have, in the electrode terminal  16   a,  the insulating portion  16   f  and the electrode portion  16   g  according to the third modification. Further, one optical module may have all of the components described in the above described embodiment and first to fourth modifications, as long as compatibility among them is achieved. 
     According to an aspect of an optical module disclosed by this application, an effect of being able to improve high frequency characteristics while ensuring strength of a connecting portion between two circuit boards is achieved. 
     All examples and conditional language recited herein are intended for pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment of the present invention has been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.