Abstract:
Disclosed is an optical modulator module including an optical modulator configured to have a signal electrode and a ground electrode; a conductive package configured to accommodate the optical modulator and have electrical continuity with the ground electrode of the optical modulator; a substrate configured to have a ground electrode on a first surface thereof electrically connected to the package by solder or a conductive adhesive and have a signal electrode on another surface thereof; and a lead pin configured to electrically connect the signal electrode of the optical modulator to the signal electrode of the substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of Japanese Patent Application No. 2010-192142, filed on Aug. 30, 2010, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiment discussed herein is related to an optical modulator module. 
       BACKGROUND 
       [0003]    Optical waveguide devices each using an electro-optic crystal such as a LiNbO 3  (LN) substrate and a LiTaO 2  substrate have been developed. In order to form such an optical waveguide device, an optical waveguide is first formed in such a manner that a metal film made of titanium or the like is formed and thermally diffused on a part of a crystal substrate or is subjected to proton exchange under benzoic acid after being patterned. With provision of electrodes near the optical waveguide, the optical waveguide device is formed. An example of such an optical waveguide device includes an optical modulator. 
         [0004]    An optical modulator is accommodated in a metal package and mounted in a transmitter as an optical modulator module. Recently, various surface-mounting type components have been developed for the purpose of improving mounting performance (see, for example, Patent Document 1). However, they give rise to a problem in high-frequency characteristics. 
         [0005]    Patent Document 2 discloses a configuration in which a spacer is interposed between a package and a flexible substrate to alleviate impedance mismatching. Patent Document 3 discloses a configuration in which a lead pin is mounted parallel on the signal electrode pad of an FPC and solder-connected.
   Patent Document 1: Japanese Laid-open Patent Publication No. 4-336702   Patent Document 2: Japanese Laid-open Patent Publication No. 2007-42756   Patent Document 1: Japanese Laid-open Patent Publication No. 2009-252918   
 
         [0009]    However, the configurations of Patent Documents 2 and 3 put limitations on space. 
       SUMMARY 
       [0010]    According to an aspect of the present invention, there is provided an optical modulator module including an optical modulator configured to have a signal electrode and a ground electrode; a conductive package configured to accommodate the optical modulator and have electrical continuity with the ground electrode of the optical modulator; a substrate configured to have a ground electrode on a first surface thereof electrically connected to the package by solder or a conductive adhesive and have a signal electrode on another surface thereof; and a lead pin configured to electrically connect the signal electrode of the optical modulator to the signal electrode of the substrate. 
         [0011]    The object and advantages of the present invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0012]    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 present invention as claimed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]      FIGS. 1A and 1B  are a schematic plan view of a Mach-Zehnder type optical modulator and a cross-sectional view taken along line A-A in  FIG. 1A , respectively; 
           [0014]      FIGS. 2A ,  2 B, and  2 C are a schematic plan view of an optical modulator module according to a first comparative example, a cross-sectional view of the optical modulator module taken along line B-B in  FIG. 2A , and a view illustrating a state in which a ground electrode is connected to a ground electrode formed at the under surface of a relay substrate by way of a via-hole, respectively; 
           [0015]      FIGS. 3A and 3B  are view for explaining an optical modulator module according to a second comparative example; 
           [0016]      FIGS. 4A through 4D  are views for explaining an optical modulator module according to a first embodiment; 
           [0017]      FIGS. 5A and 5B  are graphs illustrating calculation results of a relationship between S parameters at 30 GHz and a shortest distance dY; 
           [0018]      FIGS. 6A and 6B  are views for explaining another example of a flexible substrate according to the first embodiment; 
           [0019]      FIGS. 7A and 7B  are views for explaining another example of the flexible substrate; 
           [0020]      FIGS. 8A and 8B  are views for explaining another example of the flexible substrate; 
           [0021]      FIGS. 9A and 9B  are views for explaining another example of the flexible substrate; 
           [0022]      FIG. 10  is a view for explaining another example of a package; 
           [0023]      FIGS. 11A through 11C  are view for explaining an optical modulator module according to a second embodiment; 
           [0024]      FIGS. 12A through 12D  are views for explaining another example of a flexible substrate; 
           [0025]      FIGS. 13A through 13C  are views for explaining another example of the flexible substrate; 
           [0026]      FIGS. 14A and 14B  are views for explaining another example of the flexible substrate; 
           [0027]      FIGS. 15A and 15B  are views for explaining connection between the flexible substrate and a print substrate; 
           [0028]      FIGS. 16A and 16B  are graphs for explaining calculation results of S parameters; 
           [0029]      FIGS. 17A through 17C  are views for explaining an optical modulator module according to a third embodiment; 
           [0030]      FIG. 18  is a view for explaining the optical modulator module according to the third embodiment; and 
           [0031]      FIG. 19  is a block diagram for explaining the entire configuration of an optical transmitter according to a fourth embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0032]    Next, embodiments are described below with reference to the accompanying drawings. 
         [0033]    First, a Mach-Zehnder type optical modulator is described as an example of an optical modulator provided in an optical modulator module.  FIG. 1A  is a schematic plan view of a Mach-Zehnder type optical modulator  10 .  FIG. 1B  is a cross-sectional view taken along line A-A in  FIG. 1A . As illustrated in  FIGS. 1A and 1B , the optical modulator  10  has a substrate  14  in which an optical waveguide is formed. The substrate  14  is an electro-optic substrate having an electro-optic crystal such as a LiNbO 3  (LN) substrate and a LiTaO 2  substrate. 
         [0034]    The optical waveguide includes an incident waveguide, parallel waveguides  11   a  and  11   b  formed to branch from the incident waveguide, and an emitting waveguide in which the parallel waveguides  11   a  and  11   b  merge with each other. The optical waveguide is formed in such a manner that metal such as Ti (titanium) is thermally-diffused into the substrate  14 . 
         [0035]    As illustrated in  FIG. 1B , a buffer layer  15  is provided at a surface on the side of the optical waveguide of the substrate  14 . The optical waveguide is covered with the buffer layer  15 . The buffer layer  15  is provided to prevent light transmitted through the optical waveguide from being absorbed in electrodes described below. The buffer layer  15  is made of, for example, SiO 2  or the like and has a thickness of about 0.2 through 2 μm. 
         [0036]    On the parallel waveguide  11   b , a signal electrode  12  is provided via the buffer layer  15 . On the parallel waveguide  11   a , a ground electrode  13   b  is provided via the buffer layer  15 . Further, on the buffer layer  15 , a ground electrode  13   a  is provided on the side opposite to the ground electrode  13   b  in such a manner as to sandwich the signal electrode  12  between the ground electrodes  13   a  and  13   b . Thus, the signal electrode  12  and the ground electrodes  13   a  and  13   b  form coplanar electrodes. If a Z-cut substrate is used as the substrate  14 , the signal electrode  12  and the ground electrode  13   b  are arranged right above the parallel waveguides to make use of refractive-index fluctuations resulting from electrolysis in a Z direction. 
         [0037]    In order to drive the optical modulator  10  at high speed, the ends of the signal electrode  12  and the ground electrodes  13   a  and  13   b  are connected by a resistor to form a traveling wave electrode and a micro wave signal is applied on the input side of the traveling wave electrode. In this case, the refractive indexes of the parallel waveguides  11   a  and  11   b  fluctuate, for example, like +Δn and −Δn due to an electric field. Thus, due to fluctuations in phase difference between the parallel waveguides  11   a  and  11   b , Mach-Zehnder interference occurs. As a result, signal light having modulated intensity is output from the emitting waveguide. The effective refractive index of microwaves can be controlled with a change in the cross-sectional shapes of the electrodes, and high-speed light response characteristics can be obtained by matching the speeds of light and microwaves. 
         [0038]      FIG. 2A  is a schematic plan view of an optical modulator module according to a first comparative example.  FIG. 2B  is a cross-sectional view of the optical modulator module taken along line B-B in  FIG. 2A . As illustrated in  FIGS. 2A and 2B , the optical modulator  10  is accommodated in a metal package  20 . Although not illustrated in  FIGS. 2A and 2B , a cover may be provided above the package  20 . At one end of the package  20  is provided a connector  22   a  in which an optical fiber  21   a  penetrates. At the other end of the package  20  is provided a connector  22   b  in which an optical fiber  21   b  penetrates. The incident waveguide of the optical modulator  10  is arranged to coincide with the optical axis of the optical fiber  21   a . The emitting waveguide of the optical modulator  10  is arranged to coincide with the optical axis of the optical fiber  21   b.    
         [0039]    An end of the signal electrode  12  and first ends of the ground electrodes  13   a  and  13   b  are connected to each other via a terminal resistor  23 . Another end of the signal electrode  12  and other ends of the ground electrodes  13   a  and  13   b  are guided to an outside via a relay substrate  31 . At the top surface of the relay substrate  31 , a signal electrode  33  for the signal electrode  12  is formed. To the signal electrode  33  is connected a lead pin  36  by solder  34 . The lead pin  36  extends to the outside via a coaxial connector  35  that penetrates the side wall of the package  20 . Note that since the lead pin  36  is harder than lead wire, it can accurately maintain an interval between ground such as the metal package  20  and the lead pin  36 . Accordingly, the lead pin  36  can accurately take impedance matching as a transmission path. 
         [0040]    At the upper surface of the relay substrate  31 , a ground electrode  33   a  connected to the ground electrodes  13   a  and  13   b  is further formed. As illustrated in  FIG. 2C , the ground electrode  33   a  is connected to a ground electrode  32  formed at the under surface of the relay substrate  31  by way of a via-hole  33   b . The ground electrode  32  has electrical continuity with the package  20 . The package  20  is grounded. 
         [0041]    In the optical modulator module according to the first comparative example, it is necessary to input an electric signal output from a driver amplifier to the lead pin  36  via an edge-mount type connector or the like. Accordingly, it is difficult for the optical modulator module to be mounted. 
         [0042]      FIGS. 3A and 3B  are views for explaining an optical modulator module according to a second comparative example. The optical modulator module according to the second comparative example is a surface-mounting type module for facilitating its mounting. In the optical modulator module according to the second comparative example, an electric signal output from a driver amplifier is input from a print substrate. Accordingly, the optical modulator module has improved mounting performance. 
         [0043]      FIG. 3A  is a view corresponding to the view illustrated in  FIG. 2B . In the second comparative example, a flexible substrate  41  is used for the purpose of improving mounting performance. The flexible substrate  41  having flexibility is made of polyimide, liquid crystal polymer, or the like. In this example, an insulative glass member  25  that penetrates a package  20  is provided at the under surface of the package  20 . The glass member  25  is formed into a cylindrical shape as an example. Further, the flexible substrate  41  is provided at the under surface of the glass substrate  25 . A lead pin  36  extends to the under surface of the flexible substrate  41  while penetrating the glass substrate  25  and the flexible substrate  41 . 
         [0044]    In this example, a lead pin  37   a  ( FIG. 3B ) connected to a ground electrode  45   a  via a ground electrode  32  and a lead pin  37   b  connected to a ground electrode  45   b  via the ground electrode  32  are provided. The lead pins  37   a  and  37   b  extend to the under surface of the flexible substrate  41  while penetrating the glass member  25  and the flexible substrate  41 . The lead pins  37   a  and  37   b  are symmetrically arranged about the lead pin  36 . 
         [0045]      FIG. 3B  is a view seen from the under surface side of the flexible substrate  41 . As illustrated in  FIG. 3B , a signal electrode  43  and the ground electrodes  45   a  and  45   b  are formed at the under surface of the flexible substrate  41 . The lead pin  36  is connected to the signal electrode  43  via solder  42 . The lead pin  37   a  is connected to the ground electrode  45   a  via solder  44   a . The lead pin  37   b  is connected to the ground electrode  45   b  via solder  44   b . The ground electrodes  45   a  and  45   b  are symmetrically arranged about the signal electrode  43 . Thus, a coplanar line (CPW) is formed. 
         [0046]    According to the configuration of the second comparative example, since the ground lead pins and the signal lead pin can be soldered at the under surface of the flexible substrate  41 , the optical modulator module has high mounting performance. This configuration is particularly effective if the optical modulator module has a large number of the lead pins. However, it is necessary to set an interval of, for example, 1 mm or more between the adjacent lead pins in order to solder the lead pins. In this case, the interval between the signal electrode and the ground electrodes becomes large. Thus, an characteristic impedance locally greatly deviates from a desired value (for example 50Ω), whereby reflecting characteristics ( FIG. 5A , S 11 ) are degraded. Further, since contact areas between the ground lead pins and the electrodes are limited to only the parts of the lead pins, grounding cannot be sufficiently established for high frequency. Thus, transmitting characteristics (S 21 ) are degraded. The reflecting characteristics (S 11 ) and the transmitting characteristics (S 21 ) may present problems at a high-speed modulation band such as 20 Gbps and 40 Gbps. Further, a modulator having plural signal lines such as a DQPSK modulator and a DP-QPSK modulator may give rise to problems in that it has a difficulty in its high density and requires a large mounting space. In view of this, the following embodiments describe optical modulator modules capable of realizing both high frequency characteristics and mounting performance while accommodating limitations in space. 
         [0047]    Here, the reflecting characteristics (S 11 ) refer to the ratio of reflecting power (Pr) to input power Pin input from a driver amplifier to an optical modulator. The transmitting characteristics (S 21 ) refer to the ratio of output power Pout to the input power Pin input from the driver amplifier to the optical modulator. Specifically, the reflecting characteristics (S 11 ) are calculated by Pr/Pin (dB), and the transmitting characteristics (S 21 ) are calculated by Pout/Pin (dB). 
       First Embodiment 
       [0048]      FIGS. 4A through 4C  are views for explaining an optical modulator module  100  according to a first embodiment. The optical modulator module  100  is a surface-mounting type module for facilitating its mounting.  FIG. 4A  is a view corresponding to the view illustrated in  FIG. 3A .  FIG. 4B  is a view corresponding to the view illustrated in  FIG. 3B  and seen from the under surface sides of a flexible substrate  41  and a package  20 .  FIG. 4C  is a view seen from the top surface side of the flexible substrate  41 . 
         [0049]    As illustrated in  FIGS. 4A and 4B , a lead pin  36  penetrates a glass member  25  and extends to the under surface of the flexible substrate  41  while being in contact with the side surface of the flexible substrate  41 . In this embodiment, the glass member  25  is formed into a cylindrical shape, and the lead pin  36  penetrates the substantial center of the cylindrical shape. The lead pin  36  is connected to a signal electrode  43  at the under surface of the flexible substrate  41  via solder  42 . 
         [0050]    Note that since the lead pin  36  is provided to be in contact with the side surface of the flexible substrate  41 , a contact area between the lead pin  36  and the flexible substrate  41  may not be sufficiently obtained. However, if the cross section of the lead pin  36  is formed into a rectangular shape, it is possible to sufficiently ensure the contact area between the lead pin  36  and the flexible substrate  41 . 
         [0051]    On the top surface of the flexible substrate  41 , a ground electrode  45  having a predetermined width is formed. Since the lead pin  36  is provided along the side surface of the flexible substrate  41 , the ground electrode  45  is formed to be away from the side surface. For example, the ground electrode  45  may be provided to avoid a semi-circular region surrounding a part at which the lead pin  36  is provided. The ground electrode  45  is connected to the external wall (for example, the under surface) of the package  20  via solder  44 . For example, the ground electrode  45  may be connected to the under surface of the package  20  at a part adjacent to the glass member  25 . 
         [0052]    In this embodiment, with the provision of the lead pin  36  at the side surface of the flexible substrate  41 , the side surface of the flexible substrate  41  is away from the package  20 . This enables confirmation as to whether the solder  44  flows out from the under surface side of the flexible substrate  41 . In  FIG. 4B , flowing out of the solder is confirmed by the existence of solder  44   a  and  44   b , which enables the confirmation of the connecting state between the package  20  and the ground electrode  45  of the flexible substrate  41 . Accordingly, manufacturing yield can be maintained at high level. 
         [0053]    In this embodiment, without the use of a ground lead pin, the ground electrode  45  formed on the top surface of the flexible substrate  41  and the external wall of the package  20  are connected to each other. In this case, a contact area between the ground electrode  45  and the external wall of the package  20  become larger compared with a case in which the ground lead pin is used. Thus, grounding can be sufficiently established for high frequency. As a result, degradation of S parameters can be suppressed. Further, since there is no need to use a spacer or the like to reduce impedance mismatching, limitation in space can be suppressed. 
         [0054]    Further, in this embodiment, a micro strip line (MSL) structure is formed by the ground electrode  45  having a predetermined width on the top surface of the flexible substrate  41  and the signal electrode  43  on the under surface of the flexible substrate  41 . A characteristic impedance is controlled by the influences of the thickness of a substrate and a signal line width. Therefore, controlling the thickness of the flexible substrate  41  and the line width of the signal electrode  43  in the vicinity of their desired values provides an impedance having a desired value (for example, 50Ω). Thus, the optical modulator module  100  has improved reflecting characteristics (S 11 ). 
         [0055]    Further, the provision of the signal electrode  43  on the under surface of the flexible substrate  41  facilitates the mounting of the optical modulator module  100 . Note that the ground electrode  45  can extend to the under surface of the flexible substrate  41  via a via-hole or the like. Accordingly, the optical modulator module  100  can be surface-mounted by the flexible substrate  41 . 
         [0056]    Thus, according to this embodiment, the optical modulator module  100  can realize its high frequency characteristics and mounting performance while accommodating limitations in space. 
         [0057]    Note that the solder  44  used in this embodiment may be replaced by a conductive adhesive or the like. Further, as illustrated in  FIG. 4D , in a case where a lead pin  36  having a circular cross section is used, a part of the flexible substrate  41  at which the lead pin  36  is in contact with may be cut into a semicircular shape. In this case, a contact area between the lead pin  36  and the flexible substrate  41  can be increased. Moreover, the flexible substrate  41  used in this embodiment may be replaced by a substrate having high rigidity. 
         [0058]    Note that although a cross-sectional shape at a connection part between the package  20  and the flexible substrate  41  preferably has a MSL structure, a part of the ground electrode  45  is formed to avoid the lead pin  36  as illustrated in  FIG. 4C . This is aimed at avoiding short-circuits and local reduction of impedance. However, if the lead pin  36  is excessively away from the ground electrode  45 , a MSL mode is not established and high frequency characteristics are degraded. Therefore, a shortest distance dY between the ground electrode  45  and the lead pin  36  is preferably set to be within an appropriate range.  FIG. 5A  is a graph illustrating calculation results of a relationship between the S parameters at 30 GHz and the shortest distance dY. As illustrated in  FIG. 5A , the shortest distance dY is preferably set to be less than or equal to 260 μm in order to suppress the reflecting characteristics (S 11 ) below −20 dB. 
         [0059]    Note that in the second comparative example, the signal lead pin and the ground lead pins penetrate and protrude from the substrate. However, in this embodiment, only the lead pin  36  protrudes downward from the under surface of the flexible substrate  41 . Thus, a distance from the tip end of the lead pin  36  to the ground electrode  45  becomes large. If the protruding length of the lead pin  36  becomes large, impedance mismatching may become significant. In view of this, a relationship between the protruding length of the lead pin  36  and the S parameters at 30 GHz was calculated. The calculation results are illustrated in  FIG. 5B . As illustrated in  FIG. 5B , in order to suppress the reflecting characteristics (S 11 ) below −20 dB, the length of the lead pin  36  protruding from the flexible substrate  41  is preferably set to be less than or equal to 590 μM. 
         [0060]    According to the configuration illustrated in  FIGS. 4A through 4C , the contact area between the ground electrode  45  and the package  20  becomes large. In this case, when temperature or humidity changes, stress resulting from a difference in expansion coefficient between the flexible substrate  41 , the solder  44 , and the package  20  becomes high, which may bring about characteristic degradation or breakage in the modulator. This problem becomes remarkable particularly in a modulator having a large number of terminals such as a DP-QPSK modulator and may become a main factor that degrades the long-term reliability of the modulator. 
         [0061]    (Another Example (1) of Flexible Substrate) 
         [0062]    Therefore, it is preferable that the ground electrode  45  can be observed from the under surface side of the flexible substrate  41 .  FIGS. 6A and 6B  are views for explaining another example of the flexible substrate  41 .  FIG. 6A  is a perspective view illustrating the under surface of the flexible substrate  41  and the side surface of the flexible substrate  41  on the side of the lead pin  36 .  FIG. 6B  is a plan view illustrating the under surface of the flexible substrate  41 . 
         [0063]    As illustrated in  FIGS. 6A and 6B , a notch is formed in the flexible substrate  41  at a part at which the lead pin  36  is arranged. The lead pin  36  is arranged at the notch and connected to the signal electrode  43 . In the side surface of the flexible substrate  41  on the side of the lead pin  36 , notches  46   a  and  46   b  may be further formed. The shapes of the notches  46   a  and  46   b  include, but are not particularly limited to, a semi-circular shape as an example. The notches  46   a  and  46   b  are symmetrically provided about the notch at which the lead pin  36  is arranged. Further, the notches  46   a  and  46   b  are provided at a part at which the ground electrode  45  illustrated in  FIG. 4C  is formed. Thus, the ground electrode  45  can be confirmed from the under surface side of the flexible substrate  41 . Further, flowing out of the solder  44  that connects the ground electrode  45  to the package  20  can be confirmed. 
         [0064]    (Another Example (2) of Flexible Substrate) 
         [0065]      FIGS. 7A and 7B  are views for explaining another example of the flexible substrate  41 .  FIG. 7A  is a perspective view illustrating the under surface of the flexible substrate  41  and the side surface  41  on the side of the lead pin  36  of the flexible substrate  41 .  FIG. 7B  is a plan view illustrating the under surface of the flexible substrate  41 . 
         [0066]    As illustrated in  FIGS. 7A and 7B , a part of the ground electrode  45  may be a flying lead that protrudes from the end part of the flexible substrate  41 . In this case, the flying lead can be confirmed from the end of the flexible substrate on the side of the lead pin  36 . Thus, the ground electrode  45  can be confirmed. Further, flowing out of the solder  44  that connects the ground electrode  45  to the package  20  can be confirmed. 
         [0067]    (Another Example (3) of Flexible Substrate) 
         [0068]      FIGS. 8A and 8B  are views for explaining another example of the flexible substrate  41 .  FIG. 8A  is a perspective view illustrating the under surface of the flexible substrate  41  and the side surface of the flexible substrate  41  on the side of the lead pin  36 .  FIG. 8B  is a plan view illustrating the under surface of the flexible substrate  41 . As illustrated in  FIGS. 8A and 8B , a part of the ground electrode  45  may extend to the side surface of the flexible substrate  41 . Thus, the ground electrode  45  can be confirmed. Further, flowing out of the solder  44  that connects the ground electrode  45  to the package  20  can be confirmed. 
         [0069]    (Another Example (4) of Flexible Substrate) 
         [0070]      FIGS. 9A and 9B  are views for explaining another example of the flexible substrate  41 .  FIG. 9A  is a perspective view illustrating the under surface of the flexible substrate  41  and the side surface of the flexible substrate  41  on the side of the lead pin  36 .  FIG. 9B  is a plan view illustrating the under surface of the flexible substrate  41 . As illustrated in  FIGS. 9A and 9B , electrodes  47   a  and  47   b  may be formed one on each side of a notch at the side surface of the lead pin  36 . Moreover, the ground electrode  45  may be connected to the electrodes  47   a  and  47   b . In this case, the ground electrode  45  can be confirmed. Further, flowing out of the solder  44  that connects the ground electrode  45  to the package  20  can be confirmed. 
         [0071]    (Another Example of Package) 
         [0072]      FIG. 10  is a view for explaining another example of the package  20 .  FIG. 10  is a perspective view illustrating the under surfaces of the flexible substrate  41  and the package  20  and the side surface of the flexible substrate on the side of the lead pin  36 . As illustrated in  FIG. 10 , grooves  26   a  and  26   b  may be formed in the package  20  at a part at which the package  20  is connected to the ground electrode  45 . That is, at the under surface of the package  20 , a concave part may be formed at the part at which the package  20  is connected to the ground electrode  45 . 
         [0073]    In this case, when the package  20  is connected to the ground electrode  45  by the solder  44 , the solder  44  flows in the grooves  26   a  and  26   b . Thus, flowing out of the solder  44  can be confirmed. It is preferable that the grooves  26   a  and  26   b  be provided to cross the end part of the flexible substrate  41  on the side of the lead pin  36 . This is because it facilitates the confirmation of the flowing out of the solder  44 . 
       Second Embodiment 
       [0074]      FIGS. 11A through 11C  are views for explaining an optical modulator module  100   a  according to a second embodiment.  FIG. 11A  is a view corresponding to the view illustrated in  FIG. 4A .  FIG. 11B  is a view corresponding to the view illustrated in  FIG. 4B  and seen from the under surface side of a flexible substrate  41 .  FIG. 11C  is a view corresponding to the view illustrated in  FIG. 4C  and seen from the top surface of the flexible substrate  41 . 
         [0075]    As illustrated in  FIGS. 11A through 11C , a lead pin  36  penetrates a glass member  25  and extends to the lower surface of the flexible substrate  41  while penetrating the flexible substrate  41 . In this embodiment, the glass member  25  is formed into a cylindrical shape, and the lead pin  36  penetrates the substantial center of the cylindrical shape. The lead pin  36  is connected to a signal electrode  43  at the under surface of the flexible substrate  41  by solder  42 . As illustrated in  FIG. 11C , a ground electrode  45  is formed to have a predetermined distance between the ground electrode  45  and a part of the flexible substrate  41  at which the lead pin  36  penetrates. Thus, a short circuit of the ground electrode  45  and the lead pin  36  can be prevented. 
         [0076]    In this embodiment, since the lead pin  36  penetrates the flexible substrate  41 , the flexible substrate  41  can be connected to a package  20  to surround the glass member  25 . In this case, a contact area between the flexible substrate  41  and the package  20  is increased. Thus, adhesion between the flexible substrate  41  and the package  20  can be improved. 
         [0077]    Note that a gap is preferably formed between the flexible substrate  41  and the package  20  on the extension of the flexible substrate  41 . In this case, the connection part of the ground electrode  45  is exposed at the gap. Thus, the connection part of the ground electrode  45  can be confirmed from the under surface side of the flexible substrate  41 . Further, at a connection part between the flexible substrate  41  and the package  20 , a notch is preferably formed in the package  20  to expose the ground electrode  45 . In this case, the connection part of the ground electrode  45  can be confirmed. 
         [0078]    Further, on the top surface of the ground electrode  45 , an insulative coverlay  48  is preferably provided between the solder  44  and the lead pin  36 . In this case, a short circuit of the lead pin  36  and the ground electrode  45  due to flowing out of the solder  44  is suppressed. As the coverlay  48 , polyimide or the like can be used. 
         [0079]      FIGS. 12A through 12D  are views for explaining an example of the flexible substrate  41  according to this embodiment.  FIGS. 12A through 12D  are views seen from the under surface side of the flexible substrate  41 . As illustrated in  FIG. 12A , through-holes  49  may be formed in the flexible substrate  41  at apart at which the ground electrode  45  is formed. The through-holes  49  are formed to avoid the glass member  25 . 
         [0080]    As illustrated in  FIG. 12B , notches  49   a  may be formed in the flexible substrate  41  at the part at which the ground electrode  45  is formed. The notches  49   a  are formed to avoid the glass member  25 . As illustrated in  FIG. 12C , plural through-holes  49   b  may be formed in the flexible substrate  41  to surround the glass member  25 . The through-holes  49   b  may be the same in shape as the through-hole of the lead pin  36 . In this case, a through-hole forming process is simplified. 
         [0081]    Note that in consideration of high frequency characteristics, solder is preferably attached on the side of the lead pin  36  at the outer edge of the through-holes  49  and  49   b  and the notches  49   a . Further, in order to reduce stress, the solder is preferably not attached on the side far from the lead pin  36  at the outer edge of the through-holes  49  and  49   b  and the notches  49   a . Moreover, the through-holes  49  and  49   b  and the notches  49   a  are preferably covered with a transparent dielectric layer. In this case, climbing of the solder can be suppressed. 
         [0082]    As illustrated in  FIG. 12D , vias  49   c  may be formed in the under surface of the flexible substrate  41  on the side opposite to the signal electrode  43  about the lead pin  36 . In this case, straight traveling of a signal capable of being not linked to the lead pin can be suppressed. Further, a ground electrode  50  may be provided on the under surface of the flexible substrate  41  to be connected to the ground electrode  45  by way of the vias  49   c . In this case, since a grounded coplanar structure is formed by the signal electrode  43 , the ground electrode  45 , and the ground electrode  50 , grounding on the periphery of the lead pin  36  is enhanced. Note that the vias  49   c  may be an embedded, but through-type vias can realize the enhancement of grounding and the confirmation of the solder. Note that the configuration illustrated in  FIGS. 12A through 12D  is also applicable to the first embodiment in which the lead pin  36  is provided along the side surface of the flexible substrate  41 . 
         [0083]    In the above respective embodiments, through-holes for fixation to the package  20  may be formed in the flexible substrate  41 .  FIG. 13A  is a plan view for explaining an example in which through-holes for fixation are formed in the flexible substrate  41 .  FIG. 13A  is a view illustrating the under surface of the flexible substrate  41 . As illustrated in  FIG. 13A , the plural through-holes  51  for fixation may be formed in the flexible substrate  41 . 
         [0084]      FIG. 13B  is a plan view for explaining an example in which fixation pins  52  are inserted into the through-holes  51 .  FIG. 13C  is a cross-sectional view taken along line C-C in  FIG. 13B . As illustrated in  FIGS. 13B and 13C , insertion of the fixation pins  52  into the through-holes  51  can improve fixation strength between the flexible substrate  41  and the package  20 . Further, with the provision of the fixation pins  52  so as to protrude more than the lead pin  36  on the under surface side of the flexible substrate  41 , not only the productivity of the optical modulator module but also fixation strength of the flexible substrate  41  can be improved. 
         [0085]    Note that in a case where the flexible substrate  41  has a MSL structure, the value of an impedance in a MSL mode that transmits the flexible substrate  41  becomes important. Unlike a CPW mode, the thickness of the flexible substrate  41  and the line width of the signal electrode  43  become important parameters. If the flexible substrate  41  is thin when the flexible substrate  41  is set to have an electric resistance of 50Ω, it is necessary to reduce the line width of the signal electrode  43 . In this case, a conductor loss may be increased, and an impedance may be greatly changed due to a slight change in signal line width. Therefore, the thickness of the flexible substrate  41  is preferably in the range of about several tens through 100 μm. 
         [0086]    However, if the flexible substrate  41  impairs flexibility due to its increased thickness, it may have a part having more flexibility than the connection part at which the flexible substrate  41  is connected to the package  20 . For example, as illustrated in  FIG. 14A , the width of the ground electrode  45  on the flexible substrate  41  may be narrowed at a part other than the connection part. Further, as illustrated in  FIG. 14B , the ground electrode  45  may be formed into a meshed shape at a part other than the connection part. In these cases, flexibility of the flexible substrate  41  can be improved. 
         [0087]    Alternatively, the ground electrode  45  may be provided only on the same surface as the signal electrode  43  at a part other than the connection part. For example, as illustrated in  FIG. 15A , it is assumed that a connection part between the flexible substrate  41  and the package  20  is a region  1  and a connection part between the flexible substrate  41  and a print substrate  60  is a region  3 . The flexible substrate  41  is connected to the print substrate  60  at an end part on the side opposite to the lead pin  36 . It is assumed that a region between the regions  1  and  3  is a region  2 . Although the regions  1  and  3  have a MSL or GCPW, provision of a single-sided electrode in the region  2  can improve flexibility of the flexible substrate  41 . 
         [0088]    Note that if the flexible substrate  41  has sufficient flexibility, the lead pin  36  may be provided to be perpendicular to the side surface of the package  20  and the flexible substrate  41  may be folded by 90 degrees as illustrated in  FIG. 15B . 
         [0089]      FIG. 16A  is a graph illustrating calculation results of S parameters of the optical modulator module according to the second comparative example.  FIG. 16B  is a graph illustrating calculation results of the S parameters of the optical modulator module  100   a  according to the second embodiment illustrated in  FIG. 12D . As illustrated in  FIGS. 16A and 16B , the reflecting characteristics (S 11 ) and the transmitting characteristics (S 21 ) of the optical modulator module  100   a  according to the second embodiment are improved compared with the optical modulator module according to the second comparative example. 
       Third Embodiment 
       [0090]      FIG. 17A  is a view for explaining an optical modulator module  100   b  according to a third embodiment.  FIG. 17A  is the view corresponding to the view illustrated in  FIG. 4A .  FIGS. 17B and 17C  are views seen from the under surface side of a package  20 . 
         [0091]    In this embodiment, an external conductor  39  is provided on the periphery of a glass member  25  as illustrated in  FIG. 17A . Thus, a coaxial line (having a resistance of, for example, 50Ω) is formed by a lead pin  36 , the glass member  25 , and the external conductor  39 . In this embodiment, the coaxial line is inserted into a concave part at the under surface of the package  20 . 
         [0092]    In this case, solder  44  is preferably connected to a flexible substrate  41  to encircle the periphery of the external conductor  39 . Therefore, a groove is preferably provided along the periphery of the external conductor  39  in the package  20  as illustrated in  FIG. 17B . In this case, the solder  44  encircles the periphery of the external conductor  39  via the groove. With the provision of an inlet for flowing the solder  44  in the groove and an outlet for confirming flowing out of the solder  44 , soldering can be effectively performed. 
         [0093]    Note that the groove encircling the periphery of the external conductor  39  may have two paths as illustrated in  FIG. 17C . In this case, since two outlets for confirming the flowing out of the solder  44  are provided, the solder flowing out from both of the paths can be confirmed. Thus, confirmation as to whether the solder  44  flows in both of the paths can be made. As a result, sufficient grounding can be obtained, and degradation of the S parameters can be suppressed. 
         [0094]    Note that if adhesion between the glass member  25  and the flexible substrate  41  is reduced, a gap is formed between the glass member  25  and the flexible substrate  41 . In this case, a characteristic impedance may become large at the gap. Therefore, as illustrated in  FIG. 18 , the thickness (Wair) of the lead pin  36  at a part at which the lead pin  36  is connected to a signal electrode  43  may be greater than the thickness (Wglass) of the lead pin  36  at a part at which the lead pin  36  penetrates the glass member  25 . In this case, the impedance is corrected. Note that the cross section of the lead pin  36  at the part at which the lead pin  36  is connected to the signal electrode  43  may be a circular shape or a rectangular shape but it is not particularly limited. 
       Fourth Embodiment 
       [0095]      FIG. 19  is a block diagram for explaining the entire configuration of an optical transmitter according to a fourth embodiment. As illustrated in  FIG. 19 , the optical transmitter  200  has an optical device  210 , a data generation unit  220 , and the like. The optical device  210  is a semiconductor laser or the like having any one of the optical modulator modules described above. The data generation unit  220  transmits a driving signal for driving the optical device  210  to the optical device  210 . The optical device  210  outputs an optical modulation signal in response to the driving signal from the data generation unit  220 . The optical modulation signal is output to an outside via an optical fiber or the like. 
         [0096]    Each of the embodiments described above uses the Mach-Zehnder type optical modulator module as an optical modulator, but the optical modulator is not limited to it. Any optical modulator having a ground electrode and a signal electrode is applicable to the embodiments described above. 
         [0097]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the present invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, and the organization of such examples in the specification does not relate to a showing of the superiority or inferiority of the present 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 present invention.